p-type Ag2S

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Facile Synthesis of n-type (AgIn)xZn2(1-x)S2/p-type Ag2S Nano Composite for Visible Light Photocatalytic Reduction to Detoxify Hexavalent Chromium Hairus Abdullah, and Dong-Hau Kuo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09647 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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ACS Applied Materials & Interfaces

Facile Synthesis of n-type (AgIn)xZn2(1-x)S2/p-type Ag2S Nano Composite for Visible Light Photocatalytic Reduction to Detoxify Hexavalent Chromium

Hairus Abdullah, Dong-Hau Kuo* Department of Materials Science and Engineering, National Taiwan University of Science and Technology, No.43, Sec. 4, Keelung Road, Taipei 10607, Taiwan

1 ACS Paragon Plus Environment


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ABSTRACT: n-type (AgIn)xZn2(1-x)S2/p-type Ag2S nano composites with 10%, 20%, and 30% Ag2S loading were successfully synthesized via the simple solvothermal and sol gel methods. The as-prepared nano composites were characterized and their visible light photocatalytic reductions were tested for detoxification of hexavalent chromium (Cr(VI)). The results showed only 20 mg of the as-prepared nano composites could reduce 100 ml of 20 ppm potassium dichromate by almost 100% in less than 90 min without adding any hole scavenger agents and pH adjustment (pH = 7). The good photocatalytic reduction was related to the narrower bandgap of (AgIn)xZn2(1-x)S2 solid solution because of the hybridized orbitals of Ag, In, Zn and S and low recombination rate of photo-generated electron and hole pairs due to the effectiveness of p-type Ag2S and n-type (AgIn)xZn2(1-x)S2 nano heterojunctions. This work does not only give a contribution to the creation of visible light photocatalysis for wide-bandgap semiconductors, but also extends our technological viewpoints in designing highly efficient metal sulfide photocatalyst. To the best of our knowledge, this work is the first finding of a high photocatalytic reduction of hexavalent chromium under visible light illumination by simultaneously using both concepts of p-n nano heterojunction and solid solution in our photocatalyst design. In this present work, these concepts were used to replace the using of hole scavenger agents which were commonly used by many other works to retard the recombination rate of photo-induced electron and hole pairs for photo degradation of hexavalent chromium.

KEYWORDS: (AgIn)xZn2(1-x)S2, solid solution, Ag2S, p-n nano heterojunction, hexavalent chromium, nano composite.


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1. INTRODUCTION Heavy metal ion in aquatic environment has attracted much environmental concern because of its noxious property and calamitous effect on drinking water.1 Hexavalent chromium (Cr(VI)) as one of the heavy metals is highly toxic not only to human life but also to aqueous environment due to its highly mobile

2,3

and non biodegradability nature.4 Cr(VI) has been

classified as group I human carcinogen by the International Agency for the Research on Cancer (IARC)

5-8

with the LD50 value between 50 and 150 mg/Kg

9

and its permissible

level in drinking water is 0.05 mg/L.10 However, trivalent chromium (Cr(III)) was easy to be precipitated or adsorbed as Cr(OH)3, therefore to reduce Cr(VI) to Cr(III) with low toxicity is highly required to solve the environmental and health problems 11-14. To remediate the Cr(VI), many efforts include ion exchange, evaporation, chemical precipitation, solvent extraction, reverse osmosis, membrane process, and adsorption have been done.15-17 Tian et al. used electro reduction to remove Cr(VI) with polypyrrole-modified electrode.18 Lo et al. removed Cr(VI) by utilizing the reactivity of zero-valent iron.19 Lu et al. used the natural clino-pyrrhotite to remove Cr(VI) from aqueous solution and industrial wastewater.20 Moniem et al. used Simonkolleit-TiO2 to remediate Cr(VI) in wastewater containing organic substances.21 Sun et al. combined adsorption and photocatalysis process in removing Cr(VI) and Cr(III) with TiO2 and TiO2 nanotubes (TNTs).22 Another green method was using organic reductant, but the reaction could not be accelerated without the presence of light and catalyst.23 In all these works, they did not take advantages from photocatalytic reduction based on p-n heterojunction and solid solution concepts in narrowing material bandgap energy for the catalyst design in removing Cr(VI). Among the photocatalyst materials, TiO2 is the most popular one because of its stability, low cost, safety,



