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(a) Orthorhombic unit cell of WO 3 crystalline: 32 atoms in total, 8 W 6+ and 24 O 2− . (b) Tetragonal unit cell of WSe 2 crystalline: 6 atoms in total, 2 W 4+ and 4 O 2− . (c) Coordination geometry for W 6+ and O 2− atoms at bulk WO 3 . (d) Coordination geometry for W 4+ and Se 2− atoms at bulk WSe 2 .
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An extensive understanding of WO3 and WSe2 bulk crystalline structures and explicit solvent effects on (001)-WO3 and (100)-WSe2 facets are essential for design of efficient (photo) electrocatalysts. The atomistic level understanding of both WO3 and WSe2 bulk solids and how water solvation processes occur on WO3 and WSe2 facets are nowadays characte...
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... WO 3 structure crystallizes in a monoclinic unit cell belonging to í µí± 2 1 ∕í µí± (í µí±í µí±. 14) space group. The here (geometry and cell) optimized unit cell ( Fig. 1-a) is defined by 8 W 6+ and 24 O 2− , for a total of 32 atoms arranged in a lattice with í µí± = 7.29 Å, í µí± = 7.48 Å, í µí± = 7.62 Å sides, and í µí»¼ = í µí»½ = í µí»¾ ∼ 90 • angle, where each W 6+ is bonded to 6 oxygen atoms in an octahedral geometry and each O 2− is bonded to two W 6+ in a planar trigonal arrangement, as depicted ...
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... 1-a) is defined by 8 W 6+ and 24 O 2− , for a total of 32 atoms arranged in a lattice with í µí± = 7.29 Å, í µí± = 7.48 Å, í µí± = 7.62 Å sides, and í µí»¼ = í µí»½ = í µí»¾ ∼ 90 • angle, where each W 6+ is bonded to 6 oxygen atoms in an octahedral geometry and each O 2− is bonded to two W 6+ in a planar trigonal arrangement, as depicted in Fig. ...
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... more stable 2H-phase of WSe 2 compound crystallizes in a rhombohedral unit cell belonging to í µí± 6 3 ∕í µí±í µí±í µí± − í µí±í µí±. 164 (hexagonal) space group, as depicted in Fig. 1-b. 2 W 4+ and 4 O 2− identify the unit cell, for a total of 6 atoms arranged in a lattice of í µí± = í µí± = 3.28 Å, í µí± = 12.98 Å side, and í µí»¼ = í µí»½ = 90 • , í µí»¾ = 119.5 • angle, where the bulk coordination geometry is octahedral for W 4+ and trigonal for O 2− , respectively (see Fig. 1-d). The crystal structure shows ...
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... 164 (hexagonal) space group, as depicted in Fig. 1-b. 2 W 4+ and 4 O 2− identify the unit cell, for a total of 6 atoms arranged in a lattice of í µí± = í µí± = 3.28 Å, í µí± = 12.98 Å side, and í µí»¼ = í µí»½ = 90 • , í µí»¾ = 119.5 • angle, where the bulk coordination geometry is octahedral for W 4+ and trigonal for O 2− , respectively (see Fig. 1-d). The crystal structure shows WSe 2 layers of around 4 Å spacing to each ...
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... as possible surface sites of water adsorption. We found that only 5 water molecules (3 entire and 2 dissociated) are on average surface-adsorbed at the unsaturated W í µí±í µí±¢í µí± atoms leading to a surface termination/speciation with 30% of í µí¼ 1 -OH 2 and 20% of í µí¼ 1 -OH exposed sites over W í µí±í µí±¢í µí± atoms, as depicted in Fig. 10a, b. No water adsorptions have been detected at the 5 W í µí±í µí±í µí±í µí±í µí± surface sites during our simulation times. W í µí±í µí±í µí±í µí±í µí± sites have not been affected by the presence of water at the surface (see Fig. ...
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... of í µí¼ 1 -OH 2 and 20% of í µí¼ 1 -OH exposed sites over W í µí±í µí±¢í µí± atoms, as depicted in Fig. 10a, b. No water adsorptions have been detected at the 5 W í µí±í µí±í µí±í µí±í µí± surface sites during our simulation times. W í µí±í µí±í µí±í µí±í µí± sites have not been affected by the presence of water at the surface (see Fig. ...
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... unsaturated Se í µí±í µí±¢í µí± atoms at the (100) surface receive the 2 hydrogen atoms (from the 2 dissociated water molecules at W í µí±í µí±¢í µí± sites) leading to 2 Se-H exposed sites (Fig. ...
