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2D Transition Metal Dichalcogenides for Photocatalysis

Angewandte Chemie International Edition

January 2023
62(13)

DOI:10.1002/anie.202218016

Authors:

Ruijie Yang

The University of Calgary

Yingying Fan

The University of Calgary

Liang Mei

City University of Hong Kong

Show all 15 authorsHide

Two‐dimensional (2D) transition metal dichalcogenides (TMDs), a rising star in the post‐graphene era, are fundamentally and technologically intriguing for photocatalysis. Their extraordinary electronic, optical, and chemical properties endow them as promising materials for effectively harvesting light and catalyzing the redox reaction in photocatalysis. Here, we present a tutorial‐style review of the field of 2D TMDs for photocatalysis to educate researchers (especially the new‐comers), which begins with a brief introduction of the fundamentals of 2D TMDs and photocatalysis along with the synthesis of this type of material, then look deeply into the merits of 2D TMDs as co‐catalysts and active photocatalysts, followed by an overview of the challenges and corresponding strategies of 2D TMDs for photocatalysis, and finally look ahead this topic.

The pioneers and the annual number of publications of 2D transition metal dichalcogenides (TMDs) for photocatalysis. The number of publications (in the past decade) were collected from Google Scholar at December 22, 2022 by searching the two topical keywords of “2D Transition Metal Dichalcogenides” and “Photocatalysis”.

…

Composition and crystal phase of TMDs. a) Composition of TMDs. b) Schematic models of different phases of TMDs, including hexagonal (2H), octahedral (1T), and distorted octahedral (1T′). Top view shows the typical atomic arrangement of a single layer (top panel). Side view shows the typical packing sequences (bottom panel).

…

Electronic band structure of TMDs. a) The valence band (VB) and conduction band (CB) positions of TMDs as a function of the layer number calculated by Perdew–Burke–Ernzerhof (PBE). The vacuum level of zero is used as reference. Data collected from ref. [40]. b) Calculated band structures of bulk MoS2, and MoS2 flakes with four layers, two layers, and one layer. The arrows in each part indicate the lowest energy transitions. Reproduced with permission from ref. [41]. Copyright 2010, American Chemical Society. c) The valence band (VB) and conduction band (CB) positions of TMDs monolayers calculated by many‐body G0W0 approximation method. The vacuum level is zero used as reference. Data collected from ref. [23]. d) Spectral responses of 2D TMDs. NUV: near ultraviolet; VIS: visible; NIR: near infrared; and SIR: longwave infrared. Data collected from ref. [43]

…

Fundamentals of photocatalysis. a) Basic process of photocatalysis. b) Photocatalytic system configuration. c) Parameters for activity evaluation of photocatalysis. STH: solar‐to‐hydrogen conversion efficiency. AQY: apparent quantum yield. TOF: turnover frequency.

…

+11

Synthesis of 2D TMDs. a) Mechanical cleavage. b) Direct liquid exfoliation. c) Intercalation‐based liquid exfoliation. d) CVD growth. e) Wet‐chemical synthesis. Left panel: hydro/solvothermal synthesis. Right panel: hot‐injection synthesis.

…

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Photocatalysis

2D Transition Metal Dichalcogenides for Photocatalysis

Ruijie Yang+, Yingying Fan+, Yuefeng Zhang, Liang Mei, Rongshu Zhu,* Jiaqian Qin,

Jinguang Hu, Zhangxing Chen, Yun Hau Ng, Damien Voiry, Shuang Li, Qingye Lu,

Qian Wang,* Jimmy C. Yu,* and Zhiyuan Zeng*

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Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDs), a rising star in the post-graphene era, are

fundamentally and technologically intriguing for photocatalysis. Their extraordinary electronic, optical, and chemical

properties endow them as promising materials for effectively harvesting light and catalyzing the redox reaction in

photocatalysis. Here, we present a tutorial-style review of the field of 2D TMDs for photocatalysis to educate researchers

(especially the new-comers), which begins with a brief introduction of the fundamentals of 2D TMDs and photocatalysis

along with the synthesis of this type of material, then look deeply into the merits of 2D TMDs as co-catalysts and active

photocatalysts, followed by an overview of the challenges and corresponding strategies of 2D TMDs for photocatalysis,

and finally look ahead this topic.

1. Introduction

Two-dimensional (2D) transition metal dichalcogenides

(TMDs) constitute a charming material library.[1] They are a

promising alternative to graphene (zero band gap), and have

the potential to go beyond it in some quarters,[2] due to their

versatile chemistry, ranging from true metals (e.g., NbS2and

VSe2), semimetals (e.g., WTe2and TiSe2), and semiconduc-

tors (e.g., MoS2and WS2), to insulators (e.g., HfS2).[2]

This class of materials has recently attracted wide-

ranging interest in electronics, photonics, optoelectronics,

energy conversion, electrocatalysis, environmental remedia-

tion, biosensing, and bioimaging, to name but a few,[1–5]

owing to their extraordinary properties, including atomically

thin nature, strong spin-orbit coupling, tunable band gap,

direct energy gaps of monolayer counterparts in the near-

infrared to a visible spectral region, strong excitonic effects,

and naturally abundance.[1–5]

The past decade has also witnessed a rise in curiosity of

2D TMDs for photocatalysis both fundamentally and

technologically.[6–9] An astounding increase in publications

regarding this topic has been seen in the last few years

(Figure 1), since the pioneering work[10] published in 2008

(using MoS2as co-catalyst for photocatalytic hydrogen

production). Despite this being a young field and challenges

remain, it indeed points to a productive present and a bright

future, both as co-catalysts[10–22] and active photocatalysts

(light-harvesting materials).[23–37]

In this Review, we first introduce the fundamentals of

2D TMDs and photocatalysis as well as the synthesis of this

type of material, before delving into the advantages of them

(as co-catalysts or active photocatalysts) compared to the

classical photocatalytic materials. Then, an essential over-

view is presented toward the challenges and corresponding

strategies (including the engineering of edge site, phase,

doping, vacancy, and interface) of 2D TMDs for photo-

catalysis. Finally, we outlook the future opportunities of

these materials in the photocatalysis field.

1.1. Fundamentals of 2D TMDs

Composition, crystal phases, and electronic band structure

of 2D TMDs are introduced here, aiming to give readers a

preliminary understanding of this material family before

introducing its photocatalytic applications.

[*] R. Yang,+Y. Fan,+Y. Zhang, L. Mei, Prof. Dr. Z. Zeng

Department of Materials Science and Engineering, and State Key

Laboratory of Marine Pollution, City University of Hong Kong

83 Tat Chee Avenue, Kowloon, Hong Kong 999077 (P. R. China)

E-mail: zhiyzeng@cityu.edu.hk

R. Yang,+Y. Fan,+Prof. Dr. J. Hu, Prof. Dr. Z. Chen, Prof. Dr. Q. Lu

Department of Chemical and Petroleum Engineering, University of

Calgary

2500 University Drive, NW, Calgary, Alberta, T2N 1N4 (Canada)

Prof. Dr. R. Zhu

State Key Lab of Urban Water Resource and Environment, School of

Civil and Environmental Engineering, Harbin Institute of Technol-

ogy Shenzhen

Shenzhen 518055 (P. R. China)

E-mail: rszhu@hit.edu.cn

Prof. Dr. J. Qin

Center of Excellence in Responsive Wearable Materials, Metallurgy

and Materials Science Research Institute, Chulalongkorn University

Bangkok 10330 (Thailand)

Prof. Dr. Y. Hau Ng

Low-Carbon and Climate Impact Research Centre, School of Energy

and Environment, City University of Hong Kong

83 Tat Chee Avenue, Kowloon, Hong Kong SAR (P. R. China)

Prof. Dr. D. Voiry

Institut Européen des Membranes, IEM, UMR 5635, Université

Montpellier, ENSCM, CNRS

Montpellier (France)

Prof. Dr. S. Li

College of Polymer Science and Engineering, State Key Laboratory

of Polymer Materials Engineering, Sichuan University

Chengdu (China)

Prof. Dr. Q. Wang

Graduate School of Engineering, Nagoya University

Furo-cho, Chikusa-ku, Nagoya 464-8603 (Japan)

and

Institute for Advanced Research, Nagoya University

Furo-cho, Chikusa-ku, Nagoya 464-8601 (Japan)

E-mail: wang.qian@material.nagoya-u.ac.jp

Prof. Dr. J. C. Yu

Department of Chemistry and Materials Science and Technology

Research Centre, The Chinese University of Hong Kong

Shatin, New Territories, Hong Kong 999077 (China)

E-mail: jimyu@cuhk.edu.hk

Prof. Dr. Z. Zeng

Shenzhen Research Institute, City University of Hong Kong

Shenzhen 518057 (China)

E-mail: zhiyzeng@cityu.edu.hk

[+] These authors contributed equally to this work.

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1.1.1. Composition

TMDs, commonly marked as MX2(M: transition metal

elements from group IVB to group VIII; X: S, Se and Te

elements of group VIA) (Figure 2a), are typical 2D layered

nanomaterials. Each TMD monolayer contains three atom-

layers, forming an “X–M–X’’ sandwich structure.[1] The

adjacent monolayers, with a distance of 6–7 Å, are held

together by weak van der Waals forces, which makes them

readily isolated.[2]

1.1.2. Crystal Phases

TMDs exist in several structural phases (Figure 2b). Specif-

ically, there are three well-known polytypic structures

resulting from different coordination geometry of the

transition metal atoms, namely 2H (trigonal prismatic), 1T

(octahedral), and 1T’(distorted octahedral).[38,39] Their

stacking orders of the three atomic planes (X–M–X) are also

different. 2H-TMDs exhibit a Bernal (ABA) stacking,

whereas the stacking order of atomic planes in 1T-TMDs is

rhombohedral ABC.[1] The diversity of structural phases

gives rise to various electronic properties. For example, 2H-

Ruijie Yang received his M.S. degree

from Harbin Institute of Technology,

China, in 2019. He is currently a Ph.D.

student at the University of Calgary,

Canada. Prior to this, he worked as a

research assistant in Dr. Zhiyuan Zeng’s

group at City university of Hong Kong.

His research interests focus on 2D

materials, catalysis, and in situ charac-

terizations.

Yingying Fan received his M.S. degree

from Harbin Institute of Technology,

China, in 2020. She is currently a Ph.D.

student at the University of Calgary,

Canada. Her research interests focus on

photocatalytic hydrogen production, 2D

transition metal dichalcogenides, and

biomass photo-refining.

Rongshu Zhu studied at the Harbin

University of Technology, receiving her

Ph.D. in 2007. Since then, she has been

engaged in teaching and scientific re-

search at Harbin Institute of Technology

(Shenzhen). In 2010, she was rated as

the local leading talent in Shenzhen. She

established the “Shenzhen Key Labora-

tory for organic pollution prevention

and control” in 2016 and “International

Joint Research Center for Persistent Toxic

Substances (IJRC-PTS) Harbin Institute of

Technology (Shenzhen) Sub Center” in

2017, where she is serving as the director

and executive deputy director, respectively. Her research interests

include catalysts for environmental application and energy conversion.

Qian Wang is currently an Associate

Professor at Nagoya University, Japan.

