Photocatalytic Generation of H2O2 by Graphene Oxide in Organic

Research Article pubs.acs.org/journal/ascecg

Photocatalytic Generation of H2O2 by Graphene Oxide in Organic Electron Donor-Free Condition under Sunlight Wen-Che Hou* and Yi-Sheng Wang Department of Environmental Engineering, National Cheng Kung University, Tainan City, Taiwan 70101 S Supporting Information *

ABSTRACT: Graphene family nanomaterials are emerging, two-dimensional photocatalysts consisting of Earth-abundant elements. This study evaluated the potential of graphene oxide (GO) toward photocatalytic H2O2 production in water driven by renewable sunlight and visible light without synthetic organic electron donors. We reported for the first time that GO can efficiently photocatalyze the generation of H2O2 to millimolar levels under simulated sunlight in a few hours. The concentration of H2O2 produced is among the greatest values reported in current photocatalytic systems without organic electron donors. We showed that dissolved O2 played a pivotal role in the photoproduction of H2O2 by GO and that superoxide (O2•−) was not involved. A 2-fold increase in H2O2 photoproduction can be readily achieved by raising pH from 3 to 7. The addition of oxalate as the electron donor only enhanced H2O2 photoproduction at low pH, but not at high pH where GO suffered greater photocorrosion. As a result, GO had a greater long-term stability at low pH 4. The reduced photocatalytic activity at low pH can be fully compensated by adding oxalate while maintaining GO’s long-term photostability. Our results indicate that GO is a promising, metal-free photocatalyst to generate H2O2 in an environmentally sustainable manner. KEYWORDS: pH dependence, O2 dependence, Two-electron reduction of oxygen, Superoxide

■

INTRODUCTION

photocatalysis, as it does not involve expensive noble metals frequently used in photocatalytic systems.15−17 H2O2 is a clean oxidant as well as a fuel that generates O2 and H2O upon decomposition.18,19 It finds wide applications including fuel cells, organic synthesis, bleaching agents, and advanced oxidation processes (AOPs) such as Fenton reaction (Fe2+/H2O2), and UV 254 nm/H2O2 for generating •OH for pollution removal and disinfection.18,19 Current industrial production of H2O2 involves complex sequential hydrogenation/oxidation of anthraquinones in organic solvents with significant energy demand and waste generation.18 H2O2 can be alternatively manufactured by a direct reaction of H2 and O2 gases over high-cost catalysts (e.g., Pd or Pd/Au alloy) with a risk of explosion.18,20 Photocatalysis is an emerging process for H2O2 production. This process is attractive because the reaction can be carried out in water, rather than in organic solvents, with a possibility of using renewable sunlight energy to drive the reaction. Common photocatalysts utilized in this process are semiconducting metal oxides, such as TiO2,18,21,22 metal−organic complexes,23−25 and polymeric materials.26−29 In these systems, some organic electron donors are often required for generating notable H2O2 concentrations (millimolar levels). Recent studies

Photocatalysis enabled by graphene-family nanomaterials has received considerable attention in recent years.1,2 A common strategy to design such photocatalysts is to combine some graphene-family materials with semiconducting materials, such as TiO2, to form nanocomposite photocatalysts. It is believed that this approach promotes the flow of electrons from semiconducting photocatalysts to graphene-related materials upon photoexcitation, thereby inhibiting the electron−hole pair recombination and increasing the photocatalysis efficiency.1,2 However, in such a system, the hydroxyl radical (•OH) produced during photocatalysis may react with certain graphene materials to result in rapid decomposition of the latter.3−5 GO, structurally analogous to graphene, possesses an apparent bandgap because of its association with a range of oxygen-containing groups that concomitantly enhances its dispersion into water.6−8 While GO possesses some interesting properties similar to semiconducting materials,6,9,10 its sole use in photocatalysis has not been well delineated. Recent studies by Yeh et al. demonstrated that GO can photocatalyze the splitting of water to generate a considerable amount of H2.11−13 Hsu et al. reported a possible conversion of CO2 to methanol using GO as the photocatalyst.14 These studies suggest that GO alone may act as a potential photocatalyst. GO as a carbonaceous, metal-free nanomaterial is also attractive in © 2017 American Chemical Society

Received: November 1, 2016 Revised: February 15, 2017 Published: March 7, 2017 2994

DOI: 10.1021/acssuschemeng.6b02635 ACS Sustainable Chem. Eng. 2017, 5, 2994−3001

Research Article

ACS Sustainable Chemistry & Engineering

the PTFE septa. Another needle was also poked into the septa to avoid pressure build-up. Some experiments were carried out in a water-jacketed Pyrex glass reactor placed in the CPS+ solar simulator described earlier. During irradiation, O2 gas was continuously bubbled into the aqueous solution, and the reactor was capped with a quartz plate. For visible light experiments, the reactor was covered with a 400 nm light cutoff filter (UF-3, Spartech Corp.) instead. The reaction was thermostated at 25 °C by circulating water through the water jacket from a thermostated water bath. At predetermined time periods, aliquots of samples were taken from the reactor for chemical analysis. Analysis. UV−visible absorbances were measured on a PerkinElmer Lambda 35 spectrophotometer with a 1 cm quartz cuvette. H2O2 concentrations were quantified using a spectrophotometric method involving cupric ions and 2,9-dimethyl-1,10-phenanthroline (i.e., the DMP method).39 After irradiation, aqueous GO samples were passed through 0.2-μm cellulose acetate syringe filters to remove GO, and the filtrates containing H2O2 were analyzed by the DMP method. The lower detection limit for the DMP method is 0.7 μM.39 Samples for X-ray photoelectron spectroscopy (XPS) analysis were prepared by putting drops of aqueous GO on clean silicon wafers and drying in ambient temperature in the dark. A PHI 5000 VersaProbe spectroscopy was used to record XP spectra using Mg KR X-ray irradiation (1253.6 eV). Ejected photoelectrons were detected with a precision high energy electron analyzer operating at a constant pass energy (44.75 eV) with a scan rate of 0.125 eV/step. XPS spectra were processed by CASA XPS software. Atomic force microscopy (AFM) samples were prepared by spincoating aqueous GO suspensions on clean, freshly cleaved mica wafers (1.5 cm × 1.5 cm). This allowed GO materials to uniformly cover the mica wafer surfaces. Samples were imaged on a Dimension Icon scanning probe microscope (Bruker, Inc., Germany) operating at tapping mode. The AFM images were analyzed for equivalent diameters and surface areas using Nanoscope Analysis software (Bruker AXC, Inc.).

