AgI/AgCl/H₂WO₄ Double Heterojunctions Composites: Preparation and Visible-Light Photocatalytic Performance

Abstract

$AgI/AgCl/H_2WO_4$ double heterojunctions photocatalyst was prepared via deposition-precipitation followed by ion exchange method. The structure, crystallinity, morphology, chemical content and other physical-chemical properties of the samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectra (EDX), UV-vis diffuse reflectance spectroscopy (DRS), and photoluminescence (PL). The photocatalytic activity of the $AgI/AgCl/H_2WO_4$ was evaluated by degrading methyl orange (MO) under visible light irradiation (${\lambda}$ > 400 nm). The double heterojunctions photocatalyst displayed more efficient photocatalytic activity than pure AgI, AgCl, $H_2WO_4$ and AgCl/$H_2WO_4$. Based on the reactive species and energy band structure, the enhanced photocatalytic activity mechanism of $AgI/AgCl/H_2WO_4$ was discussed in detail. The improved photocatalytic performance of $AgI/AgCl/H_2WO_4$ double heterojunctions could be ascribed to the enhanced interfacial charge transfer and the inhibited recombination of electron-hole pairs, which was in close relation with the $AgI/AgCl/H_2WO_4$ heterojunctions formed between AgI, AgCl and $H_2WO_4$.

Introduction

Photocatalytic oxidation (PCO) technique has been used in the field of environmental pollution control because it can effectively destroy many non-biodegradable pollutants with structural stability.1,2 In addition, compared to the traditional water treatment techniques via physical methods, PCO process possesses obvious advantages of energy-saving, high-efficiency, thoroughly degrading, less secondary pollution, etc. Therefore, it has been considered to be a very promising approach of wastewater treatment. Up to now, a variety of semiconductor photocatalysts have been developed and investigated in detail. TiO2 with large band gap (3.0-3.2 eV) is one of the most widely studied photocatalysts due to its high efficiency, low cost, non-toxicity, and high stability.3-5 However, TiO2 only responds to ultraviolet light which only occupies about 4% of the solar spectrum. The limited utilization of solar energy hinders the applicability of single semiconductor photocatalyst.6 Thus, it is essential to develop visible light photocatalysts with high photocatalytic activity.

Previously, many coupled semiconductor systems have been extensively studied as efficient visible light photocatalysts that showed higher photocatalytic efficiencies than single semiconductors, such as Bi2O3/TiO2,7 CuO/ZnO,8 BiOI/Bi2O3,9 BiOCl/Bi3O4Cl,10 WO3/BiOCl,11 SrO/CuBi2O4,12 Cu2O/TiO2, ZnMn2O4/TiO2,13 AgBr/WO3,14 AgI/BiOI,15 AgI/TiO2,16 WO3/TiO2,17 AgBr/AgNbO3,18 AgBr/H2WO4,19 and so on. In these systems, two semiconductors with matching band potentials are combined and a heterojunction is constructed simultaneously between the two components, which can suppress the recombination of the electron-hole pairs, increase lifetime of the charge carriers, promote quantum efficiency, and further effectively improve the photocatalytic activity of semiconductor photocatalysts.20,21 It should be noted that most coupled semiconductor systems only focused on two components, and three components semiconductor composites with double heterojunctions are rarely reported. Wang et al. reported that ZnO/ZnWO4/WO3 composites with double heterojunctions displayed higher activity than ZnO/WO3 and proposed that the formed double heterojunctions in ZnO/ZnWO4/WO3 system presented high efficient separation of electron-hole pairs.22 We have also constructed a AgI/AgCl/TiO2 three-component system with double heterojunctions and proved that the novel photocatalyst was efficient for the improvement of photocatalytic activity of AgX and TiO2,23 which implied that the construction of double heterojunctions in composite photocatalysts may be a promising way for the enhancement of photocatalytic performance.

