Atmospheric Self-induction Synthesis and Enhanced Visible Light

Article pubs.acs.org/IECR

Atmospheric Self-induction Synthesis and Enhanced Visible Light Photocatalytic Performance of Fe3+ Doped Ag-ZnO Mesocrystals Qi Zhang,† Jin-Ku Liu,*,† Jian-Dong Wang,† Hong-Xi Luo,† Yi Lu,† and Xiao-Hong Yang‡ †

Key Laboratory for Advanced Materials, East China University of Science and Technology, Shanghai, 200237 P.R. China Department of Chemistry, Chizhou University, Chizhou, 247000 P.R. China

‡

ABSTRACT: Ag-loaded ZnO mesocrystals with Fe3+ doped (FAZ) can be quantity-produced through an atmospheric selfinduction synthesis method. This synthesis method avoids the use of a high-pressure instrument for the synthesis of mesocrystal catalysts. Compared with traditional Ag-ZnO catalysts, the threshold wavelength of 1FAZ mesocrystals was shifted to the full visible light region and the absorbance of catalyst in the visible region increased to more than 300%. The content of iron ion was found to be significant to the photocatalytic efficiency of FAZ mesocrystals. The experimental results demonstrated that the most optimal molar ratio of Fe3+ to Zn atoms was 1%, and the photocatalytic activity of 1FAZ mesocrystals was increased by 145% compared with Ag-ZnO obtained under visible light. A feasible water purification system with a continuous photodegradation reactor using FAZ mesocrystals was manufactured to utilize solar light as the energy to drive the running of the water purification system.

1. INTRODUCTION Ag-loaded ZnO (Ag-ZnO) is a potential and exciting photocatalyst in recent years because of its optical and electronic properties.1 A lot of research has been devoted to explaining the activity of Ag-ZnO under UV irradiation, which was primarily explained as the effect of the Schottky barrier.2,3 However, limited success has been achieved in the application of Ag-ZnO photocatalyst due to its nanosized structure and poor absorbance of visible light.3−5 Therefore, it is worthwhile to expand the threshold wavelength from the ultraviolet light region to the visible light region and increase the possibility of application by designing a kind of applicable structure.6,7 According to recent reports, metal ions, such as Co2+,8 Fe3+,9 and Mg2+,10 could improve the visible light absorbance of a photocatalyst when they are doped in the origin semiconductor. Among these reports, Fe3+-doped ZnO has attracted more and more interest recently because it was believed that Fe3+ doping modification directly influenced the intrinsic properties of ZnO, such as extending the photoresponse of ZnO, increasing the recombination rates of carried charges, and improving the morphology.9 However, it is a pity that few studies have been focused on the relationship between common metal ions and noble metal coexisting in ZnO and whether the coexistence will improve the performance of ZnO-based photocatalysts.8 Wang et al. studied the hybrid Pt-Co:ZnO nanostructure photocatalysts via a twostep synthetic step.8 Such synergetic effect could make use of visible photons as well as facilitate the separation of photogenerated charges to prevent recombination, which presented an efficient photocatalyst under visible light. However, the agglomeration of nanosized materials limited the application of nanostructure photocatalyst. It has been well-demonstrated that the agglomeration problem can be solved by assembling nanosized units to form mesocrystals materials.5,11 Mesocrystals, as micrometer-sized superstructures, have become noticed because of their high © 2014 American Chemical Society

activity and structure stability, which were hard to obtain simultaneously in traditional nanomaterials.11,12 Deng et al. reported a two-step method to synthesize Ag-ZnO micrometer rods.13 The rod-like structure was nanostructured and porous, simultaneously utilizing silver to improve the charge separation. Hence, they obtained an efficient and high-stability photocatalyst. However, the rare use of high-pressure instruments in the manufacturing industry limited the large-scale production of Ag-ZnO micrometer rods. In addition, the Ag-ZnO was only able to degrade dyes under visible light utilizing the effect of dye sensitization, which means it cannot be applied to colorless organic pollutants. Therefore, it is worthwhile to make Ag-ZnO visible-light-responsive and avoid the use of high-pressure instruments for better application prospects.14−16 In this work, we report a uniquely configured Fe3+-doped AgZnO (FAZ) mesocrystals synthesized through the atmospheric self-induction synthesis (ASIS) method. Compare to traditional synthesis methods for mesocrystals, the ASIS method was under atmosphere and addictive-free, therefore the mesocrystal catalysts can be quantity-produced for large-scale applications. The purposes of this study are listed as follows: (i) to improve the absorbance in visible region of Ag-ZnO photocatalysts, (ii) to develop the photocatalysts into microsized, porous, mesocrystal structures for better application in the purification of industry effluents, (iii) and to design a constinuouly water purification system utilizing 1FAZ mesocrystals as the photocatalysts. Therefore, this work presents a novel approach to enhance the efficiency and practicability of ZnO-based photocatalysts. Received: Revised: Accepted: Published: 13236

