Graphene Acting as Surface Phase Junction in Anatase–Graphene

Article pubs.acs.org/JPCC

Graphene Acting as Surface Phase Junction in Anatase−Graphene− Rutile Heterojunction Photocatalysts for H2 Production from Water Splitting Yan Yan, Chao Wang, Xu Yan, Lisong Xiao, Jianghua He, Wei Gu, and Weidong Shi* School of Chemistry and Chemical Engineering, Jiangsu University, No. 301, Xuefu Road, Zhenjiang, P. R. China, 212013 ABSTRACT: The construction of heterojunction photocatalysts has received much attention in the field of photocatalytic H2 production from water splitting. The surface phase junction of semiconductors is very important to the activity of heterojunction photocatalysts. In this study, an effective anatase−graphene−rutile heterojunction structure was designed and fabricated by using graphene as the surface phase junction between anatase and rutile particles. Results show that both the anatase/rutile ratio (A/R ratio) and graphene amount represent important influence on H2 production activities of anatase−graphene−rutile heterojunction photocatalysts. Under our experimental condition, the optimum A/R ratio is 7/3 and graphene addition amount is 2 wt %, the corresponding AR7/ 3−2%G sample has the highest H2 production rate of 1.714 mmol/h. By further experimental study, we think the high H2 production activity of anatase−graphene−rutile heterojunction photocatalyst is from the enhanced charge separation rather than other effect. This work values the expanding of surface phase junction and provides a feasible strategy to develop high-performance photocatalysts by designing and expanding the surface phase junction.

1. INTRODUCTION Photocatalytic H2 production from water splitting is a promising technique in terms of the solar energy conversion. Many strategies have been proposed by researchers from a viewpoint of the separation of photogenerated charge-carriers of semiconductor photocatalysts including the introducing of cocatalysts,1,2 intrinsic conducting polymers,3 graphene or graphene oxide4−6 and constructing homo- or heterojunction systems.7−9 Among them, the heterojunction system of semiconductors with matched energy bands or p−n junctions can effectively suppress the recombination of photogenerated charge-carriers.10−14 For example, the AgIn5S8/TiO2 heterojunction nanocomposite based on matched energy bands showed 7.7 times enhanced H2 production rate over the pristine AgIn5S8.15 The CuS/ZnS porous nanosheet based on the interfacial charge transfer also showed significant enhancement in H2 production activity.16 Among photocatalysts involved, TiO2 appears to be the most promising and suitable material to construct high-activity heterojunction photocatalytic systems due to its superior photocatalytic activity, chemical stability, low cost, and nontoxicity.17,18 In particular, the anatase−rutile TiO2 heterojunction photocatalyst attracts much attention.19−21 The coupling of anatase and rutile TiO2 shows obvious synergetic effect, which allows the photogenerated electrons migrate from the conduction band of the rutile phase to the trapping sites on the anatase surface,22−24 thereby improving the charge © XXXX American Chemical Society

separation efficiency and enhancing the photocatalytic activity. Typically, in a heterojunction structure based on semiconductors, the interface between different semiconductors is considered to be the most active part of photocatalytic reactions. The fast charge migration at the interface provides huge amount of reaction opportunities for photogenerated electrons and holes. Li et al. have already demonstrated that the surface-phase junction of a heterojunction semiconductor catalyst (anatase and rutile TiO2 in their report) directly contributes to photocatalytic reactions,19 which means it is possible to develop high-performance photocatalysts by designing and preparing specific surface-phase junctions. To enlarge the interface area of heterojunction structures, much effort has been devoted into the core−shell structures25,26 and solid-state solutions with noble metals as electron mediates.27 Fushijima et al. reported an interesting layer-bylayer heterogeneous structure with anatase nanoparticles on the rutile nanorods.20 However, the increased interface area by these strategies is still vastly constrained by the surface area of employed materials and these strategies are very difficult to realize the precise control over the amount of different compositions along the interface, which is the key to the model optimization of heterojunction photocatalysts. Recently, Received: July 16, 2014 Revised: August 26, 2014

