Solvothermal Synthesis of ZnIn2S4 by Alcohol Solvents and Visible Light Photocatalytic Activity on Selective Oxidation and Dye Degradation①

HE Yun-Hui CHEN Zhi-Xin XU Jing-Jing WU Yan XIAO Guang-Can

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Solvothermal Synthesis of ZnIn2S4by Alcohol Solvents and Visible Light Photocatalytic Activity on Selective Oxidation and Dye Degradation①

HE Yun-Huia, b, cCHEN Zhi-Xina, b, c②XU Jing-Jinga, bWU Yana, bXIAO Guang-Cana, b

a(350116)b(350116)c(351100)

A series of hexagonal ZnIn2S4samples with different morphologies have been successfully prepared via a facile solvothermal approach using different alcohol solvents with the optimum synthesis time and temperature. X-ray diffraction, field emission scanning electron microscopy, UV-vis diffuse reflection spectroscopy and photoelectrochemical measurements are employed to determine the properties of the samples. It is found that the solvent has a significant influence on the morphology, optical properties and electronic nature of the samples. The photocatalytic activities of the samples have been evaluated by selective oxidation of benzyl alcohol to benzaldehyde to benzaldehyde and the degradation of methyl orange (MO) under visible light irradiation. The results reveal that the photocatalytic activities of ZnIn2S4are closely related to the reaction solvent. The ethanol-mediated ZnIn2S4exhibits the best photocatalytic performance toward selective oxidation of benzyl alcohol to benzaldehyde and the degradation of dye MO compared to the samples prepared in other solvents, which can be attributed to the integrative effect of the enhanced light absorption intensity and the prolonged lifetime of photogenerated carriers. In addition, a possible mechanism is proposed and discussed. It is expected that our current research could promote further interest on the synthesizing efficient ternary chalcogenides semiconducting materials for environment remediation and organic transformation.

photocatalyst, solvothermal, selective oxidation, methyl orange, visible light;

1 INTRODUCTION

In the field of materials, solvothermal methods have been widely used for the fabrication of nanostructure materials because of the mild synthesis conditions and the simple control of size and morphology of nanomaterials by using different solvents[1-4]. For instance, Johnhave synthe- sized ZnO nanocrystalline materials by the solvothermal method. It is found that the solvent (such as methanol, ethanol and propanol) in the solvothermal synthesis has significant influence on the crystallite, particle sizes and photocatalytic activity of ZnO[2]. As well, different phases of nitrogen-doped titania such as anatase, rutile and brookite are prepared by the solvothermal process in TiCl3-hexamethylenetramine (HMT)-alcohol solu- tion and their photocatalytic activities for the oxidative destruction of nitrogen monoxide under visible-light irradiation are also characterized[5]. These results demonstrate that the solvothermal reaction is becoming a promising method to produce nanosize crystals with soft agglomeration and to control particle morphology and crystalline phase easily. Furthermore, the synthesis solvent has significant influence on the photocatalytic activity of materials.

ZnIn2S4, an important semiconducting material of ternary chalcogenides, has been extensively investi- gated because of its potential applications in charge storage, electrochemical recording, thermoelectricity, and photocatalysis[6-13]. Therefore, enormous atten- tion has been devoted to exploiting the simple, quickly, and clean synthesis process for various ZnIn2S4nanostructures[13-15]. Among these synthesis methods, solvothermal approach has been widely used for the fabrication of ZnIn2S4as the mild synthesis conditions and the simple control of size and morphology of ZnIn2S4by using different solvents[11-14, 16]. For instance, Shenhave reported that hexagonal ZnIn2S4with different morphology and crystallinity are prepared in aqueous-, methanol- and ethylene glycol-mediated conditions via a solvothermal/hydrothermal method. The results demonstrate that aqueous-mediated ZnIn2S4possesses the best crystallinity and micro-structures, which resulted in the highest and most stable photocatalytic activity for hydrogen evolution under visible light[16]. Recently, our group has reported that ZnIn2S4microspheres prepared in ethanol solvent show efficient visible light pho- tocatalytic activity for selective oxidation of benzyl alcohol to benzaldehyde compared to ZnIn2S4prepared in aqueous solvent. Nearly 100% conver- sion along with > 99% selectivity is reached upon ZnIn2S4prepared in ethanol solvent under visible light irradiation (> 420 nm) for 2 h, but only 58% conversion and 57% yield are achieved over ZnIn2S4prepared in aqueous solvent under the same conditions[7].

