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brookite TiO2
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Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2 Qiuling Tay, Zhong Chen* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
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abstract
Article history:
In this paper, we synthesized composites consisting of reduced graphene oxide (rGO) and
Received 1 February 2016
two-phase anatase/brookite TiO2 particles. Results showed that with the addition of rGO to
Received in revised form
anatase/brookite TiO2, the photocatalytic H2 production from aqueous methanol solution
2 April 2016
over Pt loaded catalyst is greatly enhanced. This is attributed to effective charge separation
Accepted 5 April 2016
leading to the inhibition of photogenerated electronehole recombination as evidenced
Available online xxx
from photoluminescence (PL) spectroscopic measurement. Moreover, with the rGO e anatase/brookite TiO2 composite, the H2 production is 2.3 times the amount of H2 produced
Keywords:
by rGO e P25 TiO2 (which consists of anatase and rutile particles) composite. The com-
Anatase
parison illustrates the importance of the conduction band potential position. As the con-
Brookite
duction band potential of brookite TiO2 is more cathodic than anatase and rutile TiO2, it is
Photocatalyst
energetically more favorable for reductive hydrogen production. Through the concept of
Reduced graphene oxide
compositing rGO with two-phase semiconductor photocatalyst, our work has elucidated
Charge separation
the importance of effective charge transfer as well as conduction band potential position in
Hydrogen production
photocatalytic hydrogen generation. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since the discovery of H2 generation via water splitting with TiO2 photoanode under the irradiation of ultra-violet (UV) light by Fujishima and Honda in 1972 [1], TiO2 has been a popular candidate as the photocatalyst for water splitting as it exhibits high photocatalytic activity and photochemical stability in a wide range of aqueous solution [2e5]. However, the
photocatalytic activity of TiO2 is limited by the fast recombination of the photogenerated electrons and holes. To enhance the photocatalytic activity of TiO2, several strategies can be introduced to suppress the fast electronehole recombination. One of them is to form TiO2-based nanocomposites or heterojunction where the electrons can be transferred from TiO2 to another material with less cathodic conduction band potential or to TiO2 from another material with more cathodic conduction band potential. The same applies to the photogenerated
* Corresponding author. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.ijhydene.2016.04.022 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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holes transferred between different anodic valence band potentials. As a result, this leads to charge separation and thus hindering the electronehole recombination [6]. TiO2 exists in several polymorphs. The most common ones are rutile (tetragonal, P42/mnm), anatase (tetragonal, I41/amd), and brookite (orthorhombic, Pbca). Studies have shown that with multi-phase TiO2, the photocatalytic activity is enhanced as compared to single phase TiO2 [7e9]. Bickley et al. reported that with two-phase anatase/rutile TiO2 under UV irradiation, the photoexcited electrons are transferred from the anatase phase to the rutile phase due to lower conduction band energy of the rutile phase [7]. As a result, charge recombination is inhibited and photocatalytic activity is enhanced as compared to single phase TiO2. Among the three phases, anatase and rutile are the most commonly synthesized and studied TiO2 phases due to their ease of preparation [10]. There has been much less study on brookite probably due to the difficulties in its synthesis because it is thermodynamically further away from equilibrium under ambient conditions [11]. It was reported that brookite nanocrystals show better photocatalytic activity than rutile and anatase [12,13]. In our previous studies, we have proven that the conduction band edge potential of brookite is more cathodic than anatase phase which leads to higher photocatalytic H2 production [14]. In addition, with twophase anatase/brookite TiO2, H2 production is higher as compared to one-phase brookite and anatase/rutile TiO2. In addition to the semiconductor heterojunctions formed between various oxides, sulfides, and nitrides, studies have also shown that composites of TiO2 and carbon-based materials, including carbon nanotube (CNT), fullerene, and graphene, could improve the photocatalytic performance [15e19]. Studies has shown that there are several drawbacks with TiO2CNT and TiO2-fullerene composites such as weakening of light intensity arriving at the surface of the catalyst, insolubility of CNT in common solvents and polydispersity in CNT length and diameter [15,20,21]. On the other hand, graphene, which is an allotrope of carbon with a 2D layered structure, has been found to possess superior electrical conductivity, high electron mobility, large surface area, and high transparency [22e25]. Accordingly graphene has received great amount of attention in recent years. At room temperature, graphene exhibits a high electron mobility of 2.5 105 cm2 V1 s1 due to it 2D pep conjugated network. It also possesses a large specific surface area (2630 m2 g1), and thus can absorb a high amount of reactants which are beneficial for enhancing photocatalytic performance [22,23]. Both Zhang et al. and Zhou et al. reported that graphene-TiO2 (P25) composites showed enhanced photocatalytic activities as compared to TiO2 alone [22,23]. However, one point to take note for the use of rGO-TiO2 composites in photocatalytic degradation of pollutants which is primarily driven by evolution of OH from the TiO2 valence band holes, the rGO can be photocatalytically unstable [26]. So far, there have been reports on graphene composited with single phase anatase TiO2, rutile TiO2, and two-phase anatase/rutile TiO2 [22,23,27e30], however, there has been no report on the photocatalytic performance using composite between two-phase anatase/brookite and graphene. Our intention is to make use of the known excellent photocatalytic activity of such twophase material based on our previous study to explore whether there is further room for improvement in charge
separation when it is coupled with a good electron transport material such as graphene. Herein, we synthesized reduced graphene oxide (rGO) before preparing a composite with anatase/brookite TiO2 particles. Two-phase anatase/brookite TiO2 was synthesized via a simple hydrothermal method as reported before and were reproducible [14]. Graphene oxide (GO) was synthesized via the well known Hummers method [31] and dispersed in suspension with two-phase anatase/brookite TiO2. Subsequently, GO was reduced to rGO with the use of strong reducing agent to form rGO-anatase/brookite TiO2 mixtures. Hydrazine hydrate was used in this study to reduce GO as it was found to be one of the best reducing agent in producing very thin graphene-like sheets [32]. The photocatalytic activities of rGO-anatase/ rutile TiO2 composites were evaluated via measuring the photocatalytic H2 production from aqueous methanol solution over Pt loaded catalyst. The use of sacrificial agent such as methanol and Pt co-catalyst were widely reported by researchers in enhancing the H2 production [10,33].