A

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graphene, a 2D honeycomb structure based on sp2-bonded carbon atoms, is extensively studied for applications in photocatalysis as supporting matrix of catalyst particles due to its excellent electric conductivity and superhigh surface area.4−6 The 2D morphology and superior electric properties render graphene an ideal electron mediate and supporter for heterojunction photocatalysts by extending the interface between different semiconductors to the whole surface area of graphene sheets, which extremely facilitates the charge migration in heterojunction structures and prolongs the charge lifetimes by suppressing the recombination of photogenerated electrons and holes. More importantly, it would be easy to precisely control the amount of anatase and rutile TiO2 particles along the interface in heterojunction structures if these particles were loaded on the surface of graphene sheets. However, to the best of our knowledge, no studies have been reported dealing with the preparation of anatase−graphene− rutile heterojunction photocatalyst for H2 production and the relative amount of anatase, rutile TiO2, and graphene in this heterojunction system, although they are strongly desired. Herein, we fabricate a series of anatase−graphene−rutile heterojunction photocatalysts by the surface-assembling strategy. The influence of the relative amount of anatase, rutile and graphene on the photocatalytic H2 production activity of anatase−graphene−rutile photocatalysts is systematically studied. The effect of graphene as the surface phase junction in heterojunction structures is also discussed.

2.2.3. Synthesis of Rutile TiO2 Nanoparticles. Rutile TiO2 nanoparticles were synthesized by the high-temperature calcination of P25 TiO2. In a typical synthetic procedure, P25 TiO2 was calcined at 800 °C for 3 h in a Muffle furnace. The size of obtained rutile TiO2 nanoparticles is ca. 100 nm. 2.2.4. Synthesis of Anatase−Graphene−Rutile Heterojunction Photocatalysts. Anatase−graphene−rutile heterojunction photocatalysts were synthesized through the surfaceassembling strategy. In a typical synthetic procedure, a certain amount of GO was dissolved in 100 mL water to form a transparent solution. Totally 100 mg anatase and rutile TiO2 nanoparticles in certain ratio were added into the above GO solution under vigorous stirring and then treated by ultrasonic irradiation for 30 min. The obtained suspension was exposed to the full-range irradiation of a 300 W xenon lamp for 3 h to reduce the GO into graphene. The final product collected by centrifugation, dried at 60 °C in air. The detailed synthetic information on different anatase−graphene−rutile samples are listed in Table 1. Table 1. Information of all Samples Used in Experiments

2. EXPERIMENTS 2.1. Chemicals. Tetrabutyl titanate (TBT), acetic acid (HAc), flake graphite, H2SO4, NaNO3, and KMnO4 were purchased from Sinopharm (Beijing, China). Thirty % H2O2 was purchased from Aladdin (Shanghai, China). P25 TiO2 was purchased from Degussa (Germany). All reagents were used without further purification, and the deionized water was used in all experiments. 2.2. Synthesis. 2.2.1. Synthesis of Graphene Oxide. Graphene oxide (GO) was synthesized via the modified Hummers method.28 In a typical synthesis program, 10.0 g of graphite powder and 5.0 g of NaNO3 were added to 200 mL of 98% H2SO4 under the condition of an ice bath, and then 30.0 g of KMnO4 was gradually added into the above mixture under vigorous stirring. After the obtained mixture was stirred at 35 °C for 4 h, 230 mL of deionized water was added, followed by vigorous stirring at 98 °C for 15 min. Then the suspended mixture was further diluted to 700 mL by deionized water and stirred for 30 min. The reaction was ended by adding 20 mL of H2O2 (35%) under stirring at room temperature. The resulting product was washed with deionized water and ethanol and dried at 40 ◦C in vacuum for 3 days afterward. The obtained GO sheets have many reactive oxygen species on surface. 2.2.2. Synthesis of Anatase TiO2 Nanoparticles. Anatase TiO2 nanoparticles were synthesized by a modified solvothermal method.29 In a typical synthetic procedure, 0.45 mL of TBT was added dropwise into 35 mL HAc with vigorous stirring for 30 min. The obtained mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave, and subsequently heated at 200 °C for 24 h. After the autoclave cooled to room temperature, the product was collected by centrifugation, washed with ethanol several times, dried at 60 °C in air, and calcined at 400 °C for 3 h to remove the residual organics. The size of obtained anatase TiO2 nanoparticles is ca. 200 nm.

