Surface-binding through polyfunction groups of Rhodamine B on composite surface and its high performance photodegradation

Applied Surface Science 366 (2016) 59–66

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Surface-binding through polyfunction groups of Rhodamine B on composite surface and its high performance photodegradation Yiqun Wan a,b , Xiaofen Wang a , Yun Gu a,b , Lan Guo a,b , Zhaodi Xu b,∗ a b

College of Chemistry, Nanchang University, Nanchang 330031, China Center of Analysis and Testing, Nanchang University, Nanchang 330047, China

a r t i c l e

i n f o

Article history: Received 6 November 2015 Received in revised form 30 December 2015 Accepted 6 January 2016 Available online 8 January 2016 Keywords: Photocatalyst Rh B Adsorption Waste water Dye Degradation

a b s t r a c t A kind of novel composite ZnS/In(OH)3 /In2 S3 is synthesized using zinc oxide nanoplates as zinc raw material during hydrothermal process. Although the obtained samples are composited of ZnS and In(OH)3 and In2 S3 phase, the samples possess different structure, morphology and optical absorption property depending on molar ratio of raw materials. Zeta potential analysis indicates different surface electrical property since various content and particle size of the phases. The equilibrium adsorption study confirms the composite ZnS/In(OH)3 /In2 S3 with surface negative charge is good adsorbent for Rhodamine B (Rh B) dye. In addition, the degradation of Rh B over the samples with surface negative charge under visible light ( ≥ 420 nm) is more effective than the samples with surface positive charge. The samples before and after adsorbing Rh B molecule are examined by FTIR spectra and Zetasizer. It is found that the three function groups of Rh B molecule, especially carboxyl group anchors to surface of the sample through electrostatic adsorption, coordination and hydrogen-bond. It contributes to rapid transformation of photogenerated electron to conduction band of In(OH)3 and suppresses the recombination of photogenerated carrier. The possible adsorption modes of Rh B are discussed on the basis of the experiment results. © 2016 Elsevier B.V. All rights reserved.

1. Instruction Dyes are widely used in industries and have shown significant increase to color their final products. Meanwhile industrial dyes are becoming one of the largest groups of chemical pollutants and causing significant pollution to groundwater. To prevent environmental pollution, many treatment technologies have been developed including photocatalytic degradation under the assistant of catalyst [1–10]. Since photocatalytic reaction occurs at the surface of catalyst, then the prerequisite for degradation reactions occurrence is the adsorption of dyes on catalyst surface [11,12]. To explore high effective photocatalyst, the understanding of adsorption mode and site of dyes is an important issue. For example, Liu et al. [13] proposed defect sites affected the adsorption structure of Rh B and in the end leaded to different photodegradation product and efficiency. Liu et al. [14] found that sulforhodamineB gave different degradation products when it adsorbed on TiO2 with sulfonate or diethylamino. Several typical dyes, for example Rh B had been investigated for photogradation on single or complex catalysts surface [15–21]. Degradation of Rh B on the surface of

∗ Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.apsusc.2016.01.045 0169-4332/© 2016 Elsevier B.V. All rights reserved.

catalysts was previously studied by STM, UV, ESR, X-ray diffraction, and absorption spectroscopy [22–25]. Since Rh B molecule contains three functional groups and varies adsorption mode and site depending on surface property of photocatalysts, therefore photodegradation of Rh B is a complex process and leads to different degradation products and efficiency. Moreover, two degradation mechanisms are involved, direct photodegradation reaction and the successive deethylation of the four ethyl groups [26]. Different catalysts possibly hold different degradation mechanism for Rh B under light irradiation. We have to determine the adsorption modes by analyzing in detail interaction between Rh B and catalyst surface. In this paper, we prepared the multicomponent composite ZnS/In(OH)3 /In2 S3 under the experimental condition and investigated the adsorption and photocatalytic degradation of Rh B in the surface of the samples under the visible light irradiation. When Rh B molecule tightly bonds onto ZnS, In2 S3 and In(OH)3 phase in the composite ZnS/In(OH)3 /In2 S3 through carboxyl and amino group, photogenerated electron and hole can separate rapidly and effectively so that the sample holds high performance photocatalytic degradation activity. Meanwhile, we find one new degradation pathway that after Rh B removing one ethyl group in dark, N,N,N -triethyl rhodamine can simultaneously remove three ethyl and transform to rhodamine rather than successive deethylation

