Anatomy of a Visible Light Activated Photocatalyst for Water Splitting

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Anatomy of a Visible Light Activated Photocatalyst for Water Splitting Somphonh Peter Phivilay, Charles Roberts, Andrew Gamalski, Eric A. Stach, Shiran Zhang, Luan Nguyen, Yu Tang, Anke Xiong, Alexander Puretzky, Franklin (Feng) Tao, Kazunari Domen, and Israel E. Wachs ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01388 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Anatomy of a Visible Light Activated Photocatalyst for Water Splitting Somphonh P. Phivilay1†, Charles A. Roberts1‡, Andrew D. Gamalski2, Eric A. Stach2, Shiran Zhang3, Luan Nguyen3, Yu Tang3, Anke Xiong4, Alexander A. Puretzky5, Franklin (Feng) Tao3, Kazunari Domen4 and Israel E. Wachs1* 1

Operando Molecular Spectroscopy & Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA.

2

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA.

3

Department of Chemical and Petroleum Engineering and Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.

4

Department of Chemical System Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. 5

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

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ABSTRACT.

The supported mixed oxide (Rh2-yCryO3)/(Ga1-xZnx)(N1-xOx) photocatalyst, highly active for splitting of H2O, was extensively characterized for its bulk and surface properties with the objective of developing fundamental structure-photoactivity relationships. Raman and UV-vis spectroscopy revealed that the molecular and electronic structures, respectively, of the oxynitride (Ga1-xZnx)(N1-xOx) support are not perturbed by the deposition of the (Rh2-yCryO3) NPs. Photoluminescence (PL) spectroscopy, however, showed that the oxynitride (Ga1-xZnx)(N1-xOx) support is the source of excited electrons/holes and the (Rh2-yCryO3) NPs greatly reduce the undesirable recombination of photoexcited electron/holes by acting as efficient electron traps as well as increase the lifetimes of the excitons. High Resolution-XPS and High Sensitivity-LEIS surface analyses reveal that the surfaces of the (Rh2-yCryO3) NPs consist of Rh+3 and Cr+3 mixed oxide species. In Situ AP-XPS help to reveal that the Rh+3 and surface N atoms are involved in water splitting. Dispersed RhOx species on the (Ga1-xZnx)(N1-xOx) support and on CrOx NPs were found to be the photocatalytic active sites for H2 generation and N and Zn sites from the (Ga1xZnx)(N1-xOx)

support are the photocatalytic active site for O2 generation. The current

investigation establishes the fundamental structure-photoactivity relationships of these visible light activated photocatalysts.

KEYWORDS. Photocatalysis, Water Splitting, Photocatalyst, Spectroscopy, AP-XPS, HS-LEIS


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1. Introduction. One of the great challenges facing society today is finding alternative renewable energy resources to help alleviate our reliance upon fossil fuels for energy. Hydrogen from non-carbon sources is considered to be one of the potential candidates for replacing fossil fuels for our energy needs. Solar hydrogen fuel generated from photocatalytic splitting of water has the potential to be the pinnacle of sustainable green energy because the H2O reactant is inexpensive, naturally abundant, and the reaction produces no environmentally damaging byproducts. The past 40+ years of research on photocatalytic splitting of water have led to the discovery of over 130 photocatalysts capable of producing H2 and O2.1–4 However, only a small number of these photocatalysts are capable of photocatalytic water splitting under visible light.5–12 Oxynitridebased materials are among the most promising photocatalyst systems to date,13 but a deeper understanding of both the molecular and electronic structures of the photocatalytically active surface sites is necessary to improve and rationally design future visible light activated photocatalysts with enhanced activity. The advanced bulk mixed metal oxynitride materials (Ga1-xZnx)(N1-xOx) and (Zn1+xGe)(N2Ox), synthesized by calcination of the physically mixed metal oxide starting materials in an NH3 environment, were investigated for their ability to generate electron/hole pairs under visible light irradiation for photocatalytic water splitting by the Domen research group.1,6,7 Whereas GaN absorbs in the UV region (3.3 eV), the mixed (Ga1-xZnx)(N1-xOx) oxynitride solid solution absorbs in the visible region (2.4-2.8 eV) due to the narrowing of the band gap from the addition of ZnO. The narrowing of the band gap is attributed to the presence of p-d repulsion between Zn 3d and N 2p electrons, resulting in an increase in the valence band maximum and allowing visible light absorption.14–16 Modification of the (Ga1-xZnx)(N1-xOx) by deposition of transition


