Ag3PO4 Oxygen Evolution Photocatalyst Employing Synergistic Action

Sep 4, 2012 - This increase in activity is attributed to a combination of depletion of the ..... Improving the photocatalytic performance of silver ph...
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Ag3PO4 Oxygen Evolution Photocatalyst Employing Synergistic Action of Ag/AgBr Nanoparticles and Graphene Sheets Yang Hou, Fan Zuo, Quan Ma, Chen Wang, Ludwig Bartels,* and Pingyun Feng* Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *

ABSTRACT: A graphene-supported Ag3PO4/Ag/AgBr water oxidation photocatalyst was prepared by a photoassisted deposition−precipitation reaction, followed by a hydrothermal treatment. The composite photocatalyst exhibits double the O2-production activity than that of bare Ag3PO4 under visible light irradiation. Moreover, it exhibits enhanced activity in comparison to unsupported Ag3PO4/Ag/AgBr, to graphenesupported bare Ag3PO4 powder as well as to Ag/AgBr powder. This increase in activity is attributed to a combination of depletion of the conduction band of the as-synthesized ndoped Ag3PO4 material and a downshift of the Ag3PO4 valence band due to the pinning of its conduction band at the silver Fermi level, a process that is assisted by charge transfer and distribution onto the graphene support.

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INTRODUCTION The photocatalytic splitting of water into H2 and O2 has attracted considerable attention because the process presents an avenue for direct generation of an energy carrier that is easily stored and transformed into electric energy.1−6 Crucial for its utility is the yield that can be achieved with the solar spectrum reaching the earth’s surface. As the majority of the solar energy flux is in the visible spectral range, the efficient use of visible light photons (as opposed to UV radiation) is tantamount. This situation poses a significant technological challenge, and the opportunity for catalysis to play an important role. Although the potential difference required for water splitting in a fourelectron process is only 1.23 eV (corresponding to infrared photons), kinetic considerations require a significant overvoltage:7,8 in addition to the reaction enthalpy, the elementary steps of water splitting have activation energies, which need to be overcome rapidly once a photoexcitation generates an exciton in the photocatalyst, so as to compete efficiently with wasteful exciton recombination. This poses two avenues for the improvement of photocatalysts: stabilization of the exciton to make it available longer for high reaction yield and optimization of the photocatalyst band alignment to maximize each photogenerated hole’s oxidative power. Both of these are realized in the hybrid compound described in this study. Activation of hydrogen evolution is the more facile halfreaction of water splitting, and it has been explored widely.9−13 In contrast, O2 evolution is more challenging, requiring a sequence of multiple reaction steps transferring a net of four electrons per oxygen molecule generated. Optimization of the oxygen evolution reaction is the lynchpin of efficient overall water splitting, which renders it the key target for engineering the photocatalyst band alignment to offer the largest overvoltage possible while maintaining overall energy efficiency. © 2012 American Chemical Society

Recently, Ag3PO4 has been found to be one of the very few materials that exhibit excellent visible-light-driven photooxidative capability for O2 evolution from water, even if it cannot directly split water to release H2 due to its conduction band position.14 Here, we show that the addition of metal nanoparticles in combination with wide delocalization of negative charge on reduced graphene oxide (RGO) sheets can double its photocatalytic yield. Individually, the use of metal nanoparticles15−19 and graphene20−28 has shown advantages in prior studies. In this paper, we explore concerted and synergistic action of these two materials. Through application of numerous characterization techniques, we attempt to shed a detailed light on the underlying fundamental processes. In particular, we propose that the as-synthesized n-doped Ag3PO4 nanoparticles3 are denuded of the majority of charge carriers by transfer to the Ag nanoparticles, eliminating the availability of extra conduction-band electrons for recombination with the photogenerated holes (resulting in increased hole availability for water oxidation). This leads to the pinning of the Ag3PO4 conduction band at the silver Fermi level, shifting the Ag3PO4 valence band edge downward and rendering the photogenerated holes more active in water oxidation. Charge transfer to the silver creates a substantial negative charge on the very small metal nanoparticles, limiting their beneficial effect on the photocatalyst. Charging of the nanoparticles can be reduced by distribution of the charge onto RGO sheets, further lowering the Ag3PO4 valence band position. Under irradiation with >420 nm light, the effectiveness of these processes manifests itself in Received: April 4, 2012 Revised: August 8, 2012 Published: September 4, 2012 20132

