Hierarchical Heterostructure of ZnO@TiO2 Hollow Spheres for Highly Efficient Photocatalytic Hydrogen Evolution
© The Author(s). 2017
Received: 17 July 2017
Accepted: 5 September 2017
Published: 13 September 2017
The rational design and preparation of hierarchical nanoarchitectures are critical for enhanced photocatalytic hydrogen evolution reaction (HER). Herein, well-integrated hollow ZnO@TiO2 heterojunctions were obtained by a simple hydrothermal method. This unique hierarchical heterostructure not only caused multiple reflections which enhances the light absorption but also improved the lifetime and transfer of photogenerated charge carriers due to the potential difference generated on the ZnO–TiO2 interface. As a result, compared to bare ZnO and TiO2, the ZnO@TiO2 composite photocatalyst exhibited higher hydrogen production rated up to 0.152 mmol h−1 g−1 under simulated solar light. In addition, highly repeated photostability was also observed on the ZnO@TiO2 composite photocatalyst even after a continuous test for 30 h. It is expected that this low-cost, nontoxic, and readily available ZnO@TiO2 catalyst could exhibit promising potential in photocatalytic H2 to meet the future fuel needs.
KeywordsZnO TiO2 Hollow sphere Hierarchical Heterojunction Hydrogen production
Hydrogen (H2), one of the most important clean and sustainable energy, has been regarded as a promising alternative energy for meeting future fuel needs [1–5]. Since the discovery of photoelectrochemical (PEC) water-splitting system by Fujishima and Honda in the 1970s , the production of H2 based on TiO2 semiconductor photocatalysts using sunlight has attracted increasing attention. However, the practical application of single bare TiO2 in the industry is still a challenge due to the high-rate recombination of photogenerated electrons and holes at the surface of TiO2 results in a low quantum efficiency. To date, many efforts have been made to design TiO2-based composite photocatalysts to solve the above issues, such as coupling with another semiconductor, doping transition metal ions or nonmetal atoms, and so on [7–9]. In particular, the formation of semiconductor–semiconductor heterojunctions with matching band potentials is an effective way to prevent the charge recombination and increase the lifetime of the charge carriers [10–12].
Among the various semiconductors, ZnO is also extensively studied because of its identical properties of TiO2 with non-toxicity, cheapness, high efficiency, and chemical stability [13, 14]. Since the conduction band (CB) and valence band (VB) of ZnO lie above those of TiO2, the photogenerated electrons in ZnO will be transferred to TiO2 once a heterojunction was formed between TiO2 and ZnO. This kind of ZnO@TiO2 composite heterojunction will benefit for the separation of photogenerated electron–hole pairs, thus leading more electrons accumulated on the TiO2 which will react with H2O to generate H2 [15–17].
In addition to the above we have discussed, geometric shapes and morphologies of the photocatalysts also heavily influence the hydrogen evolution reaction (HER) performance [18–20]. It is has been reported that the diffractions on the hollow spheres and the multiple reflections due to the shell structure would enhance the effectiveness of light utilization . For example, Li’s group prepared hydrogenated cage-like titania hollow spheres exhibited much higher HER activities than solid structure . Beyond that, the spherical hollow structures have the advantages of large specific surface area, reduced transport lengths for charge carriers, and good chemical and thermal stability, which all contribute to the excellent photocatalytic ability . However, most of the research has focused on the preparation of composite hollow spheres by doping transition element, such as Ce–ZnO , Ni–ZnO , Ag–TiO2 , Au–TiO2 , and so on. To the best of our knowledge, few studies reported on the synthesis of closed, complete, and intact hollow spheres composed of mixed metal oxides porous particles. Even so, most of these composites are applied in photocatalytic degradation of organic pollutants but not in the photocatalytic hydrogen production.
In this paper, we reported a facile method to synthesize hierarchically porous ZnO@TiO2 composite hollow microspheres and applied them in the photocatalytic H2. The hollow spheres enhanced the light absorption by multiple reflections, at the same time, the lifetime and transfer rate of photogenerated charge carriers were also improved due to the potential difference generated on the ZnO–TiO2 interface. The result showed that the ZnO@TiO2 composite photocatalyst exhibited enhanced H2 evolution rate, compared to the bare ZnO and TiO2. In addition, the mechanism of the photocatalytic H2 on the ZnO@TiO2 composite hollow spheres was discussed in detail.
Synthesis of the Hierarchical ZnO@TiO2 Hollow Spheres
The preparation of ZnO@TiO2 composites was based on a very facile one-step template-free hydrothermal method at ambient conditions. In a typical procedure, 0.015 mol of Ti(SO4)2, 0.015 mol of Zn(NO3)2·6H2O, 0.015 mol of NH4F, and 0.06 mol of CO(NH2)2 were added to a beaker with 50 mL deionized water. After stirring for 60 min, the mixture solution was transferred into a Teflon-lined stainless steel autoclave and heated in an electric oven at 180 °C for 12 h. After that, the white precipitate was thoroughly washed with ethanol four times and then dried at 60 °C for 12 h to obtain ZnO@TiO2 heterostructures. For comparison, bare TiO2 and ZnO were prepared under the same conditions.
