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Photocatalyst sheets composed of particulate LaMg1/3Ta2/3O2N and Mo-doped BiVO4 for Z-scheme water splitting under visible light Zhenhua Pan, Takashi Hisatomi, Qian Wang, Shanshan Chen, Mamiko Nakabayashi, Naoya Shibata, Chengsi Pan, Tsuyoshi Takata, Masao Katayama, Tsutomu Minegishi, Akihiko Kudo, and Kazunari Domen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01561 • Publication Date (Web): 17 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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ACS Catalysis

Photocatalyst sheets composed of particulate LaMg1/3Ta2/3O2N and Mo-doped BiVO4 for Z-scheme water splitting under visible light Zhenhua Pan1, Takashi Hisatomi1,2, Qian Wang1,2, Shanshan Chen1,2, Mamiko Nakabayashi3, Naoya Shibata3, Chengsi Pan4,†, Tsuyoshi Takata4,‡, Masao Katayama1,2, Tsutomu Minegishi1,2, Akihiko Kudo5, Kazunari Domen1,2,*

1 Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan. 2 Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), 2-11-9 Iwamotocho, Chiyoda-ku, 101-0032 Tokyo, Japan 3 Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, 113-8656 Tokyo, Japan 4 Global Research Center for Environmental and Energy based on Nanomaterials Science (Green), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba-shi, 305-0044 Ibaraki, Japan 5 Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan †

Current affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign,

Urbana, Illinois 61801, United States ‡

Current affiliation: Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,113-8656 Tokyo, Japan; Japan Technological

Research Association of Artificial Photosynthetic Chemical Process (ARPChem), 2-11-9 Iwamotocho, Chiyoda-ku, 101-0032 Tokyo, Japan

*Corresponding author’s E-mail address: [email protected] Abstract Z-scheme water splitting by photocatalyst sheets is a promising approach for efficient and scalable H2 production. One of the most important challenges in developing such photocatalyst sheets is the application of photocatalysts with long

1

ACS Paragon Plus Environment

ACS Catalysis

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absorption

edges.

In

this

study,

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RhCrOx-loaded

LaMg1/3Ta2/3O2N

(RhCrOx/LaMg1/3Ta2/3O2N) and Mo-doped BiVO4 (BiVO4:Mo), with

absorption

edges at 600 and 540 nm, are investigated as the hydrogen evolution photocatalyst (HEP) and oxygen evolution photocatalyst (OEP) in photocatalyst sheets, respectively. The (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet exhibits water splitting activity about five times higher than that of corresponding powder suspensions. Stable water splitting

is

achieved

under

visible

light

using

the

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet coated with an amorphous TiO2 (a-TiO2) protective layer. However, it’s shown the low H2 evolution activity of the RhCrOx/LaMg1/3Ta2/3O2N imposes a ceiling on the activity of the photocatalyst sheet. The surface modification of the LaMg1/3Ta2/3O2N with ZrO2 effectively enhances its H2 evolution activity by reducing the density of defects on the surface of LaMg1/3Ta2/3O2N. Accordingly, the activity of the photocatalyst sheet is almost doubled when employing ZrO2-modified LaMg1/3Ta2/3O2N. The solar-to-hydrogen energy conversion efficiency is still low of 1×10-3%, although this work expands the possibility for the development of photocatalyst sheets capable of functioning under irradiation by long-wavelength photons.

Keyword: Photocatalyst sheet, water splitting, Z-scheme, visible light, oxynitride

1. Introduction 2


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ACS Catalysis

Hydrogen production by photocatalytic solar water splitting is one of the most promising solutions to the energy and environmental concerns arising from the massive present-day consumption of fossil fuels. Over the past several decades, a number of photocatalysts that are active under UV irradiation have been reported to achieve one-step overall water splitting.1 However, photocatalysts that split water under visible light are still rare,2 because the photoexcited carriers in narrow band gap photocatalysts under visible light are thermodynamically less favorable for the water splitting reaction than those in UV-responsive wide band gap photocatalysts. To address this challenge, Z-scheme water splitting involving two-step excitation of a H2 evolution photocatalyst (HEP) and an O2 evolution photocatalyst (OEP) has been developed.3 In a typical Z-scheme water splitting process, electron transfer between a HEP and an OEP is mediated by the reversible reactions of redox couples,4-5 conductive additives bridging HEP and OEP particles,6 or inter particle electron transfer

between

aggregates

of

HEP

and

OEP

particles.7

Many

visible-light-responsive oxides, (oxy)nitrides, and (oxy)sulfides have been applied to Z-scheme water splitting.4-11 Recently, we developed an all-solid-state Z-scheme system by embedding Laand Rh-codoped SrTiO3 (SrTiO3:La,Rh) as the HEP and BiVO4 as the OEP into a gold layer via particle transfer.12 By taking advantage of the particle transfer method,13 charge carriers can be effectively transferred between the HEP and OEP via the underlying gold layer that formed an intimate contact with the HEP and OEP particles 3