label

anatase amount (mg)

rutile amount (mg)

GO amount (mg)

hydrogen production rate (mmol/h)

A−2%G R−2%G AR9/1−2%G AR8/2−2%G AR7/3−2%G AR5/5−2%G AR3/7−2%G AR7/3 AR7/3−0.5%G AR7/3−l%G AR7/3−3%G AR7/3−5%G AR7/3−10%G

100 0 90 80 70 50 30 70 70 70 70 70 70

0 100 10 20 30 50 70 30 30 30 30 30 30

2 2 2 2 2 2 2 0 0.5 1 3 5 10

1.083 0.592 1.463 1.571 1.714 0.866 0.827 1.112 1.128 1.207 1.080 0.889 0.670

2.3. Characterizations. Powder X-ray diffraction (XRD) patterns were obtained on a D/MAX-2500 diffract meter (Rigaku, Japan) using Cu Kα radiation source (λ = 1.54056 Å) at a scan rate of 5° min−1 to determine the crystal phase of the obtained samples. The accelerating voltage and the applied current were 50 kV and 300 mA, respectively. Transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrum (EDX) were collected on an F20 S-TWIN electron microscope (Tecnai G2, FEI Co.), using a 200 kV accelerating voltage. UV−vis absorption spectra of the samples were obtained from a UV2550 (Shimadzu, Japan) UV−vis spectrophotometer. BaSO4 was used as a reflectance standard. The AC impedance spectroscopy was measured in a typical CHI660 (Chenhua, China) electrochemical workstation using a standard three-compartment cell. A Pt wire and a calomel electrode were used as counter electrode and reference electrode, respectively. Na2SO4 (0.01 M) solution was used as electrolyte. 2.4. Photocatalytic H2 Production. The photocatalytic hydrogen production experiments were carried out in a Lab-H2 photocatalytic hydrogen production system. A 300 W xenon arc lampwas used as the light source and was positioned 20 cm away from the reactor in system. In a typical photocatalytic B

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Figure 1. TEM images in low-magnification (a) and high-magnification (b) and HRTEM images of typical individual anatase particle (c) and rutile particle (d) of the AR 7/3−2%G sample.

Figure 2. STEM image of AR 7/3−2%G (a) and the TEM-EDX pattern of the selected area (b).

experiment, 50 mg catalyst was dispersed with constant stirring in 100 mL 20% methanol aqueous solution. A certain amount of H2PtCl6·6H2O aqueous solution was dripped into the system to load 1% Pt onto the surface of the photocatalyst by a photochemical reduction deposition method. Prior to irradiation, the system was vacuumized to remove the dissolved oxygen. During the whole reaction process, vigorous agitation was performed to ensure the uniform irradiation of the catalyst suspension. A certain amount of generated gas was collected intermittently and hydrogen content was analyzed by gas chromatograph (GC-14C, Shimadzu, Japan, TCD, argon as a

carrier gas). All glassware was rigorously cleaned and carefully rinsed with distilled water prior to use.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. The morphology of anatase−graphene−rutile heterojunction photocatalysts is characterized by TEM (Figure 1, parts a and b). The lowmagnification TEM image (Figure 1a) shows that the typical sample AR7/3−2%G is composed of numerous nanoparticles with different sizes on the near transparent graphene sheet. Wrinkles of graphene sheet can be clearly observed. The highC

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Figure 3. XRD patterns of AR7/3−2%G, pure rutile and pure anatase samples (a), XRD patterns of graphene oxide (GO), AR7/3−2%GO and AR7/3−2%G samples (b).

