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Highly efficient photocatalytic hydrogen evolution of graphene/YInO3 nanocomposites under visible light irradiation† Jianjun Ding,ab Wenhao Yan,ab Wei Xie,ab Song Sun,ab Jun Baoab and Chen Gao*ab Visible-light-driven hydrogen evolution with high efficiency is important in the current photocatalysis research. Here we report for the first time the design and synthesis of a new graphene–semiconductor nanocomposite consisting of YInO3 nanoparticles and two-dimensional graphene sheets as efficient photocatalysts for hydrogen evolution under visible light irradiation. The graphene/YInO3 nanocomposites were synthesized using a facile solvothermal method in which the formation of graphene and the deposition of YInO3 nanoparticles on the graphene sheets can be achieved simultaneously. The addition of graphene as a cocatalyst can narrow the band gap of YInO3 to visible photon energy and prolong the separation and lifetime of electron–hole pairs by the chemical bonding
Received 11th November 2013 Accepted 2nd December 2013
between YInO3 and graphene. The photocatalytic reaction with this nanocomposite reaches a high H2 evolution rate of 400.4 mmol h1 g1 when the content of graphene is 0.5 wt%, over 127 and 3.7 times higher than that of pure YInO3 and Pt/YInO3, respectively. This work can provide an effective approach
DOI: 10.1039/c3nr05984g www.rsc.org/nanoscale
to the fabrication of graphene-based photocatalysts with high performance in the field of energy conversion.
Introduction Because of its high energy capacity and environmental friendliness, hydrogen energy via photocatalytic water splitting has been recognized as a clean, low-cost, and potential energy carrier for the future. Since the pioneering report by Fujishima and Honda on photoelectron-chemical splitting of water on a TiO2 electrode,1 the production of hydrogen via photocatalytic water splitting has attracted a lot of attention by utilizing solar energy. Among a variety of visible-light-driven photocatalysts for hydrogen evolution, perovskite-type oxides have been found to show photocatalytic activity under visible light excitation, such as SrTiO3,2 MCo1/3Nb2/3O3 (M ¼ Ca, Sr, Ba),3 MIn0.5Nb0.5O3 (M ¼ a
National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230029, China. E-mail: [email protected]; Fax: +86(551)6514-1078; Tel: +86(551)63602031
b
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
† Electronic supplementary information (ESI) available: XRD patterns of G/YIO nanocomposites with different graphene contents, TEM images and diameter distributions of YIO with different magnications, TEM image of Pt0.5/YIO, FTIR spectra of GO, graphene, YIO and G0.5/YIO nanocomposites with different magnications, BET surface areas of G/YIO nanocomposites, photocatalytic activities of Pt-dispersed YIO nanoparticles as a function of Pt amount, XRD patterns of G0.5/YIO nanocomposites aer photocatalytic reaction for 40 h, XPS spectra of Y 3d, In 3d and C 1s for G0.5/YIO nanocomposites aer photocatalytic reaction for 40 h. See DOI: 10.1039/c3nr05984g
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Ca, Sr, Ba),4 YInO3,5,6 etc. However, the hydrogen evolution rate achieved from these oxides is still far from satisfactory. A major limitation to achieve high photocatalytic efficiency is the quick recombination rate of photogenerated charge carriers.7,8 Generally, noble metals, such as Pt, Pd and Ru, are loaded on the photocatalytic surface as co-catalysts to promote the separation of photoexcited electrons and holes, and to enhance the activities of photocatalysts.9–11 Nevertheless, noble metals possess the disadvantages of being rare and expensive, which limits their wide application. Graphene, a two-dimensional p-conjugation sheet of carbon atoms bonded through sp2 hybridization, has been reported repeatedly to be an efficient cocatalyst for photocatalytic hydrogen evolution because of its remarkable electrical conductivity (15 000 m2 V1 S1 at room temperature), large surface area (theoretical value 2600 m2 g1) and exible structure. In the case of graphene–semiconductor composites, graphene as an excellent supporting matrix can efficiently suppress the growth of semiconductor particles and facilitate the transportation of the photogenerated electrons, both of which could improve the photocatalytic activity. Although, there have been numerous attempts to prepare various graphene–semiconductor composites (such as TiO2, CdS, BiVO4, Ag3PO4, etc.),12–19 no studies have been reported on decorating YInO3 nanoparticles on graphene sheets for water splitting. Besides, few studies have focused on the band gap narrowing effects of the semiconductor through the combination with graphene to increase the visible absorption and therefore enhancing the photocatalytic activity.
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Herein, we report for the rst time the design and synthesis of a new type of graphene–semiconductor nanocomposite consisting of YInO3 nanoparticles and graphene sheets as efficient photocatalysts for hydrogen evolution under visible light irradiation. YInO3 was reported exhibiting visible photocatalytic activity for water splitting. However, the optical absorption of this material is fairly weak in the visible region.5,6 In this work, the addition of graphene can narrow the band gap of YInO3 to visible wavelengths and prolong the separation and lifetime of electron–hole pairs by the chemical bonding between YInO3 and graphene. Signicantly enhanced performance for hydrogen evolution was achieved when graphene was used as the cocatalyst. Under visible light irradiation, a H2 evolution rate of 400.4 mmol h1 g1 was obtained at a graphene content of 0.5 wt%, which was 127 and 3.7 times higher than that of pure YInO3 and Pt/YInO3, respectively. Furthermore, the nanocomposites can produce H2 stably from water for ve consecutive runs of 40 h without obvious deactivation.
Experimental section Chemicals and materials Indium nitrate and chloroplatinic acid were purchased from Aladdin Industrial Inc. Natural graphite powder, yttrium nitrate and glycine were supplied by Sinopharm Chemical Reagent Co., Ltd. Graphene oxide (GO) was prepared from natural graphite powder using a modied Hummers' method which is described elsewhere.20 All other reagents were at least of analytical reagent grade and used without further purication. Synthesis of YInO3 YInO3 (YIO) was synthesized by a low temperature solution combustion method, which was also used in our previous study.5 The required amount of each reactant was calculated from the desired mass of the products according to the reaction. Briey, 0.77 g Y(NO3)3$6H2O, 0.6 g In(NO3)3 and 0.5 g C2H5NO2 were dissolved in 25 mL deionized water. The solution was le in air for more than 24 h for diffusion. The mixed solution was placed in an electrical furnace at 473 K for 30 min and then slowly heated to 573 K in 30 min to produce uffy powder via spontaneous combustion. Finally, the powder was annealed at 1373 K for 12 h in air. Fabrication of nanocomposite photocatalysts The synthesis procedures for graphene/YInO3 (G/YIO) nanocomposite photocatalysts are based on a one-step solvothermal method. In a typical process, a given amount of the as-prepared GO was dispersed in 100 mL CH3CH2OH by ultrasonication to obtain a homogeneous GO suspension. With 0.4 g YIO added to the suspension, the mixture was stirred for 1 h and transferred to a 120 mL Teon-lined stainless steel autoclave with up to 80% of the total volume. The autoclave was sealed and kept at 453 K for 12 h. The product was collected and washed using absolute ethanol and deionized water several times before drying at 353 K. Series of G/YIO photocatalysts were obtained by changing the amount of GO. Pure YIO or graphene was also
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prepared through a similar procedure, in the absence of GO or YIO. For comparison, 0.5 wt% Pt-dispersed YInO3 (labelled as Pt0.5/YIO) was prepared by an incipient impregnation method using chloroplatinic acid as a metal precursor. Aer impregnation, the sample was dried at 383 K for 24 h, and then reduced at 573 K in pure H2 for 2 h. Photocatalytic experiments The photocatalytic activity for water splitting was measured in a 330 mL top-irradiation gas-closed circulation reactor. Photoirradiation was carried out using a 300 W Xe arc lamp (PLS-SXE 300, ChangTuo Ltd.) through Infrared and UV cutoff lters to ensure visible illumination only (420 nm # l # 750 nm). The distance between the lamp and the solution surface was 30 cm. In a typical photocatalytic experiment, 10 mg of the photocatalyst was added into the reactor with constant stirring in a 100 mL solution containing 0.0025 mol Na2S and 0.0025 mol Na2SO3. To eliminate any thermal effect, a water jacket outside the reactor was used to keep the temperature of the solution constant at room temperature by owing cooling water. Before the reaction, the circulation system was purged with argon several times to remove the dissolved oxygen. The H2 evolved was analyzed using an online TCD gas chromatograph (Shimadzu GC 14C, argon as a carrier gas, equipped with a TDX01 column). The amount of H2 evolution was calculated versus the amount of photocatalyst in the system. Characterization Powder X-ray diffraction (XRD) patterns were measured using a Rigaku TTR III diffractometer with CuKa radiation (l ¼ 0.15148 nm) at a scan rate of 5 min1 to determine the crystal phase of the prepared samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a JEM2100F eld emission transmission electron microscope with an acceleration voltage of 200 kV. The UV/Vis diffuse reectance spectrum was recorded at room temperature using a UV/Vis spectrometer (SolidSpec-3700, Shimadzu, Japan) using BaSO4 as the reference. The BET surface area was determined by an adsorption–desorption method (Micromeritics ASAP 2000) with N2 as the adsorbent. The surface characterization was carried out using X-ray Photoelectron Spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientic) with a base pressure lower than 1.0 1010 Pa and MgKa radiation (E ¼ 1253.6 eV) operated at 150 W as the X-ray source. Raman spectra were recorded with an InVia microscopic confocal Raman spectrometer using a 514.5 nm laser beam. Fourier transform infrared (FTIR) spectroscopy was carried out on a Nicolet 8700 FTIR in a KBr pellet scanning from 4000 to 400 cm1. The photoluminescent spectra were recorded at room temperature with a JY Fluorolog-3-Tou uorospectrophotometer.
