Efficient photocatalytic water splitting using Al-doped SrTiO3 coloaded

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Efficient photocatalytic water splitting using Al-doped SrTiO coloaded with molybdenum oxide and rhodium-chromium oxide Tzu Hsuan Chiang, Hao Lyu, Takashi Hisatomi, Yosuke Goto, Tsuyoshi Takata, Masao Katayama, Tsutomu Minegishi, and Kazunari Domen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Catalysis

Efficient photocatalytic water splitting using Al-doped SrTiO3 coloaded with molybdenum oxide and rhodium-chromium oxide

Tzu Hsuan Chiang,1,2 Hao Lyu,1 Takashi Hisatomi,1 Yosuke Goto,1,† Tsuyoshi Takata,1 Masao Katayama,1 Tsutomu Minegishi,1 Kazunari Domen1,* 1

Department of Chemical System Engineering, The University of Tokyo, 7-3-1

Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2

Department of Energy Engineering, National United University, Lienda, Nan-Shi Li,

Miaoli 36006, Taiwan †

Current Affiliation: Department of Physics, Tokyo Metropolitan University, 1-1,

Minami-osawa, Hachioji 192-0397, Japan *Corresponding author: Kazunori Domen ([email protected])

Abstract Al-doped SrTiO3 loaded with a rhodium-chromium mixed oxide (RhCrOx/STO:Al) efficiently promotes photocatalytic overall water splitting with an apparent quantum yield (AQY) of 56% under 365 nm ultraviolet (UV) light. Further increasing this AQY is of vital importance, because this value determines the maximum solar-to-hydrogen energy conversion efficiency that can be achieved. Herein, we demonstrate that the AQY during overall water splitting by RhCrOx/STO:Al is improved by 20% (to 69% at 365 nm) by coloading molybdenum oxide (MoOy, 0.03

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wt% as Mo) followed by calcination. Reductively photodeposited MoOy modifies the chemical state of the RhCrOx cocatalyst and likely promotes photocatalytic H2 evolution, whereas MoOy loaded onto STO:Al catalyzes neither photocatalytic H2 nor O2 evolution. The present study indicates a means of further enhancing the water splitting activity of highly efficient cocatalyst/photocatalyst composites by loading small amounts of promoters in a facile manner. Keywords: Cocatalyst, Coloading, Molybdenum oxide, Quantum yield, Water splitting, Hydrogen evolution.

1.

Introduction Strontium titanate (SrTiO3) is a semiconductor with a band gap energy of 3.2 eV,

and has been reported to function as a photoanode for the oxidation of water to generate oxygen at a zero applied potential [1]. This material has been applied as a photocatalyst to split water into H2 and O2 under UV light irradiation since the 1980s [2-4] and as a host material to prepare doped oxides with visible light activity in the 21st century [5-7]. Early studies showed that doping with low valence cations, loading with hydrogen evolution cocatalysts, and suppressing backward reactions are all effective at enhancing the activity of SrTiO3 photocatalysts during overall water splitting. As an example, the doping of Al3+ and Ga3+ into the Ti4+ sites of this material as well as the doping of Na+ into the Sr2+ sites boosts its activity under UV


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irradiation by more than an order of magnitude, presumably because of a reduction in the concentration of Ti3+ species [8,9]. In contrast, doping with higher valence cations suppresses the water splitting activity of this catalyst. Furthermore, the loading of different cocatalysts on SrTiO3 has also been investigated, including Ni/NiO core/shell particles [2-4], Ni@NiOx [10,11], and rhodium-chromium mixed oxides (RhCrOx) [8,9,12]. Meanwhile, Takata et al. [13] demonstrated that thin metal oxide layers photodeposited from peroxide complexes on Sc-doped SrTiO3 worked as permselective layers blocking molecular oxygen and effectively suppressed backward reactions involving it on a noble metal oxide cocatalyst loaded, thereby allowing the photocatalyst to accomplish overall water splitting. In our previous study, an aluminum-doped SrTiO3 (STO:Al) photocatalyst, prepared by a flux method and loaded with a RhCrOx cocatalyst (0.1 wt% each for Rh and Cr with respect to SrTiO3:Al), exhibited overall water splitting with an apparent quantum yield (AQY) of 56% in response to irradiation at 365 nm [12]. This AQY value indicates that more than half of the photoexcited carriers contributed to the water splitting reaction and is outstanding for operation in the near-UV region [14,15]. Nevertheless, a Ga2O3 photocatalyst has been shown to generate an even higher AQY of 71%, albeit in the middle-UV region of the spectrum (254 nm) [16]. It is therefore likely that the AQY of the STO:Al photocatalyst can be increased to a similar value.