Experimental Hydrothermal synthesis of anatase/brookite TiO2 200 mg of titanium sulfide (TiS2, Sigma Aldrich) was added to 3.43 mL of 1 M of sodium hydroxide (NaOH). After stirring of 5 min, the suspension was transferred to a 23 mL Teflon-lined stainless steel autoclave and heated at 200 C for 14 h. After reaction, the autoclave was cooled to room temperature and the as-synthesized TiO2 product was isolated by centrifugation, washed several times with deionized water and dried at 80 C. Based on our previous study with variation of heating time from 1 to 24 h, the hydrogen production was the highest with 91.5 wt % of anatase and 8.5 wt% of brookite synthesized with reaction time of 14 h [14]. Thus, in this study, the two-phase anatase/ brookite TiO2 was synthesized with reaction time of 14 h.
Synthesis of graphene oxide (GO) via the Hummers method 1 g of pure graphite flakes (Sigma Aldrich) was added into the mixture of 0.75 g NaNO3 (Sigma Aldrich) and 34 mL of H2SO4 (Sigma Aldrich) solution. The mixed solution was subsequently stirred vigorously in an ice bath. 5 g of KMnO4 was slowly added into the solution. The addition caused the solution temperature to increase, but such an increase was maintained not to exceed 35 C. The solution was stirred for 2 h during which 50 mL of DI water was added. 4 mL of 30% of H2O2 was then added drop wise into the solution. A yellow solution was formed along with bubbling. The mixture was filtered and washed with 1:10 HCl aqueous solution followed by a large amount of deionized water till the solution pH reaches 7. Subsequently, the suspension was sonicated for 5 h and centrifuged to collect a stable GO solution from the supernatant.
Preparation of rGO-anatase/brookite TiO2 composite Different amount of GO (1 wt%, 2 wt% and 5 wt%) was mixed with the as-synthesized two-phase anatase/brookite TiO2 in a
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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suspension. The suspension was sonicated for 30 min before 1 mL of hydrazine hydrate was added. Subsequently, the mixture was stirred at 1000 rpm for 10 h. The resulting powder was isolated via centrifugation, washed several times with deionized water and dried at 80 C. The powders were heat treated in air at 400 C for 2 h with a ramping rate of 5 C per min. This heating temperature was chosen because a temperature higher than 400 C would decrease the surface area of TiO2 [34] while a lower heat treatment temperature such as 300 C was found insufficient for good interfacial contact between anatase/brookite TiO2 and rGO. In comparison with heat treatment temperature of 300 C and 400 C, anatase/ brookite TiO2 and rGO mixture heat treated at 400 C produces higher amount of H2 (not shown in this paper) and thus a moderate high temperature of 400 C was chosen as the heat treatment temperature. For comparison, 2 wt% of GO was mixed with the commercially available P25 (Degussa) TiO2 with the same preparation procedure to obtain rGO-anatase/ rutile TiO2 composite. Two-phase anatase/brookite TiO2 without the addition of GO was also heat treated in air 400 C for 2 h for comparison.
Characterization The crystallographic phases of the TiO2 and rGO nanostructures were identified by X-ray diffraction (XRD) patterns using a Bruker-AXS X-ray diffractometer with Cu-Ka radiation (l ¼ 1.54178 A). Raman spectroscopy was performed on a WITec a300S.R spectrometer (WITec Instrument Corp, Germany) with laser wavelength of 488 nm at room temperature. Field emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and transmission electron microscopy (TEM, JEOL JEM-2010) were used to characterize the morphologies of the samples. With excitation wavelength of 325 nm, photoluminescence (PL) spectra were collected from RF 5301 spectrofluorophotometer. Micromeritics ASAP 2010 adsorption analyzer was used to determine the surface areas of rGOanatase/rutile TiO2 composites.
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Results and discussion A series of rGO-anatase/brookite TiO2 composites are denoted as Gx-AB, where x stands for the weight percentage of GO added to the two-phase anatase/brookite TiO2 before the reduction of GO to rGO. P25 with 2 wt% of GO is denoted as G2P25. Two-phase anatase/brookite TiO2 is denoted as AB and two-phase anatase/brookite TiO2 heated treated at 400 C is denoted as AB-HT. Fig. 1 shows the XRD patterns of G1-AB, G2AB and G5-AB samples with 1 wt%, 2 wt% and 5 wt% of GO respectively. Anatase TiO2 has a characteristic 2q value at around 25.3 which corresponds to the (101) planes. Brookite TiO2 also has a characteristic 2q value at around 25.3 which corresponds to the (120) planes (See Fig. S1 for clearer brookite peak). Because there is an overlap of the anatase (101) and brookite (120) peaks, the most prominent peak to identify brookite is the characteristic 2q value at around 30.8 corresponding to its (121) plane. Based on the XRD spectra, twophase anatase/brookite TiO2 has been successfully synthesized. On the other hand, no graphene diffraction peak was observed in the diffraction patterns and this could be due to that the main characteristic peak of graphene at 25.4 is shielded by the strong intensity diffraction peaks from anatase (101) and brookite (120) planes. From the Raman spectroscopy, it could provide further evidence of the presence of rGO in the composites. Fig. 2 shows the Raman spectra of GO, rGO-anatase/brookite TiO2 composite (G1-AB) and anatase/brookite TiO2 (AB). In the Raman spectra of the AB sample, vibration peaks are observed at 314 cm1, 388 cm1, 507 cm1 and 630 cm1 which are the characteristics B1g mode of brookite phase, B1g, A1g and Eg modes of anatase phase respectively [35]. Graphene has two typical peaks at 1346 cm1 and 1586 cm1 corresponding to the D (disorder) and G (graphitic) bands. The D band arises from the disruption of the symmetrical hexagonal graphitic lattice caused by internal structural defects, edge defects and dangling bonds while the G band arises from the in-plane vibration of symmetric sp2 bonded carbon atoms [22,36,37]. The
Evaluation of photocatalytic activity The photocatalytic activities of rGO-anatase/rutile TiO2 composites were evaluated via measuring the photocatalytic hydrogen production in a Pyrex glass reactor with closed-gas circulation system. 45 mg of catalyst was added to 80 mL of deionized water and 20 mL of methanol in the glass reactor. 0.3 wt% of Pt was loaded on the rGO-anatase/brookite TiO2 mixture via photodeposition. The required amount of H2PtCl6 was added in the reactant solution and the reactor was irradiated from the top with UVevis light of intensity of 230 mW cm2 from a 800 W XeeHg lamp (Newport, USA) for 2 h. The residual air in the reactor was removed by vacuuming the system with a rotary pump and purging with argon gas for a few times. Subsequently, UVeVisible light with intensity of 230 mW cm2 was illuminated on the top quartz window of the glass reactor. With the help of external water circulation, the reaction temperature was maintained at 25 C. A gas chromatograph (Shimadzu GC-2014; Molecular sieve 5A, TCD detector, Ar carrier gas) directly connected to the reactor was used to determine the amount of hydrogen produced.