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Table 1 Description, specific surface area and apparent rate constant of the obtained samples. Molar ratio of Zn, In and S

1:1:3

2:1:3

3:1:3

4:1:3

5:1:3

2:1:1

2:1:2

2:1:4

2:1:5

Samples denoted as BET (m2 g−1 ) Apparent rate constant ()

S1 40.50 0.097

S2 44.89 0.099

S3 33.79 0.023

S4 28.34 0.0037

S5 30.66 0.00036

S6 70.42 0.008

S7 21.97 0.0013

S8 30.16 0.088

S9 69.44 0.076

of Rh B. Next the rhodamine is degradated into colorless little molecule. All of these are not reported up to now. 2. Experimental

filter membrane of a syringe to remove the catalyst particles. The concentration of Rh B in the clear supernatant solution was determined by measuring the absorbance at ca. 554 nm with UV–vis spectrophotometer.

2.1. Preparation of ZnO nanoplate

3. Result and discussion

ZnO nanoplate was synthesized by a hydrothermal process at 95◦ C for 24 h using a modified literature method [27]. Typically, 40 mmol of ZnO, 80 mmol of NaOH and 0.014 mmol of sodium citrate were dissolved in 75 mL of ultrapure water. The mixture was stirred for 30 min and then transferred to a 100 mL Teflon autoclave, treated at 95◦ C for 24 h, and then cooled to room temperature in air. The white product was separated by filtration, washed with ultrapure water, and dried at 70 ◦ C for 6 h [28].

3.1. Characterization of the samples synthesized

2.2. Preparation of ZnS-In(OH)3 -In2 S3 photocatalyst The ZnS-In(OH)3 -In2 S3 photocatalyst was synthesized via a hydrothermal method. Typically, 1.0 mmol of In2 (SO4 )3 ·6H2 O, a amount of thioacetamide (TAA) and ZnO nanoplates were added to 75 mL of ultrapure water. The mixture was stirred for 30 min and then transferred to a 100 mL Teflon autoclave. The mixture was heated to 120◦ C and maintained at this temperature for 6 h and then cooled to room temperature in air. The product was separated by filtration, washed with ultrapure water and ethanol, and dried at 70◦ C for 6 h. The obtained samples were denoted in Table 1 depending on molar ratio of raw materials.

Fig. 1 shows the XRD patterns of the samples obtained with different molar ratio of zinc and indium and TAA. From Fig. 1a, changing the molar ratio of zinc and indium, the obtained samples are composed from the cubic phase ZnS (PDF No. 77-2100) and tetragonal phase In2 S3 (PDF No. 73-1366) and cubic phase In(OH)3 (PDF No. 85-1338). Compared with the intensity and wide of diffraction peaks, when the molar ratio of zinc and indium and TAA was 2:1:3, the crystallinity and particle size of In(OH)3 phase are higher than the other four samples. Moreover, when the molar ratio of zinc and indium and TAA was 4:1:3 and 5:1:3, the obtained samples present a new obvious diffraction peak at ca. 12.40 degree,

2.3. Characterization XRD patterns were acquired with a Bede D1 system mul˚ tifunction X-ray diffractometer employing Cu Ka ( = 1.5418 A) radiation. The voltage and current were 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) images were taken with a JSM 6701F field emission scanning electron microscope. Brunauer–Emmett–Teller (BET) surface was determined by nitrogen adsorption–desorption using a Micromeritics ASAP 2020C analyzer. UV–vis diffuse reflectance spectra were obtained using a Hitachi U-4100 spectrophotometer. UV–vis absorption spectra were obtained using a Shimadzu UV-250 1PC spectrophotometer. Fourier transform infrared spectra were recorded at room temperature with a KBr pellet on Nicolet 5700 spectrometer. Zeta potential and conductivity values of the samples in ultrapure water were determined on nano ZS 90 Malvern Zetasizer. 2.4. Catalyst evaluation Photocatalytic reaction was conducted in a 400 mL Pyrex cell. A 300 W Xe lamp was used as the light source removing the ultraviolet light with a 420 nm cutoff filter. A water bath was used to keep constant temperature. In a typical photocatalytic experiment, 50 mg of the as-prepared photocatalyst was added to 250 mL 11.2 mg L−1 of Rh B (Rh B) solution. Prior to illumination, the suspension was magnetically stirred in the dark for 1 h to reach the adsorption–desorption equilibrium of Rh B on the photocatalyst surface. During the visible light irradiation, 5 mL of the reaction suspension was sampled at intervals of 5 min and separated by the

Fig. 1. XRD patterns of the obtained samples with different molar ratio of zinc and indium and thioacetamide.