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and photosensitivity. TiO2 can be used to reduce Cr(VI) to Cr(III) under UV irradiation.24-29 However, the TiO2 limitation in only 3–4 % absorbance of sunlight spectrum caused a drawback in using TiO2 as a photocatalyst. Therefore, it is necessary to develop a visible light responded photocatalyst to greatly harness the solar light energy and to convert it into chemical energy for the purpose of degrading pollutant species. To obtain visible light-driven materials, the material selection by coupling narrow bandgap with wide bandgap semiconductor materials had been proposed.30,31 Another approach to narrow the bandgap was introducing donor and acceptor level in the energy bands by forming a solid solution among semiconductor materials. The solid solution of (AgIn)xZn2(1-x)S2 was one of the best photocatalysts used for water splitting. Kudo et al. reported that the solid solution of AgInS2 and ZnS with bandgap of 2.3 eV showed a good photocatalysis in H2 evolution from aqueous solution.32 Tsuji et al. also reported that the solid solution of CuInS2 and ZnS could shift the bandgap to 2.3 eV and a high photocatalytic activity was shown.33 The conduction and valence band levels of the solid solution will continuously shift with the ratio amount (x values) in solid solution. Based on this concept, the wide bandgap energy of metal sulfide semiconductor can be tailored to visible light region to increase the efficiency of photocatalytic activity under visible light illumination. To increase the efficiency of photo reactions, the life time of photogenerated electron and hole pairs after photo excitation is crucial. Some research works took advantages from the organic compounds-containing wastewater to fast reduce Cr(VI) to Cr(III), since the organic compounds in reaction medium acted as hole scavengers leading to a better photo carrier separation.21,34 The effective photo carrier separation also can be much improved by coupling the p-type and n-type semiconductor materials.35 The p-n heterojunction concepts had been


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extensively studied for the purpose of decreasing recombination rate of photo carriers in many research works.36-43 At the p-n heterojunction in equilibrium, p-type material has negative charge and n-type material has positive charge in the depletion zone. Once the p-n heterojunction was illuminated, the built-in electric field in it will drift the photogenerated electron to conduction band of n-type material and holes to valence band of p-type material. This mechanism obviously retards the recombination rate between photogenerated electron and hole pairs. The recombination retardation of photocarriers will provide them a chance in photo redox reactions and promote a wide range of chemical reactions. It is well-known that to accomplish redox reactions, any species that had a reduction potential more positive than that of conduction band of semiconductor material can consume electrons (reduction) and the one with a oxidation potential more negative than that of valence band of semiconductor material can consume holes (oxidation).44 In this present work, the photocatalyst designs were based on both of p-n heterojunction and solid solution concepts. Both of these concepts were simultaneously applied on our photocatalysts. These special designs were applied to form p-type Ag2S nano particledecorated n-type (AgIn)xZn2(1-x)S2 nano rod (hereafter (AgIn)xZn2(1-x)S2/Ag2S). Solid solution of (AgIn)xZn2(1-x)S2 was prepared by utilizing a solution route without using the toxic H2S gas. This nano composite had not been studied for the purpose of Cr(VI) detoxification yet. Furthermore, it is expected that the narrow bandgap of Ag2S will cause more photo-excited electron and hole pairs under visible light irradiation and the formation of p-n heterojunction between Ag2S and solid solution of (AgIn)xZn2(1-x)S2 can retard the recombination rate between photo-induced carriers. Moreover, the defects in solid solution will create donor and acceptor levels in the energy bands and narrow the bandgap of (AgIn)xZn2(1-x)S2 solid



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solution. The photocatalytic reduction ability of (AgIn)xZn2(1-x)S2/Ag2S nano composites with different Ag2S contents and their reusability as well as the photo reaction mechanisms were discussed in this paper.