She obtained her Ph.D. in 2014 at the

University of Tokyo, Japan, where she

worked on the development of perov-

skite-type oxide photocatalysts for visi-

ble-light-driven water splitting. She then

worked as a postdoctoral researcher at

the Japan Technological Research Asso-

ciation of Artificial Photosynthetic Chem-

ical Processes (ARPChem) on the devel-

opment of standalone photocatalyst

devices for overall water splitting. In

2018, she became a Marie Sklodowska-

Curie Research Fellow at the University of Cambridge to develop

inorganic-organic hybrid photocatalysts. She joined Nagoya University

as an Associate Professor in May 2021 and established her research

group, which is currently developing new materials, approaches, and

technologies for solar energy storage in the form of renewable fuels via

artificial photosynthesis.

Jimmy C. Yu is an emeritus professor at

The Chinese University of Hong Kong.

Before retiring from teaching, he was

Choh-Ming Li Professor of Chemistry and

Head of United College. Professor Yu

received a B.S. degree from St. Martin’s

College and a Ph.D. from the University

of Idaho. He has been a highly cited

researcher for many years. His major

research interest is photocatalysis, and

he holds several patents on the fabrica-

tion and application of photocatalytic

nanomaterials. Prof. Yu has received

numerous honors including a State

Natural Science Award and Chang Jiang Scholar Chair Professorship.

Zhiyuan Zeng is an assistant Professor in

Department of Materials Science and

Engineering, City University of Hong

Kong. He obtained his B.E. and M.E. from

Central South University, Zhejiang Uni-

versity, in 2006 and 2008, respectively.

He completed his Ph.D. at Nanyang

Technological University, Singapore

(2013). He started his postdoctoral train-

ing at Lawrence Berkeley National Labo-

ratory (2013–2017), followed by Senior

Engineer at Applied Materials Inc. (2017–

2019, Silicon Valley); he joined CityU in

2019. His research interests include lith-

ium intercalation & exfoliation method for 2D TMDs, in situ liquid

phase TEM, environmental catalysis, water desalination, etc.

Angewandte

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MoS2shows semiconducting attributes,[38] whereas 1T-MoS2

exhibits a metallic character.[1]

1.1.3. Electronic Band Structure

TMDs possess a tunable (thickness-dependent) electronic

band gap (Figure 3a), resulting from quantum confinement

effect. With 2H-MoS2as exemplification, their calculated

electronic band gap values range from 0.88 eV (bulk) to

1.71 eV (monolayer).[1, 40] Interestingly, when the thickness

of MoS2decreases to monolayer, it changes from an indirect

band gap semiconductor to a direct band gap semiconductor,

along with a dramatic jump in luminescence (Figure 3b).[41]

Additionally, �2.16 eV is the experimental value of the

electronic band gap for monolayer MoS2.[42] Naturally, the

band gap values between different TMD monolayers are

also different (Figure 3c),[23] which leads to their different

spectral responsivities (Figure 3d).[43]

1.2. Fundamentals of Photocatalysis

Rudimentary knowledge of photocatalysis is introduced

here, aiming to help readers have a primary impression of

this catalytic technology, before focusing it on 2D TMDs.

1.2.1. Advantages

Photocatalysis employs solar energy, which is an abundant,

non-polluting and renewable natural energy source, to drive

Figure 1. The pioneers and the annual number of publications of 2D transition metal dichalcogenides (TMDs) for photocatalysis. The number of

publications (in the past decade) were collected from Google Scholar at December 22, 2022 by searching the two topical keywords of “2D

Transition Metal Dichalcogenides” and “Photocatalysis”.

Angewandte

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chemical processes, representing a promising approach to

store the solar energy into high-valued chemicals such as

hydrogen, methane and carbon monoxide.[44–46] It shows

superiorities over traditional catalysis. Unlike traditional

thermal catalysis, which typically requires high temperatures

and a high-pressure operating environment, most photo-

catalysis is operated under ambient conditions.[44, 47,48] Be-

sides, overoxidation and catalyst deactivation are common

problem of thermal catalysis, but can be avoided in

photocatalysis.[44] All these factors contribute to the advant-

age of photocatalysis, which points to a bright future.

1.2.2. Basic process

In general, the basic photocatalytic process involves three

steps (Figure 4a):[49–53] (1) light absorption by the light-

absorbing material in photocatalyst; (2) separation and

migration of the photogenerated charge carriers; (3) surface

redox reactions with the assistance of cocatalysts on the

semiconductor surface. Of note, the carrier migration

process is always accompanied by carrier recombination,

including bulk recombination and surface recombination,

which is not conducive to photocatalysis and should be

avoided as much as possible.

Figure 2. Composition and crystal phase of TMDs. a) Composition of TMDs. b) Schematic models of different phases of TMDs, including

hexagonal (2H), octahedral (1T), and distorted octahedral (1T’). Top view shows the typical atomic arrangement of a single layer (top panel). Side

view shows the typical packing sequences (bottom panel).

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Figure 3. Electronic band structure of TMDs. a) The valence band (VB) and conduction band (CB) positions of TMDs as a function of the layer

number calculated by Perdew–Burke–Ernzerhof (PBE). The vacuum level of zero is used as reference. Data collected from ref. [40]. b) Calculated

band structures of bulk MoS2, and MoS2flakes with four layers, two layers, and one layer. The arrows in each part indicate the lowest energy

(CB) positions of TMDs monolayers calculated by many-body G0W0approximation method. The vacuum level is zero used as reference. Data

collected from ref. [23]. d) Spectral responses of 2D TMDs. NUV: near ultraviolet; VIS: visible; NIR: near infrared; and SIR: longwave infrared. Data

collected from ref. [43]

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1.2.3. Thermodynamics requirements

The occurrence of photocatalytic reaction needs to meet

thermodynamics requirements:[54–57] (1) the energy of inci-

dent photons should be equal to or greater than the optical

band gap (Eg) of the semiconductor; (2) the valance band

maximum (VBM) of semiconductor needs to be more

positive than the oxidation potential of the donor (E(D/D));

(3) while the conduction band minimum (CBM) needs to be

more negative than the reduction potential of the acceptor

(E(A+/A)).

1.2.4. Kinetic Challenges

In addition to the above-mentioned thermodynamics re-

quirements, photocatalysis also faces a lot of kinetic

challenges:[58] (1) the three basic steps span a huge timescale

from 1015 to 101s (1015–109s of light absorption, <1015 s

of charge separation and transport, and 103–101s of surface

reactions),[59] which poses a great challenge to maximize the

synergy between the three-step reaction; (2) an optimum

photocatalyst needs to simultaneously hold broad light-

absorption (needing narrow band gap) in the solar radiation

spectrum, and strong redox ability (needing wide band gap),

but their realization is inherently contradictory; (3) the

semiconductor photosensitizers typically lack active sites.

Therefore, cocatalysts are normally required to be loaded

Figure 4. Fundamentals of photocatalysis. a) Basic process of photocatalysis. b) Photocatalytic system configuration. c) Parameters for activity

evaluation of photocatalysis. STH: solar-to-hydrogen conversion efficiency. AQY: apparent quantum yield. TOF: turnover frequency.

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on the semiconductor surface to promote charge separation

and transfer and reduce the activation energy.[60]

1.2.5. Photocatalytic System Configuration

Reasonable construction of a photocatalytic system is vital

for photocatalysis. In general, there are two types of system

configurations in photocatalysis:[6,45] “suspension” system

and “fixed” system (Figure 4b). In the “suspension” system,

the particulate photocatalyst is directly suspended in the

reaction solution. As the name suggests, the “fixed” system

refers to photocatalyst fixed on a substrate for photo-

catalytic reaction. For detail information about the photo-

catalytic system configuration, the reader is referred to a

current review article.[6]

1.2.6. Parameters for Activity Evaluation

The ultimate pursuit of photocatalysis is the efficient

utilization of solar energy. Thus, how to evaluate efficiency

of the system is important. Herein, we outlined the common

parameters for the evaluation of photocatalytic activities

(Figure 4c): solar-to-chemical conversion efficiency (for

example, solar-to-hydrogen conversion efficiency, STH),

apparent quantum yield (AQY), and turnover frequency

(TOF). In addition to the above-mentioned parameters,

stability is another important index for the practical

application of photocatalysis, which is worthy of attention.

The direct evaluation strategy for stability is to conduct a

long-term photocatalytic reaction test. For more information

about the efficiency accreditation and testing protocols of

photocatalysts, the reader is referred to a current author-

itative article.[61]

1.3. Synthesis of 2D TMDs

Reliable synthesis methods of 2D TMDs are presented here

to provide readers a concise guide for the synthesis of this

type of material. Ideally, the synthesis methods of 2D TMDs

can be divided into two categories: top-down exfoliation

(from bulk to 2D nanosheets; exampled by mechanical

cleavage,[41,62, 63] direct liquid exfoliation,[64, 65] and intercala-

tion-based liquid exfoliation[66–70]) and bottom-up synthesis

(from small building block molecules to 2D nanosheets;

exampled by chemical vapour deposition growth[71–73] and

wet-chemical synthesis[74–77]). The pros and cons of each

method and their main applications are listed in Supporting

Information Table 1.

1.3.1. Mechanical Cleavage

Mechanical cleavage (Figure 5a), employing scotch tape to

cleave atom-layers from their bulk crystal, is a simple and

common method for the exfoliation of TMDs.[78] The

produced 2D TMDs have the characteristics of single crystal,

high purity, and cleanliness, which are suitable for device

applications.[1] Theoretically, this method can be used to

exfoliate all the layered bulk TMDs to generate the ultrathin

2D counterparts, but it is not scalable.[79] Additionally, it is

relatively difficult for the systematic controlling of the

thickness, size, and shape of the final products.

1.3.2. Direct Liquid Exfoliation

For obtaining exfoliated 2D TMDs on a large scale

(mechanical cleavage is not feasible), liquid exfoliation is

promising.[4] Direct liquid exfoliation (Figure 5b) refers to

the direct exfoliation of the bulk TMDs into their ultrathin

2D counterparts in solvents, by breaking the weak van der

Waals interaction via sonication or shear forces.[64, 65] N-

methyl-pyrrolidone (NMP) and dimethylformamide (DMF)

are the commonly used two solvents.[64] Simple operation

steps, low costs, solution-processability, and insensitivity to

air and water make it show great potential in commercial

applications. However, direct liquid exfoliation is plagued by

the low-yield of the monolayers, the small lateral size of the

exfoliated flakes and the toxicity of the used organic

solvents.[65]

1.3.3. Intercalation-based Liquid Exfoliation

Intercalation-based liquid exfoliation[66–70,80] (Figure 5c) is an

up-and-coming alternative for direct liquid exfoliation, and

have the potential to overcome its deficiency mentioned

above.[70] The underlying principle of intercalation-based

liquid exfoliation is to enlarge the interlayer spacing and

thereby weaken the interlayer adhesion before exfoliation

by intercalating foreign species (ions or molecules).[70] The

typical procedure involves chemical or electrochemical

intercalation of foreign species into the interlayer spaces of

layered TMDs, followed by a mild exfoliation process.[70]

Lithium-ion (Li+) is a commonly used intercalant, but

always introduces basal-plane defects and induces 2H-to-1T/

1T’phase transitions.[66,67, 70] Larger ions, such as tetraalky-

lammonium cation (R4N+), can avoid such phase transitions,

but the increased size inevitably leads to the increase in

intercalation barrier, and thus reduces the intercalation rate

of bulk TMDs crystals, which thereby decrease the exfolia-

tion rate and the yield of monolayers.[68, 69] Low energetic

cost, solution-processability, and yield of large-area TMD

monolayers are advantageous of intercalation-based liquid

exfoliation strategy.