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Fig. 2. SEM images of the samples obtained by changing molar ratio of zinc and indium and thioacetamide (a, b) S2: Zn/In/S = 2:1:3; (c, d) S5: Zn/In/S = 5:1:3; (e, f) S9: Zn/In/S = 2:1:5.

which is assigned to the (101) lattice face of In2 S3 phase. We speculate that the formation and growth of In2 S3 crystal faces vary under the condition of extremely deficiency of precipitant TAA [29,30]. When fixing the molar ratio of zinc and indium, changing the concentration of TAA, the obtained samples still contain the ZnS and In(OH)3 and In2 S3 phases (Fig. 1b). Obviously, the intensity of diffraction peak of ZnS phase increasingly strengthens, however that of In(OH)3 reduces, which illustrates the crystallinity and particle size of ZnS phase increase and that of In(OH)3 decreases with the increase of concentration of TAA. Fig. 2 shows the images of the representative samples obtained varying molar ratio of raw materials. From Fig. 2a, c and e, the three samples are composited of nanoparticles, microspheres and petalslike grains. The other samples similarly contain the particles with the above three kinds of morphologies. Magnifying these images, we find there are a good number of nanoparticles stacking under the surface of the microspheres in the sample S2. And the petals in

the sample S9 are much more than in the other two samples, and these petals had transformed into marigold-like microspheres in the sample S9. The sample S5 contains least petal-like grains and large number of nanoparticles dense packs together. Therefore, the morphology of the samples reveals the difference of the specific surface area. The specific surface area of the samples S2, S5 and S9 is 44.89, 69.44 and 30.66 m2 g−1 , respectively (Table 1). Among of the three samples, the specific surface area of the sample S9 is the biggest (69.44 m2 g−1 ), and that of the sample S5 is the smallest (30.66 m2 g−1 ) (Table 1). The UV–vis diffuse reflectance spectra of samples synthesized with the different element molar ratio are shown in Fig. 3. All the as-prepared samples have steep absorption edges in the visible light, indicating the absorptions are relevant to the band gap due to the intrinsic transition of the materials rather than the transition from impurity levels [31]. From Fig. 3a, the absorption edges obviously shift to shorter wavelength with the increase of

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Fig. 4. The relationship between Zeta potential of the samples obtained with different molar ratio of zinc and indium and thioacetamide and the adsorption amount of Rh B over the samples under dark condition.

Fig. 3. The UV–vis diffuse reflectance spectra of the samples obtained with different molar ratio of zinc and indium and thioacetamide.

molar concentration of zinc. Fig. 3b shows the absorption edges of the samples red-shift as the concentration of TAA increased. Therefore proper molar ratio of zinc and indium and TAA benefits from extending optical absorption range, which might lead to better photocatalytic efficiency under the visible light irradiation. The adsorption of Rh B is related to the specific surface area and surface electric property of photocatalysts. The specific surface area of all of the samples was listed in Table 1. And combining with the variation tendency of the adsorption amount of Rh B on the obtained samples (Fig. 4), we find the bigger the specific surface area is, the larger the adsorption amount of Rh B is not. For example, the specific surface area of the sample S6 was the maximum (70.42 m2 g−1 ), but its adsorption amount of Rh B was the least. Therefore, the specific surface area is not only one effect factor of the adsorption amount. The Zeta potential also possibly influences the adsorption amount of Rh B in the surface of samples. Fig. 4 shows the relationship between Zeta potential of the samples and the adsorption amount of Rh B over the samples under dark condition. Since Rh B is a cation dye in solution, according to the electrostatic adsorption theory, the more negative charge on the surface of the sample is, the more adsorption amount of Rh B is. Fig. 4a shows the adsorption amount of Rh B decreases and the Zeta potential of the samples changes from negative to positive with the increase of concentration of zinc. And Fig. 4b shows the adverse tendency in the adsorption amount of Rh B and Zeta potential of the