Results and discussion The samples with different contents of graphene to YIO (0, 0.1, 0.3, 0.5, 0.7, 1 and 2 wt%) were labelled as G0/YIO, G0.1/YIO,
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G0.3/YIO, G0.5/YIO, G0.7/YIO, G1/YIO and G2/YIO, respectively. During the solvothermal reaction, the reduction of GO was done with the deposition of YIO nanoparticles on the graphene sheets simultaneously. As shown in Fig. 1, the XRD pattern of graphite powders shows that an interlayer spacing of 0.34 nm (2q ¼ 26.54 ) can be calculated from the (002) peak using the Bragg formula. Aer oxidation, the GO sample exhibited an intensive peak at 9.3 , corresponding to an interlayer spacing of 0.95 nm. The (002) peak shied to about 0.35 nm (2q z 25 ) aer the solvothermal process, which indicated the formation of graphene from GO.21,22 The slightly larger interlayer spacing of graphene compared to that of graphite suggests the presence of oxygen residual. Fig. S1† shows the XRD patterns of G/YIO nanocomposites. In general, there was no obvious difference in XRD patterns among the composites with different weight ratios of graphene to YIO in terms of XRD peak position and intensity. The main diffraction peaks of the samples are at 20.8 , 29.6 , 34.3 , 49.3 and 58.6 , corresponding to the diffractions of the (211), (222), (400), (440) and (622) planes of YIO (PCPDF #251172), respectively. The XRD patterns of the composites were similar to those of the pure YIO synthesized in the absence of GO. No characteristic diffraction peaks for carbon species are observed in the XRD patterns even though the content of graphene is as high as 2%, likely because of its relatively low diffraction intensity and high dispersion.23 The average grain size estimated using the Scherrer's equation and the three prominent XRD lines was about 20 nm for all G/YIO nanocomposites, which suggests little effect of GO on the formation of the YIO crystal structure. TEM measurements were performed to characterize the morphology of YIO, GO, graphene, G0.5/YIO (the highest H2 evolution activity) and Pt0.5/YIO nanocomposites. Fig. 2A, S2 and S3† display the TEM images and the diameter distributions of YIO nanoparticles with different magnications. As can be seen from the images, YIO shows a ne rod or spherical shape with the mean diameter of about 100 nm. The nanoparticles linked end-to-end to form a net structure with a large degree of porosity. The porosity was caused by the superheated gases
Fig. 1
XRD patterns of graphite, GO and graphene.
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Fig. 2 The TEM images of YIO synthesized by a solution combustion method (A), GO (B) and G0.5/YIO nanocomposites (C and D).
escaping from the combustion reaction.24,25 The net structure can be helpful to prevent the aggregation of YIO nanoparticles. As shown in Fig. 2B, the GO displays two-dimensional sheets with chiffon-like ripples and wrinkles resulting from the deformation and distortion of graphite sheets during the oxidation reaction. Fig. 2C shows the TEM image of the G0.5/ YIO composite. Aer the solvothermal reaction, graphene also shows a paper-like structure with several stacking layers of the monatomic graphene sheets. It can be seen that the net structure of YIO is basically retained and the YIO nanoparticles are dispersed on the graphene sheets with good contact. This contact allows the electronic interactions between YIO nanoparticles and graphene sheets and can, hence, enhance the charge carrier separation and photocatalytic activity of G/YIO nanocomposites. The high resolution TEM image (Fig. 2D) showed the lattice fringes of the YIO nanoparticles, suggesting that the nanoparticles were well crystallized. The lattice spacing was measured to be around 0.42 nm, which could be assigned to the (211) plane of the YIO crystal structure. In addition, Fig. S4† shows the TEM image of Pt0.5/YIO. The Pt particles with a size of about 6 nm were localized on the surface of YIO. Fig. 3 shows the FTIR spectra of GO, graphene, YIO and G0.5/ YIO nanocomposites. For GO, the peaks at 1735 cm1, 1406 cm1, 1224 cm1 and 1054 cm1 are ascribed to C]O stretching, carboxyl O–H stretching, phenolic C–OH stretching and alkoxy C–O stretching, respectively.26,27 The peak at 850 cm1 is the characteristic absorption peak of epoxide groups.28 The peak at 1621 cm1 can be assigned to the vibrations of the absorbed water molecules or the vibrations of sp2 hybridized C–C bonding.29 The broad peak at 3000–3700 cm1 (Fig. S5†) corresponds to the stretching mode of –OH, and the physically adsorbed H2O also contributes to this broad peak.30 For graphene, only weak features appear in the spectra,
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3 FTIR spectra nanocomposites.
Fig.
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of
GO,
graphene,
YIO
and
G0.5/YIO
demonstrating the reduction of GO during the solvothermal process.31 The peak at 1572 cm1 corresponds to the skeletal vibration of the graphene sheets.32 Compared with YIO and graphene, only the characteristic peaks of YIO appear in the spectrum of G0.5/YIO nanocomposites because of the low content of graphene in the nanocomposite. The peaks at 495 cm1 and 570 cm1 can be assigned to the vibrations of In–O33 and Y–O34 bonds, respectively. Fig. 4 shows a comparison of the Raman spectra of GO, graphene and G0.5/YIO nanocomposites. In the Raman spectrum of GO, two typical bands of GO can be found at 1354 and 1596 cm1, corresponding to the D and G bands, respectively. The D band is ascribed to the local defects or disorder, while the G band arises from the sp2 hybridized graphene domains. The as-prepared graphene exhibits a blue-shied G band compared to that of GO, which is a characteristic to the reduction of GO. The integrated intensity ratio ID/IG of graphene increased to 1.43 from 1.34 for GO, which indicates higher density of local defects or disorder in graphene than in GO.35,36 This is typically
Fig. 4 Raman spectra nanocomposites.
of
GO,
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graphene
and
G0.5/YIO
observed when graphene is produced by reducing GO. It is worth noting that a G band red-shied from 1596 to 1602 cm1 was observed for G0.5/YIO nanocomposites compared with GO. The phenomenon was similar to previous reports in which the p-type doping of the graphene caused red-shi of the G band because of charge transfer between graphene sheets and YIO nanoparticles.37,38 XPS has proved to be a useful tool for identifying the oxidation state of elements. In Fig. 5, the XPS spectrum of C1s from GO (Fig. 5A, solid line) can be deconvoluted into three peaks (dashed lines), which can be attributed to the following functional groups: C–C (sp2 bonded carbon, 284.8 eV), C–O (epoxy/hydroxyl, 286.9 eV) and C]O (carbonyl, 287.7 eV), indicating a considerable degree of graphene oxidation.39,40 For G0.5/YIO nanocomposites, a signicant decrease of oxygencontaining functional groups is observed (Fig. 5B), indicating the partial removal of the oxygen-containing functional groups.41 The representative In core level XPS spectrum for G0.5/YIO nanocomposites depicted in Fig. 5C shows two peaks centered at 443 and 450.5 eV, corresponding to In 3d5/2 and In 3d3/2, respectively. Fig. 5D shows a representative Y core level XPS spectrum with two peaks centered at 157.9 and 159.9 eV for Y 3d5/2 and Y 3d3/2, respectively, with 2 eV splitting between two peaks. The XPS data conrm that the oxidation states of the elements Y and In were both 3+.42,43 Fig. 6A shows the UV-Vis diffuse reectance spectra of the G/ YIO nanocomposites. The introduction of graphene affects the optical absorption properties for the G/YIO nanocomposites signicantly. As compared to the pure YIO, the absorption background in the visible region is enhanced for the G/YIO nanocomposites. Meanwhile, a red-shi to higher wavelength in the absorption edge of the G/YIO nanocomposites has also been observed, indicating a narrowing of the band gap of YIO. A plot of the transformed Kubelka–Munk function as a function of photo-energy is shown in Fig. 6B. The estimated band gaps
C 1s spectra of GO (A), XPS spectra of C 1s (B), In 3d (C), Y 3d (D) for G0.5/YIO nanocomposites.
