The Role of Ni-Based Cocatalyst in Inhomogeneous RVO4

Article pubs.acs.org/JPCC

The Role of Ni-Based Cocatalyst in Inhomogeneous RVO4 Photocatalyst Systems (R = Y, Gd) 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: We extend here our inspection, via first-principles dynamical simulations supported by experiments, to the inhomogeneous photocatalytic RVO4 systems. At variance with our former paper, we limit the present analysis to the most promising photocatalysts (R = Y, Gd) for which the addition of a Ni-based cocatalyst provides the best activities. The specific role of Ni-based cocatalysts is elucidated, showing that a surface Ni doping, as opposed to a bulk one, is the most efficient way to enhance the catalytic activities of these compounds. This is a key step in the improvement of the photocatalysts and a roadmap to optimize the amount of Ni dopant needed for both hydrogen and oxygen generation. dissociation5 although more hydrogen is produced (as shown in Table 2 later). In this paper, by using first-principles molecular dynamics, we focus on the role of NiOx cocatalyst on the adsorption properties of H2O molecules onto YVO4. On the basis of the electronic structures of RVO4 bulk crystals (R = Y, Gd) and that of the related inhomogeneous systems (including YVO4), with the addition of a NiOx cocatalyst and in the presence of water, we can figure out the origin of the high performance of the RVO4 inhomogeneous systems in terms of both oxygen and hydrogen generation.

1. INTRODUCTION In the expanding field of bio-inspired materials, photocatalysis represents a major target, particularly if such a process occurs under visible light. Encouraged by the successful discovery of the visible light response of InVO4,1 we extended here our investigation to other promising vanadates compounds,2,3 namely YVO4 and GdVO4. In the case of InVO4, the major result of the photocatalytic activity upon water decomposition is the generation of hydrogen,1 contrasting with the activity shown by BiVO4 in oxygen generation.4 Although the catalytic efficiency of the bare YVO4 and GdVO4 (i.e., without a cocatalyst) is not so remarkable, a dramatic increase in water decomposition occurs in the presence of a NiOx cocatalyst loaded on the surface of the photocatalyst grains. Unfortunately, such a photocatalytic ability is limited to the UV region, and even worse, in the absence of a suitable cocatalyst the photocatalytic activity of YVO4 is significantly reduced and even nearly suppressed. In a previous work,5 we have shown that adsorption of water molecules is possible on YVO4 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 only able to trivially generate oxygen upon water © 2014 American Chemical Society

2. YVO4 IN THE PRESENCE OF A NiOx COCATALYST Experiment. The YVO4 compound carrying a NiO x cocatalyst loaded on the grain surface was prepared following a procedure analogous to the one reported and assessed in a previous work,2 namely a solid-state reaction method. Specifically, in the first stage, stoichiometric amounts of Y2O3 and V2O5 powder were mixed in a mortar (for instance, standard amounts would be 5.539 g of Y2O3 and 4.461 g of V2O5 for 10 g of YVO4); then this mixture was heated at 700 °C for 12 h in air in an alumina crucible by means of an electric furnace. After natural cooling, the reacted powder was ground in a mortar to reduce the grain diameter below 10 μm. After this process, the resulting crystal structure of the synthesized polycrystalline YVO4 compound powder was found to possess a zircon-type cubic crystal structure, belonging to the I41/amd Received: February 28, 2014 Revised: May 22, 2014 Published: May 22, 2014 12845

dx.doi.org/10.1021/jp502099v | J. Phys. Chem. C 2014, 118, 12845−12854

The Journal of Physical Chemistry C

Article

space group. The lattice constants a (= b) and c, determined by X-ray Rietveld analysis, were 7.12 and 6.29 Å (Table 1),

