LIN Guo-Ling WU Chen LIU Min-Yi ZHANG Xio-Yi JIANG Wen-Jie SONG Xu-Chun

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AgIO3Composited with-BiVO4as Enhanced Visible-light Heterojunction Photocatalysts for the Elimination of Aqueous Organic Pollutants①

LIN Guo-LiangaWU ChenaLIU Min-Yib,c②ZHANG Xiao-YiaJIANG Wen-JiebSONG Xu-Chunc

a(350118)b(350118)c(350007)

The heterojunction strategy was used to overcome the drawbacks of AgIO3for its wide band gap and of BiVO4for its low charge-separation efficiency. The photocatalytic per-formances of the as-prepared AgIO3/BiVO4were assessed by the photodegradation of rhodamine B (RhB) and tetracycline hydrochloride antibiotics (TC) under visible-light irradiation. All the as-prepared samples showed remarkable enhanced photocatalytic efficiency than pure AgIO3and BiVO4. Especially, 40% AgIO3/BiVO4exhibited optimum photocatalytic activities among all the prepared samples. The improvements may be ascribed to the synergetic effect between AgIO3and BiVO4including matching potentials and effective charge separation. Radical trap experiments determined that h+radical plays a significant role in the photodegradation of RhB while e-is not the dominating reactive species.

AgIO3, BiVO4, heterojunction photocatalysis;

1 INTRODUCTION

Nowadays, photocatalysis as a clean technology is considered to have a broad prospect for the solution of environmental problems and energy problems. The key point to the practical application of photocatalytic lies in the improved visible light response in solar energy and enhanced separation efficiency of the photoinduced polarized. These twoexample, AgIO3[5]has been reported exhibiting goodelectrons and holes, which are two main limitations in conventional semi-conductor photocatalyst. Therefore, the design of efficient visible light responsive with high activity is still an urgent task for endeavor worldwide. Recently, metal-iodate semiconductors, such as BiIO4[1], Y(IO3)3[2]and Bi(IO3)3[3]with excellent photocatalytic performance, have attracted increasing attention due to the long pair electrons of I5+in (IO3)-anion, which can lead to the formation of layered structure and make it For photocatalytic properties for its high separation rate of photoexcited charge carriers under UV light induced. However, the wide band gap of AgIO3, which can only be excited by UV-light, restricted the further application of AgIO3. It is reported[6]that combining a wide band gap semiconductor with a narrow band gap semiconductor can improve the photocatalytic performances, such as photoluminous, the formation of different new band alignment forms, surface state change, and so on. In the previous work of our team, AgI[7], AgBr[8]and WO3[9]were ex- plored to couple with AgIO3to form the hetero- junction which can help to solve the poor visible- light response. Another literature[10]about the ultra- thin C3N4coupled with AgIO3to form a highly effi- cient heterostructure photocatalyst for enhanced visible-light photocatalytic activity was also re- ported.

Meanwhile, BiVO4semiconductor, with a band gap of ~2.3 eV, has aroused widespread concern for its high response to visible light irradiation[11]. BiVO4/BiOBr[12]core-shell hierarchical mesoporous spindles with highly enhanced visible-light were synthesized by Yaping Zhang to remove 75% norfloxacin antibiotics in 120 min. Other hetero- structures such as BiFeO3/BiVO4[13],MoS2-BiVO4[14], BiVO4/g-C3N4[15]and BiVO4@CeO2[16]were also investigated. However, the photocatalytic efficiency of pure BiVO4in organic pollutant degradation is usually not satisfactory due to the low separation efficiency of photoinduced charge carriers. Therefore, combining AgIO3with BiVO4to form AgIO3/BiVO4heterostructure may be a promising strategy to restructure the energy band structure of both semi- conductors and enlarge the spectral responsive range of AgIO3, at the same time, to improve the separation of photogenerated electron-hole pairs of BiVO4.

In this work, a novel kind of AgIO3/BiVO4heteo- junction photocatalysts were synthesized via facile hydrothermal and chemical precipitation methods. The photocatalytic properties of as-prepared AgIO3/ BiVO4heterostructures were characterized by the decolorization of RhB and TC antibiotics under the visible-light irradiation. The heterojunction of AgIO3/ BiVO4exhibited much better photocatalytic effi- ciency compared with pure AgIO3and BiVO4due to the improved separation efficiency of photogenerated carriers. The radical scavenger experiments were conducted to explore the probable active species in the process of photocatalysis. Meanwhile, a possible mechanism was discussed in details in this report.