2. EXPERIMENTAL PROCEDURE Materials. All chemical compound used in this work is commercially obtained without any further treatment. Preparation of (AgIn)xZn2(1-x)S2 Nano Rods. Based on the literature report45 to fabricate a high photocatalytic activity of (AgIn)xZn2(1-x)S2 solid solution, the molar ratios of AgNO3, InCl3, and Zn(CH3COO)2·2H2O precursors was set to 1:1:10. In our typical preparation, to prepare about 150 mg of (AgIn)xZn2(1-x)S2 powder, 21.31 mg AgNO3, 27.76 mg InCl3, and 275 mg Zn(CH3COO)2 dihydrate were first dissolved in 40 ml oleylamine. All the precursors were prepared in a three neck flask. In the other vessel, 120 mg of sulfur powder was dissolved in 20 mg oleylamine by ultrasonication treatment for 1 h. The amount of sulfur was excessive to compensate the vaporized sulfur at high temperature. The dissolved sulfur in oleylamine produced a transparent red solution. The first precursor solution was then heated at 120 ⁰C for 2 h under a pumping condition with vigorous stirring to remove all volatile elements from solution. After 2 hour heating, yellowish solution was formed. Under continuously vigorous stirring, nitrogen gas was flowed into the three neck flask and the solution temperature was increased to 220 ⁰C. When the temperature of precursor solution achieved 200 ⁰C, the red sulfur solution was slowly injected into three neck flask to initiate the reaction. The temperature of reaction solution was then held at 220 ⁰C for 1h followed by slowly cooling to room temperature. The obtained precipitate was collected by centrifugation


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and first washed using hexane followed by three time alcohol washing. The brown precipitate of (AgIn)xZn2(1-x)S2 nano rods was then dried using a rotary evaporator. Preparation of (AgIn)xZn2(1-x)S2/Ag2S Nano Composites. The (AgIn)xZn2(1-x)S2/Ag2S nano composites were prepared in three compositions with different Ag2S contents (10%, 20%, and 30%) loaded in the nano composites. In a typical procedure, 6.85 mg AgNO3 and 3.12 mg Na2S were first dissolved in two different vessels containing 10 ml DI water in each vessel. In the other vessel, the 100 mg as-prepared (AgIn)xZn2(1-x)S2 solid solution was then dispersed in 20 mg DI water by ultrasonication treatment for 30 min. After the (AgIn)xZn2(1x)S2

was well-dispersed in solution, AgNO3 solution was added into the (AgIn)xZn2(1-x)S2-

dispersed solution. After adding AgNO3, the solution was then treated with ultrasonication for 5 min. The color of solution would change to dark brown after sonication treatment. To deposit Ag2S on (AgIn)xZn2(1-x)S2 nano rod surfaces, the as-prepared Na2S solution was slowly dropped into AgNO3-added (AgIn)xZn2(1-x)S2 solution under vigorous stirring at room temperature. The reaction solution was then stirred for 10 h and the obtained precipitate was collected by centrifugation and washing with alcohol for three times. The obtained (AgIn)xZn2(1-x)S2/Ag2S nano composite powder with 10 wt% Ag2S loading was finally dried by a rotary evaporator. The Ag2S percentage value was calculated based on the (AgIn)xZn2(1x)S2

weight amount. The 20% and 30% Ag2S loaded in (AgIn)xZn2(1-x)S2/Ag2S nano

composites were also prepared with the same procedure. Characterization. The (AgIn)xZn2(1-x)S2/Ag2S nano composites were examined by fieldemission scanning electron microscopy (FE-SEM, JSM 6500F, JEOL, Tokyo, Japan) and high resolution transmission electron microscope (HRTEM, Tecnai F20 G2, Philips, Netherlands). The element mappings of nano composites were evaluated by STEM (Tecnai