1.3.4. CVD Growth

CVD growth (Figure 5d) is a classical bottom-up strategy

for the wafer-scale synthesis of 2D TMDs on substrates

(e.g., the traditional SiO2/Si).[71–73] In a typical process, the

vaporized reactive precursors (e.g., the sulphur powder and

MoO3powder[71]) pass through the surface of the target

substrate with the specific gas flow under a high temperature

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Figure 5. Synthesis of 2D TMDs. a) Mechanical cleavage. b) Direct liquid exfoliation. c) Intercalation-based liquid exfoliation. d) CVD growth.

e) Wet-chemical synthesis. Left panel: hydro/solvothermal synthesis. Right panel: hot-injection synthesis.

Angewandte

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and high vacuum environment, during which the precursors

undergo reaction, decomposition, in situ nucleation and

growth on the substrate surface, finally forming the 2D

TMD flakes.[71–73] The produced 2D TMDs are large-size,

high-purity, and high-crystallinity. However, this method is

hampered by the harsh working environment (high temper-

ature and high vacuum).[1]

1.3.5. Wet-chemical Synthesis

2D TMD flacks can also be prepared by a wet-chemical

synthesis process (another famous bottom-up strategy), such

as hydro/solvothermal synthesis (left panel of Figure 5e)[74,75]

and hot-injection synthesis (right panel of Figure 5e).[76, 77]

This method is simpler and easier to operate than CVD

growth, but the product quality and crystallinity are not as

good as those obtained by CVD growth. Hydro/solvother-

mal synthesis of 2D TMDs occurs under high temperature

and high pressure in a sealed vessel and uses water/organic

solvent as the reaction medium, during which metal

precursor (such as molybdic acid) and chalcogen precursor

(such as thiourea) reacted with each other to form TMD

crystals. The resulting 2D TMD flacks hold rich defects and

active edge sites, showing outstanding catalytic

performance,[74,75] but their thickness is not conclusively

shown to be single layers. Such a method is simple and

scalable, but the product quality is sensitive to reaction

conditions, such as operating temperature, time, type and

concentration of precursors, solvents and surfactants. Hot-

injection method is an effective strategy for 2D TMDs

colloid synthesis, which induce chemical reactions by rapidly

injecting active reactant molecules (e.g., CS2and WCl6

precursors) into a hot solution containing special surfactants

(long chain oleylamine and/or oleic acid).[76,77] Such a hot-

injection method produces 2D TMDs with good dispersity,

high purity, and uniform size and shape, but limitations still.

The uses of hard-to-remove surfactants (long chain oleyl-

amine and oleic acid) are unfavorable for catalytic applica-

tions. Besides, the special operation of the precursor

injection leads to the impossibility of mass production.

1.3.6. Others

Beyond these commonly used methods mentioned above,

the past few years also saw the emergence of some novel

strategies for the production of 2D TMDs, such as molecular

beam epitaxy (MBE),[81] atomic layer deposition (ALD),[82]

pulsed laser deposition,[83] and atomic layer etching by

planar surface plasmon polaritons (SPPs).[84] The emergence

of these technologies enriches the toolbox of 2D TMDs

preparation.

1.3.7. Analysis

Rich methodologies have been established with their own

merits for the synthesis of 2D TMDs flakes (Supporting

Information Table 1). Still, none of them are yet perfect.

Which method is more suitable for researchers (especially

new comers)? Demands decide choices. For example, acting

as active photocatalyst needs 2D TMDs hold semiconduct-

ing phase; mechanical cleavage,[41,62, 63, 78] direct liquid

exfoliation,[64,65] organic ammonium ion intercalation-based

exfoliation,[68,69] CVD growth,[71–73] and hydrothermal

method[74,75] are feasible for the synthesis of semiconducting

phase 2D TMDs. By contrast, hydrothermal method is the

most-friendly one for novices, because it is simple, easy to

operate and has a high success rate. 2D TMDs as cocatalyst,

preferably metallic phase; mechanical cleavage,[78] lithium-

ion intercalation-based exfoliation,[66,67, 70] CVD growth,[85, 86]

and hydrothermal method[87,88] are workable for the fabrica-

tion of metallic phase 2D TMDs. Our recommend one is the

lithium-ion intercalation-based exfoliation method[66,67, 70] (of

particular the electrochemical lithium-ion intercalation-

based exfoliation method[67,70]). Because this method is more

capable of large-scale preparation of metallic phase 2D

TMDs than the mechanical cleavage, it is easier to operate

than the CVD growth, and the phase of the synthesized 2D

TMDs is purer than the hydrothermal method.[70] For the

detailed operation process of electrochemical lithium-ion

intercalation-based exfoliation method, the researchers (of

particular beginners) are recommended a recently published

protocol.[70] Of note, this method needs to be carried out in

an argon-filled glovebox to prevent lithium from being

oxidized by air. The second choice for synthesizing metallic

phase 2D TMDs is hydrothermal method, which is simple

and easy to operate, although the phase synthesized is not

too pure.[87,88]

2. Why 2D TMDs for Photocatalysis

Here gives the advantages of 2D TMDs in photocatalysis

compared to the classical photocatalytic materials (including

the classical cocatalysts, Pt, Pd, Rh, Ru, and Au, the classical

semiconductor photocatalysts, TiO2, WO3, and CdS, as well

as the classical 2D photocatalytic materials, graphene and

graphitic carbon nitride), to explain why there is a possibility

to replace the classical photocatalytic materials.

2.1. Merits of 2D TMDs as Cocatalysts

2D TMDs (of particular their metallic phase crystal, for

example 1T-MoS2) are promising cocatalysts in

photocatalysis[10–22] (some representative examples of 2D

TMDs as cocatalysts are listed in Supporting Information

Table 2). They are an ideal alternative to noble metals and

even have the potential to go beyond noble metals in

performance. For example, 2D MoS2[10] and WS2[11] as

cocatalysts showed a better promoting effect on CdS for

photocatalytic H2evolution from 10 vol % lactic solution

than noble metals, including Pt, Pd, Rh, Ru, and Au

(Figure 6a). Such remarkable performance of metallic phase

2D TMDs as cocatalysts in photocatalysis results from their

extraordinary features (listed below).

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Figure 6. Merits and functions of 2D TMDs as cocatalyst in photocatalysis. a) The rate of H2evolution on CdS loaded with 0.2 wt % of MoS2, Pt,

Ru, Rh, Pd and Au. Date collected from ref. [10]. b) Merit of metallic phase 2D TMDs as cocatalyst: good conductivity. c) Function of metallic phase

2D TMDs as cocatalyst: providing trapping sites for photo-generated charges and promote charge separation, migration, and collection. d) The

electron occupations in Mo 4d orbital in 1T-MoS2and 2H-MoS2crystal fields. e) Merit of metallic phase 2D TMDs as cocatalyst: rich active sites on

both the edge and basal plane. f) Function of metallic phase 2D TMDs as cocatalyst: playing an active role as adsorption, activation, and reaction

site of reactant molecules to lower the activation energies and accelerate reaction kinetics.

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2.1.1. Good Conductivity

Metallic phase 2D TMDs hold good conductivity (Fig-

ure 6b),[1,89] enabling them to provide trapping sites for

photo-generated charges and promote charge separation,

migration, and collection, thus improving quantum efficiency

(Figure 6c). For example, metallic 1T phase MoS2(with high

electrical conductivity of 10–100 S cm1) is almost 107times

more conductive than its semiconducting 2H phase and is

close to graphene (�100 S cm1),[89] which endows it with

the role of ideal cocatalyst in photocatalysis like

graphene.[16,18] Generally, the high conductivity of materials

is related to their electronic structure and atomic

arrangement.[90] For metallic 1T phase MoS2, it has the

incomplete occupation of the lower-lying t2g orbital (Fig-

ure 6d),[90] which makes it easy for the outermost valence

electrons to break free from the atomic nucleus and become

free electrons. This property is the same as that of a metal

atom, which leads to its high conductivity like that of a

metal.

2.1.2. Rich Active Sites on Both the Edge and Basal Plane

In parallel, metallic phase 2D TMDs have rich active sites

on both the edge and basal plane (Figure 6e),[91] endowing

them to play an active role as adsorption, activation, and

reaction site of reactant molecules to lower the activation

energies and accelerate reaction kinetics (Figure 6f). This

point of view (rich active sites on both the edge and basal

plane) was confirmed by theoretical calculations and exper-

imental verification, which will be described in detail below

(with hydrogen evolution reaction as an exemplification).

In the field of photocatalysis, active sites are normally

located at the position with strong reactivity. The reactivity

of catalyst is related to adsorption energy.[7] Sabatier

principle shows that the ideal free energy of adsorption

should be zero.[92] Over free energy of adsorption always

leads to a poor binding between the catalyst and the reactant

(causing the low reaction rate).[92] Insufficient free energy of

adsorption will result in a slow desorption of atoms (leading

to the poisoning of the catalyst surface).[92] Both of these

situations lead to weak reactivity.

Pioneering theoretical calculations (based on density

functional theory) has shown a zero-nearing Gibbs free

energy of adsorption of hydrogen atom (ΔGH) for the

metallic phase TMDs edge[93, 94] and basal plane (Fig-

ure 7a,b, d),[95] reveling that metallic phase 2D TMDs have

strong reactivity (rich active sites) on both the edge and

basal plane. Numerous substantial studies have also corro-

borated this view experimentally.[91,96,97] However, for semi-

conducting phase 2D TMDs, although their edge sites have

zero-nearing ΔGH(Figure 7a,b), their basal plane’ ΔGHare

usually very high (Figure 7c).[95] This reveals that their

(semiconducting phase 2D TMDs) active sites for hydrogen

evolution reaction are only at the edges, and their basal

planes are catalytically inert.[95]

2.1.3. Dense Interface Junctions

Crucially, their (2D TMDs) atomically thin nature grants

them to form dense interface junctions with light-harvesting

semiconductors, which guarantees the rapid transfer of

photo-induced charges.[16] Such dense interface junctions are

constructed by chemical bond bridge or van der Waals

force.

