Observing the Role of Graphene in Boosting the Two-Electron

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Observing the Role of Graphene in Boosting the Two-Electron Reduction of Oxygen in Graphene−WO3 Nanorod Photocatalysts Bo Weng, Jing Wu, Nan Zhang, and Yi-Jun Xu* State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, PR China State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, New Campus, Fuzhou University, Fuzhou 350108, PR China S Supporting Information *

ABSTRACT: The new role of graphene (GR) in boosting the two-electron reduction of O2 to H2O2 has been first identified in the GR−WO3 nanorod (NR) nanocomposite photocatalysts, which are fabricated by a facile, solid electrostatic self-assembly strategy to integrate the positively charged branched poly(ethylenimine) (BPEI)-GR (BGR) and negatively charged WO3 NRs at room temperature. Photoactivity test shows that, as compared to WO3 NRs, BGR−WO3 NRs with an appropriate addition ratio of GR exhibit remarkably enhanced and stable visible-light photoactivity toward the degradation of Rhodamine B. Besides the common roles of GR observed in the GR-based composite photocatalysts in the literature, including enhancing the visible-light absorption intensity, improving the lifetime and transfer of photogenerated charge carriers, and increasing the adsorption capacity for reactants, we have observed the new role of GR in boosting the two-electron reduction of O2 to H2O2 in this specific BGR−WO3 NR photocatalyst system. Importantly, this new role of GR does contribute to the overall photoactivity enhancement of BGR−WO3 NR nanocomposites. The synergistic contribution of GR on improving the photoactivity of WO3 NRs and the underlying reaction mechanism have been analyzed by the structure−photoactivity correlation analysis and controlled experiments using radicals scavengers.

1. INTRODUCTION Since the emergence of graphene (GR) in 2004,1 GR has blossomed into a promising candidate for constructing highperformance photocatalysts owing to its unique two-dimensional (2D) structure and advantageous electronic and physicochemical properties as an ideal photocatalyst carrier.2−9 The enormous interest in utilizing GR in the field of photocatalysis has led to the massive exploration of various graphene−semiconductor composite photocatalysts for specific applications, including environmental cleanup, hydrogen evolution, and selective organic transformations. 2−27 A literature survey reveals that the common roles of graphene observed in the GR−semiconductor photocatalysts are mainly demonstrated to enhance the visible-light absorption intensity, improve the lifetime and transfer of photogenerated charge carriers, and increase the adsorption capacity for reactans.2−27 Recently, being distinctly different from the previous research on GR−semiconductor photocatalysts, for which GR is claimed to behave as an electron reservoir to capture/shuttle the electrons photogenerated from the semiconductor,2−28 our group has proposed a new photocatalytic mechanism where the role of GR in the ZnS−GR and ZnO−GR nanocomposite photocatalysts is an organic dye-like macromolecular “photosensitizer” for wide-band-gap ZnS or ZnO instead of an electron reservoir.29,30 These intriguing findings29,30 prompt us © 2014 American Chemical Society

to envisage that, besides the reported roles of graphene in GR− semiconductor nanocomposite photocatalysts, whether there are some other intrinsic functions of nonmetal material GR in the specific GR−semiconductor composite that have not been discovered in GR−semiconductor composite photocatalysts. Fundamental information on this aspect would significantly advance our understanding of the multifaced role of GR in various GR−semiconductor composites, which in turn aids the design of versatile GR-based composite photocatalysts toward specific applications.2−30 Hitherto, there have been significant efforts to explore graphene−semiconductor (e.g., TiO2, CdS, ZnS, ZnO, Bi2WO6, BiVO4, WO3, SnO2, and Sr2Ta2O7) composite photocatalysts with improved photoactivities.2−30 Among the photoactive semiconductors, considerable attention has been given to tungsten oxide (WO3), an important n-type semiconductor, due to its narrow band gap (2.4−2.8 eV), nontoxicity, stable physicochemical properties, resilience to photocorrosion, and high oxidation power of valence band (VB) holes, which make it possible to harvest visible light to degrade organic compounds or oxidize water for oxygen evolution.24−26,31−35 Received: December 23, 2013 Revised: March 26, 2014 Published: April 24, 2014 5574

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Scheme 1. Flowchart for the Preparation of BGR−WO3 NR Nanocomposites