Fig. 5
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Fig. 7 H2 evolution rate of G/YIO nanocomposites with different
contents of graphene under visible light irradiation. UV-Vis diffuse reflectance spectra of G/YIO nanocomposites (A), and the plot of transformed Kubelka–Munk function versus the energy of light (B).
Fig. 6
are 2.65, 2.58, 2.50, 2.48, 2.40, 2.34 and 3.16 eV for G0.1/YIO, G0.3/YIO, G0.5/YIO, G0.7/YIO, G1/YIO, G2/YIO and pure YIO, respectively. The band gap narrowing should be attributed to the chemical bonding between YIO and graphene, that is, the formation of COO–metal bonds, which were also found in the case of TiO2/graphene,44 CdS/graphene,45 BiFeO3/graphene18 and CuInZnS/graphene46 composites. Because of the increased absorbance, the utilization of solar energy can be more efficient. Therefore, mixing graphene with YIO nanoparticles is an effective way to improve the photocatalytic activity in visible. The inuence of graphene on the BET surface area of the G/ YIO nanocomposites was also investigated. As shown in Table S1,† G/YIO nanocomposites showed higher BET surface area than pure YIO (7.64 m2 g1). It can be seen that the BET surface area of the nanocomposites increased gradually from 8.58 to 19.29 m2 g1 with increasing the content of graphene. The larger surface area of the nanocomposites could be benecial for enhancing the photocatalytic activity of the nanocomposites studied.47,48 This hypothesis is conrmed by the following water splitting over the G/YIO nanocomposites under visible light irradiation. Photocatalytic H2 evolution activities of the prepared G/YIO and Pt/YIO nanocomposites were evaluated under visible-light irradiation using Na2SO3 and Na2S as sacricial reagents. Control experiments indicated that no appreciable H2 production was observed in the absence of either irradiation or photocatalyst, suggesting that H2 was produced by photocatalytic reaction on the photocatalyst. As shown in Fig. 7, a relatively low H2 evolution rate (3.14 mmol h1 g1) was detected when pure YIO was used as the photocatalyst. For the Pt0.5/YIO photocatalyst, the rate of H2 evolution was enhanced to 108.4 mmol h1 g1, which is 32 times higher than that of pure YIO. Although the optical absorption of YIO in the visible region is relatively low, photocatalytic activity could still be detected for H2 evolution. The changes in the photocatalytic activity with different amounts of Pt dispersed on YIO nanoparticles are
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presented in Fig. S6†. From Fig. 7, it can be seen that graphene induced an obvious enhancement in the photocatalytic activity of YIO. In the presence of a small amount of graphene (0.1 wt%), the H2 evolution rate of the G0.1/YIO nanocomposite was signicantly enhanced to 66.55 mmol h1 g1. The H2 evolution rate could reach 400.4 mmol h1 g1 when the amount of graphene was increased to 0.5 wt%, which is 127 and 3.7 times higher than that of pure YIO and Pt0.5/YIO, respectively. The present results clearly indicated that graphene could function as an efficient cocatalyst with performance superior to noble metals. With further increasing the amount of graphene to 0.7 wt%, however, the H2 evolution rate was decreased to 315.7 mmol h1 g1. A rather low H2 evolution rate was observed when the amount of graphene was increased to 1 and 2 wt%. This decrease may be attributed to the trade-off between the excellent charge transfer capability of graphene and its detrimental effect on visible light absorption.19 The observations are similar to the previous studies showing that a suitable loading content of graphene is crucial for optimizing the photocatalytic activity of G/YIO nanocomposites.49,50 Notably, the remarkable enhanced visible photocatalytic performance of G/YIO nanocomposites could be ascribed to three possible effects: (1) the two dimensional at structure of graphene sheets with large surface area acts as an excellent supporting matrix with more active adsorption and photocatalytic reaction sites,51 and simultaneously prevents the aggregation of semiconductor particles. (2) As mentioned in Fig. 6, the introduction of graphene can narrow the band gap of YIO, red-shi the absorption edge to the visible region and therefore collect solar radiation in a wider spectral range. (3) Graphene can efficiently collect and transport electrons, which reduces the probability of electron–hole recombination and improves the separation efficiency. Photoluminescent (PL) spectroscopy can be used to disclose the separation efficiency of charge carriers since the PL emission results from the recombination of free charge carriers. A lower PL intensity indicates a lower recombination rate of electron–hole pairs and hence
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Fig. 8 PL spectra of YIO, Pt0.5/YIO and G/YIO nanocomposites. The
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Fig. 10 Stability of photocatalytic H2 evolution for G0.5/YIO nanocomposites under visible light irradiation.
excited wavelength is 365 nm.
higher separation efficiency.52 Herein, we present the PL intensity for pure YIO, Pt0.5/YIO and G0.5/YIO nanocomposites, as shown in Fig. 8. The excitation wavelength is 365 nm. With increasing the graphene content, the PL intensity decreased and reached a minimum value at a graphene content of 0.5 wt%. When the graphene content exceeds 0.5 wt%, the PL intensity increased accordingly. Moreover, in comparison with pure YIO and Pt0.5/YIO, the PL intensity of G0.5/YIO nanocomposites was much lower, which means the separation efficiency of electron–hole pairs for G0.5/YIO nanocomposites should be higher than that of pure YIO and Pt0.5/YIO. The result was consistent with that of photocatalytic H2 evolution activity. Based on the above results, a tentative mechanism for the high H2 evolution activity of G/YIO nanocomposites is illustrated in Fig. 9. Under visible light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of YIO, creating holes in the VB. The photogenerated electrons
transfer from the CB of YIO via a percolation mechanism53 (eqn (1)) to graphene sheets where they participate in H2 evolution (eqn (2)). The holes le on the surface of YIO oxidize S2 and SO32 to form S22 and SO42 directly,54 according to eqn (3) and (4). The introduction of graphene in the G/YIO composite can reduce the probability of electron–hole recombination, prolonged the lifetime of the charge carriers, and thus enhance the photocatalytic activity. Moreover, the graphene sheets allow the photocatalytic reactions to take place not only on the surfaces of YIO nanoparticles, but also on the graphene sheets with signicantly increased reaction sites. G/YIO + hn / G(e) + YIO(h+)
(1)
2G(e) + 2H+ / 2G + H2
(2)
2YIO(h+) + SO32 + H2O / 2YIO + SO42 + 2H+
(3)
2YIO(h+) + 2S2 / 2YIO + S22
(4)
Because of the importance of chemical stability for practical applications, the photocatalytic activity of the G0.5/YIO nanocomposite was further investigated using cycle experiment. As shown in Fig. 10, no appreciable degradation of the photocatalyst was observed in G0.5/YIO nanocomposites for 5 runs of at least 40 h, indicating the composite photocatalyst has good photocatalytic stability for hydrogen evolution under visible light. The structure and composition of G0.5/YIO nanocomposites aer 5 cycles of reaction were also analyzed by XRD and XPS (Fig. S7 and S8†), which indicated no change in the structure and composition of G0.5/YIO nanocomposites.
Conclusions Schematic illustration of the charge separation and transportation over G/YIO nanocomposites under visible light irradiation. Fig. 9
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In summary, we have synthesized G/YIO nanocomposites using a facile solvothermal method for efficient photocatalytic water splitting under visible light illumination. The chemical bonding
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between YIO and graphene plays a critical role in narrowing the band gap of YIO to the visible light region and prolonging the separation and lifetime of electron–hole pairs. With these advantages, G/YIO nanocomposites exhibited signicantly increased photocatalytic activity over pure YIO in water splitting. A high H2 evolution rate of 400.4 mmol h1 g1 was achieved with 0.5 wt% graphene in the nanocomposite, which was 127 and 3.7 times higher than that of pure YIO and Pt0.5/YIO, respectively. Furthermore, the G0.5/YIO nanocomposite can produce H2 stably from water for ve consecutive runs of 40 h without noticeable degradation, which makes them promising candidates as advanced visible photocatalysts for solar hydrogen production.