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metal oxides (Ni, Ru, Rh, Ir and Pt) onto the surface activates this system for both H2 and O2 production.17 The photoactivity is further enhanced (between 2.5-860x) when the transition metal oxides are co-loaded with Cr2O3. The co-loaded (Rh2-yCryO3)/(Ga1-xZnx)(N1-xOx) catalyst is one of the most active photocatalysts under visible light water splitting (λ > 400nm) with an optimized apparent photonic efficiency (PE) of 5.9%.18–21 In order to provide fundamental insights into the molecular and electronic structures of this leading photocatalyst system for splitting of water, the supported (Rh2-yCryO3)/(Ga1-xZnx)(N1xOx)

photocatalyst was extensively characterized with modern surface and bulk methods. Bulk

information was accessed with in situ optical spectroscopy (Raman, UV-vis, photoluminescence (PL), and time-resolved (TR) picosecond PL) and electron microscopy (STEM and STEM-EDS) methods. Cutting edge surface characterization methods (high-resolution and in situ ambient pressure X-ray photoelectron spectroscopy (HR-XPS and AP-XPS) and high-sensitivity low energy ion scattering (HS-LEIS)), was used to provide information on the nature of the surface region (~1-3nm) and outermost atomic layer (~0.3nm), respectively and to determine the actives sites during photocatalysis. Surface analysis of the photocatalyst was performed pre- and postreaction to determine how the surface of the fresh and used photocatalyst is affected by the photocatalytic water splitting environment. 2. Experimental Section. 2.1 Catalyst Synthesis. The (Ga1−xZnx)(N1−xOx) with x=0.12 as measured from energy-dispersive (EDX) analysis was synthesized by a nitridation method.15 A mixture of Ga2O3 (High Purity Chemicals, 99.9%) and ZnO (Kanto Chemicals, 99%) in a 1:2 molar ratio was first well mixed in an agate mortar. The physical mixture was then calcined at 1123 K under NH3 flow (250 ml/min) for 15 hours in a


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custom quartz nitridation reactor. The (Ga1−xZnx)(N1−xOx) was mixed in an evaporating dish with the aqueous precursors, Cr(NO3)3•9H2O (Wako Pure Chemicals, 99.9%) and Na3RhCl6•2H2O (Kanto Chemicals, 97% as Rh), yielding 1 wt.% Rh and 1.5wt.% Cr.18 This suspension was then placed over a water bath and continuously stirred with a glass rod until complete evaporation. The powder was then collected and calcined in air at 623 K for one hour. The catalyst obtained will be referred to as (Rh2-yCryO3)/(Ga1-xZnx)(N1-xOx), so named in the literature. The postreaction samples were obtained after 7 h reaction time using a Xe arc lamp in a top down Pyrex reactor cell filled with 100 mL of distilled water in a similar manner from the literature.22 2.2 Electron Microscopy (STEM and STEM-EDS). STEM images were collected using an aberration corrected Hitachi HD2700C STEM operated at 200 kV with a spatial resolution of 0.9 Å and energy resolution 350 meV. STEM-EDS maps were collected using the Hitachi HD2700C using a Bruker Silicon Drift Detector Energy Dispersive Spectroscopy detector. The major peaks in STEM-EDS corresponded to Rh-Lα, CrKα, Ga-Kα, and Zn-Kα with 300 ms acquisition time per pixel. 2.3 Raman Spectroscopy. Raman spectroscopy was utilized to obtain the molecular structure of the fresh photocatalysts and was performed on a Lab Ram-HR Raman spectrometer (Horiba-Jobin Yvon) equipped with visible (442 nm) laser excitation and utilizing a confocal microscope (Olympus BX-30) for focusing the laser on the catalyst sample. The 442 nm visible laser excitation was generated by a He-Cd laser ( ~7 mW) with the scattered photons directed into a single monochromator and focused onto a UV-sensitive liquid-N2 cooled CCD detector (Horiba-Jobin Yvon CCD-3000V) having a spectral resolution of ∼2 cm-1 for the given parameters. About 5-10 mg of the catalyst was placed into a high temperature in situ cell (Linkam TS-1500) with a quartz window and the


