Yi-Bo Dua, Cheng-Gang Niua,∗, Lei Zhanga, Min Ruanb, Xiao-Ju Wena, Xue-Gang Zhanga, Guang-Ming Zenga,∗

a College of Environmental Science Engineering, Key Laboratory of Environmental Biology Pollution Control, Ministry of Education, Hunan University, Changsha, 410082, China

b School of Energy and Power Engineering, Changsha University of Science & Technology, Changsha, 410076, China

a b s t r a c t

A kind of Ag/AgCl hollow spheres were synthesized with the assist of Cu2O nanospheres via simple chemical methods The Cu2O nanospheres were used for template as well as raw material. The FeCl3 aqueous solution, as the Cl− and weak acid environment provider, was played an important role in the removal of Cu2O template and the formation of Ag/AgCl hollow spheres. The X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Brunauer-Emmett-Teller (BET), photoluminescence emission spectra (PL), ultraviolet-visible diffuse reflflectance spectra (UV-vis DRS) and transient photocurrents were employed to investigated the crystal structures, morphologies, specific surface area and optical property of the samples. The experiment results confirmed that the harvest of visible light and separation efficiency of electron-hole pairs were enhanced significantly owing to the large specific surface area and porous structure of hollow spheres. In comparison with common Ag/AgCl, the as-prepared Ag/AgCl hollow spheres exhibited excellent photocatalytic activity on the degradation for rhodamineB (RhB), methyl orange (MO), methylene blue (MB) oxytetracycline (OTC) and tetracycline (TC) under visible light irradiation ( > 420 nm). The possible photocatalytic mechanism was also proposed on the basis of trapping experiment.

1. Introduction

In the past few decades, noble metal nanoparticles were studied extensively on photodegradation and energy transformation for its excellent optical performance [1–5]. Up to now, many relevant materials have been synthesized and applied in photocatalytic fifield. Among them, AgCl was found to be a promising photosensitive material used for organic pollutants degradation and hydrogen production under UV-light owing to its unique photolysis characteristics and electrical properties [6–8]. However, it couldn’t be responded at visible light region for its wide band gap (Eg = 3.2 eV) [9], which inhibited its photocatalytic property largely [10]. Recently, it was reported that the harvest of visible light and photoactivity could be enhanced greatly by depositing Ag nanopar-ticles on the surface of AgCl [11,12], which primarily attributed to the surface plasmon resonance (SPR)[13,14]; and the lower recombination efficiency of photogenerated electron-hole pairs [15]. The photocatalytic performance of Ag/AgCl was still restricted to its big particle size and small specifific surface area. Changing the morphology and structure may be a feasible method to improve the photocatalytic property of materials [16–19]

Over the years, various efforts have been devoted to investigate the synthesis of semiconductormaterials withtiny size, special morphology and structure, aim at promoting their nature photocatalytic property [20–25]. The hollow sphere structures, with large specifific surface area and thin shell architecture, have attracted much interest due to its notable advantage of the harvest of visible light and separation efficiency of photoelectron-hole pairs [5,26–28]. For example, the hollow CuO microspheres were synthesized by Wang et al. and exhibited enhanced photocatalytic activity on the degradation of methyl orange (MO). In the preparation process, SiO2 was used as template and corroded with high concentration NaOH aqueous solution. [26]; ZnO hollow sphere nanostructures were prepared by Mousav et al. using carbon particles as template to reform the property of pholocatalyst. Zn salt could be complexed with functional groups on the surface of carbon spheres in the procedure, and then the hollow microspheres were got after calcination [29]. The hollow structures could effectively enhance the property of the materials based on the abundant reports. So far, diverse particles have been used as templates to construct the hollow structures, such as silica [27], carbon particles [30,31], micelles [32], polymer [33] and so on. However, Cu2O as a noveltemplate,there are few related reports in the preparation of hollow structures materials. In this paper, Ag/AgCl hollow spheres with open nanoholes on the surface were prepared for the first time with the assist of Cu2O spheres used as template. The common Ag/AgCl nanomaterial was also synthesized by a simple precipitation-photoreduction method for contrastive study. rhodamineB (RhB), methyl orange(MO), methylene blue(MB) oxytetracycline(OTC) and tetracycline(TC) were used as target pollutants to estimate the photocatalytic property and stability of samples. The results revealed that Ag/AgCl hollow spheres owned higher photocatalytic activity and stability compared to common Ag/AgCl. Furthermore, the possible mechanism of degradation was put forward based on the trapping experiments.