ACS Catalysis

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without degrading the intrinsic photocatalytic properties of the materials. In addition, backward reactions involving redox mediators can be avoided because of their absence. Furthermore, this method is applicable to various kinds of particulate photocatalysts. An optimized photocatalyst sheet fabricated in this fashion exhibited an apparent quantum yield (AQY) above 30% at 419 nm and a solar-to-hydrogen energy conversion efficiency (STH) of 1.1%.14 Unfortunately, the short absorption edge wavelength and the weak light absorption of the SrTiO3:La,Rh (λ ≤ 520 nm, resulting from excitation of the Rh3+ donor level to conduction bands) do not produce a STH that is sufficiently high to allow the practical application of this system. Therefore, it remains important to explore the development of HEPs capable of utilizing a wide visible light range for use in Z-scheme photocatalyst sheet systems. LaMg1/3Ta2/3O2N is an oxynitride photocatalyst that can directly split water into H2 and O2 under irradiation at a wavelength up to 600 nm, though its activity under visible light is still low.15 This may be because the valence band maximum of LaMg1/3Ta2/3O2N is too close to the equilibrium potential of the oxygen evolution reaction, although its conduction band maximum is sufficiently negative relative to the equilibrium potential of the hydrogen evolution reaction.15 Accordingly, we applied LaMg1/3Ta2/3O2N as the HEP in a photocatalyst sheet system, in combination with rutile-type TiO2 as the OEP.16 These (RhCrOx/LaMg1/3Ta2/3O2N)/Au/TiO2 photocatalyst sheets demonstrated stable water splitting after being coated with an amorphous titanium oxide (a-TiO2) protective layer, proving the feasibility of 4


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ACS Catalysis

LaMg1/3Ta2/3O2N as a HEP. To mitigate the challenges associated with rutile-type TiO2 (λ ≤ 400 nm), which can only be photo-excited under ultraviolet irradiation, Mo-doped BiVO4 (BiVO4:Mo) (λ ≤ 540 nm)14 was introduced in conjunction with the LaMg1/3Ta2/3O2N to construct a novel visible-light-responsive photocatalyst sheet. The results show that a stable, visible-light-driven overall water splitting process was achieved by this photocatalyst sheet system after coating with an a-TiO2 layer. In addition, the water splitting activity was found to be further improved by impregnating ZrO2 particles on the LaMg1/3Ta2/3O2N particles, indicating that surface modification is a promising approach to develop efficient LaMg1/3Ta2/3O2N-based photocatalyst sheet systems.

2. Experimental 2.1 Preparation of materials and photocatalyst sheets LaMg1/3Ta2/3O2N was prepared by thermal nitridation of an oxide precursor prepared by a citric acid method in an NH3 flow, according to our previous reports.15,16 ZrO2-modified LaMg1/3Ta2/3O2N (ZrO2(x wt%)/LaMg1/3Ta2/3O2N, where x wt% refers to the mass percentage of Zr loaded on the LaMg1/3Ta2/3O2N, was prepared by a post thermal nitridation treatment. Typically, ZrO(NO3)2·2H2O (Kanto Chemical Co., Inc. 99%) was impregnated on LaMg1/3Ta2/3O2N and the dried powder was subsequently heated under 100 mL·min−1 dry NH3 flow at 973 K for 1 h. A cocatalyst, RhCrOx, was loaded on the LaMg1/3Ta2/3O2N (or ZrO2/LaMg1/3Ta2/3O2N) by an 5


ACS Catalysis

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impregnation method.15 BiVO4 was prepared by a solid-liquid reaction by stirring the starting materials in an aqueous nitric acid solution.6 BiVO4:Mo (0.05 mol% to V) powder was synthesized as previously reported.17 The as-prepared particulate photocatalysts were embedded into a Au layer by a particle transfer method13,16 to fabricate ((RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets. The preparation procedure is presented in Figure 1, where a Au think layer was deposited as a conductor layer by vacuum evaporation (VFR-200 M/ERH, ULVAC KIKO Inc.) at an evaporation rate of approximately 15 nm·s−1. The thickness of the Au layer was measured to be 300 nm by crystal thickness monitor (ULVAC CRTM-6000). The activity of the photocatalyst sheet samples were largely reproducible within an error margin of 10% because weakly adhered photocatalyst particles were removed from the photocatalyst sheets during the sonication process. The amount of particles retained on the sheet was approximately 10 mg. RhCrOx/ZrO2/LaMg1/3Ta2/3O2N was used instead of RhCrOx/LaMg1/3Ta2/3O2N for the fabrication of (RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheets. In some experiments, photocatalyst sheets composed of only one kind of the photocatalysts was prepared by the same procedures for comparison.

6


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ACS Catalysis

RhCrOx/LaMg1/3Ta2/3O2N Vacuum evaporation

Mixing and deposition Substrate

a-TiO2

Au layer Substrate Bonding to another glass substrate

BiVO4:Mo Photodeposition Substrate

Lifting off

Substrate

Substrate Carbon tape Photocatalyst sheet

Substrate

Figure 1. Preparation procedure for (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets coated with an a-TiO2 layer.

2.2 Characterization The samples were characterized by X-ray powder diffraction (XRD, RINT-UltimaIII, Rigaku; Cu Kα), UV-visible diffuse reflectance spectroscopy (DRS; V-670, JASCO), and X-ray photoelectron spectroscopy (XPS; JPS-9000, JEOL, Mg, Kα). The morphologies of the photocatalyst particles and sheets were investigated by scanning electron microscopy (SEM; S-4700, Hitachi) and SEM with energy dispersive X-ray spectroscopy (SEM-EDS; JSM-7001FA, JEOL). Transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) analyses were conducted with a JEM-2800 (JEOL) microscope equipped with X-Max 100TLE SDD detector (Oxford Instruments). The BET specific surface areas were measured by N2 adsorption (MicrotracBEL, BELSORP-mini).