The CN bottom and the VB top of H2WO4 llie below the CB bottom and VB top of AgI and AgCl, respectively, which will result in the highly efficient separation of photoinduced electrons and holes. Inspired by this, in this work, we introduced H2WO4 as a substrate and synthesized a novel visible light composite photocatalyst AgI/AgCl/H2WO4 with double heterojunctions structure via a simple deposition-precipitation method followed by ion exchange between AgCl and KI solution. XRD, SEM, EDX and UV-Vis spectrometry were used to characterize the as-prepared AgI/AgCl/H2WO4 double heterojunctions photocatalyst. The photodegradation of methyl orange (MO) was carried out to study the photocatalytic activity of the AgI/AgCl/H2WO4 double heterojunctions under visible light irradiation (λ > 400 nm). Moreover, in attempt to explore the roles of different reactive species and the reaction mechanism, various scavengers were introduced to the photocatalytic reaction system.

 

Experimental

Materials. Sodium tungstate dihydrate (Na2WO4·2H2O), nitric acid (HNO3), ethanol, silver nitrate (AgNO3), potassium chloride (KCl), potassium iodide (KI) and methyl orange (MO) were of analytical grade without further purification. They were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used throughout this study.

Samples Preparation. H2WO4 was prepared in advance. Na2WO4·2H2O was dissolved in ethanol-water solution (1:2, V/V). After the pH of the solution was adjusted to 2.0 with HNO3 solution, the reaction mixture was subsequently stirred for 12 h at 60 °C. Finally, yellow H2WO4 precipitate was centrifuged, collected, washed with deionized water for 3 times, and dried at 65 °C for 24 h.

The procedure of AgCl/H2WO4 substrate prepared via a deposition-precipitation method was depicted as follows. H2WO4 (10.0 g) was homogeneously dispersed in deionized water (500 mL) and sonicated for 20 min. Subsequently, a quantity of AgNO3 (1.70 g) solution was added to the H2WO4 suspension and stirred for 20 min. Then a stoichiometric amount of KCl (0.745 g) was dropwise added into the suspension and the resulting suspension was strongly stirred at room temperature for 2 h. The product was centrifuged, washed and dried at 60 °C for 14 h. Finally, AgCl/H2WO4 substrate with theoretical Ag/W molar ratio of 0.25:1 was obtained.

AgI/AgCl/H2WO4 photocatalyst was synthesized by iodine ion exchange of AgCl/H2WO4 substrate. In a typical synthetic route, a stoichiometric amount of KI solution was dropped into the AgCl/H2WO4 (1.0 g) suspension that dispersed in deionized water (100 mL) and then stirred magnetically for 20 min. After that, the product was filtered, washed and dried at 65 °C for 24 h. The corresponding AgI/AgCl/H2WO4 samples with different molar percentage of AgI to initial AgCl (10%, 30%, 50% and 80%) were obtained by changing the KI amounts, respectively.

Sample Characterization. The phases of the products were identified using X-ray diffractometer (XRD) (BRUKER D8 ADVANCE X-ray powder diffractometer) with Nifiltered Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 10° s−1 from 10° to 60°. The accelerating voltage and emission current were 40 kV and 40 mA, respectively. FEI Sirion 200 field emission scanning electron microscope (SEM) with 5.00 kV scanning voltages was employed to observe the morphologies of the as-prepared catalysts. Energy-dispersive spectroscopy (EDS) was observed by using an Oxford instruments INCA X-act detector. UV–vis diffuse reflectance spectroscopy measurements were carried out to calculate the energy band gaps of the composites using a TU-1901 UV–VIS spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) equipped with an integrating sphere attachment. The scanning wavelength range was from 350 to 600 nm and BaSO4 was used as a reflectance standard. Fluorescence emission spectra were recorded on a JASCO FP-6500 type fluorescence spectrophotometer with 315 nm excited source over a wavelength range of 350-600 nm.

Photocatalytic Activity Measurements. In order to evaluate the photocatalytic activity of samples, the process of MO degradation was carried out in a batch reactor under visible light irradiation (λ > 400 nm). The initial concentration of MO solution was 10 mg/L. The reactor and procedure were referred to the previous study.23 The MO concentration collected at each specific irradiation time interval was determined from the absorbance at a wavelength of 464 nm using 722s spectrophotometer, with deionized water as a reference sample.

Terephthalic acid photoluminescence probing technique (TA-PL) was introduced to detect the formation of ·OH radicals, in which a basic TA solution set at 5 × 10−4 M in 2 × 10−3 M NaOH solution was added to the reactor instead of MO. Then the suspension was collected every 60 min to measure on a JASCO FP-6500 type fluorescence spectrophotometer after centrifugation. The excitation wavelength used was 315 nm.