attempt to look for more sustainable photocatalysts to generate H2O2 without organic electron donors.30,17,31 For example, Kaynan et al. reported a ternary photocatalyst consisting of TiO2/silicon/Au to generate H2O2 from O2 and water without sacrificial compounds.17 Similarly, Moon et al. developed a TiO2/reduced GO (rGO)/cobalt phosphate photocatalyst to generate H2O2 without organic electron donors.30 These TiO2based systems produced tens of micron-molar levels of H2O2. Other systems utilizing rare earth metal (e.g., ruthenium, scandium, or iridium)-organic complex photocatalysts can produce up to millimolar levels of H2O2.31−33 Recent studies have investigated the photoreactivity of GO to gain an understanding of the potential environmental impact and fate of this emerging nanomaterial. For example, Zhao et al. evaluated the photoproduction of H2O2 by GO under the irradiation of UVA light (300−400 nm),34 since H2O2 is a reactive oxygen species (ROS) implicated in nanotoxicity.35,36 They found that a micromolar level of H2O2 was formed (∼3.5 μM). A broader prospect of GO in photocatalytic H2O2 production is not currently available. In this work, the potential of GO in the photocatalytic H2O2 production was examined using simulated sunlight or visible light (λ ≥ 400 nm) in the absence of organic electron donors. Photoproduction of H2O2 was evaluated over a range of variables, such as the pH, GO concentration, added organic electron donors, oxygen level, and long-term stability. The reactive transient species such as O2•− and photocorrosion of GO during photocatalysis were also investigated to shed light on the observed photocatalytic activities.

■

MATERIALS AND METHODS

■

Materials. GO dispersed in pure water at 2 mg/mL was obtained from Cheap Tubes, Inc. (Brattleboro, VT). This GO sample was manufactured using the modified Hummer’s method.37 Another GO sample was prepared in-house using an improved Hummer’s method that involves oxidation of flake graphite with KMnO4, H2SO4, and H3PO4 with a greater yield of GO.38 We refer to this in-house sample as IGO in this study. All other chemicals were used as supplied from Sigma-Aldrich or J. T. Baker. All aqueous samples were prepared with water purified by a Millipore Synergy ultrapure water system (≥18.0 MΩ). Irradiation. Experiments involving simulated sunlight were carried out in an Atlas SunTest CPS+ solar simulator equipped with a 1.5 kW xenon arc lamp. The output intensity of the lamp was set at 765 W. This was to simulate the typical clear day, summer noon sunlight in Tainan city. The outdoor sunlight and simulated sunlight spectra are shown in Figure S1. Reaction vessels were 8 mL Pyrex glass tubes (13 mm O.D. × 100 mm) to which 7 mL of the GO sample was added. The sample pH was adjusted by 1 M NaOH or HCl. pH buffer solutions were not used to avoid potential interferences with the photocatalytic reaction. The pH was closely monitored, and it dropped 0.5−1.4 pH unit after photoreaction, which was relatively small compared to the range of pH (3, 7, 10) used in the study. Aqueous samples were equilibrated with ambient air before irradiation. Sample tubes were submersed in a water bath during irradiation with temperature controlled at 25 °C. For kinetic studies, a series of tubes were prepared for irradiation, and tubes were taken out from the water bath and sacrificed for analysis at predetermined time periods. Dark control tubes were wrapped by aluminum foil and irradiated simultaneously. This ensured that other factors except light exposure were identical. To keep the solutions in an oxygen-free condition for oxygendependent studies, aqueous samples were sealed with open-topped caps lined with gastight PTFE septa. Argon gas (99.9%) was purged into the samples for 30 min using a needle which was poked through

RESULTS AND DISCUSSION GO Characterization. The mean equivalent diameter of the GO sample as revealed by the AFM images was 213.0 nm with a mean areal size of ∼50,000 nm2 (Figure S2a,b). The elemental analysis of GO using the XPS spectrum (Figure S3) revealed that no elements other than oxygen and carbon were present, indicating a high purity of GO used. XPS data (Figure S2) also indicate that the oxygen-to-carbon atom ratio (O:C) in GO was 0.54, and the functional groups were 44.0% C−O, 3.0% CO, and 5.0% O−CO. The UV−visible spectrum of GO showed a strong light absorbance within the solar energy range (300−800 nm), but no characteristic absorbance peak occurred (Figure S4). The GO sample prepared in-house, IGO, had a mean size of 423.3 nm and contained 43.4% C−O, 2.9% CO, and 5.5% O−CO functionalities. Photocatalytic H2O2 Production by GO. The effect of the GO dose on the photoproduction of H2O2 was first evaluated without added organic electron donors. Figure 1 indicates that the level of H2O2 formed increased with increasing GO dose until the latter reached 320 mg/L. Beyond that, the level of H2O2 started to decrease with increasing GO dose. The decreased H2O2 formation may result from a light screening effect that becomes more intensified as the GO dose increases. The GO sample prepared in-house (i.e., IGO) was also evaluated for its photocatalytic H2O2 formation. The result (Figure S5) indicates that the two GO samples exhibited a similar photoreactivity toward H2O2 formation. Notably, GO exhibited a greater photocatalytic H2O2 formation potential than did P25 TiO2, a photocatalyst commonly used for comparison (Figure S5). Photocatalytic H2O2 generation by 2995