Acknowledgements This work was supported by National Nature Science Foundation of China (11179034, 11205159), Anhui Provincial Natural Science Foundation (1308085MB27), and National Basic Research Program of China (973 Program) (2012CB922004).
Notes and references 1 2 3 4 5
A. Fujishima and K. Honda, Nature, 1972, 238, 37. H. Kato and A. Kudo, J. Phys. Chem. B, 2002, 106, 5029. J. Yin, Z. G. Zou and J. H. Ye, J. Phys. Chem. B, 2003, 107, 4936. J. Yin, Z. G. Zou and J. H. Ye, J. Phys. Chem. B, 2003, 107, 61. J. J. Ding, J. Bao, S. Sun, Z. L. Luo and C. Gao, J. Comb. Chem., 2009, 11, 523. 6 N. Arai, N. Saito, H. Nishiyama, Y. Shimodaira, H. Kobayashi, Y. Inoue and K. Sato, Chem. Lett., 2008, 37, 46. 7 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 253, 253. 8 X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503. 9 A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2002, 124, 13547. 10 A. Dickinson, D. James, N. Perkins, T. Cassidy and M. Bowker, J. Mol. Catal. A: Chem., 1999, 146, 211. 11 M. Hara, J. Nunoshige, T. Takata, J. N. Kondo and K. Domen, Chem. Commun., 2003, 3000. 12 H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano, 2010, 4, 380. 13 T. G. Xu, L. W. Zhang, H. Y. Cheng and Y. F. Zhu, Appl. Catal., B, 2011, 101, 382. 14 Q. Li, B. D. Guo, J. G. Yu, J. R. Ran, B. H. Zhang, H. J. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878. 15 X. Q. An, J. C. Yu, Y. Wang, Y. M. Hu, X. L. Yu and G. J. Zhang, J. Mater. Chem., 2012, 22, 8525. 16 Y. H. Ng, A. Iwase, A. Kudo and R. Amal, J. Phys. Chem. Lett., 2010, 1, 2607. 17 X. F. Yang, H. Y. Cui, Y. Li, J. L. Qin, R. X. Zhang and H. Tang, ACS Catal., 2013, 3, 363. 18 Z. X. Li, Y. Shen, C. Yang, Y. C. Lei, Y. H. Guan, Y. H. Lin, D. B. Liu and C. W. Nan, J. Mater. Chem. A, 2013, 1, 823. 19 A. Mukherji, B. Seger, G. Q. Lu and L. Z. Wang, ACS Nano, 2011, 5, 3483. This journal is © The Royal Society of Chemistry 2014
Nanoscale
20 Y. X. Xu, H. Bai, G. W. Lu, C. Li and G. Q. Shi, J. Am. Chem. Soc., 2008, 130, 5856. 21 Y. H. Zhang, N. Zhang, Z. R. Tang and Y. J. Xu, ACS Nano, 2012, 6, 9777. 22 P. Y. Dong, Y. H. Wang, L. N. Guo, B. Liu, S. Y. Xin, J. Zhang, Y. R. Shi, W. Zeng and S. Yin, Nanoscale, 2012, 4, 4641. 23 W. G. Wang, J. G. Yu, Q. J. Xiang and B. Cheng, Appl. Catal., B, 2012, 119–120, 109. 24 E. J. Bosze, J. Mckittrick and G. A. Hirata, Mater. Sci. Eng., B, 2003, 97, 265. 25 J. A. Gonzalez-Ortega, E. M. Tejeda, N. Perea, G. A. Hirata, E. J. Bosze and J. Mckittrick, Opt. Mater., 2005, 27, 1221. 26 C. H. Wu, Y. Z. Zhang, S. Li, H. J. Zheng, H. Wang, J. B. Liu, K. W. Li and H. Yan, Chem. Eng. J., 2011, 178, 468. 27 J. J. Guo, Y. Li, S. M. Zhu, Z. X. Chen, Q. L. Liu, D. Zhang, W. J. Moon and D. M. Song, RSC Adv., 2012, 2, 1356. 28 G. Z. Li, S. Chen, X. Quan, H. T. Yu and H. M. Zhao, J. Mater. Chem., 2012, 22, 2721. 29 J. F. Shen, T. Li, Y. Long, M. Shi, N. Li and M. X. Ye, Carbon, 2012, 50, 2134. 30 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355. 31 S. Y. Han, I. Y. Kim, K. Y. Jo and S. J. Hwang, J. Phys. Chem. C, 2012, 116, 7269. 32 A. V. Murugan, T. Muraliganth and A. Manthiram, Chem. Mater., 2009, 21, 5004. 33 L. G. Gai, L. Ma, H. H. Jiang, Y. Ma, Y. Tian and H. Liu, CrystEngComm, 2012, 14, 7479. 34 Z. Huang, W. Guo, Y. Liu, Q. F. Huang, F. Tang and Y. G. Cao, Mater. Chem. Phys., 2011, 128, 44. 35 M. S. A. S. Shah, K. Zhang, A. R. Park, K. S. Kim, N. G. Park, J. H. Park and P. J. Yoo, Nanoscale, 2013, 5, 5093. 36 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575. 37 E. P. Gao, W. Z. Wang, M. Shang and J. H. Xu, Phys. Chem. Chem. Phys., 2011, 13, 2887. 38 A. Manna and S. K. Pati, Chem.–Asian J., 2009, 4, 855. 39 L. A. Gu, J. J. Wang, H. Cheng, Y. Z. Zhao, L. F. Liu and X. J. Han, ACS Appl. Mater. Interfaces, 2013, 5, 3085. 40 Y. S. Fu and X. Wang, Ind. Eng. Chem. Res., 2011, 50, 7210. 41 J. G. Hou, Z. Wang, W. B. Kan, S. Q. Jiao, H. M. Zhu and R. V. Kumar, J. Mater. Chem., 2012, 22, 7291. 42 A. Bhirud, N. Chaudhari, L. Nikam, R. Sonawane, K. Patil, J. O. Baeg and B. Kale, Int. J. Hydrogen Energy, 2011, 36, 11628. 43 S. Bouattour, A. M. Botelho do Rego and L. F. Vieira Ferreira, Mater. Res. Bull., 2010, 45, 818. 44 Y. H. Zhang, Z. R. Tang, X. Z. Fu and Y. J. Xu, ACS Nano, 2010, 4, 7303. 45 N. Zhang, M. Q. Yang, Z. R. Tang and Y. J. Xu, J. Catal., 2013, 303, 60. 46 X. S. Tang, Q. L. Tay, Z. Chen, Y. Chen, G. K. L. Goh and J. M. Xue, J. Mater. Chem. A, 2013, 1, 6359. 47 X. J. Dai, Y. S. Luo, W. D. Zhang and S. Y. Fu, Dalton Trans., 2010, 39, 3426. 48 H. Xu and L. Z. Zhang, J. Phys. Chem. C, 2010, 114, 940. Nanoscale, 2014, 6, 2299–2306 | 2305
View Article Online
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49 X. Y. Zhang, Y. J. Sun, X. L. Cui and Z. Y. Jiang, Int. J. Hydrogen Energy, 2012, 37, 811. 50 S. Ghasemi, S. Rahman Setayesh, A. Habibi-Yangjeh, M. R. Hormozi-Nezhad and M. R. Gholami, J. Hazard. Mater., 2012, 199–200, 170. 51 X. F. Yang, H. Y. Cui, Y. Li, J. L. Qin, R. X. Zhang and H. Tang, ACS Catal., 2013, 3, 363.
2306 | Nanoscale, 2014, 6, 2299–2306
Paper
52 S. Sun, J. J. Ding, J. Bao, C. Gao, Z. M. Qi, X. Y. Yang, B. He and C. X. Li, Appl. Surf. Sci., 2012, 258, 5031. 53 Y. F. Sun, B. Y. Qu, Q. Liu, S. Gao, Z. X. Yan, W. S. Yan, B. C. Pan, S. Q. Wei and Y. Xie, Nanoscale, 2012, 4, 3761. 54 I. Tsuji, H. Kato, H. Kobayashi and A. Kudo, J. Am. Chem. Soc., 2004, 126, 13406.