cocatalyst is expected to be similar to that around the Ni site in Ni-doped YVO4. Of course, this is true only if the cocatalyst adheres tightly to the surface in the sintering process, irrespective of the fact that a Y−Ni exchange reaction might or might not occur. In this respect, our modeling should be able to mimic at least the actual situation around the boundary between the YVO4 catalyst and the NiOx cocatalyst. The size of surface amounts to 2a × 2c (= 2 × 7.12 Å × 2 × 6.29 Å) = 179 Å2, which is roughly identical to the size of one monomolecular H2O layer composed of about 18−19 molecules. When 35 water molecules are added on top of the model surface, this results in about two H2O molecular layers. ̈ view, since the slab thickness is 7.12 Å and that of In a naive water layer in liquid phase should be 5.82 Å at standard temperature and pressure as established by IUPAC (STP), the total thickness of the system should be 12.9 Å. On these grounds, we prepared a 2a × 2c × 18.52 Å simulation box, whose height is much larger than the one expected at STP, in which the YVO4 slab was placed, prior to any Ni doping. Water layers were subsequently added at initial H2O distances larger than, at least, the typical hydrogen bond distance (∼3.0 Å) to avoid any spurious molecular adsorption which could possibly bias the system. On the (010) surface constructed in this way, seven sets of 4-fold oxygen-coordinated vanadium (4c-V) and 7c-Y in addition to one set of 6c-Y and 3c-V are present. We recall that the bulk crystal is composed of only 4c-V and 8c-Y sites. For an easy comparison of the present results and the ones of our former work on YVO4, the surface model prepared here is identical to the one in Figure 3 of ref 5. The water layer was pre-equilibrated separately at STP and then introduced on top of the surface. Then the cell thickness was shrunk gradually to about 13.0 Å by applying a constant pressure approach of the Parrinello−Rahman type7 in which the cell sizes a and c were kept fixed, whereas the height (b-axis) was allowed to vary. During this NPT simulation, all atoms except for the bottom layer, fixed to bulk crystallographic positions, are allowed to follow the dynamics. The scope is to find the optimal cell size getting rid of residual pressure effects which could be a source of biasing for the simulated catalytic processes. After this cell optimization, two Y atoms at 7c-Y sites on the exposed surface were replaced by Ni atoms (Figure 1a). Finally, NVT molecular dynamics simulations at 300 K were done without any atomic positional constraints. We underscore the fact that the substitution of Y atoms and not V atoms with Ni is not a random choice, but an educated guess based on the fact that the Y−O atomic distance in a YO8 triangular dodecahedron, present in the YVO4 crystal, ranges from 2.29 to 2.44 Å and that of V−O of VO4 tetrahedron is 1.71 Å; it is then a reasonable to assume that a Y atom can be more easily exchanged with a Ni atom, being the Ni−O atomic distance in a NiO6 octahedron (NiO crystal) 2.09 Å,8 i.e., smaller than Y− O but larger than V−O.

Table 1. Electronic Shells, Ionic Radii and Crystallographic Parameters of the Main Elements Used in This Work R

4f

5d

6s

R3+ ion size (Å)

a (Å)

c (Å)

Y Gd Lu

0 7 14

1 (4d) 1 1

2 (5s) 2 2

0.900 00 0.938 00 0.861 00

7.115 63 7.209 44 7.027 80

6.286 99 6.346 08 6.236 23

respectively, as previously reported.6 Subsequently, 1.0 wt % NiO was loaded on the grain surface according to the following procedure. The powder was impregnated with a Ni(NO3)2 aqueous solution; then it was dried at 200 °C for 5 h and subsequently exposed to hydrogen gas atmosphere (200 Torr) at 500 °C (773 K) for 2 h. Finally, the system was subjected to oxidation (100 Torr) at 200 °C for 1 h. The photocatalytic activities of YVO4 with the addition of the NiOx cocatalyst were observed upon UV light irradiation using an inner-irradiation type quartz cell with a 400 W high-pressure Hg lamp in a closed gas circulation system. More precisely, 1.0 g of catalyst powder was suspended in 370 mL of pure water using a silica glass cell connected to the closed gas circulation system. The evolved gases were analyzed with a gas chromatograph by a thermal conductivity detector (GCTCD) connected to a gas circulating line. For comparison, the photocatalytic activity of the YVO4 without the cocatalyst was also investigated with the same procedure. The results are summarized within Table 2, and they show clearly that without the NiOx cocatalyst, almost no oxygen generation was observed. The only product is hydrogen, although it is produced at a small rate of 10 μmol/h. On the other hand, in the presence of the cocatalyst, a significant improvement in both hydrogen (300 μmol/h) and oxygen (150 μmol/h) generation and 2:1 stoichiometric gas evolution rate of H2:O2 was observed. Simulation Model. To elucidate the role of the Ni-based cocatalyst, we performed dynamical simulations at finite (room) temperature within first-principles approaches of the inhomogeneous photocatalytic system, including H2O molecules at the density of the ordinary liquid water, the YVO4 metal oxide photocatalyst, and the Ni-based cocatalyst. Simulations have been done by modeling the inhomogeneous photocatalytic reaction system with a supercell approach where a Ni doped YVO4 slab, exposing the (010) surface, is in contact with H2O molecules at the typical density of bulk liquid water. The simulated system contains 2 Ni dopant atoms, 14 Y atoms, 16 V atoms, and 64 O atoms for the substrate and 35 H2O molecules above it. Ni doping was done by exchanging two surface Y atoms with two Ni atoms. We investigated thermally equilibrated model systems all along the ongoing discussion, and the coordination structure around the Ni site in NiOx