According to a lot of literature researches, to date, we find that a systematic study of the alcohol solvents, such as ethanol, methanol, isopropyl alcohol, ethylene glycol and glycerol on the properties of ZnIn2S4have not yet been carried out. As a visible-light-driven photocatalyst, it has been extensively investigated and its photocatalytic activity has been found to be influenced by a variety of factors including preparation conditions, particle size, morphology, and crystallinity. For example, features like large surface and good crystallinity have been found to favor higher photocatalytic activity[17]. Against this background, we herein report the synthesis of ZnIn2S4with different alcohol solvents, such as ethanol, methanol, isopropyl alcohol, ethylene glycol and glycerol via a facile solvothermal approach. We investigate the pho- tocatalytic activity and stability of ZnIn2S4toward photocatalytic selective oxidation of benzyl alcohol and the degradation of dye methyl orange (MO) under visible light irradiation, and the underlying photocatalytic reaction mechanism. It is hoped that our current work could widen the application of ternary chalcogenides and open promising prospects for the utilization of ternary chalcogenides as visible light photocatalyst for selective organic transforma- tions and the degradation of organic dyes.

2 EXPERIMENTAL

2. 1 Materials

Zinc chloride (ZnCl2), indium chloride (InCl3·4H2O), thiacetamide (TAA), ethanol (EtOH), methanol (MeOH), isopropyl alcohol (IPA), ethylene glycol (EG) and glycerol (GL) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All materials were used as received without further purification. Deionized water used in the synthesis was obtained from local sources.

2. 2 Preparation of photocatalysts

In a typical procedure, ZnCl2(1 mmol) and InCl3·4H2O (2 mmol) were dissolved in 27 mL of solvent respectively by stirring 0.5 h to form a homogeneous transparent solution, Then, excessive sulfur source (8 mmol) was added into the above mixture solution. After being stirred for another 0.5 h, the mixture solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 140 ℃ for 12 h. The resulting product was cooled at room temperature and recovered by filtration and washed with deionized water and absolute ethanol several times, respec- tively. The final sample was fully dried at 60 ℃ in a vacuum for characterization and pho- tocatalytic reaction. In this paper, ZnIn2S4prepared in ethanol-, methanol-, isopropyl alcohol-, ethylene glycol-, and glycerol-mediated conditions were labeled as ZIS-EtOH, ZIS-MeOH, ZIS-IPA ZIS-EG and ZIS-GL, respectively.

2. 3 Photocatalyst characterization

Crystal phase properties of the samples were analyzed with a Bruker D8 Advance X-ray diffrac- tometer (XRD) using Ni-filtered Curadiation at 40 kV and 40 mA in the 2range from 15o to 80o with a scan rate of 0.02o per second. The optical properties of the samples were characterized by UV-vis diffuse reflectance spectroscopy (DRS) using a UV-vis spectrophotometer (Cary500, Varian Co.), in which BaSO4was used as the internal reflectance standard. The morphology of the samples was determined by a field emission scanning electron microscopy (FESEM) on a FEI Nova NANOSEM 230 instrument. The irradiation source (> 420 nm) was a 300 W Xe arc lamp system, which was the light source for our photocatalytic selective oxidation of benzyl alcohol and the degradation of methyl orange (MO). Photoelectrochemical measu- rements were performed in a homemade three electrode quartz cell with a PAR VMP3Multi po- tential apparatus. Pt plate was used as the counter, and Ag/AgCl electrode as the reference electrode, while the working electrode was prepared on fluoride-tin oxide (FTO) conductor glass. The sample powder (10 mg) was ultrasonicated in 0.5 mL of anhydrous ethanol to disperse it evenly to get slurry. The slurry was spread onto the FTO glass whose side part was previously protected using Scotch tape. The working electrode was dried overnight under ambient conditions. A copper wire was connected to the side part of the working electrode using a conductive tape. Uncoated parts of the electrode were isolated with epoxy resin. The electrolyte was 0.2 M of aqueous Na2SO4solution without additive. The visible light irradiation source was a 300W Xe arc lamp system equipped with a UV cutoff filter (> 420 nm).

