Inhomogeneous RVO4 Photocatalyst Systems (R = Y, Ce, Pr, Nd, Sm

Article pubs.acs.org/JPCC

Inhomogeneous RVO4 Photocatalyst Systems (R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) Mitsutake Oshikiri,*,† Jinhua Ye,‡ and Mauro Boero§ †

Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan ‡ Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-University of Strasbourg, UMR 7504, , 23 rue du Loess, BP 43, F-67034 Strasbourg, France S Supporting Information *

ABSTRACT: By resorting on first-principles dynamical simulations and supporting experiments, we present a systematic and detailed inspection of a new series of inhomogeneous photocatalytic RVO4 systems (R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), where YVO4 is the most representative member. We evidenced a carrier separation, promoted by Pt acting as a cocatalyst, which allows for a clear understanding of the electronic properties of these inhomogeneous systems. These, in turn, are shown to be of crucial importance in enhancing the photocatalytic activity in the cases of water or methanol aqueous solutions. The presence of f electrons, acknowledged as essential for an optimal photocatalytic performance, is clearly rationalized and the possibility of using Lu, in view of its smaller ionic radius, to enhance hydrogen generation is disclosed. Experiments confirm that YVO4, GdVO4, and LuVO4 have a potentially much higher efficiency than compounds containing other rare earth lanthanides. Hence, we provide a comprehensive guideline for designing a new generation of photocatalysts possessing unprecedented efficiencies.

1. INTRODUCTION The worldwide increasing demand for energy has disclosed several problems, ranging from the production sources to costs connected to the shortage of fossil energy resources in the near future.1 In such a scenario, photocatalysis represents a fundamental process, exploited by living organisms, for instance, in photosynthesis, and a viable way to use solar light to trigger chemical reactions,2−4 like water dissociation, and to get as final products oxygen and hydrogen, instead of the environmentally unfriendly CO2. In this respect, the compounds TiO2, ZrO2, and Ta2O5 have already been shown to possess a photocatalytic ability to decompose water molecules into oxygen and hydrogen in the ultraviolet region (UV).5−7 The electron affinity (EA) levels [i.e., the level of the conduction band minimum (CBM)] of these transitionmetal-oxide photocatalysts are composed of d orbitals of the transition metal, whereas the ionization potential (IP) levels [i.e., the level of the valence band maximum (VBM)] originate mainly from oxygen 2p orbitals. Such a feature allows one to infer naively that metal oxide photocatalysts based on vanadium should possess a good photocatalytic activity for hydrogen generation in longer wavelength regions, possibly including the visible range, since the 3d level of atomic V is located at relatively lower energies than the d levels of Ti, Zr, Nb, and Ta. Although the discovery of the visible light response of BiVO4 dates back to 1998,8 it was observed that this compound can © 2014 American Chemical Society

generate only oxygen in the presence of a sacrificial reagent (such as AgNO3). The lack of hydrogen production was ascribed to the fact that the width of the conduction band of BiVO4 originating from V 3d states is too large; hence, the location of the CBM along the energy axis is below the hydrogen reduction level. From a structural point of view, this can be rationalized in a shorter V−V distance (3.9 Å) with respect to the bulk BiVO4. Later investigations focused on InVO4, because in this system the V−V atomic distance is larger (4.05 Å) than that of BiVO4, and visible light response accompanied by hydrogen evolution was indeed observed.9,10 Encouraged by this successful discovery, we extended our investigation to other vanadates and found a new series of promising compounds,11,12 namely, YVO4, GdVO4, and LuVO4. These systems possess remarkable efficiencies in decomposing water or methanol aqueous solution in the presence of a cocatalyst (Pt or NiOx11) loaded on the surface of the photocatalyst grain. Unfortunately, their photocatalytic ability is limited to the UV region, and in the case of the absence of a suitable cocatalyst, the photocatalytic activity of YVO4 is significantly reduced. In a previous work,13 we have shown that adsorption of water molecules is possible on YVO4 Received: October 25, 2013 Revised: March 28, 2014 Published: March 29, 2014 8331

dx.doi.org/10.1021/jp410565e | J. Phys. Chem. C 2014, 118, 8331−8341

The Journal of Physical Chemistry C

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Table 1. Photocatalytic Properties of RVO4 in the Presence and Absence of Pt Cocatalyst for Water and Methanol Aqueous Solution (R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) H2 evolution rate catalyst

cocatalyst (wt % Pt)

YVO4, 1 g YVO4, 1 g YVO4, 1 g GdVO4, 1 g CeVO4, 0.5 g PrVO4, 0.5 g NdVO4, 0.5 g SmVO4, 0.5 g EuVO4, 0.5 g GdVO4, 0.5 g TbVO4, 0.5 g DyVO4, 0.5 g HoVO4, 0.5 g ErVO4, 0.5 g TmVO4, 0.5 g YbVO4, 0.5 g LuVO4, 0.5 g YVO4, 0.5 g

none 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

reactant water, water, water, water, water, water, water, water, water, water, water, water, water, water, water, water, water, water,