Table 2. Photocatalytic Properties of RVO4 in the Presence and Absence of Cocatalyst for Water (R = Y, Gd) catalyst

cocatalyst

reactant

H2 evolution rate (μ mol/h)

H2 evolution rate (mol/h/RVO4 mol)

O2 evolution rate (μ mol/h)

O2 evolution rate (mol/h/RVO4 mol)

remark

YVO4 (1 g) YVO4 (1 g) GdVO4 (1 g)

none 1.0 wt % NiO 1.0 wt % NiO

water (370 mL) water (370 mL) water (370 mL)

10 300 300

0.0020 0.0611 0.0817

Tr 150 150

Tr 0.0306 0.0408

a a, b a, b

A 400 W Hg high-pressure lamp using an inner-irradiation type quartz cell, RVO4 by R2O3 + V2O5 at 700 °C 12 h in air. Measurement errors in the evolution rates are estimated to be approximately ±5%. b1.0 wt % corresponds to mole ratio of YVO4:NiO = 1:0.00273, GdVO4:NiO = 1:0.00364.

a

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Figure 1. (a) Simulation model for the inhomogeneous Ni-doped YVO4 catalyst in contact with water. The color code for the stick and balls representation, here and in all the following figures, is white for H, red for O, gray for V, cyan for Y, and blue for Ni. (b) Time evolution of the atomic distances between the Ni dopant at the surface and the O atoms belonging to the two adsorbed water molecules during the 300 K dynamics. In one case (red curve) the departure of the H2O molecule occurs after about 5.3 ps.

Dynamical simulations were done within the Car−Parrinello9 molecular dynamics (CPMD) framework, and electronic and photocatalytic properties were extracted in the related postprocessing analysis. In the CPMD, an unrestricted spin approach (LSD) was used, and gradient corrections on the exchange and correlation functional after Becke−Lee−Yang− Parr (BLYP)10,11 were used. The valence−core interaction was modeled by norm-conserving Troullier−Martins (TM) pseudopotentials12 for Y, V, Ni, and O atoms, whereas an analytical Car−von Barth pseudopotential13 was used for H. In the case of Y and V, the use of semicore states was found to be essential for a good description of both the structure and the energetics. The electrons of Y 4s, 4p, 4d, 5s; V 3s, 3p, 3d, 4s; Ni 3d, 4s; O 2s, 2p; H 1s were treated explicitly as valence electrons and expanded in a plane-wave basis set with an energy cutoff of 80 Ry, with the sampling of the Brillouin restricted to the Γ point. The temperature of the system was controlled by a velocity rescaling algorithm.14 A fictitious electronic mass of 1200 au and an integration step of 5.0 au ensured a good control of the conserved quantities. When Kohn−Sham equations were solved and electronic structure properties were investigated, normal (nonsemicore) numerical BLYP-TM pseudopotentials were employed. The electrons of Y 4d, 5s; V 3d, 4s; Ni 3d, 4s; O 2s, 2p; H 1s were treated explicitly as valence electrons and expanded in a plane-wave basis set with an energy cutoff of 80 Ry. The photoabsorption profile was computed according to the formula α(ω) =

2πe 2 2 3m2Vcell ω

∑ (fi

atoms. In Figure 1a, we have labeled the O atoms of water molecules adsorbed on the two Ni sites, Ni1 and Ni2, as O108 and O130, respectively. These represent an equilibrium condition, attained during the 300 K dynamics in about 5 ps, and those distances vary between 1.9 and 2.6 Å, the average being about 2.1 Å. No spontaneous dissociation of H2O was observed during the dynamics. Instead, after about 5.3 ps one H2O molecule (O130) was desorbed from Ni2 (see Figure 1b). Another noticeable effect is that the 3c-V exposed site, intentionally introduced, during the simulation evolves toward a 4c-V structure as a consequence of the thermal relaxation; thus, no dissociative adsorption of water molecules could be observed at V sites for this system, contrary to the findings of our former work.5 At the initial stage of the dynamics, Ni1 and Ni2 are surrounded by eight O atoms, one belonging to a nearby water molecule and the remaining seven to the substrate, with Ni−O distances ranging from 2.16 to 2.69 Å (2.39 Å on average). As long as the dynamics approaches the equilibrium, the Ni sites preserve their 8-fold coordination. However, the NiO8 coordination shell undergoes significant changes, leading to the formation of Ni sites with only five or six Ni−O distances lower than 2.7 Å, while the other distances are often larger than 3.0 Å. It looks as if the Ni atoms attempt to recover a regular NiO6 structure, similar to the octahedral structures seen in the NiO bulk crystal, pushing away the excess oxygen atoms as far as the system allows (see Table S1 in Supporting Information). The H2O sticking onto the Ni1 site was also characterized in terms of adsorption energy. In this case, to eliminate residual forces, a local geometry relaxation of the adsorbate, involving only the substrate and the adsorbed water molecule (i.e., not the rest of the equilibrated liquid water), was done by direct inversion in the iterative subspace (DIIS)17 until each force component less than 32 meV/Å. The adsorption energy (Eads) of H2O to the 7c-Ni reads then

