Improved Solar-Driven Photocatalytic Activity of Hybrid Graphene

Oct 31, 2016 - Electrochemistry and Inhibitors Group, Industrial Protection Division, ... the potential reaction pathway following a direct Z-scheme a...
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Research Article pubs.acs.org/journal/ascecg

Improved Solar-Driven Photocatalytic Activity of Hybrid Graphene Quantum Dots/ZnO Nanowires: A Direct Z‑Scheme Mechanism Mahdi Ebrahimi,† Morasae Samadi,† Samira Yousefzadeh,† Mojtaba Soltani,† Alireza Rahimi,‡ Tsu-chin Chou,§ Li-Chyong Chen,§ Kuei-Hsien Chen,⊥ and Alireza Zaker Moshfegh*,†,# †

Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Iran Electrochemistry and Inhibitors Group, Industrial Protection Division, Research Institute of Petroleum Industry, P.O. Box 14857-33111, Tehran, Iran § Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan ⊥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan # Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O.Box 14588-89694, Tehran, Iran ‡

S Supporting Information *

ABSTRACT: Herein, an electrochemical technique as a costeffective and one-step approach was utilized to fabricate graphene quantum dots (GQDs). Different amounts of GQDs (0, 0.2, 0.4, 0.8, and 1.2 wt %) were decorated uniformly on the surface of anodized ZnO nanowires (NWs) forming GQD/ZnO NWs. Transmission electron microscopy and atomic force microscopy confirmed formation of GQDs on the ZnO NWs, 12−22 nm in width and 1−3 graphene layers thick. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were employed to verify the functional groups on the surface of GQDs, and the results indicated that GQDs readily anchored on the surface of ZnO NWs. The GQD/ ZnO NWs exhibited a considerable improvement on the photocatalytic degradation of methylene blue under solar irradiation, due to efficient light absorption. In addition, the results indicated that the optimized GQD (0.4 wt %)/ZnO NWs showed the highest photoactivity with about 3-fold enhancement as compared to pure ZnO NWs. Finally, a mechanism of charge carrier generation, transport, and separation was proposed using different scavengers to probe the potential reaction pathway following a direct Z-scheme approach. KEYWORDS: Heterogeneous photocatalysis, Graphene quantum dots, ZnO nanowires, Scavengers, Z-scheme, Visible light, Photocatalytic activity

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absorption of only ∼4% of solar spectrum. Thus, surface modification of ZnO nanostructures for improving the photocatalytic activity under visible light (∼43% of solar irradiation) is still a great challenge among researchers. Various modification routes to activate ZnO photocatalysis under visible light have been employed in our group in the past few years, including sensitization, 10−12 semiconductor coupling,13−15 and doping.16 Very recently, different doping strategies have been reviewed for photocatalytic application of ZnO nanostructures under visible light.17 Modification with carbon structures such as graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphitic carbon nitride (g-C3N4), and other carbon nanostructures received a great attention.18 Their efficient roles in charge carrier separation, photoactivity in visible light, and enhancing catalytic reaction sites show promise for the design of

INTRODUCTION In recent years, environmental pollution and energy shortages have created serious social and economic issues, which caused key challenges for human society.1 Photocatalysis on various metal oxide semiconductors and related compounds has emerged as a highly efficient technique to overcome these challenges.2−6 The advantages of this well-established approach is due to its ease of process, being environmentally clean and low cost.7 Although some of the photocatalytic semiconductors have been commercialized for air or water purifications, selfcleaning procedures, and antibacterial activity, their efficiency is still low due to their wide band gap and electron−hole pair recombination.8 Among various semiconductors, ZnO has been extensively studied in the past decade, because of its unique properties such as high electron mobility, biocompatibility, photosensitivity, and low cost, which can be utilized in different applications.9 ZnO nanostructures, as widely used for photocatalytic degradation of pollutants, allow the absorption of ultraviolet (UV) light due to its wide band gap (3.37 eV), which allows the © 2016 American Chemical Society

Received: July 25, 2016 Revised: October 23, 2016 Published: October 31, 2016 367