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Figure 1. (a) Photocatalytic O2 evolution under visible light irradiation (wavelength > 420 nm) from a 0.05 M aqueous AgNO3 solution over (from bottom to top) bare Ag/AgBr (0 μmol h−1), Ag3PO4 (38 μmol h−1), Ag3PO4/RGO (43 μmol h−1), Ag3PO4/Ag/AgBr (48 μmol h−1), and Ag3PO4/ Ag/AgBr/RGO (76 μmol h−1). Rates are based on a linear fit of the performance over the first 4 h; subsequently, the photocatalytic oxygen yield tapers off. (b) Transient photocurrent responses of electrodes functionalized with the Ag3PO4-based materials in the same order (bottom to top) as in panel (a). Measurements proceeded in a 0.01 M Na2SO4 aqueous solution under visible light irradiation (wavelength > 420 nm, I0 = 64 mW cm−2) at 0.5 V bias vs SCE.

photometer operating in diffuse mode. Infrared spectra were collected with a Bruker Equinox 55 Fourier transform spectrophotometer at a resolution of 4 cm−1 with 40 scans per spectrum. Pressed pellets diluted in KBr powder were used as samples. Photoluminescence (PL) spectra were measured on a Spex Fluorolog-3 at room temperature using an excitation wavelength of 325 nm. A Dilor XY microspectrometer was used to record the Raman spectrum of the samples at a 532 nm excitation wavelength. Thermogravimetric analysis (TGA) was carried out using a TGA-Q500 thermoanalyzer with a heating rate of 5 °C min−1 in air. X-ray photoelectron spectroscopy (XPS) used a Scienta R3000 analyzer and a Mg Kα source. Photoelectrochemical Measurements. Photocurrent densities were measured using a Solartron SI 1287 electrochemical interface analysis instrument operated in a standard three-electrode configuration with a Ti electrode modified with Nafion/ethanol/Ag3PO4/Ag/AgBr/RGO as the photoanode (an effective area of 2 cm2), a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Na2SO4 (0.01 M, 30 mL) purged with N2 was used as the electrolyte. A 300 W xenon lamp (Oriel) with a filter to remove light of wavelengths below 420 nm was used as the visible light source to provide a light intensity of 64 mW cm−2. Photocatalytic O2 Production. Photocatalytic O2-production experiments were conducted in a sealed circulation system containing 0.1 g of sample suspended in 120 mL of 0.05 M AgNO3 aqueous solution (sacrificial reagent) under magnetic stirring. A 300 W Xe lamp with a 400 nm edge filter (Newport Corp.) was used as a visible light source. The amount of evolved O2 was determined by gas chromatography (Shimadzu GC-8A) using a thermal conductivity detector. The quantum yield was obtained as 4× the number of O2 molecules evolved divided by the number of incident photons using a 300 W Xe lamp with a 420 nm band-pass filter (Newport Corp.)31

a 1.3 times higher O2 evolution yield of Ag3PO4/Ag/AgBr/ RGO as compared with that of Ag3PO4/Ag/AgBr and a 2 times higher yield compared with that of bare Ag3PO4.