Research Article


earlier work has extensively examined the phototransformation of GO under solar light conditions similar to those reported in this work.41 It indicates that even without O2, GO still strongly phototransformed in a way comparable to that in oxic conditions. This indicates that H2O2 is unlikely to directly come from the byproduct formed as a result of GO phototransformation, as the H2O2 formation is minimal in anoxic conditions where phototransformation of GO remains strong. Figure 2b shows that the H2O2 formation accelerated with decreasing proton concentration. Enhanced photoproduction of H2O2 by GO can occur at the circumneutral pH range common to natural waters, a condition advantageous for practical operation. The pH value can affect the photocatalytic H2O2 production in several potential ways. For example, OH− added to increase the pH is itself a good hole scavenger that could form H2O2 via the •OH reaction.42 However, the steadystate •OH concentration measured using parachlorobenzoic acid as the chemical probe in our system was fairly small (∼10−9 μM at pH 7),5,43 suggesting that the •OH reaction is not a major pathway contributing to H2O2 photoproduction by GO. Other pH-dependent H2O2 formation pathways could be O2•− dismutation and/or proton-coupled electron transfer (PCET); however, these reactions seem unimportant because they should become more efficient with protons present (i.e., low pH),30,44,45 a result not supported by the pH trend observed here (Figure 2b). The cause for the pH dependence remains elusive. It is known that the redox potential of conduction bands is controlled by pH, which shifts −59 mV per unit increase in pH.46,47 It is speculated that the conduction band of GO becomes more reductive with increased pH to result in a greater reduction of O2 to H2O2. On the basis of the energy level diagram presented in Figure 3, the O2 reduction reaction is favorable at pH 3 and 7. The construction of the energy level diagram is explained in the Supporting Information, Text S1. The pH increase from 3 to 7 results in the shift of ECB to a more negative position of −0.87 V and increases the driving force for O2 reduction, a result consistent with the pH trend observed in Figure 2b. Considering that the increase in reduction potential with increasing pH is relatively small (ΔECB = 0.24 V between pH 3 and 7), it is likely that this potential increase just meets the overpotential requirement for the efficient O2 reduction reaction.

Figure 1. Dependence of GO dose on the photocatalytic H2O2 formation under simulated sunlight without organic electron donors added at pH 7. Open symbols indicate the dark control samples of 320 mg/L GO at pH 7.

TiO2 in pure water is rather inefficient and generally requires UV photoactivation and addition of organic electron donors.21,22,27,30 Additionally, bare, uncoated TiO2 can react with the H2O2 formed, resulting in a rapid decomposition during photocatalysis.22,30,40 It is worthwhile to mention that GO already photoproduced higher (submillimolar) levels of H2O2 in organic electron donor-free water, with water being the likely electron donor in the current system. Oxidation of H2O by photoexcited GO is favorable based on the electropotential of GO’s valence band (EVB) of 2.63 V (vs Ag/AgCl) compared to 0.6 V of H2O oxidation (see the energy level diagram in Figure 3). The photocatalyzed H2O2 formation by GO appeared fairly linear over time in contrast to systems involving TiO2 and ZnO that rapidly reach a plateau due to catalystmediated photodecomposition of the H2O2 formed.22,30 Overall, the results indicate that GO is photocatalytically active under sunlight conditions, generating submillimolar levels of H2O2 even in the absence of organic electron donors. Solution Chemistry on Photoproduction of H2O2. Important water chemistry parameters including oxygen level and pH on the photoproduction of H2O2 were evaluated. Figure 2a compares the H2O2 formation in water with airsaturated and argon-saturated GO samples. The H2O2 level increased to 150 μM with air-saturated GO samples in 10 h of irradiation, while that with argon-saturated samples increased only slightly to 10 μM, indicating that dissolved O2 plays a key role. A similar O2 dependence was also observed at lower pH 4.5 (Figure S6). The result indicates that dissolved O2 is a key precursor to the photocatalytic H2O2 production by GO. Our

Figure 2. Photocatalytic H2O2 formation by GO (40 mg/L) under simulated sunlight without organic electron donors, showing (a) the oxygen level (pH 7) and (b) pH dependence on H2O2 formation (air saturated). 2996


Research Article


For example, in the case of graphitic g-C3N4, it was suggested that O2 is reduced by electrons photogenerated by g-C3N4, forming intermediate 1,4-endoperoxide moieties across the benzene ring-like structures of g-C3N4, that are subsequently released to form H2O2.26−29 The release of H2O2 could proceed through the hydrolysis of 1,4-endoperoxide, a recognized pathway for H2O2 formation.51,52 The apparent reaction for the two-electron reduction process is shown in eq 3. O2 + 2e− + 2H+ → H 2O2

The formation of endoperoxide suppresses one-electron reduction of O2 and superoxide formation, consistent with the results in Figure 4.26,27 This can also explain the efficient H2O2 photoproduction by GO observed in this study. While the photocatalytic H2O2 production by GO observed here is consistent with two-electron reduction of O2, the actual reaction pathway could be more complex, and further research is needed to identify the hypothetical reaction intermediates such as GO endoperoxide species. Effect of Organic Electron Donor Addition. Figure 5a evaluated the addition of organic electron donors that are common promotors in semiconductor photocatalysis of H2O2 formation.22,42 Formate and oxalate are better electron donors in enhancing H2O2 generation by GO. For example, the H2O2 concentration increased by ∼2 folds (220 μM) in samples containing oxalate than that (80 μM) of GO alone in 10 h of irradiation. Further experiments (Figure S8) to examine the effect of formate concentration on H2O2 formation indicate that an increase in the formate concentration from 1 to 4 mM did not affect H2O2 formation, indicating that the reaction is not limited by the electron donor under this condition. Formate and oxalate are known to directly react with the valence band hole via one-electron transfer, resulting in photodecarboxylation (i.e., the photo-Kolbe reaction) and the formation of transient CO2•− radical and CO2 (in the case of oxalate).42,53−56 The transient CO2•− can transfer another electron to the valence band hole and form CO2 as the stable product, resulting in a current doubling effect and greater O2 reduction.42,56 In contrast, acetate lacks the current doubling mechanism as a result of the methyl group, as it forms a •CH3 radical and CO2 upon reaction.53,54 The electron-donating reaction can alternatively occur through the •OH-mediated process where •OH forms as a result of H2O or a surfacebound OH group reaction with a valence band hole and then reacts with organic electron donors through hydrogen