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Highly efficient photocatalytic hydrogen evolution of graphene/YInO3 nanocomposites under visible light irradiation† Jianjun Ding,ab Wenhao Yan,ab Wei Xie,ab Song Sun,ab Jun Baoab and Chen Gao*ab Visible-light-driven hydrogen evolution with high efficiency is important in the current photocatalysis research. Here we report for the first time the design and synthesis of a new graphene–semiconductor nanocomposite consisting of YInO3 nanoparticles and two-dimensional graphene sheets as efficient photocatalysts for hydrogen evolution under visible light irradiation. The graphene/YInO3 nanocomposites were synthesized using a facile solvothermal method in which the formation of graphene and the deposition of YInO3 nanoparticles on the graphene sheets can be achieved simultaneously. The addition of graphene as a cocatalyst can narrow the band gap of YInO3 to visible photon energy and prolong the separation and lifetime of electron–hole pairs by the chemical bonding
Received 11th November 2013 Accepted 2nd December 2013
between YInO3 and graphene. The photocatalytic reaction with this nanocomposite reaches a high H2 evolution rate of 400.4 mmol h1 g1 when the content of graphene is 0.5 wt%, over 127 and 3.7 times higher than that of pure YInO3 and Pt/YInO3, respectively. This work can provide an effective approach
DOI: 10.1039/c3nr05984g www.rsc.org/nanoscale
to the fabrication of graphene-based photocatalysts with high performance in the field of energy conversion.
Introduction Because of its high energy capacity and environmental friendliness, hydrogen energy via photocatalytic water splitting has been recognized as a clean, low-cost, and potential energy carrier for the future. Since the pioneering report by Fujishima and Honda on photoelectron-chemical splitting of water on a TiO2 electrode,1 the production of hydrogen via photocatalytic water splitting has attracted a lot of attention by utilizing solar energy. Among a variety of visible-light-driven photocatalysts for hydrogen evolution, perovskite-type oxides have been found to show photocatalytic activity under visible light excitation, such as SrTiO3,2 MCo1/3Nb2/3O3 (M ¼ Ca, Sr, Ba),3 MIn0.5Nb0.5O3 (M ¼ a
National Synchrotron Radiation Laboratory and Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230029, China. E-mail: [email protected]; Fax: +86(551)6514-1078; Tel: +86(551)63602031
b
CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China
† Electronic supplementary information (ESI) available: XRD patterns of G/YIO nanocomposites with different graphene contents, TEM images and diameter distributions of YIO with different magnications, TEM image of Pt0.5/YIO, FTIR spectra of GO, graphene, YIO and G0.5/YIO nanocomposites with different magnications, BET surface areas of G/YIO nanocomposites, photocatalytic activities of Pt-dispersed YIO nanoparticles as a function of Pt amount, XRD patterns of G0.5/YIO nanocomposites aer photocatalytic reaction for 40 h, XPS spectra of Y 3d, In 3d and C 1s for G0.5/YIO nanocomposites aer photocatalytic reaction for 40 h. See DOI: 10.1039/c3nr05984g
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Ca, Sr, Ba),4 YInO3,5,6 etc. However, the hydrogen evolution rate achieved from these oxides is still far from satisfactory. A major limitation to achieve high photocatalytic efficiency is the quick recombination rate of photogenerated charge carriers.7,8 Generally, noble metals, such as Pt, Pd and Ru, are loaded on the photocatalytic surface as co-catalysts to promote the separation of photoexcited electrons and holes, and to enhance the activities of photocatalysts.9–11 Nevertheless, noble metals possess the disadvantages of being rare and expensive, which limits their wide application. Graphene, a two-dimensional p-conjugation sheet of carbon atoms bonded through sp2 hybridization, has been reported repeatedly to be an efficient cocatalyst for photocatalytic hydrogen evolution because of its remarkable electrical conductivity (15 000 m2 V1 S1 at room temperature), large surface area (theoretical value 2600 m2 g1) and exible structure. In the case of graphene–semiconductor composites, graphene as an excellent supporting matrix can efficiently suppress the growth of semiconductor particles and facilitate the transportation of the photogenerated electrons, both of which could improve the photocatalytic activity. Although, there have been numerous attempts to prepare various graphene–semiconductor composites (such as TiO2, CdS, BiVO4, Ag3PO4, etc.),12–19 no studies have been reported on decorating YInO3 nanoparticles on graphene sheets for water splitting. Besides, few studies have focused on the band gap narrowing effects of the semiconductor through the combination with graphene to increase the visible absorption and therefore enhancing the photocatalytic activity.
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Herein, we report for the rst time the design and synthesis of a new type of graphene–semiconductor nanocomposite consisting of YInO3 nanoparticles and graphene sheets as efficient photocatalysts for hydrogen evolution under visible light irradiation. YInO3 was reported exhibiting visible photocatalytic activity for water splitting. However, the optical absorption of this material is fairly weak in the visible region.5,6 In this work, the addition of graphene can narrow the band gap of YInO3 to visible wavelengths and prolong the separation and lifetime of electron–hole pairs by the chemical bonding between YInO3 and graphene. Signicantly enhanced performance for hydrogen evolution was achieved when graphene was used as the cocatalyst. Under visible light irradiation, a H2 evolution rate of 400.4 mmol h1 g1 was obtained at a graphene content of 0.5 wt%, which was 127 and 3.7 times higher than that of pure YInO3 and Pt/YInO3, respectively. Furthermore, the nanocomposites can produce H2 stably from water for ve consecutive runs of 40 h without obvious deactivation.
Experimental section Chemicals and materials Indium nitrate and chloroplatinic acid were purchased from Aladdin Industrial Inc. Natural graphite powder, yttrium nitrate and glycine were supplied by Sinopharm Chemical Reagent Co., Ltd. Graphene oxide (GO) was prepared from natural graphite powder using a modied Hummers' method which is described elsewhere.20 All other reagents were at least of analytical reagent grade and used without further purication. Synthesis of YInO3 YInO3 (YIO) was synthesized by a low temperature solution combustion method, which was also used in our previous study.5 The required amount of each reactant was calculated from the desired mass of the products according to the reaction. Briey, 0.77 g Y(NO3)3$6H2O, 0.6 g In(NO3)3 and 0.5 g C2H5NO2 were dissolved in 25 mL deionized water. The solution was le in air for more than 24 h for diffusion. The mixed solution was placed in an electrical furnace at 473 K for 30 min and then slowly heated to 573 K in 30 min to produce uffy powder via spontaneous combustion. Finally, the powder was annealed at 1373 K for 12 h in air. Fabrication of nanocomposite photocatalysts The synthesis procedures for graphene/YInO3 (G/YIO) nanocomposite photocatalysts are based on a one-step solvothermal method. In a typical process, a given amount of the as-prepared GO was dispersed in 100 mL CH3CH2OH by ultrasonication to obtain a homogeneous GO suspension. With 0.4 g YIO added to the suspension, the mixture was stirred for 1 h and transferred to a 120 mL Teon-lined stainless steel autoclave with up to 80% of the total volume. The autoclave was sealed and kept at 453 K for 12 h. The product was collected and washed using absolute ethanol and deionized water several times before drying at 353 K. Series of G/YIO photocatalysts were obtained by changing the amount of GO. Pure YIO or graphene was also
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prepared through a similar procedure, in the absence of GO or YIO. For comparison, 0.5 wt% Pt-dispersed YInO3 (labelled as Pt0.5/YIO) was prepared by an incipient impregnation method using chloroplatinic acid as a metal precursor. Aer impregnation, the sample was dried at 383 K for 24 h, and then reduced at 573 K in pure H2 for 2 h. Photocatalytic experiments The photocatalytic activity for water splitting was measured in a 330 mL top-irradiation gas-closed circulation reactor. Photoirradiation was carried out using a 300 W Xe arc lamp (PLS-SXE 300, ChangTuo Ltd.) through Infrared and UV cutoff lters to ensure visible illumination only (420 nm # l # 750 nm). The distance between the lamp and the solution surface was 30 cm. In a typical photocatalytic experiment, 10 mg of the photocatalyst was added into the reactor with constant stirring in a 100 mL solution containing 0.0025 mol Na2S and 0.0025 mol Na2SO3. To eliminate any thermal effect, a water jacket outside the reactor was used to keep the temperature of the solution constant at room temperature by owing cooling water. Before the reaction, the circulation system was purged with argon several times to remove the dissolved oxygen. The H2 evolved was analyzed using an online TCD gas chromatograph (Shimadzu GC 14C, argon as a carrier gas, equipped with a TDX01 column). The amount of H2 evolution was calculated versus the amount of photocatalyst in the system. Characterization Powder X-ray diffraction (XRD) patterns were measured using a Rigaku TTR III diffractometer with CuKa radiation (l ¼ 0.15148 nm) at a scan rate of 5 min1 to determine the crystal phase of the prepared samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a JEM2100F eld emission transmission electron microscope with an acceleration voltage of 200 kV. The UV/Vis diffuse reectance spectrum was recorded at room temperature using a UV/Vis spectrometer (SolidSpec-3700, Shimadzu, Japan) using BaSO4 as the reference. The BET surface area was determined by an adsorption–desorption method (Micromeritics ASAP 2000) with N2 as the adsorbent. The surface characterization was carried out using X-ray Photoelectron Spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientic) with a base pressure lower than 1.0 1010 Pa and MgKa radiation (E ¼ 1253.6 eV) operated at 150 W as the X-ray source. Raman spectra were recorded with an InVia microscopic confocal Raman spectrometer using a 514.5 nm laser beam. Fourier transform infrared (FTIR) spectroscopy was carried out on a Nicolet 8700 FTIR in a KBr pellet scanning from 4000 to 400 cm1. The photoluminescent spectra were recorded at room temperature with a JY Fluorolog-3-Tou uorospectrophotometer.