Chemical bond bridges are easily formed between 2D

TMDs and light-harvesting semiconductors, such as BiS

bond bridges in Bi12O17Cl2

MoS2interface (Figure 8a),[16]

ZnS bond bridges in ZnIn2S4

MoS2interface (Fig-

ure 8b),[18] and ORe (or OMo) bond bridges in TiO2

ReS2

or TiO2

MoS2interface (Figure 8c).[20,21] Such bridges pro-

vide a high-speed channel for electron migration, thus

promoting the separation of photo-generated electron-hole

pairs, prolonging the carrier lifetime (II panel in Figure 8a)

(3446 ns of Bi12O17Cl2

MoS2; 136 ns of Bi12O17Cl2), and

improving photocatalytic efficiency (32.68 mmol·g1·h1for

H2evolution of Bi12O17Cl2

MoS2; 0.86 mmol g1h1for H2

evolution of Bi12O17Cl2). This is because the existence of

such bridges reduces the energy barrier and distance of

electron migration compared with bridge-free interface; for

example, the energy barrier and distance of electron

migration at BiS bond bridges is 5 eV and 2.3 Å, respec-

Figure 7. Theoretical calculated (based on density functional theory)

thermodynamics of hydrogen adsorption on 2D transition metal

dichalcogenides. a) The differential ΔGHas a function of the ΔGHX

(X =S) of TMD monolayers edge sites. b) The differential ΔGHas a

function of the ΔGHX (X =Se) of TMD monolayers edge sites. c) The

differential ΔGHas a function of the ΔGHX of semiconducting TMD

monolayers basal planes. e) The differential ΔGHas a function of the

ΔGHX of metallic TMD monolayers basal planes. ΔGH: Gibbs free

energy of adsorption of hydrogen atom. ΔGHX: Gibbs free energy of

adsorption of XH (X =S or Se). Reproduced with permission from

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Figure 8. Electron bridge formed between metallic phase 2D TMDs and light-harvesting semiconductors. a) BiS electron bridge formed in

Bi12O17Cl2

MoS2, showing an electron transfer from Bi to S along BiS bond bridge. Top left panel in part I: schematic illustration of the

morphology. Bottom left panel in part I: schematic illustration of the occurred photocatalytic redox reaction, showing that H2evolution reaction

(reductive reaction) occurred on MoS2layer; while, ascorbic acid conversion reaction (oxidation reaction) occurred on Bi12O17Cl2layer. Right panel

in part I: schematic illustration of the formed electron bridge. Part II: femtosecond-resolved transient absorption (TA) spectroscopy of

Bi12O17Cl2

MoS2with BiS bonds (red curve) and without BiS bonds (bule curve). Part III: the calculated electrostatic potentials of (Cl2)-(Bi12O17)-

(MoS2) with BiS bonds. Part IV: the calculated electrostatic potentials of (Cl2)-(Bi12O17)-(MoS2) without BiS bonds. Top left panel in part I and

ZnIn2S4

MoS2, showing an electron transfer from Zn to S along ZnS bond bridge. Top panel: schematic illustration of the morphology and

formed electron bridge. Bottom panel: schematic illustration of the occurred photocatalytic redox reaction, showing that H2evolution reaction

(reductive reaction) occurred on MoS2layer; while, lactic acid conversion reaction (oxidation reaction) occurred on ZnIn2S4layer. Top panel is

MoS2, showing

an electron transfer from O to Mo along OMo bond bridge. Top panel: schematic illustration of the morphology and formed electron bridge.

Bottom panel: schematic illustration of the occurred photocatalytic redox reaction, showing that H2evolution reaction (reductive reaction) occurred

on MoS2layer; while, ethanol conversion reaction (oxidation reaction) occurred on TiO2layer. Top panel is reproduced with permission from

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tively (III panel in Figure 8a); while, for BiS bond bridge

free interface, they are 17 eV and 4.5 Å, respectively (IV

panel in Figure 8a).

Alternatively, a plethora of dense interface junctions can

be formed by combine 2D TMDs with several other 2D

nanosheets in one vertical stack,[98–100] held together by van

der Waals forces[101–104] (the same forces that hold layered

crystals together). Such dense interface junctions have

ultrafast excited-state dynamics, allowing ultrafast electron

migration, long-lived spin and valley polarization in resident

carriers, and thereby promoting the separation of photo-

generated electron-hole pairs.[105]

2.1.4. Some Other Attractive Interfacial Surface Properties and

Behaviors

Metallic phase 2D TMDs also hold some other attractive

interfacial surface properties and behaviors, including active

site self-optimization,[22] rich sulfur vacancies as electron

enrichment domains,[106] induced photothermal effect,[107]

and beyond. These properties and behaviors stimulate more

potential for them as cocatalysts in photocatalysis. In the

following, we take the active site self-optimization as a

notable example to give a brief introduction.

Pioneering work by Wang et al.[22] has shown that 1T

phase MoS2(normally been considered as the main source

of active sites in hydrogen production reactions) can be

irreversibly converted to 1T’phase (a more catalytically

active phase) as true active sites as cocatalysts in photo-

catalytic hydrogen production reactions, named “active site

self-optimization”. They proposed that the adsorption of

hydrogen atoms will cause electron transfer, which is the

main driving force for this phase transition. Such an “active

site self-optimization” behaviors greatly boosted the photo-

catalytic performance of TiO2in the photochemical

environment.[22]

2.2. Merits of 2D TMDs as Active Photocatalysts

Semiconducting phase 2D TMDs (for instance 2H-MoS2)

are potential candidates of active photocatalysts (light-

harvesting materials)[24–26,28–37] (some representative exam-

ples of 2D TMDs as active photocatalysts are listed in

Supporting Information Table 3). Their advantages as active

photocatalysts are list below.

2.2.1 Narrow Band Gaps

They hold narrow band gaps (normally <2.4 eV, for

instance, 2.16 eV of MoS2monolayer[42]), endowing them

with wider spectral absorption (cut-off wavelength ranging

from near-infrared to visible spectral[43]) and more efficient

solar spectrum utilization (for example, �50% of solar

spectrum utilization of few-layer MoS2[25]) than traditional

semiconductors (e.g., TiO2–3.2 eV-4% of solar spectrum

utilization, WO3–2.8 eV-10% of solar spectrum utilization,

and CdS-2.4 eV-20 % of solar spectrum utilization) (Fig-

ure 9). Besides, their narrow band gaps resulting in their

Figure 9. Band gaps and light-absorption properties of semiconducting phase 2D TMDs and some representative photocatalysts.

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strong light-matter interactions,[108,109] ensuring their strong

electron-hole creation.

2.2.2. Well-placed Energy Levels

In parallel, their well-placed energy levels meet the

thermodynamics requirements of a variety of photocatalytic

reactions (Figure 10), including hydrogen (H2)

evolution,[23,29, 30, 35, 110] carbon dioxide (CO2) reduction,[32] and

nitrogen (N2) fixation,[27,111] sterilization,[25, 31] pollution

degradation,[24] and organic synthesis,[26,28] which have been

proven theoretically and experimentally.

2.2.3. Atomically Thin Nature

More to the point, their atomically thin nature grants them

more extraordinary properties as light-harvesting materials

in photocatalysis, including quantum confinement, short

transmission distance of carriers, large surface-area-to-

volume ratio, and so on.[6] Quantum confinement effect

endows them with tunable band structure, proven by

theoretical calculation[40] (Figure 3a) and experimental

detection[112] (Figure 11a), resulting in diversified optical

properties to meet a variety of photocatalytic reactions. The

short transmission distance of carriers from the inner to the

surface is good for the effective suppression of bulk

recombination of carriers, prolonging the lifetime of light

quanta, and thereby improving the quantum efficiency

(Figure 11b). Large surface-area-to-volume ratio is in favor

of light capturing, and makes a great contribution to rich

point contact between the reactants and catalytic sites, and

provides a good platform for the formation of doping, non-

killer defects and heterojunction, which are all beneficial for

the further enhancing of the photocatalytic performance

(Figure 11c).

2.2.4. As Ideal Platform for Fundamental Research

Additionally, semiconducting phase 2D TMDs, especially

monolayer, can act as an ideal platform to demonstrate

some fundamental principles in photocatalysis, resulting

from their unique properties mentioned above. For example,

they can be used to demonstrate the direction and time of

carrier transfer after light excitation at the heterostructure

interface.[108] A prior study has experimentally verified

this.[108] In this study, using 2D TMDs-based van der Waals

heterostructure (constructed with MoS2monolayer and WS2

monolayer) as a model, ultrafast transfer of carriers at this

van der Waals interface was demonstrated.[108] Photolumi-

nescence mapping and femtosecond pump-probe spectro-

scopy revealed that photoinduced holes migrate from the

MoS2layer to the WS2layer in a very short time ( �50 fs)

after light excitation (Figure 11d).

2.3. Unique Merits of 2D TMDs for Photocatalysis Compared

with Classical 2D Photocatalytic Materials

Why 2D TMDs can stand out in photocatalysis from many

classical 2D photocatalytic materials is explained here from

the current perspective.

2.3.1. Comparison with Graphene

Graphene, while being theoretically and practically interest-

ing for photocatalysis as classical 2D cocatalysts,[113–115] is

chemically inert and zero-band-gap.[2] This limits graphene

to acting as a cocatalyst in photocatalysis and always need to

be activated by functionalization with desired molecules.[116]

2D TMDs exhibit versatile chemistry, ranging from true

metals (zero-band-gap, like graphene), semimetals (near-

zero-band-gap), to semiconductors (narrow-band-gap).[2]

This offers 2D TMDs the opportunities for diverse funda-

mental and technological researches in a variety of photo-

catalytic reactions both acting as cocatalysts and active

photocatalysts. Such a diversity is the most obvious advant-

age of 2D TMDs over graphene in photocatalysis.

2.3.2. Comparison with Graphitic Carbon Nitride

The graphitic carbon nitride (g-C3N4),[117–119] a representative

2D semiconductor photocatalyst, has become a new research

hotspot in the field of photocatalysis. Its band gap is 2.7 eV,

theoretically resulting in 10%-20 % of solar spectrum

utilization.[117–119] As previously mentioned, 2D TMDs hold

narrower band gaps (normally <2.4 eV[42]), giving them with

wider spectral absorptions[43] and more efficient solar

spectrum utilizations (>20% of solar spectrum

utilization[25]). Such an efficient solar spectrum utilization is

the advantage of 2D TMDs over g-C3N4as active photo-

catalysts in photocatalysis.

Undeniably, from the current perspective, graphene (as

cocatalysts) and g-C3N4(as active photocatalysts) are more

popular and mature in photocatalysis. However, 2D TMDs

are promising photocatalytic materials and are expected to

become the next research hotspot, due to their diversity and

effective spectral utilization.

3. Challenges and Strategies

Although the photocatalysis of 2D TMDs is promising, it

still faces numerous challenges. The corresponding design

considerations and engineering strategies to address these

challenges are needed (Supporting Information Table 4).

3.1. Challenges

Semiconducting phase 2D TMDs as active photocatalysts

face the challenges of catalytically inert basal plane,[120]

sluggish carrier dynamics (originating from their high

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electrical resistance which hinders charge transfer),[96] and

the mutual compromise of light spectrum harvesting range

versus redox potentials. Several engineering strategies,

including the engineering of edge site, phase, doping and

Figure 10. Well-placed energy levels of 2D TMDs and the redox potentials of some common photocatalytic reactions. Top panel: valence band (VB)

and conduction band (CB) positions of TMD monolayers, as well as the redox potentials of some common photocatalytic reactions at pH =0.

Bottom panel: a summary table, reflecting whether various TMD monolayers meet the thermodynamic requirements of different reactions. The

solid circle represents meeting the thermodynamic requirements of the specified reaction. The blank represents not meeting the thermodynamic

requirements of the specified reaction. SHE: standard hydrogen electrode. The data of the VB and CB positions in the top panel are collected from

ref. .[23].

