Effects of Ta2O5 Surface Modification by NH3 on the Electronic

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Effects of Ta2O5 Surface Modification by NH3 on the Electronic Structure of a Ru-Complex/N−Ta2O5 Hybrid Photocatalyst for Selective CO2 Reduction Soichi Shirai,* Shunsuke Sato, Tomiko M. Suzuki, Ryosuke Jinnouchi, Nobuko Ohba, Ryoji Asahi, and Takeshi Morikawa Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan S Supporting Information *

ABSTRACT: This work examined a Ru-complex/N−Ta2O5 (N−Ta2O5: nitrogen-doped Ta2O5) hybrid photocatalyst for CO2 reduction. In this material, electrons are transferred from the N−Ta2O5 to the Ru-complex in response to visible light irradiation, after which CO2 reduction occurs on the complex. N-doping is believed to produce an upward shift in the conduction band minimum (CBM) of the Ta2O5, thus allowing more efficient electron transfer, although the associated mechanism has not yet been fully understood. In the present study, the effects of NH3 adsorption (the most likely surface modification following nitrification) were examined using a combined experimental and theoretical approach. X-ray photoelectron spectroscopy data suggest that NH3 molecules are adsorbed on the N−Ta2O5 surface, and it is also evident that the photocatalytic activity of the Ru-complex/N−Ta2O5 is decreased by the removal of this adsorbed NH3. Calculations show that both the occupied and unoccupied orbital levels of Ta16O40(NH3)x clusters (x = 4, 8, 12, or 16) are shifted upward as x is increased. Theoretical analyses of Ru-complex/cluster hybrids demonstrate that the gap between the lowest unoccupied molecular orbital of the Ta16O40 moiety and the unoccupied orbitals of the Ru-complex in Rucomplex/Ta16O40(NH3)12 is much smaller than that in Ru-complex/Ta16O40. The highest occupied molecular orbital of [Rucomplex/Ta16O40]− is evidently localized on the Ta16O40 moiety, whereas that of [Ru-complex/Ta16O40(NH3)12]− is spread over both the Ta16O40 and Ru-complex. These results indicate that the NH3 adsorption associated with N-doping can result in an upward shift of the CBM of Ta2O5. Additional calculations for Ta16O40−y(NH)y (y = 2, 4, 6, 8, or 10) suggest that the substitution of NH groups for oxygen atoms on the Ta2O5 surface may be responsible for the red shift in the adsorption band edge of the oxide but makes only a minor contribution to the upward shift of the CBM.

1. INTRODUCTION Artificial photosynthesis is a chemical process that converts water and carbon dioxide (CO2) to organic materials utilizing sunlight as the energy source and is expected to represent a solution to global warming and energy supply issues, both of which are recognized as major impediments to realizing sustainable societies.1 In natural photosynthesis, water is oxidized via photosystem II, while CO2 is reduced by photosystem I, and this so-called “Z-scheme” process in naturally occurring systems can be mimicked to construct artificial photosynthetic processes. Recently, a Z-scheme-type artificial photosynthesis system that does not require either the external supply of chemicals or an electrical bias was demonstrated by combining a photocathode composed of a metal-complex/semiconductor hybrid (for CO2 reduction) with a metal oxide photoanode (for water oxidation).2 A relatively high solar-to-chemical energy conversion efficiency of 4.6% has been achieved as a result of the extensive ongoing research and development of this technology.3,4 However, further improvements in the energy conversion efficiency of such systems will require a detailed understanding of the © XXXX American Chemical Society

electronic structures and mechanism associated with the photocatalytic CO2 reduction at the photoelectrodes.5−9 The first semiconductor/metal complex hybrid photocatalyst developed for CO2 reduction was a Ru-complex combined with N-doped tantalum pentoxide (Ru-complex/N−Ta2O5).10 In this material, electrons in the valence band maximum (VBM) of the N−Ta2O5 are excited to the conduction band minimum (CBM) via the absorption of visible light and subsequently transfer from the CBM to unoccupied molecular orbitals of the Ru-complex, thus promoting the CO 2 reduction reaction.5,6,10,11 To facilitate efficient electron transfer, the N− Ta2O5 CBM should be situated above the lowest unoccupied molecular orbital (LUMO) of the Ru-complex, and experimental studies have shown that Ru-complex/N−Ta 2 O5 satisfies this condition.5,6,10 The VBM of N−Ta2O5 as determined by photoelectron spectroscopy in air (PESA) was found to range from −5.7 to −5.5 eV.12 Because the absorption Received: September 29, 2017 Revised: December 13, 2017