2. Experimental

2.1. Materials

Silver nitrate(AgNO3), Copper(II) sulfate pentahydrate(CuSO4&903;5H2O), sodium hydroxide (NaOH), dextrose monohydrate (C6H12O6·H2O), polyvinyl pyrrolidone(PVP), ferric chloride(FeCl3), sodium oxalate (Na2C2O4), sodium chloride(NaCl), hydrochloric acid(HCl) rhodamineB(RhB), methyl orange(MO), methylene blue (MB) oxytetracycline(OTC) and tetracycline(TC) were all purchased from Sinopharm Chemical Reagent Co. All the reagents were analytical grade and without further treatment. Deionized water and ethanol were used as solvents in the whole experiment process.

2.2. Synthesis of samples

Cu/Cu2O template spheres were synthesized by a facile water bath method. 2.5 g CuSO4·5H2O, 2.4 g NaOH and 3.96 g C6H12O6·H2O were respectively dissolved in 50 mL deionized water (denoted as solution A, B and C). Firstly, solution B was added dropwise to the solution A (blue solution in a flask) under continuous magnetically stirring. After 15 min, the blue deposits were generated gradually in the mixture solution. Secondly, solution C was added to the above mixture rapidly and kept stirring at 50 ◦C for another 15 min until brick-red precipitate produced [34], then the mixture was placed for 5 h without stirring at 50 ◦C to ensure that pure Cu generated on the surface of Cu2O nanospheres. Similarly, when the above brick-red precipitate was placed without stirring at room temperature for 5 h, the Cu2O spheres could be achieved [34]. In the end, the products were collected and washed three times with deionized water and ethanol, then dried in vacuum drying oven at 60 ◦C for 8 h.

Ag/Cu2O spheres were prepared through simple displacement reactions at room temperature. Briefly, 0.025 g as-prepared Cu/Cu2O, 0.5 g PVP and 0.15 g Na2C2O4 were successively dissolved in 40 mL deionized water and ultrasonicated for 10 min, then different amount of 0.100 M AgNO3 (0.88, 1.75, 2.62 mL) was dripped slowly to the above brick-red solution with continuous stirring and the color of the solution changed from brick-red to milky white gradually. At last the precipitate turned into black entirely and

aqueous solution became light blue, which confirmed that pure Cu on the surface of Cu2O was translated into Cu2+ in the reaction. After stirring for 3 h, the black precipitate was filtered and washed with deionized water and ethanol for three times, and dried in vacuum drying oven at 60 ◦Cfor 8 h. The as-prepared Ag/Cu2O spheres were respectively marked as Ag/Cu2O-0.5, 1 and 1.5 according to the different mole ratio of Ag and Cu.

Ag/AgCl (HS) were fabricated via simple chemical methods according to the reported literature [36]. 0.050 g as-synthesized Ag/Cu2O-0.5, 1 and 1.5 were respectively dispersed in 40 mL deionized water and ultrasonicated for 10 min to obtain homogeneous mixture, then 25.0 mL 0.10 M FeCl3 was introduced to the above solution and stirred for 0.5 h at room temperature, then the rufous precipitates were collected and washed with deionized water and ethanol for three times. Finally the products were dried in vacuum drying oven at 60 ◦C for 8 h. The as-synthesized composites were denoted as Ag/AgCl-0.5, 1 and 1.5(HS) correspond to Ag/Cu2O-0.5, 1 and 1.5 separately.

The Ag/AgCl used for contrastive study was prepared using chemical precipitation-photoreduction method according to the literature [35]. 20.0 mL 0.100 M NaCl solution was added dropwise into 20.0 mL 0.100 M AgNO3 solution, the mixed suspension kept stirring for 2 h in the dark to gain AgCl white precipitate. Subsequently,the suspension was irradiated under a 300WXe lamp with a cutoff filter (CEL-HXF300, Beijing, λ > 420 nm, the actual intensity was about 135900LUX) for 20 min to ensure that silver ions were reduced to Ag0 species on the surface of AgCl, and the color of precipitate changed from white to gray in the process. In the end, the gray precipitate was collected and washed with deionized water and ethanol for three times, then dried in vacuum drying oven at 60 ◦Cfor 8 h.