(Shanghai, China). Branched poly(ethylenimine) (BPEI) (Mw 25 000) was obtained from Sigma-Aldrich Co., Ltd. (Shanghai, China). All materials were used as received without further treatment. Deionized (DI) water was obtained from local sources. 2.2. Catalyst Preparation. (a) Synthesis of WO3 nanorods (NRs). WO3 NRs were synthesized through a one-step hydrothermal treatment. Typically, 1.056 g of Na2WO4·2H2O and 0.935 g of NaCl were dissolved in 30 mL of DI water and kept stirring for 20 min. The pH value of the above solution was adjusted to about 2 using HCl (3.0 M) solution and stirred for 3 h. Then, the solution was transferred to a Teflon-lined autoclave and maintained at 180 °C for 24 h. Afterward, the products were cooled to room temperature, separated by centrifugation, and then washed with DI water. The sediment was dried at 60 °C and then WO3 NRs were obtained. (b) Synthesis of BPEI-graphene (BGR). Graphene oxide (GO) fabricated by a modified Hummers method was adopted as the precursor of graphene.10,12,14,27−30,37,38 Details of the typical process are presented in the Supporting Information. Four mL of BPEI (86 mg·mL−1) was added to 80 mL of the GO solution (0.4 mg·mL−1), followed by stirring for 20 min. The mixture was stirred at 90 °C for 2 h. The resulting stable black dispersion was centrifuged at 10 000 rpm and washed with DI water to remove excess polymer. BPEI−graphene (BGR) aqueous solution (0.25 mg·mL−1) was attained by redispersing them in a certain amount of DI water. (c) Synthesis of BGR−WO3 NR nanocomposites. The BGR−WO3 NR nanocomposites were fabricated by a simple and efficient electrostatic self-assembly method, as illustrated in Scheme 1. The given volume of positively charged BGR aqueous suspension (0.25 mg·mL−1) was added dropwise into a negatively charged WO3 NR aqueous dispersion (0.5 mg·mL−1). The weight addition ratios of BGR are selected to be 0.1, 0.5, 1, and 2%. The mixture was stirred for 1 h in water and then washed with DI water. The precipitate was dried in an oven at 60 °C, and the BGR− WO3 NR nanocomposites were obtained. 2.3. Catalyst Characterization. Zeta potential (ξ) measurements of the samples were determined by dynamic light scattering analysis (Zetasizer 3000HSA). Thermogravimetric (TG) analysis was conducted on a PerkinElmer TGA7 analyzer under 10 mL·min−1 flowing air at a heating rate of 10 °C·min−1. The morphology of the samples was analyzed by field-emission scanning electron microscopy (FESEM) on a FEI Nova NANOSEM 230 spectrophotometer and transmission electron microscopy (TEM) using a JEOL model JEM 2010 EX instrument. The phase composition of the samples was characterized by powder X-ray diffraction (XRD, Philip X’Pert Pro MPP) using Cu Kα radiation. An X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Thermo Scientific ESCA Lab250 spectrometer to obtain the surface composition of the sample. All of the binding energies were calibrated by the C 1s peak at 284.6 eV. The Raman spectra were recorded on a Renishaw inVia Laser Raman Co-focal microspectrometer with the laser at 532 nm. The Fourier transform infrared spectroscopy (FTIR) experiments were carried out on a Nicolet Nexus 670 FTIR spectrometer. UV−vis diffuse reflectance spectroscopy (DRS) on a UV−vis spectrophotometer (PerkinElmer Lambda 950 UV−Vis−NIR) was used to

However, the rapid recombination of photogenerated charge carriers and the relatively low conduction band (CB) edge of WO3 are two main obstacles inhibiting its photocatalytic efficiency.24−26,36 Considering that the CB level of WO3 is less negative than the potential for the single-electron reduction of oxygen, the photogenerated electrons in the CB of WO3 cannot be consumed efficiently by the oxygen molecules via the singleelectron reduction process (eq 1), which leads to the accumulation of photoinduced electrons on the surface of WO3 photocatalysts under visible-light irradiation and easy recombination of electrons and holes.24,36 Consequently, the photocatalytic activity of the WO3 semiconductor is low without any suitable optimization. O2 + e− = •O2−(aq),

−0.284 V vs NHE

(1)

In this work, we report a simple approach to construct graphene−WO3 nanorod (NR) nanocomposites via an electrostatic interaction between positively charged branched poly(ethylenimine) (BPEI)-GR (BGR) and negatively charged WO3 NRs in the aqueous phase at room temperature. In the BGR−WO3 nanorod (NR) nanocomposite photocatalysts, the new role of graphene (GR) in boosting the two-electron reduction of O2 to H2O2 has for the first time been identified. This novel function of GR along with its common roles observed in the GR-based composite photocatalysts in the literature, including enhancing the visible-light absorption intensity, improving the lifetime and transfer of photogenerated charge carriers, and increasing the adsorption capacity for the reactant, has proven to contribute to the overall visible-light photoactivity enhancement of BGR−WO3 NRs toward the degradation of Rhodamine B as compared to bare WO3 NRs. To the best of our knowledge, the effect of GR on promoting the multielectron transfer process for the activation of oxygen in GR-based semiconductor composite photocatalysts has not been reported previously.2−30 It is expected that this work could provide new insights for understanding the multifaced role of GR in the photoactivity improvement of GR-based nanocomposite photocatalysts for solar energy conversion.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium tungstate dihydrate (Na2WO4·2H2O), sodium chloride (NaCl), graphite powder, sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), potassium persulfate (K2S2O8), phosphorus pentoxide (P2O5), potassium permanganate (KMnO4), hydrogen peroxide, 30% (H2O2), ammonium oxalate ((NH4)2C2O4·H2O), tert-butyl alcohol (C4H10O), potassium peroxydisulfate (K2Cr2O7), and 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (C9H18NO2) were supplied by Sinopharm Chemical Reagent Co., Ltd. 5575

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measure the optical properties of the samples with BaSO4 as the internal reflectance standard. The photoluminescence (PL) spectra were obtained on an Edinburgh Analytical Instrument FLS 920 spectrophotometer. Photocurrent and electrochemical impedance spectroscopy (EIS) were carried out on a BAS Epsilon workstation and a CHI 660D workstation (CH Instruments, USA), respectively. The details are presented in the Supporting Information. Micromeritics ASAP2010 equipment was used to determine the nitrogen adsorption−desorption isotherms and the Brunauer−Emmett−Teller (BET) surface areas at 77 K. H2O2 was determined by a photometric method in which N,N-diethyl-p-phenylenediamine (DPD) is oxidized by a peroxidase (POD)-catalyzed reaction.39,40 2.4. Catalyst Activity. A 20 mg sample of the photocatalyst was dispersed in 40 mL of a 5 ppm RhB aqueous solution. The mixture was stirred for 1 h in the dark to allow the adsorption−desorption equilibrium between the sample and reactant before irradiation. A 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.) with a UV−CUT filter to cut off light of wavelength λ < 400 nm was used as the irradiation source. During the reaction, 3 mL of the sample was drawn from the system at a certain time interval. After the complete removal of the photocatalyst particles, the residual amount of RhB in the solution was analyzed on the basis of its characteristic optical absorption at 554 nm using a Varian UV−vis spectrophotometer (Cary 50, Varian Co.).