Fig. 1 e XRD patterns of G1-AB, G2-AB and G5-AB. Symbol a and * denote anatase (JCPDS 21-1272) and brookite (JCPDS 29-1360) respectively.
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 2 e Raman spectra of GO, G1-AB and AB. Inset shows the D and G bands of GO and G1-AB.
Raman spectra from the G1-AB sample clearly contain bands belonging to anatase, brookite TiO2 and graphene. Moreover, the rGO-anatase/brookite TiO2 composite shows an increased D/G intensity ratio as compared to GO which suggests a decrease in the average size of the in-plane sp2 domains upon reduction of the exfoliated GO. This confirms the existence of rGO sheets in composite [22,37]. The morphology of the rGO-TiO2 composites was characterized via FESEM and TEM as shown in Fig. 3. As observed, the rGO sheets are decorated with nanoparticles and nanoplates. As reported previously [14], anatase and brookite TiO2 synthesized by this hydrothermal route appear in nanoparticle and nanoplate forms respectively. From Fig. 3, the size of anatase nanoparticles, brookite nanoplates and rGO sheets ranges from 20 to 42 nm, 90e112 nm and less than 200 nm in length respectively. Fig. 3d and e show the TEM images of 0.3 wt% Pt loaded G2-AB and as observed, Pt cocatalyst was loaded on anatase/brookite TiO2 and as well as on rGO. Table 1 shows the specific surface areas of AB, AB-HT, G1AB, G2-AB, G5-AB and G2-P25. With heat treatment at 400 C, the surface area of two-phase anatase/brookite TiO2 (AB-HT) decreases from 23.77 m2 g1 to 9.11 m2 g1. In comparison with AB-HT as the rGO-anatase/brookite TiO2 mixtures were heat treated for good interfacial contact, with the addition of rGO, the surface area of the rGO-anatase/brookite TiO2 mixtures (G1-AB, G2-AB, G5-AB) increases which is due to the nanosheet structure of rGO (Fig. 3). The increase in surface area is beneficial to the photocatalytic reaction as it provides a larger area to absorb the reactants. However, adding excessive amount of rGO may not be helpful as it may block the light absorption of the photoactive material and will be discussed later. As a comparison, the amount of hydrogen evolved (per gram of catalyst per hour) from aqueous methanol solution over 0.3 wt% Pt loaded photocatalyst under UVeVisible light (UVevis) illumination is given in Fig. 4. 0.3 wt% of Pt cocatalyst was loaded on the catalyst to further enhance the
H2 production. With the addition of rGO (G1-AB, G2-AB and G5AB), the H2 evolution increases as compared to two-phase anatase/brookite TiO2 (AB and AB-HT). Two-phase anatase/ brookite TiO2 produces 1657 mmol g1 h1 of H2 whereas with the addition of rGO, the H2 evolution increased by 80% with sample G2-AB. To prove that the improvement of H2 production with rGO-anatase/brookite TiO2 mixture is mainly contributed by the effect of addition of rGO and not due to the improved crystallinity of anatase/brookite TiO2 by heat treatment, H2 production from AB and AB-HT were compared. From Fig. 4, with heat treatment of the two-phase anatase/ brookite TiO2, the amount of H2 generated is lower which could be due to decrease in surface area from 23.77 m2 g1 to 9.11 m2 g1 (Table 1). The great enhancement in hydrogen yield is proven to be due to more effective charge separation with the presence of rGO. As reported, brookite conduction band potential is more cathodic than anatase which is energetically favorable for protons to be reduced to hydrogen (Fig. 5a) [10,14]. As a result, H2 evolution is higher with pure brookite TiO2 than pure anatase TiO2 [14]. In addition, with two-phase anatase/ brookite TiO2, the H2 production is enhanced as compared to single phase brookite or anatase TiO2 due to the electrons transfer from brookite to anatase TiO2 leading to effective electronehole separation [14]. When UVevis light is illuminated on the rGO-anatase/brookite TiO2, electrons from both brookite and anatase TiO2 are photoexcited creating electronehole pairs. The photogenerated electrons from brookite could be transferred to anatase to suppress electronehole recombination. Moreover, with the addition of rGO, electrons will be further transferred from anatase TiO2 to rGO (Fig. 5b). The work function of graphene is reported to be 4.42 eV [38] while the conduction band position of anatase TiO2 is 4.21 eV with vacuum level as reference [39], thus graphene can accept electrons from anatase and brookite TiO2 which results in effective charge separation and enhancement of H2 generation. However, since photogenerated electrons could be transferred from brookite to rGO, brookite to anatase and then to rGO, and anatase to rGO (Fig. 5b), one of the unanswered questions is which electron transfer route is the dominant and effective charge separation process. McDaniel et al. reported with a smaller conduction band offset, it results in shorter charge separation time across the heterojunction [40]. To clarify the dominant charge transfer process in the current work, photoluminescence (PL) emission spectroscopic study was carried out. When the photogenerated electrons and holes recombine, photons are emitted resulting in photoluminescence. Lower PL intensity suggests lower recombination rate of photogenerated electronehole pairs. Fig. 6 shows the PL spectra for AB-HT, G1-AB, G2-AB and G5-AB. PL spectra of 2 wt% of rGO with anatase and 2 wt% of rGO with brookite were included in Fig. 6 which are denoted as G2-A and G2-B respectively. The peak at 376 nm corresponding to 3.30 eV is ascribed to direct electronehole recombination in anatase and brookite TiO2 which possess a band gap of 3.2 eV and 3.3 eV respectively [42,44]. The emission band from 400 to 500 nm corresponding to 3.10 to 2.48 eV arises from TiO2 indirect band gap and surface recombination processes [44]. With the addition of rGO, the PL intensity of G1-AB, G2-AB and G5-AB are much
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 3 e a) FESEM images of G5-AB, TEM images of b, c) G5-AB and d, e) 0.3 wt% Pt loaded G2-AB.
lower than AB-HT indicating the diminution of the electron and hole recombination process which is due to the charge transfer from brookite and anatase TiO2 to rGO leading to effective charge separation.