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

Research Article

ACS Sustainable Chemistry & Engineering nanocomposites with significant activity.18 Recently, carbonbased quantum dots (C-QDs) have opened up new possibilities in developing hybrid nanomaterials with enhanced photocatalytic activity because of their unique optical and electronic properties.19 In addition, their low-cost, intrinsic low toxicity, nonblinking emission, and water solubility hold considerable promise as an alternative to conventional semiconductor quantum dots (QDs).20,21 Recently, Yang et al. reported a Zscheme approach in the coupling of carbon nanodots with a semiconductor.22 The Z-scheme charge transfer mechanism in heterogeneous photocatalytic reactions is a two-step photoexcitation by mimicking the natural photosynthesis process, which is an effective strategy to improve the photocatalytic performance. Among C-QDs, graphene quantum dots (GQDs) are singleor few-layer graphene nanosheets with lateral size less than 100 nm.23 They exhibit semiconducting properties and strong fluorescence, which arise from the quantum confinement effect and edge effect.24 π-Plasmon absorption in GQDs provides the capability of UV/vis absorption and thus harvests a significant portion of the solar spectrum. Therefore, GQDs have been considered as a candidate for photosensitizers in the design of visible-light active photocatalysis.25−29 Moreover, it is believed that GQDs disrupt the radiative recombination of photoinduced electron−hole pairs and provide an excellent opportunity for efficient photocatalytic degradation.30−36 Considering these significant factors, photocatalytic applications of GQDs become a growing research field as reflected by the number of recent publications. Different strategies have been performed for synthesis of GQDs in nanohybrid semiconductor composites for enhanced photocatalytic activity.37,38 Among various methods, the electrochemical route is well-known as a facile, low cost, onestep, and large-scale production approach. In this research, we have applied this useful method to produce GQDs on ZnO NWs to investigate their photocatalytic activity. To the best of our knowledge, there are a few reports on photocatalytic applications of GQD/ZnO nanohybrids,33 and research in this field is still in its infancy and needs further investigation. To obtain optimum photocatalytic activity over the GQD/ZnO NWs, different weight percentage (wt %) of GQDs as a visiblelight sensitizer was added to the prepared ZnO NWs. Furthermore, for the first time, we have employed different scavenger tests to elucidate the mechanism of photocatalytic degradation of methylene blue (MB) as a model of organic pollutant under solar irradiation. At the end, we propose a model to describe the possible charge transfer mechanism at the interface of GQD/ZnO NWs for MB photodegradation enhancement through a Z-scheme approach.

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Preparation of ZnO Nanowires. Zinc (99.98%) foil with a thickness of 200 μm was used and cut into rectangular shape with a length of 3 cm and width of 1 cm as substrate. The anodization process is similar to the previously reported method with some modification.40 Prior to anodization, the foil was rinsed with ethanol and DI water to remove any dirt from the surface. Then, the cleaned foil was electropolished in ethanol/phosphoric acid solution with volume ratio of 7:3 under applied voltage of 12 V for 3 min. The anodization process was carried out in 5 mM KHCO3 solution under 12 V DC voltage for 50 min to form ZnO NWs. The prepared ZnO NWs were subjected to a subsequent annealing at 250 °C for 60 min to improve their crystallinity. Fabrication of GQD/ZnO NW Hybrids. GQD/ZnO NW hybrids were fabricated using the spin coating method. Different amounts of GQDs were deposited on the ZnO NWs at spin speed of 500 rpm for 30 s. Then, the GQD/ZnO NWs were placed into a tube furnace and annealed at 250 °C in a nitrogen environment for 60 min. Figure 1 depicts the overall fabrication process of the GQD/ZnO NW hybrids.