nanocomposites. In detail, when the positively charged BGR is added dropwise into the negatively charged WO3 NRs in the aqueous phase, floccules are gradually formed, indicating that the assembly of WO3 NRs and BGR is readily achieved by virtue of the electrostatic attraction between them. After stirring for 1 h to allow the accomplishment of the self-assembly process and then standing for several minutes, the floccules settle on the bottom of the beaker and the supernatant is totally pellucid, as shown in Figure S3 (Supporting Information). This phenomenon demonstrates that there is no loss of any component during the preparation procedure. In other words, the feedstock proportion of the materials almost equals the actual ratio of graphene to WO3 NRs, which is confirmed by the thermogravimetric (TG) results (Figure S4 in the Supporting Information). The whole preparation procedure for BGR−WO3 NRs can be pictorially illustrated in Scheme 1. Figure 1 reveals the morphology of WO3 NRs and the BGR− WO3 NR nanocomposites. It is easy to see from the scanning

3. RESULTS AND DISCUSSION The one-dimensional (1D) WO3 nanorods (NRs) are prepared through a hydrothermal reaction using sodium tungstate and hydrochloride acid as the precursors with the assistance of additive sodium chloride. As shown in Figure S1 (Supporting Information), the zeta potential (ξ) value of the as-obtained WO3 NRs dispersed in water without adding HCl or NaOH (pH 6) is around −55.8 mV, indicating that the surface of a WO3 NR is negatively charged in the aqueous phase. This can be ascribed to the formation process of WO3 NRs, during which Cl− ions could be preferentially absorbed onto the side crystal faces parallel to the c axis of the formed WO3 crystal nuclei to induce 1D growth.41 It is well known that graphene oxide (GO), a commonly used precursor of graphene (GR), is negatively charged due to the ionization of the carboxylic acid and phenolic hydroxyl groups existing on the surface of GO sheets.42,43 In view of this situation, it is hard to achieve effectively direct integration of the pristine WO3 NRs and GO because of their electrostatic repulsion. In order to realize the efficient assembly of WO3 NRs and GR, we consider tuning the surface charge of GO using the polymer, branched poly(ethylenimine) (BPEI), which can readily modify and reduce GO to the positively charged BPEI−graphene (BGR) via a lowtemperature wet chemistry approach in water.44 That is, modification and reduction of GO to the positively charged BGR are obtained simultaneously.44 The successful surface charge modification is confirmed by the zeta potential (ξ) measurement of BGR. It can be seen from Figure S1 (Supporting Information) that the BGR possesses a positively charged surface with the ξ value of +50.7 mV at pH 6 due to the incorporation of amino groups from both the reducing agent and stabilizer BPEI in the GO reduction process.44 Besides, the BGR is well redispersed and stable in water due to the electrostatic repulsions between positively charged BGR sheets (Figure S2 in the Supporting Information),44 thus offering a facile platform for integration with other components in a wet chemistry process. Under such circumstances, the negatively charged WO3 NRs and positively charged BGR can spontaneously establish a solid basis of electrostatic attraction for the self-assembly construction of the BGR−WO3 NR

Figure 1. Typical FESEM images of (a) WO3 nanorods and (b) a 1% BGR−WO3 NR nanocomposite; (c) TEM and (d) HRTEM images of 1% BGR−WO3 nanorods; the inset of c is the selected-area electron diffraction (SAED) pattern.

electron microscopy (SEM) image in Figure 1a that the characteristic one-dimensional (1D) WO3 NRs have been successfully synthesized via a facile hydrothermal reaction. In the BGR−WO3 NR nanocomposites, taking 1% BGR−WO3 NRs as an example, the 1D WO3 NRs spread on the surface of 2D BGR and some WO3 NRs are even enveloped by the BGR sheets, as revealed in Figure 1b, indicating that the intimate interfacial contact between the BGR sheets and WO3 NRs is readily obtained by such a simple electrostatic interaction approach. Furthermore, the transmission electron microscopy (TEM) image of 1% BGR−WO3 NRs in Figure 1c signifies that the WO3 NRs distribute on the surface of BGR sheets, and there is good interfacial contact formed between the 1D WO3 NRs and 2D BGR sheets. Since the transfer process of charge carriers in GR−semiconductor nanocomposites is intimately related to the interfacial interaction between graphene and a semiconductor,14,23 it could be expected that such good 5576

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Figure 2. XPS spectra of (a) C 1s XPS for GO, (b) the full spectrum, (c) C 1s, (d) N 1s, (e) W 4f, and (f) O 1s XPS for the 1% BGR−WO3 NR nanocomposite.