Results and discussion The samples with different contents of graphene to YIO (0, 0.1, 0.3, 0.5, 0.7, 1 and 2 wt%) were labelled as G0/YIO, G0.1/YIO,
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G0.3/YIO, G0.5/YIO, G0.7/YIO, G1/YIO and G2/YIO, respectively. During the solvothermal reaction, the reduction of GO was done with the deposition of YIO nanoparticles on the graphene sheets simultaneously. As shown in Fig. 1, the XRD pattern of graphite powders shows that an interlayer spacing of 0.34 nm (2q ¼ 26.54 ) can be calculated from the (002) peak using the Bragg formula. Aer oxidation, the GO sample exhibited an intensive peak at 9.3 , corresponding to an interlayer spacing of 0.95 nm. The (002) peak shied to about 0.35 nm (2q z 25 ) aer the solvothermal process, which indicated the formation of graphene from GO.21,22 The slightly larger interlayer spacing of graphene compared to that of graphite suggests the presence of oxygen residual. Fig. S1† shows the XRD patterns of G/YIO nanocomposites. In general, there was no obvious difference in XRD patterns among the composites with different weight ratios of graphene to YIO in terms of XRD peak position and intensity. The main diffraction peaks of the samples are at 20.8 , 29.6 , 34.3 , 49.3 and 58.6 , corresponding to the diffractions of the (211), (222), (400), (440) and (622) planes of YIO (PCPDF #251172), respectively. The XRD patterns of the composites were similar to those of the pure YIO synthesized in the absence of GO. No characteristic diffraction peaks for carbon species are observed in the XRD patterns even though the content of graphene is as high as 2%, likely because of its relatively low diffraction intensity and high dispersion.23 The average grain size estimated using the Scherrer's equation and the three prominent XRD lines was about 20 nm for all G/YIO nanocomposites, which suggests little effect of GO on the formation of the YIO crystal structure. TEM measurements were performed to characterize the morphology of YIO, GO, graphene, G0.5/YIO (the highest H2 evolution activity) and Pt0.5/YIO nanocomposites. Fig. 2A, S2 and S3† display the TEM images and the diameter distributions of YIO nanoparticles with different magnications. As can be seen from the images, YIO shows a ne rod or spherical shape with the mean diameter of about 100 nm. The nanoparticles linked end-to-end to form a net structure with a large degree of porosity. The porosity was caused by the superheated gases
Fig. 1
XRD patterns of graphite, GO and graphene.
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Fig. 2 The TEM images of YIO synthesized by a solution combustion method (A), GO (B) and G0.5/YIO nanocomposites (C and D).
escaping from the combustion reaction.24,25 The net structure can be helpful to prevent the aggregation of YIO nanoparticles. As shown in Fig. 2B, the GO displays two-dimensional sheets with chiffon-like ripples and wrinkles resulting from the deformation and distortion of graphite sheets during the oxidation reaction. Fig. 2C shows the TEM image of the G0.5/ YIO composite. Aer the solvothermal reaction, graphene also shows a paper-like structure with several stacking layers of the monatomic graphene sheets. It can be seen that the net structure of YIO is basically retained and the YIO nanoparticles are dispersed on the graphene sheets with good contact. This contact allows the electronic interactions between YIO nanoparticles and graphene sheets and can, hence, enhance the charge carrier separation and photocatalytic activity of G/YIO nanocomposites. The high resolution TEM image (Fig. 2D) showed the lattice fringes of the YIO nanoparticles, suggesting that the nanoparticles were well crystallized. The lattice spacing was measured to be around 0.42 nm, which could be assigned to the (211) plane of the YIO crystal structure. In addition, Fig. S4† shows the TEM image of Pt0.5/YIO. The Pt particles with a size of about 6 nm were localized on the surface of YIO. Fig. 3 shows the FTIR spectra of GO, graphene, YIO and G0.5/ YIO nanocomposites. For GO, the peaks at 1735 cm1, 1406 cm1, 1224 cm1 and 1054 cm1 are ascribed to C]O stretching, carboxyl O–H stretching, phenolic C–OH stretching and alkoxy C–O stretching, respectively.26,27 The peak at 850 cm1 is the characteristic absorption peak of epoxide groups.28 The peak at 1621 cm1 can be assigned to the vibrations of the absorbed water molecules or the vibrations of sp2 hybridized C–C bonding.29 The broad peak at 3000–3700 cm1 (Fig. S5†) corresponds to the stretching mode of –OH, and the physically adsorbed H2O also contributes to this broad peak.30 For graphene, only weak features appear in the spectra,
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3 FTIR spectra nanocomposites.
Fig.
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of
GO,
graphene,
YIO
and
G0.5/YIO
demonstrating the reduction of GO during the solvothermal process.31 The peak at 1572 cm1 corresponds to the skeletal vibration of the graphene sheets.32 Compared with YIO and graphene, only the characteristic peaks of YIO appear in the spectrum of G0.5/YIO nanocomposites because of the low content of graphene in the nanocomposite. The peaks at 495 cm1 and 570 cm1 can be assigned to the vibrations of In–O33 and Y–O34 bonds, respectively. Fig. 4 shows a comparison of the Raman spectra of GO, graphene and G0.5/YIO nanocomposites. In the Raman spectrum of GO, two typical bands of GO can be found at 1354 and 1596 cm1, corresponding to the D and G bands, respectively. The D band is ascribed to the local defects or disorder, while the G band arises from the sp2 hybridized graphene domains. The as-prepared graphene exhibits a blue-shied G band compared to that of GO, which is a characteristic to the reduction of GO. The integrated intensity ratio ID/IG of graphene increased to 1.43 from 1.34 for GO, which indicates higher density of local defects or disorder in graphene than in GO.35,36 This is typically
Fig. 4 Raman spectra nanocomposites.
of
GO,
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and
G0.5/YIO
observed when graphene is produced by reducing GO. It is worth noting that a G band red-shied from 1596 to 1602 cm1 was observed for G0.5/YIO nanocomposites compared with GO. The phenomenon was similar to previous reports in which the p-type doping of the graphene caused red-shi of the G band because of charge transfer between graphene sheets and YIO nanoparticles.37,38 XPS has proved to be a useful tool for identifying the oxidation state of elements. In Fig. 5, the XPS spectrum of C1s from GO (Fig. 5A, solid line) can be deconvoluted into three peaks (dashed lines), which can be attributed to the following functional groups: C–C (sp2 bonded carbon, 284.8 eV), C–O (epoxy/hydroxyl, 286.9 eV) and C]O (carbonyl, 287.7 eV), indicating a considerable degree of graphene oxidation.39,40 For G0.5/YIO nanocomposites, a signicant decrease of oxygencontaining functional groups is observed (Fig. 5B), indicating the partial removal of the oxygen-containing functional groups.41 The representative In core level XPS spectrum for G0.5/YIO nanocomposites depicted in Fig. 5C shows two peaks centered at 443 and 450.5 eV, corresponding to In 3d5/2 and In 3d3/2, respectively. Fig. 5D shows a representative Y core level XPS spectrum with two peaks centered at 157.9 and 159.9 eV for Y 3d5/2 and Y 3d3/2, respectively, with 2 eV splitting between two peaks. The XPS data conrm that the oxidation states of the elements Y and In were both 3+.42,43 Fig. 6A shows the UV-Vis diffuse reectance spectra of the G/ YIO nanocomposites. The introduction of graphene affects the optical absorption properties for the G/YIO nanocomposites signicantly. As compared to the pure YIO, the absorption background in the visible region is enhanced for the G/YIO nanocomposites. Meanwhile, a red-shi to higher wavelength in the absorption edge of the G/YIO nanocomposites has also been observed, indicating a narrowing of the band gap of YIO. A plot of the transformed Kubelka–Munk function as a function of photo-energy is shown in Fig. 6B. The estimated band gaps
C 1s spectra of GO (A), XPS spectra of C 1s (B), In 3d (C), Y 3d (D) for G0.5/YIO nanocomposites.