Table 1 e BET surface areas of AB, AB-HT G1-AB, G2-AB, G5-AB and G2-P25. Sample AB AB-HT G1-AB G2-AB G5-AB G2-P25
BET surface area (m2 g1) 23.77 9.11 19.87 29.73 24.77 6.87
To determine the most effective and dominant charge transfer process in the two-phase anatase/brookite-rGO mixture, PL intensity of G2-A, G2-B and G2-AB are compared. From Fig. 6, G2-B has higher PL intensity than G2-A which could be due to the larger potential offset of brookite and rGO conduction band potential than anatase and rGO conduction band potential as shown in Fig. 5a. This indicates less effective electron transfer from brookite to rGO than anatase to rGO [40]. With two-phase anatase/brookite, the PL intensity is the lowest with G2-AB than G2-A and G2-B, this means that electrons “cascade” from brookite to anatase and then to rGO could be the most effective and dominant charge separation process leading to the enhancement of H2 generation. The PL intensity of G2-AB and G5-AB are lower than G1-AB indicating more effective charge separation with the increase of rGO. However, when the amount of GO was increased to
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 4 e Comparison of hydrogen evolved per gram of catalyst per hour in aqueous methanol solution over 0.3 wt % Pt loaded photocatalysts; AB, AB-HT, G1-AB, G2-AB, G5-AB, G2-P25.
Fig. 5 e a) Band diagram of brookite, anatase, rutile TiO2 and graphene relative to redox potential of water at pH 7 [13,41e43]. b) Schematic diagram of the electron transfer in rGO-anatase/brookite TiO2 composite under UVevis light illumination.
Fig. 6 e Photoluminescence spectra of AB-HT, G1-AB, G2-AB, G5-AB, G2-A and G2-B.
5 wt%, the H2 evolution decreases as compared to 2 wt% of GO. This could be due to the shielding effect by the rGO that affects the light absorption of the photoactive TiO2 in the composite leading to a reduction in photocatalytic activity [21]. This is evidenced by the slightly darker color of the G5-AB sample than G2-AB and G1-AB shown in Fig. S2 (supporting information). Thus, with 2 wt% of GO added to two phase anatase/ brookite TiO2 before chemical reduction of GO, the amount of H2 produced is the highest with 3051 mmol g1 h1 as compared with 1 wt% and 5 wt% of GO. Another important consideration for semiconductor composite is the conduction band potential of all the constituent phase. In comparison with G2-P25, G2-AB produces 2.3 times the amount of H2 produced by G2-P25 despite G2-P25 has a higher surface area as shown in Table 1. The higher photocatalytic activity of G2-AB which consists of anatase/ brookite TiO2 over G2-P25 with anatase/rutile TiO2 is attributed to the higher cathodic potential of brookite conduction band than anatase and rutile conduction bands which energetically favors the reduction of protons to produce H2. In addition, the conduction band potential of rutile (0.07 V vs. NHE at pH 0) is less cathodic than the chemical potential for water reduction (0 V vs. NHE at pH 0) [41,45] and thus it cannot participate in water reduction (Fig. 5a). Hence, under UVevis illumination, photogenerated electrons, either transferred to rGO, or stayed in the brookite and anatase phase are able to reduce protons to H2 (Fig. 5a and b). On the other hand, for G2P25 sample, under UVevis illumination, only the photogenerated electrons in the anatase phase and electrons transferred to rGO from anatase are able to reduce water to form H2. The electrons that are either generated on rutile or transferred from anatase are not energetically favorable for hydrogen production. This can be proven from Fig. S3 (supporting information) which shows the comparison of H2 production with G2-P25 and G2-A. With the presence of rutile phase, G2-P25 produces lower amount of H2 than G2-A despite G2-A has slightly lower or almost the same surface area (36.16 m2 g1) as G2-P25 (36.87 m2 g1). Therefore, despite that
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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the addition of rGO can help to inhibit electronehole recombination, electrons in the rutile phases of G2-P25 do not contribute to hydrogen yield. As a result, the photocatalytic activity of our rGO-anatase/brookite TiO2 composite is better than G2-P25 composite. In addition, from Fig. S4 (supporting information), the rGO-TiO2 catalysts have approximately constant rate of H2 evolution per hour at each hour from 1st to 6th hour, indicating rGO-TiO2 catalysts are stable within our test duration.
Conclusions In summary, a series of rGO-anatase/brookite TiO2 composites with enhanced photocatalytic H2 production were synthesized via hydrothermal synthesis of anatase/brookite TiO2 and chemical reduction of GO. Results have shown that with the addition of rGO, the photocatalytic activity is higher with rGO-anatase/brookite TiO2 mixture as compared to anatase/ brookite TiO2 without rGO. From PL analysis, it is proven that the probability of electron e hole recombination is significantly reduced with rGO sheets acting as electron collector to separate the photogenerated charge carriers. A cascading electron transfer mechanism, i.e., electrons transfer from brookite to anatase and then to rGO, is found to be the most effective and dominant process. Furthermore, with the addition of rGO, surface area of rGO-anatase/brookite TiO2 composite increases which could also have contributed to enhanced photocatalytic activity. The optimal weight percentage of GO in the rGO-anatase/brookite TiO2 mixture was found to be 2 wt%, producing a rate of 3051 mmol g1 h1 of H2. rGO-anatase/brookite TiO2 has shown higher activity than rGO-P25. This is attributed to the more cathodic potential of brookite conduction band as compared to anatase and rutile conduction bands leading to higher reduction power to reduce water to produce H2. Overall, the H2 production is enhanced due to the synergistic effects of improved electronehole separation, higher surface area and higher reduction power to reduce water with rGO-anatase/brookite TiO2 mixture.
Acknowledgments The first author wishes to express her gratitude to Nanyang Technological University for supporting her doctoral degree study with the Nanyang President's Graduate Scholarship. Z.C. acknowledges the financial support from MOE Singapore (Grant RG 112/05) and the National Research Foundation (NRF) Singapore through its Campus for Research Excellence and Technological Enterprise (CREATE) program.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.04.022.