May 20, 2014 July 27, 2014 August 6, 2014 August 6, 2014 dx.doi.org/10.1021/ie502011h | Ind. Eng. Chem. Res. 2014, 53, 13236−13246

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Materials. Zinc acetate dehydrate (C4H6O4Zn·2H2O), oxalic acid dehydrate (C2H2O4·2H2O), silver nitrate (AgNO3), ethanol (C2H5OH), and deionized water were used for the synthesis of the sample. Rh. B was used for photocatalytic studies. All the reagents are of analytical grade and used without further purification. 2.2. Preparation of FAZ Catalysts. The FAZ catalysts were synthesized by the atmospheric self-induction synthesis (ASIS) method. Zinc acetate (10.98 g, 0.05 M), 2% molar ratio of silver nitrate, and different molar ratios iron nitrate were dissolved in ethanol (500 mL) at 70 °C and stirred for 30 min to obtain homogeneous solution in beaker. Then oxalic acid (12.55 g, 0.1 M) dissolved in ethanol (200 mL) at the same temperature was then slowly added to the prior prepared solution. Then the solution was stirring for 2 h under 80 °C water bath. The thick yellow colloidal semigel formed was allowed to dry at 80 °C overnight. The xerogel prepared in the previous steps was grinded in an agate mortar and further calcined at 600 °C for 2 h to form FAZ catalysts. Photocatalysts obtained by using 0.5%, 1%, 3%, and 5% molar ratio of iron to Zn atoms were designated as 0.5FAZ, 1FAZ, 3FAZ, and 5FAZ, respectively. In addition, the FAZ catalysts can be quantityproduced due to the facile synthesis craft, and all the steps in Scheme 1 were template-free, which saved the cost of products and decreased the pollutants in the process of production.

chosen as a model pollutant and the photodegradation of Rh. B was studied over all the samples, respectively. A 400-W Xe lamp with UV cutoff filters were used as light source. In a typical experiment, photocatalyst (0.15 g) was added to a 50-mL quartz batch reactor containing 50 mL 2 × 10−5 mol/L Rh. B aqueous solution. The reaction system was stirred for 30 min in the dark to reach the adsorption−desorption equilibrium before irradiation. Hence, the reactor was exposed to visible light (wavelength >420 nm) under stirring. The temperature of the experimental solution was maintained at 25 °C in water bath. Samples were taken hourly to test the absorbance of Rh. B solution and its concentration. The degradation rate was calculated by the absorbance ratio of the Rh. B solution taken out hourly. The degradation rate was calculated from the following expression: degradation rate =

C0 − Ct × 100% C0

where C0 is the initial concentration of Rh. B and Ct is the concentration of Rh. B after “t” hours. To prevent UV−vis absorbance of the powder in solution, catalysts were separated from the solution by centrifugation for recycling use.

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalyst. Figure 1 was the typical XRD patterns of the photocatalysts. It is clear that diffraction

Scheme 1. Synthesis Process of the FAZ Catalyst

2.3. Characterization. The crystal structure of FAZ catalyst was characterized by X-ray powder diffraction (XRD, Shimadzu XD-3Adiffractometer). The structures and morphologies were investigated by transmission electron microscopy (TEM, Hitachi-800) with an acceleration voltage of 200 kV. The structure and thermal property of precursor product were studied by Fourier transform infrared spectroscopy (FT-IR, Shimadzu, IRPrestige-21) and thermogravimetry−differential scanning calorimetry instruments (Mettler Toledo TGA/ SDTA851, heating rate = 20 °C/min). The light absorption of the samples was studied by UV−vis spectroscopy (Shimadzu, UV-2600). The surface composition of product was researched by X-ray photoelectron spectroscopy (XPS, VG ESCALAB MK II). The specific surface area (BET) of the FAZ catalysts was determined by isothermal nitrogen adsorption−desorption analysis (Micromeritics ASAP 2400). The fluorescence property was studied through fluorescence spectrophotometer (Cary Eclipse, Varian). 2.4. Photocatalytic Activity Study. To evaluate the photocatalytic activities of the synthesized catalysts, Rh. B was