Furthermore, the effects of main reactive species involved in the PCO process were investigated using a quantity of appropriate species quenchers. A series of comparison experiments were carried out between the original degradation of MO by AgI/AgCl/H2WO4 and those obtained after addition of quenchers in the initial solution, under other identical conditions. The dosages of the species quenchers were all 1 mmol/L.

 

Results and Discussion

Characterization of AgI/AgCl/H2WO4. XRD is used to investigate the changes of structure and the composition of the AgI/AgCl/H2WO4 double heterojunctions photocatalysts obtained at different AgI contents. Figure 1 shows the XRD patterns of the samples obtained at various AgI contents. The analysis of XRD patterns reveals that AgCl with the characteristic peaks of (111), (200) and (220) is of cubic structure (JCPDS 31-1238), while AgI has both β-AgI (JCPDS 85-0801) and γ-AgI (JCPDS 09-0399) structures, assigned to β-(100), (110) and γ-(111) diffraction peak, respectively. The characteristic peaks of H2WO4, (020), (111) and (131), are in good agreement with the standard card of orthorhombic structure (JCPDS 43-0679). With the increase of AgI contents in AgI/AgCl/H2WO4 composites, the intensity of peaks of AgI gradually increased, whereas those of AgCl decreased simultaneously due to its consumption, and that no other peaks of impurity were detected. Based on this result, AgI, AgCl and H2WO4 are confirmed to coexist in AgI/AgCl/H2WO4 composites and AgI/AgCl/H2WO4 double heterojunctions may be formed in the composites.

Figure 2 shows the comparison of XRD pattern of AgI/AgCl/H2WO4 before and after 180 min irradiation for the degradation of MO. It is observed that the crystal structure of the used AgI/AgCl/H2WO4 had no obvious changes except that two small peaks at 38.16° was detected and identified as the main peaks of metal silver (JCPDS 04-0783), as shown in the inset of Figure 2. This suggests that a trace amount of metal silver was formed in AgI/AgCl/H2WO4 after degradation of MO, which may affect the stability and the photocatalytic activity of AgI/AgCl/H2WO4.

Figure 1XRD patterns of (a) H2WO4, (b) AgCl/H2WO4, (c) 10% AgI/AgCl/H2WO4, (d) 30% AgI/AgCl/H2WO4, (e) 50% AgI/AgCl/H2WO4 and (f) 80% AgI/AgCl/H2WO4.

Figure 2.XRD patterns of 50% AgI/AgCl/H2WO4 before and after 180 min irradiation. The inset is the enlarged XRD results.

The typical microstructure of the pure H2WO4 and 50% AgI/AgCl/H2WO4 were analyzed by SEM and the results are shown in Figure 3(a) and (b). It can be observed that pure H2WO4 and AgI/AgCl/H2WO4 consisted of anomalous particles with diameters less than 250 nm, respectively. Moreover, the surfaces of these anomalous particles were modified with some nanoparticles (Fig. 3(b), inset). But the quantity of the nanoparticles on the surface of AgI/AgCl/H2WO4 sample decreased, which may be attributed to the aggregation of AgCl and AgI particles that deposited on the surface of H2WO4. Higher resolution images can not be obtained because AgI and AgCl are decomposed by the high energy electron beam. To further confirm the element components of the composite, the EDS of pure H2WO4 and AgI/AgCl/H2WO4 composites were obtained on an Oxford instruments INCA X-act detector, respectively. Since H element can not be detected, only W and O elements could be observed in pure H2WO4 (Fig. 3(c)). Similarly, five elements including Ag, I, Cl, W and O was appeared in AgI/AgCl/H2WO4 (Fig. 3(d)), and no other impurity element was found.

Figure 3.SEM images of (a) pure H2WO4 and (b) 50% AgI/AgCl/H2WO4. EDS of (c) pure H2WO4 and (d) 50% AgI/AgCl/H2WO4.