Fig. 5
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Fig. 7 H2 evolution rate of G/YIO nanocomposites with different
contents of graphene under visible light irradiation. UV-Vis diffuse reflectance spectra of G/YIO nanocomposites (A), and the plot of transformed Kubelka–Munk function versus the energy of light (B).
Fig. 6
are 2.65, 2.58, 2.50, 2.48, 2.40, 2.34 and 3.16 eV for G0.1/YIO, G0.3/YIO, G0.5/YIO, G0.7/YIO, G1/YIO, G2/YIO and pure YIO, respectively. The band gap narrowing should be attributed to the chemical bonding between YIO and graphene, that is, the formation of COO–metal bonds, which were also found in the case of TiO2/graphene,44 CdS/graphene,45 BiFeO3/graphene18 and CuInZnS/graphene46 composites. Because of the increased absorbance, the utilization of solar energy can be more efficient. Therefore, mixing graphene with YIO nanoparticles is an effective way to improve the photocatalytic activity in visible. The inuence of graphene on the BET surface area of the G/ YIO nanocomposites was also investigated. As shown in Table S1,† G/YIO nanocomposites showed higher BET surface area than pure YIO (7.64 m2 g1). It can be seen that the BET surface area of the nanocomposites increased gradually from 8.58 to 19.29 m2 g1 with increasing the content of graphene. The larger surface area of the nanocomposites could be benecial for enhancing the photocatalytic activity of the nanocomposites studied.47,48 This hypothesis is conrmed by the following water splitting over the G/YIO nanocomposites under visible light irradiation. Photocatalytic H2 evolution activities of the prepared G/YIO and Pt/YIO nanocomposites were evaluated under visible-light irradiation using Na2SO3 and Na2S as sacricial reagents. Control experiments indicated that no appreciable H2 production was observed in the absence of either irradiation or photocatalyst, suggesting that H2 was produced by photocatalytic reaction on the photocatalyst. As shown in Fig. 7, a relatively low H2 evolution rate (3.14 mmol h1 g1) was detected when pure YIO was used as the photocatalyst. For the Pt0.5/YIO photocatalyst, the rate of H2 evolution was enhanced to 108.4 mmol h1 g1, which is 32 times higher than that of pure YIO. Although the optical absorption of YIO in the visible region is relatively low, photocatalytic activity could still be detected for H2 evolution. The changes in the photocatalytic activity with different amounts of Pt dispersed on YIO nanoparticles are
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presented in Fig. S6†. From Fig. 7, it can be seen that graphene induced an obvious enhancement in the photocatalytic activity of YIO. In the presence of a small amount of graphene (0.1 wt%), the H2 evolution rate of the G0.1/YIO nanocomposite was signicantly enhanced to 66.55 mmol h1 g1. The H2 evolution rate could reach 400.4 mmol h1 g1 when the amount of graphene was increased to 0.5 wt%, which is 127 and 3.7 times higher than that of pure YIO and Pt0.5/YIO, respectively. The present results clearly indicated that graphene could function as an efficient cocatalyst with performance superior to noble metals. With further increasing the amount of graphene to 0.7 wt%, however, the H2 evolution rate was decreased to 315.7 mmol h1 g1. A rather low H2 evolution rate was observed when the amount of graphene was increased to 1 and 2 wt%. This decrease may be attributed to the trade-off between the excellent charge transfer capability of graphene and its detrimental effect on visible light absorption.19 The observations are similar to the previous studies showing that a suitable loading content of graphene is crucial for optimizing the photocatalytic activity of G/YIO nanocomposites.49,50 Notably, the remarkable enhanced visible photocatalytic performance of G/YIO nanocomposites could be ascribed to three possible effects: (1) the two dimensional at structure of graphene sheets with large surface area acts as an excellent supporting matrix with more active adsorption and photocatalytic reaction sites,51 and simultaneously prevents the aggregation of semiconductor particles. (2) As mentioned in Fig. 6, the introduction of graphene can narrow the band gap of YIO, red-shi the absorption edge to the visible region and therefore collect solar radiation in a wider spectral range. (3) Graphene can efficiently collect and transport electrons, which reduces the probability of electron–hole recombination and improves the separation efficiency. Photoluminescent (PL) spectroscopy can be used to disclose the separation efficiency of charge carriers since the PL emission results from the recombination of free charge carriers. A lower PL intensity indicates a lower recombination rate of electron–hole pairs and hence
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Fig. 8 PL spectra of YIO, Pt0.5/YIO and G/YIO nanocomposites. The
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Fig. 10 Stability of photocatalytic H2 evolution for G0.5/YIO nanocomposites under visible light irradiation.
excited wavelength is 365 nm.
higher separation efficiency.52 Herein, we present the PL intensity for pure YIO, Pt0.5/YIO and G0.5/YIO nanocomposites, as shown in Fig. 8. The excitation wavelength is 365 nm. With increasing the graphene content, the PL intensity decreased and reached a minimum value at a graphene content of 0.5 wt%. When the graphene content exceeds 0.5 wt%, the PL intensity increased accordingly. Moreover, in comparison with pure YIO and Pt0.5/YIO, the PL intensity of G0.5/YIO nanocomposites was much lower, which means the separation efficiency of electron–hole pairs for G0.5/YIO nanocomposites should be higher than that of pure YIO and Pt0.5/YIO. The result was consistent with that of photocatalytic H2 evolution activity. Based on the above results, a tentative mechanism for the high H2 evolution activity of G/YIO nanocomposites is illustrated in Fig. 9. Under visible light irradiation, electrons are excited from the valence band (VB) to the conduction band (CB) of YIO, creating holes in the VB. The photogenerated electrons
transfer from the CB of YIO via a percolation mechanism53 (eqn (1)) to graphene sheets where they participate in H2 evolution (eqn (2)). The holes le on the surface of YIO oxidize S2 and SO32 to form S22 and SO42 directly,54 according to eqn (3) and (4). The introduction of graphene in the G/YIO composite can reduce the probability of electron–hole recombination, prolonged the lifetime of the charge carriers, and thus enhance the photocatalytic activity. Moreover, the graphene sheets allow the photocatalytic reactions to take place not only on the surfaces of YIO nanoparticles, but also on the graphene sheets with signicantly increased reaction sites. G/YIO + hn / G(e) + YIO(h+)
(1)
2G(e) + 2H+ / 2G + H2
(2)
2YIO(h+) + SO32 + H2O / 2YIO + SO42 + 2H+
(3)
2YIO(h+) + 2S2 / 2YIO + S22
(4)
Because of the importance of chemical stability for practical applications, the photocatalytic activity of the G0.5/YIO nanocomposite was further investigated using cycle experiment. As shown in Fig. 10, no appreciable degradation of the photocatalyst was observed in G0.5/YIO nanocomposites for 5 runs of at least 40 h, indicating the composite photocatalyst has good photocatalytic stability for hydrogen evolution under visible light. The structure and composition of G0.5/YIO nanocomposites aer 5 cycles of reaction were also analyzed by XRD and XPS (Fig. S7 and S8†), which indicated no change in the structure and composition of G0.5/YIO nanocomposites.
Conclusions Schematic illustration of the charge separation and transportation over G/YIO nanocomposites under visible light irradiation. Fig. 9
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In summary, we have synthesized G/YIO nanocomposites using a facile solvothermal method for efficient photocatalytic water splitting under visible light illumination. The chemical bonding
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between YIO and graphene plays a critical role in narrowing the band gap of YIO to the visible light region and prolonging the separation and lifetime of electron–hole pairs. With these advantages, G/YIO nanocomposites exhibited signicantly increased photocatalytic activity over pure YIO in water splitting. A high H2 evolution rate of 400.4 mmol h1 g1 was achieved with 0.5 wt% graphene in the nanocomposite, which was 127 and 3.7 times higher than that of pure YIO and Pt0.5/YIO, respectively. Furthermore, the G0.5/YIO nanocomposite can produce H2 stably from water for ve consecutive runs of 40 h without noticeable degradation, which makes them promising candidates as advanced visible photocatalysts for solar hydrogen production.