2. 4 Photocatalytic activity

The photocatalytic activities of the samples were evaluated by selective oxidation of benzyl alcohol to benzaldehyde and the degradation of MO under the irradiation of visible light.

The photocatalytic selective oxidation of benzyl alcohol was performed as follows. A mixture of benzyl alcohol (0.1 mmol) and 8 mg of catalyst was dissolved in the solvent of benzotrifluoride (BTF, 1.5 mL), which was saturated with pure molecular oxygen. The above mixture was transferred into a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa and stirred for half an hour to make the catalyst blend evenly in the solution. The suspensions were irradiated by a 300W Xe arc lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) with a UV-CUT filter to cut off light of wavelength < 420 nm. After reaction, the mixture was centri- fuged at 12000 rmp for 20 min to completely remove the catalyst particles. The remaining solution was analyzed with an Aglient Gas Chromatograph (GC-7820). Controlled photoactivity experiments using different radical scavengers (ammonium oxalate as a scavenger for photogenerated holes, AgNO3as a scavenger for electrons, tert-butyl alcohol as a scavenger for hydroxyl radicals, and benzoquinone as a scavenger for superoxide radical species) were performed, similar to the above photocatalytic oxidation of alcohols, except that the radical scavengers (0.1 mmol) were added to the reaction system. The conversion of benzyl alcohol, the yield of benzaldehyde, and the selectivity for benzaldehyde were defined as below:

Conversion (%) = [(0–benzylalcohol)/0] × 100

Yield (%) =benzaldehyde/0× 100

Selectivity (%) = [benzaldehyde/(0–benzylalcohol)] × 100

where0is the initial concentration of benzyl alcohol,benzylalcoholandbenzaldehydeare the con- centration of substrate benzyl alcohol and the cor- responding benzaldehyde, respectively at a certain time after the photocatalytic reaction.

The photocatalytic degradation of MO was performed as follows. A 10 mg portion of catalyst was suspended in 40 mL of 10 ppm MO solution. Before irradiation, the suspensions were stirred in the dark for 2 h to ensure the establishment of adsorption-desorption equilibrium. Under ambient conditions and stirring, the quartz tube was exposed to the visible light irradiation (> 420 nm) produced by a 300 W Xe arc lamp. A 3 mL sample solution was taken at a certain time interval during the experiment and centrifuged to remove the catalyst completely. The solution was analyzed on a Varian UV-vis spectrophotometer (Cary-50, Varian Co. Ltd). The percentage of degradation of MO is reported as/0. Here,is the concentration of MO solution at each irradiated time interval, while0is the initial concentration of MO solution after the adsorption- desorption equilibrium is reached. Controlled photoactivity experiments using different radical scavengers (ammonium oxalate as scavenger for photogenerated holes, AgNO3as scavenger for electrons, tert-butyl alcohol as scavenger for hydroxyl radical species, and benzoquinone as scavenger for superoxide radical species) were performed similar to the above photocatalytic experiments except that the radical scavengers (20 mg) were added to the reaction system.

3 RESULTS AND DISCUSSION

Fig. 1 presents the XRD patterns of ZnIn2S4pre- pared in EtOH-, MeOH-, IPA-, EG- and GL-me- diated conditions, respectively. It is clear to see that all samples present similar profiles and the peaks of scattering angles 2values located at 21.6, 27.7, 39.8, 47.8, 52.5, 55.6 and 75.6 can be indexed to (006), (102), (108), (112), (1012), (202) and (213) facet crystal planes of hexagonal ZnIn2S4(JCPDS No. 65-2023), respectively. No other impurities such as ZnS, In2S3, oxides or organic compounds related to reactants, are detected, which is similar to our previous reports[18-20].

