A large number of non-biodegradable pollutants in wastewater have prompted the development of new and effective methods to remove environmental contaminants [1, 2]. In the past decades, nano-semiconductor photocatalysis has received enormous attention because of its excellent properties i.e., non-secondary pollution, high functionality, and low- selectivity of pollutants [3]. The key to nano-semiconductor photocatalysis is the development of highly efficient photocatalysts. TiO2 and ZnO are the most widely researched photocatalysts [4, 5, 6]. ZnO is a type II-VI semiconductor with a band gap of 3.37 eV and an excitation binding energy of 60 eV at room temperature [7]. Owing to its non-toxicity and low-cost advantages, ZnO presents great potential for application in photocatalysis. However, the low quantum efficiency and low visible light absorption limit the practical application of ZnO in photocatalysis [8].
Unlike ZnO semiconductor that features a wide band gap, AgX (X = I, Br) is a type of plasmonic semiconductor with a narrow band gap, thereby enabling excellent visible light sensitivity and photocatalytic performance under visible light illumination. AgX has thus obtained widespread attention in the field of photocatalysis [9, 10]. Wang et al. [11] have synthesized Ag/AgBr particles by a double jet method. The reaction rate constant for the photodegradation of methyl orange (MO) under the visible light irradiation over Ag/AgBr was 102 and 16 times higher than that over commercial TiO2 and mesoporous N-doped TiO2, respectively. In another study, Lin et al. [1] synthesized AgI/Ag/AgBr through a facile in situ ion exchange method. It also displayed excellent photocatalytic activity for the degradation of MO under visible light illumination. Owing to the characteristics of AgX, it is also frequently used to modify wide band gap semiconductors to shift the light absorption edge of the semiconductor from the UV to visible light region. Consequently, the composite nanostructures exhibit good visible light sensitivity and visible light-driven photocatalytic activity [12, 13]. Vignesh et al. [14] prepared AgI/ZnO powder. The highest decolorization efficiency of rosaniline hydrochloride dye under visible light irradiation was 88%. Wang et al. [15] synthesized AgBr/TiO2 nanoparticles using a deposition method. The composite nanostructure afforded effective separation and transfer of the photogenerated electrons, subsequently generating a high photocatalytic performance.
Moreover, AgX can absorb a light photon to simultaneously generate an electron and a positive hole under visible light irradiation. The photogenerated electrons can then react with interstitial Ag+ ions to form metallic Ag0. The process is shown as follows [14, 15]:
Owing to the surface plasmon resonance (SPR) of Ag and its synergistic effect with AgBr, Ag/AgX exhibits high photocatalytic activity and stability [16, 17]. Cui et al. [18] synthesized Ag/AgBr/K4Nb6O17 photocatalyst via a microwave-assisted ion exchange method. The photocatalytic performance of K4Nb6O17/Ag/AgBr towards the degradation of Rhodamine B was effectively maintained over five cyclic experiments. It has been reported that the ratio of Ag to AgX, determined by the photoreduction time, greatly affects the photocatalytic activity of Ag/AgX [19]. However, further studies are required. In general, current studies are mainly focused on the preparation of Ag/AgX or Ag/AgX-modified wide band gap semiconductor powders that are commonly used in suspension systems for wastewater treatment. Because of the difficulty in recycling suspension systems, their practical application is limited. In contrast, immobilization of the photocatalysts on substrates to produce an immobilized system can easily address the issues associated with suspension systems. However, the resulting low specific surface area of such a system may reduce the photocatalytic activity of the immobilized photocatalyst. Thus, the improvement of the visible light-driven photocatalytic activity of immobilized systems is the first step to the practical application of the photocatalysis technology.
Compared with ZnO powder, a ZnO nanorod array film has the advantages of higher specific surface areas, improved capture efficiency, and faster photogenerated electron transfer rates [21, 22]. Furthermore, ZnO nanorod arrays are an ideal immobilized system owing to their simple preparation process. Herein, sol gel method was first used to prepare a ZnO seeded layer on glass substrates. Second, hydrothermal method was used to prepare ZnO nanorod array films. Finally, the ZnO nanorod array films were immersed in a mixed solution of AgNO3 and KI or NaBr to respectively form AgI or AgBr nanoparticles-surface modified ZnO nanorod arrays. Ag/AgBr/ZnO nanostructures were synthesized using an UV light illumination pretreatment. By studying the visible light-driven photocatalytic activity of AgI/ZnO, AgBr/ZnO, and Ag/AgBr/ZnO nanostructures, and the stability of AgBr/ZnO and Ag/AgBr/ZnO nanostructures, the optimal nanostructure, featuring high photocatalytic performance and stability were obtained. This represents an important step in promoting the use of solar energy for the development of green technology.
The precursor solution was prepared by dissolving 1.1 g of zinc acetate dihydrate (AR) and 1.5 g of polyvinyl alcohol (AR) in 40 mL of deionized water. The mixed solution was heated at 80 °C with constant stirring for 1 h, generating a viscous sol that was then allowed to stand at room temperature. Clean glass substrates were cut into pieces (2.5 cm × 2.5 cm), and then placed into the SC-1B spin coater (Genesis Wiener, Beijing, China) for preparing the ZnO seed layer. The spin coater successively ran at 600 r/min for 5 s and 3000 r/min for 20 s to ensure that the viscous sol uniformly covered and deposited on the substrates. After spin coating, the wet films were dried at 100 °C for 10 min. The above steps were repeated for six times to obtain the ZnO seeded layer. Then, the seed layer was annealed at 450 °C for 30 min. After that, the as-prepared ZnO seed layer was immersed in a mixed solution of zinc nitrate hexahydrate and methenamine at 60 °C for 6 h to prepare the ZnO nanorod arrays. Both concentrations of zinc nitrate hexahydrate and methenamine were 0.03 mol/L. Finally, the obtained ZnO nanorod arrays were washed with deionized water and dried at 60 °C for further use.