Figure 1. XRD patterns of (a) pure ZnO, (b) Ag-ZnO, (c) 0.5FAZ, (d) 1FAZ, (e) 3FAZ, and (f) 5FAZ.

peaks of 32.30, 34.09, 36.14, and 56.25 corresponding to the (100), (002), (101), and (110) planes of wurtzite ZnO (JCPDS No. 79-2205) were found in all samples. It indicated the formation of good crystallinity of all samples. Diffraction peaks at 37.84, 44.15, and 64.85 observed in samples b−f correspond to the (111), (200), and (220) planes of metallic silver (Ag0) (JCPDS No. 89-3722), which demonstrate that silver had been successfully incorporated in lattice of ZnO.17 However, the 13237

dx.doi.org/10.1021/ie502011h | Ind. Eng. Chem. Res. 2014, 53, 13236−13246


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Figure 2. TEM images of the as-obtained FAZ samples with different molar ratio of Fe3+ ions: (a) 0.5FAZ; (b) 1FAZ; (c) 3FAZ; (d) 5FAZ.

material owning high porosity and high crystallinity simultaneously, which is in conflict with traditional mesoporous materials.12 Small particle size, corresponding to the large specific surface area, allows better performances in photocatalysis.24,25 For more information about the mesocrystal structure, the corresponding nitrogen adsorption−desorption isotherms (Figure 3) showed Type IV isotherm shape according to the IUPAC classification. As shown in Figure 3, the minimum BET surface area, 15.21 m2/g, belonged to 1FAZ, which was due to the overall micrometer size of 1FAZ mesocrystals. Consistent with the observed result in TEM images, 0.5FAZ, 3FAZ, and 5FAZ owned irregular nanoparticles structure, and no obvious assembly can be found in their structures. So, the nanosize structure owned a larger portion of the BET surface area than microsize structure. The thermal properties of the precursor of 1FAZ mesocrystals were stuied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) curves and shown in Figure 4. The endothermic peak observed around 170 °C can be assigned to the removal of structural water present in the ZnC2O4·2H2O.10 The endothermic peaks around 220 °C were attributed to the decomposition of residual oxalic acid. The endothermic peaks around 310 and 380 °C represented the removal of oxalate species.3 Therefore, the selected calcination temperature, 600 °C, was enough to completely decompose the precursors. The FT-IR spectra of the precursor and 1FAZ mesocrystals are shown in Figure 5 taken in the range of 400−4000 cm−1. From curve a, the peak at 3406 cm−1 was assigned to O−H stretching band of the hydroxyl groups and adsorbed water molecules, and the peak at 1631 cm−1 was attributed to the bending vibration of H2O adsorbed on the surface of catalyst. The doublet at 1372 and 1326 cm−1 stand for the asymmetric and symmetric stretching vibrations of the hydroxyl groups of the carboxylic group coordinated to Zn as a bidentate ligand. Furthermore, the difference of the doublet (Δν = 46 cm−1) suggested that oxalate worked preferentially as a bidentate

intensity of peaks corresponding to metallic silver was relatively low due to the low content. No obvious intensity peak of iron was found in the patterns of all the samples since the content of Fe3+ was too low to be detectable.18 At the same time, a small part of Fe3+ added might be doped in the crystal lattice of ZnO, which also lead to immeasurable peaks of Fe3+ in the XRD analysis.19 It can be seen that the peaks of 5FAZ (Figure 1f) were lower and wider than that of other samples, suggesting the decreased crystallinity of catalyst after too much iron doping. It was in good agreement with other groups.9 The TEM images of FAZ samples with different content of Fe3+ are shown in Figure 2. Images a−d showed the structures of 0.5FAZ, 1FAZ, 3FAZ, and 5FAZ samples, respectively. The 0.5FAZ, 3FAZ, and 5FAZ (Figure 2a, c, and d) samples were irregular from the low magnification, while the 1FAZ samples were found to be rod-like structure with diameter of about 120 nm and a length of about 700−800 nm formed by ordered array of many nanoparticles. The porous structure marked with circles in Scheme 2 was beneficial to heterogeneous catalysis, 20,21 similar facts applied in the mesoporous catalysts.22,23 However, it is hard to obtain a mesoporous Scheme 2. Formation Reactions of 1FAZ Precursor in the ASIS Procedure