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spectrums were obtained under ambient conditions. The spectral acquisition time employed was 5 scans of 5 seconds/scan for each spectrum. System alignment was verified using a silica reference standard line at 520.7 cm-1. 2.4 UV-Vis NIR Diffuse Reflectance Spectroscopy (DRS). Ultra Violet-visible-Near Infrared (UV-vis-NIR) diffuse reflectance spectroscopy (DRS) was utilized to obtain the optical edge energy, Eg, values for the fresh photocatalysts. Spectra were obtained using a Varian Cary 5E UV-vis spectrophotometer with a diffuse reflectance attachment (Harrick Praying Mantis Attachment, DRA-2). The finely ground powder catalyst samples (~20 mg) were loaded into an in situ cell (Harrick, HVC-DR2) and measured in the 200-800 nm spectral region with a magnesium oxide reflectance standard used as the baseline. A filter (Varian, 1.5ABS) was employed to minimize the background noise. A magnesium oxide white reflectance standard baseline was collected under ambient conditions. Determination of the Kubelka-Munk function, F(R∞), was obtained from the UV-vis DRS absorbance and processed with Microsoft Excel software. The UV-vis edge energy (Eg) was determined by finding the intercept of the straight line in the low-energy rise of a plot of [F(R∞)hν]1/n, where n =0.5 for the direct allowed transition versus hν, where hν is the energy of the incident photon.23,24 2.5 High Resolution X-ray Photoelectron (HR-XPS) Spectroscopy. The HR-XPS spectra of the fresh and used photocatalysts were obtained on a Scienta ESCA 300 spectrometer equipped with a 300 mm hemispherical electrostatic analyzer and a monochromatic Al Kα X-ray source with energy of 1486.6 eV generated from a rotating anode. This allows for improved chemical selectivity by narrowing the spectral peaks of elements and greatly reducing the spectral background signal compared to conventional XPS spectrometers. Each spectrum was calibrated using a binding energy (BE) value of 285.0 eV for carbon in the


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C1s region. The atomic concentration ratios were calculated by correcting the measured peak area ratios with relative sensitivity factors employed in the Casa XPS software version 2.3.15. 2.6 High Sensitivity Low Energy Ion Scattering (HS-LEISS) Spectroscopy. Analysis of the outermost surface layer of the fresh and used photocatalysts was obtained on the Qtac100 HS-LEIS Spectrometer (ION-TOF) equipped with a highly sensitive double toroïdal analyzer, 3000 times higher sensitivity than conventional LEIS spectrometers, which allows for static depth profiling. Charge compensation was achieved using an electron flood gun. The photocatalyst samples were first gently cleaned with atomic oxygen to remove surface hydrocarbon contamination from the atmosphere prior to being transferred inside the analysis chamber. The HS-LEIS spectra were taken using both 4000 eV 4He+ with 7245 pA current and 3000 eV

20

Ne+ with 2959 pA current as ion sources. TOF mass filters were also utilized for

spectra obtained with Ne+ as an ion source for reduced flux background at low kinetic energies. For depth profiling, the surface was sputtered by Ar+ gas at 500 eV at a sputter yield of 1x1015 ions/cm2 corresponding to ~ 1 surface layer (0.4 nm )/cycle. Metallic Rh and Cr standards were also analyzed for quantifying the elemental composition of the photocatalysts. 2.7 Photoluminescence (PL) Spectroscopy and Time-Resolved (TR) PL Spectroscopy. Spectrally resolved PL spectra and transient PL lifetime measurements of the fresh photocatalysts were conducted using a Ti:sapphire laser (Coherent Mira 900), tunable in the 6851000 nm spectra range, generating 5 ps pulses with a 76 MHz repetition rate, pumped with a frequency-doubled Nd:YVO4 laser (Coherent Verdi V-18). The output of the laser was frequency doubled using an ultrafast harmonic generator (Coherent 5-050). To perform the luminescence measurements, the excitation light at 400 nm was directed toward a microscope of a tunable micro-, macro-Raman/photoluminescence system (Jobin Yvon Horiba, T6400) and was focused


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using a long distance objective (50x, N/A=0.5) onto a sample to a spot size of ~ 2 µm. The photocatalyst sample was placed into a high temperature in situ microscopy stage (Linkam, TS1500) and pretreated as follows: The samples were heated at 10 oC/min to 673 K in flowing 10% O2/N2 (30 sccm) to remove water, since moisture causes quenching of the PL signal,25 and to fully oxidize the samples. Upon cooling to room temperature in flowing inert gas (N2, 30 sccm), the PL decay measurements were made. The luminescence light was collected through the same objective in backscattering geometry and focused onto a slit of the triple-monochromator equipped with a fast gated intensified charge coupled device (ICCD) camera collecting in the 350-900 nm range (LaVision, Picostar HR12). The ICCD camera was gated using a sequence of 76 MHz pulses propagating with a variable delay relative to the original train of trigger pulses (76 MHz) from a photodiode in a Ti:sapphire laser. The minimum gate width was 300 ps and the maximum delay was defined by the laser repetition rate (~13.2 ns). The laser energy at the sample was maintained at approximately 1.6 mW to prevent photo-degradation of the photocatalyst sample. The grating was set to monitor decay centered at emission wavelengths from 500 to 700 nm. The grating setting used to monitor PL lifetimes decay was based on the PL peak maximum from the PL spectra. Experimental decay curves were then fit to a double firstorder exponential decay model to account for an observed “fast” (t1) and “slow” (t2) components:26,27

 −t   −t  y = A1 exp   + A2 exp   + y0  t1   t2 

(1)