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F20 G2, Philips, Netherlands). Powder X-ray diffraction (XRD) pattern of nano powders and (AgIn)xZn2(1-x)S2/Ag2S nano composites were recorded by Bruker D2-phaser diffractometer using Cu Kα radiation with a wavelength of 1.5418 Å. The UV-Vis diffuse reflectance spectra (DRS) and peak absorbance of Cr(VI) degradation at 542 nm were recorded using a Jasco V-670 UV-Visible-Near IR Spectrophotometer. Photoluminescence (PL) emission spectra were examined by a Jasco FP-8500 UV-Vis spectrofluorometer. X-ray photoelectron spectroscopy (XPS) measurements of nano composite powders before and after being used for photocatalytic experiments were done on a VG ESCA Scientific Theta Probe spectrometer system with Al Kα (1486.6eV) source and 15 ~ 400 mm X-ray spot size by using ion gun operated at 3kV and 1 mA. Photocatalytic Reduction Experiments. To test the photocatalytic reduction of (AgIn)xZn2(1-x)S2/Ag2S nano composite, potassium dichromate was used as a simulated wastewater of Cr(VI) pollutant. Halogen lamp with power of 150 watt was used as a source of light. The lamp was jacketed by annular quartz tube glass which was continuously circulated by cooling water to prevent the heating from halogen lamp during photocatalytic reduction experiment. The whole set of halogen lamp was immersed into a quartz reactor containing potassium dichromate solution. The experiment was done under vigorous stirring to accelerate diffusion of chromium ions to catalyst surfaces and prevent sedimentation of catalyst particles in solution. The photocatalytic reduction experiments were conducted with commercial ZnS, Ag2S, (AgIn)xZn2(1-x)S2, and 10%, 20%, and 30% Ag2S-loaded (AgIn)xZn2(1-x)S2/Ag2S nano composites under visible light illumination. Before starting the photocatalytic reduction, the catalysts were first dispersed into potassium dichromate solution and stirred for 30 min in dark to ensure the equilibrium of adsorption and desorption


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between catalyst particles and chromium ions. The time when the solution was irradiated by light was set at t = 0 min, while the time when the catalyst was added into solution was set at t = - 30 min. To ensure the Cr(VI) could be reduced in the presence of nano composite catalyst only under visible light illumination, the 20% Ag2S loaded (AgIn)xZn2(1-x)S2/Ag2S nano composite was dispersed into chromium solution and kept in dark for 90 min under vigorous stirring. The chromium solution then was sampled at different time intervals. After centrifugation, the supernatant sample solution was separated from the catalyst particles. To confirm the reduction of chromium ion, three drops of 5 g/L 1.5-diphenyl carbazide solution and three drops of sulfuric acid were added into the sample solutions46. After mixing with 1.5-diphenyl carbazide and sulfuric acid, the existence of Cr(VI) would give pink color to the solution. The concentration of reduced chromium ions was colorimetrically detected by the UV-Vis absorbance intensity at 542 nm. The amounts of removed Cr(VI) were calculated in terms of C/Co by comparing the peak intensity after t min irradiation (C) and peak intensity at t = 0 min (Co).

3. RESULTS AND DISCUSSION



30

40

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55

60

♦

(311)

♣

(220)

Used 20% (AgIn) x Zn 2(1-x)S 2 /Ag 2 S

Com mercial ZnS

♣

(311)

♦

(311)

♣

(220)

Fresh 20%(AgIn) x Zn 2(1-x)S 2 /Ag 2 S

(111)

Intensity (a.u)

(111)

(a)

35

(220)

25 (111)

20

♣

♦

(311)

(AgIn) x Zn 2(1-x)S 2

(220)

(111)

♦

Ag 2 S

20

25

30

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55

60

2 Theta

46.5 47.0 47.5 48.0 48.5 49.0 55.0 55.5 56.0 56.5 57.0 57.5 58.0

(b) δ = 0.4°

Intensity

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(220)

δ = 0.4°

(311)

Commercial ZnS

Commercial ZnS

(220)

(311)

(AgIn)xZn2(1-x)S2

(AgIn)xZn2(1-x)S2

46.5 47.0 47.5 48.0 48.5 49.0 55.0 55.5 56.0 56.5 57.0 57.5 58.0

2θ Figure 1. (a) XRD pattern of Ag2S, (AgIn)xZn2(1-x)S2, ZnS, and the fresh and used (AgIn)xZn2(1-x)S2/20% Ag2S nano composite catalyst and (b) XRD pattern of ZnS and (AgIn)xZn2(1-x)S2 at (220) and (311) reflections.

X-Ray Diffraction Patterns of the as-prepared Ag2S, (AgIn)xZn2(1-x)S2, (AgIn)xZn2(1x)S2/Ag2S,

and commercial ZnS powder. Figure 1a shows XRD patterns of Ag2S,

(AgIn)xZn2(1-x)S2, (AgIn)xZn2(1-x)S2/Ag2S before and after testing, and commercial ZnS 10 ACS Paragon Plus Environment