7


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2.3 Photocatalytic reactions Water splitting reactions using the photocatalyst sheets were carried out in a top-irradiation-type reaction vessel connected to a closed gas circulation system made of Pyrex.16,18 Prior to the reaction, distilled water (40 mL) was evacuated several times for thoroughly removing air. The photocatalyst sheet was then irradiated with a Xe lamp (300 W) in conjunction with an all-reflection mirror (λ ≥ 300 nm) or a cut-off filter (λ ≥ 420 nm). During water splitting half reactions, a methanol (20 vol%) aqueous solution and a silver nitrate (20 mmol L-1) aqueous solution were used for water reduction and oxidation, respectively, instead of pure water. In some comparative experiments, suspensions of RhCrOx/LaMg1/3Ta2/3O2N powder (10 mg) either alone or with BiVO4:Mo powder (10 mg) dispersed by ultrasonication and magnetic stirring were used for the photocatalytic reactions. An a-TiO2 protective layer was coated on photocatalyst sheet samples by photodeposition from titanium peroxide species according to our previous work.15,19 Typically, the optimized quantity of a titanium peroxide solution (approximately 0.3 wt% as TiO2, 200 µL)16 was added to a reaction cell. The peroxide species were decomposed by irradiation from a 300 W Xe lamp (λ ≥ 300 nm).

2.4 Quantum efficiency measurement The same experimental set-up was used to measure the AQY of the Z-scheme water splitting which proceeded in a two-step photoexcitation process, except for the 8


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ACS Catalysis

use of a band-pass filter with central wavelength 418.6 nm and full-width at half maximum 9.5 nm. The number of incident photons illuminated on the photocatalyst sheet was 2.4 × 1020 photon h-1. The AQY value was calculated using the following equation: AQY(%) = [4 × n(H2)]/n(photons) × 100

(1)

Where n(H2) and n(photons) denote the number of produced H2 molecules and the number of incident photons, respectively.

2.5 Solar-to-hydrogen conversion efficiency measurement The gas evolution rates in water splitting reaction was measure in the same experimental apparatus, except under illumination from a solar simulator (Asahi Spectra, HAL-320). The STH was calculated as follows: STH (%) = [R(H2) × ∆Go]/[P × S ] × 100

(2)

where R(H2), ∆Go, P, and S denote the rate of hydrogen evolution (mol s-1) in the Z-scheme water splitting system, the change in Gibbs free energy during water splitting reaction (237 × 103 J mol-1), the energy intensity of the solar light irradiation (100 mW cm-2), and the irradiation area (9 cm2), respectively.

3. Results and Discussion The successful preparation of both LaMg1/3Ta2/3O2N and BiVO4:Mo was confirmed by XRD as shown in Figure S1. The surface areas of the materials were 9


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12.7 and 2.0 m2 g-1, respectively. The time courses of the water splitting reaction using (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets with and without the a-TiO2 modification are summarized in Figure 2. Simultaneous evolution of H2 and O2 occurred on the RhCrOx/LaMg1/3Ta2/3O2N and BiVO4:Mo, respectively, when these materials were embedded in Au layers to form a photocatalyst sheet, although N2 was also generated due to self-oxidation of the LaMg1/3Ta2/3O2N (Figure 2a). It indicates that LaMg1/3Ta2/3O2N was not stable in water splitting reaction without surface modification. After applying a protective a-TiO2 layer over the surface of the sheet, the formation of N2 was suppressed and stable overall water splitting was achieved (Figure 2b), similar to that observed in our previous work,15, 20 although the water splitting activity decreased slightly probably because of hindrance of mass transport through the a-TiO2 layer. It is expected that the surface nitrogen species were stabilized as nitride ions after deposition of a-TiO2.21 However, further research is still necessary to uncover the mechanism of preventing N2 evolution. Note that the use of undoped BiVO4 instead of BiVO4:Mo as an OEP resulted in a slightly lower activity (Figure S2). It was reported that substituting V5+ partially with Mo6+ for n-type doping increased the charge carrier concentration and electrical conductivity of BiVO4 and improved the PEC oxygen evolution activity of BiVO4 photoanodes.22 The overall

water

splitting

reaction

also

proceeded

steadily

by

the

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet under visible light irradiation (λ ≥ 420 nm), as shown in Figure 2c. The corresponding visible-light-driven hydrogen and 10


Page 11 of 37

oxygen evolution rates were 0.6 and 0.3 µmol h−1, respectively. The activity was one order

of

magnitude

higher

than

that

of

sheets

composed

of

only

RhCrOx/LaMg1/3Ta2/3O2N as shown in Table S1. In addition, suspensions of RhCrOx/LaMg1/3Ta2/3O2N (10 mg) alone and with BiVO4:Mo (10 mg) both generated H2 no faster than 0.2 µmol h−1, even under UV and visible light irradiation (λ ≥ 300 nm), as shown in Figure S3. The improvement of the activity by loading RhCrOx/LaMg1/3Ta2/3O2N and BiVO4:Mo particles on the Au layer could not be attributed to the surface plasmon resonance effect, because the diffuse reflectance spectrum of the Au/glass sheet prepared in the identical condition exhibited absorption only below 550 nm and no absorption peak at around 600 nm typical of surface plasmon resonance of nanoparticulate Au (Figure S4).12 . a

12

Gas evolution / µmol

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

ACS Catalysis

H2

10

10

8

8

O2

6 4

b

12

H2

N2

0 2

4

6

8

6

O2

Time

H2

4

O2

2

2

N2 0

Time / h

2.