Figure 1. Schematic view of the fabrication processes of (a) GQDs, (b) ZnO NWs, and (c) GQD/ZnO NWs. Characterization and Electrochemical Measurements. Surface morphology of the samples was characterized by field emission scanning electron microscopy, FESEM (ZEISS, SIGMA VP), and transmission electron microscopy, TEM (Philips CM30 TEM). In addition, atomic force microscopy (AFM) using a commercial microscope (Veeco Autoprobe CP-research) was also performed in contact mode with a Si tip of nominal radius of 10 nm to observe the surface roughness. Optical absorption spectra of the all samples were studied by UV−visible diffuse reflectance spectroscopy (DRS) from 200 to 1000 nm wavelength with resolution of 1 nm. X-ray diffraction (XRD) analysis with Cu Kα radiation source (λ = 1.5410 Å) was also employed to determine crystallinity and phase behavior of the annealed samples. Photoluminescence (PL) spectroscopy (Varian Cary Eclipse fluorescence spectrophotometer) was used to study the luminescence characteristics providing evidence for the charge carrier lifetime. Fourier transform infrared spectroscopy (FTIR) was recorded on a Bruker VERTEX 70 spectrophotometer within a range of 800− 4000 cm−1 with accuracy of ±1 cm−1. X-ray photoelectron spectroscopy (XPS) with a radiation source of Al Kα (1486.6 eV) in an ultrahigh vacuum (UHV) system was employed to investigate surface chemical compositions of the samples. All binding energy values were calibrated by fixing the C (1s) core level peak at 284.6 eV, and the obtained XPS spectra peaks were deconvoluted using SDP

EXPERIMENTAL SECTION

Synthesis of GQDs. The GQDs were prepared by an electrochemical method as reported by Markovic et al.39 The electrolyte of the electrochemical method was prepared by combining 20 mL of ethanol with 1 mL of DI water to make a 21 mL solution containing 370 mg of NaOH. Two similar graphite rods (with diameters of about 2 mm) were used as the anode and cathode and inserted into the prepared electrolyte with a distance of 2 cm. Static potential of 25 V was applied to both electrodes using a direct current (DC) power supply. After 40 min, gradually, a dark-yellow solution appeared in the container. The prepared solution was centrifuged at 20 000 rpm for 30 min to remove graphite particles with large size (see Figure S1 in the Supporting Information). Finally, the GQD solution was diluted with water for further use. 368

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

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ACS Sustainable Chemistry & Engineering software (version 4.1) with 80% Gaussian−20% Lorentzian peak fitting. The cyclic voltammetry (CV) measurement was recorded in a 0.1 M KCl buffer in the presence of the GQD suspension (1 mg/mL) in the electrochemical station, Pt wire used as working electrode, graphite plate as counter electrode, and Ag/AgCl as the reference. The surface ζ potential of samples was measured by Zetasizer Nano ZS (red badge) ZEN 3600 at the pH of 7. Photocatalytic Activity. The photocatalytic activities of samples were measured using MB degradation as a model organic compound in aqueous solution under solar irradiation (80 Lux). The experiments were carried out in a 50 mL beaker charged with 40 mL of 10−5 M MB and 15 mg of the photocatalyst. All the photocatalyst layers were scraped from the zinc foil substrate to obtain powders for photocatalytic degradation tests. All experiments were carried out at room temperature (27 °C). At first and before irradiation, the mixture of MB solution was kept in a dark place for 60 min to obtain adsorption−desorption equilibrium. Then, MB solution was exposed to solar irradiation and 1 mL of its solution was removed from the reaction container at a constant time interval and loaded into the UV− vis spectrophotometer (Jasco V-530) to study the MB degradation. To investigate the mechanism of the MB photodegradation, similar photocatalytic activity tests were performed in the presence of various scavengers. The concentration of scavengers was controlled at 0.1 mol L−1. Further details of a typical photocatalytic measurement is given elsewhere.41

NWs were observed with diameters in the range of 170−250 nm and up to 10 μm in lengths. In Figure 2b, FESEM analysis revealed that there was an appreciable change in the morphology of ZnO NWs after spin coating of GQDs (0.4 wt %). The average length of GQD/ZnO NWs decreased due to the partial breaking of the ZnO NWs, while, the average diameter of the GQD/ZnO NWs was similar to that of the pure ZnO NWs. The uniform deposition of GQDs on the overall surface of the ZnO NWs is evident in the inset of Figure 2b. Further investigations on morphology of the samples were performed with AFM and HRTEM. Figure 3a shows AFM