samples with the increase of dosage of thioacetamide compared with Fig. 4a. When the Zeta potential of the samples is positive, the adsorption amount of Rh B is very low. Fig. 4a and b illustrates electrical property of the samples seriously affects the adsorption amount of Rh B. 3.2. Rh B photodegradation on the synthesized samples Fig. 5 shows three representative temporal UV–vis absorption spectral changes for the Rh B solution on the samples S2, S5 and S9 as a function of time under visible light irradiation. From Fig. 5a, the absorption peak of Rh B at around 554 nm over the sample S2 undergoes a fairly large decrease with irradiation time whereas the hypsochromic shifts of the absorption band are considerably insignificant. From the inset of Fig. 5a, continually decrease in absorbance in the UV region of UV–vis spectra can be attributed to degradation of smaller aromatic fragments of Rh B, for example, benzoquinone [32]. It is presumed that there is negligible deethylation, in other word the direct photodegradation of Rh B is predominant. The photodegradation of Rh B over the sample S5 is shown in Fig. 5b, after the light on, the maximum absorption band of the solution gradually shifted from 554 to 532 nm. And the intensity of absorption peak decreases insignificantly. The gradual hypsochromic shifts of the absorption maximum are caused by the N-deethylation of Rh B during light irradiation, which has been confirmed by Watanabe and coworkers [26]. And the literature reported the products obtained by gradually deethylation of Rh B corresponded to the N,N,N triethyl rhodamine (TER, 539 nm), N,N -diethyl rhodamine (DER, 522 nm), N-ethylrhodamine (MER, 510 nm), and rhodamine at 498 nm. Therefore, over the sample S5, according to the maximum absorption wavelength shift range, the deethylation of Rh B to TER

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Fig. 6. The concentration change of Rh B (or its de-ethylated products) during photocatalytic degradation over the samples under visible light irradiation ( ≥ 420 nm). (a) The sample obtained by varying the molar concentration of ZnO and fixing amount of In2 (SO4 )3 ·6H2 O and TAA; (b) the sample obtained by varying the molar concentration of TAA and fixing amount of In2 (SO4 )3 ·6H2 O and ZnO.

Fig. 5. Temporal UV–vis absorption spectral changes for the Rh B solution in the presence of samples S2 (a), S5 (b) and S9 (c) as a function of time under visible light irradiation.

only possibly occurs. The photodegradation of Rh B over the sample S9 is shown in Fig. 5c, while the irradiation time is 0 min, the maximum absorption peak is at 534 nm. There are two absorption peaks of solution corresponding to 534 and 499 nm after 5 min for visible light irradiation. The two absorption peaks intensity gradually decrease with the increase of irradiation time. It is presumed the Rh B molecule removed one ethyl and completely transformed to N,N,N -triethyl rhodamine (TER) before light on. After light on, the TER simultaneously removed three ethyl and transformed to

rhodamine, and then the rhodamine molecule over the sample S9 was photodegraded into colorless species or the other small molecules [11,12,23,32–34]. The molar absorption coefficients εmax of the Rh B and TER and rhodamine is 11.5 × 10−4 , 5.5 × 10−4 and 8.4 × 10−4 [26], respectively. Therefore, according to the Lambert–Beer law and the detected absorbance, we can determine the concentration of the partially de-ethylated Rh B, as is shown in Fig. 6. For the samples S1 and S2, the concentration of Rh B decreases continually under visible light irradiation and ca. 95% Rh B was degraded after 25 min. Their absorption spectra are in accord with Fig. 5a and the maximum wavelength shift is negligible during visible light irradiation. But for the samples S3, S4 and S5, their absorption spectra are in accord with Fig. 5b. The concentrations of Rh B (and/or its deethylated products) first increase during irradiation for 5 min and then slowly decrease, which is instructive of weaker adsorption ability to de-ethylated products than Rh B. During the irradiation for 25 min, degradation efficiency of the samples S1 and S2 is far superior to the samples S3–S5. Changing the molar amount of thioacetamide, the degradation activity of the obtained samples is shown in Fig. 6b. The absorption spectra of the samples S6 and S8 agree with that of the sample S2, which is just like Fig. 5a. And those of the samples S7 and S9 correspond to Fig. 5b and c, respectively. From Fig. 6b, when the

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Fig. 7. Plot of ln(C/C0 ) vs. irradiation time for all of the as-prepared samples.