10

6

0 0

c

8

4

2

Figure

12

2

4

6

8

N2

0 0

of

Z-scheme

4

6

8

Time / h

Time / h

courses

2

overall

water

splitting

using

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets (a) without and (b and c) with a protective a-TiO2 layer. Reaction conditions: reactant solution, 40 mL water; light source, a 300 W xenon lamp (λ ≥ 300 nm for (a, b) and λ ≥ 420nm for (c)); 11


ACS Catalysis

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irradiation area, 9.0 cm2.

The

morphology

and

elemental

distribution

of

the

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet coated with a-TiO2 layer was investigated by SEM-EDS. As can be seen from Figure 3, LaMg1/3Ta2/3O2N particles tens of nanometers in size aggregated to form larger particles while the BiVO4:Mo particles were primarily micrometer-sized with a plate-like shape. These two types of particles were both distributed over the Au layer. The Ti EDS signal for the LaMg1/3Ta2/3O2N appears to be stronger, because the Ti-K signal (4.51 keV) from the a-TiO2 and La-L signal (4.65 keV) from the LaMg1/3Ta2/3O2N could not be decoupled completely. Nevertheless, a Ti signal is observed over the entire mapping image (Figure 3e). XPS data obtained from photocatalyst sheets before and after coating with an a-TiO2 layer are given in Figure S5. Ti species are observed on the photocatalyst sheet after coating with the a-TiO2 layer, and both the Bi and Ta signals are seen to be much weaker after coating. This demonstrates that the surfaces of both the LaMg1/3Ta2/3O2N and the BiVO4:Mo on the photocatalyst sheet were covered with the

a-TiO2

layer,

similar

to

the

results

from

our

previous

work

on

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/rutile photocatalyst sheets.16 The cross-sectional SEM-EDS images (Figure S6) revealed that the thickness of the particle layer was around 3 µm. The structure of the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet coated with 12


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ACS Catalysis

an a-TiO2 layer is shown in Figure 4. Photodeposited amorphous oxide layers have been previously found to rectify the migration of O2 molecules.20 Specifically, H+ ions and H2O molecules are able to penetrate the hydrated a-TiO2 layer to reach the surface of the photocatalyst, where the evolution of H2 and O2 takes place. H2 molecules can penetrate thin hydrated amorphous oxide layers and diffuse to the reaction solution because of their small size.23 O2 molecules are also believed to diffuse to the reaction solution through the a-TiO2 layer because the partial pressure of O2 molecules confined at the photocatalyst surface by the a-TiO2 layer can be very high. However, the permeation of O2 molecules in the opposite direction is unfavorable because the partial pressure of O2 in the outer phase is lower than that inside the layer. In the case of (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheets, H2 and O2 are evolved on the surfaces of the RhCrOx/LaMg1/3Ta2/3O2N and BiVO4:Mo, respectively, beneath the a-TiO2 serving as a protective layer for the LaMg1/3Ta2/3O2N and are subsequently released to the reaction solution through the a-TiO2 layer.

13


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(a)

(b)

(c)

(d)

Page 14 of 37

(e)

3 μm

Figure 3. Top-view images of a (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet coated with an a-TiO2 layer. (a) SEM and (b-e) EDS mapping images of (b) the superimposition of (c-e), (c) Bi, (d) La, and (e) Ti.

14


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ACS Catalysis

a-TiO2 layer

O2

2H2O

2H2

λ ≥ 420 nm

Au layer RhCrOx/LaMg1/3Ta2/3O2N

BiVO4:Mo

Photocatalyst sheet Figure

4.

Schematic

working

mechanism

of

an

a-TiO2-coated

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet.

The schematic band diagram of the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet is presented in Figure 5, in which the band bending at the interface is omitted for the sake of simplicity. The proton reduction potential (H+/H2, 0 V vs. NHE at pH 0) was more positive than the bottom of conduction band of the LaMg1/3Ta2/3O2N (−0.68 V vs. NHE at pH 0). As such, electrons generated in the LaMg1/3Ta2/3O2N under illumination and transported to the RhCrOx-catalyst could reduce protons to H2 molecules.24 The potential of the water oxidation (O2/H2O, +1.23 V vs. NHE at pH 0) was more negative than the top of the valence band of the BiVO4:Mo (−2.5 V vs. NHE at pH 0),14 and holes generated in the BiVO4:Mo and transported to the surface were able to oxidize water to O2 molecules. Some of the holes remaining in the LaMg1/3Ta2/3O2N may also have been used for water oxidation 15


ACS Catalysis

because this material is active for the overall water splitting reaction. However, the observation that the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet (Figure 2a) exhibits eight times higher activity than the (RhCrOx/LaMg1/3Ta2/3O2N)/Au sheet without BiVO4:Mo (Figure S7) indicates that the majority of the holes in the LaMg1/3Ta2/3O2N recombined with electrons remaining in BiVO4:Mo via the underlying Au layer.