370 370 320 320 220 220 220 220 220 220 220 220 220 220 220 220 220 220

mL mL mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL; mL;

methanol 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL methanol, 50 mL

O2 evolution rate

μmol/h

mol/h/RVO4 mol

mol/h

mol/h/RVO4 mol

10 140 1500 2000 4 33 47 82 38 812 9 52 55 50 41 52 840 663

0.0020 0.0285 0.3057 0.5444 0.002 0.017 0.024 0.044 0.020 0.442 0.005 0.029 0.031 0.028 0.023 0.030 0.487 0.270

trace trace trace trace − − − − − − − − − − − − − −

trace trace trace trace − − − − − − − − − − − − − −

remark a a a,c a,c b b b b b b,d b b b b b b b,d b,d

400-W Hg high-pressure lamp using an inner-irradiation type quartz cell; RVO4 by R2O3 + V2O5 at 700 °C for 12 h in air. 400-W Hg high-pressure lamp using an inner-irradiation type quartz cell; RVO4 by R2O3 + V2O5 at 700−800 °C for 12 h in air. Oxygen production was not checked. cMole ratio of H2O:CH3OH of the solution (320 mL of water and 50 mL of methanol) corresponds to 14.3:1.00, at 20 °C, since 320 × 0.9982/18 = 17.74577, 50 × 0.793/32.04 = 1.2375 (see ref 31). d0.2 wt % corresponds to a mole ratio of YVO4:Pt = 1:0.00209, GdVO4:Pt = 1:0.00279, LuVO4:Pt = 1:0.00297. a

b

Table S1) a (=b) and c were determined by X-ray Rietveld analysis and confirmed to be nearly identical to reported data.14 In particular, the lattice parameters a = 7.40 Å and c = 6.49 Å of CeVO4 decrease gradually to a = 7.03 Å and c = 6.24 Å of LuVO4 by going across the sequence Ce, Pr, Nd, ..., Gd, ..., Ho, Y, Er, ..., Lu.15 The photocatalytic activities of the system YVO4 plus the Pt cocatalyst were observed following a procedure described in Experimental Section B of the SI using a highpressure Hg lamp and a GC−TCD. For comparison, the photocatalytic activity of the YVO4 without the cocatalyst was also investigated in the framework of the same procedure above. With the addition of the Pt cocatalyst, almost no oxygen generation was observed, but hydrogen was produced. In the case with pure water, the hydrogen evolution rate was 140 μmol/h, and when a methanol aqueous solution was used, it increased to 1500 μmol/h. Along the same guidelines summarized here, the photocatalytic activity of the whole RVO4 series with the Pt cocatalyst was also monitored following the same protocol of YVO4. The results are summarized in Table 1. At a glance, we can notice that the photocatalytic activities of the YVO4, GdVO4, and LuVO4 are extremely high and H2 generation was drastically suppressed when the component Y, Gd, or Lu was replaced by another lanthanide. To rationalize the origin of such a behavior, we inspected the electronic structure of our inhomogeneous photocatalyst. This will be the target of the next paragraphs. Computer Modeling. To get a better insight into the role of the Pt−based cocatalyst, we focused on the roomtemperature-equilibrated system and related the electronic structure. The inhomogeneous photocatalytic system was modeled by placing in a supercell a Pt−plated YVO4 slab substrate and H2O molecules at standard bulk water density or methanol aqueous solution. The exposed surface of YVO4 was selected to be the (010) surface, according to our former

without the help of any cocatalyst. In that work, we pointed out that oxygen coordination defects of V atoms at the surface were the trigger for H2O dissociative adsorption, giving rise to −OH and −H adducts in a way similar to the case of TiO2. From the electronic point of view, we observed that the energy levels of occupied states originating mainly from O 2p orbitals of the adsorbed H2O molecules were located at rather lower energies with respect to the highest occupied state of the inhomogeneous photocatalytic system. This observation provides a clue to understand why the YVO4 system, at least without a cocatalyst, is able to generate only hydrogen and almost no oxygen upon water dissociation.13 In this paper, by using first-principles molecular dynamics, we focus on the role of Pt cocatalyst on the adsorption properties of reactant molecules onto YVO4. On the basis of the electronic structures of RVO4 bulk crystals (where R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and those of inhomogeneous photocatalytic systems, including YVO4, with the addition of a Pt cocatalyst, in the presence of water or methanol aqueous solution, we infer a general relationship between the compound R and the photocatalytic activities for hydrogen generation. Special attention will be given to RVO4 systems in which a Pt cocatalyst is loaded on the surface and methanol aqueous solution are used, since this represents the actual photocatalytic system used in experiments.

2. RVO4 IN THE PRESENCE OF A PT COCATALYST Experimental Results. The RVO4 (R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) system with Pt loaded on the grain surface was prepared by following the procedure described in Experimental Section A of the Supporting Information (SI). The crystal structure of the produced RVO4 polycrystalline powder is zircon-type belonging to I41/ amd of cubic system (No. 141). The lattice constants (see SI, 8332