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RESULTS AND DISCUSSION Structural, Morphological, and Optical Absorption Properties of GQDs and GQD/ZnO NW Hybrids. To determine surface morphology of the samples, FESEM was implemented. Figure 2a illustrates low-magnification FESEM image of the zinc foil anodized at 12 V voltage. As shown in this figure, ZnO NWs were grown vertically on the substrate. A higher-magnification FESEM image of ZnO NWs is also demonstrated in the inset of Figure 2a. Large quantities of ZnO Figure 3. (a) AFM image of the GQDs. The inset shows the height profile of GQDs corresponding to the white line. (b) TEM image of the GQDs. Inset panels show HRTEM image (top) and size distribution (bottom). (c,d) TEM images of GQD (0.4 wt %)/ZnO NWs with different magnifications.

image of the GQDs on a mica substrate. Accordingly, the size of the prepared GQD particles was measured in a range of several tens of nanometers. In addition, the AFM height profile in the inset of Figure 3a reveals that the topographic heights were about 1 nm, which corresponds to 1−3 graphene layers. Moreover, TEM images provide further insight and detailed morphology of the GQDs and GQD/ZnO NWs. Figure 3b shows the GQD particles with uniform size, and in the inset, their size distribution histogram shows that the size of the asprepared GQDs is in the range of 12−22 nm. In the inset of Figure 3b, the crystalline nature of the GQDs was examined by HRTEM measurements, and the results clearly exhibit the lattice spacing of the crystal (0.32 nm), which corresponds to the (002) crystal phase of graphite.20,33 In addition, Figures 3c,d shows the TEM images of a single GQD (0.4 wt %)/ZnO NW hybrid with a length of about 1.2 μm. As demonstrated by SEM images, the GQDs are distributed uniformly along the ZnO NWs with large aspect ratio. Figure S2 shows the TEM images of a single GQD, the pure ZnO NW, and GQD/ZnO NW with different loadings of GQDs. Figure 4a indicates the optical properties and UV−vis absorption spectrum of prepared GQDs. The UV−vis spectrum displays a strong optical absorption peak in the UV region,

Figure 2. FESEM images of (a) pristine ZnO NWs and (b) GQD/ ZnO NWs with 0.4 wt % GQDs. Corresponding inset figures are shown with higher magnifications. 369

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

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reported by other researchers.34 The results indicate that the surface of the GQDs are strongly functionalized with OH groups, which could act as active surface sites for the surface adsorption of organic molecules and cause a good solubility in water for further use. This in turn would increase the photocatalytic activity. XPS analysis was utilized to investigate the surface chemical states and bonding configuration of the samples. The survey spectrum in Figure 5a shows the presence of Zn, C, and O

Figure 4. (a) UV−visible spectrum and (b) PL spectra of GQDs under different excitation wavelengths from 350 to 450 nm. Inset shows luminescence of the GQD aqueous solution taken under UV light (350 nm).

which extends into the visible range. The absorption peak at about 240 nm corresponds to the π → π* transition of aromatic sp2 domains42 and another weak absorption peak as a shoulder centered in a wavelength range 330−380 nm is associated with n → π* transition of CO.36 Therefore, this absorption spectrum is further evidence of fabrication of graphene quantum dots. Further study on the optical propertyies of GQDs was performed by the PL technique using different excitation wavelengths from 350 to 450 nm (Figure 4b). As shown in the figure, the increase of excited wavelengths causes a decrease in the PL peak intensity and induces a shift to longer wavelengths. Therefore, the PL spectra reveal a Stokes shift in the peak position and a rapid decrease in the intensity with increasing excitation amplitude, which corresponds to the confinement effect of GQDs.43 The characteristic excitation dependence shows down-converted PL of GQDs under the excitation wavelength from 350 to 450 nm, which is consistent with results of previous reports.32,44 The inset of Figure 4b shows bright blue fluorescence under UV light at 350 nm, which indicates the down-converted property of GQDs. Figure S3 illustrates the XRD spectra of the ZnO NWs and GQD-modified ZnO NWs. The presence of sharp zinc peaks was due to the metal zinc foil used in the process based on JCPDS card no. 04-0831. In addition, the formation of hexagonal wurtzite ZnO structure was confirmed from the characteristic diffraction peaks based on JCPDS card no. 361451. No phases corresponding to GQDs were detected in the XRD pattern due to the low concentration of GQDs in the samples.19 FTIR analysis also carried out to investigate the effect of functional groups on the surface of the GQDs (Figure S4). The peak observed at 1660 cm−1 represents the CC group. The broad and intense peak centered at 3400 cm−1 and the bands appearing in the range of 1000−1400 cm−1 indicate the existence of a large amount of residual hydroxyl group and C− O groups on the surface of the GQDs, respectively. The obtained values in the FTIR show good agreement with values