2.8 In Situ Ambient Pressure X-ray Photoelectron (AP-XPS) Spectroscopy. The surface chemistry of the photocatalyst during photocatalytic water splitting was investigated using an in-house ambient-pressure X-ray photoelectron spectrometer equipped with


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a monochromated Al Kα source (Microfocus 600, Specs) and hemispherical energy analyzer (Phoibos 150, Specs).28 A certain amount of catalyst powder was dispersed in water and a small droplet of the mixture was dripping onto an Au foil that was deliberately roughened using a SiC knife for an increased adhesion, followed by drying in air at 50 oC to evaporate all water. Such preparation allowed for the formation of a thin layer of catalyst on the Au substrate to effectively minimize surface charging. The Au foil was loaded on a sample holder and transferred onto a sample stage located in a designed reaction cell. The reaction cell has a glass window that allows visible light outside the instrument to penetrate through and reach the sample stage. A 1000 W Xenon arc lamp (Newport) was used as light source. Deionized water was contained in a sealed glass tube attached to the gas inlet controlled by a leak valve. Prior to the introduction of water vapor, the deionized water was degassed for several times to ensure no dissolved residue gases. 0.2 mbar of water vapor was introduced to the reaction cell monitored by ion gauges at the inlet and outlet of the reaction cell as well as an online mass spectrometer. Photoemission features of Rh 3d, Ga 2p, Ga 3d, Cr 2p, N 1s, and O 1s were tracked during light-on and light-off conditions. All spectra were calibrated using Au 4f7/2 at 84.0 eV. Spectral analysis was performed using CasaXPS software. 3. Results and Discussion. 3.1 Bulk Structure and Morphology (STEM and STEM-EDS) Representative scanning transmission electron microscopy (STEM) images of the preand post-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalysts is shown in Fig. 1A and Fig. 1B, respectively. These micrographs show the photocatalyst particles are >100 nm in size and covered by smaller ~10 nm particles of the deposited oxides of rhodium and chromium.


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No significant change in particle size and overall morphology was observed between the pre- and post-reaction catalysts.

Figure 1. STEM micrographs of (A) pre-reaction and (B) post-reaction supported (Rh2yCryO3)/(Ga1−xZnx)(N1−xOx) yCryO3)

photocatalysts. In Fig. 1A, the diameter of one of the deposited (Rh2-

NPs on the oxynitride support is indicated. The contrast and brightness in Fig. 1A has

been adjusted to make the deposited surface NPs clearer in the image. The scale bar of 100 nm applies to both STEM images. The STEM micrographs and STEM electron dispersive spectroscopy (EDS) maps for the prereaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst are shown in Fig. 2. Similar to Fig. 1A-B, Fig. 2A shows the presence of large (>100 nm) particles with smaller (~10 nm) particles deposited on the surface. The Rh STEM-EDS and Cr STEM-EDS maps are shown in Fig. 2B and Fig. 2C, respectively, and the overlapping Rh and Cr EDS maps showing the distribution of Rh and Cr on the oxynitride support are depicted in Fig. 2D. Although there is some correlation of the Rh and Cr signals indicating that they are found at the same spot


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(presumably as (Rh2-yCryO3) NPs), the Rh and Cr signals are not strongly correlated, indicating that Rh and Cr are not always present at the same location (presumably as separate Rh2O3 and Cr2O3 NPs). The Cr EDS signal (Fig. 2C) does correlate with the ~10 nm diameter surface particles, suggesting that the ~10 nm particles are predominately CrOx. Similar STEM-EDS data for the post-reaction catalysts are shown in Fig. S1. By comparing the pre- and post-reaction STEM-EDS maps of the catalysts, there does not appear to be any significant difference in the spatial distribution of Rh or Cr before and after the photocatalytic reaction. The STEM-EDS data in Fig. 2 and Fig. S1 suggests Rh and Cr are distributed differently on the larger (Ga1−xZnx)(N1−xOx) oxynitride support (>100 nm): CrOx is concentrated in particles 510 nm in diameter and Rh is relatively uniformly distributed across both the support and the CrOx particles. STEM-EDS data is inconclusive as to whether or not the Rh distribution is truly uniform, or dispersed as small (< 2 nm diameter) oxide Rh particles over both the oxynitride support and CrOx particles. These findings are in contrast to the previous study,18 which showed only the presence of larger 5–20 nm (Rh2-yCryO3) NPs using HR-TEM, and which were posited as the photocatalytic active sites. Because the current STEM-EDS data shows that the oxides of Rh and Cr can also be distributed as separate dispersed species on the surface of the (Ga1−xZnx)(N1−xOx) support, such RhOx and CrOx surface species cannot be ruled out as possible photocatalytic active sites.