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work13 and experimental guidelines, and the five different cases were investigated. In the first one, the supercell contained the photocatalyst substrate composed of 16 Y, 16 V, and 64 O atoms with 7 Pt atoms added to the surface. The resulting exposed surface area on the slab was 2a × 2c (=2 × 7.12 Å × 2 × 6.29 Å = 179 Å2). The supercell thickness was set to 22.23 Å, in which the slab representing the substrate occupies about ∼6 Å [∼b (=a)] of this cell size and the remaining empty space separates periodically repeated images or is available to accommodate liquid water or water−methanol mixtures upon proper tuning to control the available volume with respect to the number of additional liquid molecules inserted. Hereafter, this pristine model will be referred to as YVO4−Pt. The second simulation system was prepared from YVO4−Pt by addition of 35 H2O molecules, at bulk water density, in contact with the slab surface; hereafter, we shall refer to this wet model as YVO4−Pt−35Wt. The third model system contained 58 H2O molecules and 4 methanol CH3OH molecules on the surface and will be named YVO4−Pt−58Wt−4Met hereafter. The fourth model, hereafter indicated as YVO4−Pt−56Wt−4Met− 1Hyd, was the former one in which two H2O molecules are subtracted and replaced by one H2 molecule. Finally, a fifth system containing 54 H2O molecules, four CH3OH molecules, and two H2 molecules on the surface of YVO4−Pt was simulated and called YVO4−Pt−54Wt−4Met−2Hyd in the reminder of the paper. Since different amounts of molecules, from the 35-molecule pure liquid water to the methanol−water solutions indicated above, are used, the thicknesses of the various supercells were tuned by controlling the actual pressure via an NPT set of simulations in which only the cell axis in the direction orthogonal to the slab surface was allowed to vary.16 As a result, the thicknesses of the various simulation cells, namely, the YVO4−Pt −35Wt, the YVO4−Pt−58Wt−4Met, the YVO4− Pt−56Wt−4Met−1Hyd, and the fifth YVO4−Pt−54Wt− 4Met−2Hyd model, turned out to be 14.13, 18.86, 18.79, and 18.72 Å, respectively. The systems containing 58, 56, and 54 H2O molecules carry approximately three monomolecular water layers in ordinary room-temperature liquid phase, and the molecular ratios of H2O:CH3OH in these models are selected to reproduce the concentration of the real methanol aqueous solution used in the experiments. The surface exposes seven sets of fourfold oxygen coordinated V (hereinafter called 4c-V) and 7c-Y sites, in addition to one set of 6c-Y and 3c-V, although the bulk crystal is composed of only 4c-V and 8c-Y sites. In order to clarify why YVO4, GdVO4, and LuVO4 display a higher efficiency with respect to the other members of the RVO4 family, the latter system has been included in the ongoing discussion. However, because of the intrinsic difficulty in the treatment of f electrons in DFT-based approaches, we try to rationalize the experimental results on the basis of the electronic structure of the YVO4−Pt + water system at 300 K and the electronic structure of bulk RVO4. To this aim, the electronic structure of bulk RVO4 was computed by a spinunrestricted linear muffin tin orbital approach with atomic sphere approximation (LMTO-ASA).17 Dynamical simulations were done within the Car−Parrinello18 (CPMD) framework and auxiliary electronic structure calculations were done by LMTO-ASA. Further details can be found in the Computational Details section of the SI (CD-SI).

3. THE PT-PLATED YVO4 SUBSTRATE WITHOUT LIQUID REACTANTS Atomic-Level Insight on the Plated Pt. As a first preparatory step, we focused on the dynamical properties of a seven-atom Pt cluster placed on the YVO4 surface. In a bulk metallic state, Pt crystallizes in the form of a face-centered cubic structure belonging to the space group Fm3m ̅ (No. 225), and the reported unit cell lattice constant is 3.9231 Å, the shortest Pt−Pt distance being 2.77 Å.19 The slab carrying the Pt cluster was initially prepared in such a way that the shortest distance between a Pt atom located at the corner of the tetrahedral quarried Pt cluster and the surficial O atom of the YVO4 slab surface was 2.8 Å, and then the system was equilibrated at 300 K for 2.7 ps. Since the expected Pt−O bond length is about 2.0 Å, as in typical platinum oxide materials (for example, H8PtO8),20 the initial Pt−O distance in our model is not too short; thus, no biasing is introduced by such a choice. When the simulation approaches the thermal equilibrium, the Pt cluster displaces closer to the O atom at the surface, and eventually a Pt atom of the cluster is adsorbed to this O site, forming a stable Pt−O dynamically oscillating around 2.0−2.1 Å and keeping the Pt cluster anchored to the surface. The trends of the distances between Pt and O atoms belonging to the photocatalyst surface at 300 K and at a higher temperature are reported in Figure S2 of the SI. On the other hand, the initial shape of the Pt cluster changed, accompanied by a shrinking of the Pt−Pt distances from 2.8 to about 2.4 Å. At the same time, no geometrical changes were observed for the exposed 3c-V, 6c-Y, 4c-V, and 7c-Y sites. The equilibrium geometry is shown in Figure 1.

Figure 1. Equilibrium geometry of the Pt cluster on the YVO4 surface. The color code is red for O, cyan for Y, silver for V, and blue for Pt. All distances are in Å.

Electronic Properties. The electronic properties21 were investigated in detail for the equilibrated system. A projection of the Kohn−Sham orbitals onto atomic wave function for the YVO4−Pt system has given the result shown in Figure 2a−h, the atomic configuration being the one shown in Figure 1 (at point A in Figure S2 in the SI). The picture provided by such a projection is that the occupied states mainly due to Pt 5d orbitals are situated in the energy range from −6.5 to −4.0 eV, namely, in the range from the top of the O 2p band to the center of the YVO4 fundamental band gap. Looking at the region around the 8333



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Figure 2. (a−h) Electronic structure, obtained upon projection29 onto atomic orbitals, of the Pt-plated YVO4 photocatalyst at 300 K in the absence of any reactant. (i) Optical absorption spectrum.30

54Wt−4Met−1Hyd model. A representative snapshot of the dynamics is shown in Figure 3.

lowest unoccupied states, we notice that these levels are composed by V 3d and Pt 5d contributions. A noticeable feature is that among the V 3d components, the one coming from the site V24 was the largest one contributing to the lowest unoccupied state; now, this specific V site is located near the Pt cluster. At the same time, the major contribution to these lowest unoccupied levels of Pt 5d states was found for the Pt sites Pt34, Pt35, and Pt36, all of them located at distances from V24 significantly larger than other Pt atoms (Figure 1). So, we can infer that an electron excited from the valence band (mainly O 2p orbitals) to the states at around the lowest unoccupied levels immediately should spread up to Pt34, -35, and -36 via V24, and therefore, the excited electron carrier is spatially well separated from the hole that is left around the O sites, providing a remarkable carrier separation.