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Improvements in the AQY at longer wavelengths will promote the development and activation of visible-light-driven photocatalysts that currently suffer from low AQY values (in the vicinity of 5%) [17-20]. Coloading of cocatalysts has been shown to enhance both the activity and stability of certain photocatalysts [21]. Mu et al. [22] studied coloading of Pt and Co3O4 onto 6-facet and 18-facet SrTiO3 photocatalysts as hydrogen and oxygen evolution cocatalysts, respectively. It was found that coloading materials such as these with different functionalities in a site-selective manner improved the water splitting activity of the SrTiO3, although the AQY of this material remained moderate (0.8% at 365 nm) because of the low activity of the SrTiO3 used. As a result, it is presently of significant interest to determine whether or not water splitting on the highly-active RhCrOx/STO:Al photocatalysts can be further enhanced via the loading of additional cocatalysts. Molybdenum oxides have recently been demonstrated to function as electrocatalysts for hydrogen evolution [23, 24]. Some molybdenum oxides have also been applied as cocatalysts for photocatalytic water splitting. Busser et al. [25] studied CuOz/CrOy/MoOx-modified Ga2O3 photocatalysts meant for overall water splitting and reported that these materials showed excellent activity and stability. The application of a MoOx coating on a Pt/SrTiO3 photocatalyst was shown to suppress


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the backward reactions on Pt in a similar manner to a Cr2O3 coating [26]. Therefore, the loading of molybdenum oxides is also expected to enhance the water splitting activity of various photocatalysts. In the present work, the coloading of MoOy cocatalysts onto RhCrOx/STO:Al photocatalysts was studied in an attempt to improve the activity of the original materials. It was found that coloading MoOy by photodeposition and calcination did indeed enhance the water splitting activity of these photocatalysts, such that the AQY was improved by 20%, to 69%, at 365 nm. The characteristics and photocatalytic water splitting activities of these MoOy and RhCrOx-coloaded STO:Al photocatalysts are discussed herein.

2.

Experimental

2.1

Preparation

of

Al-doped

SrTiO3

powder

loaded

with

RhCrOx

(RhCrOx/STO:Al) STO:Al was prepared using a flux method previously reported by our group [9,12]. SrTiO3 (Wako Pure Chemicals Industries, Ltd, 99.9%), Al2O3 (Sigma-Aldrich Co, LLC., nanopowder) and SrCl2 (Kanto Chemicals Co., Inc., 98.0%, anhydrous) were mixed at a molar ratio of 1:0.02:10 using an agate mortar. The mixture was heated in an alumina crucible at 1423 K for 10 h and then cooled to room temperature. The product was put into a large volume of distilled water, stirred and retrieved by


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filtration to remove impurities associated with the SrCl2. This process was repeated three times. Finally, the resulting STO:Al powder was dried at 313 K in an oven. The STO:Al powder was loaded with RhCrOx (0.1 wt% Rh and 0.1 wt% Cr) by the impregnation method [12]. Na3RhCl6·nH2O (Mitsuwa Chemistry Co., Ltd., 17.8 wt% as Rh) and Cr(NO3)3·9H2O (Kanto Chemicals Co., Inc., 98.0–103.0%) were used as Rh and Cr sources, respectively. The STO:Al powder was dispersed in an aqueous solution containing the necessary concentrations of Na3RhCl6 and Cr(NO3)3, after which the solution was evaporated to dryness while being stirred, and the resulting solid was calcined in air at 623 K for 1 h.

