Hydrothermal synthesis of In2O3 nanoparticles hybrid twins hexagonal disk ZnO heterostructures for enhanced photocatalytic activities and stability
© The Author(s). 2017
Received: 11 April 2017
Accepted: 16 July 2017
Published: 25 July 2017
In2O3 nanoparticles hybrid twins hexagonal disk (THD) ZnO with different ratios were fabricated by a hydrothermal method. The as-obtained ZnO/In2O3 composites are constituted by hexagonal disks ZnO with diameters of about 1 μm and In2O3 nanoparticles with sizes of about 20–50 nm. With the increase of In2O3 content in ZnO/In2O3 composites, the absorption band edges of samples shifted from UV to visible light region. Compared with pure ZnO, the ZnO/In2O3 composites show enhanced photocatalytic activities for degradation of methyl orange (MO) and 4-nitrophenol (4-NP) under solar light irradiation. Due to suitable alignment of their energy band-gap structure of the In2O3 and ZnO, the formation of type п heterostructure can enhance efficient separation of photo-generate electro-hole pairs and provides convenient carrier transfer paths.
KeywordsZnO In2O3 Heterostructures Photocatalytic efficiency
In recent years, environmental pollution and energy shortage have created serious social and economic issues for human society. Semiconductor-based photocatalysis has been widely employed as a highly efficient technique to overcome these issues [1–3]. Among these semiconductor metal oxides, zinc oxide (ZnO) has been recognized as a promising photocatalyst owing to its outstanding electrical and optical properties, low cost, high biological safety, versatile shapes and structures, environment benign and strong photocatalytic degradation ability of organic pollutants under UV light. However, ZnO with a wide band gap (Eg = 3.3 eV) can only be activated by ultraviolet (UV) light, which restricts its practical applications for solar energy [4–8]. Another main drawback of ZnO is rapid recombination of photo-induced electron-hole pairs, which results in the low quantum yield for any photocatalytic reactions [9–12]. Therefore, how to extend absorption edge of ZnO to visible light region for the utilization of about 43% solar spectrum meanwhile suppress the photo-generated electron-hole pairs recombination is still a great challenge for scientists. Various modification strategies to activate ZnO photocatalysis under visible light have been employed in the past few years, including sensitization, semiconductor coupling and doping. An efficient strategy is coupling ZnO with another narrowband-gap semiconductor (e.g. CdS , CdSe , Cu2O , C3N4 , ZnFe2O4 , Ag3PO4 , CuInS2 , AgBr  and BiVO4 ) to form ZnO/narrow-band-conductor type п heterostructures. The formation of type II heterostructures has been recognized as an attractive route to overcome the limitations of ZnO because it promotes efficient charge separation, enlarges the effective contact interfaces and improves the optical absorption [22, 23].
In2O3 with a band gap of 2.56 eV has been proved as efficient sensitizer to extent the light absorption spectra by coupling other semiconductor. Also, its valence and conduction band alignments are staggered relative to those of ZnO [24, 25]. A lot of researches on In2O3-ZnO composite have been reported for degradation of organic compounds and hydrogen production by photocatalysis [26–28]. These results show that the incorporation of In2O3 in ZnO nanostructure can remarkably inhibit recombination of photo-generated electron-hole pairs and thus improve the photocatalytic activity. To the best of our knowledge, there has rarely been reported on the fabrication and improvement ZnO photocatalytic activities and stability by In2O3 nanoparticles hybrid.
In this paper, In2O3 nanoparticles hybrid THD ZnO with different ratios were fabricated by a hydrothermal method. The microstructure and optical properties of ZnO/In2O3 heterostructures were examined. The photocatalytic activity and photo-stability of ZnO/In2O3 composites were evaluated by MO and 4-NP under light irradiation. Finally, the charge transfer and probable photocatalytic mechanism have been discussed and proposed on the basis of optical characterization, band gap structure and reactive species reaction.
Formation of ZnO/In2O3 heterostructure
First, 0.1 mol of ZnAc and a specific molar of In(NO 3 ) 2 with a designed atom percent of In to Zn (about 2.0, 5.0, 8.0, 12.0 and 15.0 atom%) were dissolved in 50 ml deionized water to form a clear solution. Then, 15 ml of triethanolamine (TEA) was dropwise into the above solution under magnetically stirring. After that, the mixed solution was heated at 90 °C for 4 h, the obtained precipitates were centrifuged and washed by deionized water and ethanol for several times and dried in an oven at 60 °C. The final ZnO/In2O3 composites were thus obtained by annealing at 200 °C for 1 h. According to the In/Zn molar ratios of 0, 2, 5, 8, 12 and 15%, the composites were marked as Zn-In-0, Zn-In-1, Zn-In-2, Zn-In-3, Zn-In-4 and Zn-In-5, respectively. For comparison, pure In2O3 were also fabricated under the same condition.