A

DOI: 10.1021/acs.jpcc.7b09670 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Section. Ta2O5 powder was synthesized via sol−gel method using TaCl5 (Mitsuwa Chemicals Co., Ltd.) and aqueous ammonia. N−Ta2O5 powder was prepared by annealing the Ta2O5 powder in a flow of an NH3/Ar mixture at 848 K for 6 h, as in a previous study.10,12 The flow rates were 0.4 L/min for NH3 and 0.2 L/min for Ar, respectively. The resulting N−Ta2O5 powder was confirmed to exist as an orthorhombic phase by X-ray diffraction (XRD) analysis.12 NH3 adsorption on the N−Ta2O5 powder was demonstrated by X-ray photoelectron spectroscopy (XPS, Quantera SXM, Ulvac-Phi), using monochromatic Al Kα radiation, which determined a N concentration of 5.3 at. %. To allow for comparison trials, the NH3 molecules on the N−Ta2O5 surface were removed by dispersing a portion of the material in pure water and adding 0.1 M HCl with stirring until the pH of the aqueous phase was 5−6. The product was recovered by filtration. This acid-treated N−Ta2O5 is hereafter referred to as a-N−Ta2O5. N 1s XPS data were also acquired to confirm the removal of N from the a-N−Ta2O5 powder. Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2 was synthesized by a previously reported method21 and subsequently deposited on samples of the Ta2O5, N−Ta2O5, and a-N−Ta2O5 powders by stirring dispersions of the complex and the respective oxides in MeOH overnight. The resulting powders were subsequently dried under vacuum at 323 K. The photocatalysts prepared in this manner are henceforth referred to as Ru/Ta2O5, Ru/N− Ta2O5, and Ru/a-N−Ta2O5, respectively. In each case, the amount of metal complex absorbed on the catalyst can be calculated from the absorption spectrum of the filtrate, and the concentration of absorbed Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2 was found to be 1.0, 0.69, and 0.82 wt % for Ru/Ta2O5, Ru/ N−Ta2O5, and Ru/a-N−Ta2O5, respectively. Photocatalytic activities at ambient temperature and pressure were assessed by preparing a dispersion of 10 mg of the photocatalyst powder in 4 mL of acetonitrile/triethanolamine (5:1, v/v) in a test tube, followed by purging with CO2 for 10 min. Each specimen was irradiated with light over the wavelength range of 410−750 nm using a 500 W Xe lamp with a UV cutoff filter (99.99% cutoff at 410 nm, 42L, Sigmakoki) combined with a UV/IR cutoff filter allowing light from 390 to 750 nm to pass (SCF-1, Ushio). The primary reaction product (formic acid, HCOOH) was quantified using ion chromatography (IC, Dionex ICS-2000) with IonPacAS15 and IonPacAG15 columns, whereas CO and H2 (gaseous byproducts) were analyzed by gas chromatography with a thermal conductivity detector (GC-TCD, Shimadzu GC-14A) in conjunction with an activated carbon column held at 308 K. The IC eluent was 3 mM KOH for the first 10 min, with a gradual transition to 10 mM KOH over the next 5 min and a further transition to 30 mM KOH over the last 5 min. The photocatalytic activities of Ru/N−Ta2O5 and Ru/a-N−Ta2O5 were examined for 3 runs (at 8 and 16 h). Standard deviation error was less than 5% even after 16 h reaction of 3 runs. Fourier transform infrared (FTIR) spectra of the Ru/Ta2O5 and Ru/N−Ta2O5 catalysts were acquired with a Bruker VERTEX80 spectrometer to determine the charge states of the Ru-complex, employing the attenuated total reflectance (ATR) method. 2.2. Calculations. Molecular geometrical structure optimizations were carried out using density functional theory (DFT) with the B3LYP functional.22 Vibrational analyses and

band edge of N−Ta2O5 is 2.4 eV, the CBM is estimated to be located in the range of −3.3 to −3.1 eV, while electrochemical data have shown that the LUMOs of Ru-complexes are located from −3.8 to −3.5 eV, depending on the molecular structure.10,13 Therefore, the CBM of N−Ta2O5 is situated above the LUMO of the Ru-complex. The VBM and CBM of nondoped Ta2O5 have been reported to appear at −7.9 and −4.0 eV, respectively, based on estimations using ultraviolet photoelectron spectroscopy (UPS) data.14 These values imply that N-doping moves the Ta2O5 CBM upward.10,13 This doping also results in a red shift in the absorption band edge that is consistent with the decreased band gap.12 The VBM of N− Ta2O5 is 2.2 eV higher than that of Ta2O5, while the CBM of the former is 0.7 eV higher than that of the latter. The decrease in the band gap can therefore be primarily attributed to the upward shift of the VBM, as a result of the hybridization of N and O p orbitals at high levels of doping, similar to the results obtained from the N-doping of TiO2.15 The mechanism responsible for the upward shift of the CBM is, however, not sufficiently understood. Jinnouchi et al. performed a theoretical study utilizing first-principles calculations and reported that the surface dipole moment induced by a combination of doped N atoms and oxygen defects in the surface layer can raise the CBM.16 This result suggests that structural changes on the surface of the Ta2O5 associated with N-doping could be responsible for raising the CBM and the associated activation of the photocatalyst. In any case, because N−Ta2O5 has an elevated CBM level, it is very effective at promoting the CO2 reduction reaction under visible light irradiation with the aid of metal complex catalysts.10 Recently, other semiconductors, such as C3N417,18 and CdS,19 were found to be applicable to metal complex/ semiconductor hybrids intended for the CO2 reduction reaction. Therefore, understanding the electronic structure near the interface between the metal complex and the semiconductor, which in turn affects the electron transfer and catalysis of the material, has become more important. It is wellknown that the catalytic activity at the solid surfaces of nanostructured metal cocatalysts is strongly influenced by surface acidity and basicity as well as the presence of surface defects. In addition, the local surface states of the semiconductor could significantly affect the performance of a metal complex catalyst because the metal complex is present in the form of minute particles that are locally connected to the semiconductor through linkages. In the case of N−Ta2O5, structural changes other than oxygen defects are also induced by N-doping and may affect the photocatalytic activity of the material by modifying electronic structures and band alignments.20 In a typical doping procedure, N−Ta2O5 is prepared by heating nanoscale Ta2O5 in a flow of gaseous ammonia (NH3) at 848 K for 6 h.12 Thus, it is most likely that NH3 and related species are present on the catalyst surface, even when the material is subsequently exposed to solvents during the photocatalytic reaction. Nevertheless, the effects of N-doping on the electronic structure of the oxide have not yet been examined. In the present study, we therefore carried out experiments and computations to elucidate the effects of NH3 adsorption on the photocatalytic activity of the catalyst and to determine the mechanism responsible for such effects. The results suggest that NH3 adsorption may be responsible for the upward shift of the CBM as well as the increased photocatalytic activity during the light-induced CO2 reduction reaction. B