reduction of GO is collected from the FT-IR analyses. In Figure 4, characteristic bands of GO can be observed in GO and AR7/

magnification TEM image (Figure 1b) shows the size of the large elliptic nanoparticle is ca. 200 nm and that of small undefined nanoparticle is ca. 100 nm. HRTEM images (Figure 1, parts c and d) provide further insight into the crystallinestructural details of these two kinds of nanoparticles. Figure 1c shows the representative HRTEM image of the selected area of a 200 nm-sized elliptic nanoparticle. From which, clear lattice fringes can be observed with interplanar spacing of 0.185 nm, which corresponds to the (200) plane of anatase TiO2 phase (JCPDS No. 21−1272), indicating these 200 nm sized elliptic nanoparticles are anatase TiO 2. Figure 3d shows the representative HRTEM image of the selected area of a 100 nm-sized undefined nanoparticles. Clear lattice fringes can also be observed in Figure 3d with interplanar spacing of about 0.229 nm, which corresponds to the (200) plane of rutile TiO2 phase (JCPDS No. 21−1276). It indicates these 100 nm-sized undefined nanoparticles are rutile TiO2. Combined with TEM images, we can conclude that anatase and rutile nanoparticles were successfully loaded on the graphene surface. The chemical composition of the typical anatase−graphene− rutile nanocomposite sample (AR7/3−2%G) is determined by the STEM-EDX (Figure 2). Figure 2a shows the STEM image of sample AR7/3−2%G, from which 200 nm-sized anatase nanoparticles and 100 nm-sized rutile nanoparticles scattered on the near transparent graphene sheet can further be confirmed. Figure 2b is the EDX pattern of the selected area in STEM image. Besides Cu and Si signals from the substrate, signals of Ti, O and C are detected in the EDX pattern. The phase purity and crystalline compositions of different samples are detected by XRD analyses. Figure 3a shows XRD patterns of as-prepared anatase, rutile and AR7/3−2%G samples. Peaks of anatase and rutile samples can be readily indexed into corresponding planes of the anatase TiO2 phase (JCPDS No. 21-1272) and rutile TiO2 phase (JCPDS No. 21-1276), respectively. The AR7/3−2%G sample exhibits both anatase and rutile peaks. This result coincides with the HRTEM analysis. Figure 3b shows XRD patterns of GO, AR7/3−2%GO and AR7/3−2%G samples. Note that the AR7/3−2%GO sample is prepared in the same method with AR7/3−2%G sample except for the photoreduction treatment. In XRD patterns of GO and AR7/3−2%GO, the peak at 2θ = 10° (marked with an asterisk) can be ascribed to the presence of GO. However, this peak is vanished in the pattern of the AR7/ 3−2%G sample, indicating that GO is reduced into graphene after the photoreduction treatment. More evidence of the

Figure 4. FT-IR spectra of GO, AR7/3−2%GO and AR7/3−2%G.

3−2%GO samples at 972 cm−1 (epoxy stretching), 1057 cm−1 (alkoxy C−O stretching), 1224 cm−1 (phenolic C−OH stretching), 1402 cm−1 (carboxyl O−H stretching) and 1724 cm−1 (CO stretching vibrations of carboxyl or carbonyl groups). The broad absorption at around 1620 cm−1 is related to H−O−H bending band of the adsorbed H2O molecules or the in-plane vibrations of sp2-hybridized C−C bonding. As compared with GO and AR7/3−2%GO samples, the IR spectrum of AR7/3−2%G sample has no observable peaks of GO functional groups. This result also proves that GO is reduced into graphene after the photoreduction treatment. 3.2. UV−Vis Absorption Spectra. The light absorption ability of pure anatase, pure rutile, AR7/3, and AR7/3−2% G samples is characterized by the UV−vis absorption analyses. As shown in Figure 5, the pure anatase sample has absorption edge at around 380 nm, which corresponds to the band gap energy of 3.2 eV. Compared with the anatase sample, the rutile sample has a red-shifted absorption edge at around 410 nm, which corresponds to the band gap energy of 3.0 eV. It is well acknowledged that the band gap of rutile TiO2 is narrower than that of anatase TiO2, which coincides with our results. The AR7/3 sample is the mixture of anatase and rutile samples in the ratio of 7:3, which has a moderate light absorption edge between the anatase and rutile samples. In particular, after the sample is decorated by 2% graphene, the light absorption spectrum of AR7/3−2% G sample has a strong absorption over D

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Figure 5. UV−vis absorption spectra of pure rutile, pure anatase, AR7/3, and AR7/3−2%G samples (a) and the corresponding calculated band gap energy of pure rutile, anatase and AR7/3−2%G sample (b).