Figure 3. Schematic energy level diagram showing the O2 reduction and water oxidation by electron−hole pair of GO.

It is notable that at pH 10 the formation of H2O2 reached a plateau, which correlated with the enhanced photocorrosion of GO at high pH shown later (Figure 6). Role of Superoxide. Prior studies have indicated that photocatalytic H2O2 formation could potentially proceed through one-electron or two-electron reduction of dissolved O2.26,27,42 Conduction band electrons (e−) are photogenerated by GO that subsequently reduce O2. Consequently, O2•− is generated as the intermediate in the one-electron reduction pathway (eq 1), which disproportionates (with another O2•−) to form H2O2 (eq 2). O2 + e− → O•− 2

(1)

+ 2O•− 2 + 2H → H 2O2 + O2

(2)

(3)

Further experiments were therefore conducted to evaluate the role of O2•−. Superoxide dismutase (SOD) catalyzes the dismutation of O2•− to H2O2,48 and its presence is expected to increase H2O2 yield. However, results presented in Figure 4a do not agree with this expectation. We also used 2,3-bis(2methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) to detect O2•− formation.49,50 XTT reacts with O2•− to form pink-colored products that absorb light at 470 nm. Figure 4b shows no absorbance at 470 nm. The results thus indicate that one-electron reduction is unlikely to be the major pathway leading to photoproduction of H2O2 by GO samples used in the study. Pathway of H2O2 Photoproduction. Recent research has increasingly indicated that H2O2 can be efficiently generated by two-electron reduction of O2 over photocatalysts.19,21,26−28,31

Figure 4. (a) Photoproduction of H2O2 by GO (40 mg/L) under simulated sunlight at pH 7 in the presence of superoxide dismutase (SOD) at 40 U/mL. (b) Photoproduction of O2•− detected by XTT (0.1 mM) under simulated sunlight at pH 7 and GO (40 mg/L). 2997


Research Article


Figure 5. Photoproduction of H2O2 by GO (40 mg/L) under simulated sunlight irradiation in the presence of (a) different organic electron donors (1 mM) at pH 4.5 and (b) in the presence of 1 mM oxalate at varied pH.

abstraction.53,54,57 This •OH reaction pathway is important for compounds with α-hydrogens such as methanol and acetate.53,57 Our measurement result indicates that •OH formation is fairly small, consistent with the data showing methanol and acetate as inefficient electron donors. The pH effect in the presence of oxalate on H2O2 formation was further evaluated (Figure 5b). Interestingly, the pH dependence with oxalate present became less notable, in large contrast to the case where the organic electron donor was absent (Figure 2b). The photoproduction of H2O2 by GO with oxalate present lacked enhancement at higher pH (7 and 10) (Figure S9), thereby closing the difference in H2O2 formation between high versus low pH under organic electron donor-free conditions. GO has ionizable functionalities such as carboxyl groups, and therefore, it is more negatively charged at higher pH.58 Given that oxalate has carboxyl groups and is negatively charged at high pH (pKa = 1.25, 4.14),59 it is possible that greater electrostatic repulsion between GO and oxalate at higher pH results in inhibited interaction and H2O2 formation. Alternatively, the stronger photocorrosion of GO observed at high pH with oxalate present as discussed later (Figure 6) could also lead to the decay of GO’s photocatalytic activity. Photocorrosion of GO. The structural changes of GO during photocatalysis were examined by XPS to shed light on the photocatalytic activity observed in earlier results. The percent changes in functionalities of irradiated GO samples normalized to their corresponding dark control samples are presented in Table 1, and the raw fitting data of XPS are shown in Table S1. Table 1 and Figure 6 indicate that GO was photoreduced to a greater extent at higher pH range (7 and 10) as indicated by the O:C ratio. The photoreduction can be mainly attributed to the loss of the C−O functional group (∼286.5 eV). Dark control GO samples were also chemically reduced at high pH (7 and 10) as indicated by the strong decrease in C−O signal compared to that of CC/CH (∼284.6 eV). A similar observation has been reported previously and was attributed to the base-catalyzed hydrolysis.60 The presence of oxalate at high pH (7 and 10) further enhanced the photocorrosion of GO, while the GO sample at pH 4 showed a small change (Figure 6). For example, CO (∼287.6 eV) and O−CO (∼288.6 eV) functional groups considerably increased at pH 10 (13.6% to 290.9% and 120.0% to 712.0%, respectively) (Figure 6c and Table 1). The strong reduction of C−O at high pH 10 (from −33.6% without oxalate to −72.3% with oxalate) was also notable. It is noted that the XPS samples were analyzed after dialysis (1 kDa dialysis membranes) to remove residual oxalate to avoid spectral interferences.

Figure 6. XPS spectra showing the structural evolutions of GO in the presence and absence oxalate (1 mM) at pH 4, 7, and 10. Irradiation time was 6 h.