To synthesize AgI/ZnO, the ZnO nanorod array was used as substrate and immersed in a mixed solution of AgNO3 and KI at the same concentration (10 mmol/L) and volume for 8 h. The resulting AgI/ZnO sample was washed with deionized water several times, and referred to as AgI/ZnO-8.
AgNO3 and NaBr were used to prepare AgBr/ZnO nanostructures with the same preparation procedure as that of AgI/ZnO. To study the effect of concentration on the photocatalytic performance of AgBr/ZnO, different concentrations of 2.5, 5, 10, and 15 mmol/L were investigated. The resulting samples are denoted as 2.5mMAgBr/ZnO, 5mMAgBr/ZnO, 10mMAgBr/ZnO, and 15mMAgBr/ZnO, respectively. The immersion time was fixed at 12 h.
For comparison purposes, an AgBr particle film was prepared. The ZnO seed layer that was used as substrate was immersed in a mixed solution of AgNO3 and NaBr at a concentration of 5 mmol/L for 12 h. The preparation of a continuous AgBr particle film was impossible in the absence of the ZnO seed layer on the glass substrate. Owing to poor adhesion, AgBr particles would easily fall off the substrate.
To study the effect of immersion time on the photocatalytic performance of AgBr/ZnO, different immersion time of 4, 8, 12, and 16 h was selected. The impregnating solution consisted of AgNO3 and NaBr at the same concentration (5 mmol/L) and volume. The resulting samples are denoted as AgBr/ZnO-4, AgBr/ZnO-8, AgBr/ZnO-12, and AgBr/ZnO-16, respectively.
Ag/AgBr/ZnO was synthesized by irradiating AgBr/ZnO, which was prepared from a mixed solution of AgNO3 and NaBr at a concentration of 5 mmol/L and a 12 h immersion time, with a 254 nm UV light source in a black box (Jia Peng, Shanghai, China). Illumination time was 1, 1.5, 2, and 3 h. The obtained samples are denoted as Ag/AgBr/ZnO-1, Ag/AgBr/ZnO-1.5, Ag/AgBr/ZnO-2, and Ag/AgBr/ZnO-3, respectively.
The crystal phase of the samples was characterized by an X’Pert PRO MPD X-ray diffractometer (Philips, the Netherlands). The scan range was 2θ = 20°-80° and scan speed was 2°/min. Surface morphology of the films was assessed on a Quanta 450 field-emission scanning electron microscope (FESEM, FEI, USA). The chemical species of the samples were analyzed on a VG Multilab 2000 X-ray photoelectron spectrometer (XPS). A U-3900 UV-Vis spectrophotometer (Hitachi, Japan) was used to evaluate the diffuse reflectance properties of the catalysts. The analysis range was set between 200 and 700 nm.
The visible light-driven photocatalytic activity of the prepared nanostructures was assessed by examining the degradation of MO dye solution over 180 min using a 300 W Xenon light with a 400 nm filter. The sample films were immersed in 20 mL MO solution (5 mg/L). During the photocatalytic process, aliquots of 2 mL of MO solution were taken out for quantitative analysis at 30 min intervals. The concentration of MO was monitored by a 2012PC UV-Vis spectrophotometer at an emission peak of 464 nm.
For the stability tests, AgBr/ZnO-12 and Ag/AgBr/ZnO-1.5 were selected using the same testing conditions above. After each photoreaction, the substrate was washed with deionized water for several times, and then dried in an oven at 60 °C. The photocatalytic experiment was repeated five times.
Figure 1 shows the XRD patterns of the ZnO, AgI/ZnO, 5mMAgBr/ZnO, AgBr/ZnO-8, and Ag/AgBr/ZnO-1.5 samples. All samples exhibit three strong diffraction peaks at 2θ = 31.7°, 33.4°, and 36.2°, which can be indexed as ZnO. The peak at 2θ = 33.4° corresponds to the (001) plane of ZnO, which is the strongest and sharpest peak among the three peaks. This may be attributed to the preferential growth of ZnO nanorods along the <001> direction. The diffraction peaks associated with AgI and AgBr are also observed in the XRD patterns of AgI/ZnO and 5mMAgBr/ZnO, respectively. This indicates the successful synthesis of the crystalline AgI and AgBr phases on the surface of the ZnO nanorod arrays using the current impregnation method. Ag/AgBr/ZnO was synthesized by UV light pretreatment of AgBr/ZnO according to the literature procedures [11, 22]. However, no peaks corresponding to Ag phases were detected in the XRD patterns of the as-synthesized samples. It is believed that small quantity of Ag in Ag/AgBr/ZnO would be difficult to detect from the XRD analysis.
Figure 2 shows the FESEM images of pure ZnO, AgBr/ZnO-12, and Ag/AgBr/ZnO-1.5. The as-prepared ZnO nanorod arrays are uniform; the nanorods have an average diameter of -50 nm. For AgBr/ZnO-12, the AgBr particles are mainly distributed on the top and top side surfaces of the ZnO nanorods, and no particles were observed at the base of the ZnO nanorods. AgBr particles that formed on the side surfaces of the ZnO nanorod arrays were in close proximity and connected to each other, generating a porous network structure. The average diameter of the AgBr particles is about 120 nm. Unlike dense granular membrane structures, the current porous structure may afford a high specific surface area and enhancement of the photocatalytic activity of AgBr/ZnO-12. Many small nanoparticles formed on the AgBr surface following UV light pretreatment; this is consistent with the synthesis of Ag/AgBr reported by Yan et al. [16]. Based on the XRD analysis and the morphology changes as a result of the irradiation pretreatment, the small particles might correspond to Ag nanoparticles that formed upon Ag+ reduction.