The UV-vis diffuse reflectance spectra of AgI, AgCl, H2WO4, AgCl/H2WO4 and 50% AgI/AgCl/H2WO4 are compared in Figure 4. It can be seen that the absorption edge of AgI was located at about 475 nm while AgCl only absorbed visible light slightly, whereas H2WO4 exhibited an absorption edge near 540 nm. However, AgCl/H2WO4 and 50% AgI/AgCl/H2WO4 composites had the similar absorption edges that were just a little shorter than H2WO4. The result indicates that the dominant factor for the enhanced photocatalytic performance of AgI/AgCl/H2WO4 may be not due to the light absorption ability but the role of AgI-AgCl-H2WO4 double heterojunctions that will facilitate the transfer of the photoinduced electrons and holes. Besides, the used AgI/AgCl/H2WO4 displayed much stronger absorption in the visible region than fresh AgI/AgCl/H2WO4, which may result from the formation of metal silver on the surface of AgI/AgCl/H2WO4.19,24,25 according with the result of Figure 2.

In addition, the energy band gap (Eg) of a semiconductor can be calculated using Eq. (1):23,26,27

where α, ν, Eg and A are absorption coefficient, light frequency, band gap energy, and a constant, respectively. Among them, n depends on the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). The values of n for AgI, AgCl and H2WO4 are 1, 4 and 1, respectively.15,19,23,28 According to Eq. (1), the Eg of AgI, AgCl and H2WO4 are determined from the plot of (αhv)2/n vs. hv based on their absorption spectra and obtained to be 2.78 eV, 2.93 eV and 2.47 eV, separately, which are shown in Figure 5.

Figure 4.UV-vis diffuse reflectance spectra of the as-prepared samples and the used 50% AgI/AgCl/H2WO4.

Photocatalytic activity of AgI/AgCl/H2WO4. Methyl orange (MO) that commonly existed in dyeing effluent is a non-biodegradable azo dye and often selected as a model pollutant to evaluate the photocatalytic performance of photocatalytic materials. The degradation of MO was used to evaluate the photocatalytic performance of the prepared catalysts under visible light irradiation (λ > 400 nm). MO was not degraded after 180 min illumination in the absence of photocatalyst, or in the dark with the photocatalyst. The results presented in Figure 6(a) demonstrate that H2WO4 did not possess photocatalytic activity for MO and AgCl/H2WO4 only decomposed 2.5% of MO. However, the composites AgI/AgCl/H2WO4 (10%-80%) exhibited much higher degradation efficiencies than pure H2WO4 and AgCl/H2WO4. Furthermore, it can be seen that the degradation efficiency of the composites increased first as the content of AgI increased to 50%, then decreased with the persistent increase of AgI content in the AgI/AgCl/H2WO4. That is to say, 50% AgI/AgCl/H2WO4 showed the best photocatalytic activity, at which 62.2% of MO was decomposed. This is because fewer interfaces of AgI/AgCl and AgCl/H2WO4 heterojunctions will be formed when the content of AgI is too low or excessive, which leads to limitations in separating the electron-hole pairs and further impedes the photocatalytic activity. Therefore, with an appropriate ratio of heterojunctions (50% AgI/AgCl/H2WO4), the composites can exhibit perfect performance in photocatalysis. In this case, the effect of AgI/AgCl/H2WO4 double heterojunctions on the enhancement of photocatalytic activity is the key factor. What’s more, the changes of UV-vis spectra of MO photodegradation over AgI/AgCl/H2WO4 under visible light irradiation were carried out to test the photocatalytic performance of 50% AgI/AgCl/H2WO4, shown in Figure 6(b). It can be observed that the intensity of the typical absorption peak of MO at 464 nm decreased gradually with increasing irradiation time, which indicates that AgI/AgCl/H2WO4 could exhibit excellent visible light photocatalytic activity for the MO degradation.

Figure 5.(a) The band gaps (Eg) of AgI and H2WO4, (b) The band gaps (Eg) of AgCl.

Figure 6.(a) Effect of different catalysts on MO degradation under visible light irradiation (Initial concentration of MO is 10 mg/L); (b) UV-vis spectral changes of MO photodegradation over 50% AgI/AgCl/H2WO4 under visible light irradiation.