2 EXPERIMENTAL

2. 1 Preparation of BiVO4 precursor

Pure monoclinic BiVO4was synthesized by hydro- thermal method. 5 mmol NH4VO3was dissolved in 10 mL NaOH (2 mol/L) solution and stirred for 30 min, which was named as solution A. 5 mmol Bi(NO3)3·5H2O was added into 10 mL HNO3(4 mol/L) and then vigorously stirred for 10 min to form a transparent solution named solution B. Subsequently, the prepared solution B was dropped slowly into solution A and ultrasonic treated for another 30 min to make sure NH4VO3blended sufficiently with Bi(NO3)3. After adjusting the acidity to pH 7 under stirring, the mixture was transferred into a Teflon-lined stainless-steel autoclave and heated to 150 °C and kept for 12 h, and finally cooled down naturally. The yellow solid was filtered and washed thoroughly, and then dried at 80 °C for 12 h.

2. 2 Preparation of a series of AgIO3/BiVO4

The AgIO3/BiVO4heterojunction was synthesized by a precipitation method. In a typical procedure, 2 mmol BiVO4was dispersed uniformly in 20 mL distilled water to form suspension. Then a certain amount of AgNO3was add into the suspension and stirred vigorously to ensure its complete dissolution. Meanwhile, KIO3with the same molar was also dissolved in 20 mL distilled water and then dropped slowly into the above corresponding suspension with intense stirring. Then the AgIO3/BiVO4composites with different ratios of 20%, 40% and 80% AgIO3were successfully achieved with continuous stirring for another 30 min. All the processes were operated at room temperature. The products were washed with distilled water, collected and dried at 60 ℃ over- night. For comparison, pure AgIO3photocatalyst was also prepared without adding BiVO4.

2. 3 Characterization

X-ray powder diffraction (XRD) patterns of the as-synthesized samples were recorded on an Rigaku MiniFlex 600 diffractometer with Curadiation (40 KV, 15 mA and?= 0.154056 nm) in a range of 2= 10～80°. The morphology of the samples was acquired from the scanning electron microscope on a Hitachi S-3400N SEM spectrometer. The compo- sitions of the samples were characterized by energy dispersive X-ray detector on a FEI-quantax 200 with an accelerating voltage of 20 kV. UV-vis absorption spectra of the as-prepared samples were performed on an Agilent Cary 4000 spectrophotometer. A CHI 660b workstation was used for electrochemical analysis of the samples with the Pt wire, a calomel electrode, and the samples serving as the counter electrode, reference electrode, and working electrode in a three-electrode cell, respectively. For photo- current time analysis, the 300WXe lamp (cutting off< 420 nm) and Na2SO4(0.5 M) were employed as light source and electrolyte. The absorption of RhB and TC in solution was detected by an ultraviolet- visible spectrophotometer (UV759S).

2. 4 Photocatalytic experiments

The photocatalytic activities of the as-fabricated samples were evaluated by the photodegradation of RhB and TC under visible light irradiation. Briefly, 200 mL of TC solution with an initial concentration of 20 mg/L in the presence of solid photocatalyst (50 mg) was stirred for 30 min in a dark place to ensure the adsorption-desorption equilibrium between the antibiotic and photocatalyst. After that, the sus- pension was illuminated by a 300 W Xe lamp (1900 mW/cm2) working as the visible-light source with a 420 nm cut off filter. 3 mL suspension was extracted at certain time intervals and then centrifugated to get rid of the photocatalyst powders, which was then analyzed on a UV759S UV-vis spectrophotometer. The photodegradation of RhB was conducted at the same procedure with the only difference of con- centration of the RhB solution (2 × 10-5mol/L).

2. 5 Radical scavenger experiments

The radical scavenger experiments were repeated by adding 2 mm ethylenediamine tetraacetic acid disodium salt (EDTA-2Na+), 15 mL isopropyl alco- hol (IPA), and 2 mm silver nitrate (AgNO3) which acted as hole (h+) scavenger, hydroxyl radical (·OH) scavenger, and electrons (e-) scavenger, respectively.