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Figure 3. Nitrogen adsorption−desorption isotherms of FAZ samples: (a) 0.5FAZ, (b) 1FAZ, (c) 3FAZ, and (d) 5FAZ.

rather than as a bridging ligand between two zinc atoms.11 The peaks at 823 cm−1 were attributed to the C−H bond. Formation of a Zn−O−Zn network in the precursor was detected from the peak at 503 cm−1. In curve b, the main bonds of 1FAZ mesocrystals were located at 3444, 1638, 1402, 1036, and 499 cm−1, respectively standing for the O−H stretching mode, O−H bending vibration, C−H scissoring mode, C−H bending mode, and Zn−O bond.26 However, no peak could be found at 1326 and 1372 cm−1 due to the decomposition of carboxyl groups in the process of calcination. On the basis of the difference of structures among samples a−d in Figure 2, the formation of 1FAZ precursor and the possible formation process of the 1FAZ mesocrystals were illustrated in Schemes 2 and 3, respectively. At the initial step of the process, precursors were formed through the acidolysis of [Zn(C2H5O)4]2− under the existence of oxalic acid. Due to the acceleration effect of ligand, the Fe3+ was activated from the Fe (C2O4)33−, which promoted the occurrence of the esterification in the reaction system.27 Due to the direction of the interaction between the outer carboxyl and the alcohol, the ordered rodlike arrays of FAZ nanoparticles were formed. When dried in oven, some diethyl oxalate molecules were adsorbed by the FAZ arrays in the reaction system due to hydrogen bonds between carboxyl and diethyl oxalate, which were generated in the process of esterification. The esters occupied part of the space in the arrays, which guaranteed the porosity of the FAZ mesocrystal being removed by calcined at 600 °C. Herein, the role of diethyl oxalate can be described as “template” which was the key factor in the formation the rod-like mesocrystals. In addition, the activated Fe3+ as a catalyst accelerated the formation of diethyl oxalate as well as the “template”, which explained the fact that the rod-like structure could only be found in 1FAZ samples but no where else since only the

Figure 4. DSC-TG curves of the precursor of 1FAZ mesocrystals.

Figure 5. FT-IR spectra of the precursor (a) and 1FAZ mesocrystals (b).

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Scheme 3. Schematic of the 1FAZ Mesocrystals by the ASIS Method

Figure 6. UV−vis spectrum (a) and Kubelka−Munk plot (b) of 1FAZ mesocrystals and Ag-ZnO. 780

optimal content could result in the fast generation of diethyl oxalate. UV−visible absorption spectrum is shown in Figure 6a, which presented solar spectrum, 1FAZ absorption, and pure ZnO absorption spectrum at the same coordinate axis. Compared to Ag-ZnO with an absorption edge of ca. 420 nm, the 1FAZ had an extended absorption edge up to ca. 700 nm covering almost the full visible region in solar spectrum region, which meant that more efficient absorption overlap with the solar spectrum was generated after incorporating Fe3+ and metallic silver. From the curves shown in Figure 6b, the band gap energy was estimated from the intersection of tangent line with the x-axis in Kubelka−Munk plot. It was obvious that the 1FAZ had a decreased band gap energy (3.20 eV) compared with that of Ag-ZnO (3.28 eV). In order to quantify the observed enhancement of absorbance due to Fe3+ doping, the average solar spectrum absorbance (Aavg) was used,25 which can be given by eq 1.