− f j )|⟨ψi|p|ψj⟩|2 δ(εi − εj − ℏω)

i,j

As explained elsewhere,15,16 ψi are the Kohn−Sham eigenstates, εi the corresponding eigenvalues, f i the Fermionic occupation number, e the electron charge, Vcell the volume of the simulation box, m the electron mass, and p the momentum. As usual, to avoid the intrinsic gauge problems of the position operator x (dipole approximation), we make use of the commutation relationship p/m = i[H,x]. Atomic-Level Insight near the Ni Sites. We first focus on the fate of water molecules in proximity to the exposed Ni

Eads = (Erelaxed slab only + Erelaxed one water molecule) − Erelaxed slab and adsorbed one water molecule· 12847



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with two additional O atoms protruding out of the octahedron at Ni−O distances of about 3.3−3.4 Å. We remark that these distances are much larger than the Y−O ones observed in the case of the YO8 triangular dodecahedron of YVO4 and ranging from 2.29 to 2.44 Å. Since the positions of the hole levels in the energy spectrum originating from the Ni dopant are affected by the O coordination structure around Ni atoms, in view of the forthcoming discussion about the electronic structure, we extend here the analysis to the case that Y atoms in bulk YVO4 are replaced with Ni atoms. Additional calculations, within the computational framework used here, confirmed that a substitutional bulk Ni atom is 8-fold coordinated forming a NiO8 polyhedron with Ni−O distances in the range 2.10−3.09 Å and an average value is 2.34 Å (see Supporting Information, Table S1), almost identical to the average Y−O atomic distance in the YO8 triangular dodecahedron of YVO4 (2.37 Å). Summarizing Ni tends to mimic a YO8-like polyhedron in the bulk, whereas at the surface a NiO5−6 polyhedral structure appears with the remaining O atoms coordinated at distances larger than 3 Å. Electronic Properties. The detailed electronic structure18 of the system including Y14Ni2V16O64 plus 35 water molecules was analyzed by sampling along the trajectory several uncorrelated configurations. Since the presence of Ni dopants provides remarkable improvements in the experimental photocatalytic activity, we shall put particular emphasis on the Ni-

According to this estimation, the adsorption energy is 0.95 eV/ molecule and the relaxed Ni−Owater distance is 2.07 Å. This adsorbate structure is shown in Figure 2 (data listed in Table S1

Figure 2. Stable configuration of the Ni-doped YVO4 surface in which the Ni atom exposed at the surface restores its 6-fold coordination by binding one water molecule. All numbers indicate the corresponding atomic distances in Å, and the color code is identical to Figure 1a. Computational errors in the atomic distances are estimated to be approximately ±1%.

of the Supporting Information). The NiO8 (Ni−O = 2.03−3.40 Å, 2.43 Å on average) structure might be regarded as a NiO6 octahedral structure (Ni−O = 2.03−2.43 Å, 2.13 Å on average)

Figure 3. Electronic structure18 of the Ni-doped YVO4 in contact with water. The various profiles indicate the weights associated with the most relevant atoms, obtained as projections19 of each eigenfunction of the Kohn−Sham Hamiltonian onto the atomic orbitals. Panels a−g refer to the electronic structure profiles at the point C of Figure 1b, where the distance between the O atom of adsorbed water molecule and the exposed Ni is relatively large (O108−Ni1 = ∼2.61 Å). A black solid line in the last plot (h) refers to the optical absorption spectrum.16 12848



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Figure 4. Electronic structure18 of the Ni-doped YVO4 in contact with water. Panel a indicates a superposition of panels a, b, and d of Figure 3. The profiles b, c, and d indicate the weights associated with the most relevant atoms, obtained as projections19 of each eigenfunction of the Kohn−Sham Hamiltonian onto the atomic orbitals. All panels correspond to the point C of Figure 1b, where the distance between the O atom of adsorbed water molecule and the exposed Ni is relatively large (O108−Ni1 = ∼2.61 Å).