Potential / V vs. NHE at pH 0

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-1 0

RhCrOx

CBM CBM

H + /H 2

Ef

1 2

O 2 /H 2O

VBM VBM

3

LaMg1/3Ta2/3O2N / Au / BiVO4:Mo

Figure 5. Schematic band diagram of the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet. Here, CBM, VBM and Ef indicate the conduction band minimum, valence band maximum and Fermi level, respectively.

Z-scheme overall water splitting was successfully achieved when employing the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo

photocatalyst

sheet.

However,

the

performance of this sheet was still far below that of previously reported 16


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ACS Catalysis

SrTiO3:La,Rh/Au/BiVO4:Mo.12, 14 This fact suggests that the bottleneck for overall water splitting over the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet is associated with the RhCrOx/LaMg1/3Ta2/3O2N. To investigate the bottleneck of the water

splitting

reaction

on

the

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo

photocatalyst sheet further, water splitting half reactions were assessed in the presence of a sacrificial electron donor (CH3OH) or acceptor (Ag+). Photocatalyst sheets without a-TiO2 coating were used for water splitting half reactions, because the sacrificial reagents may not penetrate the a-TiO2 layer according to a previous study.20 In the presence of these sacrificial reagents, the photocatalytic reaction was free from the limitations of charge transfer via the Au layer, and so the upper-bound activity of the photocatalyst sheet for water reduction or oxidation could be determined. As shown in Table 1, the O2 evolution rate over the photocatalyst sheets (entry 2 and 3) in the presence of Ag+ was higher by an order of magnitude than that observed during overall water splitting (entry 1). However, the H2 evolution rates were virtually identical for the sacrificial H2 evolution reaction in methanol aqueous solution (entry 4 and 5) and the overall water splitting reaction (entry 1). This observation supports the consideration that the low activity of RhCrOx/LaMg1/3Ta2/3O2N as the HEP limits the net water splitting activity of the photocatalyst sheets even though BiVO4:Mo can efficiently evolves oxygen. Several factors may be responsible for the low performance of the RhCrOx/LaMg1/3Ta2/3O2N as the HEP in the photocatalyst sheets. Firstly, primary 17


ACS Catalysis

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

Page 18 of 37

particles of the LaMg1/3Ta2/3O2N (tens of nanometers in size) were found to have aggregated to form secondary particles. In these photocatalyst sheets, holes from LaMg1/3Ta2/3O2N should recombine with electrons from the Au layer for the net reaction to occur. However, only particles close to the Au layer can participate in this process because it is unlikely that charge transfer will proceed efficiently across a number of primary particles. Secondly, the properties of LaMg1/3Ta2/3O2N prepared in the present method may not be suitable for H2 evolution, because the H2 evolution activity itself was found to be rather low despite the modification with RhCrOx that was

the

most

effective

H2

evolution

cocatalyst

currently

available

for

LaMg1/3Ta2/3O2N.21 It is therefore necessary to modify the properties of the LaMg1/3Ta2/3O2N

photocatalyst

to

improve

the

activity

of

the

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet.

Table 1. Photocatalytic water splitting activity over various photocatalyst sheets in the presence of sacrificial reagents before coating a-TiO2 layers. Entry

Photocatalyst sheetsa

Sacrificial reagents

Activity

(µmol h-1)

H2

O2

1

HEP/Au/OEP

Noneb

1.2

0.6

2

HEP/Au/OEP

AgNO3c

—

23

3

OEP/Au

AgNO3c

—

17

4

HEP/Au/OEP

CH3OHd

1.2

— 18


Page 19 of 37

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

ACS Catalysis

5

HEP/Au

CH3OHd

1.4

—

Reaction condition: reactant solution, 40 mL water; light source, a 300 W xenon lamp λ ≥ 300 nm; irradiation area, 9.0 cm2. a

RhCrOx/LaMg1/3Ta2/3O2N and BiVO4:Mo were used as a HEP and an OEP,

respectively. b

Pure water, 40 mL.

c

Aqueous AgNO3 solution (20 mmol L−1), 40 mL.

d

Aqueous CH3OH solution (20 vol%), 40 mL.

ZrO2 has been used as a surface modifier for TaON to suppress the formation of reduced Ta species during the nitridation process and thus effectively enhance the H2 evolution activity.25 Based on this, LaMg1/3Ta2/3O2N was modified with ZrO2 at 973 K in an attempt to improve its H2 evolution activity. As shown in Figure S8, the H2 evolution rate obtained from (RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au photocatalyst sheets in the presence of methanol as a hole scavenger increased in proportion to the loading amount of ZrO2, in agreement with the results of a previous study.25 This effect reached a maximum at a Zr loading of approximately about 2 wt%. Figure 6 presents the XRD patterns and DRS data obtained from LaMg1/3Ta2/3O2N samples with and without ZrO2 modification at 973 K. Following the ZrO2 modification, the diffraction peaks became stronger because of the additional thermal nitridation treatment. However, the FWHM of the peaks were almost 19


ACS Catalysis

unchanged. No peak assignable to ZrO2 are observed, likely because the amount of ZrO2 was too low to generate significant peaks. With regard to the DRS results, the ZrO2 modification evidently lowered the background absorption beyond the absorption edge of the LaMg1/3Ta2/3O2N. This background absorption is believed to be associated with anion defects and reduced Ta species in the oxynitride.25 Loading of ZrO2 with high ionicity makes Ta ions in Ta-based oxynitrides more oxidized (i.e., more cationic) because of formation of Ta-O-Zr bonds. The amount of reduced Ta species on the surface of LaMg1/3Ta2/3O2N is thus considered to be decreased by loading ZrO2. With the decrease in the density of reduced Ta species, the background absorption associated with defects became weaker clearly. This will support the enhancement in the H2 evolution activity of LaMg1/3Ta2/3O2N by ZrO2 loading. The apparent blue-shift of the absorption edge after ZrO2 modification is due to the decrease in the background and shoulder absorptions as a result of lower defect density on LaMg1/3Ta2/3O2N rather the reduction of the band gap energy.