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Figure 2. STEM-EDS maps of pre-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalysts: (A) STEM image of the catalyst, (B) red dots indicate the Rh EDS signal map,


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(C) green dots indicate the Cr EDS signal map and (D) the overlaid Rh + Cr EDS map. Scale bar of 20nm applies to all four micrographs. 3.2 Bulk Molecular and Electronic Structure The Raman spectrum of the synthesized supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst is dominated by the vibrations from the (Ga1−xZnx)(N1−xOx) phase (see Fig. S2). Raman bands from the supported RhOx and CrOx components are not detected due to their relatively small quantity and possible overlap with the strong vibrations from the dominant (Ga1−xZnx)(N1−xOx) support phase. Although Raman spectroscopy does not provide information about the supported Rh2-yCryO3 mixed oxide and RhOx/CrOx NPs, it does reveal that the bulk molecular structure of the (Ga1−xZnx)(N1−xOx) support is not modified by the addition of the Rh2yCryO3

NPs.

The bulk band gap energy (Eg), as determined by UV-vis spectroscopy, for the (Ga1−xZnx)(N1−xOx) is 2.6 eV (477 nm) and falls in the reported range of 2.4-2.8 eV (442 - 517 nm) for visible light active (Ga1−xZnx)(N1−xOx) materials.15 The lower Eg value confirms that the (Ga1−xZnx)(N1−xOx) oxynitride phase is a solid solution and is not just a physical mixture of Ga2O3, ZnO and GaN. Deposition of Rh2-yCryO3 mixed oxide and RhOx/CrOx NPs on the (Ga1−xZnx)(N1−xOx) support essentially does not perturb the overall band gap energy of the composite photocatalyst. 3.3 Atomic Composition of Surface Region (~1-3nm) The atomic composition of the surface region (~1-3nm) is obtained from the XPS survey spectra for the bulk (Ga1−xZnx)(N1−xOx) and the supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx ) photocatalysts are shown in Table 1. Residual Na and Cl contaminants from the Na3RhCl6•2H2O precursor were found in the surface region of the pre-reaction supported (Rh2-


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photocatalyst. After being used as a photocatalyst for the splitting of

water, the surface Na contaminant is no longer present and the surface Cl concentration decreased by more than 50% most probably because of their aqueous solubility. These contaminants have been shown to not influence the photoactivity of the catalysts.18 The Cr 2p and Rh 3d transitions for the pre- and post-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalysts from XPS are presented in Fig. S3 and reveal the presence of only Cr3+ and Rh3+ species in the surface region. Table 1. XPS surface region atomic composition (~1-3nm) of (Ga1−xZnx)(N1−xOx) and supported (Rh2-yCryO3) /(Ga1−xZnx)(N1−xOx ) photocatalysts.

(Ga1−xZnx)(N1−xOx ) Element O 1s N 1s Ga 2p 3/2 Zn 2p 3/2 Cr 2p 3/2 Rh 3d Cl 2p Na 1s

16% 60% 22% 1.7% 0.0% 0.0% 0.0% 0.0%

(Rh2-yCryO3) /(Ga1−xZnx)(N1−xOx ) Pre-Reaction Post-Reaction 24% 50% 11% 2.4% 5.2% 0.8% 3.5% 3.4%

31% 49% 11% 1.9% 5.1% 1.1% 1.5% 0.0%

3.4 Atomic Composition of Outermost Surface Layer (~0.3nm) The atomic composition of the outermost surface layer (~0.3 nm) and the layers beneath the topmost surface layer of the photocatalysts were analyzed by HS-LEIS spectroscopy depth profiling.

The

HS-LEIS

depth

profiles

for

the

pre-reaction

supported

(Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst using both 4He+ and 20Ne+ as ion gas sources are presented in Fig. 3A and Fig. 3B. The HS-LEIS resolvable peaks are for the O, Na, Cl, Cr, Zn/Ga, and Rh in the outermost surface layer with the 4He+ gas ion source are shown in Fig. 3A.


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The absence of a peak for N is possibly due to the slow velocity of the 4He+ gas ions which makes it difficult to obtain good elemental sensitivity for low mass elements like N without the use of 3He+ as the ion gas source.29 The evolution of the HS-LEIS signals for Cl, Na and Zn/Ga normalized against the relatively constant O signal during the dynamic depth profiling are shown in Fig. S4. The Cl and Na contaminants slightly decrease with depth profiling reflecting some surface enrichment of Na and Cl. The Zn/Ga signal significantly increases during the depth profiling as expected for the bulk nature of the (Ga1−xZnx)(N1−xOx). The HS-LEIS signals for Cr and Rh are more easily resolved with the heavier 20Ne+ gas ions as seen in Fig. 3B. The Rh and Cr depth profiles (Inset Fig. 3B) show that the outermost surface layer contains slightly more Rh than Cr, and that the Cr concentration markedly increases while the Rh concentration decreases with depth profiling. Consequently, the Cr/Rh ratio is initially ≈ 1, with Cr/Rh = 3 expected from the nominal catalyst loading, and markedly increases to Cr/Rh ≈ 5 at the end of the depth profile. These concentration profiles reveal that Rh is surface segregated, while Cr becomes more prevalent with increasing depth from the surface.