Figure 5. X-ray photoelectron spectra of the ZnO NWs and the GQD (0.4 wt %)/ZnO NWs: (a) survey spectrum, (b) Zn (2p) spectrum, (c) C (1s) spectrum of ZnO NWs, and (d) C (1s) spectrum of the GQD (0.4 wt %)/ZnO NWs.

elements in the GQD/ZnO NWs without existence of any other impurities. The Zn 2p core level of XPS spectra of the GQD/ZnO and ZnO NWs are shown in Figure 5b. In the pure ZnO, the peaks at 1023 and 1046 eV correspond to the Zn 2p3/2 and Zn 2p1/2, respectively, indicating the presence of oxidation state of Zn2+.45 However, in the GQD/ZnO NWs, the double spectral lines of the Zn 2p were shifted to lower binding energy at about 0.6 eV, which can be attributed to the change in the bond character of Zn that is associated with formation of the Zn−O−C bond.46−49 Therefore, the chemical interaction between ZnO and GQDs confirms that the GQDs are decorated on the ZnO NWs. In addition, the chemical bond between the GQDs and ZnO NWs is believed to favor the transfer and separation of the photogenerated charge carriers and thus influence the photocatalytic activity. The deconvoluted C 1s XPS spectrum of the ZnO is presented in Figure 5c. The main peak of the C 1s spectrum located at 284.6 eV is attributed to the C−C bond, and the other peak at 285.6 eV is ascribed to the C−O bond. However, for the GQD/ZnO NWs, two new peaks appeared at 287.4 and 288.9 eV (Figure 5d) corresponding to the CO and COOH, respectively.50 These peaks confirm the presence of GQDs on the ZnO surface, and thus surface chemical composition of the catalyst was determined for the hybrid GQD/ZnO NWs. Figure 6 shows UV−vis absorption spectra of the pure ZnO NWs and the GQD/ZnO NWs with different GQD loadings. As shown in the figure, ZnO NWs show a strong absorption peak in the UV region and a weak absorption in the visible region due to light scattering of the ZnO NWs. The absorption 370

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

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ACS Sustainable Chemistry & Engineering ⎛C ⎞ ln⎜ 0 ⎟ = kt ⎝C⎠

(1)

where C0 is the primary concentration of MB, C is the MB concentration at any time t (ppm), and k is the first-order degradation rate constant. The k values were obtained from the slope of the straight line by linear fitting of ln(C0/C) vs irradiation time. Figure 7a shows the photocatalytic activities of

Figure 6. UV−visible absorption spectra of the pure ZnO NWs and GQD/ZnO NWs with different loadings of GQDs.

band-edge of the pure ZnO NWs was estimated at 400 nm corresponding with band gap energy of 3.1 eV, whereas, the absorption band-edges shift to higher wavelength (lower band gap energy) for the samples containing GQDs. Based on our XPS results, formation of Zn−O−C bond in the GQD/ZnO NWs caused reduction in the band gap energy due to the additional energy states induced below the conduction band level and above the valence band of ZnO NWs. Recently, this behavior was also reported by the Li group for the GQD/ TiO2.51 According to Figure 6, the addition of GQDs to ZnO improves light absorption and shifts absorption edge. By adding a proper amount of GQDs up to 0.4 wt %, the optical absorption of the GQD/ZnO NW hybrid increased with increasing GQD concentrations as seen in our optical absorption measurements. But, for higher GQD concentration (>0.4 wt %), light absorption decreased due to agglomeration of smaller particles forming larger ones. This phenomenon is also supported by our TEM observations as shown in Figure S2. Therefore, the optimum amount of GQDs (0.4 wt %) showed a uniform distribution on ZnO NWs providing a good interfacial chemical bond with a lower band gap energy. The energy band gaps of the GQD/ZnO NWs were obtained, and the values are listed in Table 1. Accordingly, the presence of