While solar light-driven photoreduction of GO has been reported previously,5,41 enhanced photoreduction mediated by pH is new. The photoreduction of GO can be attributed to the photogenerated electron that reacts with GO itself.41 Photo2998


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Functionality Changes of GO during Photocatalysis pH

treatment

ΔO:Ca (%)

ΔCC/CHa (%)

ΔC−Oa (%)

ΔCOa (%)

ΔO−COa (%)

4

light light w/oxalate light light w/oxalate light light w/oxalate

−3.6 −9.1 −5.8 −1.9 −16.7 8.3

6.4 7.6 5.7 2.3 14.7 −8.7

−9.3 −11.8 −16.8 −31.6 −33.6 −72.3

4.0 20.0 82.1 214.3 13.6 290.9

18.0 20.0 45.9 156.8 120.0 712.0

7 10 a

The % changes of functional groups or oxygen-to-carbon (O:C) ratio of irradiated samples normalized to those of corresponding dark control samples. For example, ΔO:C (%) = (O:Cirradiated − O:Cdark) × 100/O:Cdark. Here, ΔO:C is the % change of O:C ratio in the photoirradiated sample, O:Cirradiated is the O:C ratio in the photoirradiated sample, and O:Cdark is the O:C ratio in the corresponding dark control sample.

reported for organic electron donor-free photocatalytic systems.17,30,62 Our H2O2 production rate under visible light is also comparable to the well-known visible light photocatalyst g-C3N4 and metal−organic complexes in organic donor-free conditions.27,31 Collectively, the results indicate that GO has a great potential toward photocatalytic H2O2 production in organic electron donor-free conditions under sunlight or visible light. Long-Term Stability. As observed earlier in Figure 6 where GO was photocorroded to a lesser extent at pH 4, we explored the long-term photocatalytic activity under acidic pH with the expectation that GO’s photoactivity could be better preserved. As organic electron donors can promote H2O2 photoproduction at low pH (Figure 5a), oxalate (10 mM) was added to GO at pH 4. The result (Figure 7) indicates that

corrosion occurs with many semiconductor photocatalysts through reduction and/or oxidation by photogenerated electrons and holes.47,61 The stronger photoreduction of GO at higher pH observed here correlated with the greater photoreductive H2O2 production by GO under similar conditions (Figure 2b). As discussed earlier, this could be attributed to the negative shift of redox potential of the conduction band at high pH. In other words, while the photoreductive potential of GO was enhanced at high pH, leading to greater H2O2 generation, it also resulted in detrimental GO photocorrosion and more rapid loss of photocatalytic activity as shown previously at pH 10 (Figure 2b). The addition of oxalate at pH 7 and 10 resulted in an even greater alteration/photocorrosion of GO compared to the case without oxalate (Figure 6b,c). The formation of carboxyl functionality and increased reduction of C−O were particularly notable. The mechanism for carboxyl group formation is currently speculative. It is known that oxalate reacts with photogenerated holes, forming carboxyl radicals.55,56 Carboxyl radicals may subsequently react with GO to form the carboxyl adducts as detected by XPS. Such strong alteration/photocorrosion at high pH with oxalate present could result in greater decay of GO’s photocatalytic activity and mask the benefit of oxalate as a promoter for H2O2 formation that only occurred at low pH 4, where GO’s photocorrosion was small (Figure 6a). Optimized Photocatalytic Conditions. We performed optimized photocatalytic H2O2 production based on the parameters evaluated earlier (i.e., pH, O2 level, and organic electron donor addition). pH 7.0 was selected as it is within the pH range of common natural waters, and H2O2 production is greater than that at low pH. We bubbled O2 into the solution during photocatalysis as O2 is needed for this reaction (Figure 2a). Organic electron donors were not added to the experiments at pH 7 as the overall enhancement of H2O2 formation with added organic electron donors was ineffective (Figure S9). Figure S10 indicates that GO (at 640 and 800 mg/L) rapidly photocatalyzed H2O2 formation to 1 mM with a production rate of ∼2.88 μM/min. The dose dependence of GO observed here is slightly different from that reported earlier (Figure 1), as the experiments here were constantly stirred with O2 bubbling in the water-jacketed reactor, while those in Figure 1 were static in Pyrex tubes. Vigorous mixing could mitigate the lightscreening effect by GO samples. Photoproduction of H2O2 also occurred within the visible light range of the solar spectrum (with a 400 nm cutoff filter) at a rate of ∼0.75 μM/min. To our knowledge (Table S2), both the production rate and the achieved H2O2 concentration are among the highest values

Figure 7. Long-term stability of GO (640 mg/L) under simulated sunlight irradiation in the presence and absence of oxalate (10 mM) with O2 bubbling.

H2O2 was photoproduced to a significant level of 4 mM in 20 h without the loss of photocatalytic activity of GO. This contrasts the case where GO without oxalate at pH 7 showed a great loss of photoactivity in 10 h, consistent with the greater photocorrosion of GO at higher pH (Figure 6). The results indicate that while GO’s photocatalytic activity is smaller at acidic pH, its stability can be better preserved. The smaller photoactivity of GO at acidic pH can be fully compensated by adding organic electron donors that can increase H2O2 concentration to a level comparable to that at higher pH without oxalate. In other words, using low pH conditions with organic electron donors could preserve GO’s stability while maintaining good H2O2 photoproduction.