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A

10

Na

Intensity (cts/nC)

8

O

Cl 6

Cr

4

Zn/Ga 2

Rh 0

De p

th

(n m

)

0

5.6

1000

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2500

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Energy (eV)

Cl

B

20

Cr

5 Cr Rh

4

15 3 2

10 1 0 0

1

2

3 4 Depth (nm)

5

6

5

Intensity (cts/nC)

Zn/Ga

% of Pure Metal Monolayer

Rh

(n m )

0 0

De pt h

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5.6 500

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2500

Energy (eV)

Figure 3. HS-LEIS depth profile for the pre-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst using (A) He+ ion gas and (B) Ne+ ion gas. Inset shows Rh and Cr depth profiles. (Arrow indicates increasing depth from the surface) The HS-LEIS depth profiles for the post-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst are presented in Fig. 4. A very small shoulder for N is visible in the depth profile of Fig. 4A indicating that some N becomes exposed on the surface of the supported (Rh2yCryO3)/(Ga1−xZnx)(N1−xOx)

photocatalysts during the water splitting reaction. The Na and Cl

contaminants are not detected during HS-LEIS depth profiling, unlike the XPS analysis of the


16

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surface region, which suggests that the Cl contaminant is not present on the outermost surface layers of this used photocatalyst. The Rh and Cr depth profiles for the post-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst (Inset Fig. 4B) reveal comparable amounts of Rh and Cr on the outermost surface and the Cr concentration increases while the Rh concentration decreases

with

depth

profiling


as

found

for

the

pre-reaction

supported

(Rh2-

photocatalyst. The similar depth profile trends for the Rh and Cr in

the pre- and post-reaction supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalysts suggests that the supported (Rh2-yCryO3) NPs are unaltered by the photocatalysis reaction conditions.

A

10

O

N

Cr 6

4

Zn/Ga

2

Intensity (cts/nC)

8

Rh

De pt h

(n m )

0 0

5.6

2000

2500

3000

Energy (eV)

3500

4000

B

30 5

Zn/Ga

Cr Rh

25

4 3

20 2

15

1 0

Cr

0

1

2

3

4

5

6

10

Depth (nm)

5

Intensity (cts/nC)

1500

Rh

(n m )

0 5.6

De pt h

1000

% of Pure Metal Monolayer

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

500

1000

1500

2000

2500

Energy (eV)


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Figure

4.

HS-LEIS


Depth

Profile

for

Page 18 of 33

the

post-reaction

supported

(Rh2-

photocatalyst using (A) He+ ion gas and (B) Ne+ ion gas. Inset shows

Rh and Cr depth profiles. (Arrow indicates increasing depth from the surface)

3.5 Dynamics of Photoexcited Electrons and Holes The population of electron/hole recombination centers (self-trapped electrons, oxygen vacancies, defect sites, impurities, reduced metal ions, etc.) in the bulk phase of excited photocatalysts was measured with photoluminescence (PL) emissions spectroscopy.25,30 The addition of the (Rh2-yCryO3) mixed oxide NPs and pure oxides (RhOx and CrOx) greatly diminishes the PL emissions of (Ga1−xZnx)(N1−xOx), as shown in Fig. 5. The diminished emission is interpreted as a decrease in electron/hole recombination in the presence of the supported oxide NPs because the trapping of electrons prevents recombination with holes in the bulk phase to produce emission. This suggests that the supported (Rh2-yCryO3) mixed oxide and RhOx/CrOx NPs are efficient electron traps that minimize electron and hole recombination and, thus, allow for their consumption at the surface for photocatalytic water splitting.25,31 In Fig. 5, the much lower PL emissions intensities for bulk ZnO and Ga2O3 are a consequence of the 400 nm excitation energy being lower than their optical band gap energy values (388 and 264 nm). The optical absorption edge values for bulk ZnO and Ga2O3 are in the UV-range, thus, the lack of PL emissions is due to the use of visible light irradiation causing a decreased population of excited electrons being able to recombine.