The photocatalytic activity of the powders can be quantitatively evaluated by comparing their apparent reaction rate constants. Most of the photocatalytic oxidation of organic pollutants in aqueous suspensions follows the classical Langmuir–Hinshelwood kinetic model,26 which is described as follows:29,30

where kapp is the apparent pseudo-first-order rate constant (min−1); C is MO concentration in aqueous solution at time t (mg/L); C0 is initial MO concentration (mg/L).

To further investigate the effect of heterojunction formed between AgI, AgCl and H2WO4 on the photocatalytic activity of AgI/AgCl/H2WO4, the control experiments for MO degradation were carried out with different catalysts that contained the same weight of active component. The kapp values of different catalysts calculated from the Langmuir-Hinshelwood kinetics model are presented in Figure 7.

Obviously, the photocatalytic activity of heterojunction AgI/AgCl was much higher than that of the single AgI and AgCl. And the photocatalytic activity of AgCl/H2WO4 was also superior to the pure AgCl and H2WO4. Among the different samples, the 50% AgI/AgCl/H2WO4 had the highest activity with kapp of 5.52 × 10−3 min−1. The rate constants of the AgI/AgCl/H2WO4 are 14.5, 1.64 and 62.7 times higher than that of pure AgI, AgI/AgCl and AgCl/H2WO4, respecively. Therefore, the 50% AgI/AgCl/H2WO4 double heterojunctions photocatalyst exhibit strong photocatalytic activity for MO decomposition under visible-light irradiation.

The activity enhancement of AgI/AgCl/H2WO4 was likely to result from a synergistic effect of several factors, including the light absorption intensity, morphology, recombination rate of the electron-hole pairs, crystallite size, and so on. Based on the comprehensive analysis of above, the decrease in the recombination rate of the electron-hole pairs was finally regarded as the key factor for the highest photocatalytic activity of AgI/AgCl/H2WO4, which can be attributed to the function of double heterojunctions between AgI, AgCl and H2WO4 formed in the composite. That is to say, the existence of heterojunctions in the composites can facilitate the separation of the electrons and holes.

Figure 7.The comparison of degradation efficiency of MO by different photocatalysts containing the same weight of each active component.

The reuse and stability of the highly efficient AgI/AgCl/H2WO4 composites were evaluated by recycled runs in the photocatalytic degradation of MO under visible light irradiation. The catalysts were separated from the reaction mixture by centrifugation after each reaction cycle. It is found that the photocatalytic activity of AgI/AgCl/H2WO4 declined and the used catalyst became darker than the newly prepared catalyst, which demonstrates that metal silver was formed after irradiation. The comparative XRD patterns of the used AgI/AgCl/H2WO4 with the newly one in Figure 2 show that two small peaks of metal Ag were detected at 38.16° and 44.07° whereas the crystal structures of AgI and AgCl were well maintained. However, the trace amount of silver has decreased the photocatalytic activity of AgI/AgCl/H2WO4 in the cyclic experiments. Similar phenomenon has also been observed in previous studies,17,31,32 which is different from the Ag/AgX plasmonic photocatalysts,33-35 because the promotion or inhibition role of photoinduced Ag to the photocatalytic activity of the photocatalysts is based on many parameters of the Ag particles, such as morphology, position, size, etc.35-37 Herein, the stability of AgI/AgCl/H2WO4 should be further improved in this study.

Photocatalytic Mechanism of AgI/AgCl/H2WO4. In order to investigate the mechanism of photocatalysis of AgI/AgCl/H2WO4, the effect of active radical species (h+, ·OH and ·O2−) suspected to be involved in the PCO process were examined under visible light.

Generally, the formation of ·OH can be detected by terephthalic acid photoluminescence probing technique (TA-PL).38 Figure 8 shows the PL emission spectra excited at 315 nm from TA solution, which were measured every 60 min of illumination. Apparently, the PL intensity of ·OH at about 425 nm did not increase gradually with the irradiation time, which signifies that no ·OH radicals were formed during the PCO process. This confirms that ·OH radicals are not the dominant active species for the degradation of MO.

Figure 8.·OH trapping PL spectral changes observed during irradiation of AgI/AgCl/H2WO4 in a 5 × 10−4 M terephthalic acid solution (excitation at 315 nm).

Figure 9.The influence of different quenchers on the MO degradation over 50% AgI/AgCl/H2WO4.