This reflects the undercoordination affecting O sites at the surface. These atoms form the thick band of the so-called surface levels. On the other hand, the weight of O_2p orbitals of adsorbed water molecules in the states around the highest occupied level is relatively small (panels b of Figures 3 and 5). (iv) Ni_3d atomic orbital components are well merged in the set of occupied levels in the range from −4 to −8 eV. As shown in both panels d of Figures 3 and 5, Ni dopants are responsible for characteristic features in terms of unoccupied states. The first one is the arising of four empty states located in the energy gap of the pristine substrate (Ni-undoped), located roughly around −1 to −2 eV. These states are mainly contributions (>60%) of Ni_3d orbitals, as evidenced by atomic-orbital projections. Hereafter we shall refer to these states as MGU-Ni 3d (mostly genuine unoccupied Ni 3d). A second important contribution originates from two unoccupied states of mixed character in proximity of the highest occupied level (about −3.8 eV in panel d of Figure 3 and about −3.9 eV in panel d of Figure 5), namely about 20−30 meV above it, in which the mixed character still contains a contribution from Ni_3d orbitals, but such a contribution can vary between 0 and 40% during the dynamics, according to the structural fluctuations of the system around the Ni sites. Such a feature is responsible for the formation of the so-called holes level, indicated in panels d of Figure 3 and 5, and is a clear indication that Ni dopants can

related electronic structure. More precisely, we looked at the projected electronic states 19 for the complete system Y14Ni2V16O64 + 35 H2O molecules in two crucial moments of the dynamics, when the distance between H2O molecules and the exposed Ni is large (Figures 3 and 4 concerned with the point C in Figure 1b) and when it is small (Figures 5 and 6 concerned with the point D in Figure 1b). Some common features hold along the whole dynamics; these can be summarized as follows: (i) The unoccupied levels located at −1 eV are mainly due to V_3d orbitals (∼65%) and O_2p orbitals (∼25%) belonging to the photocatalyst substrate (see panels c and a of Figure 3 and panels c and a of Figure 5). The residual 10% comes, instead, from Y atoms (Y_5d (∼5%) and Y_6s (∼3%)) plus negligible contributions from Ni_4s and Ni_4p. (ii) The occupied levels, starting from the highest occupied level, located at about −3.8 eV in Figures 3 and 5, and extending up to −8.5 eV are mostly formed by O_2p orbitals (∼60%) belonging to the substrate. Contributions from O_2p orbitals of H2O molecules mix with the O_2p of the substrate in the energy range from −3.8 to −11.5 eV. (iii) The relative weight of O_2p orbitals of exposed O atoms at the surface (for instance, O66 and O72 in Figure 1a) is rather large in proximity of the highest occupied level (see panels d and c of Figure 4 and panels d and c of Figure 6) in spite of the presence of protons of water molecules close to the concerned O atoms. 12849



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Figure 5. Electronic structure18 of the Ni-doped YVO4 in contact with water. The various profiles indicate the weights associated with the most relevant atoms, obtained as projections19 of each eigenfunction of the Kohn−Sham Hamiltonian onto the atomic orbitals. Panels a−g refer to the electronic structure profiles at the point D of Figure 1b, where the distance between the O atom of adsorbed water molecule and the exposed Ni is relatively small (O108−Ni1 = ∼1.89 Å). A black solid line in the last plot (h) refers to the optical absorption spectrum.16

(i) Since the occupied levels between −4 and −9 eV are mostly due to O_2p orbitals and the system contains 99 O atoms, on average the weight of O_2p orbital per each state should be 1/99 (i.e., ∼1%), assuming a homogeneous distribution. However, occupied states around the highest occupied level often contains more than 10% of O_2p components per each surface O atom (for instance, 9.6% and 11% come from O66_2p at the surface in both panels d of Figures 4 and 6, respectively). On the other hand, O_2p components due to the H2O molecule adsorbed onto Ni are smaller than 1% since they are buried under the surface levels. However, when the Ni−Owater distance becomes shorter than about 2.0 Å, the component around the highest occupied level can increase beyond its average value of 1.0%. (This occurs, for instance, at points B and D along the trajectory of Figure 1b, where the values reaches 6.1% and 2.6%, respectively.) When this situation is realized, we can expect an enhancement of the oxidization probability for adsorbed water molecules. (ii) The thermal motion is responsible for the shifting of the hole levels upon dynamics. Specifically, the energy gap between the highest occupied level and the shallow hole levels shows variations in the range 1−61 meV, with an average value of ∼26 meV, identical to kBT at 300 K, thus ascribable to typical energy fluctuations of the finite temperature dynamics. On average, about 73% of the hole states originate from 2p orbitals of O atoms of the substrate (Ocat_2p) plus a contribution of about 15% coming from Ni_3d and 10% from 2p orbitals of O atoms