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The Journal of Physical Chemistry C natural population analyses23 were executed for the optimized geometries, and the absence of an imaginary number frequency was confirmed for all optimized structures. Time-dependent DFT (TDDFT) calculations24−28 were performed with the B3LYP and CAM-B3LYP functionals.29 When using an ordinary hybrid functional, such calculations tend to underestimate the transition energies of those excited states having a charge transfer character.30 Adopting a range-separated functional is considered to be one means of counteracting this underestimation,31 and the CAM-B3LYP functional is one of the most successful and widely used range-separated functionals for this purpose.32 The Lanl2DZ basis set was used for H, C, N, O, Ru, and Ta,33−35 whereas the 6-31G(d) basis set was employed for P and Cl.36,37 All calculations were carried out using the Gaussian09 program.38 2.2.1. Ta16O40(NH3)n and Ru-Complex Connected Models. Both NH3 adsorption and the deposition of the Ru-complex on the surface of the bulk Ta2O5 induce local structural changes without periodicity. Thus, in this study, a Ta2O5 cluster cut from the bulk system was employed as a locally modified surface model. Cluster models are often used to investigate nanoparticles or bulk surfaces in association with standard quantum chemical methods. However, this approach requires the user to select the size and shape of the model and to determine the termination of the dangling bonds, all of which are largely arbitrary yet have a significant effect on the results of the calculations.39−47 Nakatsuji et al. proposed the following three principles for the construction of clusters: (i) neutrality, (ii) stoichiometry, and (iii) coordination.48,49 In the work presented herein, we prepared a Ta2O5 cluster having the formula Ta16O40 that satisfied each of these three conditions. That is, this cluster was charge neutral, had a stoichiometric Ta:O ratio of 2:5 equal to that of the bulk system, and had no dangling bonds, since the coordination numbers of the tantalum atoms were five or more, while those of the oxygen atoms were two or three. The molecular structures of Ta16O40 and Ta16O40(NH3)x (x = 4, 8, 12, or 16) were initially optimized using DFT, and the orbitals of these compounds were analyzed to determine the effects of NH3 adsorption on the band structure of Ta2O5. In addition, the lowest 10 singlet excited states were calculated using TDDFT to estimate the absorption energy values, using the calculated S0−S1 transition energies as the absorption band edges. Subsequently, models of the hybrid Ru/Ta16O40 and Ru/ Ta16O40(NH3)12 photocatalysts were employed to assess the electronic structural changes induced by NH3 adsorption. Ru/ Ta16O40 was prepared by connecting Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2 to Ta16O40 via two anchoring groups. This structure was suggested by experimental work on Ru-complexes connected to TiO2 surface via the same anchoring groups50 and also assumed in a previous computational study regarding this photocatalyst.51 The dissociations of protons from PO3H2 groups were also assumed, and the dissociated protons were attached to the oxygen atoms of the clusters to maintain the charge neutrality of the models.52−61 Following this, Ru/ Ta16O40(NH3)12 was constructed by modeling the adsorption of 12 NH3 molecules on those Ta atoms of Ru/Ta16O40 not connected to the Ru-complex. The calculation results for Ta16O40(NH3)x indicated that the electronic structural changes become more significant with increasing x values (Section 3.2.1). Therefore, Ru/Ta16O40(NH3)12 was considered suitable for estimations of the effects of NH3 adsorption. The molecular structures of the hybrids were optimized using DFT, and the