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EXPERIMENTAL SECTION Preparation of Ag3PO4/Ag/AgBr/RGO Hybrid Composite. Graphene oxide (GO) was synthesized through chemical exfoliation of graphite powder via a modified Hummers’ method.29 In a typical synthesis of Ag3PO4/Ag/AgBr/RGO, 20 mg of GO was first dissolved in 40 mL of water by ultrasonic treatment for 1 h to obtain a suspension (0.5 mg mL−1). A 1.2 g portion of AgNO3 was then added and stirred for another 30 min, resulting in a homogeneous suspension. Subsequently, 0.576 g of NaH2PO4·2H2O was slowly added and a golden yellow precipitate formed immediately. A 0.2 g portion of cetylmethylammonium bromide (CTAB) was added quickly to the mixture, followed by magnetically stirring for 24 h at room temperature. The above solution was further mixed with 20 mL of methanol and irradiated for 20 min with filtered light (wavelength > 420 nm) from a 300 W xenon lamp (Ag3PO4/ Ag/AgBr/GO). The resultant mixture was then transferred to a Teflon-lined autoclave and heated at 180 °C for 6 h to achieve reduction of GO. A dark green precipitate (Ag3PO4/Ag/AgBr/ RGO) resulted, which was centrifuged and then washed with distilled water and ethanol before being dried at 70 °C in air overnight. The metallic Ag nanoparticles of the Ag3PO4/Ag/ AgBr/RGO hybrid are formed from AgBr by light-induced chemical reduction.30 Additionally, a small part of Ag3PO4 is possibly decomposed to form metallic Ag during the process as most of the Ag-based materials are photosensitive and unstable. In the control experiment, Ag3PO4/Ag/AgBr and Ag3PO4/ RGO were prepared by a similar method without RGO and Ag/AgBr, respectively. Detailed information can be found in the Supporting Information. Characterization. The morphology and composition of the Ag3PO4/Ag/AgBr/RGO hybrid was characterized using a Philips FEI XL30 scanning electron microscope (SEM) with a PGT-IMIX PTS EDX system and a Tecnai T12 transmission electron microscope (TEM). Powder X-ray diffraction (XRD) data were collected using a Bruker D8-Advance powder diffractometer operating at voltages and currents of 40 kV and 40 mA, respectively, for Cu Kα radiation (λ = 1.5406 Å). A Brunauer−Emmett−Teller (BET) specific surface area analysis was performed using a Micromeritics ASAP 2020 surface area and pore size analyzer. The UV−visible absorption spectra were recorded on a Shimadzu UV-3101PC UV−vis-NIR spectro-

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RESULTS The photocatalytic performances of Ag3PO4, Ag3PO4/Ag/ AgBr, Ag3PO4/RGO, and Ag3PO4/Ag/AgBr/RGO as well as bare Ag/AgBr powders were evaluated under irradiation with wavelength > 420 nm light, and Figure 1a compares the photocatalytic oxygen yield. All materials (Ag3PO4, Ag3PO4/ RGO, Ag3PO4/Ag/AgBr, and Ag3PO4/Ag/AgBr/RGO) show nearly constant rates of photogeneration of oxygen for at least the first 4 h of operation before gradually tapering off. The yield is 38 μmol h−1 for bare Ag3PO4, and it increases with the addition of either RGO or Ag/AgBr to 43 and 48 μmol h−1, 20133

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Figure 2. Model of the synergistic increase of photocatalytic activity of Ag3PO4 upon functionalization with Ag/AgBr and RGO.

Figure 3. SEM images of (a) as-prepared Ag3PO4 powder showing a particle size of ∼0.2 μm, (b) the RGO sheets, and (c) the complete Ag3PO4/ Ag/AgBr/RGO composite, which is shown schematically in (d). EDX measurements within the area of the blue box reveal particles of Ag3PO4 (e), AgBr (f), and pure Ag (g).