Fig. 8. FTIR spectra of the samples S2, S5 and S9.

molar ratio of indium and thioacetamide was below 1:3, in other word, the amount of thioacetamide further increased, the obtained samples have good adsorption ability and photodegradation activity. The degradation of Rh B follows pseudo first-order reaction ln(C/C0 ) = kt, herein k is the apparent reaction rate constant, as illustrated in Fig. 7, and kapp values are listed in Table 1. When increasing the amount of zinc oxide, the value of kapp decreases from 0.099 to 3.6 × 10−4 min−1 . However when increasing the amount of TAA, the value of kapp increases from 8.5 × 10−3 to 0.088 min−1 . It illustrates proper molar ratio of zinc and indium and thioacetamide causes the catalyst higher photoactivity. 3.3. Chemical interaction between Rh B and the interface of photocatalysts Since the interfacial electron transfer from the Rh B to photocatalyst is a critical step in the process of photodegradation, then the study of the adsorption site and mode of Rh B on the photocatalyst is necessary. Dissolving Rh B in pure water, the zwitter ionic form prevails, as illustrated in Scheme 1. The carboxylic acid group exists in the deprotonated state, leading to Rh B solution with value of almost 4.7 for pH [16]. According to the inner structure of Rh B, three functional groups (deprotonated carboxylic acid group and two amino groups) possibly interact with the interface of photocatalyst through electrostatic attraction and coordination with metal ions and H-bonding. Infrared vibrational spectroscopy usually provides a powerful tool to distinguish these binding modes. Fig. 8 shows the FTIR spectra of the samples S2, S5 and S9 in the range of high wavenumbers. A broad peak appears at the region 3300–2000 cm−1 due to the strong stretching vibrations of hydrogen bonded, including the adsorbed water molecule (3430 cm−1 ), OH group of In(OH)3

Scheme 1. Existence form of Rh B in pure water.

Fig. 9. FTIR spectra of Rh B molecules adsorbed onto three samples S2 (a), S5 (b), S9 (c) and Rh B powder (d).

(3244 cm−1 ) and hydrogen bond [35]. It implies the three samples contain In(OH)3 phase, which agrees with the results of XRD. Fig. 9 shows the FTIR spectra of the samples S2, S5 and S9 after adsorbing Rh B dye using the Rh B solid powder as a reference. Preparation of the samples adsorbing Rh B is as followed: specifically, 0.050 g of the samples S2, S5 and S9 was added into the 250 mL of Rh B solution with the concentration of 11.2 mg L−1 , respectively. And the suspension was stirred for 1 h in the dark condition in order to reach the adsorption equilibrium and then filtered and dried in the air. For the pure dye, the stretching vibration of the carboxyl group is located at 1710 cm−1 (Fig. 9d). The OH stretching vibration of the adsorbed water was at ca. 3420 cm−1 , the N-aryl band at 1340 cm−1 , the C N band at 1645 cm−1 [36]. After adsorption of Rh B on the samples, the stretching vibration of the carboxyl group at 1710 cm−1 over the three samples disappears. The two new peaks appear at ca. 1630 cm−1 and 1384 cm−1 , which attribute to antisymmetric (as ) and symmetric (a ) stretching vibration of the carboxyl group [37]. This splitting vibration was also observed in the Rh B coordination with TiO2 and Al modified TiO2 [38,39]. The literature reported the frequency difference between the antisymmetric and symmetric stretching vibration ( = as − a ) of sodium salt of Rh B was 182 cm−1 [40]. Generally, the frequency difference between the antisymmetric and symmetric stretching vibration of carboxyl group in various chemical environment is in the

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Scheme 2. The possible adsorption mode and site of the adsorbed Rh B molecule over the sample S2 (I) and S5 (II) and S9 (III).