content of BGR and the possible overlap of its main diffraction peak at ca. 25.6° with the peak of hexagonal WO3 at 26.8°. X-ray photoelectron spectroscopy (XPS), as an effective surface-analyzing technique, is often utilized to investigate the surface element composition of samples and the corresponding valence state.45 As shown in Figure 2a, the high-resolution C 1s XPS spectrum of GO can be fitted into four peaks, corresponding to C atoms in different functional groups, i.e., the nonoxygenated ring C (C−C, CC and C−H) at 284.6 eV, the C in C−OH bonds at 286.2 eV, the C in C−O−C and CO bonds at 287.5 eV, and the carbonyl C at 289.4 eV.14,21,29,46−48 The direct comparison of C 1s XPS spectra between GO (Figure 2a) and the 1% BGR−WO3 NR nanocomposites (Figure 2c) indicates that the number of oxygen-containing functional groups on the surface of BPEI-GR is significantly decreased as compared to that on GO, suggesting that during the low-temperature surface charge modification process for GO, the effective deoxygenation and partial restoration of the sp2 carbon site can also be achieved. Notably, a peak at 285.7 eV appearing in the C 1s XPS of 1% BGR−WO3 NRs can be distinguished, which is ascribed to the C bound to nitrogen.49 Besides, the peak at 289.4 eV of GO is shifted to 288.6 eV in the C 1s spectrum of 1% BGR−WO3 NRs. This may be induced by the formation of the amide (N−

interfacial contact for the BGR−WO3 NRs should favor the charge carriers’ transfer process and thus the improvement of photoactivity. The lattice spacing measured in the highresolution TEM (HRTEM) image (Figure 1d) equals 0.391 nm, which corresponds to the (001) crystal plane of hexagonal WO3. The selected-area electron diffraction (SAED) pattern of the 1% BGR−WO3 NRs shows a set of well-defined spots (the inset of Figure 1c), which is indexed to the [010] zone axis of hexagonal WO3 with a preferred growth direction along the (001) crystal facet. The XRD spectra of the WO3 NRs and BGR−WO3 NRs are displayed in Figure S5 (Supporting Information). The asprepared BGR−WO3 NR nanocomposites with different weight addition ratios of BGR exhibit a similar pattern to that of bare WO3 NRs, and all of the samples possess high crystallinity. All diffraction peaks of the samples match well with the standard data of hexagonal-phase WO3 with lattice constants of a = b = 7.298 Å and c = 3.899 Å (JCPDS card no. 33-1387). The main characteristic peaks at 2θ values of 14.0, 22.7, 24.3, 26.8, 28.2, 36.6, 50.0, and 55.3° correspond to the (100), (001), (110), (101), (200), (201), (220), and (202) crystal planes of hexagonal WO3, respectively. No apparent diffraction peak for BGR can be observed, which may be ascribed to the low 5577

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with neighboring oxygen atoms [υ(O−W−O)].55−58 The peak at around 927 cm−1 is attributed to the terminal −WO bonds,57 whereas those at around 240, 326, and 665 cm−1 arise from the O−W−O bending modes [δ(O−W−O)].25,55,57,58 The peak at the low wavenumber of 106 cm−1 comes from the lattice modes of crystalline WO3.56 These characteristic peaks can be also found in the Raman spectrum of BGR−WO3 NRs; besides, the typical D band (1346 cm−1) and G band (1597 cm−1) of graphene are detected, indicating the presence of both graphene and WO3. Compared to the spectrum of the bare WO3 NRs, the peaks at 665, 757, and 814 cm−1 corresponding to the O−W−O vibration are broadened in the BGR−WO3 NR Raman spectrum; in addition, the peak at 814 cm−1 is shifted to 806 cm−1 for the WO3 in the BGR−WO3 NR nanocomposites, probably due to the strong interaction between the WO3 NRs and BGR, making the initial W−O bond weaker.22 Such similar phenomena have also been reported in previous reports.22,59 These results confirm the formation of BGR−WO3 NR nanocomposites and indicate that WO3 NRs are bonded to BGR rather than physically adsorbed on GR sheets. This kind of structure is beneficial for effective charge transfer between WO3 NRs and BGR upon light excitation.22 In addition, GO and 1% BGR−WO3 NRs are further characterized by infrared spectroscopy (FTIR). As shown in Figure S6 (Supporting Information), the FTIR spectrum of GO shows a broad absorption band at 3427 cm−1 which is related to the O−H stretching vibrations. The absorption band appearing at around 1600 cm−1 is indexed to the unoxidized graphitic vibrations, i.e., the skeletal vibration of the GO sheets.60 In the FTIR spectrum of 1% BGR−WO3 NRs, the bands in the fingerprint region below 1000 cm−1 result from the presence of WO3 NRs.61 More specifically, the band at 827 cm−1 can be attributed to the W−O units and that at 746 cm−1 corresponds to the stretching vibrations of the bridging oxygen atoms O− W−O.62 It can be seen that the intensity of the −OH group absorption band in the 1% BGR−WO3 NR FTIR spectrum is decreased as compared to that for GO, indicating the effective removal of the oxygen functional groups in GO after the mild modification treatment, which is consistent with the XPS results. Therefore, based on the above discussion, it can be concluded that the BGR−WO3 NR nanocomposites have been successfully prepared via the self-assembly approach.