A avg =

∫380 A(λ) dλ 780

∫380 dλ

(1)

The calculated results of 1FAZ and Ag-ZnO were 12.91% and 3.95% respectively, which demonstrate that the absorbance of catalyst increased to more than 300% after iron doping. Hence, the low activity of the Ag-ZnO photocatalysts under visible irradiation28,29 was greatly improved after Fe3+ doping, which was beneficial to the photocatalytic reaction, especially under visible light. The photoluminescence spectra of Ag-ZnO and FAZ catalysts have been shown in Figure 7. The PL spectrum was useful to disclose the efficiency of charge carrier trapping, immigration, and transfer and to understand the fate electron− hole pairs in semiconductor particles. The PL spectra was investigated in the wavelength range of 380−500 nm. It can be observed that the intensities are different among the samples. The lowest intensity belonged to Ag-ZnO, which suggested that 13240



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corresponded to the chemisorbed oxygen (OH) caused by the surface hydroxyl, lattice oxygen of ZnO and oxygen connected with Fe (III).30 The Fe−O bond observed at 530.2 eV suggested Fe3+ had doped into the lattice of Zn2+.30 The binding energy peak of Ag at 367.8 and 373.9 eV correspond to 3d3/2 and 3d5/2 of Ag (Figure 8c). The 367.8 and 373.9 eV peaks observed can be attributed to metallic silver (Ag0) in the photocatalyst.31,32 The Fe 2p XPS spectrum presented the binding energy peaks at 710.9 and 717.5 eV, suggesting the Fe species were present in the oxidation state.18 3.2. Experiment of Photocatalytic Degradation. The photocatalytic performance of the FAZ catalysts with different Fe3+ contents was evaluated by the decomposition reaction of Rh. B (Figure 9). After being sonicated for 30 min in the dark, a small decrease in concentration of the dye solution was found as a result of its adsorption by the catalysts.24 It is clear that only 8.8% loss of Rh. B might be caused by its selfdecomposition or polymerization reaction under irradiation.33,34 From the curves in Figure 9, FAZ samples presented better activity than pure ZnO and Ag-ZnO, because of the expanded absorption edge after Fe3+ modified. However, AgZnO displayed similar activity compared with pure ZnO due to its similarly poor absorbance in visible region. What’s more, the best photoactivity under visible irradiation belonged to 1FAZ with 90.2% of Rh. B degraded within 5 h, followed by the 3FAZ, 5FAZ, 1Fe-ZnO, 0.5FAZ, Ag-ZnO, and pure ZnO, which had the degradation efficiencies of 76.9, 68.3, 67.6, 66.1, 44.4, and 36.8% during the same time, respectively. For better comparison of the influence with different Fe3+ modification, Table 1 listed the rate compared with Ag-ZnO. The degradation rate of 1FAZ mesocrystal increased remarkably

Figure 7. Photoluminescence spectra of FAZ catalysts and Ag-ZnO (tested at room temperature under the excitation light at 325 nm).

the recombination rate was lowest in the Ag-ZnO. When Fe3+ was doped into Ag-ZnO, the intensity was increased. And the emission intensity increased with an increase in Fe3+ content indicating the increased electron−hole recombination. To investigate the chemical state of Ag and Fe present in the catalyst, XPS spectra of 1FAZ were studied (Figure 8). The binding energies presented are calibrated by using C 1s (284.8 eV) as the reference. Figure 8a−d presented the binding energy peaks of Zn, O, Ag, and Fe, respectively. The peaks of Zn 2p at 1044.0 and 1021.5 eV (Figure 8a) proved the presence of Zn2+ in the sample. In Figure 8b, the asymmetric O 1s profile can be fitted to three symmetrical peaks located at 532.0, 530.4, and 530.2 eV, indicating the existence of three different kinds of O species in the catalyst. The three symmetrical peaks may

Figure 8. XPS spectra of 1FAZ photocatalyst: (a) high-energy-resolution Zn 2p core-level spectra, (b) high-energy-resolution O 1s core-level spectra, (c) high-energy-resolution Ag 3d core-level spectra, (d) high-energy-resolution Fe 2p core-level spectra. 13241