3 RESULTS AND DISCUSSION

The phase purity and structure of the prepared samples are presented in Fig. 1 by the charac- terization of X-ray diffraction (XRD). The diffraction peaks of AgIO3and BiVO4samples are in good agreement with pure orthorhombic phase AgIO3(JCPDS 71-2494) and monoclinic phase BiVO4(JCPDS 14-0688) without impure peaks found, indicating the target materials were fabricated suc- cessfully with high purity and crystallinity. For the series of AgIO3/BiVO4photocatalysts, both their characteristic structure patterns match well with those AgIO3and BiVO4, which manifested the coexistence of both AgIO3and BiVO4phases in the AgIO3/BiVO4composites. Meanwhile, it can be seen that the relative intensity of some diffraction peaks, such as (041), (211), (232) and (061), enhanced with the increase of AgIO3loaded on the BiVO4from 20% to 80%, which is in accordance with the synthesis process. The results indicated that the AgIO3and BiVO4were well coupled.

Fig. 1. XRD patterns of BiVO4, AgIO3and AgIO3/BiVO4heterojunctions with different AgIO3contents

Fig. 2a～d show the SEM images of pure AgIO3, BiVO4and 40% AgIO3/BiVO4heterojunction samples. Pure BiVO4particles display mainly cube-like shape with an average length of 1～2 μm, while the shape of pure AgIO3is cubold sheets with the max side length of 3 um and the width of almost 1 μm. For the 40% AgIO3/BiVO4heterojunction (Fig. 2c), the sheet structure of AgIO3adheres well to the surface of the cube BiVO4abundantly scattered, and no obvious changes in size can be observed. The intimate interfaces formed between AgIO3and BiVO4have contributed to the separation of photogenerated charges between two semiconductors, leading to the advance in photocatalytic ef?ciency. Besides, the elemental constitution of the 40% AgIO3/BiVO4heterojunction was further examined by EDS analysis. As shown in Fig. 2d, 40% AgIO3/BiVO4contains O, Bi, Ag, I and V elements, suggesting the heterojunction was composed of both AgIO3and BiVO4. The Al element appearing in EDS is due to the aluminum foil used as the substrate in the EDS test. The results of EDS confirmed that the molar ratio of Ag/Bi equals to 37%, which accords with the expected value in the experiment.

Fig. 2. SEM images of (a) BiVO4 (b) AgIO3 and (c) 40% BiVO4/AgIO3 (d) EDS of 40% BiVO4/AgIO3 heterojunction

Moreover, the absorption performance of a semi- conductor can be evaluated based on the band gap energy of the obtained samples, which can be cal- culated by the following equation based on the Kubelka-Munk theory[17]:

Fig. 3. (a) UV-Vis absorption spectra of AgIO3, BiVO4, 20% AgIO3/BiVO4, 40% AgIO3/BiVO4and 80% AgIO3/BiVO4, respectively; and (b) plot of the ()0.5.of AgIO3and BiVO4

The photocatalytic activities of all as-prepared samples for the degradation of RhB and TC are illustrated in Fig. 4. As shown in Fig. 4a, the degra- dation catalyzed by pure AgIO3and BiVO4could be neglected, whereas all the AgIO3/BiVO4hetero- structures exhibit more excellent visible light photo- catalytic properties than pure AgIO3and BiVO4. Moreover, 40% AgIO3/BiVO4composite exhibits the optimal photocatalytic activity for RhB decolo- rization with nearly 100% photodecomposed rate when the irradiation time sustained to 25 min (As can be seen in Fig. 4c). The degradation rates of 20% AgIO3/BiVO4and80% AgIO3/BiVO4composites for RhB also come to 100% after irradiation for 30 min, just only slightly inferior to that of 40% AgIO3/ BiVO4. However, the photodegradation of RhB couldn’t complete in 30 min in the presence of 5% AgIO3/BiVO4, which may be due to the lower amount of AgIO3doped in the heterojunctions and the interface between AgIO3and BiVO4is not well formed. As is known to all, the interface of the heterojunction formed between two different semi- conductors can greatly affect the photoelectric properties of the hybrid materials. With the content of AgIO3increasing in the composites, more AgIO3/ BiVO4heterojunction interface formed could sup- press the recombination of photoinduced electron- hole pairs, leading to the enhanced phtotcatalytic activity. Nonetheless, when excessive amount of AgIO3with narrow band gap was introduced, the interface between AgIO3and BiVO4may act as recombination centers of electron-hole pairs, which ultimately restrained the photocatalytic activity. Therefore, there must be an optimum ratio in the AgIO3/BiVO4heterostructure, which can induce the best photocatalytic activities of the heterojunctions, just as the experiment results demonstrated[6].