Figure 6. Time-dependent H2 production results of samples with different A/R ratios and the same graphene amount of 2% (a), time-dependent H2 production results of samples with different graphene amount and the same A/R ratio of 7/3 (b), the bar chart of H2 production rate of samples with different A/R ratios and the same graphene amount of 2% (c), and the bar chart of H2 production rate of samples with different graphene amount and the same A/R ratio of 7/3 (d).

420 nm. Similar phenomena were also observed by other researchers, which can be ascribed to the absorption of graphene.5,6 3.3. Photocatalytic H2 Production. Photocatalytic H2 production activities of different samples were evaluated under the irradiation of a 300 W xenon lamp. The influence of the anatase/rutile (A/R) ratio on the photocatalytic H2 production activity is investigated by varying the A/R ratio with

the graphene amount (2%) unchanged. From Figure 6, parts a and c, although the rutile TiO2 has better light absorption ability than the anatase TiO2 (Figure 5), the A-2%G sample (1.083 mmol/h) exhibits higher photocatalytic H2 production rate than that of the R-2%G sample (0.592 mmol/h). It is well acknowledged that the crystalline structure of anatase TiO2 is more favorable to the separation of photogenerated electrons and holes than the rutile TiO2. Thus, it is reasonable that the E

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Figure 7. EIS data fitted in R[Q(RW)] model of samples with different AR ratios and the same graphene amount of 2% (a), the EIS data fitted in R[Q(RW)] model of samples with different graphene amount and the same A/R ratio of 7/3 (b), the comparison of Rct value and H2 production rate of samples with different A/R ratios and the same graphene amount of 2% (c), the comparison of Rct value and H2 production rate of samples with different graphene amount and the same A/R ratio of 7/3 (d).

amount from 2% to 10% would decrease the photocatalytic H2 production rate. In particular, the AR7/3−10%G sample has the lowest H2 production rate of 0.670 mmol/h. The decrease of H2 production rate of samples with relatively large graphene amount (over 2%) may result from the limited light absorption. Although a large amount of graphene leads to enlarge the interface area of the anatase−rutile junction, too much graphene may shield the catalyst surface and dramatically decrease the light intensity through the depth of the reaction solution, which can be called as the “shielding effect”.30,31 As a consequence, the suitable graphene amount is crucial for optimizing the photocatalytic H2 production activity of anatase−graphene−rutile heterojunction photocatalysts. 3.4. Electrochemical Impedance Spectroscopy (EIS) Analyses. In order to explore the origin of the positive effect of the graphene as surface phase junction in anatase− graphene−rutile heterojunction catalysts, EIS analyses were systematically conducted. Before analyses, all EIS data were well fitted in the R[Q(RW)] model, as shown in Figure 7, parts a and b. These fitted results allow one to derive kinetic parameters of the charge transfer process as will be demonstratewd below. In accordance with the results obtained an equivalent circuit, as shown in insets of Figure 7, parts a and b, where Rs stood for the resistance of the electrolyte solution;

activity of A-2%G is much higher than that of R-2%G. However, anatase and rutile exhibit excellent synergetic effect in this system. When the A/R ratio is reduced from ∞ to 7/3, the photocatalytic H2 production rate is increased. In particular, the AR7/3−2%G sample shows the highest H2 production rate of 1.714 mmol/h, which is about 58% higher than the A-2%G sample and about 3 times of the R-2%G sample. Further reduce the A/R ratio would decrease the photocatalytic H2 production rate of samples as AR5/5−2%G (0.866 mmol/h) and AR3/7− 2%G (0.827 mmol/h), but they are still higher than the R-2%G sample. Thus, the optimal A/R ratio in our system is 7/3. The influence of graphene amount on the photocatalytic H2 production activity is also investigated by varying graphene addition amount from 0 to 10% with the unchanged A/R ratio of 7/3. As shown in Figure 6, parts b and d, without the addition of graphene, the H2 production rate of AR7/3 sample is 1.112 mmol/h. When the graphene addition amount is increased from 0 to 2%, the photocatalytic H2 production rate of samples is increased to 1.714 mmol/h, which is about 54% higher than the AR7/3 sample. This result indicates that graphene has a strong effect in promoting the H2 production rate of anatase−rutile junction. We think the increased activity is caused by the enlarged surface phase junction between anatase and rutile nanoparticles. Further increase the graphene F

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Figure 8. Scheme of the surface phase junction in anatase−graphene−rutile heterojunction photocatalysts: (i) electrons excited from VB to CB; (ii) electrons moved to Pt particles on TiO2 surface; (iii) electrons moved from rutile CB to Pt particles on graphene surface; (iv) electrons moved from rutile CB to the surface-defect state of anatase; (v) proton reduction on Pt particles.