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DISCUSSION We propose a model to understand the increase in photocatalytic activity caused by addition of Ag/AgBr and RGO to Ag3PO4. Our discussion uses concepts of semiconductor physics, where effects of doping and formation of semiconductor/metal contacts have been explored in great detail. In particular, we attribute the improved O2 yield to two key factors: (a) depletion of the conduction band of the assynthesized n-doped Ag3PO4 by contact formation with the silver nanoparticles, leading to a longer lifetime of photogenerated holes, and (b) a downward shift of the Ag3PO4 valence band due to charge transfer to Ag and (subsequently) RGO, resulting in a higher water oxidation power. Figure 2 shows a schematic representation: in the absence of silver nanoparticles, Ag3PO4 has its conduction band Esc at 0.45 V and its valence band Esv at 2.85 V with respect to the standard hydrogen electrode,14,34 whereas silver’s standard electrochemical potential is EAgF is 0.8 V,35 ΔE = 0.35 V below Esc (Figure 2a). Ag3PO4 is n-doped as prepared, so the conduction band is partially occupied.36 Once the Ag3PO4 particles are decorated with Ag nanoparticles, it is energetically favorable for the Ag3PO4 conduction band electrons to transfer into the silver nanoparticles (Figure 2b). We will argue below that all conduction band electrons are transferred, thus aligning the Ag3PO4 band edge with the silver Fermi level. This is different from the standard picture of a Schottky barrier, where a majority of charge carriers are depleted exclusively in the immediate vicinity of the contact, leading to band bending. Because of this electron transfer, the silver particles are negatively charged and their Fermi level shifts up by ΔEpot. The addition of RGO allows delocalization of the transferred charge over a much larger volume than that of the silver nanoparticles, thus practically eliminating ΔEpot. Because of

respectively. Synergistic effects between Ag/AgBr and RGO double the yield of bare Ag3PO4 powder to 76 μmol h−1, which is significantly greater than that of most semiconductor photocatalysts as well.32 Additionally (Figure S1, Supporting Information), Ag3PO4/Ag/AgCl/RGO (prepared similarly to the AgBr-based compound) yields 51 μmol h−1, a value lower than that of Ag 3 PO 4 /Ag/AgBr/RGO under the same experimental conditions, but higher than that of the incomplete compounds, thus suggesting that the effect observed is not specific to AgBr only. Using a band-pass filter centered at 420 nm, we deduce for Ag3PO4/Ag/AgBr/RGO a quantum yield of 18.7% calculated as 1/4 of the photon flux per O2 molecule evolved, which is higher than previously reported quantum yield values for Ag3PO4.2 Bare Ag/AgBr powder showed no photocatalytic activity under the same experimental conditions, which is consistent with the results of previous work by Hashim.33 We also measured the photocurrent produced by our catalyst compositions. While not directly corresponding to the oxygen evolution yield, photocurrent can be measured more rapidly and thus can reveal transients of photoelectrochemical activity when the illumination is switched on and off. To measure the photocurrent density achievable with the different materials under visible light irradiation (wavelength > 420 nm), we fabricated photoanodes by drop-casting the samples dispersed in a Nafion/alcohol solution onto a Ti substrate. The photocurrent traces of Figure 1b show rapid response both at the start and at the end of illumination and an improved photocurrent density of the Ag3PO4/Ag/AgBr/RGO hybrid over all other composites. To illustrate the synergistic nature of RGO and Ag/AgBr loading, Figure 1b includes the data for both Ag3PO4/Ag/AgBr and Ag3PO4/RGO. 20134

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Figure 4. XPS spectra on Ag3PO4/Ag/AgBr/RGO: (a) Survey spectrum showing the presence of all catalyst components; (b) oxygen 1s level at 531.6 eV is in good agreement with literature data;42 (c) Ag+ 3d3/2 and 3d5/2 peaks with shoulders confirming the presence of Ag0.