molecular orbitals were analyzed. During the photocatalytic reaction, the photocatalyst is excited by light irradiation and obtains electrons from sacrificial reagents.10 For this reason, [Ru/Ta16O40]− and [Ru/Ta16O40(NH3)12]− were modeled so as to visualize and estimate the distributions of the excess electrons that are used for the CO2 reduction reaction. 2.2.2. Ta16O40−y(NH)y. Variations in the surface structural modification may have different effects on the electronic structure. For this reason, the effects of the substitution of NH groups for surface oxygen atoms were estimated as an example of a structural modification other than NH3 adsorption. NH groups are commonly found in N-doped TiO220,62 and originate from reactions between adsorbed NH3 molecules and surface oxygen atoms.63,64 The molecular geometries of various Ta16O40−y(NH)y species (y = 2, 4, 6, 8, or 10) were optimized using DFT, and the excited states were calculated using TDDFT.

3. RESULTS AND DISCUSSION 3.1. Experimental Results. The pH of pure water was increased from 6.8 to 8.1 following the dispersion of the N− Ta2O5 powder, suggesting that NH3 and/or alkaline groups adsorbed on the surface of the powder were partially dissolved in the water. The N 1s XPS spectra of the N−Ta2O5 powder and the a-N−Ta2O5 are presented in Figure 1. The peak

Figure 1. N 1s XPS spectra of N−Ta2O5 powder (red) and a-N− Ta2O5 powder (blue). A difference spectrum corresponding to N 1s core level is shown as an inset.

intensities at 396 eV generated by the N−Ta2O5 and a-N− Ta2O5 were equivalent, indicating that doped N atoms substituted at O sites were maintained after the acid treatment. In contrast, the intensity in the range from 397 to 404 eV, a very broad peak around 401 eV assignable to either surface N species or N species at interstitial sites in the bulk,12,65,66 declined in the a-N−Ta2O5. N 1s peaks of adsorbed NH3 molecules are expected to be observed in the vicinity of this energy value. Hence, the reduced peak intensity observed around 401 eV likely resulted from a decrease in the concentration of NH3 species on the surface of the a-N− Ta2O5, which is consistent with the change in pH described above. Figure 2 presents the results of photocatalytic CO2 C


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bands can be assigned to monocarbonyl Ru complexes, which are minor species but inevitably produced during the syntheses of these photocatalysts. Although the detailed molecular structures of the monocarbonyl species were presently unclear, their absorption bands are expected to be lower than those of dicarbonyl species.67 The difference in wavenumber between symmetric and antisymmetric vibrational modes is about 50 cm−1.67 Thus, the absorption bands having the highest and the second highest wavenumbers should be assigned to Ru(bpy)(CO)2Cl2 moieties. The peaks derived from Ru(bpy)(CO)2Cl2 moieties were at 2076.8 and 2023.7 cm−1 in the Ru/Ta2O5, whereas those were at 2074.5 and 2022.4 cm−1 in the Ru/N− Ta2O5. The results indicate that the electron density on the Rucomplex moieties in Ru/N−Ta2O5 was different from those in Ru/Ta2O5, in which Ru center was more negatively charged in the Ru/N−Ta2O5. 3.2. Computational Results. 3.2.1. Ta 16 O 40 and Ta16O40(NH3)x. The optimized Ta16O40 structure was found to have Cs molecular symmetry with no dissociation of the Ta− O bond (see Table S1 for the Cartesian coordinates). The Ta− O bond lengths were within the range of 1.899−2.139 Å with an average value of 1.988 Å and thus in good agreement with reported experimental results.68 The calculation results also suggested that Ta16O40 has electronic properties similar to those of bulk Ta2O5. Figure 4a shows the highest occupied

Figure 2. Turnover numbers (TON) of HCOOH (orange), CO (green), and H2 (gray) converted from CO2 as a function of irradiation time during the photocatalytic reaction. The catalysts used were (a) Ru/N−Ta2O5 (0.69 wt %) and (b) Ru/a-N−Ta2O5 (0.82 wt %). TONs were calculated on the basis of amount of adsorbed Rucomplex. Standard deviation error bars calculated are shown for the plots at 8 and 16 h, which suggest that experimental error was less than 5% even after 16 h reaction of 3 runs.

reactions under visible light irradiation and shows that the photocatalytic activity of the Ru/N−Ta2O5 for HCOOH and CO generations was higher than that of the Ru/a-N−Ta2O5. In contrast, the amount of H2 production over Ru/a-N−Ta2O5 was higher than that of Ru/N−Ta2O5. CO2 reduction products were not observed during the reaction using Ru/Ta2O5 because this material does not have an absorption band in the visible light region (Figure S1). The photocatalytic CO2 reactions using the samples synthesized in another batch exhibit similar results (Figure S2), in which the photocatalytic activity of Ru/ N−Ta2O5 for HCOOH and CO generations was higher than that for Ru/a-N−Ta2O5. In this case, the turnover numbers (TONs) for HCOOH and CO generations were two or three times higher than those shown in Figure 2, which is originated from difference in N-doping conditions of Ta2O5 for each batch synthesized. However, the tendency for the high CO2 reduction rate over Ru/N−Ta2O5 was maintained for both Figure 2 and Figure S2. The IR spectra of Ru/Ta2O5 and Ru/N−Ta2O5 in the range from 1950 to 2150 cm−1 are presented in Figure 3, each of which exhibits three peaks. The absorption bands derived from the CO-stretching vibrational modes of Ru(bpy)(CO)2Cl2 moieties are supposed to be observed in this wavenumber region. Ru(bpy)(CO)2Cl2 has two CO-stretching vibrational modes: symmetric and antisymmetric. The additional