CO) linkage, in which the binding energy of C is larger than that in HO−CO bonds due to the electron donation from the adjacent nitrogen atom.50 As shown in Figure 2d, the N 1s XPS spectrum of 1% BGR−WO3 NRs gives rise to three peaks at 399.3, 400.2, and 402. eV that are attributed to the amino groups −NH2, N−C bond, and N−O bond, respectively,49,51 indicating that BPEI is successfully linked to the graphene surface covalently. In the high-resolution spectra of W 4f for the 1% BGR−WO3 NR nanocomposites (Figure 2e), there are two peaks at 35.3 and 37.5 eV which can be ascribed to W 4f7/2 and W 4f5/2, respectively. These results signify that W is in the +6 oxidation state with the typical binding energies.52 Notably, the binding energy values of W 4f7/2 and W 4f5/2 in the 1% BGR− WO3 NR nanocomposites are slightly lower than those for bare WO3.52−54 Such a shift may be attributed to the interaction between WO3 NRs and BGR.52 The O 1s peak (Figure 2f) centered at 530.5 eV is associated with the lattice oxygen in WO3, and the other O 1s peak at 531.8 eV indicates the presence of the −OH group and O−C bonds in the 1% BGR− WO3 NR nanocomposite.52 From the full spectrum of 1% BGR−WO3 NRs in Figure 2b, it can be seen that the sample consists of W, O, and N elements without impurities. Raman spectroscopy is an effective technique for characterizing the microstructure of nanocrystalline materials owing to its high sensitivity to the electronic structure.15,21,25,44,55 As shown in Figure 3, peaks centered at 757 and 814 cm−1 are

Figure 3. Raman spectra of bare WO3 NRs and 1% BGR−WO3 NR nanocomposites at room temperature.

observed in the Raman spectrum of bare WO3 NRs, which can be assigned to the stretching vibration of the tungsten atom

Figure 4. Photocatalytic performance of bare WO3 NRs and BGR−WO3 NR nanocomposites with different weight addition ratios of BGR for the degradation of RhB in the aqueous phase under visible-light irradiation (λ > 400 nm): (a) time-online degradation curves composed of C/C0 versus irradiation time and (b) pseudo-first-order linear fitting results (ka, apparent kinetic rate constant). 5578

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Figure 5. (a) Emission photoluminescence (PL) spectra with the excitation wavelength λ at 380 nm, (b) transient photocurrent responses, and (c) electrochemical impedance spectroscopy (EIS) Nyquist diagrams of bare WO3 NRs and 1% BGR−WO3 NR nanocomposites.

To understand the causes of the photoactivity enhancement of BGR−WO3 NR nanocomposites toward the degradation of RhB under visible-light irradiation as compared to WO3 NRs, a collection of characterizations have been performed. The optical properties of the WO3 NRs and BGR−WO3 NR nanocomposites have been investigated. Figure S8 (Supporting Information) displays the UV−vis diffuse reflectance spectra (DRS) of the WO3 NRs and BGR−WO3 nanocomposites. It is easy to see that the light absorption intensity in the visible-light region with the range of 450−800 nm for the BGR−WO3 nanocomposites is generally enhanced with the increase in the weight addition ratios of BGR as compared to that for bare WO3 NRs. Evidently, the introduction of BGR into the WO3 NRs is able to promote the visible-light response of the BGR− WO3 NR nanocomposites, which can be ascribed to two reasons. One is due to the background light absorption of BGR in the visible-light region. The other could be related to the enhancement of the surface electric charge of WO3 NRs, which results from the possible electronic transition of π → π* of BGR and n → π* between the n orbit of the oxygen species of WO3 NRs and BGR.38,66,67 Such an observation has also been reported in other carbon−semiconductor composite photocatalysts.10,14,38,66,67 The photoluminescence (PL) is a useful tool for investigating surface processes involving the fate of charge carriers photoexcited from semiconductors.21,38,63 As shown in Figure 5a, the PL intensity obtained from 1% BGR−WO3 NR nanocomposites is diminished as compared to that of WO3 NRs, demonstrating that the more efficient inhibition of charge carriers recombination in the 1% BGR−WO3 NR nanocomposites is realized. This should be ascribed to the introduction of graphene with high electron conductivity which can serve as a 2D network of an electron reservoir to accept and shuttle electrons photogenerated from WO3 NRs.2,10,12,14,21,22,27,42 As a result, the separation and lifetime of electron−hole pairs are improved, which thus contributes to the photoactivity enhancement of the BGR−WO3 NR nanocomposites. The thermodynamic permissibility for the

The photocatalytic performance of the BGR−WO3 NR nanocomposites and WO3 NRs is assessed by the degradation of aqueous Rhodamine B (RhB), a popular probe molecule in the heterogeneous photocatalytic degradation of dyes.63,64 Figure 4a illustrates the photocatalytic activity for the degradation of RhB under visible-light irradiation over the BGR−WO3 NR nanocomposites and WO3 NRs. Under dark conditions without light illumination, the concentration of RhB almost does not change in the presence of WO3 NRs and BGR−WO3 NR nanocomposites after the adsorption− desorption equilibrium is achieved, indicating that the reaction is indeed induced by a photocatalytic process. It is clear to see that the photocatalytic performance of WO3 NRs is enhanced by the incorporation of an appropriate amount of BGR. Among all of the samples, 1% BGR−WO3 NR nanocomposites exhibits the best visible-light photocatalytic activity toward the degradation of RhB. With 2 h of visible-light irradiation, ca. 96% of RhB is removed over 1% BGR−WO3 NRs, whereas less than 14% RhB is removed in the case of WO3 NRs. However, when the weight addition ratio of BGR is increased to 2% in the nanocomposite, the photoactivity declines, which may be due to the shielding effect of BGR,10,11,14,23,27 but it is still higher than that of bare WO3 NRs. Such an optimal synergistic effect between GR and the semiconductor has been widely observed in previous works.10,14,21,23,27,42 As displayed in Figure 4b, the curves for all of the samples are fitted based on the fact that the degradation of dyes can be ascribed to a pseudo-first-order reaction with a simplified Langmuir−Hinshelwood model when the initial concentration of the dyes (C0) is very small.63,65 The apparent kinetic rate constants (ka) follow the order 1% BGR− WO3 NRs (0.02252 min−1) > 0.5%BGR−WO3 NRs (0.01665 min−1) > 0.1% BGR−WO3 NRs (0.01424 min−1) > 2% BGR− WO3 NRs (0.01102 min−1) > WO3 NRs (0.00657 min−1). Furthermore, the recycled photoactivity test (Figure S7 in the Supporting Information) shows that the photocatalytic performance of the used 1% BGR−WO3 NRs is similar to that of fresh 1% BGR−WO3 NRs during four tests, indicating that the photocatalyst is stable and reusable. 5579