2.2 Preparation of MoOy-loaded RhCrOx/STO:Al (MoOy/RhCrOx/STO:Al) MoOy was loaded onto the RhCrOx/STO:Al by photodeposition. The RhCrOx/STO:Al photocatalyst (0.20 g) was dispersed in distilled water (100 mL) containing varying amounts of Na2MoO4·2H2O (Wako Pure Chemical Industries, Ltd) as the Mo precursor. The amount of Na2MoO4 added was nominally 0–10 wt% with respect to the RhCrOx/STO:Al powder. The actual loading amount of Mo species was determined by inductively-coupled plasma optical emission spectroscopy (ICP-OES; ICPS-8100, Shimadzu). Photodeposition was carried out using a closed gas-circulation system. The


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suspension was contained in a top-irradiation reactor and evacuated to completely remove air, after which it was irradiated using a 300 W xenon lamp (300 nm < λ < 500 nm), equipped with a dichroic mirror, through a Pyrex window for 4 h. The powder was subsequently collected and dried in an oven at 313 K, following which the MoOy/RhCrOx/STO:Al was calcined at 573 K in air for 1 h. In some cases, the calcination temperature was varied within the range from 313 K (that is, no calcination) to 673 K.

2.3 Measurement of the photocatalytic activity The water splitting activity of each photocatalyst sample was determined under UV irradiation from a 300 W xenon lamp (300 nm < λ < 500 nm) using a closed gas circulation system, similarly to the procedure employed for the photodeposition of MoOy described above. A 0.10 g quantity of the photocatalyst was used and the reaction was performed in distilled water. The half reactions of the photocatalyst samples were also examined in aqueous methanol (10 vol% MeOH, 100 mL) and aqueous silver nitrate (10 mM AgNO3, 100 mL) as the sacrificial electron donor and acceptor, respectively. In the case of the oxygen evolution reaction, a neutral density filter (transmittance = 6%) was attached to the xenon lamp to slow the exhaustion of Ag+ ions in the solution via deposition on the photocatalyst. The temperature of the reactant solution was maintained at 288 K by a flow of cooling water during the


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reaction. The evolved gases were analyzed using a gas chromatograph (Shimadzu, GC-8A) equipped with a molecular sieve 5Å column and a thermal conductivity detector with Ar as the carrier gas. AQY values were determined under irradiation from a xenon lamp equipped with a 365 nm band-pass filter having a full width at half maximum of 10 nm. Power spectra were acquired with a spectroradiometer (LS-100, EKO Instruments, Japan) at various positions using a manual XY stage. The resulting photon fluxes were integrated over the irradiation area (61 cm2), with the irradiation of the photocatalyst estimated to be at 1.2×1017 photons s-1. The AQYs were calculated according to the following equation.

2.4 Characterization of photocatalysts Scanning transmission electron microscopy (STEM; JEM-2800, JEOL) in conjunction with energy-dispersive X-ray spectroscopy (EDS; X-MAX 100TLE SDD detector, Oxford Instruments) was conducted to investigate the distributions of the cocatalyst species. X-ray photoelectron spectroscopy (XPS) data were obtained with a JEOL