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Figure 11. Advantages of 2D TMDs as photosensitizers: quantum confinement, short transmission distance of carriers, large surface-area-to-

volume ratio, and an ideal platform for fundamental research. a) Angle-resolved photoemission spectroscopic (ARPES) spectra of MoSe2with one

Publishing Group. b) Schematic illustration of short transmission distance of carriers in 2D TMDs. c) Schematic illustration of large surface-area-

to-volume ratio of 2D TMDs. d) An example of 2D TMDs as ideal platform for fundamental research. Left panel: a schematic illustration of the

theoretically predicted band alignment and carrier transfer direction at the heterostructure composed of MoS2monolayer and WS2monolayer.

Right panel: a schematic illustration of the formed heterostructure composed of MoS2monolayer and WS2monolayer. Reproduced with permission

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vacancy, as well as interface have been applied to deal with

these issues and are introduced below.

Additionally, 2D TMDs are prone to agglomeration and

chemical changes in the storage process due to their high

surface energy and rich active sites, lacking long-term

stability and durability. It is feasible to cope with this

challenge by storing 2D TMDs in the form of suspension

solution, along with stabilizing them via rheological

refinements,[121] or covalent functionalization.[122] Besides,

agglomeration of 2D TMDs also occurs frequently during

catalytic reactions. The effective way to solve this problem is

to grow 2D TMDs on the substrate.

3.2. Edge Site Engineering

Semiconducting phase 2D TMDs hold catalytically inert

basal planes,[120] and their catalytic active centers are often

located at the edges, where rich unsaturated atoms are

exposed.[29] Therefore, creating well-exposed active edge

sites (named edge site engineering) is a reasonable approach

to enhance the photocatalytic activity of 2D TMDs.

Using CVD method to construct vertically grown 2D

TMDs on the substrate has proved to be an effective

strategy for creating well-exposed active edge sites (Fig-

ure 12a).[25,29, 31] Such 2D MoS2is ~ 15 times as effective as

bulk MoS2in photocatalytic sterilization (log inactivation

efficiency of the indicator bacteria).[25] Besides, such 2D

ReS2also shows boosted photocatalytic performance for H2

production[29] and sterilization.[31] Alternatively, introducing

pores on the surface of 2D TMDs by a ball-milling method

is also a powerful tool for enriching boundary and edge sites

(Figure 12b).[120] Recent study shows that such MoS2with

rich pores (or said with fully activated basal planes) displays

excellent photocatalytic performance for H2evolution.[120] In

addition, there are also some potentially feasible strategies

for creating well-exposed active edge sites to boost their

photocatalytic performance, such as building a spiral

screwed structure 2D TMDs (Figure 12c).[123]

It is clear that creating well-exposed active edge sites

(edge site engineering) is a viable way to boost the photo-

catalytic activity of 2D TMDs.[25,29,31, 120] Still, this strategy is

in its infancy and its implementation often requires harsh

conditions (such as the high temperature and high vacuum

conditions[25,29, 31]). Continuous efforts should be devoted to

this field (edge site engineering of 2D TMDs for photo-

catalysis), with more emphasis on clever structural design

and easy edge site synthesis.

3.3. Phase Engineering

Previous theoretical[38,124] and experimental studies[125, 126]

demonstrated the diverse properties of 2D TMDs with

different phases. For example, 2H-MoS2monolayer is a

semiconductor with catalytically inert basal plane[120] and

sluggish carrier dynamics due to high electrical resistance;[96]

1T-MoS2monolayer holds zero band gap, showing metallic

nature, catalytically active basal plane, and high

conductivity;[97] 1T’-MoS2possesses only a tiny band gap,

being a quasi-metallic phase and sharing properties similar

to those of the metallic 1T phase.[97]

Therefore, theoretically, introducing metallic/quasi-met-

allic phase 2D TMDs (as cocatalysts) into semiconducting

phase 2D TMDs (as photosensitizers), named phase engi-

neering, is expected to achieve complementary properties

and synergetic catalytic effects, thereby boosting the photo-

catalytic activity of semiconducting phase 2D TMDs.

Among them, metallic phase TMDs, acting as cocatalysts,

can accelerate the carrier dynamics and enrich the catalytic

sites. Besides, at the formed in-plane interface, photo-

induced electrons can quickly migrate from semiconducting

phase to metallic phase, promoting the separation of photo-

generated electron and hole and improving the quantum

efficiency (Figure 12d).

The feasibility of phase engineering to improve photo-

catalytic activity has been verified experimentally.[97,111, 126, 127]

For example, 2H-1T-1T’multiphasic MoS2nanosheets[97]

and 2H/1T mixed-phase Mo1xWxS2nanosheets[111] show

boosting photocatalytic performance for H2production from

proton reduction, and ammonia (NH3) synthesis from nitro-

gen (N2) reduction, respectively.

The specific implementation methods of phase engineer-

ing include direct synthesis of 2D TMDs with multiphase by

wet chemical method (e.g., hydrothermal method),[111] or

controlled synthesis of 2D TMDs with multiphase by partial

phase transition.[97,126] The ways to realize partial phase

transition (from 2H to 1T) include lithium intercalation[97]

and supercritical CO2inducing.[126]

Undoubtedly, phase engineering is effective for boosting

the photocatalytic performance of 2D TMDs,[97,111, 126] but its

implementation faces two substantive challenges. First, the

metal phase 2D TMDs (for example 1T/1T’phase MoS2) are

normally metastable,[66,128] which are easy to occur phase

transition[66,128] when placed in the ambiance or at the

catalytic process. This will denature the catalysts and bring

uncertainty to their photocatalytic researches. Second, it is

not easy to precisely control the synthesis of multiphase 2D

TMDs with accurate and on-demand phase ratio, either

direct synthesis (via hydrothermal method[111]) or fabrication

through partial phase transition (via lithium intercalation[97]

or supercritical CO2inducing[126]). Therefore, efforts should

be paid to address these two main bottlenecks in future

researches. For example, improve the stability of metal

phase 2D TMDs via surface modifications to solve the

challenge of phase transition. In parallel, enhance the

controllability of phase transition to synthesize catalyst with

accurate phase ratio. Of note, electrochemical lithium-ion

intercalation-based exfoliation is a promising strategy, which

is expected to achieve precise regulation of phase transition

by controlling the current or cut-off voltage.[70]

3.4. Doping and Vacancy Engineering

Introducing heteroatom (named doping engineering, divided

into nonmetallic element doping and metal element doping,

or substitute doping and interstitial doping, Figure 13a) into

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Figure 12. Edge site engineering and phase engineering of 2D TMDs for boosting photocatalytic performance. a) A effective strategy for creating

well-exposed active edge sites of 2D TMDs: constructing vertically grown 2D TMDs on the substrate. Left panel: a schematic illustration of the

constructed vertically grown 2D MoS2on the substrate and their photocatalytic sterilization application. Right panel: a schematic illustration of the

strategy for creating well-exposed active edge sites of 2D TMDs: introducing pores on the surface of 2D TMDs. c) A effective strategy for creating

well-exposed active edge sites of 2D TMDs: building a spiral screwed structure of 2D TMDs. Reproduced with permission from ref. [123]. Copyright

2022, John Wiley & Sons. d) Phase engineering of 2D TMDs. Left panel: a schematic illustration of the constructed heterophase interface

composed of 1T/1T’TMDs and 2H TMDs. Right panel: a schematic illustration of the electron transfer at the heterophase interface composed of

1T’MoS2and 2H MoS2.

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semiconducting 2D TMDs could bring various attractive

advantages for photocatalytic application.[129–133] Theoretical

calculation shows that the generated defect levels caused by

the introduction of heteroatoms can adjust the band gaps of

2D TMDs, thereby tuning their redox ability.[129, 130] Subse-

quent experimental studies confirmed this.[131–133] For in-

stance, carbon-doped SnS2holds a larger band gap value of

2.54 eV than that of its pristine counterparts (2.43 eV)

(Figure 13b).[131] Besides, introducing heteroatom can also

effectively modify their optoelectronic properties (e.g.,

effectively strengthening the light absorption and prolonging

carrier lifetime, Figure 13c), thus boosting their photocata-

lytic performance (Figure 13d).

Creating vacancy (named vacancy engineering, divided

into metal vacancy and chalcogen vacancy, Figure 14a) is

also a prevalent strategy to unlock the large potential of 2D

TMDs for photocatalytic property improvement.[134–137] This

is because it can tune the electronic structure (Figure 14b)

and neighboring atomic arrangement of 2D TMDs to boost

the intrinsic activity.[134,135] Besides, acting as a carrier trap, it

is also beneficial for the separation of photogenerated

electrons and holes, prolonging their lifetimes, and thereby

improving the photocatalytic performance (Fig-

ure 14c).[136,137]

Notably, the degree of modification (e.g., the concen-

tration of doping and vacancy) needs to be moderate.

Because over modification may lead to the opposite results.

For example, excess vacancies may become the center of

carrier recombination and inhibit photocatalysis.[138]

Doping and vacancy engineering are indeed useful for

the enhancement of photocatalytic performance of 2D

TMDs. Still, the exploitation of such strategies in 2D TMDs-

based photocatalysis[131–133,136, 137] is just starting to emerge.

Although these strategies enjoy great popularity in the field

of catalysis.[139] Additional researches toward this topic

(doping and vacancy engineering in 2D TMDs-based photo-

catalysis) should be carried out in the future. This is a

research field full of opportunities. For example, modern

technical toolkits for doping and vacancy engineering are

constantly enriched and improved (such as plasma induced

doping,[140] laser-assisted doping,[141] neutron-transmutation

doping,[142] electron beam lithography inducing vacancy[143]),

Figure 13. Doping engineering of 2D TMDs for boosting photocatalytic performance. a) Doping categories. b) Band edge position of SnS2and

CSnS2. Data collected from ref. [131]. c) Optical properties of SnS2and CSnS2. Top panel: UV/Vis diffuse reflectance spectra of SnS2and CSnS2.

Bottom panel: normalized photoluminescence (PL) spectra of SnS2and CSnS2. d) Photocatalytic CO2reduction (the formation of acetaldehyde)

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Figure 14. Vacancy engineering of 2D TMDs for boosting photocatalytic performance. a) Vacancy categories. Left and middle panel: top view and

side view of the atomic structure model of TMD monolayer with vacancy. Right panel: a scanning transmission electron microscope (STEM) image

structures of perfect monolayer MoS2, monolayer MoS2with Mo-vacancy, and with S-vacancy. Reproduced with permission from ref. [134].

photocatalytic performance (degradation of methylene blue, right panel) of MoS2and MoS2with S-vacancy. NHE: normal hydrogen electrode.

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which provide more sophisticated techniques to introducing

heteroatoms and vacancies into photocatalysts. Applying

these state-of-the-art techniques in 2D TMDs to boost their

photocatalytic performance is a promising research topic

and is worth making efforts.