Figure 7. (a) Variation of ln(C0/C) vs reaction time under solar irradiation for the ZnO NWs and GQD/ZnO NWs with different GQD loadings; (b) k values of the GQD (0.4 wt %)/ZnO NW photocatalyst with different scavengers under solar light irradiation.

the pure ZnO NWs as compared with GQD/ZnO NWs with different amounts of GQD loading. A linear behavior is observed for the samples, and the results obey LH kinetics. As shown in the Figure 7a, MB degradation under solar illumination due to photolysis and in the absence of the catalysts is negligible. Therefore, the MB photocatalytic degradation is mainly due to the property of the NWs. The photocatalytic activity of GQD/ZnO NWs was enhanced as compared with the pure ZnO NWs. Based on our data analysis, the reaction rate constants (k) were obtained, and the results are listed in Table 1. Therefore, the optimum value of GQD loading was determined at 0.4 wt %, which shows about 2.8 times higher photocatalytic activity than the pure ZnO NWs, which had a lower band gap energy. As can be seen in Figure 7a, further increasing GQD contents to 1.2 wt % leads to an obvious decrease in the degradation efficiency, which is due to the lower light absorption in higher GQD concentration (see Figure 6). In the other words, only the uppermost layer of the GQDs absorbs the light. During the photocatalytic degradation within 180 min, the degradation efficiency of MB was found about 34% for the pure ZnO NWs, while using GQD (0.4 wt %)/ZnO NWs led to an increased degradation efficiency of 78%. Therefore, GQDs are acting as a good optical absorber resulting in higher MB photodegradation activity as compared to pure ZnO NWs. This is consistent with other reports that indicated an optimum amount of GQD required in order to enhance the photocatalytic activity of AgVO335 and ZnS.34 As discussed

Table 1. Band Gap Energy and Reaction Rate Constant for the ZnO NWs and GQDs/ZnO NWs sample ZnO NWs GQD (0.2 wt GQD (0.4 wt GQD (0.8 wt GQD (1.2 wt

%)/ZnO %)/ZnO %)/ZnO %)/ZnO

NWs NWs NWs NWs

band gap energy (eV)

k (min−1)

3.10 2.71 2.65 2.75 2.83

0.0025 0.0054 0.0070 0.0043 0.0033

GQDs extends the absorption edge of GQD/ZnO NWs to the visible-light region and enhances the light harvesting property, which is useful for photocatalytic applications. The extension of absorption edge into the visible region was also seen for the GQD/TiO2 photocatalyst, very recently.36 Photocatalytic Activities of ZnO NWs and GQD/ZnO NWs. To study the photocatalytic degradation rate of semiconductor materials, MB has been employed as a model dye molecule. The photocatalytic activity of the ZnO NWs and GQD/ZnO NWs was investigated by measuring decomposition rate of MB dye at pH = 7 under the solar light source (irradiation intensity of 800 W/m2). It is commonly accepted that the photocatalytic degradation of MB is categorized as the first-order Langmuir−Hinshelwood (LH) kinetics described by the following equation:52 371

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

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decrease in the rate of electron−hole recombination.11 Therefore, the addition of GQDs to the ZnO NWs could improve the photocatalytic activity due to retardation of electron−hole recombination. To elucidate the mechanism of photocatalytic degradation of MB over the GQD/ZnO NWs samples and determine the HOMO and LUMO orbitals, CV characteristics of the GQDs were studied (Figure S6). The peak observed at −0.12 V is due to the oxidation potential (Eox) of GQDs. However, this peak was not found in the electrolyte (0.1 M KCl) without the GQDs. The HOMO energy level relative to the vacuum level was determined at −4.28 eV using the following relation:47