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JPS-9000 instrument, using the binding energy of the C 1s line at 284.6 eV as a reference. All spectra were acquired with a data interval of 0.1 eV, with excitation via Mg-Kα X-rays. ICP-OES was performed for a MoOy/RhCrOx/SrTiO3:Al photocatalyst liquidized by molten-salt decomposition. A portion (0.0105 g) of the photocatalyst was weighed, added with 1 g of anhydrous Na2CO3 and H3BO3 (3:1 mixture by weight, respectively), and mixed in a Pt crucible. This mixture was melted in a muffle furnace at 1273 K in air for a few hours. After cooled down, the molten salt was washed out in 4 mL of HCl (18.6 %w/w) mixed with 1 mL of H2O2 (30%w/w) and 10 mL of 5 %w/w aqueous tartaric acid solution, and adjusted at 100 mL by distilled water. For ICP-OES intensity calibration, a matrix solution was prepared by melting the blank Na2CO3 and H3BO3 (3:1 mixture by weight, 2 g in total) and washing in the double amount of the same solution, and adjusted at 100 mL. Portions (usually 25 mL) of this matrix solution were added with some known amounts of standard Mo and W solutions (1000 ppm ICP standard solutions, Wako, Japan) at the same time, and adjusted with distilled water at the double volume (usually 50 mL) of the initial portions. Note that the calibration solutions contain the same amount of molten salts as those in the sample solution. ICP spectral intensities for Mo and W were recorded altogether with these calibration solutions and the test solution, and the amounts of


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Mo and W in the sample were quantified.

3.

Results and Discussion

3.1 Photocatalytic activity of MoOy/RhCrOx/STO:Al Figure 1 shows the water splitting rates obtained from MoOy/RhCrOx/STO:Al samples added with different amounts of Na2MoO4 and calcined at 573 K under UV irradiation. The H2 and O2 evolution rates were increased with increases in the added amount of Mo from 0 to 5 wt%. However, excessive addition of Na2MoO4 (more than 8 wt% Mo) lowered the gas evolution rates, presumably because the MoOy blocked surface active sites on the RhCrOx/STO:Al photocatalyst and also shaded the photocatalyst The loading of an excess of a cocatalyst generally decreases the activity of photocatalysts [27]. Similar phenomena were observed by Du et al. [28] for a Zn0.5Cd0.5S photocatalyst loaded with a MoO2 cocatalyst and by Ma et al. [30] for a TiO2 photocatalyst loaded with a MoO3 cocatalyst. The actual loading amount of MoOy for the most active sample, which was irradiated in aqueous Na2MoO4 solution containing 5 wt% Mo, was determined to be 0.03 wt% as Mo by ICP-OES. The Mo species that was not photodeposited on the photocatalyst remained in the reaction solution. Note that sequential loading of MoOy (0.03 wt% as Mo) by an impregnation and calcination did not improve the water spiting activity of the RhCrOx/STO:Al photocatalyst as shown in Fig. S1.


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Figure 1. The photocatalytic activity of MoOy/RhCrOx/STO:Al under UV irradiation (300 nm < λ < 500 nm) as a function of the amount of Mo added. All samples were calcined at 573 K.

In

the

following

experiments,

MoOy/RhCrOx/STO:Al

photocatalysts

photodeposited with Mo 0.03 wt% were employed. Figure 2 presents the water splitting rates obtained from MoOy/RhCrOx/STO:Al photocatalysts calcined at different temperatures for 1 h. The gas evolution rates evidently increased with the calcination temperature up to 573 K. However, the sample calcined at 673 K showed almost the same activity as that of the pristine RhCrOx/STO:Al and thus was not improved by the coloading of MoOy. This result can most likely be attributed to the aggregation of the RhCrOx cocatalyst during calcination at 673 K [30]. Consequently, the MoOy/RhCrOx/STO:Al calcined at 573 K exhibited the highest water splitting activity, with an AQY of 69 ± 1.4% at 365 nm. This value is 20%


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higher than that reported in our previous study [12], and is outstanding among photocatalysts active at this wavelength. Figure 3 plots the evolutions of H2 and O2 during overall water splitting using the MoOy/RhCrOx/STO:Al photocatalyst calcined at 573 K in conjunction with intermittent evacuation. The photocatalyst can be seen to have maintained a high level of activity over the 16 h reaction.

2.0 Rate of gases evoluition / mmol h-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H2 H2 H2

1.6 H2

H2

1.2

0.8

O2

O2

O2

O2

O2

0.4

0.0

(a)

(b)

(c)

(d)

(e)

Samples Figure 2. Dependence of the photocatalytic activity of MoOy/RhCrOx/STO:Al under UV irradiation (300 nm < λ < 500 nm) on the calcination temperature: (a) RhCrOx/STO:Al,

(b)

MoOy/RhCrOx/STO:Al

before

calcination,

MoOy/RhCrOx/STO:Al after calcination at (c) 473, (d) 573, and (e) 673 K. The MoOy loading of each sample was 0.03 wt% as Mo.