Fig. 4. Photocatalytic activity of pure AgIO3, BiVO4and as-prepared AgIO3/BiVO4heterojunctions assessed by the degradation of RhB (a) and TC (b) under visible light irradiation. UV-vis absorption spectra of RhB (c) and TC (d) solutions after being irradiated with visible light for different time intervals in the present of 40% AgIO3/BiVO4heterojunction

In order to exploit the potential for further application, the degradation experiment of TC anti- biotic was also conducted under visible light irra- diation (> 420 nm). Fig. 4b shows the photo- catalytic degradation rate (C/C0) of TC over the AgIO3, BiVO4and 40% AgIO3/BiVO4. The results were obtained according to the TC normalization concentration in the solution versus the irradiation time from the optical absorbance measurement at 365 nm. All of the as-prepared samples have a positive effect on the photodegradation of TC, as shown in Fig. 4b. At the first 25 min of irradiation time, the TC solution containing 40% AgIO3/BiVO4presents a much faster peak-descending tendency than AgIO3and BiVO4with almost 80% TC elimination, but the degradation rate maintained after 25 min without further removal. The continually reduced absorption indicates the gradually decreased concentration of TC with increasing the reaction time (Fig. 4d). The experiments of degradation of RhB and TC both confirmed that the photocatalytic activity is enhanced after the hybrid of AgIO3and BiVO4. The stability of 40% AgIO3/BiVO4for photodegradation of RhB was conducted by reclaiming and re-examining for three extra cycles, which is displayed in Fig. 5. It can be seen that the degradation rate maintained at about 91.64% after three cycles, indicating that the photo- catalyst has excellent stability and repeatability. A little reduction of the photocatalytic activity may be ascribed to the loss of photocatalyst during the washing and recycling processes.

As is known to all that the reactive species generated in the photocatalytic oxidation process from seminconductors will directly determine the performance of catalyst[18-20]. To investigate the photocatalysis mechanism of RhB degradation in detail, several active species trapping experiments were conducted. AgNO3(2 mm), isopropanol (IPA, 15 mL) and EDTA-2Na+(2 mm) worked as electronics (e-) scavenger, hydroxyl radical (·OH) scavenger and holes (h+) scavenger respectively in RhB degradation under visible light. As depicted in Fig. 6, the RhB was completely decomposed on 40% AgIO3/BiVO4heterojunction after 25 min light irradiation, while the addition of EDTA caused the obvious deactivation of the as-prepared photocatalyst with only 48% of RhB degraded. The degradation rates of RhB with IPA, AgNO3added were 70%, 83% respectively. It can be summarized that h+was the most reactive radical promoted photo-oxidization of RhB under visible light irradiation. On the contrary, the trapping of e-is good for the separation of carried pairs, which is advantage to the photo- catalytic reaction. The most generated radicals responsible for photocatalysis were found to follow a sequence of h+> ·OH > e-.

Fig. 5. Cycling runs in the photocatalytic degradation of RhB in the presence of 40%AgIO3/BiVO4

Fig. 6. Influence of various radical scavengers on the visible-light photocatalytic activity of 40%AgIO3/BiVO4

Photocurrents were measured for as-prepared samples to investigate the transfer efficiency of photogenerated charge[21]. As shown in Fig. 7, pure AgIO3shows almost no photocurrent re- sponse to visible-light owning to the wide band gap. All of the heterojunctions were prompt in generating photocurrent with a reproducible re- sponse to 3 cycles of on/off visible-light irradia- tion. With the increase ratio of AgIO3coupled with BiVO4, the photocurrent responses were 2 and 2.5 times higher than that of the pure BiVO4when the dopant amounts are 20% and 40%, indicating that photogenerated charge carriers in AgIO3are able to separate and subsequently transfer to BiVO4because of the interface created between the AgIO3and BiVO4semiconductors. The continuing in- crease of AgIO3causing the decrease of photo- induced current density may be ascribed to the large number of AgIO3on the surface of BiVO4, which blocks the transport of electrons and turns into the recombination of photoexcited electron- hole pairs. The results are in good accordance with the UV-Vis absorption spectra and photodegra- dation activities described above.