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Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2 Qiuling Tay, Zhong Chen* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
article info
abstract
Article history:
In this paper, we synthesized composites consisting of reduced graphene oxide (rGO) and
Received 1 February 2016
two-phase anatase/brookite TiO2 particles. Results showed that with the addition of rGO to
Received in revised form
anatase/brookite TiO2, the photocatalytic H2 production from aqueous methanol solution
2 April 2016
over Pt loaded catalyst is greatly enhanced. This is attributed to effective charge separation
Accepted 5 April 2016
leading to the inhibition of photogenerated electronehole recombination as evidenced
Available online xxx
from photoluminescence (PL) spectroscopic measurement. Moreover, with the rGO e anatase/brookite TiO2 composite, the H2 production is 2.3 times the amount of H2 produced
Keywords:
by rGO e P25 TiO2 (which consists of anatase and rutile particles) composite. The com-
Anatase
parison illustrates the importance of the conduction band potential position. As the con-
Brookite
duction band potential of brookite TiO2 is more cathodic than anatase and rutile TiO2, it is
Photocatalyst
energetically more favorable for reductive hydrogen production. Through the concept of
Reduced graphene oxide
compositing rGO with two-phase semiconductor photocatalyst, our work has elucidated
Charge separation
the importance of effective charge transfer as well as conduction band potential position in
Hydrogen production
photocatalytic hydrogen generation. Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since the discovery of H2 generation via water splitting with TiO2 photoanode under the irradiation of ultra-violet (UV) light by Fujishima and Honda in 1972 [1], TiO2 has been a popular candidate as the photocatalyst for water splitting as it exhibits high photocatalytic activity and photochemical stability in a wide range of aqueous solution [2e5]. However, the
photocatalytic activity of TiO2 is limited by the fast recombination of the photogenerated electrons and holes. To enhance the photocatalytic activity of TiO2, several strategies can be introduced to suppress the fast electronehole recombination. One of them is to form TiO2-based nanocomposites or heterojunction where the electrons can be transferred from TiO2 to another material with less cathodic conduction band potential or to TiO2 from another material with more cathodic conduction band potential. The same applies to the photogenerated
* Corresponding author. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.ijhydene.2016.04.022 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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holes transferred between different anodic valence band potentials. As a result, this leads to charge separation and thus hindering the electronehole recombination [6]. TiO2 exists in several polymorphs. The most common ones are rutile (tetragonal, P42/mnm), anatase (tetragonal, I41/amd), and brookite (orthorhombic, Pbca). Studies have shown that with multi-phase TiO2, the photocatalytic activity is enhanced as compared to single phase TiO2 [7e9]. Bickley et al. reported that with two-phase anatase/rutile TiO2 under UV irradiation, the photoexcited electrons are transferred from the anatase phase to the rutile phase due to lower conduction band energy of the rutile phase [7]. As a result, charge recombination is inhibited and photocatalytic activity is enhanced as compared to single phase TiO2. Among the three phases, anatase and rutile are the most commonly synthesized and studied TiO2 phases due to their ease of preparation [10]. There has been much less study on brookite probably due to the difficulties in its synthesis because it is thermodynamically further away from equilibrium under ambient conditions [11]. It was reported that brookite nanocrystals show better photocatalytic activity than rutile and anatase [12,13]. In our previous studies, we have proven that the conduction band edge potential of brookite is more cathodic than anatase phase which leads to higher photocatalytic H2 production [14]. In addition, with twophase anatase/brookite TiO2, H2 production is higher as compared to one-phase brookite and anatase/rutile TiO2. In addition to the semiconductor heterojunctions formed between various oxides, sulfides, and nitrides, studies have also shown that composites of TiO2 and carbon-based materials, including carbon nanotube (CNT), fullerene, and graphene, could improve the photocatalytic performance [15e19]. Studies has shown that there are several drawbacks with TiO2CNT and TiO2-fullerene composites such as weakening of light intensity arriving at the surface of the catalyst, insolubility of CNT in common solvents and polydispersity in CNT length and diameter [15,20,21]. On the other hand, graphene, which is an allotrope of carbon with a 2D layered structure, has been found to possess superior electrical conductivity, high electron mobility, large surface area, and high transparency [22e25]. Accordingly graphene has received great amount of attention in recent years. At room temperature, graphene exhibits a high electron mobility of 2.5 105 cm2 V1 s1 due to it 2D pep conjugated network. It also possesses a large specific surface area (2630 m2 g1), and thus can absorb a high amount of reactants which are beneficial for enhancing photocatalytic performance [22,23]. Both Zhang et al. and Zhou et al. reported that graphene-TiO2 (P25) composites showed enhanced photocatalytic activities as compared to TiO2 alone [22,23]. However, one point to take note for the use of rGO-TiO2 composites in photocatalytic degradation of pollutants which is primarily driven by evolution of OH from the TiO2 valence band holes, the rGO can be photocatalytically unstable [26]. So far, there have been reports on graphene composited with single phase anatase TiO2, rutile TiO2, and two-phase anatase/rutile TiO2 [22,23,27e30], however, there has been no report on the photocatalytic performance using composite between two-phase anatase/brookite and graphene. Our intention is to make use of the known excellent photocatalytic activity of such twophase material based on our previous study to explore whether there is further room for improvement in charge
separation when it is coupled with a good electron transport material such as graphene. Herein, we synthesized reduced graphene oxide (rGO) before preparing a composite with anatase/brookite TiO2 particles. Two-phase anatase/brookite TiO2 was synthesized via a simple hydrothermal method as reported before and were reproducible [14]. Graphene oxide (GO) was synthesized via the well known Hummers method [31] and dispersed in suspension with two-phase anatase/brookite TiO2. Subsequently, GO was reduced to rGO with the use of strong reducing agent to form rGO-anatase/brookite TiO2 mixtures. Hydrazine hydrate was used in this study to reduce GO as it was found to be one of the best reducing agent in producing very thin graphene-like sheets [32]. The photocatalytic activities of rGO-anatase/ rutile TiO2 composites were evaluated via measuring the photocatalytic H2 production from aqueous methanol solution over Pt loaded catalyst. The use of sacrificial agent such as methanol and Pt co-catalyst were widely reported by researchers in enhancing the H2 production [10,33].