4. THE PT-LOADED YVO4 SYSTEM IN CONTACT WITH LIQUID WATER Stuctural Properties. Thirty-five H2O molecules, amounting to about two molecular layers of standard liquid water at room temperature, were placed on the surface of the YVO4−Pt, as explained in the section on computer modeling. Upon equilibration at 300 K, we found that the Pt cluster was stably bound to the catalyst surface by a few Pt atoms making bonds of ∼2 Å with an exposed O atom, hereafter labeled Ocat, and some water molecules were adsorbed to the Pt sites in undissociated form. The average atomic distance between the Pt and the O atoms of the adsorbed H2O molecules (Owt) oscillated slightly around 2.15 Å during the finite-temperature dynamics. In a test case in which only one H2O molecule is adsorbed, while the other are removed from the simulation cell, the Pt−Owt equilibrium distance was 2.22 Å (see Figure S3 in SI) and the estimated adsorption energy was about 1.41 eV/ molecule. A dissociative adsorption of an H2O molecule to the 3c-V site, accompanied by nondissociative adsorptions to the 6c-Y and 7c-Y sites, was also observed, consistent with ref 13. Conversely, no H2O dissociation at Pt sites was observed on the time scale of the simulation for this specific system. We can anticipate here that this is not the case for the YVO4−Pt−

Figure 3. Snapshot of the equilibrium structure of the Pt-plated YVO4 system in the presence of water at 300 K. The color code is identical to that of Figure 1 with the addition of H (white). All numbers listed beside the figure indicate the corresponding atomic distances in Å.

Electronic Properties. The electronic structure of the Ptloaded YVO4 substrate in contact with liquid water is shown in Figure 4. At a first glance, the electronic structure is basically similar to that of the dry Pt-loaded YVO4 slab on which the electronic structure of the bulk water is superposed. However, a closer look reveals that something interesting is occurring to the surface levels. Some of these levels originating from the surficial O atoms in the bare Pt-plated YVO4 system disappeared due to the presence of positively charged H atoms belonging to water molecules near these surficial oxygen atoms. But the O 2p components of water molecules around the top of the Owt 2p band (about −3.5 eV in Figure 4c) are smaller than those due to the substrate (Ocat 2p in Figure 4b). From this observation, we can conclude that strong surface levels still exists in this inhomogeneous system. In this case it seems not easy for the excited hole to spread into the occupied 8334



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Figure 4. Electronic structure, obtained upon projection29 onto atomic orbitals, of the Pt-plated YVO4 photocatalyst in contact with liquid water at 300 K. The black solid lines in the last (k) plot shows the optical absorption spectrum.30

2p orbitals of H2O molecules; thus, it becomes much more difficult to oxidize water molecules, and as a consequence, a low efficiency in oxygen generation is expected. Specifically, photons having energies of approximately 2.9 eV (see Figure 4a,b) close to the band edge (Figure 4k) should be inactive for oxygen generation. On the other hand, since the O 2p components of water molecules located in the energy range from −4.0 to −5.0 eV around the top of the occupied Owt 2p band are relatively large, photons having energies larger than 3.5 eV may be able to create a hole in the water Owt 2p band. The drawback, in this case, could be the presence of Pt 5d electrons, which might hinder the creation of holes in the occupied O 2p band, hence suppressing oxygen generation in the Pt-plated system. The created hole at O sites can recombine with the electron provided by the occupied Pt 5d states, and the hole may go up to the top of the occupied Pt 5d band. However, the wave function mixing of Pt 5d and the O 2p of the catalyst or the water is not so favorable (for example, the weight of the Ocat 2p or Owt 2p is less than 12% or 4%) at the top of the occupied band (around −1 eV), in comparison with contributions generally larger than 60% in the Ocat 2p or Owt 2p band around −4 eV (Figure 4b or c). Given this scenario, a hole would rapidly recombine with the excited electron to the V 3d−Pt 5d hybridized unoccupied band, without giving rise to any photocatalytic activity. Instead, hydrogen generation can still be promoted by the presence of unoccupied Pt 5d components. Similar to the case of the bare Pt-loaded slab, the unoccupied V 3d states are well-hybridized with the unoccupied Pt 5d states around the bottom of the unoccupied band. By looking at the components 3d of V26, 5d of Pt33, and 5d of Pt34 (Figure 4h,j,i), we notice that the unoccupied 5d components of the Pt33 (Figure 4j) are mainly concentrated in the lower part of the unoccupied band than those of Pt34. What is interesting here is the fact that the V26−Pt33 distance (∼4.2 Å) is much larger than that of V26−Pt34 (∼2.6 Å) (Figure 3). This implies that an electron excited into the unoccupied band, in the energy range close to the bottom containing large V26 3d and Pt 5d components, would spread not only to the V26 site but also to Pt33, more distant than