create a hole not only in the substrate but also partially in the water band structure. In the YVO4 system, generally three electrons are transferred from Y to O, so that yttrium is present in the system as Y3+. Conversely, the inclusion of Ni introduces a chemical species which is less electron rich and can donate only two electrons. As a consequence, one hole is introduced in the valence band for each Y−Ni substitution. Since in our case two Y atoms are replaced by Ni, two hole states are expected. Indeed, this is what we find in the analysis of the electronic structure, where these states with opposite spins exist. The interesting feature is the fact that the Ni_3d components are often included in the hole levels and in the energy range of the surface levels of the system around the highest occupied level. This implies that the holes created at O sites on the surface or nearby by photoexcitation, are able to move toward Ni sites. Focusing on the electronic structure changes occurring upon dynamics, two specific features can be retained. On a first instance, (i) the O_2p component belonging to the water molecule adsorbed onto the Ni site is distributed more homogeneously in the valence band when the atomic distance Ni−Owater is less than 2.0 Å (Figure 6, panel b) rather than when such a distance is larger (Figure 4, panel b). (ii) The location of both the hole levels and the MGU-Ni 3d levels are sensitive to the thermal motion. The influences due to (i) and (ii) upon the redox mechanism are worthy of note. Here below we discuss in more detail these two features. 12850



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Figure 6. Electronic structure18 of the Ni-doped YVO4 in contact with water. Panel a indicates a superposition of panels a, b, and d of Figure 5. The profiles b, c, and d indicate the weights associated with the most relevant atoms, obtained as projections19 of each eigenfunction of the Kohn−Sham Hamiltonian onto the atomic orbitals. All panels correspond to the point D of Figure 1b, where the distance between the O atom of adsorbed water molecule and the exposed Ni is relatively small (O108−Ni1 = ∼1.89 Å).

the average value is larger than that in the bulk by 3−9% (further details in Table S1 of the Supporting Information). Concerning the MGU-Ni 3d levels, although the bandwidth somewhat depends on the Ni−Owt distance, the levels are situated just below the unoccupied V_3d band bottom. This feature seems to be advantageous for excited electron carrier separation in proximity of a Ni site, where H2 generation should occur according to our experimental results. Indeed, a Pt plated YVO4 system is able to produce hydrogen in larger amount with respect to the bare YVO4 (i.e., without Pt); this, again, seems to be related to the presence of unoccupied levels originating from Pt_5d orbitals, located below unoccupied V_3d band bottom.20 A first conclusion that can be drawn is that Ni doping can provide a shallow hole state if it is exposed at the surface of YVO4 while if it is located inside the YVO4 bulk a deep hole state is created, and this makes such a state energetically less accessible. Generally, oxygen defects are unavoidably present in metal oxides, and these defects, in turn, make the system an ntype semiconductor. Thus, around the conduction band bottom some electrons are left, and this raises the chemical potential of the system, suppressing at the same time the oxidization activity for the adsorbed species on the photocatalyst surface. Conversely, if the system has a p-type character and the hole concentration is sufficiently high, since the electron carriers brought by O defects can be compensated and the hole−

belonging to water molecules (Owt_2p). Nonetheless, we have to remark that the relative weight of the components Ni_3d and Owt_2p can undergo large fluctuations from 0.3 to 36% and from 3 to 26%, respectively. Generating holes in water molecules might also promote H2O dissociation; however, this is an entirely different dissociation mechanism with respect to the one realized by approaching to a 3c-V site, as shown in our former work.5 When just one water molecule is adsorbed at the Ni site (Figure 2), this general scenario does not undergo significant changes: the hole state is again originating from Ocat_2p by (71%), with Ni_3d (19%) and Owt_2p (only 5%) as minor contributions. Since the dominant component of the hole state is still Ocat_2p, the hole is not so localized and distributed in proximity of the highest occupied level as in the case of normal Ocat_2p valence states. The situation is entirely different, instead, in the case of substitutional Ni in the bulk. In this case the hole level is located energetically far from the highest occupied level and the related gap increases to about 286 meV, i.e., about a factor of 10 with respect to the case of surficial Ni (∼26 meV). The hole state in the bulk crystal is mainly due to Ocat_2p (53%) and Ni_3d (40%). This difference can be ascribed to the difference in the coordination shell around substitutional Ni atoms. In fact, the Ni−O distances in Ni-doped YVO4 bulk crystal range from 2.10 to 3.09 Å (average = 2.34 Å), whereas for the Ni exposed at the surface those same distances vary between 1.9 and 3.6 Å and 12851