3.5. Interface Engineering

When the interface is formed, the carrier migration will

show strong directivity.[43,144, 145] Such strong directivity con-

tributes to the effective separation of photogenerated

carriers, thus prolonging their lifetime and improving optical

quantum efficiency.[146] Therefore, a plethora of opportuni-

ties for boosting photocatalytic performance appear when

we start constructing a 2D TMDs-based heterojunction

interface.[43]

A well-designed 2D TMDs-based heterojunction inter-

face should be energy band matched, dimension (geometry)

matched, and chemically compatible.[43]

Theoretically, several 2D TMDs-based heterojunction

interfaces can be formed, according to the band alignments

and directivity of electron transfer, including type I (strad-

dling gap), type II (alternate gap), Z-scheme (alternate gap),

type III (broken gap), and Schottky junction (or ohmic

junction) (Figure 15a).[43] Among them, type II,[100,147, 148] Z-

scheme,[32,36, 149–151] and Schottky junction[28, 33, 152–155] (or ohmic

junction[156]) are energy band well matched. At these

heterojunction interface (type II, Z-scheme, and Schottky

junction or ohmic junction), photoinduced electrons or holes

are transfer from one building block to another and

enriched in different building block (as shown in Fig-

ure 15a).[157] Such a feature is conducive to the separation of

carriers. Besides, it also benefits the occurrence of photo-

catalytic oxidation and reduction reactions in different

places, thus improving the reaction efficiency.[157]

2D TMDs-based type II and Z-scheme heterojunctions

are formed between two semiconductors (semiconducting

phase 2D TMDs and another semiconductor). For Z-scheme

heterojunction, the redox potential of photoexcited elec-

tron–hole pairs are increased, resulting in higher redox

capability, but for type II heterojunctions, the opposite is

true (as shown in Figure 15a). Therefore, constructing 2d

TMDs-based Z-scheme heterojunction is more conducive to

the improvement of photocatalytic performance.

2D TMDs-based Schottky junction is composed of semi-

conducting phase 2D TMDs and near-zero band-gap crystal

(e.g., metals[28,33, 152, 153] and carbon materials[154, 155]). Near-

zero band-gap crystal normally hold high electrical con-

ductivity and can act as electron traps as well as provide rich

active reaction sites, thereby enhancing the photocatalytic

performance of 2D TMDs. It is worth noting that when

stacking 2D TMDs and plasmonic metals (e.g., Au,[152]

Ag,[153] Pd,[28] Cu[33]) together, some other fascinating syner-

getic effects appear. On the one hand, plasmonic metals can

enhance the light-absorption capability of 2D TMDs

through localized surface plasmon resonance (LSPR).[152]

LSPR can induce a large electric dipole (left panel of

Figure 15b), and thus form an intense electric field nearby

the plasmonic nanoparticles, called near-field enhancement

phenomenon.[43] The near-field enhancement phenomenon

leads to a strong local optical field and thereby enhances the

light absorption of 2D TMDs.[152] On the other hand, owing

to the plasmon resonance effects, plasmonic nanoparticles

can create high energy (“hot”) electrons, which can migrate

to the conduction band of 2D TMDs via the formation of a

Schottky junction (right panel of Figure 15b).[33] Such hot

electrons can drive the photocatalytic reaction.[33]

Beyond the energy band matching (mentioned above),

dimension (geometry) matching is another key factor for

building an efficient heterojunction interface. A well-

matched dimension (geometry) is beneficial for the forma-

tion of dense interface junctions between the two materials,

providing a high-speed channel for carrier migration.[43]

Figure 15c–f display some representative examples of well-

designed 2D TMDs-based heterojunction with 0D–2D,[28]

1D–2D,[158] 2D–2D,[16] and 3D–2D[159] matched dimension.

Among them, the 2D–2D heterojunction interface is the

famous one,[160–162] and can be divided into in-plane hetero-

junction interface (exemplified by in-plane MoS2

WS2

heterojunction interface,[163] Figure 15g) and vertical hetero-

junction interface (exemplified by PtSe2/Pt vertical hetero-

structure interface,[164] Figure 15h).

Additionally, the chemical compatibility of the two

building blocks (2D TMDs and another material) should

also be emphasized. In terms of 2D TMDs, metal chloride

(such as CdS,[158,165–169] ZnIn2S4,[17, 36, 170] and TMD

themselves[163,171, 172]) would be most chemically compatible

with them to make functional hybrid photocatalysts. This is

because they have the same or familiar non-metallic

elements (for example, the S element in MoS2and CdS),

which makes it easy for them to build heterostructures with

good interfaces by epitaxial growth.[158, 159,165, 173] At such an

interface, chemical bonds (for example, MoSCu bond

formed between MoS2and CuS interface[158]) are often

formed, connecting the two building blocks, which will

facilitate the rapid transfer of carriers and improve optical

quantum efficiency.[158,165]

Despite a plethora of opportunities appear when stack-

ing 2D TMDs with other units into a heterostructure, we

have not yet obtained an ideal heterostructure for practical

photocatalytic applications. It has also been challenging to

directly characterize the specific path of electron migration

in situ or operando at such heterojunction interfaces. It is

not yet possible to control the charge transfers and built-in

electric fields with atomic precision at such heterojunction

interfaces. These are both challenges and opportunities, and

researchers in the future are worth studying.

3.6. Analysis

Rich engineering strategies have been established to deal

with the challenges of intrinsic 2D TMDs faced for photo-

catalysis. These strategies have their own merits and bottle-

necks (Supporting Information Table 4), but none of them

are yet perfect. Of the strategies reviewed here, the most

readily and widely applicable one is interface engineering

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Figure 15. Interface engineering. a) Type of heterojunction interface, classified according to the band alignments and directivity of electron transfer,

including type I (straddling gap), type II (alternate gap), Z-scheme (alternate gap), type III (broken gap), Schottky junction, and ohmic junction.

Note that Schottky junction is formed by the contact between a semiconductor (n-Type) with a lower work function and a conductor with a higher

work function, and form upward band bending. The ohmic junction is formed by the contact between a semiconductor with a higher work function

and a conductor with a lower work function, resulting in the downward bending of the energy band. b) Localized surface plasmon resonance and

hot electrons transfer at interface. Left panel: schematic illustration of localized surface plasmon resonance for a plasmonic nanoparticle. Right

panel: schematic illustration of the formation of hot electrons and interfacial charge transfer at the interface of plasmonic nanoparticles and 2D

Chemistry Society. e) TEM image of 2D–2D Bi12O17Cl2

Group. f) TEM image of 3D–2D TEM image of Cu2χS0.8Se0.2

& Sons. g) High resolution annular-dark-field (ADF) scanning transmission electron microscopy (STEM) image and atomic model of the interface

of WS2

Chemical Society.

Angewandte

Chemie

Reviews

(or named hybrid engineering, including, building hetero-

junction or coating cocatalyst). Although this strategy also

faces numbers bottlenecks as mentioned above, it is indeed

the most-friendly one for novices, owing to its easy

operation and obvious effect.

Notably, it is sometimes impossible to achieve a

satisfactory improvement effect by using only one strategy.

At this time, two or more strategies need to be applied

cooperatively. For example, creating well-exposed active

edge sites (edge site engineering) to deal with the challenge

of catalytically inert basal plane of semiconducting 2D

TMDs, along with coating conductors (such as Pt and

graphene) on 2D TMDs to cope with the challenge of

sluggish carrier dynamics.

4. Outlook

Undoubtedly, 2D TMDs, whether as cocatalysts or active

photocatalysts (light-harvesting materials), point to a pro-

ductive past, a moving present, and a bright future. Yet,

challenges remain, but opportunities abound. Here, we look

ahead the appealing directions of 2D TMDs for photo-

catalysis (Figure 16).

4.1. Extending Members

The family members of 2D TMDs used for photocatalysis

are continuously growing, which started with MoS2[10, 25]

Figure 16. 2D TMDs for photocatalysis: looking ahead. Panel of ‘Extending members’ shows that the family members of 2D TMDs using for

photocatalysis are continuously growing. Panel of ‘Improving existing strategies’ shows an example of existing strategy (edge site engineering) that

should be improved in the future for boosting photocatalytic performance of 2D TMDs. Reproduced with permission from ref. [179]. Copyright

2022, Nature Publishing Group. Panel of ‘Developing new strategies’ shows an example of new strategy (metal intercalation) that can be developed

in the future for boosting photocatalytic performance of 2D TMDs. Panel of ‘Understanding mechanisms’ shows an available in situ

characterization (in situ liquid cell transmission electron microscope) technology for understanding the photocatalytic mechanisms of 2D TMDs.

Panel of ‘Developing device’ shows a notable device example: flexible 100 cm2perovskite-BiVO4device. Reproduced with permission from

Angewandte

Chemie

Reviews

(MoSe2[17,36]) and WS2[11, 24] (WSe2[34, 37]), then ReS2[29, 149] and

SnS2,[131] have now evolved to PtS2,[174] PtSe2,[164] and beyond.

It looks like this process is only beginning, more members of

2D TMDs (e.g., ZrS2,[175] HfS2,[176] PdS2[177]) may pour into

photocatalysis in the near future predicted by high through-

put calculations. In the future, these theoretically feasible

TMDs need to be verified experimentally. Of note, although

ReS2has exciting physicochemical properties for photo-

catalytic applications, its metal element (Re) is not earth-

abundant (one of the rarest elements in the Earth’s

crust).[178] Therefore, its practical application prospect will

be limited by economic feasibility.

4.2. Improving Existing Strategies

Big promotion of photocatalytic performance achieves when

modifying 2D TMDs by the engineering of edge

site,[25,29, 31, 120] phases,[97, 111, 126] doping,[131–133] vacancy,[136, 137]

and interface. Still, compared with electrocatalysis, most of

the engineering technologies for photocatalysis of 2D TMDs

are in their infancy. In the future, continuing to dig deeper

into these engineering categories is essential. For instance,

fabricating ladder 2D TMDs to enrich edge sites;[179]

performing local phase transition to build periodic hetero-

phase interface;[125] applying laser patterning, thermal etch-

ing, and endo-epitaxial growth process to construct mosaic

heterostructures.[181]

4.3. Developing New Strategies

In parallel, developing new engineering strategies for photo-

catalysis using 2D TMDs also requires passion and effort.

For example, introducing intercalated metals into the

interlayers of 2D TMDs to boost their photocatalytic

performance by synergistic catalysis and confined catalysis

effect (confined catalysis: a new catalysis concept, to provide

a constrained environment for the catalytic reaction system

at the nanoscale, so as to achieve precise regulation of

catalytic performance and make catalysis “fast and

good”[182]).

4.4. Understanding Mechanisms

Understanding the photocatalytic mechanisms of 2D TMDs

should not be ignored. Clarifying some natures of science is

also necessary, such as who is the real active site? Whether

the phase of TMDs changes during the catalytic reaction?

How do electrons migrate at the interface? State-of-the-art

in situ characterization technologies offer powerful tools for

exploring these mechanisms and natures deeply in real-time.

These available in situ techniques include but are not limited

to, in situ liquid cell transmission electron microscope

(TEM), in situ X-ray absorption spectroscopy (XAS), and in

situ Raman. For example, in situ liquid cell TEM can

provide real-time visual detection of catalyst surface at the

atomic scale,[183,184] may open the black box of real photo-

catalytic sites of 2D TMDs-based materials. Beyond

advanced in situ characterization technologies, ultrafast

carrier detection (e.g., femtosecond pump-probe

spectroscopy[108]) and high throughput first-principles calcu-

lations are also effective tools for photocatalytic mechanism

research.