Figure 3 shows the XPS spectra of AgBr/ZnO-12 and Ag/AgBr/ZnO-1.5. Peaks corresponding to Ag 3d and Br 3d were observed in both samples. The Ag 3d peaks of Ag/AgBr/ZnO-1.5 shift by 0.2 eV with respect to that of AgBr/ZnO-12. This may be caused by interactions between Ag0 and Ag+ [16]. Ag/AgBr/ZnO-1.5 displays peaks at 367.56 and 373.53 eV that correspond to Ag 3d5/2 and Ag 3d3/2, respectively, and are deconvoluted to two different peaks. The peaks at 367.40 and 373.30 eV correspond to Ag+ of AgBr, and those at 368.20 and 373.82 eV are attributed to metallic Ag0 of Ag nanoparticles [16, 23, 24]. Figure 3(b) shows the Br 3d spectra of AgBr/ZnO-12 and Ag/AgBr/ZnO-1.5 that display a single peak at 68.22 eV [23, 24]. The XPS results confirm that metallic Ag0 and AgBr co-exist in Ag/AgBr/ZnO-1.5.
Figure 4 shows the UV-Vis diffuse reflectance spectra and band gap energy profiles of pure ZnO, 5mMAgBr/ZnO, and Ag/AgBr/ZnO-1.5. After AgBr or Ag/AgBr is loaded onto the ZnO nanorod arrays, the optical absorption edge of the composite nanostructures shows a distinct red shift relative to that of pure ZnO. This confirms that 5mMAgBr/ZnO composite has stronger absorption ability in the visible light region than pure ZnO. The spectrum of Ag/AgBr/ZnO shows a strong absorption peak in the 400-525 nm regions that may be attributed to interactions between the surface plasmon Ag nanoparticles, and the synergistic effect of the Ag and AgBr nanoparticles.
The band gap values of the photocatalysts were calculated using the following equation [15]:
where α, h, ν, Eg, and A represent the absorption coefficient, Planck constant, the optical frequency, the band gap energy, and constant, respectively. In this equation, the value of n is 4, which is determined by the number of optical transitions in a semiconductor. The Eg values of ZnO, 5mMAgBr/ZnO, and Ag/AgBr/ZnO-1.5 are 3.16, 2.94, and 2.82 eV, respectively. The narrow band gap can improve the absorption properties of the composite nanostructure in the visible light region. Thus, AgBr/ZnO and Ag/AgBr/ZnO-1.5 composites are expected to possess visible light-driven photocatalytic activity.
Figure 5 shows the degradation curves of MO over AgI/ZnO-8 and AgBr/ZnO-8 samples under visible light illumination. Both samples were prepared under the same conditions, where the volumes and the concentrations (10 mmol/L) of AgNO3, NaBr, and KI were the same. It shows that the difference in the MO photodegradation efficiency of AgBr/ZnO-8 and AgI/ZnO-8 is 33% after irradiation for 180 min; AgBr/ZnO-8 exhibits higher photocatalytic activity than AgI/ZnO. This is consistent with the reported results [1]. It should be noted that AgBr/ZnO-8 and AgI/ZnO-8 were prepared under the same conditions, although the exact content of AgBr or AgI deposited onto the surface of the ZnO nanorod arrays could not be determined.
The photocatalytic activity of AgBr/ZnO prepared from impregnating solutions with different concentrations was studied. Degradation curves of MO using different nanostructures including pure ZnO, AgBr, and AgBr/ZnO were obtained and shown in Fig. 6(a). The pure ZnO nanorod array sample shows negligible visible light-driven photocatalytic activity, whereas the AgBr sample that was prepared in the presence of the ZnO seed layer as a substrate achieves a photodegradation efficiency of 17.9%. In contrast, the AgBr/ZnO composite nanostructures that were prepared in the presence of the ZnO nanorod arrays as substrates showed considerably improved photocatalytic performance. This indicates that the substrate has an important role in the visible light-driven photocatalytic activity of the AgBr/ZnO composites. As shown in Fig. 6(a), the 5mMAgBr/ZnO nanorod array film has the highest degradation efficiency; its MO photodegradation efficiency is at least four times higher than that of AgBr. Figure 6(b) shows the relationship between ln(C0/C) and MO degradation time over the samples. The reaction profiles obey the pseudo-first-order kinetics law as indicative of the linear graphs (R2 > 0.99) [25].
The initial reaction rates (v0), pseudo-first-order rate constants (kapp), and half-lives (t1/2) were calculated and listed in Table 1. Increased impregnating solution concentrations led to an initial increase in the reaction constants up to a concentration of 5 mmol/L, after which the values of the reaction constants decreased.
The photocatalytic performance of AgBr/ZnO samples prepared at different immersion time was investigated. Figure 7(a) shows the MO degradation efficiency of AgBr/ZnO-4, AgBr/ZnO-8, AgBr/ZnO-12, and AgBr/ZnO-16 under visible light irradiation. As observed, in general, samples prepared at increasing immersion time showed higher photocatalytic degradation rates. The MO degradation efficiency over AgBr/ZnO-12 is the highest among all composites, reaching 78.6%. In contrast, AgBr/ZnO-16 that was prepared at a longer immersion time of 16 h showed a reduced MO degradation efficiency. The degradation trend can be explained in terms of the AgBr component content in the AgBr/ZnO composite that plays a key role in the degradation process. Samples that were prepared at immersion time shorter than 12 h display a low MO degradation efficiency because of the low AgBr content in these samples. In contrast, samples prepared at longer immersion time feature a higher AgBr content that enhance the photocatalytic performance of the AgBr/ZnO composite. However, further increase in the immersion time leads to samples with reduced photodegradation efficiency, this is may because that the excessive AgBr content result in the disappearance of the porous structure and decrease in the specific surface area of AgBr/ZnO. The presence of ZnO is possibly also an active component in the AgBr/ZnO composite. Moreover, it may inhibit the adsorption of MO molecules onto ZnO and the diffusion of the MO molecules from ZnO to the surface of the AgBr nanoparticles may be limited.