The crystal structures were studied by powder X-ray diffraction (XRD) with a 0.154178 nm Cu-Kα radiation. The morphologies and size of the ZnO/In2O3 composites were measured by field emission scanning electron microscopy (FESEM; JSM-6700F, Japan). Chemical compositions were analyzed by X-ray energy-dispersive spectroscopy (EDS) equipped to the SEM. The detailed microstructures of samples were characterized by high resolution transmission electron microscopy (FE-SEM SUPRA™ 40). Chemical states of the samples were analyzed using X-ray photoelectron spectroscopy (XPS; PHI-5300, ESCA, USA). The UV-vis diffused reflectance spectra (UV-vis DRS) of samples were measured on a UV-3600 spectrophotometer. Photoluminescence (PL; Renishaw1000, UK) spectra were measured at room temperature using a He-Cd laser as the excitation light source at 325 nm. The •OH-trapping PL spectra was collected in 5 * 10−3 M terephthalic acid solutions containing 0.01 M NaOH solution with different irradiation time; the excitation wavelength was 325 nm.
The photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic degradation of MO and 4-NP. The wavelength distribution of Xenon lamp was similar to that of solar light; thus, a 500 W Xenon lamp was employed as the light source. For each photocatalytic activity measurement, typically, 10 mg of the photocatalyst was dispersed in 50 ml of MO (5 mg/l) or 4-NP (1 mg/l) aqueous solution and then stirred in the dark for 30 min to achieve an adsorption-desorption equilibrium. The photocatalytic reaction was carried out by Xenon lamp as the solar light source with continuous stirring. At the given intervals, 3 mL of the aliquots was sampled and analyzed by recording variations in the absorption band (464 and 317 nm) in the UV-vis spectra of MO or 4-NP, respectively. To probe the photo-stability of the Zn-In-4 catalyst, cycle degradation was carried out. In this case, Zn-In-4 was repeatedly used, which was separated and collected by centrifugation. After being washed with water and ethanol for several times and dried at 60 °C overnight, the Zn-In-4 catalyst was reused with a fresh MO aqueous solution (5 mg/l) for subsequent reactions under the identical conditions.
Trapping experiments were performed to probe the main active species in the photocatalytic process. The experimental apparatus and procedures were identical to that of the photocatalytic activity tests except that different types of scavengers (1 mM) were added into the MO solution. Herein, a fluorescence technique was employed to detect the formation of free hydroxyl radicals (•OH) and terephthalic acid (TPA) was used as the probe molecule. In detail, the as-synthesized Zn-In-4 (0.025 g) was dispersed into 50 mL mixed solution of 0.25 mmol TPA and 1 mmol NaOH under magnetically stirring. After Xenon lamp (500 W) irradiation for 90 min, the supernatant of reaction solution was collected and examined by a FP-6500 fluorescence spectrophotometer with an excitation wavelength of 315 nm.
Results and discussion
Morphology and phase structure analysis
Weights and atomic percentages of elements in ZnO/In2O3 composites
As we all known, the mass ratio of components has a great effect on the photocatalytic performance in heterostructural photocatalyst system [27, 36]. With mass ratio increasing of In2O3 in ZnO/In2O3 composites, there was no significant difference in the degradation tendency of MO and 4-NP, and appears the maximum degradation efficiency for the sample Zn-In-4. However, PL intensities of the samples show an opposite variation tendency. This result indicates that an appropriate amount of In2O3 in the composites was beneficial to the fast separation of photo-generated charge carriers and thus enhanced the photocatalytic activity [5, 37].
Proposed photocatalytic mechanism
In summary, In2O3 nanoparticles hybrid THD ZnO with different ratios were fabricated via the hydrothermal process. Significantly, compared with pure ZnO, the fabricated ZnO/In2O3 exhibits much better photocatalytic activities for the degradation of MO and 4-NP under simulated solar light irradiation, which can be ascribed to the synergetic effect between ZnO and In2O3, including the maximum heterostructure interface with intimate contact and excellent solar light response in the composite, which both can enhanced photogenerated charge separation efficiency. This work could give insights into the importance of rational design of heterostructure systems and provide a potential method for the construction of efficient heterostructure photocatalysts with controllable sizes and space distributions.
This work was supported by the National Natural Science Foundation of China (No. 51502081); Basic and Frontier Research Programs of Henan Province (No. 152300410088).
HL carried the main part of the experimental work and XRD measurements. HZ carried the XPS tests. CH and JY participated in the preparation of the samples. ZL carried SEM images measurements. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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