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Figure 7. Projected density of states (PDOS) for the various RVO4 (R = Y (a), Gd (b), Lu (c)) bulk systems.20 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. Weights of dominant atomic orbital components at CBM and VBM are inserted in panels a−c. (d) Optical absorption spectra of RVO4 (R = Y, Gd, Lu) obtained in experiments.20

the linear muffin-tin orbital method in the atomic-spheres approximation (LMTO-ASA).22 This type of calculation was performed by including the orbitals 4f, 5d, 5p, and 6s for R (R = Gd, Lu); 4d, 4p, and 5s for Y; 3d and 4s for V; and 2s and 2p for O in the valence states. Spin polarization was accounted for in each case, while the spin−orbit interaction was not taken into account. All calculations have been done on crystallographic identical unit cells having lattice vectors (−a/2, a/2, c/ 2), (a/2, −a/2, c/2), (a/2, a/2, −c/2). Comparing the electronic structures of GdVO4 bulk crystals with that of YVO4, we can infer that the electronic structure of the bulk GdVO4 crystal is nearly identical to the bulk YVO4 crystal on top of which Gd_4f bands are added. In particular, the electronic structure of GdVO4 near the valence band top is almost identical to that of YVO4. The band gap of GdVO4 is 3.14 eV, slightly smaller than that of YVO4 (3.45 eV) due to the Gd_4f upper band. Inspection of the dispersion E(k) of the electronic bands has confirmed that the conduction band minima of YVO4 and GdVO4 (and also LuVO4) are located at Γ points, but the valence band maxima are slightly shifted away from Γ as shown by calculations making use of 8 × 8 × 8 kpoints in the first Brillouin zone. The differences between the highest occupied energies at Γ points and those at the k-points having VBM of YVO4, GdVO4, and LuVO4 are as small as 40, 34, and 7 meV, respectively. These indirect band gaps would result in a longer lifetime for the photoexcited carriers. We remark that the band gap of bulk YVO4 matches the one of our slab model. From these observations, we can deduce that the electronic structure of the inhomogeneous Ni-doped GdVO4 system (including water) should be similar to that of Ni-doped YVO4. The main difference between GdVO4 and YVO4 is the presence of the unoccupied Gd_4f upper band and the corresponding occupied lower band. The f−f transition is normally forbidden because of the parity selection rule. However, in the present case, the band gap between the upper Gd_4f band and the lower Gd_4f band is large. As a consequence, wave functions can undergo non-negligible

electron recombination rate for holes created by photoexcitations can be reduced, the suppression of the oxidization activity of the photocatalyst could be avoided. Moreover, as a second conclusion, the presence of Ni as a dopant is likely to contribute to the electron carriers separation, thus enhancing proton reduction, i.e., H2 generation.

3. GdVO4 IN THE PRESENCE OF A NiOx COCATALYST Experiment. Sample fabrication and measurements are similar to the ones reported in section 2 for YVO4, namely starting from a mixture of stoichiometric amounts of Gd2O3 and V2O5 powder in a mortar (6.659 g of Gd2O3 and 3.341 g of V2O5 for 10 g of GdVO4); the identical procedure described above gives the final catalyst sample. For the ongoing discussion, we just recall the main structural features. Specifically, the crystal structure of the synthesized polycrystalline GdVO4 compound powder was found to be similar to the crystal structure of YVO4. The differences are mainly in the lattice constants a (= b) and c, slightly larger than those of YVO4 (see Table 1).21 As shown in Table 2, the GdVO4 with NiOx cocatalyst shows a better performance in terms of both hydrogen and oxygen generation than YVO4 by roughly 30%. The difference in the performance may be ascribed mainly to the atomic ratio in the number of RVO4 units and the amount of Ni-based cocatalyst (see Table 2, remark b). Nevertheless, other factors might partly concur to this experimental result, as pointed out in the next section. Discussions. On the basis of the electronic structures of the inhomogeneous photocatalytic systems we investigated, including YVO4 with the addition of a NiOx cocatalyst (in the presence of water) and with the auxiliary information given by the electronic structures of YVO4 and GdVO4 bulk crystals, we try to extract the general picture concerning the photocatalytic activity. Concerning the bulk, the electronic structures of YVO4 and GdVO4 are shown in Figures 7a and 7b, respectively. These (and that of LuVO4 for later discussion, Figure 7c) were computed using a DFT local spin density approximation within 12852