Figure 7(b) displays the relationship between ln(C0/C) and MO degradation time, which obeys the pseudo-first-order kinetics law [25]. AgBr/ZnO-12 achieved the highest reaction rate kapp = 0.0091 min-1, whereas AgBr/ZnO-16 achieved the lowest reaction rate kapp = 0.0065 min-1.
The AgBr/ZnO composite prepared under optimum conditions was irradiated with 254 nm UV light in a black box for different time. During the irradiation process, some of the Ag+ in AgBr nanoparticles is reduced into Ag0 nanoparticles. Finally, Ag/AgBr/ZnO photocatalysts were obtained. Figure 8(a) displays the degradation curves of MO over Ag/AgBr/ZnO samples. With increasing photoreduction time, the photocatalytic activity of Ag/AgBr/ZnO initially increases and then decreases. Ag/AgBr/ZnO-1.5 displays the highest photocatalytic activity at a photoreduction time of 1.5 h. The MO degradation efficiency over Ag/AgBr/ZnO-1.5 is 83.5% after irradiation for 3 h.
The kinetics study for the degradation of MO over Ag/AgBr/ZnO samples is shown in Fig. 8(b). Ag/AgBr/ZnO-1.5 displays the highest kapp for MO degradation. This result indicates that the content ratio of Ag0 to Ag+ changes with increasing photoreduction time. An optimum Ag0/Ag+ ratio is expected at a photoreduction time of 1.5 h [19]. Consequently, Ag/AgBr/ZnO-1.5 features the highest photocatalytic activity. Similar results have also been observed for the degradation of tetrachlorophenol under visible light irradiation using Ag/AgCl@TiO2-t nanostructures as photocatalysts (t represents the light reduction time). Ag/AgCl@TiO2-20 exhibited the best photocatalytic performance [19].
Figure 9 shows the repeated cycles for degradation of MO over AgBr/ZnO-12 and Ag/AgBr/ZnO-1.5. After five successive cycles under the same conditions, the degradation efficiency of AgBr/ZnO-12 considerably decreased to about 40%, whereas the degradation efficiency of Ag/AgBr/ZnO-1.5 slightly decreased by about 12%. This result confirms that Ag/AgBr/ ZnO-1.5 catalyst exhibits higher photocatalytic stability than AgBr/ZnO-12.
Based on the results of the photocatalytic tests, the photocatalytic reaction mechanism over AgBr/ZnO and Ag/AgBr/ZnO nanostructures is proposed. As observed in Figure 2, the AgBr nanoparticles are in close proximity to each other on the top surface of the ZnO nanorod arrays. Modification with the AgBr nanoparticles results in extension of the light response from the UV region to visible light region for the resulting AgBr/ZnO. Furthermore, the Ag/AgBr/ZnO-1.5 nanostructure, obtained by UV light pretreatment, exhibits a stronger absorption in the visible light region than AgBr/ZnO because of the SPR effect exerted by the metallic Ag0. As the properties of materials are determined by their microstructures, the above experimental results confirm that the visible light-driven photocatalytic activity of the AgBr/ZnO and Ag/AgBr/ZnO nanostructure is highly related to their chemical compositions and morphology.
The conduction band and valence band potentials of AgBr are positioned at 0.05 and 2.57 eV, respectively. However, the conduction band and valence band potentials of ZnO are at -0.31 and 2.89 eV, respectively; hence, the band potentials of AgBr and ZnO do not match [24]. Under visible light irradiation, AgBr would generate electron hole pairs because of its narrow band gap, whereas ZnO may not produce any electron hole pairs because of its wide band gap. This indicates that component ZnO in AgBr/ZnO may only act as a substrate. Owing to its large specific surface area and good connection with AgBr, the surface of the ZnO nanorod arrays can adsorb large amounts of MO molecules that can then easily move from the ZnO nanorod surface to the AgBr nanoparticle surface. Therefore, the ZnO nanorod arrays substrate can accelerate the photocatalytic reaction rate. Furthermore, the porous structure of AgBr/ZnO and Ag/AgBr/ZnO can increase the specific surface area and visible light illumination area, consequently improving the photocatalytic activity of the composites. In addition, the photogenerated electrons in the AgBr nanoparticles can react with the interstitial Ag+ to generate metallic Ag0. This can enhance the visible light absorption because of the SPR effect. Furthermore, the electron hole pairs generated by AgBr under visible illumination can be effectively separated because of the strong electron trapping effect exerted by the metallic Ag0 nanoparticles. After that, the electrons can react with dissolved O2 molecules to form -O2-. Some -O2- can convert to -OH. Under visible light illumination, -O2-, -OH, and h+ concurrently degrade the MO molecules. Therefore, Ag/AgBr/ZnO exhibits excellent visible-light-driven photocatalytic activity. The proposed photocatalytic reaction mechanism is shown in Fig. 10.
Unlike AgBr/ZnO, the Ag/AgBr/ZnO nanorod arrays film has higher photocatalytic stability. This may be caused by the metallic Ag0 nanoparticles that formed upon UV light pretreatment that can effectively prevent decomposition of AgBr during the photocatalysis process [26]. The photocatalytic process involves two steps relating to the consumption and formation of metallic Ag0. For instance, Ag+ can combine with the photogenerated electrons or reactant radicals to generate metallic Ag0, and Ag0 can react with the photogenerated holes to generate Ag+. The studies revealed that the formation of Ag+ from reaction between Ag0 and the holes is the main reaction process that occurs in the presence of dissolved oxygen. The dissolved oxygen can easily capture electrons, thus inhibiting the combination of electrons and Ag+ and promoting the formation of holes [27]. In AgBr/ZnO, the consumption of Ag+ prevails because of the lack of Ag0. Then, AgBr decomposes continuously under irradiation, which results in the decrease of the photocatalytic activity. In Ag/AgBr/ZnO, Ag0 forms upon the UV light pretreatment, thus ensuring the stability of AgBr during the photocatalysis process. It has been reported that the strength of the Ag0 peak in the associated XPS spectrum decreases after the photocatalytic process using Ag/AgBr/TiO2 for organic pollutants degradation. This showed that Ag0 can inhibit the decomposition of AgBr [26]. Therefore, the existence of Ag0 is likely to be the main cause of the improvement of the photocatalytic stability of Ag/AgBr/ZnO prepared in this study.