Figure 4. Frontier orbitals of (a) Ta16O40, (b) Ta16O40(NH3)4, (c) Ta16O40(NH3)8, (d) Ta16O40(NH3)12, and (e) Ta16O40(NH3)16.

molecular orbital (HOMO) and LUMO of Ta16O40, which are derived from the O 2p and Ta 5d orbitals, respectively. The HOMO, LUMO, and HOMO−LUMO gaps (ΔH−L) calculated using the B3LYP functional were −8.16, − 4.02, and 4.14 eV (Table 1), respectively, which are in good agreement with the respective experimental values: −7.9, −4.0, and 3.87 eV.12,14 The HOMO and LUMO of Ta16O40(NH3)x were also derived from O 2p and Ta 5d orbitals, respectively (Figure 4), and their energy levels were shifted upward with increasing x (Table 1). Since the HOMO and LUMO levels simultaneously shifted upward, ΔH−L was not significantly dependent on x. Similarly, the S0−S1 transition energies determined using the TDDFT calculations varied only within a narrow range with changes in x. The S0−S1 transition energy of Ta16O40 (3.46 eV) calculated using the TD-B3LYP functional was lower than the experimentally measured band edge of bulk Ta2O5 (3.9 eV). This underestimation can be attributed to the charge transfer character of the S0−S1 transition, which corresponds to O 2p → Ta 5d excitation. The transition energy obtained using the CAM-B3LYP functional (4.02 eV) was thus in better agreement with the experimental value. The transition energies determined using the TD-CAM-B3LYP approach also exhibited little dependence on x.

Figure 3. IR spectra of Ru/Ta2O5 (black) and Ru/N−Ta2O5 (red). The absorption bands assigned to the CO-stretching vibrational modes of Ru(bpy)(CO)2Cl2 are indicated by arrows (see also text). D


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Table 1. Calculated Molecular Orbital Energy Levels, HOMO−LUMO Gap (ΔH−L), and S0−S1 Transition Energies of Ta16O40 and Ta16O40(NH3)x (x = 4, 8, 12, and 16) in eV S0−S1 transition energy

natural charges

cluster

HOMO

LUMO

ΔH−L

B3LYP

CAM-B3LYP

Ta16O40

(NH3)x

(NH3)x/x

Ta16O40 Ta16O40(NH3)4 Ta16O40(NH3)8 Ta16O40(NH3)12 Ta16O40(NH3)16

−8.16 −7.11 −6.27 −5.39 −4.79

−4.02 −2.96 −1.89 −1.06 −0.43

4.14 4.15 4.38 4.33 4.36

3.46 3.41 3.63 3.64 3.67

4.02 4.09 4.19 4.32 4.35

0.000 −1.133 −2.147 −3.069 −3.966

0.000 +1.133 +2.147 +3.069 +3.966

0.000 +0.283 +0.268 +0.256 +0.248

Table 2. B3LYP Calculated Energy Levels of the tHOMO, HOMO, LUMO, and rLUMO of Ru/Ta16O40 and Ru/ Ta16O40(NH3)12 in eV and the Calculated HOMO and LUMO Levels of Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2

3.2.2. Ru/Ta16O40 and Ru/Ta16O40(NH3)12. Herein, for the sake of convenience, the highest occupied orbital distributed on the Ta16O40 moiety and the lowest unoccupied orbital distributed on the Ru-complex moiety in each model are termed tHOMO and rLUMO, respectively. Figure 5 presents

model Ru/Ta16O40

Ru/Ta16O40(NH3)12

Ru{4,4′(PO3H2)2bpy} (CO)2Cl2

molecular orbitala

distribution

rLUMO (LUMO+10) LUMO HOMO tHOMO (HOMO−9) rLUMO (LUMO+1) LUMO HOMO tHOMO (HOMO−4) LUMO

Ru-complex Ta16O40 Ru-complex Ta16O40 Ru-complex Ta16O40 Ru-complex Ta16O40

energy level −3.35 −4.32 −6.16 −8.33 −1.96 −1.99 −5.17 −6.17 −3.42 −6.18

HOMO a

The highest occupied orbital distributed on the Ta16O40 moiety and the lowest unoccupied orbital distributed on the Ru-complex moiety in each model are termed tHOMO and rLUMO, respectively.