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that GdVO4 can give rise to additional optical transitions between the occupied valence band and the Gd_4f unoccupied band along with the more standard O_2p-V_3d transition and from the possible excited electron carrier separation exerted by unoccupied Gd_4f upper band in addition to the upper unoccupied Ni_3d band.

deformation with respect to pure atomic levels, and then the selection rule can be broken to some extent. Indeed, looking at the optical absorption profiles of RVO4 (R = Y, Gd, Lu) in Figure 7d, the profile of GdVO4 shows a characteristic strong optical absorption in the range 340−400 nm, in addition to the main O_2p−V_3d transition common to the three systems. This unique strong absorption can be ascribed to the transitions between the occupied lower Gd_4f band and the unoccupied upper Gd_4f band accompanied by transitions between occupied O_2p and unoccupied Gd_4f states. 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. As mentioned in the discussion about the Ni-doped YVO4 system, the unoccupied Ni_3d bands are located at positions similar to the Gd_4f upper band. Consequently, the excited electron carriers can be well separated from V_3d band by populating the Gd_4f upper band, which is spatially well localized; additionally, excited carriers can go populating also the Ni_3d unoccupied band, provided that they originate from V sites around the Ni site whenever Ni dopants are present. This should favor proton reduction (i.e., hydrogen generation). Since the top of the valence band of GdVO4 is similar to that of YVO4 (see atomic orbital components at VBM inserted into Figures 7b and 7a), a hole can be created by the Ni dopant also in the GdVO4 system. Nonetheless, we cannot expect that the exposed Gd ion can contribute to water dissociation, mainly because of the large Gd3+ ionic radius (larger than Y3+) which is generally a disadvantage for hydrogen and oxygen generation.5,23−25 The better optical coupling and available electronic levels for carrier separation translate into a the better performance in production of hydrogen and oxygen of GdVO4 in comparison with YVO4. Since the ionic radius of Lu3+ is smaller than Y3+ (see Table 1), it would be also interesting to compare Ni-doped LuVO4 with Ni-doped YVO4 and GdVO4 in forthcoming photocatalysts investigations.

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ASSOCIATED CONTENT

* Supporting Information S

Additional details about the geometrical parameters of the Nidoped system and comparison of the BLYP and PBE functionals for the Ni doped YVO4 in contact with water. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Ph +81-29-863-5414, Fax +81-29-863-5599, e-mail [email protected] (M.O.). Notes

The authors declare no competing financial interest.

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

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REFERENCES

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4. CONCLUSIONS Our systematic exploration of the photocatalytic properties of the inhomogeneous RVO4 systems provides a clear and comprehensive roadmap to engineer this class of system to work efficiently. More specifically, a first general conclusion that can be drawn is that the Ni dopants on the surface of the YVO4 catalyst represents a viable way to get a p-type metal-oxide semiconductor also in the presence of water, preventing the system to slide into an n-type one, which would be very disadvantageous for oxidizing water to generate oxygen. Taking into account the location of the acceptor levels, doping with Ni the catalyst surface is preferable than incorporating Ni in the bulk. Furthermore, the presence of Ni as a dopant is likely also to contribute to the electron carriers separation, thus enhancing proton reduction, i.e., H2 generation. However, the addition of Ni to the exposed surface turns out to be still insufficient to promote the dissociation of water, which is promoted at the exposed 3-fold V sites.5 Hence, an optimum ratio in the number of 4c-V, 3c-V sites, and Ni sites at the surface exist to optimize simultaneously the surface stability, the dissociation ability, and the hole concentration. The presence of an indirect band gap characterizing both YVO4 and GdVO4 can also be regarded as an advantage in the sense that allows for a longer lifetime of the photoexcited carriers. The reason why GdVO4 has a better performance in terms of hydrogen and oxygen generation than YVO4 can be inferred from the observation 12853



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The Role of Ni-Based Cocatalyst in Inhomogeneous RVO4

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