■

CONCLUSION The utility of GO toward photocatalytic production of H2O2 has been explored in this study under a range of conditions in 2999


Research Article


(5) Hou, W.-C.; Henderson, W. M.; Chowdhury, I.; Goodwin, D. G., Jr.; Chang, X.; Martin, S.; Fairbrother, D. H.; Bouchard, D.; Zepp, R. G. The contribution of indirect photolysis to the degradation of graphene oxide in sunlight. Carbon 2016, 110, 426−437. (6) Eda, G.; Lin, Y.-Y.; Mattevi, C.; Yamaguchi, H.; Chen, H.-A.; Chen, I.-S.; Chen, C.-W.; Chhowalla, M. Blue Photoluminescence from Chemically Derived Graphene Oxide. Adv. Mater. 2010, 22 (4), 505−509. (7) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39 (1), 228. (8) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New insights into the structure and reduction of graphite oxide. Nat. Chem. 2009, 1 (5), 403−408. (9) Chien, C.-T.; Li, S.-S.; Lai, W.-J.; Yeh, Y.-C.; Chen, H.-A.; Chen, I.-S.; Chen, L.-C.; Chen, K.-H.; Nemoto, T.; Isoda, S.; et al. Tunable Photoluminescence from Graphene Oxide. Angew. Chem., Int. Ed. 2012, 51 (27), 6662−6666. (10) Guo, L.; Shao, R.-Q.; Zhang, Y.-L.; Jiang, H.-B.; Li, X.-B.; Xie, S.Y.; Xu, B.-B.; Chen, Q.-D.; Song, J.-F.; Sun, H.-B. Bandgap Tailoring and Synchronous Microdevices Patterning of Graphene Oxides. J. Phys. Chem. C 2012, 116 (5), 3594−3599. (11) Yeh, T.-F.; Chen, S.-J.; Yeh, C.-S.; Teng, H. Tuning the Electronic Structure of Graphite Oxide through Ammonia Treatment for Photocatalytic Generation of H2 and O2 from Water Splitting. J. Phys. Chem. C 2013, 117 (13), 6516−6524. (12) Yeh, T.-F.; Chan, F.-F.; Hsieh, C.-T.; Teng, H. Graphite Oxide with Different Oxygenated Levels for Hydrogen and Oxygen Production from Water under Illumination: The Band Positions of Graphite Oxide. J. Phys. Chem. C 2011, 115 (45), 22587−22597. (13) Yeh, T.-F.; Syu, J.-M.; Cheng, C.; Chang, T.-H.; Teng, H. Graphite Oxide as a Photocatalyst for Hydrogen Production from Water. Adv. Funct. Mater. 2010, 20 (14), 2255−2262. (14) Hsu, H.-C.; Shown, I.; Wei, H.-Y.; Chang, Y.-C.; Du, H.-Y.; Lin, Y.-G.; Tseng, C.-A.; Wang, C.-H.; Chen, L.-C.; Lin, Y.-C.; Chen, K.-H. Graphene oxide as a promising photocatalyst for CO2 to methanol conversion. Nanoscale 2013, 5 (1), 262−268. (15) Tsukamoto, D.; Shiro, A.; Shiraishi, Y.; Sugano, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Photocatalytic H2O2 Production from Ethanol/ O2 System Using TiO2 Loaded with Au−Ag Bimetallic Alloy Nanoparticles. ACS Catal. 2012, 2 (4), 599−603. (16) Teranishi, M.; Naya, S.; Tada, H. In Situ Liquid Phase Synthesis of Hydrogen Peroxide from Molecular Oxygen Using Gold Nanoparticle-Loaded Titanium(IV) Dioxide Photocatalyst. J. Am. Chem. Soc. 2010, 132 (23), 7850−7851. (17) Kaynan, N.; Berke, B. A.; Hazut, O.; Yerushalmi, R. Sustainable photocatalytic production of hydrogen peroxide from water and molecular oxygen. J. Mater. Chem. A 2014, 2 (34), 13822−13826. (18) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process. Angew. Chem., Int. Ed. 2006, 45 (42), 6962−6984. (19) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. Seawater usable for production and consumption of hydrogen peroxide as a solar fuel. Nat. Commun. 2016, 7, 11470. (20) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process. Science 2009, 323 (5917), 1037−1041. (21) Shiraishi, Y.; Kanazawa, S.; Tsukamoto, D.; Shiro, A.; Sugano, Y.; Hirai, T. Selective Hydrogen Peroxide Formation by Titanium Dioxide Photocatalysis with Benzylic Alcohols and Molecular Oxygen in Water. ACS Catal. 2013, 3 (10), 2222−2227. (22) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Photocatalytic production of hydrogen peroxides and organic peroxides in aqueous suspensions of titanium dioxide, zinc oxide, and desert sand. Environ. Sci. Technol. 1988, 22 (7), 798−806. (23) Fukushima, M.; Tatsumi, K.; Tanaka, S.; Nakamura, H. Photocatalytic Production of Hydrogen Peroxide by Tris(2,2′bipyridine) Ruthenium(II) Using Humic Acids as Electron Donor. Environ. Sci. Technol. 1998, 32 (24), 3948−3953.

simulated sunlight. Photoproduction of 1 mM H2O2 can be achieved in 6 h of solar irradiation under the optimized condition (pH 7 and O2 saturation) without organic electron donors. Dissolved O2 is required for the H2O2 photoproduction by GO. Photocatalytic H2O2 production by GO does not require adding organic electron donors, indicating that water is the likely electron donor. Two-electron photoreduction of O2 by GO is the hypothetical pathway toward H2O2 formation. H2O2 photoproduction by GO can be readily enhanced by raising pH. While photocatalytic activity of GO is greater at higher pH range, the photostability of GO decays more rapidly at basic pH where reductive photocorrosion of GO is greater. Overall GO is a promising, noble-metal free photocatalyst that efficiently produces H2O2 without organic electron donors in water under sunlight and visible light irradiation. This suggests that photocatalysis using GO can be a new, more environmentally sustainable process for H2O2 production.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02635. Additional information on solar simulator and outdoor sunlight spectra, AFM images and size distribution of GO, XPS data and UV−visible absorbance spectrum of GO, photoproduction of H2O2 by P25 TiO2 and IGO, O 2 and formate concentration effects on H 2 O 2 production at pH 4.5, Tauc plot of GO, raw fitting data of XPS spectra, presence of oxalate on H2O2 photoproduction by GO at pH 7 and 10, optimized H2O2 photoproduction, and summary of recent studies on photocatalytic H2O2 production. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +886 62757575, ext. 65842. Fax: +886 6 2752790. Email: [email protected]. ORCID

Wen-Che Hou: 0000-0001-9884-2932 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support provided by the Ministry of Science and Technology (MOST) of Taiwan (for W.-C. Hou) under Grants MOST 103-2221-E-006-015-MY3 and MOST 104-2628-E006-001-MY2 is acknowledged.