Pt34 ones. Furthermore, the wave function of H 1s components are mixed to some extent with the unoccupied Pt 5d band (Figure 4e,f). The conclusion that can be drawn is that the presence of the Pt cluster on the YVO4 surface can promote the carrier separation, which, in turn, would enhance the excited electron lifetime and the probability to transfer excited electrons to the protons, thus leading to hydrogen generation. Since a similar excited electron carrier separation has been evidenced also in the bare Pt-loaded system, the carrier separation feature cannot be ascribed to the presence of water. The present detailed analysis of the electronic structure suggests clearly that this photocatalytic system is well-suited for hydrogen production but not for oxygen generation. This is corroborated by an experimental confirmation from our inhomogeneous Pt-loaded YVO4 photocatalyst (including water), for which only hydrogen production was detected, beside negligible traces of oxygen.

5. THE PT-LOADED YVO4 SYSTEM IN CONTACT WITH AQUEOUS METHANOL SOLUTION Structural Properties. As mentioned in the section on computer modeling, the simulation cell contains four methanol molecules solvated into 58 H2O molecules in contact with the YVO4−Pt surface. The exposed surface was 2a × 2c and the whole simulation cell has dimensions 2 × 7.12 Å × 2 × 6.29 Å × 18.86 Å. During the 300 K NVT dynamics, the system equilibrated, giving rise to the dissociation of one CH3OH molecule adsorbed to the Pt39 site, one nondissociated CH3OH molecule adsorbed to the Y10 (bottom surface because of periodic boundary conditions), and one nondissociated water molecule adsorption to the Pt36 site. Other methanol molecules were not adsorbed to any site and were just moving around in the water solvent. The O atom of the CH3OH molecule was first attracted to the Pt site, just like water molecules do, and the H atom of the hydroxyl group formed a hydrogen bond (H-bond) with a nearby water molecule. Eventually, the H atom of the hydroxyl group of methanol was extracted by the O atom of the approaching H2O molecule, forming a hydronium H3O+ species. The snapshot of 8335



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the range between −9.5 and −2.0 eV mixing with the Owt 2p band (Figure 6d,e) and with the occupied Ocat 2p band due to the O atoms of the catalyst (Figure 6c). On the other hand, the C 2p occupied methanol states are located in the range from −9.5 to −5 eV, with the major components concentrated around −8 to −7 eV (Figure 6h). Contributions coming from Omet 2p of the dissociatively adsorbed CH3OH molecule are spread in the energy range from −4.5 to +1 eV, in addition to the band between −8.5 and −7 eV (Figure 6g), and mixed with the Pt 5d band in both the occupied and unoccupied bands of the whole system (Figure 6i). The empty levels around the bottom of the unoccupied band are mainly contributions of V 3d and Pt 5d, with minor components of H 1s (Figure 6a,i,j). An interesting point about the distribution of the H 1s weights (Figure 7) is the fact that there are almost no contributions from the hydrogen atoms of nondissociated CH3OH molecules and of the H3O+ hydronium to the unoccupied band bottom (Figure 7d−f). Instead, we can find some contributions from the H atoms of the −CH3 moiety and from the hydrogen of the water molecule (re-)formed after accepting a proton from the CH3OH undergoing dissociation (Figure 7c,g,h). Another peculiar feature we noticed is that a few states originating from O 2p of water molecules exist just above the highest occupied level of the O 2p band of the catalyst substrate (compare panels c and d of Figure 6).

the dissociated methanol molecule adsorbate structure is shown in Figure 5. The methanol molecule was composed of the

Figure 5. Snapshot of the equilibrium structure of the Pt-plated YVO4 system in contact with a methanol aqueous solution at 300 K. The color code is identical to that of Figure 3 with the addition of C (gray). Numbers listed beside the figure refer to the corresponding atomic distances in Å.

atoms labeled as H294, H295, H296, H297, C42, and O168 at the initial stage, but during the simulation the proton H297 is transferred to the H-bonded water molecule (H297−O60− H202) via a proton wire mechanism22−24 and the methanol molecule dissociates to form the adsorbate −Pt39−O168− C42−(H294, H295, H296). Electronic Properties. Since the main difference between the model used in this simulation and the former ones is just the inclusion of methanol molecules, we shall focus here only on the effect of their presence. Such an effect translates into the rising of Omet 2p peaks, due to the oxygen atom of CH3OH in

6. THE PT-LOADED YVO4 SYSTEM IN CONTACT WITH AQUEOUS METHANOL SOLUTION PLUS SOLVATED H2 MOLECULES Structural Properties. This section, dedicated to the model systems labeled as YVO4−Pt−56Wt−4Met−1Hyd and −54Wt−4Met−2Hyd, focuses on the inhomogeneous catalyst system at 300 K in which one or two hydrogen molecules have been added to the methanol−water solution, as experimentally expected to occur. At variance with the system previously discussed, the inclusion of H2 molecules has the net effect of

Figure 6. Electronic structure, obtained upon projection29 onto atomic orbitals, of the Pt-plated YVO4 photocatalyst in contact with a methanol aqueous solution at 300 K. 8336



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Figure 7. Electronic structure in terms of relative weights obtained upon projection29 of the various Kohn−Sham eigenfunctions onto the H 1s atomic orbital for the Pt-plated YVO4 photocatalyst in contact with a methanol aqueous solution at 300 K comparing with that of Pt 5d.