Fig. 1. XRD pattern of the ZnIn2S4samples prepared in different alcohols solvents

Fig. 2. UV-vis diffuses reflectance spectra (a) and the (h)1/2versus energy (h) curves (b) of the samples prepared in different alcohols solvents

DRS is a useful tool for characterizing the optical absorption property of photocatalyst, which is viewed as a critical factor to influence its pho- tocatalytic activity[9]. The DRS results acquired from the ZnIn2S4samples are presented in Fig. 2a. All of the spectra exhibit similar absorption intensity in the UV region. However, regarding the visible light region, it is found that the samples show slight red shift with the order of alcohol solvents GL, EG, IPA, MeOH and EtOH, respectively. As shown in Fig. 2b, the estimated band gaps of the as-synthesized samples are about 2.72, 2.73, 2.75, 2.81 and 2.86 eV, approximately, corresponding to ZIS-EtOH, ZIS- MeOH, ZIS-IPA, ZIS-EG and ZIS-GL, respectively. The results demonstrate that the band gaps of ZnIn2S4prepared in different alcohol solvents are obviously different, and the variation in the band gap might lead to different degree of delocalization of photogenerated electron-hole pair, which probably resulted in different photocatalytic efficiency[21].

Fig. 3 shows FESEM images of ZnIn2S4samples with different morphologies synthesized in different alcohol solvents. It can be seen that the solvent species obviously diversified the morphologies of the products when taking a view of the overall images. Fig. 3a presents marigold-like ZIS-EtOH microsphere with numerous nanosheets structure synthesized in ethanol solvent which is similar to the reports in our earlier literatures[18-20]. Similarly, as shown in Fig. 3b, ZIS-MeOH sample is also mainly composed of asymmetric microspheres with dimensions in the range of 5～6m, yet there are some small anomalous nanoparticles appearing on the surface of the microsphere, compared to the novel marigold-like microsphere structure of ZIS-EtOH (Fig. 3a). Morphologies of ZnIn2S4prepared with isopropyl alcohol, ethylene glycol, and glycerol are shown in Fig. 3c～e, which exhibit nanoparticle structures with diameters in the range of 40～50 nm. Comparing these different structures, it can be indicated that the morphologies of the samples are strongly affected by the reaction solvents. And the ethanol solvent is in favor of improving the formation of ZnIn2S4microspheres in present conditions.

Fig. 3. FESEM images of ZnIn2S4samples prepared with different alcohol solvents: (a) EtOH, (b) MeOH, (c) IPA, (d) EG, (e) GL

The photocatalytic activities of the samples are evaluated by selective oxidation of benzyl alcohol to benzaldehyde and the degradation of MO under the irradiation of visible light. As shown in Fig. 4a, ZIS-EtOH exhibits the best visible light photocata- lytic performance toward selective oxidation of benzyl alcohol to benzaldehyde. Under the irradia- tion of 2 h, nearly 100% selectivity along with 98% conversion is reached. For comparison purpose, the photocatalytic performance of ZnIn2S4prepared in other solvents, such as MeOH, IPA, EG and GL, is also performed. For ZIS-GL, under the same reaction condition, the conversion is only 20% along with 18% yield. This manifests that the ZIS-EtOH sample is nearly five times more active than ZIS-GL, and the photocatalytic activities follow the order of ZIS-EtOH > ZIS-MeOH > ZIS-IPA > ZIS-EG > ZIS-GL. The results corroborate that the solvent has a significant influence on the photocatalytic activity of ZnIn2S4for selective oxidation benzyl alcohol to benzaldehyde under visible light irradiation.

In addition, the photocatalytic activity of ZnIn2S4samples also has been evaluated by photocatalytic degradation of dyes, methyl orange (MO), which is well-known organic dye pollutant in wastewater produced from textile and other industrial pro- cesses[22, 23]. Fig. 4b illustrates the photocatalytic activity for the degradation of MO under visible light irradiation over ZnIn2S4samples. When taking a view of the overall activities, it can be clearly seen that ZnIn2S4samples all exhibit obvious catalytic activity for the degradation of MO. The MO solution is gradually photocatalytic degraded by ZnIn2S4samples. The solution is nearly colorless and the value of/0is about zero after 12 min irradiation. ZIS-EtOH shows the best photocatalytic activity compared with other samples under the same conditions. The above photocatalytic activities of ZnIn2S4samples prepared in different alcohol solvents demonstrate that ZIS-EtOH exhibits not only the highest activity for selective oxidation benzyl alcohol to benzaldehyde, but also the best photocatalytic activity for the degradation of MO under the ambient conditions.