pinning of the RGO Fermi level and Esc to the silver Fermi level, this causes a net downshift of the Ag3PO4 valence band by ΔE, which is expected to lead to a significant increase in its oxidation power (Figure 2c). For this scenario to be valid, a number of conditions must be met: in the following, we will show supporting experimental evidence acquired with a broad range of characterization methods. Most important for the understanding of the chargetransfer processes at the foundation of our model is knowledge about the particle size of the components of our composite (so that we can estimate how many electrons are involved in the charge-transfer process) and verification of their intermixing/ contact formation (so that the charge transfer can occur). Figure 3a shows SEM images of the Ag3PO4 particles produce in this study. They form a loose powder with a grain size of ∼0.2 μm. Successful exfoliation of the graphite is evidenced by SEM images of individual sheets resembling slightly crumpled paper (Figure 3b). In SEM images of the composite catalyst, the RGO sheets and the Ag3PO4 nanoparticles are intimately intermixed and can be made out clearly (Figure 3c). Close attachment of AgBr and Ag is confirmed by local electron spectroscopy (EDX), in which we find nearly pure Ag (g), AgBr (f), and Ag3PO4 (e) in immediate proximity (note that the silicon signal in (g) originates from the underlying support). From the synthetic procedure and the EDX beam diameter used to obtain exclusively silver, we estimate the Ag nanoparticles to be 2 × rAg ∼ 5−50 nm in diameter. TEM also shows Ag3PO4/Ag/AgBr nanoparticles well-scattered on the RGO sheets (Figure S2, Supporting Information). Attempts at high-resolution imaging are precluded by decomposition of the composite in the electron beam, a common property of silver salts.37 TGA analysis is employed to determine the content of RGO present in Ag3PO4/Ag/AgBr/RGO (Figure S3, Supporting Information). Assuming that the final residue is only Ag3PO4/Ag/AgBr after heating the composite to about 500 °C, we calculate that the content of RGO in the composite is 1.84%. This value is close to the expected value of 1.91% based on amounts of GO (20 mg) used and Ag3PO4/Ag/AgBr/ RGO (1.04 g) obtained. Measurement of the BET specific surface area yields 1.5025, 1.2301, and 14.3592 m2 g−1 for Ag3PO4, Ag3PO4/Ag/AgBr, and Ag3PO4/Ag/AgBr/RGO, respectively (Table S1, Supporting Information): as expected, we find a marked increase in surface area when RGO is introduced.38 XPS (Figure 4) confirms the presence of all of the elements in the composite. Moreover, decomposition of the silver 3d spectrum into components for Ag0 and Ag+ using literature binding energies39−41 of 373.3 and 367.4 eV for the Ag+ 3d3/2 and 3d5/2 levels, respectively, and 374.3 and 368.4 for Ag0 indicates that a small fraction of the silver in the composite is in

the metallic state, whereas the overwhelming majority is cationic. This further validates our assumption of silver particles being present in the composite, decorating the Ag3PO4 crystallites. XRD measurements confirm the crystallinity of the Ag3PO4, AgBr, and Ag in the actual composite materials (Figure 5). The

Figure 5. XRD data (from bottom to top) of (a) Ag3PO4, (b) Ag3PO4/Ag/AgBr, (c) Ag3PO4/Ag/AgBr/RGO, and (d) Ag3PO4/Ag/ AgBr/RGO after use as an oxygen evolution catalyst for 5 h. Known diffraction peaks for Ag3PO4, AgBr, and Ag are indicated as ⧫, ∗, and ▲, respectively.14,44 The insets show a magnification of the region, in which the dominant Ag crystal peak is expected, and reveal the presence of small amounts of highly dispersed crystalline silver.

comparatively weak signal for metallic silver reflects its low fraction in the composite and the high dispersion of Ag nanoparticles on the composite surface.43 We also measured the diffraction pattern of Ag3PO4/Ag/AgBr/RGO after use as an oxygen evolution catalyst for 5 h. Except for the metallic silver peak, the diffraction pattern changed only minimally, suggesting good stability of our composite. The contents of metallic silver increased during use as an oxygen evolution photocatalyst due to precipitation of metallic silver from the sacrificial AgNO3, which was confirmed by XPS (Figure S8, Supporting Information). We also performed experiments of methylene blue oxidation (Figures S6 and S7, Supporting Information), for which no sacrificial AgNO3 is required, and find no increase in metallic silver, further attesting to the absence of Ag3PO4 decomposition in our composite during photocatalytic action. In combination, the results of Figures 3−5 confirm that the precondition for our model, the presence of Ag3PO4, Ag/AgBr nanoparticles, and RGO in close contact, is met. The measured size of the Ag3PO4 particles has some implications for their electronic setup: at ∼0.2 μm, they contain some N ∼ 4 × 107 Ag3PO4 units. Approximating the intrinsic charge carrier concentration as N × exp{−[(Egap)/(2kT)]} ∼ 7 × 10−21, it 20135