previously, the UV−vis absorption spectra of the nanohybrids confirmed that the presence of GQDs could extend their absorption edge to the visible-light region. Therefore, these results show that the appropriate amount of GQDs could enhance the separation efficiency of electron−hole pairs and thus improve the photocatalytic activity of the GQD (0.4 wt %)/ZnO NWs. Furthermore, the influence of surface charges on the ZnO NWs and GQD (0.4 wt %)/ZnO NWs was investigated by the ζ potential measurements. The obtained values are 8.1 and 10.4 mV for the ZnO NWs and GQD (0.4 wt %) /ZnO NWs at the pH of 7, respectively. According to these values, it is expected that the surface charges of the ZnO NWs and the GQD (0.4 wt %)/ZnO NWs are similar positive charges. Therefore, it showed that they have a similar absorption behavior toward MB dye. Reaction Mechanism over the GQD/ZnO NWs. To understand the possible reaction mechanism for the photodegradation of MB over the GQD/ZnO NWs, the effects of different scavengers and N2 purging on the rate of MB photodegradation were investigated. Here, KI, AgNO3, and tBuOH were used as hole (h+), electron (e−), and hydroxyl radical (•OH) scavengers, respectively.53,54 For a better comparison, the sample with the highest photodegradation activity (GQD (0.4 wt %)/ZnO NWs) was examined with all these scavengers, and the results are shown in Figure 7b. It can be seen that the addition of t-BuOH, as •OH scavenger, greatly decreased the photocatalytic activity, indicating that •OH is the dominant active species in the degradation reaction. In contrast, the photocatalytic degradation rate is obviously increased after the addition of KI (•OH and h+ scavenger). This finding reveals that the consumption of h+ in the KI solution leads to a reduction in the rate of electron−hole recombination. Therefore, the photodegradation occurs due to the increase in the electron concentration. To elucidate the role of electron in the photocatalytic degradation, the experiment was also carried out in the presence of AgNO3 as an e− scavenger. The result shows almost no effect on the degradation rate, indicating that e− does not directly play a key role for the degradation. The experiment in the presence of KI indicates that the e− could have an important role, but different results are revealed by AgNO3 scavenger. Therefore, further investigation was conducted to understand the role of electrons in O2•− generation through reaction with dissolved O2. To verify this assumption, the degradation reaction was performed without any dissolved oxygen (purging with pure N2 gas). A significant decrease in the photocatalytic activity was observed upon N2 purging indicating that O2 plays an important role in the photocatalytic reaction. Consequently, the photogenerated electrons do not participate in the degradation process, but they react with O2 molecules to produce reactive O2•− species for MB degradation. In summary, the main reactive species involved in the photocatalytic degradation of MB over the GQD (0.4 wt %) /ZnO NWs are •OH and also O2•−. Their exact roles in the photocatalytic degradation will be discussed below. PL technique was also used to investigate the radiative recombination rate of excitons.55 Figure S5 shows PL spectra of both ZnO NWs and GQD (0.4 wt %)/ZnO NWs with an excitation source at λ = 330 nm. For the pure ZnO NWs, we have found that the PL peak at 390 nm corresponds to the recombination between holes in the valence and electrons in the conduction band and a visible emission peak for the transition in defect states.55 The PL intensity of the GQD/ZnO NWs is much lower than that of ZnO NWs indicating a

E(HOMO) = −e(Eox + 4.4)

(2)

The GQDs possess a band gap energy about of 2.2 eV obtained from UV−vis absorption spectrum of its solution (Figure 4a). Thus, the LUMO energy level relative to the vacuum level is −2.08 eV. In addition, according to the other published report, ZnO NWs with band gap of around 3.1 eV have a conduction band (CB) of 4.2 eV and a valence band (VB) of 7.3 eV vs vacuum.24 Based on the obtained energy levels, PL investigation and experiments carried out in the presence of scavengers, the charge transfer pathway during the photocatalytic process over the GQD/ZnO NWs can be proposed in Figure 8. The GQDs and ZnO NWs were excited

Figure 8. Schematic illustration of charge transfer process for the GQDs/ZnO NWs under solar light irradiation.