Acknowledgements This work was supported by National Nature Science Foundation of China (11179034, 11205159), Anhui Provincial Natural Science Foundation (1308085MB27), and National Basic Research Program of China (973 Program) (2012CB922004).
Notes and references 1 2 3 4 5
A. Fujishima and K. Honda, Nature, 1972, 238, 37. H. Kato and A. Kudo, J. Phys. Chem. B, 2002, 106, 5029. J. Yin, Z. G. Zou and J. H. Ye, J. Phys. Chem. B, 2003, 107, 4936. J. Yin, Z. G. Zou and J. H. Ye, J. Phys. Chem. B, 2003, 107, 61. J. J. Ding, J. Bao, S. Sun, Z. L. Luo and C. Gao, J. Comb. Chem., 2009, 11, 523. 6 N. Arai, N. Saito, H. Nishiyama, Y. Shimodaira, H. Kobayashi, Y. Inoue and K. Sato, Chem. Lett., 2008, 37, 46. 7 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 253, 253. 8 X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503. 9 A. Ishikawa, T. Takata, J. N. Kondo, M. Hara, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2002, 124, 13547. 10 A. Dickinson, D. James, N. Perkins, T. Cassidy and M. Bowker, J. Mol. Catal. A: Chem., 1999, 146, 211. 11 M. Hara, J. Nunoshige, T. Takata, J. N. Kondo and K. Domen, Chem. Commun., 2003, 3000. 12 H. Zhang, X. J. Lv, Y. M. Li, Y. Wang and J. H. Li, ACS Nano, 2010, 4, 380. 13 T. G. Xu, L. W. Zhang, H. Y. Cheng and Y. F. Zhu, Appl. Catal., B, 2011, 101, 382. 14 Q. Li, B. D. Guo, J. G. Yu, J. R. Ran, B. H. Zhang, H. J. Yan and J. R. Gong, J. Am. Chem. Soc., 2011, 133, 10878. 15 X. Q. An, J. C. Yu, Y. Wang, Y. M. Hu, X. L. Yu and G. J. Zhang, J. Mater. Chem., 2012, 22, 8525. 16 Y. H. Ng, A. Iwase, A. Kudo and R. Amal, J. Phys. Chem. Lett., 2010, 1, 2607. 17 X. F. Yang, H. Y. Cui, Y. Li, J. L. Qin, R. X. Zhang and H. Tang, ACS Catal., 2013, 3, 363. 18 Z. X. Li, Y. Shen, C. Yang, Y. C. Lei, Y. H. Guan, Y. H. Lin, D. B. Liu and C. W. Nan, J. Mater. Chem. A, 2013, 1, 823. 19 A. Mukherji, B. Seger, G. Q. Lu and L. Z. Wang, ACS Nano, 2011, 5, 3483. This journal is © The Royal Society of Chemistry 2014
Nanoscale
20 Y. X. Xu, H. Bai, G. W. Lu, C. Li and G. Q. Shi, J. Am. Chem. Soc., 2008, 130, 5856. 21 Y. H. Zhang, N. Zhang, Z. R. Tang and Y. J. Xu, ACS Nano, 2012, 6, 9777. 22 P. Y. Dong, Y. H. Wang, L. N. Guo, B. Liu, S. Y. Xin, J. Zhang, Y. R. Shi, W. Zeng and S. Yin, Nanoscale, 2012, 4, 4641. 23 W. G. Wang, J. G. Yu, Q. J. Xiang and B. Cheng, Appl. Catal., B, 2012, 119–120, 109. 24 E. J. Bosze, J. Mckittrick and G. A. Hirata, Mater. Sci. Eng., B, 2003, 97, 265. 25 J. A. Gonzalez-Ortega, E. M. Tejeda, N. Perea, G. A. Hirata, E. J. Bosze and J. Mckittrick, Opt. Mater., 2005, 27, 1221. 26 C. H. Wu, Y. Z. Zhang, S. Li, H. J. Zheng, H. Wang, J. B. Liu, K. W. Li and H. Yan, Chem. Eng. J., 2011, 178, 468. 27 J. J. Guo, Y. Li, S. M. Zhu, Z. X. Chen, Q. L. Liu, D. Zhang, W. J. Moon and D. M. Song, RSC Adv., 2012, 2, 1356. 28 G. Z. Li, S. Chen, X. Quan, H. T. Yu and H. M. Zhao, J. Mater. Chem., 2012, 22, 2721. 29 J. F. Shen, T. Li, Y. Long, M. Shi, N. Li and M. X. Ye, Carbon, 2012, 50, 2134. 30 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355. 31 S. Y. Han, I. Y. Kim, K. Y. Jo and S. J. Hwang, J. Phys. Chem. C, 2012, 116, 7269. 32 A. V. Murugan, T. Muraliganth and A. Manthiram, Chem. Mater., 2009, 21, 5004. 33 L. G. Gai, L. Ma, H. H. Jiang, Y. Ma, Y. Tian and H. Liu, CrystEngComm, 2012, 14, 7479. 34 Z. Huang, W. Guo, Y. Liu, Q. F. Huang, F. Tang and Y. G. Cao, Mater. Chem. Phys., 2011, 128, 44. 35 M. S. A. S. Shah, K. Zhang, A. R. Park, K. S. Kim, N. G. Park, J. H. Park and P. J. Yoo, Nanoscale, 2013, 5, 5093. 36 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575. 37 E. P. Gao, W. Z. Wang, M. Shang and J. H. Xu, Phys. Chem. Chem. Phys., 2011, 13, 2887. 38 A. Manna and S. K. Pati, Chem.–Asian J., 2009, 4, 855. 39 L. A. Gu, J. J. Wang, H. Cheng, Y. Z. Zhao, L. F. Liu and X. J. Han, ACS Appl. Mater. Interfaces, 2013, 5, 3085. 40 Y. S. Fu and X. Wang, Ind. Eng. Chem. Res., 2011, 50, 7210. 41 J. G. Hou, Z. Wang, W. B. Kan, S. Q. Jiao, H. M. Zhu and R. V. Kumar, J. Mater. Chem., 2012, 22, 7291. 42 A. Bhirud, N. Chaudhari, L. Nikam, R. Sonawane, K. Patil, J. O. Baeg and B. Kale, Int. J. Hydrogen Energy, 2011, 36, 11628. 43 S. Bouattour, A. M. Botelho do Rego and L. F. Vieira Ferreira, Mater. Res. Bull., 2010, 45, 818. 44 Y. H. Zhang, Z. R. Tang, X. Z. Fu and Y. J. Xu, ACS Nano, 2010, 4, 7303. 45 N. Zhang, M. Q. Yang, Z. R. Tang and Y. J. Xu, J. Catal., 2013, 303, 60. 46 X. S. Tang, Q. L. Tay, Z. Chen, Y. Chen, G. K. L. Goh and J. M. Xue, J. Mater. Chem. A, 2013, 1, 6359. 47 X. J. Dai, Y. S. Luo, W. D. Zhang and S. Y. Fu, Dalton Trans., 2010, 39, 3426. 48 H. Xu and L. Z. Zhang, J. Phys. Chem. C, 2010, 114, 940. Nanoscale, 2014, 6, 2299–2306 | 2305
View Article Online
Published on 05 December 2013. Downloaded by University of Massachusetts - Amherst on 02/09/2014 07:56:26.
Nanoscale
49 X. Y. Zhang, Y. J. Sun, X. L. Cui and Z. Y. Jiang, Int. J. Hydrogen Energy, 2012, 37, 811. 50 S. Ghasemi, S. Rahman Setayesh, A. Habibi-Yangjeh, M. R. Hormozi-Nezhad and M. R. Gholami, J. Hazard. Mater., 2012, 199–200, 170. 51 X. F. Yang, H. Y. Cui, Y. Li, J. L. Qin, R. X. Zhang and H. Tang, ACS Catal., 2013, 3, 363.
2306 | Nanoscale, 2014, 6, 2299–2306
Paper
52 S. Sun, J. J. Ding, J. Bao, C. Gao, Z. M. Qi, X. Y. Yang, B. He and C. X. Li, Appl. Surf. Sci., 2012, 258, 5031. 53 Y. F. Sun, B. Y. Qu, Q. Liu, S. Gao, Z. X. Yan, W. S. Yan, B. C. Pan, S. Q. Wei and Y. Xie, Nanoscale, 2012, 4, 3761. 54 I. Tsuji, H. Kato, H. Kobayashi and A. Kudo, J. Am. Chem. Soc., 2004, 126, 13406.
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