Synthesis of Pt–ZnO@TiO2 Samples
In a typical synthesis process of Pt–ZnO@TiO2 samples, the ZnO@TiO2 hollow spheres were put into a container containing 10 vol% triethanolamine and H2PtCl6 solution. Then, the system was bubbled with nitrogen for 30 min to remove the air. Finally, the Pt was in situ photodeposited on the ZnO@TiO2 hollow spheres under a full arc light irradiation (λ > 300 nm) for 2 h. The Pt content can be tuned by the concentration of H2PtCl6 and the reaction time, which was determined by inductively coupled plasma (ICP, PE5300DV).
The morphology of ZnO@TiO2 heterostructures was characterized via field emission scanning electron microscope (FESEM, Hitachi, Japan), transmission electron microscopy (TEM, Tecnai F20), high-angle annular dark field scanning TEM (STEM, Tecnai F20), and high-resolution TEM (HRTEM, Tecnai F20). The energy-dispersive X-ray spectroscopy (EDS) mapping images were captured on a Tecnai G2 F20 S-TWIN atomic resolution analytic microscope. The crystal phase properties of the samples were characterized using an X-ray diffractometer with Cu–K radiation (XRD, M21X, MAC Science Ltd., Japan). The BET specific surface areas were measured on Belsorp-mini II analyzer (Japan).
Photocurrent studies were performed on a CHI 660D electrochemical workstation, using a three-electrode configuration where fluorine-doped tin oxide (FTO) electrodes deposited with the samples as working electrode, Pt as counter electrode, and a saturated calomel electrode (SCE) as reference. The electrolyte was 0.35 M/0.25 M Na2S–Na2SO3 aqueous solution. For the fabrication of the working electrode, 0.25 g of the sample was grinded with 0.06 g polyethylene glycol (PEG, molecular weight 20,000) and 0.5 mL ethanol to make a slurry. Then, the slurry was spread onto a 1 × 4 cm FTO glass by the doctor blade technique and then allowed to dry in air. A 300 W xenon arc lamp served as a simulated solar light irradiation source (Perfectlight, PLS-SXE 300C, Beijing, China). The incident light intensity was tuned to be 100 mW/cm2 measured by NOVA Oriel 70260 with a thermodetector.
Photocatalytic Hydrogen Production Tests
Photocatalytic hydrogen production experiments were performed in a sealed quartz flask at ambient temperature and under atmospheric pressure. A 300 W xenon arc lamp (Perfect light, PLS-SXE 300C, Beijing, China) was used as the light source to trigger the photocatalytic reaction. The evolved H2 were collected and online-analyzed by a H2-solar system (Beijing Trusttech Technology Co., Ltd.) with a gas chromatogram equipped with a thermal conductivity detector (TCD), 5A molecular sieve column, and nitrogen as the carrier gas. All photocatalytic experiments over 100 mg photocatalyst were performed in an aqueous solution containing H2O (80 mL) and alcohol (20 mL). Prior to irradiation, the system was deaerated by bubbling nitrogen for 15 min. During the photocatalytic reaction, the reactor was tightly sealed to avoid gas exchange.
Results and Discussion
In summary, the hierarchical heterostructure of ZnO@TiO2 hollow spheres has been successfully prepared via a simple hydrothermal method. Compared to bare ZnO and TiO2, the ZnO@TiO2 composite photocatalyst exhibited high hydrogen production rated up to 0.152 mmol h−1 g−1 under simulated solar light. It is believed that hierarchical heterostructure increased the surface area which proving more active sites for effective HER and simultaneously improved the lifetime and transfer of photogenerated charge carriers due to the potential difference generated on the ZnO–TiO2 interface. Moreover, the ZnO@TiO2 composite photocatalyst exhibited good durability even after being reused five times. This work demonstrated a good prospect for photocatalytic H2 evolution from water based on the rational use and preparation of high activity, inexpensive, and chemical stability of ZnO and TiO2.
This research was supported by the National Natural Science Foundation of China (No. 51608175), the Key Project of Science and Technology of the Education Department of Henan Province (No. 17B610004), and the Doctor Foundation of Henan Institute of Engineering (Nos. D2015020, D2014021).
The concept was developed by WLL and LY. This manuscript was written by LY and WLL. The experiment and the data analysis were carried out by GFX, WLL, and LY. The preparation of samples is performed by LJ, DP, and YT. The characterization of samples are made by YK and GFX. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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