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is clear that, within each nanoparticle, there is generally no intrinsic charge carrier. The n-doping induced during synthesis thus aligns the Fermi level of the Ag3PO4 with its conduction band edge. Given the size of our crystallites, we were not able to obtain their majority charge carrier concentration independently (e.g., via Hall effect measurements). Assuming typical dopant concentrations of 1016−1018 1/cm3, each particle contains 1−100 electrons in the conduction band. From the relative amount of silver, we estimate that there is at least one silver particle (of 105−106 atoms) decorating each Ag3PO4 particle. Given that silver has one electron in its metallic sp band per silver atom, the silver particles will readily pin the Fermi energy of the system at their native Fermi level; this will, however, on average shift due to charging of the particles by (1)/ (4πεεwaterrAg) ∼ 2 mV per electron transferred to them (with ε the permittivity, εwater the dielectric constant of the solution approximated as that of water (80), and rAg assumed to be 10 nm, corresponding to, on average, 20 nm silver particles). Assuming the “worst case” (i.e., high doping density and only one Ag particle per Ag3PO4 particle), ∼100 e− need to be transferred to completely deplete the Ag3PO4 particles from conduction band electrons. This will reduce the silver reduction potential by ΔEpot ∼ 0.2 V, smaller than the difference ΔE = 0.35 V of the Ag3PO4 conduction band and the silver Fermi level. Therefore, we expect complete Ag3PO4 conduction band depletion in our composite photocatalysts (with and without addition of RGO). Note that, at complete conduction band depletion, the band bending typically associated with a metal− semiconductor Schottky junction degenerates to alignment of the metal Fermi level with the majority charge carrier (here: conduction) band. Consequently, in our compounds containing Ag/AgBr, we expect two outcomes: (a) the hole lifetime to be longer because the conduction band is depleted of electrons and (b) the Ag3PO4 valence band edge to be downshifted by ΔE − ΔEpot. Both conditions are expected to increase the photocatalytic oxygen yield, as observed. To corroborate these predictions, we measured I−V curves of the various compounds in the dark and under illumination. Figure 6a shows the measurements in the dark. While our electrochemical measurements do not allow us precise determination of the Fermi level, we note that neutrality versus the standard calomel electrode is reached at 0.12 V for the Ag3PO4 compound, but at 0.19 V for the composites, including also Ag/AgBr. This downshift in neutrality is consistent with the model we propose. We would like to stress, however, that quantitative reproduction of the band shift is not to be expected from our measurements. Various effects (e.g., surface potential) were not corrected for, and these effects are particularly relevant when the RGO is added due to the ensuing very large increase in surface area.45,46 Figure 6b shows the increase of current density under illumination by >420 nm light. Ag3PO4/Ag/AgBr and Ag3PO4/ RGO, respectively, yield a photoinduced increase in current density of 0.132 and 0.077 mA cm−2 at 0.5 V (vs SCE), as compared to pure Ag3PO4, which exhibits in our setup 0.059 mA cm−2 at the same applied voltage. Thus, these materials produce a roughly 30.5% increase of photoinduced current density. In comparison, the complete Ag3PO4/Ag/AgBr/RGO composite yields 0.211 mA cm−2 of photoinduced increase of current density at that applied voltage, nearly 3.6 times the original value. We also measured the reversible oxygen

Figure 6. (a) Variation of current density with bias potential (vs SCE) for Ag3PO4 (1), Ag3PO4/Ag/AgBr (2), and Ag3PO4/Ag/AgBr/RGO (3) electrodes in the dark. (b) Photoinduced increase in current density under visible light irradiation (wavelength > 420 nm, I0 = 64 mW cm−2). Additionally, data for Ag3PO4/RGO (4) are shown.

