Flower-like ZnO-Ag2O composites: precipitation synthesis and photocatalytic activity
© Xu et al.; licensee Springer. 2013
Received: 23 September 2013
Accepted: 6 December 2013
Published: 19 December 2013
Ag2O-decorated flower-like ZnO composites were fabricated through a chemical precipitation process. X-ray diffraction analysis confirms the co-existence of cubic Ag2O and wurtzite ZnO phases. Scanning electron microscopy images reveal Ag2O nanoparticles located on the rough surface of ZnO flowers. The photocatalytic activities of the composites with various mole ratios were evaluated by the degradation of methyl orange (MO) under ultraviolet irradiation, which confirms that the composite shows superior activity to that of pure ZnO and Ag2O. The improvement can be ascribed to the deposited Ag2O forming the p-n junction at the interface of ZnO and Ag2O, resulting in the transfer of photocarriers and suppressing the electron–hole recombination rate.
Semiconductor photocatalysts for clean hydrogen energy production and environment decontamination have attracted much interest [1, 2]. When the excitation energy is higher than or equal to the band gap energy of the semiconductor, photoinduced electron–hole pairs would be generated and utilized in initiating oxidation and reduction of organic compounds. ZnO can be used as a photocatalyst and has drawn increasing attention because its photocatalytic activity is comparable to that of TiO2[3, 4]. It has been reported that the photocatalytic activity is closely correlated with the morphology of photocatalysts . Hierarchical ZnO with flower-like morphology shows promising application in decomposition of organic pollutant due to the increased optical absorption efficiency and large specific surface area [6, 7]. However, due to the wide band gap of ZnO (3.2 eV), only a few part of natural solar radiation can be utilized and the photogenerated electron and hole pairs are liable to recombination, leading to low quantum yields. To improve its photocatalytic activity, one of strategy is to complex ZnO with a narrow-band semiconductor forming composites with a wider range light absorption and a reduced rate of the recombination of photogenerated electrons and holes.
Many reports focused on the enhanced photocatalytic performance of ZnO composites by coupling with suitable semiconductors, such as TiO2, ZnS, Bi2O3, and CuO [8–12]. The efficiency improvement on the degradation of organic dye can be ascribed to the effective separation of photoinduced carriers. Furthermore, the separation of photoinduced electrons and holes would be greatly enhanced and more efficient especially in the inner electric field, which was formed by a p-n-type semiconductor composite, such as CuO/ZnO and NiO/ZnO [12, 13]. Ag2O is a p-type semiconductor with a band gap of about 1.3 eV. Recently, the modification of TiO2 and Bi2O3 was carried out using Ag2O nanoparticles decorated on the surface of photocatalysts [14–17]. Based on the heterojunction of Ag2O and TiO2, the recombination of photogenerated electrons and holes was greatly inhibited by transferring for the energy band matching and the build-up inner electric field, resulting in the photocatalytic activity enhancement [15, 16]. However, to the best of our knowledge, there is no report in the literature on the photocatalytic properties of the p-n junctions of hierarchical mesoporous ZnO-Ag2O composites.
In this paper, flower-like ZnO-Ag2O composites were fabricated through a chemical co-precipitation process. The as-prepared composite including Ag2O particles deposited on the petal surfaces of ZnO flowers shows high crystallization. Compared with ZnO flowers and Ag2O particles, the photocatalyst ZnO-Ag2O composites with wide mole ratios exhibited enhanced photocatalytic properties that was confirmed by the degradation of methyl orange (MO) under ultraviolet irradiation.
Preparation of flower-like ZnO
All the chemicals used for the synthesis of flower-like ZnO are analytical grade reagents. Zinc nitrate solution (0.001 M) was prepared by dissolving a proper amount of Zn (NO3)2 in deionized water. The materials - 20 mL of Zn (NO3)2 solution, 20 mL of deionized water, 0.25 g of sucrose, and 1.2 g of urea - were added into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed, heated at 90°C for 2 h, and finally cooled to room temperature naturally. The white precipitation (precursor) was filtered and washed several times with deionized water, followed by drying in air at 90°C for 2 h. The precipitations were heat-treated at 600°C in air for 2 h (heating rate of 5°C min−1) in a muffle furnace to obtain the final hierarchical ZnO flowers.