■

REFERENCES

(1) Xiang, Q.; Yu, J.; Jaroniec, M. Graphene-based semiconductor photocatalysts. Chem. Soc. Rev. 2012, 41 (2), 782−796. (2) Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J. Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115 (18), 10307−10377. (3) Radich, J. G.; Krenselewski, A. L.; Zhu, J.; Kamat, P. V. Is Graphene a Stable Platform for Photocatalysis? Mineralization of Reduced Graphene Oxide With UV-Irradiated TiO2 Nanoparticles. Chem. Mater. 2014, 26 (15), 4662−4668. (4) Radich, J. G.; Kamat, P. V. Making Graphene Holey. GoldNanoparticle-Mediated Hydroxyl Radical Attack on Reduced Graphene Oxide. ACS Nano 2013, 7 (6), 5546−5557. 3000


Research Article

ACS Sustainable Chemistry & Engineering (24) Yamada, Y.; Nomura, A.; Miyahigashi, T.; Ohkubo, K.; Fukuzumi, S. Acetate Induced Enhancement of Photocatalytic Hydrogen Peroxide Production from Oxalic Acid and Dioxygen. J. Phys. Chem. A 2013, 117 (18), 3751−3760. (25) Zhuang, H.; Yang, L.; Xu, J.; Li, F.; Zhang, Z.; Lin, H.; Long, J.; Wang, X. Robust Photocatalytic H2O2 Production by Octahedral Cd3(C3N3S3)2 Coordination Polymer under Visible Light. Sci. Rep. 2015, 5, 16947. (26) Shiraishi, Y.; Kanazawa, S.; Sugano, Y.; Tsukamoto, D.; Sakamoto, H.; Ichikawa, S.; Hirai, T. Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light. ACS Catal. 2014, 4 (3), 774−780. (27) Shiraishi, Y.; Kanazawa, S.; Kofuji, Y.; Sakamoto, H.; Ichikawa, S.; Tanaka, S.; Hirai, T. Sunlight-Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal-Free Photocatalysts. Angew. Chem., Int. Ed. 2014, 53 (49), 13454−13459. (28) Shiraishi, Y.; Kofuji, Y.; Sakamoto, H.; Tanaka, S.; Ichikawa, S.; Hirai, T. Effects of Surface Defects on Photocatalytic H2O2 Production by Mesoporous Graphitic Carbon Nitride under Visible Light Irradiation. ACS Catal. 2015, 5 (5), 3058−3066. (29) Kofuji, Y.; Ohkita, S.; Shiraishi, Y.; Sakamoto, H.; Tanaka, S.; Ichikawa, S.; Hirai, T. Graphitic Carbon Nitride Doped with Biphenyl Diimide: Efficient Photocatalyst for Hydrogen Peroxide Production from Water and Molecular Oxygen by Sunlight. ACS Catal. 2016, 6, 7021−7029. (30) Moon, G.; Kim, W.; Bokare, A. D.; Sung, N.; Choi, W. Solar production of H 2 O 2 on reduced graphene oxide−TiO 2 hybrid photocatalysts consisting of earth-abundant elements only. Energy Environ. Sci. 2014, 7 (12), 4023−4028. (31) Kato, S.; Jung, J.; Suenobu, T.; Fukuzumi, S. Production of hydrogen peroxide as a sustainable solar fuel from water and dioxygen. Energy Environ. Sci. 2013, 6 (12), 3756−3764. (32) Isaka, Y.; Oyama, K.; Yamada, Y.; Suenobu, T.; Fukuzumi, S. Photocatalytic production of hydrogen peroxide from water and dioxygen using cyano-bridged polynuclear transition metal complexes as water oxidation catalysts. Catal. Sci. Technol. 2016, 6 (3), 681−684. (33) Isaka, Y.; Kato, S.; Hong, D.; Suenobu, T.; Yamada, Y.; Fukuzumi, S. Bottom-up and top-down methods to improve catalytic reactivity for photocatalytic production of hydrogen peroxide using a Ru-complex and water oxidation catalysts. J. Mater. Chem. A 2015, 3 (23), 12404−12412. (34) Zhao, Y.; Jafvert, C. T. Environmental photochemistry of single layered graphene oxide in water. Environ. Sci.: Nano 2015, 2 (2), 136− 142. (35) Fu, P. P.; Xia, Q.; Hwang, H.-M.; Ray, P. C.; Yu, H. Mechanisms of nanotoxicity: Generation of reactive oxygen species. J. Food Drug Anal. 2014, 22 (1), 64−75. (36) Kamat, J. P.; Devasagayam, T. P. A.; Priyadarsini, K. I.; Mohan, H. Reactive oxygen species mediated membrane damage induced by fullerene derivatives and its possible biological implications. Toxicology 2000, 155 (1−3), 55−61. (37) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80 (6), 1339−1339. (38) Marcano, D. C.; Kosynkin, D. V.; Berlin, J. M.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L. B.; Lu, W.; Tour, J. M. Improved Synthesis of Graphene Oxide. ACS Nano 2010, 4 (8), 4806−4814. (39) Kosaka, K.; Yamada, H.; Matsui, S.; Echigo, S.; Shishida, K. Comparison among the Methods for Hydrogen Peroxide Measurements To Evaluate Advanced Oxidation Processes: Application of a Spectrophotometric Method Using Copper(II) Ion and 2,9-Dimethyl1,10-phenanthroline. Environ. Sci. Technol. 1998, 32 (23), 3821−3824. (40) Li, X.; Chen, C.; Zhao, J. Mechanism of Photodecomposition of H2O2 on TiO2 Surfaces under Visible Light Irradiation. Langmuir 2001, 17 (13), 4118−4122. (41) Hou, W.-C.; Chowdhury, I.; Goodwin, D. G.; Henderson, W. M.; Fairbrother, D. H.; Bouchard, D.; Zepp, R. G. Photochemical Transformation of Graphene Oxide in Sunlight. Environ. Sci. Technol. 2015, 49 (6), 3435−3443.