evolution potential to be at ∼0.77 V in the dark (Figure S4, Supporting Information). These findings underline the photoactive properties of our compounds and the synergistic effects of silver and RGO, yet they offer the question why RGO can by itself not serve the same purpose as Ag/AgBr/RGO, that is, depleting the Ag3PO4 conduction band and shifting it downward. Here, the particular electronic structure of graphene is relevant: graphene’s charge density near its Fermi level is very low because of the Diraccone shape of its band structure. This leads to its inability to pin the Fermi level effectively; as a consequence, graphene can be charged readily and its workfunction can be altered widely by electrostatic effects, as many studies have shown.47 This effect becomes less pronounced, if graphene is chemically modified and has dangling bonds or is bonding to other materials, as the states associated with these bonds can themselves pin the Fermi level. Thus, a comparatively low density of defects in RGO is crucial for our model so that the silver nanoparticles can determine the energetic position of the Ag3PO4 conduction band. The quality of graphene-based materials can be estimated from measurement of the ratio of the intensity of the D and G modes in Raman spectroscopy and its infrared absorption characteristics. Figure 7a shows Raman spectra for our GO, RGO, and the complete composite photocatalyst. The intensity ratio is 0.8 for the former, confirming gentle exfoliation leading to large GO flakes. Reduction increases the ratio to 1.0, and mixing with the Ag3PO4/Ag/AgBr nanoparticles causes a further increase to 1.1. This value is clearly worse than what is found for pristine graphene (0),48 yet a ratio of ∼1.0 still indicates a comparatively well-ordered material. Moreover, we observe a shift of both the D and the G modes to lower energy upon formation of the composite. Charge transfer 20136

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explanations for an increased photocatalytic yield. UV−vis diffuse reflection spectroscopy permits us to measure the absorption edge of the materials, thus indicating which portion of the solar spectrum the different composites can utilize. We find the Ag3PO4 absorption edge at ∼497 nm, consistent with a band gap of ∼2.4−2.5 eV (Figure 8a). Addition of Ag/AgBr and RGO successively generates additional broad absorbance (e.g., through the silver surface plasmon30); however, the native AgBr adsorption edge at 476 nm is not revealed, suggesting that the absorption and photocatalytic activity are dominated by the Ag3PO4. Subtracting the ensuing linear backgrounds indicated in Figure 8a and scaling the intensity of the RGO-containing compound by a factor of 1.9 lead to the data displayed in panel b. There is clearly no significant spectral shift of the absorption edge. Thus, the spectral range accessible to the Ag/AgBr/RGObased composite is practically identical to that of native Ag3PO4 and any increase in yield cannot be attributed to access to a broader wavelength range of incoming photons. The large broad range absorption caused by the addition of RGO may actually reduce the net photocatalytic efficiency of the complete composite. Thus, the increase in activity for Ag3PO4/Ag/AgBr/RGO reported here may actually be the net result of an even larger increase of efficiency per photon absorbed in Ag3PO4 countered by a smaller number of photons available due to parasitic absorption by RGO. The notion that the RGO native light adsorption is parasitic to the catalytic activity of the compound is supported by photoluminescence (PL) measurements (Figure 8c): PL spectroscopy shows two emission peaks for Ag3PO4. Addition of Ag/AgBr and RGO does not alter the spectral position or relative intensity of the peaks, but successively reduces the total emission intensity. The PL reduction upon addition of Ag/AgBr is predicted by our model, because the Ag/AgBr-decorated Ag3PO4 nanoparticles are depleted from conduction band electrons, which otherwise can contribute to radiative decay of the photoinduced valence band holes. Our model predicts, however, no further reduction of the PL yield upon addition of RGO, as the conduction band is already empty after Ag/AgBr addition. However, the parasitic absorption of the RGO renders less photons available for adsorption by Ag3PO4, as shown in Figure 8a. For the background-corrected absorbance curves to match in Figure 8b, we needed to rescale the curve for Ag3PO4/Ag/AgBr/RGO by a factor of 1.9, suggesting that approximately half of the light was absorbed by RGO. Using this same scaling constant on the PL curve for Ag3PO4/Ag/AgBr/RGO, we generate the thin black line in Figure 8c, which traces the intensity for Ag3PO4/ Ag/AgBr quantitatively. Thus, the reduction in PL intensity

Figure 7. (a) Raman spectroscopy of RGO modes. The position and ratio of the D and G bands are indicated. We ascribe the increase in signal for the composite to enhancement by the silver surface plasmon. (b) Infrared absorption spectroscopy of the materials used in this study. Note that the spectrum for the complete composite was scaled with a factor of 1.9 discussed in the text.