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145.1% compared with that of Ag-ZnO. The substantial enhancement in photocatalytic activity can be explained as follow: the absorbance of Ag-ZnO in visible region was increased after Fe3+ doping. At the same time, only 1FAZ sample had the porous mesocrystal structure in Figure 2b, and the porous rod-like superstructure was beneficial to the photocatalytic reaction because of its large specific area.20 Meanwhile, the optimal Fe3+-doping concentration for degrading Rh. B is when Fe/Zn = 1%. When the doping concentration was higher than 1%, Fe3+ doping became detrimental. The effect of Fe3+ doping can be explained as follows: photocatalytic activity under visible light depends on the absorption of visible light and electron−hole recombination rate. When the doping concentration was lower than 1%, the absorption dominated the activity. However, if the doping concentration was once higher than 1%, the recombination rate would increase due to the decreased distance between recombination centers and played the dominant role in the photocatalytic activity. In addition, some reports also regarded the reduction of surface hydroxyl groups as one of the reasons for poor photocatalytic performance in the presence of transition metal.18,35 In a word, the best photocatalytic activity of 1FAZ mesocrystals was the result of synergistic effects of these factors. The degradation of Rh. B with 1FAZ under UV irradiation was also investigated. A 300 W high pressure Hg lamp was used as a UV light source with visible light cutoff filters (λ < 380 nm). From the UV−vis curves in Figure 10b, the Rh. B with initial concentration of 2 × 10−5 mol/L was removed completely within 25 min, which presented high efficiency. The chemical oxygen demand (COD) was used to test the

Figure 9. Photodegradability of Rh. B under visible light [Rh. B] = 2 × 10−5 mol/L, catalyst suspended = 3 g/L, pH = 9, temperature of solution = 25 °C: (a) 1Fe-ZnO; (b) Ag-ZnO; (c) 0.5FAZ; (d) 1FAZ; (e) 3FAZ; (f) 5FAZ; (g) pure ZnO; (h) no catalyst.

Table 1. Degradation Rate of Rh. B Enhanced after Different Fe3+ Modification at the Fifth Hour samples

degradation rate (%)

rate increase (%)

Ag-ZnO 0.5FAZ 1FAZ 3FAZ 5FAZ blank

36.8 66.1 90.2 76.9 68.3 8.8

0 79.6 145.1 109.0 85.6 -

Figure 10. Rh. B degradation UV−vis curves with 1FAZ mesocrystals under visible light irradiation (a) and UV irradiation (b). The durability of 1FAZ mesocrystals for photodegradation of Rh. B under visible light (c) and UV light (d). [Rh. B] = 2 × 10−5 mol/L, catalyst suspended = 3 g/L, pH = 9, temperature = 25 °C. 13242



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Figure 11. Effects of mass concentration of catalyst (a) and the initial solution pH (b) using 1FAZ; [Rh. B] = 2 × 10−5 mol/L, visible irradiation time = 5 h, temperature = 25 °C.

Figure 12. Rh. B degradation UV−vis curves with 1FAZ mesocrystals with different trapping agents (a) isopropyl alcohol; (b) methanol. [Rh. B] = 2 × 10−5 mol/L, catalyst suspended = 3 g/L, pH = 9, temperature = 25 °C.

greatly influenced by the pH of the reaction system. Therefore, it is worthwhile to investigate the effect of initial pH on the degradation of Rh. B. As shown in Figure 11b, in a typical experiment, the degradation rates of Rh. B under different pH conditions were 30.2%, 49.2%, 86.2%, 90.2%, and 80.4%, respectively. Among all the samples, the sample working under pH = 9 owned the best activity. The effect of catalyst suspended in the solution was studied by varying the mass concentration of the catalyst from 1.0 to 5.0 g/L, and the result is shown in Figure 11a. The degradation rates are 38.7%, 67.2%, 90.2%, 82.5%, and 72.1%, respectively. The 1FAZ catalyst exhibited the highest efficiency in the degradation reaction when its mass concentration was 3 g/L. When the mass concentration of 1FAZ was lower than 3 g/L, the particles in Rh. B solution could not provide sufficient surface area for adsorption; however, higher mass concentration of 1FAZ would reduce the absorbance of light and cause a decrease in photocatalytic efficiency. To verify the photoexcited electrons and holes are the dominating factors in the degradation reaction of Rh. B. Isopropyl alcohol and methyl alcohol were used as the trapping agents to trap photoexcited electrons and holes, respectively. It is shown in Figure 12 that different activities of 1FAZ mesocrystals were found in the two parallel photocatalytic experiments, the higher degradation rate belonged to the samples using methanol alcohol to catch photoexcited holes rather than that using isopropyl alcohol to catch photoexcited electrons. Hence, the photoexcited electron was the dominating factor in the process of degradation.38