In addition, the control experiments were performed by adding different scavengers in a manner similar to the photodegradation experiment. The corresponding dosages were referred to the previous studies.32 The results are displayed in Figure 9. It is clear to observe that the degradation efficiency of MO decreased remarkably to 5.9% with the addition of benzoquinone (BQ, a quencher of ·O2−).32 When ammonium oxalate (AO, a quencher of h+)39 or isopropanol (IPA, a quencher of ·OH)32 was added to the solution independently, the degradation efficiency of MO decreased slightly compared with the highest efficiency of 62.2% in the absence of quencher. Considering the results of both TA-PL detection of ·OH and quenching effect of scavengers, we can conclude that ·O2− radicals, as the dominating active species, play a more important role than h+, while ·OH radicals can be neglected for the degradation of MO under visible light irradiation.

In general, when constructing a composite semiconductor including two or more components with matching band potentials, heterojunction interfaces will be formed between them, which facilitate the transfer of the photogenerated carriers and further enhance the photocatalytic activities of semiconductors. In this experiment, the band potentials of AgI, AgCl and H2WO4 can be calculated by the following empirical:38,40,41

where EVB is the valence band (VB) potential, ECB is the conduction band (CB) potential, X is the electronegativity of the semiconductor (which is the geometric mean of the electronegativity of the constituent atoms), Ee is the energy of free electrons on the hydrogen scale (~4.5 eV), and Eg is the band gap of the semiconductor. The relative CB edge potentials of AgI, AgCl and H2WO4 are −0.41, 0.11 and 1.16 eV respectively, whereas the VB edge potentials of them are 2.37, 3.04 and 3.63 eV differently. The above results of band calculation indicate that AgI, AgCl and H2WO4 have the matching band potentials.

Figure 10.Schematic diagram of electron-hole pairs separation and the possible reaction mechanism over AgI/AgCl/H2WO4 photocatalyst under visible light irradiation.

According to the results above, a possible mechanism for MO degradation over AgI/AgCl/H2WO4 double heterojunctions photocatalyst was proposed and illustrated in Figure 10. When AgI, AgCl and H2WO4 were combined, AgI/AgCl/H2WO4 double heterojunctions were formed in the composite. Two corresponding active regions, the area where the three substances intersected each other, emerged along with the double heterojunctions. At the active regions, generation and separation of electron-hole pairs occurred and the charges separated more easily. Under visible light irradiation (λ > 400 nm), AgI, AgCl and H2WO4 can be simultaneously excited to form electron-hole pairs. Then the photogenerated electrons could sequentially transfer from the CB of AgI to that of H2WO4 via the CB of AgCl, which can be further trapped by molecular oxygen adsorbed on the surface of the catalyst to yield ·O2− that effectively react with dyes, shown as Eq. (5):

Meanwhile, the holes at the VB of H2WO4 migrate to the VB of AgCl, and then to that of AgI, which could directly oxidize a portion of dyes. Through this way, electrons and holes recombined with each other more difficultly and separated more easily in the AgI/AgCl/H2WO4 double hetero-junctions. In consequence, the photocatalytic activity of AgI/AgCl/H2WO4 heterostructure is greatly enhanced.

 

Conclusion

In summary, the AgI/AgCl/H2WO4 system developed in this work is a new kind of double heterojunction photocatalyst. It was synthesized by deposition-precipitation method followed by iodine ion exchange of AgCl/H2WO4 substrate, which worked efficiently under visible light irradiation. The AgI/AgCl/H2WO4 displayed much higher visible light photocatalytic activity than AgI, AgCl or H2WO4 for the degradation of MO. It was also found that the molar ratio of AgI to initial AgCl in the AgI/AgCl/H2WO4 composites played an important role in the photocatalytic property and the optimized ratio was obtained at 50%. The studies of photocatalytic mechanism demonstrated that the ·O2− radicals and h+, especially ·O2−, were regarded as the primary active species that were generated by the charge separation and transfer during the photodegradation process. The contribution of the double heterojunctions AgI/AgCl/H2WO4 in the composite, which could depress the recombination of photogenerated electron-hole pairs, was responsible for the enhancement of photocatalytic activity.