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... 2 : on average during the DFT-MD, 5 water molecules are surface-adsorbed (dissociated) at the unsaturated W í µí±í µí±¢í µí± atoms leading to a surface termination/speciation with 50% of í µí¼ 1 -OH exposed sites over W í µí±í µí±¢í µí± atoms, as depicted in Fig. 11-a. Also here, no water adsorptions have been detected at the 5 W í µí±í µí±í µí±í µí±í µí± surface sites during our simulation time (Fig. 11-b). The unsaturated Se í µí±í µí±¢í µí± atoms at the (100) surface receive the 5 hydrogen atoms (from the 5 dissociated water molecules at W í µí±í µí±¢í µí± sites) leading to 5 Se-H exposed ...
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... (dissociated) at the unsaturated W í µí±í µí±¢í µí± atoms leading to a surface termination/speciation with 50% of í µí¼ 1 -OH exposed sites over W í µí±í µí±¢í µí± atoms, as depicted in Fig. 11-a. Also here, no water adsorptions have been detected at the 5 W í µí±í µí±í µí±í µí±í µí± surface sites during our simulation time (Fig. 11-b). The unsaturated Se í µí±í µí±¢í µí± atoms at the (100) surface receive the 5 hydrogen atoms (from the 5 dissociated water molecules at W í µí±í µí±¢í µí± sites) leading to 5 Se-H exposed sites (Fig. ...
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... 11-a. Also here, no water adsorptions have been detected at the 5 W í µí±í µí±í µí±í µí±í µí± surface sites during our simulation time (Fig. 11-b). The unsaturated Se í µí±í µí±¢í µí± atoms at the (100) surface receive the 5 hydrogen atoms (from the 5 dissociated water molecules at W í µí±í µí±¢í µí± sites) leading to 5 Se-H exposed sites (Fig. ...
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... possible sites of water adsorption/dissociation, neither adsorptions nor dissociations have been detected in our simulation. It turns out that the (001)-WO 3 surface is not affected by the presence of water leading to surface speciation (of equilibrium) with exposed W í µí±í µí±¢í µí± and O í µí±í µí± sites as described before and depicted in Fig. ...
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... 2 : in our model, ∼18 water molecule are present in the first water layer and no one has H-bonds with the surface revealing a surface-water average distance of ∼2.5 Å, as revealed by the position of the first peak in the radial distribution function (RDF) in Fig. 13. 2 : same interfacial H-bond water environment as found at the (100)-WSe 2 : ∼16-18 water molecules are present in the first water layer and no one makes H-bonds with the surface revealing a surface-water (first layer) average distance of ∼2.5 ...
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... 3 : despite ∼24 water molecules are present in the first water layer, only 3 water molecules are surface H-bonded to the surface as HB-donor to O í µí±í µí± surface sites as depicted in Fig. 14-a, with an average surface-water distance of ∼1.8 Å (see first peak position of RDF in Fig. 14). We found a preferential in-plane H-bond organization of water molecules where the majority (85%) of the interfacial water molecules are H-bond connected in a sort of 2D-HBond-network (see Fig. 14-b), as already detected in literature in ...
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... 3 : despite ∼24 water molecules are present in the first water layer, only 3 water molecules are surface H-bonded to the surface as HB-donor to O í µí±í µí± surface sites as depicted in Fig. 14-a, with an average surface-water distance of ∼1.8 Å (see first peak position of RDF in Fig. 14). We found a preferential in-plane H-bond organization of water molecules where the majority (85%) of the interfacial water molecules are H-bond connected in a sort of 2D-HBond-network (see Fig. 14-b), as already detected in literature in several water interfaces [102,103, Fig. 13. RDF between W í µí±í µí±¢í µí± atoms (at the surface ...
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... HB-donor to O í µí±í µí± surface sites as depicted in Fig. 14-a, with an average surface-water distance of ∼1.8 Å (see first peak position of RDF in Fig. 14). We found a preferential in-plane H-bond organization of water molecules where the majority (85%) of the interfacial water molecules are H-bond connected in a sort of 2D-HBond-network (see Fig. 14-b), as already detected in literature in several water interfaces [102,103, Fig. 13. RDF between W í µí±í µí±¢í µí± atoms (at the surface of (100)-WSe 2 ) and oxygen atoms of water molecules in the first water layer. 105,112], with an average surface-water distance of ∼2.5 Å (see second peak position of RDF in Fig. ...