The synthesis and visible light-driven photocatalytic activity of plasmonic AgX nanoparticles-surface modified ZnO nanorod arrays using the impregnation method were reported. The effects of impregnating solution concentration, immersion time, and UV light illumination pretreatment on the photocatalytic activity of AgX/ZnO were investigated. The morphology of the AgBr/ZnO composite features a porous network, whereby the AgBr nanoparticles are interconnected and homogeneously distributed on the surface of the ZnO nanorods. The MO photodegradation tests showed that both the AgBr/ZnO and AgI/ZnO samples exhibited better photocatalytic performance than the pure ZnO nanorod arrays. More specifically, the MO degradation efficiency of AgBr/ZnO was higher by -33% when compared with that of AgI/ZnO. The AgBr/ZnO nanostructure synthesized under the optimal reaction conditions (impregnating solution concentration: 5 mmol/L; immersion time: 12 h) displayed the highest photocatalytic activity. Upon UV light illumination pretreatment, metallic Ag0 nanoparticles were formed on the surface of AgBr through the photoreduction of interstitial Ag+ in AgBr nanoparticles, generating Ag/AgBr/ZnO. The MO photodegradation efficiency over Ag/AgBr/ZnO (prepared under a UV light pretreatment for 1.5 h) was 83.5%. Furthermore, Ag/AgBr/ZnO exhibited higher photocatalytic stability than AgBr/ZnO. The high photocatalytic activity of Ag/AgBr/ZnO may be attributed to the high surface area of the ZnO nanorod array, the visible light sensitivity of AgBr, and the surface plasmon resonance effect of Ag/AgBr.
废水中含有大量具有非生物降解特性的污染物迫使人们开发新的有效方法来消除环境污染[1, 2].纳米半导体光催化具有无二次污染、操作性强、对污染物选择性小等优点,因而得到了研究者的广泛重视[3].纳米半导体光催化技术的核心就是光催化剂的研发.ZnO和TiO2是目前研究最为广泛的两种光催化剂[4, 5, 6].其中,ZnO是II-VI族化合物半导体,室温下有3.37eV的禁带宽度和60eV的激子结合能[7],无毒且价格低廉,在光催化方面有较大的应用潜力.然而,其较低的光量子效率与可见光利用率限制了ZnO光催化技术的实用化进程[8].
与宽带半导体ZnO相比,AgX(X=I, Br等)等离子基元材料具有较窄的禁带宽度及良好的可见光活性,因而在光催化研究领域中得到了重视[9, 10].Wang等[11]通过双喷法合成了Ag/AgBr颗粒,在可见光照射下,其降解甲基橙得到的速率常数是同样条件下TiO2的102倍,是N掺杂TiO2的16倍.Lin等[1]通过原位离子交换法制备出AgI/Ag/AgBr,同样发现其在可见光下对甲基橙降解时表现出优良的光催化活性.基于此,AgX也常用来修饰宽带半导体纳米材料,将宽带半导体的光吸收范围从紫外移至可见光区域,使其具有良好的可见光活性[12, 13].Vignesh等[14]制备出AgI/ZnO粉末,在可见光照射下对玫瑰苯胺盐酸盐进行降解,降解率达到了88%.Wang等[15]采用沉淀法制得AgBr/TiO2粉末,它可有效地分离和转移光生电子,从而提高光催化性能.
另一方面,在光照下吸收光子的同时,AgX会释放电子与空穴,光生电子会与内部Ag+结合形成纳米Ag颗粒.反应过程如下[13, 14]:
由于Ag表面等离子共振效应(SPR)及其与AgX的协同作用,Ag/AgX在光催化过程中表现出了更好的光催化活性及光稳定性[16, 17].Cui等[18]通过微波辅助法合成了Ag/AgBr/K4Nb6O17光催化材料,发现它在可见光催化降解罗丹明B反应中循环使用五次后,其活性依然保持稳定.但是在研究Ag/AgX光降解过程中,却很少有人考虑光还原时间的不同,导致Ag在Ag/AgBr中的不同含量对光催化性能的影响[19].除此之外,在目前的研究当中,以AgX修饰宽带半导体用于光降解有机污染物以悬浮体系为主,由于该体系存在回收再利用等方面的困难,因而限制了其实际使用.相比较而言,负载体系可实现回收再利用的目的,但由于与有机污染物接触的有效比表面积减小,使得光催化活性降低,因而对负载体系可见光光催化活性的提高是光催化技术真正走上实际应用的关键一步.
相比于ZnO粉末,ZnO纳米柱状阵列具有更高的比表面积、高的光捕获效率以及光生电子转移速率[20, 21];同时,它的制备工艺简单,因而是一种理想的负载体系.因此本文首先通过溶胶-凝胶法制备晶种层,其次采用水热法制备出纳米ZnO柱状阵列膜,然后通过浸渍法,以AgNO3,KI和NaBr为反应物,在ZnO纳米柱状阵列膜上沉积出AgI和AgBr纳米颗粒进行表面修饰,并采用紫外光光照预处理的方法获得Ag/AgX/ZnO纳米复合结构,研究不同纳米复合体系AgI/ZnO,AgBr/ZnO,Ag/AgBr/ZnO的可见光活性及其光稳定性,以期获得一种具有高可见光活性、高稳定性的纳米复合结构,对太阳能的有效利用以及绿色环保技术的发展具有一定的促进意义.