Fig. 4. Photocatalytic selective oxidation of benzyl alcohol to benzaldehyde (a) degradation MO (b) over ZnIn2S4samples prepared in different alcohol solvents under visible light irradiation (> 420 nm)

Fig. 5. Transient photocurrent response (a) and (b) Mott-Schottky plots for ZnIn2S4samples in 0.2 M Na2SO4aqueous solution (pH = 6.8) without bias versus Ag/AgCl

In order to clarify the photocatalytic mechanism of a series of ZnIn2S4samples, the photoelec- trochemical characterization was carried out. Fig. 5a shows the photocurrent transient response for the electrodes of ZnIn2S4prepared in different solvents upon visible light irradiation. It can be seen that, with the light switched -on and -off the cycles, ZIS-EtOH has the highest photocurrent transient response under visible light irradiation among these five samples, suggesting the remarkably efficient separation of charge carrier,., electron-hole pairs, which is in agreement with the highest photoactivity of ZIS-EtOH toward selective oxidation of benzyl alcohol to benzaldehyde and degradation of MO[22]. Furthermore, Fig. 5b exhibits the Mott-Schottky plots for ZnIn2S4samples prepared in different alcohol solvents. The positive slope of the-2-plots suggests the expected n-type semiconductor of ZnIn2S4in the nanomaterials[24]. The flatband potentials of ZIS-EtOH, ZIS-MeOH, ZIS-IPA, ZIS-EG and ZIS-GL obtained by the extrapolation of Mott-Schottky plots approximately equals –0.54, –0.58, –0.69, –0.72 and –0.76 V, respectively, which are more negative than the standard reduction potential of O2/·O2-(–0.15 V. NHE)[25]. Therefore, they are thermodynamically permissible for the transformation of photogenerated electrons to the absorbed O2for producing superoxide radicals (·O2-).

To further understand the role of primary active species and underlying reaction mechanism involved for selective oxidation benzyl alcohol to benzaldehyde and the degradation of dye MO over ZIS-EtOH sample under visible light irradiation, we have carried out control experiments with adding scavengers for superoxide radicals (·O2-), electrons (e-), holes (h+) and hydroxyl radicals (·OH), respectively[25, 26]. Fig. 6a shows the photocatalytic activities of ZIS-EtOH for selective oxidation benzyl alcohol to benzaldehyde in the presence of different radical scavengers,, AgNO3scavenger for electrons, ammonium oxalate (AO) scavenger for holes, benzoquinone (BQ) scavenger for ·O2-, and tert-butyl alcohol (TBA) scavenger for ·OH, respectively. It is obvious that when the radical scavenger, benzoquinone (BQ) for superoxide radicals (·O2-) is added into the reaction system, the conversion of benzyl alcohol is significantly inhibited (shown in entry a of Fig. 6a). A similar and obvious inhibition phenomenon for the pho- tocatalytic reaction is also observed when the AgNO3scavenger for electrons is added into the reaction system (shown in entry b of Fig. 6a). The addition of scavenger, AO for holes has a smaller effect on the conversion of benzyl alcohol compared with the influence of AgNO3scavenger for electrons, as reflected by entry c in Fig. 6a. However, there is no significant change for the conversion of benzyl alcohol when using TBA as the radical scavenger for ·OH (see entry d in Fig. 6a, respectively), which is consistent with the absence of hydroxyl radicals in the BTF solvent. However, with regard to the photocatalytic activities of ZIS-EtOH for the degradation of MO under visible light irradiation, the case is quite different, as reflected in Fig. 6b. The adding of BQ into the reaction almost does not affect the photocatalytic degradation of MO at ZIS-EtOH. Obviously, the order of affecting the rate for degradation of MO follows that of h+> ·OH >-> ·O2-. Namely, the h+and ·OH play the most important key role in the degradation of MO, and this observation is in agreement with the common viewpoint on the photocatalytic degradation of organic dyes over semiconductor[9].