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(Ga1-xZnx)(N1-xOx)

Intensity (Counts)

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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(Rh2-yCryO3)/(Ga1-xZnx)(N1-xOx)

β-Ga2O3 x2

ZnO x2

500

550

600

650

700

750

Wavelength (nm)

Figure 5. PL spectra of bulk β-Ga2O3, ZnO and oxynitride catalysts at 400 nm excitation. Time-resolved photoluminescence emissions (TR-PL) spectroscopy was applied to determine the lifetimes of these excited electrons. The TR-PL emission decay curves for the bulk ZnO, Ga2O3 and oxynitride photocatalysts are plotted in Fig. S5. The TR-PL emission decays for all the photocatalysts were fitted with a double first-order exponential decay model that is based on two different species of electrons decaying at different rates and the fit parameters are given in Table 2. The parameters t1 and A1 refer to decay constant and amplitude of the “fast” component of electron decay while t2 and A2 refer to the decay constant and amplitude of the “slow” component of electron decay. TR-PL spectroscopy is able to measure the lifetime of the excited electrons/holes from the decay emissions parameters based on the “slow” t2 component and A2 where the ratio A1 + A2 is proportional to the relative population of these long lived electrons

with slow emissions decay.32,33


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The (Ga1−xZnx)(N1−xOx) support phase is found to have a higher relative population of long lived electrons compared to the bulk ZnO and Ga2O3 phases (see Fig. S5). The addition of (Rh2yCryO3)

mixed oxide and RhOx/CrOx NPs were able to prolong the lifetimes of excited

electrons/holes as shown in Table 2 from the increased lifetimes not only from the “slow” t2 component of decay but also for the “fast” t1 component. The relative population of long-lived excited electrons was also found to be enhanced by the surface (Rh2-yCryO3) mixed oxide and RhOx/CrOx NPs. PL and TR-PL spectroscopy demonstrate that the (Ga1−xZnx)(N1−xOx) support with surface modified (Rh2-yCryO3) and RhOx/CrOx NPs possess all of the measurable desired properties for an efficient photocatalyst system. The (Rh2-yCryO3) and RhOx/CrOx NPs decreased the recombination of electron/holes in the bulk by transferring them to the surface resulting in longer lifetimes and higher populations of the excited electrons and holes. This greatly increases the probability for the photocatalytic splitting of H2O into H2 and O2 at the surface. It has been considered that CrOx facilitates charge transfer from the (Ga1−xZnx)(N1−xOx) support to the photocatalytic active sites.34 The HS-LEIS depth profiling revealed that the CrOx is more concentrated than the RhOx towards the bulk (Inset Fig. 3B) suggesting that the CrOx may indeed be involved in electron charge transfer.


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Table 2. Eg values, peak emission wavelengths, and decay fit parameters of the catalysts at 400 nm excitation. The decay parameters displayed are those found in the PL spectra taken in the emission range of the peak value. Excitation 400 nm Catalyst Eg Peak t1 (ps) (eV) Maximum fast (nm) 4.7 713 4.0 Ga2O3 ZnO

3.2

637

(Ga1−xZnx)(N1−x Ox ) (Rh2-yCryO3) /(Ga1−xZnx)(N1− xO x )

2.6

657

2.5

725

y=A1*exp(-t/t1)+A2*exp(-t/t2)+y0 A1 fast t2 A2 A 1/ A 2/ (ps) slow (A1+A2) (A1+A2) slow 1.3 x 10

5

7.9 x 2 10 9.4 x 2 10

2.9 x 10

1

1.8 x 10

2

1.1 x 3 10

2.4 x 10

1

2.4 x 3 10 6.8 x 3 10 7.4 x 3 10

1.2

0.99

0.94 x 10

7.6

0.79

0.21

57

0.76

0.24

7.5 x 3 10

12

0.66

0.34

-6

3.6 In Situ Surface Composition during Photocatalysis The photoemission features of the supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst exposed to 0.2 mbar water vapor under “light-on” and “light-off” conditions are presented in Fig. 6. The photoemission peaks of Ga 2p, Ga 3d, Rh 3d, Cr 2p and O 1s (the Zn 2p signal is too weak for detection) are shifted to lower binding energies during light-on conditions, which indicates that Ga, Rh, Cr and O atoms become partially negatively charged. In contrast, the XPS peak of N 1s does not shift. For the (Ga1−xZnx)(N1−xOx) support, DFT calculations show that the bottom of the conduction band is composed of Ga 4s and 4p and the top of the valence band is mainly composed of N 2p and Zn 3d orbitals.14,35,15 Thus, when the light is on, electrons at the valence band will be excited to the conduction band of the (Ga1−xZnx)(N1−xOx) support, which leads to a rich electron density at Ga atoms and an accumulation of positive holes at N atoms and possibly Zn atoms. In the presence of water vapor during photocatalysis, however, the N atoms