content of organic residue in the dye solution which had been degraded to be colorless, and the result showed that the value of COD was zero, which indicated that there was no organic residue present in the degraded solution. In other words, the Rh. B was completely degraded into CO2 and H2O under the catalysis of as-prepared sample. The good performance ensured the application of 1FAZ mesocrystals under both UV irradiation and visible light. The conditions of applied photocatalysts included not only high photocatalytic activity but also good photostability. Hence, the durability of 1FAZ mesocrystals was studied and the recycle curves are shown in Figure 10c and d. The drop in efficiency of 1FAZ mesocrystals from 91.2% (first run) to 87.8% (fourth run) under visible light within 5 h is shown in Figure 8c, and the drop in efficiency from 100% (first run) to 88.6% (fourth run) under UV light was also shown in Figure 10d. The good stability of 1FAZ mesocrystals under both visible light and UV light was beneficial to the application in treating wastewater. There were two possible reasons responsible for the admirable stability of 1FAZ mesocrystals. On the one hand, the 1FAZ mesocrystals existed as the micrometer-sized superstructure, which possessed both high activity (nanoparticles) and structure stability (micrometer-sized materials).36 On the other hand, silver in the 1FAZ mesocrytals decreased the defect sites in the ZnO lattice and restrained the process of photocorrosion, leading to the favorable photostability of 1FAZ mesocrystals.37 It is known that the photocatalytic reaction depends on the adsorption of dye molecules on the surface of catalysts, which is 13243



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Scheme 4. Mechanism of Degradation of Rh. B by FAZ Catalyst

3.3. Mechanism of Degradation of Rh. B under Visible Light. The degradation of Rh. B over FAZ catalysts under visible light irradiation can be illustrated in Scheme 4. The AgZnO without Fe3+ modified has the enhanced photocatalytic activity compared to pure ZnO, which has been proposed by others.39−41 Noble metals were recognized as electron sinks, when ZnO was modified by metallic silver, part of excited electrons transferred from conductance band of ZnO to the surface of silver and was trapped by silver due to the formation of Schottky barrier between the ZnO and metallic silver.42 As a result, the recombination rate of excited electrons and holes decreased considerably. However, the poor absorbance of visible light region limited the excitation of Ag-ZnO under visible light irradiation due to the wide band gap. After Fe3+ doped, the threshold wavelength was shifted to the full visible light region, which has been shown in UV−vis spectrum. It was obvious that Fe3+ doping was the key factor here because the acceptor level served as a mediator for the transition of excited electrons with lower energy. As a result, the band gap was decreased and the band edge was shifted to the full visible region. Once the electrons was transferred to the band of ZnO, further behaviors such as transition to the Fermi level of silver and reaction with dye molecules would also occur as described in the Ag-ZnO.43 In addition, as we can see in the degradation experiment, compared with Fe-ZnO, the modification of silver improved the quantum yield of FAZ, which led to its better activity in photocatalytic reaction under visible irradiation. What’s more, the ASIS method constructed a more practical mesocrystal structure, and porous structure and large specific surface area were significant to the degradation process of Rh. B.24,25 The silver loading and Fe3+ have been proved to be beneficial to the photocatalytic performance under visible light, which has been studied a lot. However, the single modification cannot meet the requirements of an efficient visible-light-driven photocatalyst, which needs good absorbance in visible light region as well as the low electron−hole recombination rate. For solving this problem, some new attempts have been made to combine noble metals with ion-doped semiconductor. Lu and coworks introduced a novel hybrid Pt-Co:ZnO nanostructure photocatalyst, and the synergetic effect could make use of visible photons as well as facilities the separation of photogenerated charges to prevent recombination.8 Hence, it is a promising way to design and fabricate novel catalysts with rapid electron transfer and visible photoresponse capabilities. However, no more reports have been focused on this method, and more matched configuration should be found and used into the system of semiconductor photocatalysts. Based on this idea, Fe3+ doping and silver loading was matched to improve the properties of ZnO photocatalyst. From the results of degradation of Rh. B, the synergetic effect between Fe3+ and silver made the FAZ catalysts more efficient than Ag-ZnO or