Figure 8. Water molecule (a) and hydrogen molecule (b) dissociation at a Pt site for the Pt-plated YVO4 photocatalytic system in contact with a methanol aqueous solution. Panel a refers to the case of a single H2 molecule added, whereas panel b refers to the presence of additional H2 molecules, corresponding to a very high concentration. All numbers listed beside the snapshots indicate the relevant atomic distances in Å.

phenomenon seems then to be related to the concentration of hydrogen molecules, and the estimated dissociation energy (per molecule) is about 0.84 eV, although a single event does not provide enough statistics to corroborate this value. Electronic Properties. Here the only difference with respect to the systems discussed in sections 4 and 5 is the addition of H2 molecules, one of which undergoes dissociation in the YVO4−Pt−54Wt−4Met−2Hyd system. For this reason, we start our analysis by focusing on the H 1s components of the hydrogen molecules. Figure 9 shows the relative atomicorbital contributions of the whole system, where the four main components, V 3d, Ocat 2p, Owt 2p, and Omet 2p, are sketched in panel a. In panel b, the Pt 5d contributions from the Pt cocatalysts are reported, and in panel c, the hydrogen 1s contribution Hwt 1s due to water is displayed separately from

inhibiting the methanol dissociation. In fact, we still found a few H2O molecules sticking onto Pt sites, along with one CH3OH molecule, and in the case of the YVO4−Pt−56Wt−4Met− 1Hyd, one H2O molecule, they eventually dissociate on the Pt cluster (Figure 8a, H180, O50, H181), but no methanol dissociation occurred on the time scale of the simulation, indicating that a higher barrier exists and that it must be larger than thermal fluctuations occurring during the NVT dynamics. The microscopic reason for this can be ascribed to the insufficient help to proton extraction caused by the strong Hbond network of water molecules around the CH3OH molecule, enforced by the presence of H2. Instead, we observed the dissociation of one of the two H2 molecules in proximity to a Pt site of the cocatalyst cluster (Figure 8b, H274, H275) in the case of the YVO4−Pt−54Wt−4Met−2Hyd model. Such a 8337



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Figure 9. Electronic structure of the Pt-plated YVO4 photocatalyst in contact with a methanol aqueous solution plus two hydrogen molecules at 300 K. As in the former cases, the profiles refer to wave function projections onto atomic orbitals.29

structure of bulk YVO4 crystal with the electronic structure of the inhomogeneous Pt-loaded YVO4 system in contact with an aqueous methanol solution at 300 K, it seems possible to roughly guess the actual electronic structure of the general RVO4 compound from the corresponding bulk, at least to some extent. Specifically, the bulk electronic structure shows that the band gaps of YVO4, GdVO4, and LuVO4 are semiconductorlike ones, having values of 3.45, 3.14, and 3.56 eV, respectively. We remark that the first value matches well the gap of our slab model catalyst. These three materials are characterized by a slightly indirect band gap (see CD-SI), and such a feature is likely to favor longer lifetimes for the excited electron carriers. Moreover, these three systems present O 2p orbitals around the top of the occupied band (Table 2), and their chemical potentials are the lowest among all the members of the RVO4 family. In view of the electronic structure properties summarized here, these systems are expected to possess a high photocatalytic activity (Table 1). We remark that the absolute position of the 4f band is often badly reproduced in DFT-based approaches. Nonetheless, these effects are likely to be negligible in our case for two reasons: Firstly, we are not interested in the absolute value of the 4f levels but in their relative positions when they are partly occupied or empty. Indeed, ad hoc corrections to heal this problems (see ref 25) consist generally of a rigid shift, and in any case, being empirical corrections, they do not allow for any dynamical simulation. Secondly, we benchmarked our theoretical findings with available experimental data, and such a comparison shows that the above-mentioned problems do not invalidate our conclusions. A point that has to be underscored is the fact that whenever a system has a partially occupied R 4f band, along with the Pt 5d band, excited holes in the O 2p band would easily recombine, thus suppressing any oxidation activity. Analogously, an excited electron would relax to the empty state of the not fully occupied R 4f band, hindering any coupling to protons and thus suppressing also any reduction reaction. Although one

the contributions of molecular hydrogen. Such a contribution is shown in panel d for the H 1s state of the nondissociated H2 molecule, while panel e shows the H 1s contribution of the dissociated H2 molecule. All 1s contributions coming from methanol molecules not adsorbed onto Pt sites are grouped in panel f, whereas panels g and h report the H 1s contributions from the hydroxyl group and the methyl group of the adsorbed CH3OH molecule. In line with former findings, the general trend is a dispersion of H 1s components not belonging to H2 molecules over a broad range of energies from −9 to +1 eV, mixing with Pt 5d components, and the hybridized states distribute up to the unoccupied band region. Coming to hydrogen, most of the H 1s components are confined in a narrow energy range between −8 and −7 eV. A different scenario is offered by the dissociation of an H2 molecule: in this case the H 1s components are widely spread in both the occupied and unoccupied bands of the system. Despite the fact that only two H atoms are involved, their relative H 1s weight in the unoccupied band (upper circle in Figure 9e) reaches roughly 10% with respect to the analogous total contributions due to all the water molecules (circle in Figure 9c). The general picture that we can extract from this simulation and the previous ones is that the presence of methanol molecules in aqueous solution may induce a shift to lower energies of the H 1s component in the antibonding states of H2O molecules, providing support to the observed higher H production whenever CH3OH is added to the solvating water.