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higher adsorption capacity toward dye molecules over the BGR−WO3 NR nanocomposites than WO3 NRs indicates a higher accumulating concentration of reactants over the photocatalyst surface, which is beneficial to the photoactivity enhancement. To learn the involvement of active radical species in the photocatalytic process over BGR−WO3 NRs for the degradation of RhB under visible-light irradiation, controlled experiments have been conducted with the addition of radicals scavengers. It can be seen from Figure 6 that the concentration

transformation of electrons from the conduction band (CB) of WO3 to graphene is faithfully verified by their matched energy band. We can acquire from the Mott−Schottky plot (Figure S9 in the Supporting Information) for 1% BGR−WO3 NRs that (i) the positive slope of the C−2−E plot suggests the expected n-type semiconductor of WO3;68 (ii) its flat-band potential (Vfb) obtained by extrapolation of the Mott−Schottky plot equals ca. −0.46 V vs Ag/AgCl, −0.26 V vs NHE, which is more negative than the work function of graphene (−0.08 V vs NHE) and less negative than the standard reduction potential of O2/•O2− (−0.284 V vs NHE).23,33,61 Therefore, the transport of photoinduced electrons from the CB of WO3 NRs to graphene is thermodynamically favorable. However, the single-electron reduction of O2 to superoxide radicals (•O2−) is not feasible in the present system, which is in accordance with the result of electron spin resonance (ESR) analysis. As shown in Figure S10 (in the Supporting Information), there is no detectable signal of •O2− over that of the 1% BGR−WO3 NR nanocomposites. To further confirm the effect of graphene on the lifetime of the photoexcited charge carriers, the transient photocurrent responses of WO3 NRs and 1% BGR−WO3 NRs have also been studied under intermittent visible-light illumination with the exact wavelength range used in the photocatalytic reactions. The results in Figure 5b reveal that the addition of BGR is able to enhance the photocurrent remarkably, indicating a more efficient separation of the electron−hole pairs obtained in the 1% BGR−WO3 NR nanocomposites in contrast to that in bare WO3 NRs. The photocurrent decay is observed for the samples during the first few switch-on and -off cycles, indicating that the recombination process of photogenerated electron−hole pairs is occurring. After the equilibration of competitive separation and recombination of photogenerated electron−hole pairs, the photocurrent reaches a relatively constant value.14,69 Besides, electrochemical impedance spectroscopy (EIS) Nyquist plots signify that the introduction of BGR can facilitate the interfacial charge transfer more effectively than WO3 NRs can, as reflected by the decreased semicycle arc at high frequencies in the EIS Nyquist plots (Figure 5c).27,38,70 Therefore, BGR in the BGR− WO3 NR nanocomposite photocatalysts is able to act as an electron reservoir to enhance the fate and transfer of photoinduced charge carriers, thus contributing to the visiblelight photoactivity improvement for the degradation of RhB. Furthermore, the Brunauer−Emmett−Teller (BET) surface area and porosity structure of WO3 NRs and 1% BGR−WO3 NRs have also been analyzed. As reflected in Figure S11 (Supporting Information), both WO3 NRs and 1% BGR−WO3 NRs give rise to a type IV adsorption−desorption isotherm with a typical H3 hysteresis loop according to the IUPAC classification.71 The BET surface area and total pore volume of 1% BGR−WO3 NRs are ca. 30 m2·g−1 and 0.063 cm3·g−1, which are larger than that of bare WO3 NRs (ca. 25 m2·g−1 and 0.059 cm3·g−1), as listed in Table S1. This could contribute to the higher adsorption capacity of 1% BGR−WO3 NRs toward RhB dye molecules. Besides, the π−π bonds formed between the benzene ring of BGR and the aromatic rings of RhB could also facilitate the adsorption process on the surface of BGR− WO3 NRs, whereas such attractive interaction does not exist in the case of bare WO3 NRs. The results of adsorption experiments in the dark for the RhB molecule over WO3 NRs and 1% BGR−WO3 NR nanocomposites confirm the difference of their adsorption capacity due to the introduction of BGR (Figure S12, in the Supporting Information). The

Figure 6. Controlled experiments with different conditions including the addition of ammonium oxalate (AO) as a photogenerated hole scavenger, tert-butyl alcohol (TBA) as a scavenger for hydroxyl radicals, Cr(VI) as an electron scavenger, and 4-hydroxy-2,2,6,6tetramethylpiperidinyloxy (TEMPOL) as a scavenger for superoxide radicals, H2O2, and with an N2-saturated atmosphere for the photocatalytic degradation of RhB in the aqueous phase over 1% BGR−WO3 NR nanocomposites under visible-light irradiation (λ > 400 nm).