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Figure 3. Cyclic water splitting using the MoOy/RhCrOx/STO:Al photocatalyst calcined at 573 K under UV irradiation (300 nm < λ < 500 nm) with intermittent evacuation.

3.2 Structure and chemical states of MoOy The distribution of the cocatalyst on the SrTiO3:Al photocatalyst was analyzed by STEM-EDS, as shown in Figure S2. It was not possible to observe MoOy directly by TEM because of the low loading amount and high degree of dispersion. However, Rh, Cr and Mo signals were detected evenly over the SrTiO3 particles represented by the Ti signal, suggesting the absence of significant aggregates of RhCrOx and MoOy. This point may also be supported by TEM and XPS analyses. It was shown in our previous work [12] that RhCrOx particles a few nanometers in size were well dispersed on


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STO:Al particles.

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The surface Rh/Ti and Cr/Ti ratios estimated by XPS were 0.22

and 0.12, respectively.

The RhCrOx cocatalyst had also been loaded on gallium zinc

oxynitride ((Ga0.88Zn0.12)(N0.88O0.12)) by impregnation and calcination [31], yielding Rh/Ga and Cr/Ga ratios of approximately 0.1 and 0.4, respectively.

Note that the

loading amount of the RhCrOx cocatalyst was 10 times greater (1.0 wt% Rh and 1.5 wt% Cr) for (Ga0.88Zn0.12)(N0.88O0.12) than for the present SrTiO3:Al photocatalyst while the specific surface area was also 2–3 times greater, respectively.

Thus, the Rh

and Cr surface atomic ratios at the comparable orders indicate comparable or superior dispersion of the RhCrOx cocatalyst on STO:Al to that on (Ga0.88Zn0.12)(N0.88O0.12). On the other hand, the Mo/Ti ratio on the MoOy/RhCrOx/SrTiO3:Al with Mo loading of 0.03 wt% was 0.05. This value is lower than the Rh/Ti and Cr/Ti ratios but is reasonable taking the difference in the loading amounts into account. In contrast, Mo was not detected by either EDS or XPS when assessing the RhCrO/SrTiO3:Al before photodeposition.

Thus,

Mo

species

were

undoubtedly

deposited

on

the

RhCrOx/STO:Al photocatalyst by the photodeposition process. Rh 3d, Cr 2p and Mo 3d XPS data for MoOy/RhCrOx/STO:Al (Mo 0.03 wt%) specimens calcined at different temperatures are shown in Fig. 4. The Rh 3d5/2 and Cr 2p3/2 peaks for RhCrOx/STO:Al could be seen at 309.1 and 576.8 eV, respectively. This observation was consistent with the characterization of RhCrOx loaded on


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gallium zinc oxynitride reported by Maeda et al. [30]. After 0.03 wt% Mo was loaded via the photodeposition of [MoO4]2-, doublet Mo 3d peaks were also observed at 231.2 and 234.6 eV. These binding energies can be attributed to Mo5+ species because they are lower than those for MoO3 and Na2MoO4 but higher than those for MoO2. This result suggests that the Mo6+ in MoO42- was reduced during photodeposition to form HzMoO3 [32] or partially-reduced MoO3 [33] on the surface of the RhCrOx/STO:Al. For simplicity, the deposited Mo species are designated as MoOy throughout this work. The Cr 2p3/2 peak of Cr3+ tended to be shifted toward higher binding energies, while the Rh 3d5/2 peak moved from 309.1 to 308.7 eV upon loading of the MoOy. Such a change was not observed when RhCrOx/SrTiO3:Al was calcined again in air at 573 K (Figure S3 in Supporting Information). Thus, this is presumably because of changes in the coordination environments in the RhCrOx cocatalyst nanoparticles. Changes in the binding energy values of cocatalysts following the photodeposition of a second component have also been reported by Busser et al. [25]. The MoOy was fully oxidized to MoO3 upon calcination at 573 K or above. Mo 3d5/2 and 3d3/2 peaks appear at 232.3 and 235.7 eV, and these values are similar to those for Mo6+ (232.5 ± 0.2 and 235.7 ± 0.2 eV for Mo 3d5/2 and 3d3/2, respectively) [34]. The Cr3+ 2p3/2 and Rh3+ 3d5/2 peaks resulting from the RhCrOx cocatalyst tended to shift