Experimental Hydrothermal synthesis of anatase/brookite TiO2 200 mg of titanium sulfide (TiS2, Sigma Aldrich) was added to 3.43 mL of 1 M of sodium hydroxide (NaOH). After stirring of 5 min, the suspension was transferred to a 23 mL Teflon-lined stainless steel autoclave and heated at 200 C for 14 h. After reaction, the autoclave was cooled to room temperature and the as-synthesized TiO2 product was isolated by centrifugation, washed several times with deionized water and dried at 80 C. Based on our previous study with variation of heating time from 1 to 24 h, the hydrogen production was the highest with 91.5 wt % of anatase and 8.5 wt% of brookite synthesized with reaction time of 14 h [14]. Thus, in this study, the two-phase anatase/ brookite TiO2 was synthesized with reaction time of 14 h.
Synthesis of graphene oxide (GO) via the Hummers method 1 g of pure graphite flakes (Sigma Aldrich) was added into the mixture of 0.75 g NaNO3 (Sigma Aldrich) and 34 mL of H2SO4 (Sigma Aldrich) solution. The mixed solution was subsequently stirred vigorously in an ice bath. 5 g of KMnO4 was slowly added into the solution. The addition caused the solution temperature to increase, but such an increase was maintained not to exceed 35 C. The solution was stirred for 2 h during which 50 mL of DI water was added. 4 mL of 30% of H2O2 was then added drop wise into the solution. A yellow solution was formed along with bubbling. The mixture was filtered and washed with 1:10 HCl aqueous solution followed by a large amount of deionized water till the solution pH reaches 7. Subsequently, the suspension was sonicated for 5 h and centrifuged to collect a stable GO solution from the supernatant.
Preparation of rGO-anatase/brookite TiO2 composite Different amount of GO (1 wt%, 2 wt% and 5 wt%) was mixed with the as-synthesized two-phase anatase/brookite TiO2 in a
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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suspension. The suspension was sonicated for 30 min before 1 mL of hydrazine hydrate was added. Subsequently, the mixture was stirred at 1000 rpm for 10 h. The resulting powder was isolated via centrifugation, washed several times with deionized water and dried at 80 C. The powders were heat treated in air at 400 C for 2 h with a ramping rate of 5 C per min. This heating temperature was chosen because a temperature higher than 400 C would decrease the surface area of TiO2 [34] while a lower heat treatment temperature such as 300 C was found insufficient for good interfacial contact between anatase/brookite TiO2 and rGO. In comparison with heat treatment temperature of 300 C and 400 C, anatase/ brookite TiO2 and rGO mixture heat treated at 400 C produces higher amount of H2 (not shown in this paper) and thus a moderate high temperature of 400 C was chosen as the heat treatment temperature. For comparison, 2 wt% of GO was mixed with the commercially available P25 (Degussa) TiO2 with the same preparation procedure to obtain rGO-anatase/ rutile TiO2 composite. Two-phase anatase/brookite TiO2 without the addition of GO was also heat treated in air 400 C for 2 h for comparison.
Characterization The crystallographic phases of the TiO2 and rGO nanostructures were identified by X-ray diffraction (XRD) patterns using a Bruker-AXS X-ray diffractometer with Cu-Ka radiation (l ¼ 1.54178 A). Raman spectroscopy was performed on a WITec a300S.R spectrometer (WITec Instrument Corp, Germany) with laser wavelength of 488 nm at room temperature. Field emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and transmission electron microscopy (TEM, JEOL JEM-2010) were used to characterize the morphologies of the samples. With excitation wavelength of 325 nm, photoluminescence (PL) spectra were collected from RF 5301 spectrofluorophotometer. Micromeritics ASAP 2010 adsorption analyzer was used to determine the surface areas of rGOanatase/rutile TiO2 composites.
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Results and discussion A series of rGO-anatase/brookite TiO2 composites are denoted as Gx-AB, where x stands for the weight percentage of GO added to the two-phase anatase/brookite TiO2 before the reduction of GO to rGO. P25 with 2 wt% of GO is denoted as G2P25. Two-phase anatase/brookite TiO2 is denoted as AB and two-phase anatase/brookite TiO2 heated treated at 400 C is denoted as AB-HT. Fig. 1 shows the XRD patterns of G1-AB, G2AB and G5-AB samples with 1 wt%, 2 wt% and 5 wt% of GO respectively. Anatase TiO2 has a characteristic 2q value at around 25.3 which corresponds to the (101) planes. Brookite TiO2 also has a characteristic 2q value at around 25.3 which corresponds to the (120) planes (See Fig. S1 for clearer brookite peak). Because there is an overlap of the anatase (101) and brookite (120) peaks, the most prominent peak to identify brookite is the characteristic 2q value at around 30.8 corresponding to its (121) plane. Based on the XRD spectra, twophase anatase/brookite TiO2 has been successfully synthesized. On the other hand, no graphene diffraction peak was observed in the diffraction patterns and this could be due to that the main characteristic peak of graphene at 25.4 is shielded by the strong intensity diffraction peaks from anatase (101) and brookite (120) planes. From the Raman spectroscopy, it could provide further evidence of the presence of rGO in the composites. Fig. 2 shows the Raman spectra of GO, rGO-anatase/brookite TiO2 composite (G1-AB) and anatase/brookite TiO2 (AB). In the Raman spectra of the AB sample, vibration peaks are observed at 314 cm1, 388 cm1, 507 cm1 and 630 cm1 which are the characteristics B1g mode of brookite phase, B1g, A1g and Eg modes of anatase phase respectively [35]. Graphene has two typical peaks at 1346 cm1 and 1586 cm1 corresponding to the D (disorder) and G (graphitic) bands. The D band arises from the disruption of the symmetrical hexagonal graphitic lattice caused by internal structural defects, edge defects and dangling bonds while the G band arises from the in-plane vibration of symmetric sp2 bonded carbon atoms [22,36,37]. The
Evaluation of photocatalytic activity The photocatalytic activities of rGO-anatase/rutile TiO2 composites were evaluated via measuring the photocatalytic hydrogen production in a Pyrex glass reactor with closed-gas circulation system. 45 mg of catalyst was added to 80 mL of deionized water and 20 mL of methanol in the glass reactor. 0.3 wt% of Pt was loaded on the rGO-anatase/brookite TiO2 mixture via photodeposition. The required amount of H2PtCl6 was added in the reactant solution and the reactor was irradiated from the top with UVevis light of intensity of 230 mW cm2 from a 800 W XeeHg lamp (Newport, USA) for 2 h. The residual air in the reactor was removed by vacuuming the system with a rotary pump and purging with argon gas for a few times. Subsequently, UVeVisible light with intensity of 230 mW cm2 was illuminated on the top quartz window of the glass reactor. With the help of external water circulation, the reaction temperature was maintained at 25 C. A gas chromatograph (Shimadzu GC-2014; Molecular sieve 5A, TCD detector, Ar carrier gas) directly connected to the reactor was used to determine the amount of hydrogen produced.