of RhB in the aqueous phase decreases slightly with the irradiation of visible light in the absence of the photocatalyst, which is ascribed to the self-photosensitized degradation of the RhB molecule.72 With adding ammonium oxalate (AO) as a hole scavenger to the solution,63 the rate of degradation for RhB over 1% BGR−WO3 NRs is remarkably decreased, suggesting that the photogenerated holes play an important role in the photocatalytic process for the degradation of RhB. When tert-butyl alcohol (TBA), as a scavenger for hydroxyl radicals (•OH) is added,29,40 the rate of degradation for RhB is also decreased but with less inhibition than that with addition of AO. The presence of hydroxyl radicals in the current system can be evidenced by electron spin resonance (ESR) spectra, as displayed in Figure S13 (in the Supporting Information). These results demonstrate that the holes photogenerated from WO3 NRs play a more significant role than hydroxyl radicals toward the photocatalytic degradation of RhB over BGR−WO3 NR nanocomposites. The degradation efficiency is almost not affected by the addition of 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPOL) as a superoxide radical scavenger.73 This is consistent with the ESR results (Figure S10 in the Supporting Information) that superoxide radicals are not detected in the present system. The results obtained with the addition of Cr(VI) as an electron scavenger73,74 and under N2 saturation reveal that the electrons and oxygen molecule also contribute to the photocatalytic degradation process over BGR−WO3 NR 5580

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The following controlled experiment corroborates the positive function of H2O2 in improving the photoactivity of BGR−WO3 NR nanocomposites. It is easy to see that the degradation rate over 1% BGR−WO3 NR nanocomposites is increased with the addition of H2O2, indicating that H2O2 does contribute to the enhanced photoactivity of BGR−WO3 NR composites. In other words, besides the common roles of GR in enhancing the photoactivity of GR-based composite photocatalysts in the literature, including enhancing the visible-light absorption intensity, improving the lifetime and transfer of photogenerated charge carriers, and increasing the adsorption capacity for a reactant, we for the first time identify the new role of GR in boosting the two-electron reduction of O2 to H2O2 in this specific BGR−WO3 NR photocatalyst system which is favorable to improved photocatalytic performance. In addition, it should be noted that H2O2 can be produced from the coupling of hydroxyl radicals (eq 3).78 To exclude the possible contribution of hydroxyl radicals to the enhanced formation of H2O2 over 1% BGR−WO3 NRs, the effect of the introduction of BGR to WO3 NRs on the development of hydroxyl radicals has been investigated. The ESR spectra of a hydroxyl radical adduct trapped by DMPO (DMPO−OH·) over a bare WO3 NRs aqueous suspension are shown in Figure S14 (in the Supporting Information), which demonstrates the presence of hydroxyl radicals over WO3 NRs under visible-light irradiation. The comparison of the ESR spectra obtained in 1% BGR−WO3 NRs and the bare WO3 NR aqueous dispersion (Figures S13 and S14 in the Supporting Information, respectively) reveals that there is no significant difference with respect to the signal intensity of hydroxyl radicals in these two systems under the same experimental conditions. Considering that the water used in the ESR analysis is purified and deoxygenated, the possible contribution of the decomposition of H2O2 formed by the two-electron reduction of oxygen to the hydroxyl radicals can be excluded. Under such circumstances, it can be concluded that the introduction of graphene does not enhance the formation of hydroxyl radicals via the reaction of water with the photogenerated holes. Therefore, hydroxyl radicals are not the primary origin of the enhanced formation of H2O2 over the BGR−WO3 NR nanocomposites in the present system. This result along with the controlled experiments indicates that the promoted formation of H2O2 over BGR−WO3 NRs should be ascribed to the accelerated two-electron reduction of oxygen due to the introduction of graphene into the matrix of WO3 NRs.

nanocomposites. However, as discussed above, the photoexcited electrons of WO3 hardly activate the oxygen molecule via the single-electron reduction process due to the fact that the CB potential of WO3 is less negative than the potential of single-electron reduction of O2.24,36 In view of this, we would wonder how oxygen plays its role during the photocatalytic process over BGR−WO3 NRs. In 2008, Abe et al. reported that loaded Pt on the surface of WO3 can promote the multielectron reduction of oxygen to produce H2O2 (eq 2), thus significantly lessening the recombination of charge carriers.33 Such a phenomenon has also been observed in WC/WO3,75 CuO/WO3,76 and Cu(II)/ WO3.77 This two-electron reduction process is thermodynamically permissible due to the fact that the CB level of WO3 (−0.26 V vs NHE) is more negative than the standard reduction potential of O2/H2O2 (+0.682 V vs NHE). This discovery encourages us to imagine (i) whether nonmetal material graphene possesses the function to accelerate the multielectron reduction of O2 to H2O2 and (ii) if such an effect of graphene exists, does it favor the photoactivity enhancement of WO3 ? To date, the investigation of the role of graphene in this respect has not been available, although the roles of that GR play in GR-based composite photocatalysts have been widely studied in the literature.2−30 O2 + 2H+ + 2e− = H 2O2 (aq),

+0.682 V vs HE

(2)

In the present photocatalytic system, H2O2 is detected by the DPD-POD method.39,40 As shown in Figure 7, with 30 min of

Figure 7. Detection of H2O2 in the WO3 NRs and 1% BGR−WO3 NR aqueous dispersions as well as the counterpart without any photocatalysts for comparison; the curves are obtained by the addition of DPD and POD to the solution after 30 min of visible-light irradiation (λ >400 nm).