Fig. 7. Transient photocurrents of AgIO3, BiVO4, 20% AgIO3/BiVO4, 0% AgIO3/BiVO4and 80% AgIO3/BiVO4under visible-light irradiation

The photocatalytic mechanism of the enhanced photocatalytic activity of AgIO3/BiVO4heterojunc- tions under visible light involved the separation efficiency of photoinduced charge carriers. Therefore, it is necessary to calculate the CB and VB potentials of AgIO3and BiVO4to analyze the flowchart of photoexcited charge carrier pairs in heterojunctions, which is assisted by Eqs. (2) and (3) on the basis of Mulliken electronegativity theory[7].

E= X – Ec – 0.5(2)

E= E+ Eg (3)

whereEandEare the CB edge potential and VB edge potential, respectively,is the band gap energy of the semiconductors,Eis the energy of free electrons on the hydrogen scale (4.5 eV), and X is the electronegativity of the semiconductor. All of the values are illustrated in Table 1

Table 1. Absolute Electronegativity, Calculated CB Edge, Calculated VB Position and Band Gap Energy for BiVO4 and AgO3 at the Point of Zero Charge

Fig. 8. Schematic electronic band energy diagram of the AgIO3/BiVO4system

It is generally accepted that photocatalytic activity of photocatalysts depends on transfer and separation of photogenerated electron-hole pairs, which were determined by the band edge structure of two com- ponents in heterostructure. Based on the above results and discussion, the possible degradation mechanism of AgIO3/BiVO4under visible light is illustrated in Fig. 7.

The band gaps of AgIO3and BiVO4were calculated to be 3.1 and 2.3 eV, respectively, and both of CB and VB levels of BiVO4are higher than those of AgIO3. Although there is a unfavorable situation that the CB potential of BiVO4is just a little more cathodic than that of AgIO3, the photogenerated electrons can be forced to transfer from the CB of BiVO4to AgIO3under the irradiation of visible-light, while the photoinduced holes would spontaneously migrate from VB of AgIO3to the VB of BiVO4, which signi?cantly contributes to reduce the charge recombination within the BiVO4nanoparticles. The holes at the VB of BiVO4reacted with the H2O absorbed on the surface of the catalyst to generate ·OH to degrade the RhB. As a result, photoinuced electrons and holes were separated effectively by the heterojunction interface between BiVO4and AgIO3, resulting in the enhancement of visible-light photocatalytic activities[8].

4 CONCLUSION

In summary, a series of AgIO3/BiVO4hetero- structure photocatalysts with different ratios of AgIO3have been fabricated via hydrothermal and chemical precipitation methods. All of the as- prepared AgIO3/BiVO4samples showed remarkable enhanced photocatalytic efficiency than pure AgIO3and BiVO4in the photodegradation of RhB and TC antibiotics under visible-light irradiation. Especially, 40% AgIO3/BiVO4exhibited optimum photocatalytic activities among all of the samples. The improved photocatalytic efficiency may be attributed to the synergetic effect between AgIO3and BiVO4in- cluding matching potentials and effective charge separation. Additionally, h+is considered to be the main reactive species in the photodegradation course. This study facilitates the application of AgIO3/BiVO4catalyst in environmental issues.

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4 March 2018;

21 May 2018 (CCDC 1484395)

①This work was supported by the National Natural Science Foundation of China (41672278), the Projects of Fujian Provincial Department of Education (JAT160340), the Fujian Provincial Natural Science Foundation Projects (2017Y4001), the Science and Technology Plan Projects of Fuzhou (2017-G-96, 2017-G-90) the Scientific Research Projects Funded Start-up Funds of Fujian University of Technology (GY-Z160065) and National Undergraduate Training Program for Innovation and Entrepreneurship (201710388074)

. E-mail: mili302@163.com

10.14102/j.cnki.0254-5861.2011-2033