Figure 5. B3LYP calculated molecular orbitals of (a) Ru/Ta16O40 and (b) Ru/Ta16O40(NH3)12. The highest orbital of the occupied orbitals distributed on the Ta16O40 moiety and the lowest orbital of the unoccupied orbitals distributed on the Ru-complex moiety are denoted by tHOMO and rLUMO, respectively.

tHOMO, HOMO, LUMO, and rLUMO drawings for the Ru/ Ta16O40 and Ru/Ta16O40(NH3)12. The molecular orbitals spanning the range from tHOMO to rLUMO are collected in Figures S3 and S4. In each model, the orbital energy levels were in the order of tHOMO < HOMO < LUMO < rLUMO, and the LUMO was primarily distributed on the Ta16O40. The predicted orderings of orbital levels were the same in both models; however, their alignments were quite different (Table 2). In the case of Ru/Ta16O40, the rLUMO was the LUMO+10, and the gap between the LUMO and rLUMO was 0.97 eV. In contrast, in Ru/Ta16O40(NH3)12, the rLUMO was the LUMO +1 and the LUMO−rLUMO gap was considerably reduced, to only 0.03 eV. This change in electronic structure reflected the distribution of excess electrons in the negatively charged hybrids. The HOMO of α space (α-HOMO) of [Ru/Ta16O40]− was localized on the Ta16O40, as was the LUMO of Ru/ Ta16O40, whereas the α-HOMO of [Ru/Ta16O40(NH3)12]− was spread over both the Ta16O40 and Ru-complex (Figure 6). It is therefore evident that NH3 adsorption greatly affected the distribution of excess electrons. The calculated CO-stretching vibrational frequencies and natural charges are summarized in Table 3, together with the natural charges of the Ru(bpy)(CO)2Cl2 moieties (except for

Figure 6. Calculated α-HOMO of (a) [Ru/Ta16O40]− and (b) [Ru/ Ta16O40(NH3)12]−.

Table 3. Calculated Vibrational Frequencies (cm−1) of CO Stretching Modes and Natural Charges of Ru(bpy)(CO)2Cl2 Moieties ν(CO) molecule Ru/Ta16O40 Ru/Ta16O40(NH3)12 Ru{4,4′-(PO3H2)2bpy} (CO)2Cl2 [Ru{4,4′-(PO3H2)2bpy} (CO)2Cl2]+ [Ru{4,4′-(PO3H2)2bpy} (CO)2Cl2]−

E

natural charge of

symmetric

antisymmetric

Ru(bpy)(CO)2Cl2 moiety

2014.7 2005.4 2013.4

1962.3 1949.6 1961.3

−0.640 −0.725 −0.662

2081.3

2035.1

+0.257

1978.6

1919.5

−1.503


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3.3. Discussion. The XPS data indicate that fewer NH3 species were present on the surface of the a-N−Ta2O5 compared to the N−Ta2O5 (Figure 1). In addition, the photocatalytic activity of the Ru/N−Ta2O5 was higher than that of the Ru/a-N−Ta2O5. These experimental results suggest that NH3 molecules adsorbed on the surface of the material play an important role in promoting photocatalytic activity (Figure 2). The calculation results also generally support this hypothesis. The calculated energy levels for the frontier orbitals of Ta16O40(NH3)x suggest an upward shift of the CBM of the bulk Ta2O5 due to NH3 adsorption (Table 1), which can be understood by considering the natural charges that are also presented in Table 1. The Ta16O40 moiety became more negatively charged with increasing x, reflecting increasing Ta16O40δ−−(NH3)xδ+ polarization. The upward shifts of the orbital levels of Ta16O40(NH3)x can thus be attributed to the partial negative charge on the Ta16O40 moiety, resulting from the significant electron-accepting character of this group. The HOMO and LUMO levels of NH3 molecules calculated using the same procedure as applied to the Ta16O40(NH3)x were found to be at −6.20 and 2.67 eV, respectively, and therefore higher than those of the original Ta16O40. These results explain why the Ta16O40 more readily accepts electrons compared to (NH3)x, resulting in the Ta16O40δ−−(NH3)xδ+ polarization. It is also likely that NH3 adsorption on the bulk Ta2O5 induces this Ta2O5δ−−(NH3)xδ+ polarization. The Ta2O5δ−−(NH3)xδ+ surface dipole, which is normal to the surface, is able to shift the CBM upward, similar to a combination of doped nitrogen atoms and oxygen defects.16 The predicted effect of NH3 adsorption can be also consistent with the results in which the amount of H2 production over Ru/a-N−Ta2O5 was higher than that over Ru/N−Ta2O5, in contrast to decreased HCOOH and CO productions. The removal of the adsorbed NH3 molecules (aN−Ta2O5) leads to lowering (positive shift) of the CBM level and suppression of the excited electron transfers from surface of N−Ta2O5 to the Ru-complex moieties. It is known that N− Ta2O5 itself has photocatalytic activity for H2 generation.13 Thus, the excited electrons can be used for H2 generation on the a-N−Ta2O5. The LUMO−rLUMO gap of the Ru/Ta 16 O 40 was considerably reduced in the Ru/Ta16O40(NH3)x hybrid (Table 2), potentially promoting electron transfer between these two orbitals as a result of a decrease in the associated ΔG value.69 Thus, the results suggest that NH3 species on the semiconductor surface can promote electron transfer from the semiconductor to the Ru-complex. The distributions of αHOMOs in the [Ru/Ta16O40]− and [Ru/Ta16O40(NH3)12]−, as visualized in Figure 6, also support the promotion of electron transfer to the Ru-complex moiety through NH3 adsorption.