7. ELECTRONIC PROPERTIES OF THE RVO4 BULK SYSTEMS AND RELATIONSHIP WITH THE PHOTOCATALYTIC ACTIVITY To provide an exhaustive and systematic scenario, as a complement we discuss briefly the relationship between our experimental results and the electronic structure of the various RVO4 bulk crystals, for R = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The projected density of the states (PDOS) are reported in Figure 10. By comparing the electronic 8338



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Figure 10. Projected density of states (PDOS) for the various RVO4 (R = Y or lanthanide) bulk systems. The highest occupied level of each system is indicated by the horizontal solid red line labeled “ef”, and its value is reported in the figures (in eV) for each system. Since we could not prepare any PmVO4 compound (Pm atomic number = 61) in our experiments, the average lattice constants and atomic positions of NdVO4 (Nd atomic number = 60) and SmVO4 (Sm atomic number = 62) were used for PmVO4 calculations.

unit and the amount of Pt cocatalyst. Indeed, the major amount of Pt cocatalyst was loaded in LuVO4 in atomic %, which translates into the best efficiency in Table 1 (see footnote d). Nevertheless, we infer that other factors partly support the experimental results. By looking at the optical properties furthermore (see Figure 11c), we noticed that the GdVO4 displays a stronger adsorption in the wavelength range from 340 to 400 nm than the other two compounds. This seems to be due to two additional contributions coming from optical transitions (i) between the O 2p band and the Gd 4f upper band and (ii) between the Gd 4f lower band and the Gd 4f upper band. This is a possible explanation for the slightly better efficiency of GdVO4 in comparison with YVO4 in terms of hydrogen generation per unit mol. Although Gd 4f−Gd 4f transitions should be normally forbidden, the selection rule based on the parity should be broken to some extent, since the gap between the Gd 4f lower band and the Gd 4f upper band is large. An additional important feature is the position of the Gd 4f upper band. This band is located at slightly lower energy with respect to the unoccupied V 3d band. Consequently, the excited electron carriers can be well-separated from the V 3d band by populating the Gd 4f upper band, which is spatially well-localized and is expected to enhance the lifetime of excited electrons to some extent.

Table 2. Projection of the Kohn−Sham Orbitals onto Atomic Wavefunctions at CBM and VBM for YVO4, GdVO4, and LuVO4 YVO4 GdVO4 LuVO4

CBM

VBM

V 3d 73%, Y 4d 8% Gd 4f 77%, V 3d 18% V 3d 72%, Lu 5d 8%

O 2p 83% O 2p 77%, Gd 4f 7% O 2p 80%, Lu 4f 2%

might expect that compounds with only partially occupied R 4f bands would display visible light response to some extent (Figure 11a,b), they are not visible light active photocatalysts, since the systems cannot create holes and electrons at appropriate energy levels to promote oxidation and reduction, and even in the case in which this possibility exists, the lifetime of electrons and holes would be too short to give rise to any activity and an immediate recombination would occur. Consequently, the only possible candidates not affected by these recombination phenomena are YVO4, GdVO4, and LuVO4. Table 1 indicates apparently that LuVO4 is the best in terms of H2 evolution rate per one RVO4 unit per hour, GdVO4 is the second, and the YVO4 is the third. However, this order is mainly established by the atomic ratio of the number of RVO4 8339



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Figure 11. Optical absorption spectra of CeVO4, PrVO4, NdVO4, SmVO4, and EuVO4 (a); CeVO4 and TmVO4 (b); and GdVO4, YVO4, and LuVO4 (c). Further details are given in the text.

the bulk GdVO4 and LuVO4 crystals. The low efficiency in hydrogen generation of RVO4 systems, apart from the cases R = Y, Gd, Lu, seems to be due to the presence of electrons in the 4f band, which is located in the midrange between the occupied O 2p band and the unoccupied V 3d band. Those electrons annihilate the holes created in the O 2p band by the photoexcitation and then suppress the oxidization process. The fact that YVO4, GdVO4, and LuVO4 systems have indirect band gaps is also prone to confer a longer lifetime to the photoexcited electron carriers, thus improving the photocatalytic performance. The reason why the GdVO4 and LuVO4 have a slightly better performance in terms of hydrogen generation than YVO4 can be inferred from the observation that GdVO4 has optical transitions between the occupied valence band and the Gd 4f unoccupied band in addition to the main O 2p−V 3d transition and the possible atomic scale carrier separation of the excited electron due to the presence of the localized Gd 4f upper band appearing just below the unoccupied V 3d band, while LuVO4 is likely to possess a higher ability in dissociating adsorbed molecules because of the smaller ionic radius of Lu3+ with respect to Y3+ at the surface.

Concerning LuVO4, attention should be paid to the Lu−O and V−O distances. The Lu3+ ionic radius is smaller by 4.3% than that of Y3+ (see Table S1, SI). According to the crystallographic database, the atomic distances V−O and Lu− O in LuVO4, V−O and Gd−O in GdVO4, and V−O and Y−O in YVO4 are 1.62, 2.26, 1.71, 2.34, 1.71, and 2.29 Å, respectively. The general trend we evidenced indicates that the adsorbed water molecule (or hydroxyl group) is more easily dissociated and consequently easier to be reduced to hydrogen, if the atomic distance (−Mcat−Owt−) of the adsorbate is the shorter than 2.2−2.3 Å.13,26−28 Hence, LuVO4 seems to be better suited to this aim with respect to YVO4.