A

30

40

50

60

70

(d)

(d)

(c)

(c)

(b)

(b)

(a)

(a) 30

31

32

33

Absorbance / a.u.

1.0

Intensity / a.u.

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

Page 20 of 37

B

0.8 0.6 0.4

(c)

(d)

(a)

(b)

0.2 0.0 400

500

600

700

800

Wavelength / nm

2θ/ degree

Figure 6. (A) XRD patterns and (B) DRS of (a) pristine LaMg1/3Ta2/3O2N and those modified with ZrO2 at (b) 0, (c) 1 and (d) 3 wt%. 20


Page 21 of 37

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ACS Catalysis

The distribution of the ZrO2 loaded on the LaMg1/3Ta2/3O2N was investigated by assessing HR-TEM images (Figure S9). At a lower loading of 1 wt%, neither ZrO2 particles nor layers were observed (data not shown), and only few amorphous ZrO2 particles were seen on the LaMg1/3Ta2/3O2N particles at a higher ZrO2 loading proportion (3 wt%). In order to examine the distribution of Zr more clearly, a ZrO2(3 wt%)/LaMg1/3Ta2/3O2N sample was selected and characterized by STEM-EDS, with the results shown in Figure 7. Using the distribution of La (associated with the LaMg1/3Ta2/3O2N particles, Figure 7c) as a reference, the Zr was evidently concentrated near the outlines of the LaMg1/3Ta2/3O2N particles but not at specific locations, presenting a sort of shell structure (Figure 7d). This feature demonstrates that ZrO2 was added solely to the exterior of the LaMg1/3Ta2/3O2N particles, and suggests that the ZrO2 species were both in the form of small particles and well dispersed over the LaMg1/3Ta2/3O2N at loading amounts up to 1 wt%. At higher loadings, the ZrO2 may have segregated to form small numbers of amorphous ZrO2 domains (Figure S9).

21


ACS Catalysis

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(a)

(b)

(c)

(d)

Page 22 of 37

Figure 7. (a and b) STEM-EDS mapping and HAADF STEM images of a RhCrOx/ZrO2(3 wt%)/LaMg1/3Ta2/3O2N sample, and (c and d) STEM-EDS mapping images of (c) La and (d) Zr. The scale bar in each image indicates 50 nm.

The XPS spectrum obtained from ZrO2 (3 wt%)/LaMg1/3Ta2/3O2N powder along with those of LaMg1/3Ta2/3O2N and ZrO2 powders are presented in Figure S10. Two peaks are observed in the Zr 3d spectrum of the ZrO2/LaMg1/3Ta2/3O2N but not in that of the LaMg1/3Ta2/3O2N (Figure S10a). The Zr 3d peaks for the ZrO2/LaMg1/3Ta2/3O2N sample are in good agreement with those for ZrO2, indicating that the valence states of the Zr species on the LaMg1/3Ta2/3O2N surface and in the ZrO2 were similar. The Ta 22


Page 23 of 37

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ACS Catalysis

4f XPS spectra are shown in Figure S10b. The binding energy of the Ta 4f peaks does not exhibit a shift, indicating that the chemical environments of the Ta atoms were unchanged between the two samples. Figure

8

summarizes

the

water

splitting

rates

over

the

(RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets as a function of the ZrO2 content. The modification of the LaMg1/3Ta2/3O2N with ZrO2 at 973 K effectively enhanced the water splitting activity. The activity is seen to have peaked at a loading amount of 1 wt%, at which point it was approximately twice that obtained from the (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo, as shown in Figure 9. Note that the photocatalyst sheet composed of RhCrOx/LaMg1/3Ta2/3O2N without ZrO2 but with the same thermal treatment presented little improvement in the water splitting activity as shown in Figure S11. The higher water splitting activity of the photocatalyst sheet is

likely

primarily

due

to

the

higher

H2

evolution

activity

of

the

ZrO2/LaMg1/3Ta2/3O2N. However, the AQY of the water splitting reaction using the (RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo sheet was only 0.07% under monochromatic light irradiation at 418 nm and the STH was about 1×10-3% on the basis of the rate of gas evolution measured under simulated sunlight (Figure S12). Therefore, the efficiency of the LaMg1/3Ta2/3O2N still needs to be improved with regard to the photocatalytic H2 evolution. In addition, excessive loading gradually decreased the activity. This is distinct from the case of the H2 evolution reaction in the presence of methanol, presumably because excess ZrO2 segregated on the surface of 23


ACS Catalysis

the LaMg1/3Ta2/3O2N. ZrO2 has a band gap energy as large as 5.0 eV and therefore it cannot be excited under the present experimental condition (λ ≥ 300 nm). As well, because the band gap of ZrO2 straddles that of LaMg1/3Ta2/3O2N,26 charge transfer between LaMg1/3Ta2/3O2N and the Au or the RhCrOx across ZrO2 grains is considered to be energetically unfavorable. 2.5


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

Page 24 of 37

2.0

H2

1.5

1.0

O2

0.5

N2

0.0 0

1

2

3

Zr / wt%

Figure

8.