the PO3 or PO3H2 groups). The wavenumbers were in the order of Ru/Ta16O40 > Ru/Ta16O40(NH3)12 and were also found to appear at lower wavenumbers as the result of the adsorption of NH3 molecules on the Ta16O40 surface. The wavenumbers of the Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2 correlate with the molecular charge and are in the order of [Ru{4,4′(PO3H2)2bpy}(CO)2Cl2]+ > Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2 > [Ru{4,4′-(PO3H2)2bpy}(CO)2Cl2]−. These data demonstrate that NH3 adsorption on the Ta16O40 increases the electron density on the Ru-complex within the hybrid. The calculation results also suggested that the symmetric CO-stretching vibrational modes have higher frequencies than the antisymmetric modes. The difference in frequency between these modes predicted was about 50 cm−1, which is close to the reported value67 and also supports the assignment of the IR peaks with the highest and the second highest frequencies to Ru(bpy)(CO)2Cl2 species (Figure 3). 3.2.3. Ta16O40−y(NH)y. The Ta16O40−y(NH)y frontier orbitals are illustrated in Figure 7. In contrast to Ta16O40 and

Figure 7. Frontier orbitals of (a) Ta16O38(NH)2, (b) Ta16O36(NH)4, (c) Ta16O34(NH)6, (d) Ta16O32(NH)8, and (e) Ta16O30(NH)10.

Ta16O40(NH3)x, the HOMOs of Ta16O40−y(NH)y are derived from N 2p and O 2p orbitals, while the LUMOs originate from Ta 5d orbitals. The calculation results are summarized in Table 4 and indicate that the substitution of NH groups for oxygen atoms results in substantially different effects on the electronic structure of Ta2O5 from those of NH3 adsorption. The HOMO levels are shifted upward significantly with increasing y, whereas the LUMO levels move upward only slightly, such that ΔH−L was significantly decreased with increases in y. Similarly, the S0−S1 transition energies determined by TDDFT calculations were decreased and generally lower than those of the Ta16O40 and Ta16O40(NH3)x. The CAM-B3LYP values were higher than the B3LYP values, as seen in the Ta16O40 and Ta16O40(NH3)x. The S0−S1 transition energies calculated using the CAMB3LYP functional were within the range of 2.72−2.83 eV and thus close to the absorption band edge of N−Ta2O5 (2.5 eV).12

Table 4. Calculated Molecular Orbital Energy Levels, HOMO−LUMO Gap (ΔH−L), S0−S1 Transition Energies, and Natural Charges of Ta16O40 and Ta16O40−y(NH)y (y = 2, 4, 6, 8, and 10) in eV S0−S1 transition energy