Rct stood for the electron transfer resistance, which reflected the speed of overall interfacial charge transfer at semiconductor/ electrolyte interface; Zw represented the Warburg impedance attributed to the contribution of diffusion; Q represented the constant phase elements which were associated with the capacitance of the double layer. Figure 7a presents the EIS differences between samples with different A/R ratios and the same graphene amount of 2%, from which the pure A sample has the Rct value of 712.3 Ω and pure R sample has the Rct value of 1371 Ω. The Rct value changed dramatically with the A/R ratio change, and the AR7/3−2% G sample has the smallest Rct value of 310.9 Ω. By comparing the change of Rct and H2 production rates at different A/R ratio in Figure 7c, we find the change trends of Rct and H2 production rates are opposite. Considering that a small Rct often reflects a fast electron transfer in samples, this means that the charge transfer rate is the major effect that influences the H2 production rate of different A/R samples. Figure 7b presents the EIS differences between samples with different graphene amount and the same A/R ratio of 7/3. The AR7/3 sample without the addition of graphene has a Rct value of 1444 Ω. With the addition of graphene from 0.5% to 10%, the Rct value of samples decreased gradually from 548.2 Ω to 115.8 Ω. From Figure 7d, the Rct value decreased gradually with the graphene amount from 0 to 20%. This decrease of Rct value can be ascribed to the enhanced electron transfer from rutile CB to the surface-defect state of anatase and the enhanced proton reduction on graphene surface. However, the change trends of H2 production rate and Rct values of different graphene amount are not totally opposite. When the addition of graphene is more than 2%, the H2 production rate decreased rapidly although the Rct value still decreased. This result further demonstrates that the reduction of activity with the large graphene amount is not from the charge separation ability, and the “shielding effect” mentioned above is a possible explanation. 3.5. Tentative Mechanism. The surface junction of anatase and rutile TiO2 has been reported for years, but the limited surface phase junction area vastly constrained the activity of that kind systems. Our work is aimed to increase the surface phase junction area between anatase and rutile by using

graphene as the supporting materials. Graphene has the incredible surface area and excellent electronic conductivity at the same time, which is the perfect candidate for interfacial mediate in anatase−rutile heterojunction system. By loading anatase and rutile particles on graphene sheets, the transfer of conduction band electrons from rutile to anatase is accelerated by the excellent electronic conductivity, which would further improve the charge separation efficiency and therefore enhance the H2 production activity. Moreover, graphene provides plenty of reactive sites of proton reduction not on particles but on the surface phase junction between anatase and rutile particles, which is the most active part in this system.

4. CONCLUSIONS In this work, we design a feasible anatase−graphene−rutile heterojunction system (Figure 8) to achieve the higher H2 production activity. The key is to use graphene as the supporting material to enlarge the surface phase junction area between anatase and rutile. Our experimental results show that both the A/R ratio and graphene amount have huge influence on the H2 production rate of samples. Under our experimental condition, the optimum A/R ratio is 7/3 and optimum graphene addition amount is 2 wt %. EIS results demonstrate that the enhancement of activity is from the effective charge separation rather than other effect. This work values the expanding of surface phase junction area and provides a feasible strategy to develop high performance photocatalysts by designing and expanding the surface phase junction area.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86 511 8879 0187 Fax.: +86 511 8879 1108 Email: [email protected] (W.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for grants from the National Natural Science Foundation of China (21276116, 21301076, 21303074, G

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and 21201085), the Natural Science Foundation of Jiangsu Province (BK20131257 and BK2012294), the Postdoctoral Science Foundation of China (2014M551508 and 2014M551517), and the Program for New Century Excellent Talents in University (NCET-13-0835), Henry Fok Education Foundation (141068), and Six Talents Peak Project in Jiangsu Province (XCL-025).

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