order of (monodentate) > (ionic) ∼ (bridging) > (bidentate) [22,41–43]. In our work, the value (246 cm−1 ) of carboxyl group is far larger than the ionic type COO− (182 cm−1 ). It is instructive that carboxyl group links with the samples by monodentate coordination rather than ionic type. For the adsorbed Rh B on the sample S2, the intensity of vibration at 1384 cm−1 is far bigger than that of the other two samples and the peak is sharp, which possibly illustrates formation of hydrogen-bond between COO− and OH− of In(OH)3 [44]. As for the samples S5 and S9, after adsorbing Rh B, the vibrations of OH− at ca. 3244 cm−1 and 3126 cm−1 is not obvious. It illustrates the content of In(OH)3 phase in the samples S5 and S9 seriously decreases. This is due to the numerous In(OH)3 phase in the samples S5 and S9 were dissolved in the Rh B acid solution. The particle size of In(OH)3 in the samples S2, S5 and S9 were determined 33.5, 13.5 and 20.8 nm from the XRD patterns (Fig. 1), respectively, according to the scherrer equation, D = 0.89/ˇ cos , where D is mean crystallite size,  is the X-ray wavelength, ˇ is the value of the full width at half maximum of the diffraction peaks, and  is the diffraction angle at the peak maxima [45]. Since the smaller In(OH)3 particle is, its surface energy is higher [46,47], and therefore the easier the particles are dissolved in the acid solution. The surface electric property of the samples after adsorbing Rh B is instructive of adsorption site. Therefore, we determinate the zeta potential and conductivity of the three samples (S2, S5 and S9) before and after adsorbing Rh B molecule. The conductivities of the samples S2, S5 and S9 in ultrapure water were 0.0198, 0.00719 and 0.00427 S cm−1 , respectively. And after the three samples with same mass concentration adsorbing Rh B in ultrapure water, the conductivity values transformed to 0.239, 0.0885 and 0.0379 S cm−1 , respectively. The results show the conductivity of the samples in ultrapure water greatly increases after adsorbing Rh B molecule, which imply the charge concentration of ions in solution magnify. Charge concentration of ions in solution possibly affects the zeta potential of samples. The Zeta potential was 14.2, 1.2 and −16.8 mV for the samples S2, S5 and S9 after adsorbing Rh B, respectively. Comparing with the Zeta potential of the samples S2, S5 and S9 in ultrapure water (Fig. 4), as for the sample S2, the Zeta potential changed from −20.1 mV to 14.2 mV, which indicates that adsorption of Rh B over the sample S2 transforms the surface electron charge from negative to positive. The reasons are as follows: firstly, negative surface charges lead to strong interaction between the sample S2 surface and the positive charged amino groups, in other word, the partial negative charges on surface of sample S2 are neutralized.

Additionally, hydrogen ions are released from carboxyl group when Rh B powder is resolved into ultrapure water. Third, a large number of positive charges in solution are adsorbed onto the surface of the sample Rh B/S2 according to the conductivity data. In conclusion, three functional groups bind to the surface of the sample S2 by electrostatic attraction, hydrogen bonding, and chemical coordination. The adsorption mode can be illustrated in Scheme 2(I). For the sample S2, under the visible light illumination, the photogenerated electrons in conduction band of indium sulfide transfer to the conduction band of In(OH)3 [28]. In the whole process of photogenerated electrons transmission and photodegradation, the adsorbed Rh B molecule is just like a wire, which links In2 S3 and In(OH)3 phase, and speeds up the photogenerated electron transmission [48–50]. Most hydroxyl radicals formed by O2 capturing photogenerated electrons attack the nearest chromophoric system to carboxylic acid group and lead to direct and effective degradation of adsorbed Rh B. For the sample S5, the value of Zeta potential before and after adsorbing Rh B is basically constant, and close to zero, which illustrates the unchanged surface electrical property. Almost surface charge neutrality leads to weak adsorption of Rh B molecule on the sample (see Fig. 4a). Due to electrostatic repulsion, the linkage of positive C N+ to the sample S5 is impossible. However, the surface hydroxyl groups from the adsorbed water may act as the center for the adsorption of Rh B through hydrogen bond interaction with N-aryl group. This argument is verified the slow deethylation step from Fig. 5b. Additionally, the carboxylic group links to the surface of the sample by monodentate coordination (Fig. 9). Therefore, the Rh B molecule should stand up the surface of the sample S5. The adsorption mode can be illustrated in Scheme 2(II). For the sample S9, after adsorption of Rh B molecule, the value of Zeta potential changes from −23.4 mV to −16.8 mV. And the surface electrical property remains unchanged. According to electrostatic adsorption theory, the particular interaction between the sample surface and the positive amino groups of the dye will be favorable. Although the reason of removing an ethyl from the adsorbed Rh B molecule under dark condition is still not clear, N,N,N -triethyl rhodamine (TER, 539 nm) was removed three ethyl by one step and formed rhodamine (see Fig. 5c) during the visible light irradiation. Therefore, we deduce the charged amino and uncharged amino groups should simultaneously interact with the surface of the sample S9. According to the previous analysis result that In(OH)3 phase in the sample S9 is few and without hydrogen bond formation. The Rh B molecule is possibly flat-on the surface of the sample S9. The adsorption mode and site are proposed in Scheme 2(III).