将1.1g的乙酸锌(分析纯)和1.5g的聚乙烯醇(分析纯)混合加入到40ml的去离子水中,在80ºC下搅拌1h,获得粘稠溶胶,室温下静置1d.在一定尺寸(25mm×25mm)的干燥洁净载玻片上涂抹晶种溶液,放入SC-1B型匀胶机(北京,创世维纳)中,先用600r/min速度旋转5s,使溶液铺满整个衬底;再用3000r/min速度旋转20s,使溶液均匀附着在衬底上,最后取出,于100ºC干燥10min.重复上述步骤6次后于450ºC焙烧30min,获得ZnO晶种层.配制0.03mol/L硝酸锌(分析纯)和0.03mol/L六次甲基四胺(分析纯)的混合溶液,放入ZnO晶种薄膜,在60℃水浴中放置6h,取出用去离子水清洗,干燥备用.
配制0.01mol/L的AgNO3(分析纯)溶液和KI(分析纯)溶液等体积混合,放入ZnO柱状阵列薄膜,浸渍8h取出,用去离子水清洗数次,获得AgI/ZnO-8样品.
将AgNO3溶液和NaBr(分析纯)溶液等浓度等体积混合,放入ZnO纳米柱状阵列薄膜,浸渍12h时取出,用去离子水清洗.采用的浓度分别为2.5、5、10、15mmol/L,取出后用去离子水清洗.制备出的AgBr/ZnO分别记为2.5mMAgBr/ZnO、5mMAgBr/ZnO、10mMAgBr/ZnO和15mMAgBr/ZnO.作为对比样,AgBr颗粒膜由未参与水热过程的ZnO晶种薄膜浸渍在AgNO3溶液和NaBr溶液(5mmol/L)中12h获得.
将5mmol/L的AgNO3溶液和NaBr溶液等浓度等体积混合后放入ZnO纳米柱状阵列薄膜,分别浸渍4,8,12和16h,取出用去离子水清洗,制备出的AgBr/ZnO分别表示为AgBr/ZnO-4,AgBr/ZnO-8,AgBr/ZnO-12和AgBr/ZnO-16.
将AgBr/ZnO放入ZF-7型暗箱三用紫外线分析仪(上海,嘉鹏)中,采用254nm紫外光照射一段时间后取出获得Ag/AgBr/ZnO薄膜.光照时间分别为1,1.5,2和3h,之后的样品分别表示为Ag/AgBr/ZnO-1,Ag/AgBr/ZnO-1.5,Ag/AgBr/ZnO-2和Ag/AgBr/ZnO-3.
采用X’pert PRO MPD型X射线粉末衍射(XRD)仪(荷兰公司,PHILIPS)分析样品晶相组成,扫描范围2θ=20º-80º,扫描速度为2º/min.通过Quanta 450型扫描电子显微镜(SEM)(美国公司,FEI)观察分析样品的形貌.采用VG Multilab 2000 X型射线光电子能谱仪(XPS)分析样品的表面状态;通过U-3900型紫外-可见分光光度计(日本日立公司)对催化剂进行紫外漫反射光谱(UV-Vis)分析,扫描范围为200-700nm.
配制5mg/L的甲基橙(MO)溶液.300W氙灯作为光源,用400nm滤波片滤去紫外光,将玻璃基片放入甲基橙溶液,光照180min,每隔30min取一次试样,用UV-2102PC型紫外-可见分光分度计(上海,尤尼柯)对MO溶液进行测试,在464nm处取得其吸光度值,计算其浓度.
光稳定性实验中,将AgBr/ZnO-12和Ag/AgBr/ZnO-1.5分别放入MO溶液中,采用180min照射测试MO浓度变化,之后采用去离子水清洗,放入60ºC的干燥箱内干燥备用.循环五次后获得试样光稳定特性.
图1为ZnO,AgI/ZnO,5mMAgBr/ZnO,AgBr/ZnO-8和Ag/AgBr/ZnO-1.5样品的XRD谱.可以发现,所有试样在2θ=31.7º,33.4º和36.2º处出现的衍射峰,对应于ZnO,其中2θ=33.4º处衍射峰最强且尖锐,对应于ZnO(001)面.这主要是因为ZnO纳米棒沿<001>方向优先生长的缘故.在AgI/ZnO与5mMAgBr/ZnO样品中还分别出现Ag和AgBr的衍射峰,表明采用浸渍法在ZnO表面成功合成了具有良好结晶特性的AgI和AgBr相.对AgBr/ZnO纳米柱状阵列进行紫外光光照预处理后得到Ag/AgBr/ZnO,但在它的XRD谱中未出现Ag衍射峰,原因可能是其中Ag含量极少,而不易检测出,研究表明AgBr/ZnO经光还原后可以得到Ag/AgBr/ZnO[11, 22].
图2为纯ZnO,AgBr/ZnO-12和Ag/AgBr/ZnO-1.5样品的FESEM照片.可以看到,所制ZnO纳米棒阵列分布均匀,直径大约为50nm;对于AgBr/ZnO-12样品,AgBr颗粒主要分布于ZnO纳米柱状阵列的顶端和顶端侧面.吸附在ZnO纳米棒顶端侧面的AgBr颗粒之间相互接触形成网状结构,颗粒平均直径约为120nm;ZnO纳米柱状阵列底端并未出现AgBr颗粒.相比于致密的AgBr颗粒膜,这种多孔网状结构应该具有较大的比表面积,从而有利于提高AgBr/ZnO-12光催化活性.AgBr/ZnO-12经光照预处理后,AgBr表面形成很多细小的颗粒,这与Yan等[16]制备的Ag/AgBr形貌一致.结合图1可认为,该细小颗粒可能是Ag+还原后形成的纳米Ag颗粒.