Fig. 1 e XRD patterns of G1-AB, G2-AB and G5-AB. Symbol a and * denote anatase (JCPDS 21-1272) and brookite (JCPDS 29-1360) respectively.
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 2 e Raman spectra of GO, G1-AB and AB. Inset shows the D and G bands of GO and G1-AB.
Raman spectra from the G1-AB sample clearly contain bands belonging to anatase, brookite TiO2 and graphene. Moreover, the rGO-anatase/brookite TiO2 composite shows an increased D/G intensity ratio as compared to GO which suggests a decrease in the average size of the in-plane sp2 domains upon reduction of the exfoliated GO. This confirms the existence of rGO sheets in composite [22,37]. The morphology of the rGO-TiO2 composites was characterized via FESEM and TEM as shown in Fig. 3. As observed, the rGO sheets are decorated with nanoparticles and nanoplates. As reported previously [14], anatase and brookite TiO2 synthesized by this hydrothermal route appear in nanoparticle and nanoplate forms respectively. From Fig. 3, the size of anatase nanoparticles, brookite nanoplates and rGO sheets ranges from 20 to 42 nm, 90e112 nm and less than 200 nm in length respectively. Fig. 3d and e show the TEM images of 0.3 wt% Pt loaded G2-AB and as observed, Pt cocatalyst was loaded on anatase/brookite TiO2 and as well as on rGO. Table 1 shows the specific surface areas of AB, AB-HT, G1AB, G2-AB, G5-AB and G2-P25. With heat treatment at 400 C, the surface area of two-phase anatase/brookite TiO2 (AB-HT) decreases from 23.77 m2 g1 to 9.11 m2 g1. In comparison with AB-HT as the rGO-anatase/brookite TiO2 mixtures were heat treated for good interfacial contact, with the addition of rGO, the surface area of the rGO-anatase/brookite TiO2 mixtures (G1-AB, G2-AB, G5-AB) increases which is due to the nanosheet structure of rGO (Fig. 3). The increase in surface area is beneficial to the photocatalytic reaction as it provides a larger area to absorb the reactants. However, adding excessive amount of rGO may not be helpful as it may block the light absorption of the photoactive material and will be discussed later. As a comparison, the amount of hydrogen evolved (per gram of catalyst per hour) from aqueous methanol solution over 0.3 wt% Pt loaded photocatalyst under UVeVisible light (UVevis) illumination is given in Fig. 4. 0.3 wt% of Pt cocatalyst was loaded on the catalyst to further enhance the
H2 production. With the addition of rGO (G1-AB, G2-AB and G5AB), the H2 evolution increases as compared to two-phase anatase/brookite TiO2 (AB and AB-HT). Two-phase anatase/ brookite TiO2 produces 1657 mmol g1 h1 of H2 whereas with the addition of rGO, the H2 evolution increased by 80% with sample G2-AB. To prove that the improvement of H2 production with rGO-anatase/brookite TiO2 mixture is mainly contributed by the effect of addition of rGO and not due to the improved crystallinity of anatase/brookite TiO2 by heat treatment, H2 production from AB and AB-HT were compared. From Fig. 4, with heat treatment of the two-phase anatase/ brookite TiO2, the amount of H2 generated is lower which could be due to decrease in surface area from 23.77 m2 g1 to 9.11 m2 g1 (Table 1). The great enhancement in hydrogen yield is proven to be due to more effective charge separation with the presence of rGO. As reported, brookite conduction band potential is more cathodic than anatase which is energetically favorable for protons to be reduced to hydrogen (Fig. 5a) [10,14]. As a result, H2 evolution is higher with pure brookite TiO2 than pure anatase TiO2 [14]. In addition, with two-phase anatase/ brookite TiO2, the H2 production is enhanced as compared to single phase brookite or anatase TiO2 due to the electrons transfer from brookite to anatase TiO2 leading to effective electronehole separation [14]. When UVevis light is illuminated on the rGO-anatase/brookite TiO2, electrons from both brookite and anatase TiO2 are photoexcited creating electronehole pairs. The photogenerated electrons from brookite could be transferred to anatase to suppress electronehole recombination. Moreover, with the addition of rGO, electrons will be further transferred from anatase TiO2 to rGO (Fig. 5b). The work function of graphene is reported to be 4.42 eV [38] while the conduction band position of anatase TiO2 is 4.21 eV with vacuum level as reference [39], thus graphene can accept electrons from anatase and brookite TiO2 which results in effective charge separation and enhancement of H2 generation. However, since photogenerated electrons could be transferred from brookite to rGO, brookite to anatase and then to rGO, and anatase to rGO (Fig. 5b), one of the unanswered questions is which electron transfer route is the dominant and effective charge separation process. McDaniel et al. reported with a smaller conduction band offset, it results in shorter charge separation time across the heterojunction [40]. To clarify the dominant charge transfer process in the current work, photoluminescence (PL) emission spectroscopic study was carried out. When the photogenerated electrons and holes recombine, photons are emitted resulting in photoluminescence. Lower PL intensity suggests lower recombination rate of photogenerated electronehole pairs. Fig. 6 shows the PL spectra for AB-HT, G1-AB, G2-AB and G5-AB. PL spectra of 2 wt% of rGO with anatase and 2 wt% of rGO with brookite were included in Fig. 6 which are denoted as G2-A and G2-B respectively. The peak at 376 nm corresponding to 3.30 eV is ascribed to direct electronehole recombination in anatase and brookite TiO2 which possess a band gap of 3.2 eV and 3.3 eV respectively [42,44]. The emission band from 400 to 500 nm corresponding to 3.10 to 2.48 eV arises from TiO2 indirect band gap and surface recombination processes [44]. With the addition of rGO, the PL intensity of G1-AB, G2-AB and G5-AB are much
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 3 e a) FESEM images of G5-AB, TEM images of b, c) G5-AB and d, e) 0.3 wt% Pt loaded G2-AB.
lower than AB-HT indicating the diminution of the electron and hole recombination process which is due to the charge transfer from brookite and anatase TiO2 to rGO leading to effective charge separation.