4.5. Developing Device

Methods and protocols should be established to develop 2D

TMDs-based photocatalytic devices, realizing an integration

of photo(electric)catalytic reaction, product collection and

real-time monitoring. Recently developed perovskite-BiVO4

devices for scalable solar fuel production can provide a

reference.[180] The development of modern micro-nano-

manufacturing technology (e.g., photolithography, physical

vapor deposition), solution-based deposition technology

(e.g., inkjet-printing, industrial roll-to-roll coating), and the

continuous development of functional materials (e.g., flexi-

ble material, superconductor) provide guarantee for the

manufacturing of such devices.

4.6. Lab to Fab Transition

The ultimate goal of photocatalytic technology is to serve

production and life. Realizing the transition of photo-

catalytic application of 2D TMDs-based photocatalysts from

lab to fab is therefore essential. Yet, there is a long way to

make this transition. Steps to be completed in turn in the

future for achieving such a transition include: enlarging the

developed devices, and/or assembling them in series or in

parallel, followed by placing them in compatible industrial

systems. During these steps, the evaluation of security

factors, economic factors, and market factors should not be

ignored. The recently realized solar-to-hydrogen farm is

such a pioneer, which is worth learning from.[185]

4.7. Others

In addition to seizing the opportunities mentioned above, it

is also meaningful to devote efforts to the following issues.

Building a complete and environment-friendly system for

the production, storage and recovery of 2D TMDs-based

photocatalysts should be emphasized. Because, most of the

2D TMDs, whether prepared or discarded, pollute the

environment. Constructing efficiency accreditation and

standardized testing protocols for 2D TMDs-based photo-

catalysts toward various photocatalytic reaction is urgent.

Currently, the experimental data of each research group

lacks the credibility of horizontal comparisons.[61] Because

their experimental conditions (such as the light condition,

ambient temperature, and pH value of the reaction solution)

are often different. This normally leads to the accumulation

of unverifiable and often misleading data, hindering the

advance in the research field.[61] Therefore, the efficiency

accreditation and standardized testing protocol are required.

Angewandte

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In all, opportunities and challenges coexist in this

fascinating field. We anticipate that 2D TMDs becoming a

new material-central for photocatalysis in the coming

decades, and photo(electro)catalysis becoming one of the

technologies central for solving the energy and environ-

mental crisis in the coming era.

Acknowledgements

Z.Y. Zeng thanks ECS scheme (CityU9048163) from RGC

in Hong Kong and the Basic Research Project from

Shenzhen Science and Technology Innovation Committee in

Shenzhen, China (No. JCYJ20210324134012034). Q. Wang

thanks JSPS Leading Initiative for Excellent Young Re-

searchers program, the JST Fusion Oriented Research for

disruptive Science and Technology program, and the JSPS

Grant-in-Aid for Young Scientists (Start-up) (No.

21 K20485). Q.Y. Lu thanks Natural sciences and engineer-

ing research council of Canada (NSERC) Discovery Grant

and Alberta Innovates Advance Program-NSERC Alliance

Grant.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: 2D TMDs ·Active Photocatalyst ·Cocatalyst ·

MoS2·Photocatalysis

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Manuscript received: December 7, 2022

Accepted manuscript online: January 2, 2023

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Photocatalysis

R. Yang, Y. Fan, Y. Zhang, L. Mei, R. Zhu,*

J. Qin, J. Hu, Z. Chen, Y. Hau Ng, D. Voiry,

S. Li, Q. Lu, Q. Wang,* J. C. Yu,*

Z. Zeng* e202218016

2D Transition Metal Dichalcogenides for

Photocatalysis

The fundamentals of 2D transition metal

dichalcogenides (TMDs), their synthe-

sis, their advantages in photocatalysis,

and the strategies for boosting their

photocatalytic performance are summar-

ized in this review. Currently problems

and their solutions are presented.

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Jun 2024

Atomically thin two-dimensional transition metal dichalcogenides (TMDCs) have been regarded as ideal and promising nanomaterials that bring broad application prospects in extensive fields due to their ultrathin layered structure, unique electronic band structure, and multiple spatial phase configurations. TMDCs with different phase structures exhibit great diversities in physical and chemical properties. By regulating the phase structure, their properties would be modified to broaden the application fields. In this mini review, focusing on the most widely concerned molybdenum dichalcogenides (MoX2: X = S, Se, Te), we summarized their phase structures and corresponding electronic properties. Particularly, the mechanisms of phase transformation are explained, and the common methods of phase regulation or phase stabilization strategies are systematically reviewed and discussed. We hope the review could provide guidance for the phase regulation of molybdenum dichalcogenides nanomaterials, and further promote their real industrial applications.

Synthesis of Binder-Free, Low-Resistant Randomly Orientated Nanorod/Sheet ZnS–MoS 2 as Electrode Materials for Portable Energy Storage Applications

Article

Jun 2024

Defect‐Rich Metastable MoS 2 Promotes Macrophage Reprogramming in Breast Cancer: A Clinical Perspective

Article

Jun 2024

Tumor‐associated macrophages (TAMs) play a crucial function in solid tumor antigen clearance and immune suppression. Notably, 2D transitional metal dichalcogenides (i.e., molybdenum disulfide (MoS 2 ) nanozymes) with enzyme‐like activity are demonstrated in animal models for cancer immunotherapy. However, in situ engineering of TAMs polarization through sufficient accumulation of free radical reactive oxygen species for immunotherapy in clinical samples remains a significant challenge. In this study, defect‐rich metastable MoS 2 nanozymes, i.e., 1T2H‐MoS 2 , are designed via reduction and phase transformation in molten sodium as a guided treatment for human breast cancer. The as‐prepared 1T2H‐MoS 2 exhibited enhanced peroxidase‐like activity (≈12‐fold enhancement) than that of commercial MoS 2 , which is attributed to the charge redistribution and electronic state induced by the abundance of S vacancies. The 1T2H‐MoS 2 nanozyme can function as an extracellular hydroxyl radical generator, efficiently repolarizing TAMs into the M1‐like phenotype and directly killing cancer cells. Moreover, the clinical feasibility of 1T2H‐MoS 2 is demonstrated via ex vivo therapeutic responses in human breast cancer samples. The apoptosis rate of cancer cells is 3.4 times greater than that of cells treated with chemotherapeutic drugs (i.e., doxorubicin).

Inorganic crystal-supported precious metal single-atom catalysts for photo/electrocatalysis

Article

Jun 2024

Self-Rolled-Up WSe2 One-Dimensional/Two-Dimensional Homojunctions: Enabling High-Performance Self-Powered Polarization-Sensitive Photodetectors

Article

Jun 2024
NANO LETT

Efficient catalytic oxidation of formaldehyde by defective g-C3N4-anchored single-atom Pt: A DFT study

Article

Jun 2024
CHEMOSPHERE

MXene for photocatalysis and photothermal conversion: Synthesis, physicochemical properties, and applications

Article

Jun 2024
COORDIN CHEM REV

Ultrathin 2D Alloyed Cu−Ga−Zn−S Curved Nanobelts for Boosting Visible‐Light Photocatalytic Hydrogen Evolution

Article

Full-text available

May 2024
ADV FUNCT MATER

2D semiconductor nanomaterials have received significant attention as photocatalysts for solar‐to‐hydrogen conversion due to their robust light absorption capacity, large specific surface area, and superior electron transport characteristics. Nevertheless, it is challenging to develop 2D quaternary copper‐based sulfides (QCSs) with functional integration of conducive structural features for photocatalysis from multiple elements and intricate control conditions. Herein, ultrathin 2D alloyed Cu−Ga−Zn−S (CGZS) curved nanobelts (NBs) are fabricated by using a facile one‐pot colloidal method. Subsequently, the derived Cu31S16‐CGZS Janus heterostructures are designed by increasing Cu concentrations. The formation mechanism of 2D curved alloys and Janus heterostructures is systematically investigated. Without cocatalysts, curved alloyed CGZS NBs demonstrated superior photocatalytic activities of 1264.2 µmol g⁻¹ h⁻¹ compared to Janus heterostructured Cu31S16−CGZS (463.1 µmol g⁻¹ h⁻¹) under visible light (> 400 nm). Experimental and theoretical results unveiled that the improved photocatalytic activities of curved CGZS NBs can be attributed to enhanced charge–carrier separation efficiency resulting from high crystallinity and ultrathin 2D structure, abundant active sites from curved structure, high‐active (0001) crystal facet with the lowest reaction Gibbs energy, work function, and metal‐like properties. This work offers new insights into functional integration into ultrathin 2D QCSs with enhanced visible‐light photocatalytic performance.

Interlayer Vibronic Interactions and Phonon Anharmonic Decay in Few-Layer WS 2 Nanoflakes

Article

May 2024

Two-Dimensional Materials for Highly Efficient and Stable Perovskite Solar Cells

Article

Full-text available

May 2024

Perovskite solar cells (PSCs) offer low costs and high power conversion efficiency. However, the lack of long-term stability, primarily stemming from the interfacial defects and the susceptible metal electrodes, hinders their practical application. In the past few years, two-dimensional (2D) materials (e.g., graphene and its derivatives, transitional metal dichalcogenides, MXenes, and black phosphorus) have been identified as a promising solution to solving these problems because of their dangling bond-free surfaces, layer-dependent electronic band structures, tunable functional groups, and inherent compactness. Here, recent progress of 2D material toward efficient and stable PSCs is summarized, including its role as both interface materials and electrodes. We discuss their beneficial effects on perovskite growth, energy level alignment, defect passivation, as well as blocking external stimulus. In particular, the unique properties of 2D materials to form van der Waals heterojunction at the bottom interface are emphasized. Finally, perspectives on the further development of PSCs using 2D materials are provided, such as designing high-quality van der Waals heterojunction, enhancing the uniformity and coverage of 2D nanosheets, and developing new 2D materials-based electrodes.

Fabrication of liquid cell for in situ transmission electron microscopy of electrochemical processes

Article

Full-text available

Nov 2022

Fundamentally understanding the complex electrochemical reactions that are associated with energy devices (e.g., rechargeable batteries, fuel cells and electrolyzers) has attracted worldwide attention. In situ liquid cell transmission electron microscopy (TEM) offers opportunities to directly observe and analyze in-liquid specimens without the need for freezing or drying, which opens up a door for visualizing these complex electrochemical reactions at the nano scale in real time. The key to the success of this technique lies in the design and fabrication of electrochemical liquid cells with thin but strong imaging windows. This protocol describes the detailed procedures of our established technique for the fabrication of such electrochemical liquid cells (~110 h). In addition, the protocol for the in situ TEM observation of electrochemical reactions by using the nanofabricated electrochemical liquid cell is also presented (2 h). We also show and analyze experimental results relating to the electrochemical reactions captured. We believe that this protocol will shed light on strategies for fabricating high-quality TEM liquid cells for probing dynamic electrochemical reactions in high resolution, providing a powerful research tool. This protocol requires access to a clean room equipped with specialized nanofabrication setups as well as TEM characterization equipment. This protocol describes the procedure for the fabrication of electrochemical liquid cells for in situ liquid cell transmission electron microscopy. This allows direct visualization of complex electrochemical reactions at the nano scale in real time.