图3为AgBr/ZnO-12与Ag/AgBr/ZnO-1.5样品的XPS谱.由图可见,这2种样品均含有Ag3d和Br3d峰.比较而言, Ag/AgBr/ZnO-1.5的Ag 3d的峰位置上偏移约0.2eV,这可能是因为Ag0与Ag+的共同作用所致[16].图3(a)中Ag/AgBr/ZnO-1.5试样的Ag 3d5/2和Ag 3d3/2峰的位置分别位于367.56和373.53eV,对这两处的峰进行分峰拟合.其中367.40和373.30eV处峰属于AgBr中的Ag+,368.20和373.82eV处则归属于Ag0[16, 23, 24].图3(b)为AgBr/ZnO-12与Ag/AgBr/ZnO-1.5的Br 3d XPS谱.可以看到,Br 3d峰在68.22eV处[23, 24].结果表明,Ag/AgBr/ZnO中存在Ag和AgBr.
图4为纯ZnO,5mMAgBr/ZnO和Ag/AgBr/ZnO-1.5样品的UV-Vis谱和带系能量图.可以看出,ZnO纳米阵列表面负载AgBr后,其光吸收范围发生了明显的红移,这表明它在可见光区域将具有更好的吸光能力.Ag/AgBr/ZnO-1.5样品在400-525nm区间内出现纳米Ag颗粒表面等离子共振效应峰,该样品在可见光区域具有较强的吸收可能是由于Ag纳米粒子等离子共振效应和AgBr光吸收的共同作用.
半导体带系能量可以由以下公式计算[15]:
其中α,h,υ,Eg和A分别为吸收系数、普朗克常数、光频、带系能量和常数.对于AgBr/ZnO,n=4,因为它是由半导体光学跃迁类型所决定.因此,可以算出ZnO,5mMAgBr/ZnO和Ag/AgBr/ZnO-1.5的带系能量分别为3.16,2.94和2.82eV.AgBr/ZnO以及Ag/AgBr/ZnO-1.5试样较小的带系能量,可以促进该纳米复合结构在可见光区域的吸收,从而获得可见光光催化活性.
图5为在相同工艺条件下所制备出的AgI/ZnO-8和AgBr/ZnO-8可见光降解MO曲线图.浸渍时,AgNO3与KI及NaBr等体积等浓度(0.01mol/L)混合.由图可见,这2个样品光催化降解MO效果相差较大.经过180min可见光照射,MO降解率相差33%.尽管无法准确测出AgBr和AgI的负载量,但AgBr/ZnO可见光光催化活性要好于AgI/ZnO,与文献[1]报道一致.
基于上述结果,我们研究了不同浸渍浓度所制AgBr/ZnO的光催化活性.图6(a)为ZnO,AgBr与AgBr/ZnO在可见光照射下对MO的降解曲线.由图可见,纯ZnO纳米棒阵列在可见光照射下对MO几乎无降解效果,ZnO晶种膜浸渍制备的AgBr颗粒膜降解MO效率只有17.9%,而在ZnO纳米柱状阵列表面浸渍所得的AgBr/ZnO纳米结构光催化降解MO的效率却有很大提高,这说明基体对AgBr/ZnO纳米复合结构的可见光光催化活性有重要影响.当浸渍浓度为5mmol/L时,制备的AgBr/ZnO阵列膜可见光光催化性能最佳,其降解MO效率至少是AgBr的4倍.图6(b)为不同浸渍浓度所制AgBr/ZnO阵列膜ln(C0/C)与降解时间的关系.由于它们之间的关系可以用直线表示(R>0.99),因此为准一级反应[25].
表1为不同浸渍浓度时的初始降解速率v0、反应速率常数kapp和半衰期t1/2.由表可知,随着浸渍浓度增加,反应常数逐渐增加;当超过5mmol/L后,反应速率常数开始下降.
在最优浸渍浓度为5mmol/L的条件下,考察不同浸渍时间下获得AgBr/ZnO的光催化性能.图7(a)为AgBr/ZnO-4,AgBr/ZnO-8,AgBr/ZnO-12和AgBr/ZnO-16可见光下对MO的降解曲线.由图可知,随着浸渍时间的不断增加,所得样品光催化降解率不断提高;至12h时MO降解率达到78.6%;继续增加浸渍时间,降解效率开始下降.这可能是由于MO降解过程中主要是AgBr起作用,浸渍时间较短时,ZnO柱状阵列上沉积的AgBr含量较低,因而其降解效果也较差.随着浸渍时间的延长,AgBr含量的不断增加促进了AgBr/ZnO的光催化效果.因此在浸渍12h时,光催化性能达到最佳.随着浸渍时间的继续增加, AgBr含量的增加可能会导致表面多孔结构的消失,从而降低催化剂的比表面积;同时,AgBr/ZnO纳米结构中ZnO也可能是活性成分,过量的AgBr会降低ZnO纳米棒的比表面积,使得光催化过程中ZnO吸附的MO量减少,向AgBr颗粒表面转移的MO分子数也相应降低,因此AgBr/ZnO的光催化活性下降.
图7(b)为不同浸渍时间下,AgBr/ZnO阵列膜ln(C0/C)与降解时间关系,其满足一级反应特征[25].由图可知,以AgBr/ZnO-12的反应速率常数最大(kapp = 0.0091),AgBr/ZnO-16的最小(kapp = 0.0065).
在浸渍浓度为5mmol/L、浸渍时间为12h条件下制备出AgBr/ZnO阵列膜后,将其放入暗箱三用紫外线分析仪中254nm紫外光照射不同时间.照射过程中,AgBr/ZnO阵列膜中的AgBr发生分解形成Ag/AgBr/ZnO.图8为不同照射时间下制备出的Ag/AgBr/ZnO对MO的降解率图.由图可知,在光还原1.5h时,Ag/AgBr/ZnO的光催化性能最佳,在3h可见光照射下,MO降解率达83.5%.