The

water

splitting

activity

over

(RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets as a function of the Zr loading at 973 K. Reaction conditions: reactant solution, 40 mL water; light source, a 300 W xenon lamp (λ ≥ 300 nm); irradiation area, 9.0 cm2.

24


Page 25 of 37

20


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

ACS Catalysis

20 a

H2

16

b 16

12

12

8

O2 8

4

4

O2

0

N2 0

N2

0

2

4

6

8

of

Z-scheme

Time / h

Figure

9.

Time

courses

H2

0

2

4 6 Time / h

overall

water

8

splitting

using

(RhCrOx/ZrO2/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets with an a-TiO2 protective layer. Reaction conditions: reactant solution, 40 mL water; light source, a 300 W xenon lamp (λ ≥ 300 nm for (a) and λ ≥ 420 nm for (b)); irradiation area, 9.0 cm2.

4. Conclusion RhCrOx/LaMg1/3Ta2/3O2N and BiVO4:Mo, embedded together in a Au layer, were studied as the HEP and OEP, respectively, for photocatalyst sheets applied to visible-light-driven water splitting. Stable water splitting was achieved using these photocatalyst sheets under visible light irradiation after coating with an a-TiO2 protective layer. It was determined by SEM and SEM-EDS analyses that both the photocatalyst particles were spread over the Au layer, and the XPS data indicated that the surface of the photocatalyst sheet was covered with an a-TiO2 layer. However, the performance of the sheet system was still low, mainly due to the low H2 evolution 25


ACS Catalysis

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Page 26 of 37

activity of the RhCrOx/LaMg1/3Ta2/3O2N. Loading ZrO2 on the surface of the LaMg1/3Ta2/3O2N was found to effectively enhance the H2 evolution activity by a factor of more than three in the presence of CH3OH. Consequently, the use of ZrO2/LaMg1/3Ta2/3O2N as the HEP approximately doubled the overall water splitting activity of the photocatalyst sheet under optimized conditions as expected, although the excessive loading of ZrO2 lowered the activity because it prevented charge transfer between the LaMg1/3Ta2/3O2N and the Au layer. It is concluded that further improvements in the photocatalytic proton reduction performance of the LaMg1/3Ta2/3O2N could enhance the overall water splitting activity of

the

visible-light-responsive

(RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo

photocatalyst sheet system.

Supporting Information XRD patterns, reaction time courses, DRS, XPS spectra, SEM-EDS, TEM, and STEM-EDS images, and comparison of photocatalytic activity.

Acknowledgements This work was supported by Grants-in-Aid for Specially Promoted Research (No. 23000009) and Young Scientists (A) (No. 15H05494) and by the International Exchange Program of the A3 Foresight Program of the Japan Society for the Promotion of Science (JSPS), and the Artificial Photosynthesis Project of the New 26


Page 27 of 37

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ACS Catalysis

Energy and Industrial Technology Development Organization (NEDO) and development of Environmental Technology using Nanotechnology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). A Part of this work was conducted at the Research Hub for Advanced Nano Characterization at the University of Tokyo, under the support of the “Nanotechnology Platform” (project No.12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Z. P. also wishes to acknowledge the support of the China Scholarship Council (CSC) (No. 201306160079).

References 1. Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278. 2. Lee, Y.; Terashima, H.; Shimodaira, Y.; Teramura, K.; Hara, M.; Kobayashi, H.; Domen, K.; Yashima, M. J. Phys. Chem. C 2006, 111, 1042-1048. 3. Bard, A. J. J. Photochem. 1979, 10, 59-75. 4. Maeda, K.; Higashi, M.; Lu, D.; Abe, R.; Domen, K. J. Am. Chem. Soc. 2010, 132, 5858-5868. 5. Chen, S.; Qi, Y.; Hisatomi, T.; Ding, Q.; Asai, T.; Li, Z.; Ma, S. S. K.; Zhang, F.; Domen, K.; Li, C. Angew. Chem. Int. Ed. 2015, 54, 8498-8501. 6. Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. J. Am. Chem. Soc. 2011, 133, 11054-11057. 7. Sasaki, Y.; Nemoto, H.; Saito, K.; Kudo, A. J. Phys. Chem. C 2009, 113, 17536-17542. 8. Higashi, M.; Abe, R.; Takata, T.; Domen, K. Chem. Mater. 2009, 21, 1543-1549. 9. Abe , R.; Higashi, M.; Domen, K. ChemSusChem 2011, 4, 228-237. 10. Sasaki, Y.; Kato, H.; Kudo, A. J. Am. Chem. Soc. 2013, 135, 5441-5449. 11. Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. J. Am. Chem. Soc. 2015, 137, 604-607. 12. Wang, Q.; Li, Y.; Hisatomi, T.; Nakabayashi, M.; Shibata, N.; Kubota, J.; Domen, K. J. Catal. 2015, 328, 308-315. 13. Minegishi, T.; Nishimura, N.; Kubota, J.; Domen, K. Chem. Sci. 2013, 4, 1120-1124. 14. Wang, Q.; Hisatomi, T.; Jia, Q.; Tokudome, H.; Zhong, M.; Wang, C.; Pan, Z.; 27