(42) Hoffman, A. J.; Carraway, E. R.; Hoffmann, M. R. Photocatalytic Production of H2O2 and Organic Peroxides on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28 (5), 776−785. (43) Hou, W.-C.; He, C.-J.; Wang, Y.-S.; Wang, D. K.; Zepp, R. G. Phototransformation-Induced Aggregation of Functionalized SingleWalled Carbon Nanotubes: The Importance of Amorphous Carbon. Environ. Sci. Technol. 2016, 50 (7), 3494−3502. (44) Qiang, Z.; Chang, J.-H.; Huang, C.-P. Electrochemical generation of hydrogen peroxide from dissolved oxygen in acidic solutions. Water Res. 2002, 36 (1), 85−94. (45) Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. Reactivity of HO2/O−2 Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 1985, 14 (4), 1041−1100. (46) Photoelectrochemical Hydrogen Production; van de Krol, R., Grätzel, M., Eds.; Electronic Materials: Science & Technology; Springer US: Boston, MA, 2012; Vol. 102. (47) White, J. L.; Baruch, M. F.; Pander, J. E., III; Hu, Y.; Fortmeyer, I. C.; Park, J. E.; Zhang, T.; Liao, K.; Gu, J.; Yan, Y.; et al. Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes. Chem. Rev. 2015, 115 (23), 12888−12935. (48) Fukai, T.; Ushio-Fukai, M. Superoxide Dismutases: Role in Redox Signaling, Vascular Function, and Diseases. Antioxid. Redox Signaling 2011, 15 (6), 1583−1606. (49) Hotze, E. M.; Labille, J.; Alvarez, P. J. J.; Wiesner, M. R. Mechanisms of Photochemistry and Reactive Oxygen Production by Fullerene Suspensions in Water. Environ. Sci. Technol. 2008, 42 (11), 4175−4180. (50) Lyon, D. Y.; Brunet, L.; Hinkal, G. W.; Wiesner, M. R.; Alvarez, P. J. J. Antibacterial Activity of Fullerene Water Suspensions (nC60) Is Not Due to ROS-Mediated Damage. Nano Lett. 2008, 8 (5), 1539− 1543. (51) Haag, W. R.; Hoigné, J.; Gassman, E.; Braun, A. Singlet oxygen in surface waters  Part I: Furfuryl alcohol as a trapping agent. Chemosphere 1984, 13 (5−6), 631−640. (52) Braun, A. M.; Dahn, H.; Gassmann, E.; Gerothanassis, I.; Jakob, L.; Kateva, J.; Martinez, C. G.; Oliveros, E. 2 + 4)-Cycloaddition with Singlet Oxygen. 17O-lnvestigation of the Reactivity of Furfuryl Alcohol Endoperoxide¶. Photochem. Photobiol. 1999, 70 (6), 868−874. (53) Carraway, E. R.; Hoffman, A. J.; Hoffmann, M. R. Photocatalytic Oxidation of Organic Acids on Quantum-Sized Semiconductor Colloids. Environ. Sci. Technol. 1994, 28 (5), 786−793. (54) Choi, W.; Hoffmann, M. R. Photoreductive Mechanism of CCl4 Degradation on TiO2 Particles and Effects of Electron Donors. Environ. Sci. Technol. 1995, 29 (6), 1646−1654. (55) Perissinotti, L. L.; Brusa, M. A.; Grela, M. A. Yield of Carboxyl Anion Radicals in the Photocatalytic Degradation of Formate over TiO2 Particles. Langmuir 2001, 17 (26), 8422−8427. (56) Forouzan, F.; Richards, T. C.; Bard, A. J. Photoinduced Reaction at TiO2 Particles. Photodeposition from NiII Solutions with Oxalate. J. Phys. Chem. 1996, 100 (46), 18123−18127. (57) Mao, Y.; Schoeneich, C.; Asmus, K. D. Identification of organic acids and other intermediates in oxidative degradation of chlorinated ethanes on titania surfaces en route to mineralization: a combined photocatalytic and radiation chemical study. J. Phys. Chem. 1991, 95 (24), 10080−10089. (58) Konkena, B.; Vasudevan, S. Understanding Aqueous Dispersibility of Graphene Oxide and Reduced Graphene Oxide through pKa Measurements. J. Phys. Chem. Lett. 2012, 3 (7), 867−872. (59) Sillén, L. G.; Martell, A. E.; Bjerrum, J. Stability Constants of Metal-Ion Complexes.; Chemical Society: London, 1964. (60) Dimiev, A. M.; Alemany, L. B.; Tour, J. M. Graphene Oxide. Origin of Acidity, Its Instability in Water, and a New Dynamic Structural Model. ACS Nano 2013, 7 (1), 576−588. (61) Gerischer, H. On the stability of semiconductor electrodes against photodecomposition. J. Electroanal. Chem. Interfacial Electrochem. 1977, 82 (1), 133−143. (62) Cai, R.; Kubota, Y.; Fujishima, A. Effect of copper ions on the formation of hydrogen peroxide from photocatalytic titanium dioxide particles. J. Catal. 2003, 219 (1), 214−218. 3001


Photocatalytic Generation of H2O2 by Graphene Oxide in Organic

Recommend Documents