•OH + •OH = H 2O2

visible-light irradiation, there are two absorption maxima that emerge at ca. 510 and 551 nm in both the WO3 NRs and 1% BGR−WO3 NRs aqueous dispersion, indicating the presence of H2O2 in this system according to the literature.39 However, under the same condition but without any photocatalysts, no characteristic peak ascribed to H2O2 is detected, which suggests that the presence of photocatalysts is indispensable to the formation of H2O2 under visible-light irradiation. Obviously, the amount of H2O2 formed in the 1% BGR−WO3 NR aqueous dispersion is higher than that in WO3 NRs. These results show that the introduction of BGR into the matrix of WO3 NRs is effectively able to promote the formation of H2O2 via the two-electron reduction of O2. Notably, to the best of our knowledge, such a prominent role of GR in boosting the twoelectron reduction of oxygen has not been reported previously,2−30 and for the first time it is identified in the current BGR−WO3 NRs composite photocatalyst system.

(3)

Accordingly, the reaction mechanism for the photocatalytic degradation of RhB over the BGR−WO3 NRs can be portrayed in Scheme 2. Upon the visible-light irradiation, the electrons are excited from the valence band (VB) to the conduction band (CB) of WO3 NRs, creating photogenerated electron−hole pairs. In the absence of GR, the photogenerated charge carriers quickly recombine without performing an efficient degradation of RhB, leading to low photoactivity. As for BGR−WO3 NRs, the photogenerated electrons can transfer from WO3 to BGR nanosheets owing to their matchable energy band and intimate interfacial contact, which leads to the improved lifetime and transfer of photogenerated charge carriers. The adsorbed oxygen molecule can be reduced to H2O2 via a two-electron path, while the holes can not only oxidize the adsorbed dye molecule directly but also react with hydroxyl groups to generate hydroxyl radicals. Besides, due to the introduction of 5581

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Schottky plot of 1% BGR−WO3 NR nanocomposites, ESR spectra of a radical adduct trapped by DMPO over the 1% BGR−WO3 NR nanocomposites suspension without or with visible-light irradiation, nitrogen adsorption−desorption isotherms of bare WO3 NRs and 1% BGR−WO3 NR nanocomposites, summary of the physicochemical properties of bare WO3 NRs and 1% BGR−WO3 NR nanocomposites, remaining concentration fraction of RhB after the adsorption−desorption equilibrium is achieved over bare WO3 NRs and BGR−WO3 NR nanocomposites in the aqueous phase, and ESR spectra of a radical adduct trapped by DMPO over a bare WO3 NR suspension without or with visible-light irradiation. This material is available free of charge via the Internet at http:// pubs.acs.org.

Scheme 2. Model Picture Demonstrating the Proposed Reaction Mechanism for the Degradation of RhB over BGR−WO3 NR Nanocomposites under Visible-Light Irradiation (λ > 400 nm)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86 591 83779326. E-mail: [email protected].

BGR, the accumulating concentration of RhB over the surface of BGR−WO3 NRs is increased. As a result, the adsorbed RhB molecule can be effectively degraded by these reactive oxidative species (ROSs), including holes, hydroxyl radicals, and H2O2, over the BGR−WO3 NR composite photocatalyst under visible-light irradiation.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the National Natural Science Foundation of China (NSFC) (21173045, 20903023, and 20903022), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), the Program for Returned High-Level Overseas Chinese Scholars of Fujian Province, the Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), the project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, and the project supported by National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (no. J1103303) is gratefully acknowledged.

4. CONCLUSIONS The graphene−WO3 nanorod (BGR−WO3 NR) nanocomposites have been assembled from positively charged branched poly(ethylenimine) (BPEI)-GR (BGR) and negatively charged WO3 NRs through a facile, solid, electrostatic self-assembly approach. In this nanocomposite photocatalyst system, the new role of graphene (GR) in boosting the two-electron reduction of O2 to H2O2 has been first identified. As compared to bare WO3 NRs, the BGR−WO3 NR nanocomposites with an appropriate addition ratio of BGR exhibit enhanced the visiblelight photocatalytic performance for the degradation of RhB. A series of characterizations and controlled experiments reveal that graphene demonstrates quadruple roles in improving the visible-light photoactivity of BGR−WO3 NR nanocomposites: (i) enhanced visible-light absorption intensity, (ii) boosted fate and transfer of photogenerated charge carriers, (iii) improved adsorption capacity for reactants, and, more importantly identified in this specific composite photocatalyst, (iv) the accelerated two-electron reduction of O2 to produce H2O2. It is hoped that this work could provide new insight and promote ongoing interest in understanding the multifaced role of graphene in GR-based composite photocatalysts for light harvesting, thereby aiding the fabrication of versatile GRbased composite photocatalysts toward targeting applications.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for the preparation of graphene oxide (GO) and electrochemical analysis, zeta potential (ξ) of WO3 nanorods and the BPEI-GR aqueous dispersion, digital photographs of the BPEI-GR aqueous dispersion and the samples during the preparation procedure for 1% BGR−WO3 NR nanocomposites, thermogravimetric (TG) analysis of WO3 NRs and 1% BGR−WO3 NR nanocomposites, XRD patterns and UV−vis diffuse reflectance spectra (DRS) of bare WO3 NRs and BGR−WO3 NR nanocomposites with different weight addition ratios of BGR, FTIR spectra of graphene oxide (GO) and 1% BGR−WO3 NR nanocomposites, recycled photoactivity test of 1% BGR−WO3 NR nanocomposites, Mott− 5582

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dx.doi.org/10.1021/la4048566 | Langmuir 2014, 30, 5574−5584