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Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y.; Sharp, I. D.; Kudo, A.; Yamada, T.; Domen, K. Nat. Mater. 2016, 15, 611-615. 15. Pan, C.; Takata, T.; Nakabayashi, M.; Matsumoto, T.; Shibata, N.; Ikuhara, Y.; Domen, K. Angew. Chem. Int. Ed. 2015, 54, 2955-2959. 16. Pan, Z.; Hisatomi, T.; Wang, Q.; Nakabayashi, M.; Shibata, N.; Pan, C.; Takata, T.; Domen, K. Appl. Catal., A 2016, 521, 26-33. 17. Iwase, A.; Ito, H.; Jia, Q.; Kudo, A. Chem. Lett. 2016, 45, 152-154. 18. Ma, G.; Hisatomi, T.; Domen, K. in From Molecules to Materials: Pathways to Artificial Photosynthesis, Rozhkova, E. A., Ariga, K., Eds.; Springer: Cham, 2015; pp. 1–56. 19. Xu, J.; Pan, C.; Takata, T.; Domen, K. Chem. Commun. 2015, 51, 7191-7194. 20. Takata, T.; Pan, C.; Nakabayashi, M.; Shibata, N.; Domen, K. J. Am. Chem. Soc. 2015, 137, 9627-9634. 21. Pan, C.; Takata, T.; Domen, K. Chem. Eur. J. 2016, 22, 1854-1862. 22. Zhou, M.; Bao, J.; Xu, Y.; Zhang, J.; Xie, J.; Guan, M.; Wang, C.; Wen, L.; Lei, Y.; Xie, Y. ACS Nano 2014, 8, 7088-7098. 23. Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem. Int. Ed. 2006, 45, 7806-7809. 24. Pan, C.; Takata, T.; Kumamoto, K.; Khine Ma, S. S.; Ueda, K.; Minegishi, T.; Nakabayashi, M.; Matsumoto, T.; Shibata, N.; Ikuhara, Y.; Domen, K. J. Mater. Chem. A 2016, 4, 4544-4552. 25. Maeda, K.; Terashima, H.; Kase, K.; Higashi, M.; Tabata, M.; Domen, K. Bull. Chem. Soc. Jpn. 2008, 81, 927-937. 26. Sayama, K.; Arakawa, H. J. Photochem. Photobiol., A 1994, 77, 243-247.

TOC graphic a-TiO2 layer

O2

2H2O

2H2

λ ≥ 420 nm

Au layer RhCrOx/LaMg1/3Ta2/3O2N

BiVO4:Mo

Photocatalyst sheet

28


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ACS Catalysis

Figure 1. Preparation procedure for (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheets coated with an a-TiO2 layer. Figure 1 895x367mm (98 x 98 DPI)


ACS Catalysis

a

12

Gas evolution / mol

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

H2

12

10

10

8

8

O2

6 4

Page 30 of 37

b

12

H2

N2

0

O2

2

N2

0 0

2

4

Time / h

6

8

10 8 6

6 4

2

c

0

2

4

6

8

Time / h


H2

4 2

O2

0

N2 0

2

4

Time / h

6

8

Page 31 of 37

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ACS Catalysis

(a)

(b)

(c)

3. 0 µm

IMG1

(e)

3. 0 µm

Bi M

(d)

3. 0 µm

3. 0 µm

3 μm

3. 0 µm

ACS Paragon Plus Environment Ti K

La L

ACS Catalysis

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

Figure 4. Schematic working mechanism of an a-TiO2-coated (RhCrOx/LaMg1/3Ta2/3O2N)/Au/BiVO4:Mo photocatalyst sheet. Figure 4 713x399mm (123 x 123 DPI)


Page 32 of 37

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ACS Catalysis

Potential / V vs. NHE at pH 0

Page 33 of 37

-1

0

RhCrOx

CBM CBM

H + /H 2

Ef

1

2

O 2 /H 2O

VBM VBM

3

LaMg1/3Ta2/3O2N / Au / BiVO4:Mo


ACS Catalysis

A (d) (c) (b) (a)

30

40

50

60

70

80

Absorbance / a.u.

1.0

Intensity / a.u.

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

Page 34 of 37

B

0.8 0.6 0.4

(c)

(d)

(a)

0.2 0.0 400

500

600

700

Wavelength / nm

2/ degree


(b)

800

Page 35 of 37

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ACS Catalysis

(a)

(b)

(c)

(d)


ACS Catalysis

-1

2.5

Gas evolution rate / mol h

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

Page 36 of 37

2.0 1.5 1.0

H2

O2

0.5 N2

0.0 0

1

2 Zr / wt%


3

Page 37 of 37

20

Gas evolution / mol

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

ACS Catalysis

20 a

H2

16 12

b

16 12

H2

8

O2 8

4

4

O2

0

N2 0

N2

0

2

4 Time / h

6

8

0

2

4 6 Time / h


8

Photocatalyst Sheets Composed of Particulate ... - ACS Publications

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