by solar light ,and the electron−hole pair was generated in their CB and VB, respectively. Then electron transfer occurred from the CB of ZnO to the HOMO of GQDs; therefore, recombination between the electrons in the ZnO CB and the holes in the GQD HOMO occurred. Moreover, the electrons in the GQD LUMO and holes in the VB of ZnO remained constant. The results confirm the observation of the PL spectra in Figure S5, which indicated the retardation in electron−hole recombination of ZnO. Consequently, the photoinduced electron−hole separation process is proposed via the Z-scheme mechanism. A similar proposed mechanism was also reported by other researchers for nanohybrid photocatalytic activity with similar band energy positions.56,57 Moreover, the electrons in the LUMO of GQDs react with molecular oxygen to produce superoxide (O2•−) for MB degradation. Also, Hao et al. showed the generation of hydroxyl radical (•OH) through O2•− on the surface of GQDs in photocatalytic reaction over GQDs incorporated into mesoporous Bi2MoO6 frameworks.58 On the other hand, holes in the VB of ZnO react with molecular water to produce hydroxyl radical. As a result, MB was efficiently degraded by attacking reactive hydroxyl radicals and superoxide on the GQD/ZnO NWs surface, and the chemical reactions can be described by the following: 372

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

ACS Sustainable Chemistry & Engineering GQD/ZnO + hν → e−(GQD) + h+(ZnO)

H 2O + h+ → H+ + •OH −

O2 + e → O2

•−

*Phone: (+98)-21-6616-4516. Fax: (+98)-21-6601-2983. Email: [email protected].

(4) (5) (6)

H 2O2 → 2•OH

(7)

OH + MB → intermediates → CO2 + H 2O

(8)

O2•− + MB → intermediates → CO2 + H 2O

(9)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Research Council of the Sharif University of Technology for financial support of the project. The authors also thank the Iran National Science Foundation (INSF) for the financial support through a Grant No. 92026525. Assistance of Mr Rafiee for XPS measurement and Mrs. Vaseghinia for AFM images is greatly acknowledged.

We have also performed stability tests of the both ZnO NWs and GQD (0.4 wt %)/ZnO NWs up to 3 cycles. The results shown in Figure S7 indicate that the reaction rate constant (k) of the ZnO NWs decreased after 3 cycles, which shows poor stability, while the reaction rate constant measured for the GQD (0.4 wt %)/ZnO NWs remained constant and showed a good stability after 3 cycles. The improved durability is due to formation of the Zn−O−C bonds in the GQD (0.4 wt %)/ZnO NWs, which reduced the activation of the surface oxygen over the ZnO NWs leading to photostability enhancement.18

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REFERENCES

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CONCLUSIONS In summary, an electrochemical process was utilized to prepare GQDs with the most frequent size of 17 nm and with an absorption edge in the visible light. ZnO NWs with diameters in the range of 170−250 nm and lengths up to 10 μm have been formed by anodizing zinc foil at room temperature. In order to obtain the maximum rate of photocatalytic degradation, different amounts of the GQDs were added to the ZnO NWs to synthesize novel GQD/ZnO NW hybrids. A firm and effective contact between the GQDs and ZnO NWs was confirmed by FESEM, TEM, XPS, and DRS analyses. Formation of Zn−O−C chemical bond identified by XPS is indicative of their good contact at the interface. Based on the DRS analysis, the band gap energy of the GQD/ZnO NWs reduced to 2.65 eV with increasing GQD loads (up to 0.4 wt %), while further increasing GQDs (up to 1.2 wt %) resulted in an increase in the band gap energy of the hybrids as compared with the pure ZnO NWs (3.1 eV). Photocatalytic activity of MB degradation reaction was investigated under solar light, and the sample with GQD (0.4 wt %)/ZnO NWs exhibited the highest photocatalytic activity. In order to understand the photodegradation mechanism, the CV curve of GQDs was measured and the photocatalytic activity of the GQD (0.4 wt %)/ZnO NWs was studied using different active scavengers. The enhanced photocatalytic efficiency was determined based on the Z-scheme mechanism, as is well-established behavior in the plant photosynthetic systems. These results can also confirm that the photocatalytic degradation of pollutants occurs mainly through participation of the photogenerated hydroxyl (•OH) radicals in the reaction.

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O2•− + 2H+ → H 2O2

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01738. Additional information from TEM, XRD, CV, FTIR, and PL techniques (PDF) 373

DOI: 10.1021/acssuschemeng.6b01738 ACS Sustainable Chem. Eng. 2017, 5, 367−375

Research Article

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