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... distance of ∼1.8 Å (see first peak position of RDF in Fig. 14). We found a preferential in-plane H-bond organization of water molecules where the majority (85%) of the interfacial water molecules are H-bond connected in a sort of 2D-HBond-network (see Fig. 14-b), as already detected in literature in several water interfaces [102,103, Fig. 13. RDF between W í µí±í µí±¢í µí± atoms (at the surface of (100)-WSe 2 ) and oxygen atoms of water molecules in the first water layer. 105,112], with an average surface-water distance of ∼2.5 Å (see second peak position of RDF in Fig. ...
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... of 2D-HBond-network (see Fig. 14-b), as already detected in literature in several water interfaces [102,103, Fig. 13. RDF between W í µí±í µí±¢í µí± atoms (at the surface of (100)-WSe 2 ) and oxygen atoms of water molecules in the first water layer. 105,112], with an average surface-water distance of ∼2.5 Å (see second peak position of RDF in Fig. ...
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... electric field profiles (see Section 2 for the computational details) as a function of the í µí± §-coordinate perpendicular to the (001)-WO 3 and (100)-WSe 2 surfaces are shown in Fig. 15. We compare the bare surfaces at the interface with vacuum (profile at the top) to the surfaces in contact with the explicit water environment (profile at the bottom) Electric field profiles have been calculated extracting configurations either from geometry optimizations (for the bare surfaces) or from the DFT-MD simulations (40 ...
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... surfaces) or from the DFT-MD simulations (40 configurations extracted spaced equally in time from 25 ps DFT-MD) at finite temperature when liquid water is in contact with the hydrated surfaces. For the sake of clarity, only the electric field peak located at the surface layer height (upper (001)-WO 3 and (100)-WSe 2 surface) has been displayed in Fig. ...
Citations
... Computer simulations can complement experiments, provide indispensable understanding of, and contribute to the determination of reaction mechanisms of catalytic reactions and shed light on the identification and understanding of chemical properties, the role of controlling factors such as experimental conditions, and thermodynamic quantities of the WO 3 metal oxide surface. [12,13] In this section, we discuss recent computational studies on the performance of WO 3 surfaces for the OER and other oxidation reactions. Methods derived from quantum mechanics have been proven to provide a promising avenue for understanding the catalytic activity of metal oxide surfaces at an atomistic level, [14,15] such as the role of oxygen defects causing the formation of oxygen vacancies on the (001) surface of monoclinic WO 3 . ...
... Performance of WO 3 A previous study by some of the authors using the sophisticated AIMD technique has revealed that the WO 3 surface is hydrophobic, showing weak interaction between water bulk and the surface, as shown schematically in Figs 2a and 2c, and which can be quantified by means of the radial distribution function between oxygen atoms of water molecules and tungsten atoms of the (001) surface (see Fig. 2b). [13] In addition to that, we recently found by thermodynamically required for the overall water splitting reaction. ...
... [34,36] Moreover, Ping and Galli investigated computationally orthorhombic WO 3 which is stable at high temperature and found that the orthorhombic crystal system can improve the carrier mobilities for both electron and hole channels. The surface is stabilized, and Radial distribution function between oxygen atoms of water molecules and tungsten atoms at the surface, [13] and c) Enlarged visualization of the vacancy between water molecules and the surface (W atoms are pink). Figures reproduced from ref. [13], copyright 2022, the authors. ...
The efficient and inexpensive conversion of solar energy into chemical bonds, such as in H2 via the photoelectrochemical splitting of H2O, is a promising route to produce green industrial feedstocks and renewable fuels, which is a key goal of the NCCR Catalysis. However, the oxidation product of the water splitting reaction, O2, has little economic or industrial value. Thus, upgrading key chemical species using alternative oxidation reactions is an emerging trend. WO3 has been identified as a unique photoanode material for this purpose since it performs poorly in the oxygen evolution reaction in H2O. Herein we highlight a collaboration in the NCCR Catalysis that has gained insights at the atomic level of the WO3 surface with ab initio computational methods that help to explain its unique catalytic activity. These computational efforts give new context to experimental results employing WO3 photoanodes for the direct photoelectrochemical oxidation of biomass-derived 5-(hydroxymethyl) furfural. While yield for the desired product, 2,5-furandicarboxylic acid is low, insights into the reaction rate constants using kinetic modelling and an electrochemical technique called derivative voltammetry, give indications on how to improve the system.