toward slightly higher binding energies simultaneously. In contrast, the Rh,


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Cr and Mo binding energies did not change significantly as the calcination temperature was raised from 573 to 673 K. In addition, 2p3/2 peaks attributable to hexavalent Cr [35, 36] were generated by all samples. The XPS results for MoOy/RhCrOx/STO:Al calcined at 573 K before and after a 16 h water splitting reaction are presented in Figure S4. The weakened signal at the higher binding energy side suggests that, following the reaction, a part of the Mo6+ in the calcined MoOy/RhCrOx/STO:Al was reduced to Mo5+ in compounds such as HzMoO3 [32, 37]. Presumably, MoOy was involved in reaction processes involving photoexcited electrons, with hydrogen atoms coordinated to oxygen atoms in MoO3 via a hydrogen bonding network and forms HzMoO3 [38], according to the equation MoO3 + ze‒ + zH+ f HzMoO3.

On the other hand, the chemical states of Cr and Rh

species in the RhCrOx were largely unchanged.


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Figure 4. (A) Mo 3d, (B) Cr 2p, and (C) Rh 3d XPS spectra for (a) RhCrOx/STO:Al, and (b) MoOy/RhCrOx/STO:Al photocatalysts before calcination, and calcined at (c) 473, (d), 573, and (e) 673 K. The Cr, Mo and Rh loadings were 0.10, 0.03 and 0.10 wt%, respectively.

3.3 Roles of MoOy To elucidate the roles of the MoOy species in enhancing the activity of the RhCrOx/STO:Al photocatalyst, H2 and O2 evolution reactions in aqueous solutions containing either a sacrificial electron donor or acceptor were examined [39,40]. Figure

5

shows

the

photocatalytic

H2

evolution

over

time

using

MoOy/RhCrOx/STO:Al photocatalysts in aqueous methanol solution. The unmodified STO:Al demonstrates negligible activity for the H2 evolution reaction because of a lack of hydrogen evolution sites. And loading solely MoOy onto the STO:Al photocatalyst does not enhance the H2 evolution appreciably. In contrast, the RhCrOx cocatalyst increases the H2 evolution activity of the STO:Al photocatalysts drastically because this material provides efficient hydrogen evolution sites [12,31]. In addition, the RhCrOx/STO:Al photocatalyst produced a higher H2 evolution rate after being modified with MoOy, and calcination of the MoOy/RhCrOx/STO:Al photocatalyst at 573 K further increased the H2 evolution rate. It is noteworthy that oxygen evolution was observed even in the presence of methanol, which is known to be a very efficient hole scavenger for oxide photocatalysts [39–42]. This result demonstrates the extremely high water splitting activity of the MoOy/RhCrOx/SrTiO3:Al.


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Figure 6 shows the time courses of O2 evolution on these photocatalysts in aqueous silver nitrate solutions under weak UV irradiation. The pristine STO:Al, RhCrOx/STO:Al and MoOy/RhCrOx/STO:Al photocatalysts exhibited similar initial oxygen evolution rates, and calcination of the MoOy/RhCrOx/STO:Al photocatalyst did not significantly affect the oxygen evolution rate. In addition, the oxygen evolution rate was not enhanced but rather decreased when the STO:Al photocatalyst was loaded with MoOy alone.