8. CONCLUSIONS Our systematic exploration of the photocatalytic properties of the inhomogeneous RVO4 systems provides a clear and comprehensive roadmap to engineer this class of systems to work efficiently. More specifically, the major outcome of the present investigation is the observation that water molecules adsorbed to the Pt cluster added to the YVO4 surface as a cocatalyst do not dissociate systematically, but dissociation eventually occurs even on the short time scale of the simulations. Analogously, molecular hydrogen has also been observed to undergo dissociation on the time scale accessible by our computer experiments. All these facts indicate that a modest dissociation barrier, of a few kBT, has to be overcome and that dissociation events are likely to occur at finite temperature. Third, inspection of the electronic structure of the catalytic system in the presence of the Pt cocatalyst wetted with water, methanol aqueous solutions, and molecular hydrogen has shown that the peculiarity of Pt is its ability to delocalize electrons excited to the unoccupied V 3d band and to transfer them to the edge or surface of the Pt far from the original V site, thus avoiding an immediate subsequently vain electron− hole recombination. This, in turn, allows the transfer of the electron to the proton provided by molecular dissociations and adsorbed on the Pt atom. Finally, the excellent photocatalytic efficiency in H2 generation of the Pt-loaded GdVO4 and LuVO4 systems has been reinterpreted by a comparison of the electronic properties of the Pt-loaded YVO4 system and with

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S Supporting Information *

Additional details about the computational approach, the experimental procedure adopted for the preparation of samples, graphs of the time evolution of relevant geometrical parameters during the simulations, and related tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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*Phone: +81-29-863-5414. Fax: +81-29-863-5599. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

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Principles Tight-Binding Theory. Phys. Rev. Lett. 1984, 53, 2571− 2574. (18) (a) Car, R.; Parrinello, M. Unified Approach for MolecularDynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471−2474. (b) CPMD code. Copyright IBM Corp. 1990−2013; Copyright MPI für Festkörperforschung Stuttgart 1997−2001. (19) Swanson, H. E.; Tatge, E. National Bureau of Standards (U.S.). Circular 1953, 539, 1−95 [Prior to any simulation, we checked the performance of the Pt pseudopotential on a conventional bulk Pt crystal, finding a lattice constant of 3.844 Å (only 2% smaller than the experimental value) and the shortest Pt−Pt distances of 2.72 Å]. (20) Bandel, G.; Platte, C.; Troemel, M. Hydroxoplatinic Acid and Its Ammonium Salt. Z. Anorg. Allg. Chem. 1981, 472, 95−101. (21) Although band gaps are generally underestimated by DFT, the global electronic structure is not affected by this systematic error (see e.g. Johnson, B. G.; Gill, P. M. W.; Pople, J. A. The Performance of a Family of Density Functional Methods. J. Chem. Phys. 1993, 98, 5612− 5626 ). (22) Pomès, R.; Roux, B. Theoretical Study of H+ Translocation along a Model Proton Wire. J. Phys. Chem. 1996, 100, 2519−2527. (23) Boero, M.; Ikeshoji, T.; Terakura, K. Density and Temperature Dependence of Proton Diffusion in Water: A First-Principles Molecular Dynamics Study. ChemPhysChem 2005, 6, 1775−1779. (24) Marx, D. Proton Transfer 200 Years after von Grotthuss: Insights from Ab Initio Simulations. ChemPhysChem 2006, 7, 1848− 1870. (25) Zhang, Y.; Yang, Y.; Jiang, H. 3d−4f Magnetic Interaction with Density Functional Theory Plus U Approach: Local Coulomb Correlation and Exchange Pathways. J. Phys. Chem. A 2013, 117, 13194−13204. (26) Oshikiri, M. Japanese Patent No. 4859217, 2011. (27) Oshikiri, M.; Boero, M.; Matsushita, A.; Ye, J. Dissociation of Water Molecule at Three-Fold Oxygen Coordinated V Site on the InVO4 (001) Surface. Appl. Surf. Sci. 2008, 255, 679−681. (28) Oshikiri, M.; Boero, M.; Matsushita, A.; Ye, J. Water Molecule Adsorption Properties on Surfaces of MVO4 (M = In, Y, Bi) PhotoCatalysts. J. Electroceram. 2009, 22, 114−119. (29) For instance, the weight of V 3d is defined as ∑|αi|2 according to the expansion of the eigen wave function Φn(r) in terms of atomic orbitals: Φn(r) = ∑αi ϕV 3d(r-RVi) + ∑βi ϕO 2p(r-ROi) + ∑γi ϕPt 5d(rRPti) + ..., where ϕV 3d(r-RVi) is the 3d atomic wave function of the atom Vi located at the RVi site, etc. (30) Boero, M. Excess Electron in Water at Different Thermodynamic Conditions. J. Phys. Chem. A 2007, 111, 12248−12256. (31) 2008 Chronological Scientific Tables (in Japanese); edited by National Astronomical Observatory; Maruzen Syuppan: Tokyo, 2007.

ACKNOWLEDGMENTS We acknowledge computational facilities at National Institute for Materials Science (NIMS)-Tsukuba and at HPC of the Equip@Meso at the University of Strasbourg. This work was partly supported by JSPS KAKENHI (Grants-in-Aid for Scientific Research (C)) Grant Number 25410245.

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Inhomogeneous RVO4 Photocatalyst Systems (R = Y, Ce, Pr, Nd, Sm

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