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4. Conclusions In this study, composites ZnS-In(OH)3 -In2 S3 were synthesized by simple hydrothermal method. The effect of the molar ratio of raw materials on the composition, structure and light absorption ability of products was investigated. The photodegradation efficiency of Rh B dye solution of the obtained products under visible light irradiation was explored. It was observed that the adsorption of Rh B molecule was dependent on the surface electrical property of the samples rather than the specific surface area. The samples with negative charged surface had better adsorption capability for Rh B. The degradation efficiency of Rh B solution was dependent on the light absorption ability of the samples, adsorption mode and site of Rh B over the sample, which were affected by composition, particle size and surface electrical property of samples. When the molar ratio of zinc and indium and thioacetamide was 2:1:3, the obtained product had better adsorption capability and degradation efficiency for Rh B dye solution under the visible light irradiation ( ≥ 420 nm). And the molar ratio was 2:1:5, the obtained product had the best adsorption capability due to negative charge surface and bigger specific surface area. In addition, a new degradation pathway of Rh B over the sample was found. Therefore, our work is important to understanding the adsorption and degradation of organic dye on composite photocatalyst. Acknowledgements The financial support of this study from the National Natural Science Foundation of China (21563020, 21465017, 21366022), the Key Technology R&D Program of Jiangxi Province (20133ACG70002), Jiangxi Province Science and Technology University Ground Plan Project (KJLD 14007), the Research Program of the State Key Laboratory of Food Science and Technology in Nanchang University (SKLF-ZZA-201302), the Research Program of the State Key Laboratory of Food Science and Technology in Nanchang University (SKLF-ZZB-201304) are gratefully acknowledged. References [1] E.S. Aazam, J. Alloy Compd. 644 (2015) 1–6. [2] N. Yusoff, S.A. Ong, L.N. Ho, Y.S. Wong, W. Khalik, Desalin. Water Treat. 54 (2015) 1621–1628. [3] T. Velegraki, E. Hapeshi, D. Fatta-Kassinos, I. Poulios, Appl. Catal. B: Environ. 178 (2015) 2–11. [4] X.H. Liu, Y.L. Yang, X.X. Shi, K.X. Li, J. Hazard. Mater. 283 (2015) 267–275. [5] N. Doss, P. Bernhardt, T. Romero, R. Masson, V. Keller, N. Keller, Appl. Catal. B: Environ. 154 (2014) 301–308. [6] J.H. Shariffuddin, M.I. Jones, D.A. Patterson, Chem. Eng. Res. Des. 91 (2013) 1693–1704. [7] X.J. Lang, X.D. Chen, J.C. Zhao, Chem. Soc. Rev. 43 (2014) 473–486. [8] J.Y. Xu, Y.X. Li, S.Q. Peng, G.X. Lu, S.B. Li, Phys. Chem. Chem. Phys. 15 (2013) 7657–7665. [9] Z.R. Zhu, F.Y. Liu, H.B. Zhang, J.F. Zhang, L. Han, RSC Adv. 5 (2015) 55499–55512.

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