5

Amount of evolved gases / mmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

H2

3

2

1 O2

0 0

1

2

3

4

5

Reaction time / h Figure 5. Photocatalytic activity of STO:Al loaded with different cocatalysts in a 10 vol% aqueous methanol solution. Triangles ( ¸ ), crosses (✖), squares ( ¦ ), diamonds( Ì ), and circles ( Õ ) indicate data for STO:Al, MoOy/STO:Al, RhCrOx/STO:Al, MoOy/RhCrOx/STO:Al and MoOy/RhCrOx/STO:Al calcined at 573 K, respectively. Filled and unfilled symbols represent H2 and O2, respectively.


ACS Catalysis

0.10

Amount of evolved O2 / mmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.08

0.06

0.04

0.02

0.00 0

20

40 60 80 Reaction time / min

100

120

Figure 6. Photocatalytic activity of STO:Al samples loaded with different cocatalysts in a 10 mM aqueous AgNO3 solution. Squares ( § ), stars (ü ), triangles ( ¹ ), diamonds ( Í ), and circles ( Ñ ) indicate data for STO:Al, MoOy/STO:Al, RhCrOx/STO:Al, MoOy/RhCrOx/STO:Al and MoOy/RhCrOx/STO:Al calcined at 573 K, respectively.

The results shown in Figs. 5 and 6 suggest that the MoOy itself catalyzes neither the hydrogen nor oxygen evolution reactions, but rather promotes the hydrogen evolution process on the RhCrOx/STO:Al. It also appears that the enhancement is more effective when the Mo species are oxidized to MoO3 by calcination at 573 K. This observation is consistent with the XPS results suggesting that the MoOy species are subjected to a reductive environment during the photocatalytic water splitting


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reaction. However, it should be noted that the H2 and O2 evolution rates of the MoOy/RhCrOx/STO:Al photocatalysts in aqueous methanol and silver nitrate solutions tended to be lower than those in distilled water, which is the opposite to the behavior of NiO-loaded NaTaO3 doped with La, one of the most efficient photocatalysts active in the middle-UV region [43,44].

It might be considered that

the RhCrOx cocatalysts were poisoned and possibly corroded by products resulting from the reaction of methanol and therefore lost their selectivity toward hydrogen evolution compared to oxygen reduction. In fact, the hydrogen evolution rate in aqueous methanol decreased as the reaction progressed. Moreover, the photocatalyst showed lower water splitting activity when it was reused in distilled water after being employed for a reaction in aqueous methanol. However, the use of formic acid (1 vol%) as a sacrificial electron donor did not affect the durability of RhCrOx-loaded or MoOy/RhCrOx-loaded SrTiO3:Al significantly as shown in Figure S5. These results implies degradation of the cocatalyst during the hydrogen evolution reaction in the presence of methanol. Rh 3d XPS spectra of the RhCrOx/STO:Al before a reaction and after reactions in distilled water and in aqueous methanol solution are presented in Figure S6.

The

XPS spectra did not change significantly before and after a reaction in distilled water. However, tailing toward lower binding energy was noticed after a reaction in aqueous


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methanol solution.

This is indicative of reduction of Rh3+ species in RhCrOx as a

result of a higher population of electrons in the presence of methanol as an electron donor.

We believe that the photocatalytic hydrogen evolution on RhCrOx/STO:Al

was slower in aqueous methanol than in distilled water because the structure of the RhCrOx cocatalyst was changed. On the other hand, it is reasonable to expect that oxygen evolution in an aqueous silver nitrate solution would be suppressed as a result of the rapid consumption of Ag ions via deposition as Ag particles on surface of photocatalysts. Accordingly, the functionality of the MoOy species may not be determined solely by the use of sacrificial reagents. As noted, direct observation of MoOy species by TEM was not successful because of the low loading amount and extensive dispersion. However, the functionality of MoOy and RhCrOx cocatalysts on STO:Al may be determined in detail through transient absorption spectroscopy by assessing carrier dynamics under the working conditions.

4.