averaged natural charge of each element

cluster

HOMO

LUMO

ΔH−L

B3LYP

CAM-B3LYP

Ta

O

N

H

Ta16O40 Ta16O38(NH)2 Ta16O36(NH)4 Ta16O34(NH)6 Ta16O32(NH)8 Ta16O30(NH)10

−8.16 −7.38 −7.15 −7.04 −6.84 −6.62

−4.02 −4.16 −3.94 −3.69 −3.78 −3.71

4.14 3.22 3.21 3.35 3.06 2.90

3.46 2.32 2.33 2.44 2.37 2.35

4.02 2.72 2.73 2.80 2.80 2.83

+2.103 +2.080 +2.057 +2.028 +2.006 +1.973

−0.841 −0.841 −0.838 −0.833 −0.833 −0.827

−1.101 −1.125 −1.119 −1.117 −1.116

+0.446 +0.443 +0.435 +0.436 +0.439

F


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The Journal of Physical Chemistry C The α-HOMO of [Ru/Ta16O40(NH3)12]− is spread over both the Ta16O40 and the Ru-complex because the LUMO and rLUMO are nearly degenerate in Ru/Ta16O40(NH3)12. This predicted effect of NH3 adsorption is consistent with the experimental results in which the removal of NH3 leads to decreased photocatalytic activity (Figure 2). However, it should be noted that adsorbed NH3 is not the sole cause of the upward shift of the CBM, based on the photocatalytic activity of the Ru/a-N−Ta2O5. Other structural changes on the Ta2O5 surface, such as a combination of doped nitrogen atoms and oxygen defects,16 could contribute to both the shift of the CBM and the photocatalytic activity. The mechanism suggested by the results presented above implies that the band alignment in the Ta2O5 semiconductor/ Ru-complex hybrid system may be different from those of the individual components. The LUMO levels of the individual components were in the order of Ta16O40(NH3)x > Ru{4,4′(PO3H2)2bpy}(CO)2Cl2 > Ta16O40 (Tables 1 and 2). This ordering is in agreement with the experimentally observed results. That is, the CBM or LUMO are in the order of N− Ta2O5 > Ru-complex > Ta2O5.10 In contrast, in the case of the Ru/Ta16O40(NH3)12 hybrid, the rLUMO is also shifted upward, resulting in the order rLUMO > LUMO, even though these two orbitals are nearly degenerate (Table 2). These results imply that the electronic properties of both the individual components and the combined assembly must be carefully examined to fully understand the catalytic reaction over the metal complex/semiconductor hybrid system. The COstretching vibrational frequencies also suggest a correlation between the electronic properties of the semiconductor and those of the attached Ru-complex. The experimentally determined vibrational frequencies generated by the Ru/N− Ta2O5 were at lower wavelengths than those of the Ru/Ta2O5 (Figure 2). Because Ta16O40(NH3)x donates electrons more readily than Ta16O40 (as suggested by the orbital energies in Table 1), the Ru-complex in the Ru/Ta16O40(NH3)12 hybrid is more negatively charged than that in the Ru/Ta16O40(NH3)12 hybrid (Table 3). The calculation results for Ta16O40(NH3)x in Table 1 suggest that NH3 adsorption barely affects the red shift of the absorption band edge of the bulk Ta2O5. In contrast, NH substitution could be responsible for the red shift of the adsorption band edge of the Ta2O5, while making only a small contribution to the upward shift of the CBM (Table 4). The shifts of the orbital levels in Ta16O40−y(NH)y can be understood by considering the natural charges. Since N atoms are more negatively charged than O atoms (because of Nδ−−Hδ+ polarization), the HOMO levels are shifted upward with increasing y values. The Ta 5d orbital levels are also but only slightly shifted upward because of the negative charge on the N atoms. An upward shift of the CBM and a red shift of the absorption band edge have both been experimentally observed following the N-doping of bulk Ta2O5.10,12 A combination of surface NH3 adsorption and NH substitution could simultaneously induce these changes in the band alignment. Thus, it is clear that varying molecular structural modifications will have different effects on the electronic properties of the photocatalyst. In future, the effects of structural modifications that can potentially be induced by N-doping should be individually investigated so as to obtain a more detailed understanding and better control of the electronic properties of photocatalysts.

4. CONCLUSION This work investigated the mechanism associated with the upward shift of the CBM of bulk Ta2O5 and enhancement of the visible-light-induced CO2 reduction reaction in conjunction with a Ru-complex subsequent to N-doping. The study especially concentrated on the effect of NH3 adsorption, and NH3 adsorption on the N−Ta2O5 surface was experimentally confirmed. The photocatalytic activity of the Ru/N−Ta2O5 hybrid catalyst during the CO2 reduction reaction was found to be lower subsequent to the removal of adsorbed NH3, indicating that adsorbed NH3 contributes to the activity. Quantum chemical calculations suggest that the energy levels of the unoccupied orbitals of the Ta2O5 moiety are shifted upward following NH3 adsorption, and the energy gap between the Ta16O40 LUMO and the unoccupied orbitals on the Rucomplex is significantly reduced in Ru/Ta16O40(NH3)12. The αHOMO of [Ru/Ta16O40]− is evidently localized on the Ta16O40 moiety, while that of [Ru/Ta16O40(NH3)12]− is spread over both the Ta16O40 and the Ru-complex. These results suggest that electron transfer from the Ta2O5 to the Ru-complex is promoted by NH3 adsorption on the surface. The calculations performed during this work also appear to demonstrate that the substitution of NH groups for surface oxygen atoms contributes to the red shift of the absorption band edge of Ta2O5, while making only a minor contribution to the upward shift of the CBM.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09670. Results of photocatalytic reactions using Ru-complex/ Ta2O5 (Figure S1); results of photocatalytic reactions using another batch of Ru/N−Ta2O5 and Ru/a-N− Ta2O5 (Figure S2); Cartesian coordinates of Ta16O40 (Table S1); molecular orbitals of hybrid models (Figures S3 and S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Soichi Shirai: 0000-0001-6932-4845 Shunsuke Sato: 0000-0001-8178-7367 Tomiko M. Suzuki: 0000-0003-3763-4245 Ryosuke Jinnouchi: 0000-0002-0822-1161 Nobuko Ohba: 0000-0001-9779-6401 Ryoji Asahi: 0000-0002-2658-6260 Takeshi Morikawa: 0000-0002-4985-0925 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Ms. Naoko Takahashi at Toyota Central R&D Labs., Inc. for her technical support on the XPS measurements. This work was partially supported by the Advanced Catalytic Transformation Program for Carbon Utilization (ACT-C) of the Japan Science and Technology Agency (JST). G


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