2.3. Characterization methods

The X-ray diffraction patterns (XRD) of samples were tested by a Bruker D-8 advance X-ray diffractometer using Cu Ka radiation ( = 0.15,406 nm) in the range of 2 = 5–80◦. The structure and morphology ofthe products were obtained by a fifield emission scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HRTEM) was used to further analyze the lattice plane of as-prepared samples. The size distribution of as-synthesized samples were analyzed by a Nanoparticle size and zeta potential analyzer (Zetasizer 3000HS). The UV–vis diffuse reflflection spectra of the samples were examined by a UV–vis spectrophotometer (Hitachi U-4100) in the range of 200 to 800 nm and BaSO4 was used as reference in the process. The recombination ratio of electronhole pairs was evaluated via a photoluminescence spectrum (PL) at the excitation wavelength 400 nm on a PerkinElmer LS-55 spectroflfluorimeter. The specifific surface area (BET) of the composites was characterized by a specific surface and pore analyzer (NOVA 2200e, Quantachrome).

2.4. Photocurrents intensity evaluation

The photocurrents were measured on an electrochemical workstation (CHI760D) with a standard three-electrode system. In the typicalprocedure, 10 mg samples were loadedonthe FTO glass with the effective area around 1 cm2 as a working electrode, platinum electrode and Ag/AgCl electrode were used as auxiliary electrode and reference electrode. The 300W Xe lamp equipped with a cutoff filter (λ > 420 nm, the actual intensity was about 135900LUX) and 0.50 M Na2SO4 aqueous solution were used as visible light source and electrolyte in the procedure.

2.5. Photocatalytic activity measurement

The photocatalytic activities of as-prepared samples were evaluated with degradation experiment on removing the organic contaminants in the aqueous solution. In brief, 20 mg photocatalyst was separately dispersed into 100 mL 10 mg/L organic dyes (RhB, MO and MB), 100 mL 20 mg/L colourless organic compounds (TC,OTC) and kept stirring in the dark for 30 min to ensure the establishment of an adsorption-desorption balance. A 300W Xe lamp equipped with a cutoff filter (CEL-HXF300, Beijing,  λ > 420 nm, the actual intensity was about 135900LUX) was used as visible light source. 1.5 mL solution was sampled every 10 min (for RhB, TC, OTC) or 20 min (for MO and MB), then centrifuged for 5 min at 10,000 rpm. The degradation efficiency was estimated by measuring the absorbance of supernatant with the absorption band (554 nm for RhB,357 nm for TC, 464 nm for MO, 664 nm for MB, 355 nm for OTC) using the UV–vis spectrophotometer (Shi-madzu 2550, Japan). Furthermore, we had researched the effects of different pHs (pH = 3.0,6.0,10.0) and light wavelengths(λ  > 420 nm, λ  > 320 nm) to the degradation process in the experiments. In order to study the stability, the recycled photocatalyst was tested for 5 times at the same condition. Mechanism test was also carried out to analyze the process of pollutants degradation. In this paper, the degradation efficiency was calculated by the (C0–Ct)/C0, where C0 and Ct represented the concentration of organic pollutants before the irradiation and at time t respectively.

3. Results and discussion

3.1. The illustration of preparing the Ag/AgCl hollow spheres

The schematic illustration was shown in Scheme 1, CuSO4·5H2O, NaOH, and C6H12O6·H2O were used to synthesize precursor Cu/Cu2O nanospheres which acted as the originaltempletin the following process. Subsequently, the Ag/Cu2O spheres were achieved through the replacement reaction in which a thin layer of pure Cu on the surface of Cu2O spheres was replaced by Ag nanoparticles after adding the AgNO3 solution. Ultimately, excessive FeCl3 solution was introduced to construct the Ag/AgCl hollow spheres. As the Cl− and acid environment provider, the FeCl3 solution played an important role on the formation of Ag/AgCl hollow structure. On the one hand, the uniform Ag/AgCl nanoparticles were in situ generated on Ag surface by the reaction between Ag and FeCl3. On the other hand, the Cu2O cores were corroded gradually in the dispro-portionation reaction process, in which the acid environment was achieved due to the strong hydrolysis degree of Fe3+.

According to numerous previous reports with template-assisted synthesis of hollow structures, the hollow structures were normally obtained by removing the core with high concentration acid or alkali, high temperature, calcination in the procedure. In addition, the by-products produced in the process may lead to latent damage to health besides the harsh reaction conditions. Apart from the preeminent catalytic performance, the Ag/AgCl (HS) were fabricated and expressed apparent advantages on synthetic condition and method. In this work, the reaction occurred at a mild condition without any harmful substance produced in the process. The precursor Cu/Cu2O nanospheres were used for template as well as the raw material in the whole experiment.Above all,Ag/AgCl(HS) were formed and the core of Cu2O spheres could be corroded thoroughly at the same time when FeCl3 solution engaged in the reaction.