Floating perovskite-BiVO4 devices for scalable solar fuel production

Article

Full-text available

Aug 2022
NATURE

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability1, whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H2 g−1 h−1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO2 reduction attained a high CO:H2 selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO4 PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H2) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm2 stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm2 counterparts. Bubbles formed under operation further enabled 30–100 mg cm−2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use. This work introduces lightweight, leaf-like photoelectrochemical devices for unassisted water splitting and syngas production, which could be used in the fabrication of floating systems for solar fuel production.

Selective‐Epitaxial Hybrid of Tripartite Semiconducting Sulfides for Enhanced Solar‐to‐Hydrogen Conversion

Article

Full-text available

Aug 2022
SMALL

The design and synthesis of advanced semiconductors is crucial for the full utilization of solar energy. Herein, colloidal selective‐epitaxial hybrid of tripartite semiconducting sulfides CuInS2Cd(In)SMoS2 heteronanostructures (HNs) via lateral‐ and vertical‐epitaxial growths, followed by cation exchange reactions, are reported. The lateral‐epitaxial CuInS2 and Cd(In)S enable effective visible to near‐infrared (NIR) solar spectrum absorption, and the vertical‐epitaxial ultrathin MoS2 offer sufficient edge sulfur sites for the hydrogen evolution reaction (HER). Furthermore, the integrated structures exhibit unique epitaxial‐staggered type II band alignments for continuous charge separation. They achieve the H2 evolution rate up to 8 mmol h⁻¹ g⁻¹, which is ≈35 times higher than bare CdS and show no deactivation after long‐term cycling, representing one of the most efficient and robust noble‐metal‐free photocatalysts. This design principle and transformation protocol open a new way for creating all‐in‐one multifunctional catalysts in a predictable manner.

A laser-assisted chlorination process for reversible writing of doping patterns in graphene

Article

Full-text available

Aug 2022

Chemical doping can be used to control the charge-carrier polarity and concentration in two-dimensional van der Waals materials. However, conventional methods based on substitutional doping or surface functionalization result in the degradation of electrical mobility due to structural disorder, and the maximum doping density is set by the solubility limit of dopants. Here we show that a reversible laser-assisted chlorination process can be used to create high doping concentrations (above 3 × 1013 cm−2) in graphene monolayers with minimal drops in mobility. The approach uses two lasers—with distinct photon energies and geometric configurations—that are designed for chlorination and subsequent chlorine removal, allowing highly doped patterns to be written and erased without damaging the graphene. To illustrate the capabilities of our approach, we use it to create rewritable photoactive junctions for graphene-based photodetectors. Two laser beams with different energies and configurations can be used to reversibly dope graphene via chlorination and chlorine removal, allowing rewritable graphene photodetectors to be fabricated.

2D WSe2/MoSi2N4 type-II heterojunction with improved carrier separation and recombination for photocatalytic water splitting

Article

Nov 2022
APPL SURF SCI

The use of semiconductor photocatalysis for hydrogen production is an ideal approach for achieving solar energy and conversion. In this work, we systematically examine the photocatalytic properties of WSe2/MoSi2N4 van der Waals heterojunctions using first-principles calculation and nonadiabatic molecular dynamics. The results demonstrate that the WSe2/MoSi2N4 heterojunction possesses a 1.81 eV indirect band gap and type-II band alignment, which ensures that the photoexcited electron–hole (e–h) pairs are spatially separated. The carrier lifetime of the photoexcited e–h pairs is 278 ps significantly longer than the interlayer hole transfer time of 335 fs, implying that the heterojunction has the high quantum efficiency. In addition, this heterojunction possesses superior optical absorption (10⁵ cm⁻¹), low overpotential for OER (0.60 V), and outstanding light absorption properties. These findings indicate that this heterojunction can achieve highly efficient and spontaneous photocatalytic water splitting.

Decrease in Tumor Interstitial Pressure for Enhanced Drug Intratumoral Delivery and Synergistic Tumor Therapy

Article

Nov 2022

Currently, one of the main reasons for the ineffectiveness of tumor treatment is that the abnormally high tumor interstitial pressure (TIP) hinders the delivery of drugs to the tumor center and promotes intratumoral cell survival and metastasis. Herein, we designed a "nanomotor" by in situ growth of Ag2S nanoparticles on the surface of ultrathin WS2 to fabricate Z-scheme photocatalytic drug AWS@M, which could rapidly enter tumors by splitting water in interstitial liquid to reduce TIP, along with O2 generation. Moreover, the O2 would be further converted to reactive oxygen species (ROS), accompanied by increased local temperature of tumors, and the combination of ROS with thermotherapy could eliminate the deep tumor cells. Therefore, the "nanomotor'' could effectively reduce the TIP levels of cervical cancer and pancreatic cancer (degradation rates of 40.2% and 36.1%, respectively) under 660 nm laser irradiation, further enhance intratumor drug delivery, and inhibit tumor growth (inhibition ratio 95.83% and 87.61%, respectively), and the related mechanism in vivo was explored. This work achieves efficiently photocatalytic water-splitting in tumor interstitial fluid to reduce TIP by the nanomotor, which addresses the bottleneck problem of blocking of intratumor drug delivery, and provides a general strategy for effectively inhibiting tumor growth.

Rationally designed WS 2 /C 2 N layered heterostructures for enhanced photocatalytic hydrogen evolution: Interface and bandgap engineering

Article

Sep 2022
APPL SURF SCI

Constructing layered heterostructured photocatalysts based on two dimensional semiconductor materials, such as transition metal dichalcogenides (TMDs) and graphitic carbon nitride (CxNy), has become a research hotspot in photocatalysis with the remarkable improvement of photoexcited charge separation efficiency. Herein, the electronic properties and photocatalytic activities of WS2/C2N bilayer and trilayer heterojunctions have been comparatively investigated using first-principles calculations. The results demonstrate that the WS2/C2N, WS2/C2N/WS2, and C2N/WS2/C2N are type-II van der Waals heterostructures, with an indirect bandgap of 1.79 eV, 1.72 eV, and 1.79 eV, respectively. Compared to WS2/C2N bilayer, the two sandwiched heterojunctions demonstrate stronger interfacial interactions, charge transfer, and visible-light absorption. The free energy calculations for redox reactions of the sandwiched heterojunctions further demonstrate high catalytic activity for hydrogen evolution reaction. More interestingly, biaxial strains can adjust the bandgap width (semiconductor→metal) and induce the band type transition (indirect→direct) in WS2/C2N sandwiched heterojunctions. For example, a weak stress about 1% leads to direct band gap both in type-II WS2/C2N/WS2 and C2N/WS2/C2N heterojunctions, without degrading their excellent abilities for water decomposition. This work not only provides rational strategies to design TMDs/CxNy-based photocatalysts in view of interface and bandgap engineering, but also reveals their potential applications in electronic and optoelectronic devices.

Advances and challenges in developing cocatalysts for photocatalytic conversion of carbon dioxide to fuels

Article

Aug 2022

The global adoption of efficient sustainable energy sources is a crucial step toward meeting energy demands while achieving carbon emission reduction targets. Solar energy has become a clean and cost-competitive alternative to traditional fossil fuels, but the intermittent nature of sunlight results in challenges associated with energy storage and transport. Photocatalytic carbon dioxide reduction intends to mimic natural photosynthesis for utilizing sunlight to chemically convert water and CO2 into fuels. In this process, the solar energy is captured and stored in fuels, so-called solar fuels, for widespread on-demand use. Heterogeneous solar fuel production systems are multi-component, comprising light-harvesting (photosensitizer) and catalytic (cocatalyst) units. Cocatalysts are indispensable for photocatalytic CO2 reduction systems, which promote charge carrier separation and transport, reduce the reaction activation energy, and alter the reaction route, thereby enhancing the activity and selectivity of the photocatalytic reactions. This review presents a comprehensive summary of the recent advancements in cocatalysts for photocatalytic CO2 reduction reaction (CO2RR), with the purpose of providing new insights and guidance to the field with regard to research directions and best practices. We summarize how various cocatalysts including inorganic nanoparticles, metal complexes, enzymes, and bacteria can be combined with semiconductor photosensitizer for light-driven photocatalytic CO2RR. Side-by-side comparisons reveal the strengths and limitations of each kind of cocatalysts and how lessons extracted from studying natural photosynthetic systems can be applied to investigations of artificial photosynthesis, presenting an outlook discussing possible future concepts for a more effective photocatalytic CO2 reduction process. [Figure not available: see fulltext.]

Van der Waals heterostructures

Article

Dec 2022

The integration of dissimilar materials into heterostructures has become a powerful tool for engineering interfaces and electronic structure. The advent of 2D materials has provided unprecedented opportunities for novel heterostructures in the form of van der Waals stacks, laterally stitched 2D layers and more complex layered and 3D architectures. This Primer provides an overview of state-of-the-art methodologies for producing such van der Waals heterostructures, focusing on the two fundamentally different strategies, top-down deterministic assembly and bottom-up synthesis. Successful techniques, advantages and limitations are discussed for both approaches. As important as the fabrication itself is the characterization of the resulting engineered materials, for which a range of analysis techniques covering structure, composition and emerging functionality are highlighted. Examples of the properties of artificial van der Waals structures include optoelectronics and plasmonics, twistronics and unique functionality arising from the generalization of van der Waals assembly from 2D to 3D crystalline components. Finally, current issues of reproducibility, limitations and opportunities for future breakthroughs in terms of enhanced homogeneity, interfacial purity, feature control and ultimately orders-of-magnitude increased complexity of van der Waals heterostructures are discussed. Van der Waals epitaxy provides numerous opportunities for materials integration in heterostructures. This Primer provides an overview of methodologies for producing van der Waals heterostructures, focusing on top-down assembly and bottom-up synthesis, and discusses future opportunities for their continued development.

Covalent organic frameworks: Fundamentals, mechanisms, modification, and applications in photocatalysis

Article

Jul 2022

Covalent organic frameworks (COFs) as efficient photocatalysts have attracted extensive attention due to their high crystallinity and tunable optical and electronic properties. This review first summarizes the fundamental aspects of COF photocatalysts—including the basic physical, chemical, optical, and electronic properties—and key considerations in the synthesis of crystalline COFs. Subsequently, deep insights into the photocatalytic mechanisms of COF-based photocatalysts (i.e., adsorption and activation mechanisms of reagents, the identifications of exact active sites, and the surface reaction mechanisms, as well as the kinetics of charge-carrier separation and transport) and engineering-modification strategies of COF-based photocatalysts are thoroughly addressed. Finally, various applications of COF-based photocatalysts are discussed, including photocatalytic hydrogen production, CO2 reduction, pollutant degradation, and organic transformation. A perspective on the challenges and future exploration of COF-based photocatalysts is also proposed. It is expected that this review will inspire new thoughts and will trigger exciting progress in COF-based photocatalysis.

2D Transition Metal Dichalcogenides for Photocatalysis

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