Preparation of Ag2O nanoparticles
Ag2O nanoparticles were synthesized from AgNO3, NaOH, and polyethylene glycol 8000 (PEG-8000) aqueous solution by the precipitation method. Firstly, 1.75 g of AgNO3 and 0.2 g of PEG-8000 were dissolved in 100 mL of deionized water. After a continuous stirring for 15 min, 0.05 M NaOH aqueous solutions were dropped into the above aqueous solution with the final pH = 14. Finally, Ag2O nanoparticles were washed thoroughly with deionized water followed by drying in air at 90°C for 2 h.
Chemical synthesis of flower-like ZnO-Ag2O composites
Flower-like ZnO-Ag2O composites with different mole ratios were prepared by the chemical precipitation method. A typical experimental process for the composite with a mole ratio of 1:1 is given as follows: 0.4 g of flower-like ZnO was dispersed in 100 mL of deionized water, and 2 g of PEG-8000 was added into the mixture in order to immerse the ZnO thoroughly. Subsequently, 1.8 g of AgNO3 was added to the suspension, and the mixture was stirred magnetically for 30 min. Then 0.2 M of NaOH was dropped into the above mixture with the final pH value of 14. Finally, flower-like ZnO decorated by Ag2O nanoparticles was washed repeatedly with deionized water followed by a filtration and drying in air at 90°C for 2 h. In order to assess the relationship between the component and the photocatalytic activity of the composites, variable mole ratios of ZnO to Ag2O composites were prepared through a similar process.
Characterizations and photocatalytic testing
X-ray diffraction (XRD) measurement was carried out using a Rigaku-D/max 2500 diffractometer (Rigaku, Shibuya-ku, Japan) with Cu-Kα radiation (λ = 0.15418 nm) for crystallization identification. The morphology, particle size, and chemical composition of the product were examined by scanning electron microscopy (SEM; Hitachi S-4800, Chiyoda-ku, Japan). X-ray photoelectron spectroscopy (XPS) experiments were performed with a Thermo Fisher K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using Al Kα radiation (12 kV, 6 mA). The binding energies of elements were calibrated using C 1s (284.6 eV) as reference. Room-temperature ultraviolet–visible (UV–vis) absorption spectrum was recorded on a spectrophotometer (PerkinElmer Lambda-35, Waltham, MA, USA) in the wavelength range of 300 to 800 nm. The UV–vis diffuse reflectance spectra (DRS) were measured using a Shimadzu UV-2550 spectrophotometer (Shimadzu, Kyoto, Japan). Room-temperature photoluminescence (PL) spectra were collected with a laser micro-Raman (JY HR800, HORIBA, Kyoto, Japan).
MO was employed as a representative dye pollutant to evaluate the photocatalytic activity of ZnO-Ag2O composites. Next, 0.02 g of ZnO-Ag2O composites was suspended into 60-mL 2 × 10−5 M of MO aqueous solution and stirred for 30 min in a 200-mL beaker in the dark to reach an adsorption/desorption equilibrium for MO on the surface of ZnO-Ag2O composites. Then the mixture was irradiated by 16-W ultraviolet irradiation (Philips, Amsterdam, The Netherlands) at room temperature. After the reaction mixture was irradiated for a given time, the samples of 4 mL were withdrawn at each time and centrifuged for 20 min. The quantitative determination of MO was performed by measuring its absorption with a UV–vis spectrophotometer (PerkinElmer Lambda-35).
Results and discussion
where C0 represents the initial concentration, ΔC represents the changed concentration, C represents the reaction concentration, A0 represents the initial absorbance, ΔA represents the changed absorbance, and A represents the reaction absorbance of the MO at the characteristic absorption wavelength of 464 nm.
The results in this paper show that ZnO-Ag2O composites have higher photocatalytic activities than pure ZnO and pure Ag2O, which is mostly attributed to the inner electric field introduced by the n-type ZnO and p-type Ag2O effectively separating the photoinduced electrons and holes.
Flower-like ZnO-Ag2O composites were prepared by a chemical co-precipitating method. The XRD profiles confirm that the composite is composed of cubic-phase Ag2O and wurtzite-phase ZnO. Ag2O particles decorated on ZnO composite flowers show higher photocatalytic activity than pure components under UV irradiation for the degradation of MO. The activity dependence on the component reveals that the increased Ag2O deposited on the composite greatly enhanced the photocatalytic activity, which can be attributed to the p-n junction in the composite effectively inhibiting the recombination of electron–hole pairs.
This work was supported by a fund from Heilongjiang Provincial Committee of Education (12511164).
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