Fe-ZnO. More importantly, the 1FAZ mesocrystal was constructed with highly desirable properties, including highly efficient, recyclable, porous, and easy to synthesize. In most studies, the importance of a noble metal deposited well-distributed on semiconductor nanoparticles was emphasized generally because the plasmon resonance of the nanosize noble metal.44−46 The plasmon resonance would lead to a strong absorption in visible region and increased the visible light absorption of the ZnO catalyst. It is a pity that no obvious absorbance peaks were found in the near-by region in the UV− visible spectrum, which might be the result of badly distributed deposition of silver particulate shown in TEM images. Therefore, it is worth to investigate how to develop a synthesis method to obtain an Fe3+-doped ZnO photocatalyst with silver well deposited on the surface and make comparisons between the two kinds of catalysts. It is believed that more synergistic effects among Fe3+, silver, zinc oxide, and dye molecules can be found and a more efficient photocatalyst can be constructed. 3.4. Application Exploration. Figure 13 presented a scaleup continuous effluents purification system using 1FAZ

Figure 13. Continuous effluents purification reactor under solar light using 1FAZ mesocrystals as the photocatalysts.

mesocrystals as photocatalysts and the solar light as free energy. The process of the water purification system consisted of three steps. In step 1, a filter unit was used for separating the solid particles in the sewage. In step 2, several fluidized bed reactors (the number of reactors was up to the actual situation of degradation) using 1FAZ mesocrystals were used to photodegrade the organic pollutants in the sewage continuously under solar light. For a good capture of incident photons, the reactors were made of quartz or glass. In addition, the photocatalysts were fastened on collodion for better dispersity and stability. In step 3, polymeric membrane was used to 13244



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dislodge the residual metal ions in the water. A simulation degradation experiment was taken using Rh. B as the model pollutant. Three reactors for photodegradation were manufactured, and the calculated inner reaction area of every reactor was 0.04 m2. The initial condition of the reaction system was elected to be the best conditions found: [Rh. B] = 2 × 10−5 mol/L, catalyst suspended = 3 g/L, pH = 9, temperature = 25 °C. Then, the photodegradation process was conducted between 9:00 am and 16:00 pm. The simulate effluent of Rh. B solution was degraded under sunlight continuously, and clear liquid was obtained from the initial red solution. For testing the degradation of solution at the inlet and outlet of the reactor, a UV-2450 (Shimadzu) spectrometer was used to record the absorbance solution of the solution samples. It was shown in the tested results that the degradation rate of every reactor was 38, 72, 100% when the flow velocity of simulate effluent was 50 mL/min. The results showed a promising application of 1FAZ mesocrystals in the manufactured reactor. Moreover, different organic effluents can be realized to be purified flexibly in the reactor by changing effective photocatalysts.

REFERENCES

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4. CONCLUSIONS In the present work, a novel Ag-loaded and Fe3+-doped ZnO mesocrystal was synthesized through the ASIS method. Fe3+ plays a double role in the synthesis process, the dopant source, and the catalyst for the forming of diethyl oxalate. UV−vis spectrum showed that the threshold wavelength was shifted to the full visible light region and the absorbance of catalyst in visible region increased to more than 300% after 1% molar ratio Fe3+. Photocatalytic activity of FAZ was tested by photodegrading Rh. B dye, and the effects of preparatory conditions and operational parameters were investigated. In addition, the doping content of Fe3+ was found to be significant to the photocatalytic activity of FAZ catalysts, and the optimum content was 1%. The degradation rate of 1FAZ mesocrystal increased remarkably 146.1% compared with that of Ag-ZnO under visible light, which was the result of the increased absorbance in visible region and the porous rod-like superstructure. A mechanism for degrading Rh. B under visible light was given to show the synergistic effect between Fe3+ and silver modified. What’s more, a continuous effluent purification system was manufactured to utilize free solar light, as well as expand application range. We conjecture that 1FAZ mesocrystals could be used as a kind of potential photocatalyst in utilizing sunlight effectively due to its advantages of “low cost”, “reusability”, “visible-light-responsive”, and “practical”.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21341007), Fundamental Research Funds for the Central Universities (Grant 222201313005), and State Key Laboratory of Pollution Control and Resource Reuse Foundation (Grant 13019). 13245



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