图8(b)为Ag/AgBr/ZnO阵列膜降解MO的动力学曲线.可以看出,Ag/AgBr/ZnO-1.5的反应速率常数最大.这可能是由于随着光还原时间的不断增加,纳米Ag粒子的含量增加,Ag0/Ag+的比例也发生变化;至1.5h时,Ag0/Ag+比可能达到最佳[19].因此,Ag/AgBr/ZnO-1.5的光催化性能最好.在Ag/AgCl@TiO2-t(t为光还原时间)纳米结构可见光降解四氯苯酚实验中.当t=20min时,所得样品的光催化活性最高[19],与本文结果较为一致.
图9为AgBr/ZnO-12和Ag/AgBr/ZnO-1.5循环降解MO溶液的柱状图.由图可知,样品循环使用5次后,AgBr/ZnO-12上MO降解效率降低程度较大(40%);而Ag/AgBr/ZnO-1.5上的仅为12%.可见Ag/AgBr/ZnO-1.5表现出较好的光催化稳定性.
根据光催化实验结果,可以分析AgBr/ZnO与Ag/AgBr/ZnO纳米复合结构的光催化反应机理.由图2和图4可知,AgBr纳米颗粒紧密附着ZnO柱状阵列顶端表面;经AgBr修饰后,AgBr/ZnO吸收光谱范围由紫外区域扩展到可见区;同时经过光照预处理所形成的Ag/AgBr/ZnO纳米复合结构在可见光区域由于Ag纳米颗粒的等离子共振效应具有更强烈的吸收.这说明AgBr/ZnO和Ag/AgBr/ZnO的可见光活性与其化学成分组成及其形貌密切相关.
AgBr导带和价带的电位值分别为0.05和2.57eV,ZnO导带和价带的电位值分别为-0.31和2.89eV,因此AgBr与ZnO带系并不匹配[24].在可见光照射下,由于AgBr的禁带宽度较窄,它会产生光生电子和空穴,但是在ZnO中则不会产生.因此在AgBr/ZnO和Ag/AgBr/ZnO复合材料中,ZnO纳米柱状阵列可能只是起到基体作用,但由于其具有较大的比表面积,并与AgBr颗粒的接触良好,在降解有机物过程中其表面将吸附大量的有机污染物MO分子,这对于光催化反应过程中MO分子向AgBr纳米颗粒进行转移非常有利,从而加速反应过程.同时,AgBr/ZnO与Ag/AgBr/ZnO表面形成的多孔结构,可以增加催化剂的比表面积,能够增加光接触面积,提高光催化效率.另一方面,在可见光照射下,附着于ZnO纳米阵列表面的AgBr纳米颗粒被激发产生电子-空穴对,光生电子会与内部的Ag+接触形成Ag0单质,Ag0颗粒的生成不仅能加大复合材料对可见光的吸收,同时具有很强的电子捕获能力,因而使得AgBr产生的电子迅速被转移,达到光生电子与空穴分离的目的.AgBr价带上的电子被迅速转移后,与溶液中的O2形成-O2-,部分的-O2-会转变成具有强氧化性的-OH.在O2-,-OH和h+的共同作用下,Ag/AgBr/ZnO在可见光降解MO的过程中表现出优异的光催化活性.图10为可能的光催化反应机理示意图.
相比于AgBr/ZnO纳米结构,Ag/AgBr/ZnO柱状阵列薄膜表现出更高的光催化稳定性,其稳定性的提高可能与紫外光光照预处理所形成Ag0可有效阻止AgBr在光催化反应过程中的分解有关[26].在光催化过程中,同时存在Ag+的消耗及形成两个过程.一方面,Ag+可能与光生电子结合或与反应激子形成Ag0;另一方面,Ag0可与空穴结合形成Ag+.在溶解氧存在的情况下,Ag0与空穴结合形成Ag+将成为主要反应过程,这是因为溶解氧会捕捉光生电子,抑制电子与Ag+的结合,同时还会促进空穴的形成[27].对于AgBr/ZnO纳米结构,由于无Ag0存在,Ag+的消耗将占导,AgBr不断发生光分解导致其催化活性降低;而对于Ag/AgBr/ZnO纳米结构,紫外光光照预处理形成的Ag0会与空穴形成Ag+,从而保证反应过程中AgBr的稳定特性.研究表明,在Ag/AgBr/TiO2光催化降解有机污染物过程中,XPS中Ag0峰的强度在光催化反应前比光催化反应后要弱,证明了Ag0的存在对AgBr的分解具有抑制作用[26].因此,反应过程中Ag/AgBr/ZnO表面Ag0的存在可能是其光催化稳定性提高的主要原因.
采用浸渍法在ZnO纳米柱状阵列表面制备了AgX/ZnO纳米复合结构,并研究了浸渍时间、浸渍浓度以及紫外光光照预处理对纳米复合结构可见光催化活性的影响.经过浸渍处理所形成的AgBr纳米颗粒在ZnO纳米柱状阵列表面均匀分布,并形成多孔网状结构.可见光光催化降解MO结果显示,在相同工艺条件下AgI/ZnO与AgBr/ZnO均具有良好的可见光光催化活性,但后者更优,在3h内MO降解率提高了约33%.在浸渍盐浓度为5mmol/L,浸渍时间为12h时,采用等体积等浓度方法所制备的AgBr/ZnO纳米复合结构光催化活性达到最佳.通过紫外光光照预处理,AgBr/ZnO转变为Ag/AgBr/ZnO纳米复合结构,光还原1.5h时其可见光活性最佳,3h内MO的降解率达到83.5%.同时,Ag/AgBr/ZnO具有较好的光催化稳定性.该纳米复合结构的可见光催化反应活性可能与ZnO纳米柱状阵列较大的比表面积、AgBr的可见光响应特性以及Ag/AgBr的表面等离子共振效应有关.
2014, Vol. 35 Issue (7): 1113-1125
PDF (1019KB)
2014, Vol. 35 Issue (7): 1113-1125
PDF (1019KB)