Conclusion This study demonstrated that the water splitting activity of highly-active

RhCrOx/STO:Al photocatalysts can be enhanced by loading a relatively low amount (0.03 wt%) of Mo species. Calcination of the MoOy/RhCrOx/STO:Al photocatalyst at


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573 K further improved their water splitting activity. The resulting AQY of 69% at 365 nm is the highest yet reported for overall water splitting in this wavelength region. Mo species were reductively deposited on the RhCrOx/STO:Al photocatalyst by photodeposition, oxidized to hexavalent MoO3 upon calcination at 573 K, and partly reduced during the overall water splitting reaction. It appears that these Mo species modified the chemical states of Rh and Cr species in the RhCrOx cocatalyst and thus promoted hydrogen evolution. However, the Mo species alone deposited on STO:Al did not catalyze either the hydrogen or oxygen evolution reactions. Conventional investigations using sacrificial reagents did not give decisive results because the addition of such reagents lowered the photocatalytic reaction rate unlike the cases for other photocatalysts. Thus, further studies concerning the structures and chemical states of the cocatalyst species are needed to elucidate the functions of MoOy species coloaded on RhCrOx/STO:Al photocatalysts. Nevertheless, the present work demonstrates an opportunity to upgrade the activity of photocatalysts during overall water splitting by coloading promoters using a simple process. This technique gives increased AQY values and should assist in the future development of photocatalysts for one-step excitation overall water spitting under visible light.


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Supporting Information. Reaction time courses, STEM-EDS images, XPS spectra.

Acknowledgments This work was financially supported by the Taiwan Ministry of Science and Technology from Taiwan (MST 105-2918-I-239-001), by Grants-in-Aid for Scientific Research (A) (No. 16H02417) and for Young Scientists (A) (No. 15H05494) from the Japan Society for the Promotion of Science (JSPS), and by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO). A part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors wish to thank Prof. Naoya Shibata and Ms. Mamiko Nakabayashi of The University of Tokyo for their assistance in acquiring STEM-EDS data and Dr. Taro Yamada and Ms. Keiko Kato of The University for conducting ICP-OES.

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nanostructure. J. Am. Chem. Soc. 2003, 125, 3082–3089.

TOC Figure


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The photocatalytic activity of MoOy/RhCrOx/STO:Al under UV irradiation (300 nm < λ < 500 nm) as a function of the amount of Mo added. All samples were calcined at 573 K. 525x522mm (67 x 67 DPI)


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Dependence of the photocatalytic activity of MoOy/RhCrOx/STO:Al under UV irradiation (300 nm < λ < 500 nm) on the calcination temperature: (a) RhCrOx/STO:Al, (b) MoOy/RhCrOx/STO:Al before calcination, MoOy/RhCrOx/STO:Al after calcination at (c) 473, (d) 573, and (e) 673 K. The MoOy loading of each sample was 0.03 wt% as Mo. 177x178mm (192 x 192 DPI)


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Figure 3. Cyclic water splitting using the MoOy/RhCrOx/STO:Al photocatalyst calcined at 573 K under UV irradiation (300 nm < λ < 500 nm) with intermittent evacuation. 178x182mm (192 x 192 DPI)


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Figure 4A. Mo 3d XPS spectra for (a) RhCrOx/STO:Al, and MoOy/RhCrOx/STO:Al photocatalysts (b) before calcination and calcined at (c) 473, (d), 573, and (e) 673 K. The Cr, Mo and Rh loadings were 0.10, 0.03 and 0.10 wt%, respectively. 297x420mm (150 x 150 DPI)


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Figure 4B. Cr 2p XPS spectra for (a) RhCrOx/STO:Al, and MoOy/RhCrOx/STO:Al photocatalysts (b) before calcination and calcined at (c) 473, (d), 573, and (e) 673 K. The Cr, Mo and Rh loadings were 0.10, 0.03 and 0.10 wt%, respectively. 268x379mm (150 x 150 DPI)


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297x420mm (150 x 150 DPI)


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176x181mm (192 x 192 DPI)


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Efficient photocatalytic water splitting using Al-doped SrTiO3 coloaded

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