Table 1 e BET surface areas of AB, AB-HT G1-AB, G2-AB, G5-AB and G2-P25. Sample AB AB-HT G1-AB G2-AB G5-AB G2-P25
BET surface area (m2 g1) 23.77 9.11 19.87 29.73 24.77 6.87
To determine the most effective and dominant charge transfer process in the two-phase anatase/brookite-rGO mixture, PL intensity of G2-A, G2-B and G2-AB are compared. From Fig. 6, G2-B has higher PL intensity than G2-A which could be due to the larger potential offset of brookite and rGO conduction band potential than anatase and rGO conduction band potential as shown in Fig. 5a. This indicates less effective electron transfer from brookite to rGO than anatase to rGO [40]. With two-phase anatase/brookite, the PL intensity is the lowest with G2-AB than G2-A and G2-B, this means that electrons “cascade” from brookite to anatase and then to rGO could be the most effective and dominant charge separation process leading to the enhancement of H2 generation. The PL intensity of G2-AB and G5-AB are lower than G1-AB indicating more effective charge separation with the increase of rGO. However, when the amount of GO was increased to
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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Fig. 4 e Comparison of hydrogen evolved per gram of catalyst per hour in aqueous methanol solution over 0.3 wt % Pt loaded photocatalysts; AB, AB-HT, G1-AB, G2-AB, G5-AB, G2-P25.
Fig. 5 e a) Band diagram of brookite, anatase, rutile TiO2 and graphene relative to redox potential of water at pH 7 [13,41e43]. b) Schematic diagram of the electron transfer in rGO-anatase/brookite TiO2 composite under UVevis light illumination.
Fig. 6 e Photoluminescence spectra of AB-HT, G1-AB, G2-AB, G5-AB, G2-A and G2-B.
5 wt%, the H2 evolution decreases as compared to 2 wt% of GO. This could be due to the shielding effect by the rGO that affects the light absorption of the photoactive TiO2 in the composite leading to a reduction in photocatalytic activity [21]. This is evidenced by the slightly darker color of the G5-AB sample than G2-AB and G1-AB shown in Fig. S2 (supporting information). Thus, with 2 wt% of GO added to two phase anatase/ brookite TiO2 before chemical reduction of GO, the amount of H2 produced is the highest with 3051 mmol g1 h1 as compared with 1 wt% and 5 wt% of GO. Another important consideration for semiconductor composite is the conduction band potential of all the constituent phase. In comparison with G2-P25, G2-AB produces 2.3 times the amount of H2 produced by G2-P25 despite G2-P25 has a higher surface area as shown in Table 1. The higher photocatalytic activity of G2-AB which consists of anatase/ brookite TiO2 over G2-P25 with anatase/rutile TiO2 is attributed to the higher cathodic potential of brookite conduction band than anatase and rutile conduction bands which energetically favors the reduction of protons to produce H2. In addition, the conduction band potential of rutile (0.07 V vs. NHE at pH 0) is less cathodic than the chemical potential for water reduction (0 V vs. NHE at pH 0) [41,45] and thus it cannot participate in water reduction (Fig. 5a). Hence, under UVevis illumination, photogenerated electrons, either transferred to rGO, or stayed in the brookite and anatase phase are able to reduce protons to H2 (Fig. 5a and b). On the other hand, for G2P25 sample, under UVevis illumination, only the photogenerated electrons in the anatase phase and electrons transferred to rGO from anatase are able to reduce water to form H2. The electrons that are either generated on rutile or transferred from anatase are not energetically favorable for hydrogen production. This can be proven from Fig. S3 (supporting information) which shows the comparison of H2 production with G2-P25 and G2-A. With the presence of rutile phase, G2-P25 produces lower amount of H2 than G2-A despite G2-A has slightly lower or almost the same surface area (36.16 m2 g1) as G2-P25 (36.87 m2 g1). Therefore, despite that
Please cite this article in press as: Tay Q, Chen Z, Effective charge separation towards enhanced photocatalytic activity via compositing reduced graphene oxide with two-phase anatase/brookite TiO2, International Journal of Hydrogen Energy (2016), http://dx.doi.org/ 10.1016/j.ijhydene.2016.04.022
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the addition of rGO can help to inhibit electronehole recombination, electrons in the rutile phases of G2-P25 do not contribute to hydrogen yield. As a result, the photocatalytic activity of our rGO-anatase/brookite TiO2 composite is better than G2-P25 composite. In addition, from Fig. S4 (supporting information), the rGO-TiO2 catalysts have approximately constant rate of H2 evolution per hour at each hour from 1st to 6th hour, indicating rGO-TiO2 catalysts are stable within our test duration.
Conclusions In summary, a series of rGO-anatase/brookite TiO2 composites with enhanced photocatalytic H2 production were synthesized via hydrothermal synthesis of anatase/brookite TiO2 and chemical reduction of GO. Results have shown that with the addition of rGO, the photocatalytic activity is higher with rGO-anatase/brookite TiO2 mixture as compared to anatase/ brookite TiO2 without rGO. From PL analysis, it is proven that the probability of electron e hole recombination is significantly reduced with rGO sheets acting as electron collector to separate the photogenerated charge carriers. A cascading electron transfer mechanism, i.e., electrons transfer from brookite to anatase and then to rGO, is found to be the most effective and dominant process. Furthermore, with the addition of rGO, surface area of rGO-anatase/brookite TiO2 composite increases which could also have contributed to enhanced photocatalytic activity. The optimal weight percentage of GO in the rGO-anatase/brookite TiO2 mixture was found to be 2 wt%, producing a rate of 3051 mmol g1 h1 of H2. rGO-anatase/brookite TiO2 has shown higher activity than rGO-P25. This is attributed to the more cathodic potential of brookite conduction band as compared to anatase and rutile conduction bands leading to higher reduction power to reduce water to produce H2. Overall, the H2 production is enhanced due to the synergistic effects of improved electronehole separation, higher surface area and higher reduction power to reduce water with rGO-anatase/brookite TiO2 mixture.
Acknowledgments The first author wishes to express her gratitude to Nanyang Technological University for supporting her doctoral degree study with the Nanyang President's Graduate Scholarship. Z.C. acknowledges the financial support from MOE Singapore (Grant RG 112/05) and the National Research Foundation (NRF) Singapore through its Campus for Research Excellence and Technological Enterprise (CREATE) program.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.04.022.
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