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become neutrally charged showing that positive charges on N atoms from light excitation are consumed completely and are involved in the oxidation reaction converting H2O to O2. It cannot be ruled out that Zn also plays a role in the oxidation of H2O to O2 since the valence band also consists of Zn. It has been claimed in the literature that CrOx sites can also play a role in the oxidation of H2O to O2 on various oxide supports (Ba5Ta4O15, Ta2O5, and Ga2O3),36,37 however those photocatalyst reactions were operated under UV-irradiation and may not be applicable for our reaction conditions (visible light and oxynitride support). The addition of (Rh2-yCryO3) mixed oxide NPs and dispersed surface RhOx and CrOx sites, will be able to trap the excited electrons that induce partially negative charges at Rh, Cr and O atoms during photocatalysis which is observed. The Rh 3d peak shifts less than the Cr 2p and O 1s peaks which suggests that excited electrons on the Rh are responsible for hydrogen evolution.


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Figure 6. Photoemission features of Ga 2p (A), Ga 3d (B), Rh 3d (C), Cr 2p (D), N 1s (E), and O 1s (F) of supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst in 0.2 mbar H2O at lighton/light-off condition. 3.7 Model of the Supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) Mixed Oxide Photocatalyst The new insights from application of the above modern characterization measurements suggest a revised model of the supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst as schematically depicted in Fig. 7. The surface region of the (Ga1−xZnx)(N1−xOx) oxynitride support exists as a thin GaZnOx film with a low concentration of surface N. The HS-LEIS surface analysis of the outermost surface layer, however, was only able to detect N on the outermost surface layer of the (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst after the reaction (Fig. 4A). The HR-XPS surface region for the supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst was also found to be enriched with O relative to N (surface ~0.5 vs. bulk ~ 0.14), which is not too surprising since the deposited (Rh2-yCryO3) mixed oxide NPs, RhOx/CrOx NPs and surface CrOx and RhOx species do not possess any N. The in situ AP-XPS data shows that the minority N atoms on the outmost surface layer are involved in O2 evolution from the photocatalyst. The increased Ga/Zn concentration with depth profiling indicates that Ga and Zn are not surface enriched (see Fig. 3 and 4). The lower XPS ratio of Ga/Zn in the surface region, ~5, compared to the bulk, ~7,


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suggests that some Ga is depleted relative to Zn in the surface region of the oxynitride support (see Table 1). Unfortunately, the in situ AP-XPS was unable to measure a signal for Zn in the surface region that can also be involved in O2 evolution for the photocatalyst. The STEM-EDS showed that dispersed RhOx species can exist on both the GaZnOx as well as CrOx NPs. Experiments for the water splitting half reaction using methanol as the sacrificial reagent showed that (Ga1−xZnx)(N1−xOx) could not evolve H2 only when Rh or (Rh2-yCryO3) was added could H2 be produced.38 Thus, the dispersed RhOx species are the active sites for H2 evolution. The role of the CrOx NPs appears to be to facilitate charge transfer between the bulk (Ga1−xZnx)(N1−xOx) and the photocatalytic active sites. The current HS-LEIS surface analysis also reveals that the optimal surface composition for the (Rh2-yCryO3) mixed oxide and RhOx/CrOx NPs, as well as the surface CrOx and RhOx species, consists of a ~ 1:1 ratio of Rh:Cr on the outermost surface layer for the supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalysts. The revised model in Fig. 7 is quite different than that previously proposed for this photocatalyst system where (Rh2-yCryO3) NPs were directly supported on the oxynitride (Ga1−xZnx)(N1−xOx) support.18


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Figure 7. Schematic model of supported (Rh2-yCryO3)/(Ga1−xZnx)(N1−xOx) photocatalyst system and its photocatalytic splitting of water.

ASSOCIATED CONTENT Supporting Information. Supplemental text: Raman spectroscopy, STEM-EDS image, Raman spectra, HR-XPS spectra, HS-LEIS depth profile, and PL decay curves. AUTHOR INFORMATION


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Corresponding Author *[email protected] Present Addresses †Present address: Department of Chemistry, University of Wisconsin-Madison, 1101 University Ave., Madison, WI 53706, USA. ‡Present address: Toyota Motor Engineering & Manufacturing North America, Inc., 1555 Woodridge Ave., Ann Arbor, MI 48105, USA. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was financially supported by the Department of Energy grant: DOE-FG0293ER14350. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support by the Department of Energy grant: DOE-FG02-93ER14350. The PL and TR-PL measurements were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. A.G. acknowledges support through Laboratory Directed Research and Development funds at


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Brookhaven National Laboratory. The assistance of Dr. A. Miller at Lehigh University in obtaining and interpreting the HR-XPS and HS-LEIS data is also gratefully acknowledged.


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SYNOPSIS


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Anatomy of a Visible Light Activated Photocatalyst for Water Splitting

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