to graphene (as predicted in our model) is commonly associated with a softening of these modes, providing further support of our interpretation of the increase in photocalatytic activity of our compound. Infrared spectroscopy (Figure 7b) reveals that our preparation leads to GO that is decorated with alcohol and aldehyde groups and that the reduction procedure quantitatively removes these groups, leading to unfunctionalized RGO. Thus, the D/G ratio observed in Raman spectroscopy is likely due to disorder introduced in the graphene itself during oxidation and reduction, and not due to remaining functionalization. Addition of the photocatalyst leads to an infrared spectrum that shows exclusively the features observed only in the photocatalyst itself, yet the intensity appears somewhat reduced (by a factor of ∼1.9), as discussed later. No indications of the oxidation/functionalization of graphene can be made out. Unfortunately, we are not able to provide additional direct evidence of the validity of our model; in the following, we will present results that rule out a number of alternative

Figure 8. (a) UV−vis diffusive reflectance spectra. (b) Spectra normalized by subtraction of the linear background indicated in panel (a) and scaling of the data for the complete composite by 1.9. (c) PL spectra of Ag3PO4 (top), Ag3PO4/Ag/AgBr (middle), and Ag3PO4/Ag/AgBr/RGO (bottom). The thin black line shows the PL yield for the complete composite scaled by the same scaling constant required in panel (b) (1.9). 20137

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The Journal of Physical Chemistry C upon addition of RGO is readily explained by the parasitic absorption of RGO, suggesting no additional decrease (or increase) in PL activity due to the presence of RGO, as predicted by our model. Incidentally, the same factor also provides good agreement in the feature amplitude in the infrared spectra (Figure 7b). The similarity of the PL spectra in the presence and absence of AgBr suggests that AgBr has a very limited role in the photocatalytic activity itself, also highlighted by its exchangeability with AgCl (Figure S1, Supporting Information). We note that, in hydrogen evolution catalysts, the plasmon associated with small silver particles has been credited with enhancing activity through facilitating optical absorption and, potentially, injecting electrons into the catalyst electronic states responsible for causing hydrogen evolution.24 We are not aware of any scenario where this is likely to apply to oxygen evolution; oxygen evolution relies on the presence of holes in the catalyst valence band, which is separated energetically from the plasmon states by >2 eV. Thus, crossover of any carrier appears not plausible. However, we note that the plasmon is associated with high local electric fields. These may be ancillary to the water oxidation in a fashion that is not accessible to this study.

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CONCLUSION In summary, a Ag3PO4/Ag/AgBr/RGO hybrid composite with high visible light photocatalytic O2-production activity was successfully prepared by a photoassisted deposition−precipitation strategy, followed by facile hydrothermal treatment. The composite exhibits improved photoelectrochemical and photocatalytic performances. We attributed this improvement to conduction band depletion and valence band lowering of Ag3PO4 caused by the addition of Ag/AgBr; RGO supports this effect in a synergistic manner through delocalization of the transferred charge. At the same time, RGO provides significant parasitic absorption that partially counters the observed increase in efficiency. Thus, optimization of the RGO loading remains as an objective for preparatory improvement beyond the scope of this paper. This study motivates new developments in graphene-based hybrid materials and associated preparatory methodology. ASSOCIATED CONTENT

S Supporting Information *

Additional information about experimental procedures, BET specific surface area (Table S1), photocatalytic O2-production activity (Figure S1), TEM images (Figure S2), TGA curves (Figure S3), measurement of photoelectrochemical activity (Figures S4 and S5), XRD patterns (Figure S6), stability of Ag3PO4/Ag/AgBr/RGO (Figure S7), XPS spectra (Figure S8), and photographs of oxygen evolution from photoelectrochemical measurements (Figure S9) are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We are grateful for funding of this work by the U.S. National Science Foundation (CHE-1213795, P.F.). XPS measurements and some data analysis were supported by the U.S. Department of Energy (DE-FG02-07ER15842, L.B.).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.F.), [email protected] (L.B.). Notes

The authors declare no competing financial interest. 20138

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