Fig. 6. Controlled experiments using different radical scavengers for the photocatalytic selective oxidation of benzyl alcohol (a) and photocatalytic degradation of MO (b) over ZIS-EtOH under visible light irradiation for: a) reaction with benzoquinone (BQ) as a scavenger for superoxide radicals, b) reaction with AgNO3as a scavenger for photogenerated electrons, c) reaction with ammonium oxalate (AO) as a scavenger for photogenerated holes, d) reaction with tert-butyl alcohol (TBA) as scavenger for hydroxyl radicals, and e) the reaction in the absence of radical scavengers

To evaluate the photocatalytic stability of ZIS- EtOH, the recycled experiments for photo- catalytic selective oxidation benzyl alcohol and degradation MO have been performed, and the results are shown in Fig. 7. During the recycled experiments for four times, as shown in Fig. 7a, it is striking to find that the photocatalyst of ZIS-EtOH shows almost no deactivation for the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradia- tion. Namely, the ZIS-EtOH exhibits relatively stable photocatalytic activity during the selective oxidation process. Similarly, it is interesting to observe that the ZIS-EtOH exhibits relatively stable photocatalytic activity for photocatalytic degradation of MO. These results demon- strate that the ZIS-EtOH is relatively stable in the photocatalytic reaction, which complies with the concept of environmental protection and energy saving.

Fig. 7. Recycled photoactivity testing of ZIS-EtOH toward the photocatalytic selective oxidation of benzyl alcohol to benzaldehyde (a) and degradation MO (b) under visible light irradiation

Scheme 1. Illustration of the proposed reaction mechanism for selective oxidation of benzyl alcohol to benzaldehyde and the degradation of MO over the ZnIn2S4sample under the visible light irradiation

Based on the above experiments, a tentative photocatalytic reaction mechanism for selective oxidation benzyl alcohol to benzaldehyde and the degradation of MO over the ZIS-EtOH sample can be proposed, with the specific process shown in Scheme 1. Under the visible light irradiation, the electrons are generated from the valence band (VB) of semiconductor ZIS-EtOH and photo-induced electrons transfer to the conduction band (CB) of ZnIn2S4semiconductor quickly, leaving the holes in the conduction band (CB) of ZnIn2S4. The EtOH solvent in the synthesis procedure is beneficial for improving the lifetime of photogenerated charge carriers, which is evidenced by the above photoe- lectrochemical analysis. This consequently contri- butes to increasing the overall photocatalytic activity. For selective oxidation benzyl alcohol to benzalde- hyde, the photo-stimulated electrons rapidly migrate to the surface of ZnIn2S4, which can be further trapped by molecular oxygen to form superoxide radicals. Ultimately, the benzyl alcohol can be rabidly absorbed on the surface of ZnIn2S4and then be oxidized by the active species, such as holes and activated oxygen (·O2-), forming the benzaldehyde, whereas for the photocatalytic degradation of dye MO, the photo-induced electrons on the surface of ZnIn2S4mainly react with H2O to form the hydroxyl radicals (·OH) because of the different reaction surroundings, and then the adsorbed dye MO can be oxidized by the hydroxyl radicals and holes rapidly to form byproducts.

4 CONCLUSION

In summary, a series of ZnIn2S4samples prepared in different alcohol solvents have been synthesized via a facile solvothermal method. It is found that the reaction solvent has significant effect on the morphology, optical property and electron nature of the final product. Furthermore, ZnIn2S4micros- pheres prepared in ethanol solvent exhibit the enhanced photocatalytic activity for selective oxidation benzyl alcohol to benzaldehyde and the degradation of MO under ambient condition. Such a reaction solvent effect leads more efficient photocatalytic activity. The highest photoactivity of ZnIn2S4microspheres prepared in ethanol solvent can be attributed to the integrative effect of enhanced light absorption intensity and the prolonged lifetime of photogenerated electron-hole pairs. In addition, we have also used the radical scavenger technique to study the role of photoactive species involved in the photocatalytic selective oxidation benzyl alcohol to benzaldehyde and degradation MO. It is expected that our current research could promote further interest on the synthesizing efficient semiconducting materials of ternary chalcogenides for environment remediation.

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27 January 2017;

26 February 2018

① This work was financially supported by the Key Projects of Youth Natural Fund in Fujian Universities, China (JZ160414)

. Tel: +86-591-22863872. E-mail: czx@fzu.edu.cn

10.14102/j.cnki.0254-5861.2011-1902