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powder. To make the peak analysis easier, the peaks of as-prepared Ag2S were compared to those of (AgIn)xZn2(1-x)S2/Ag2S nano composite. The peaks of as-prepared Ag2S nanoparticle were well agreed with reference profile (PDF#140072) of Ag2S. The structure of Ag2S was identified as an acanthite (β-Ag2S) which had a monoclinic structure47,48. The major diffraction pattern of as-prepared (AgIn)xZn2(1-x)S2 nano rod was still attributed to sphalerite ZnS (PDF#050566) with the 2 theta at (220) and (311) shifting to lower angles, as marked in the Figure 1a. To clearly show the shifting peaks, XRD patterns at (220) and (311) were examined carefully with a slower scan rate. The results in Figure 1b show that the peaks of (AgIn)xZn2(1-x)S2 solid solution at (220) and (311) have shifted 0.4⁰ to lower angle. The peak shifting indicated the Ag+ and In3+ cations occupied the Zn2+ lattice sites of ZnS and imposed the lattice parameter became larger, which was caused by substituting the smaller Zn2+ (effective r[Zn2+] = 74 pm of crystal radius)49 with the larger Ag+ (effective r[Ag+] = 115 pm) 49

and In3+ (effective r[In3+]= 80 pm)

49

. Based on Bragg law, the calculated values of d-

spacing at (220) and (311) were increased 0.6% and 0.5%, respectively. The peak shifting was not easy to be observed due to the small amounts (value of x = 0.1 in (AgIn)xZn2(1-x)S2) of Ag and In doped in ZnS lattice. The added amount of Ag and In doped in ZnS lattice was calculated to be 5 at%, but the actual amount was only 2.96 at% based on the EDS results as shown in Figure 3a. The pattern of as-prepared (AgIn)xZn2(1-x)S2 nano rod was well agreed with that of previous work by Wu et al.45. The (AgIn)xZn2(1-x)S2 sphalerite had a face centered cubic structure with a = 5.41 Å. After coating with Ag2S nano particles, the additional peaks of Ag2S were appeared in the (AgIn)xZn2(1-x)S2/Ag2S nano composite patterns. There were no peak differences for the (AgIn)xZn2(1-x)S2/Ag2S nano composites before and after the



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consecutive photoreduction of Cr(VI) for four runs. This result indicated the nano composites of (AgIn)xZn2(1-x)S2/Ag2S were quite stable in our reusability experiment.

Figure 2. FE-SEM microstructure images of (a) (AgIn)xZn2(1-x)S2 nano rods, (b) (AgIn)xZn2(1-x)S2/Ag2S nano composites, and (c) TEM microstructure image of (AgIn)xZn2(1x)S2/Ag2S nano composites. 12 ACS Paragon Plus Environment

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(b)

(c)

Figure 3. (a)(AgIn)xZn2(1-x)S2/Ag2S nano composite with its element mappings and EDS spectra obtained from FE-SEM, (b) Interface between (AgIn)xZn2(1-x)S2 nano rod and Ag2S nano particle with their lattice fringes, and (c) selected area electron diffraction (SAED) pattern.



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(a) Ag+ ions attached to nano rod surfaces

(b) S2- ions from (AgIn)xZn2(1-x)S2 nano rod reacted with the attached Ag+ ions.

(c) Zn2+ ions dissolved and diffused through interfaces

(d) S2- ions from Na2S reacted with Ag+ to form Ag2S on the Ag2S nucleation.

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(e) The complete formation of Ag2S and ZnS solid solution after reacting with S2- ions. 14 ACS Paragon Plus Environment

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Figure 4. Reaction kinetic illustration in forming Ag2S nanoparticles on (AgIn)xZn2(1-x)S2 nano rod surfaces.

Morphology and Microstructure Analysis of (AgIn)xZn2(1-x)S2 Nano Rods and (AgIn)xZn2(1-x)S2/Ag2S Nano Composites. Figure 2 shows the microstructure images of (AgIn)xZn2(1-x)S2/20%Ag2S nano composites taken with field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). No differences were observed between (AgIn)xZn2(1-x)S2 nano rods and (AgIn)xZn2(1-x)S2/Ag2S nano composites from FE-SEM images shown in Figures 2a and 2b. With the TEM assistance, we easily saw the nano rods but were unable to distinguish Ag2S from (AgIn)xZn2(1-x)S2 nano rod in Figure 2c. The difficult-to-distinction nature indicated the Ag2S nanoparticles that deposited on (AgIn)xZn2(1-x)S2 nano rods surfaces were very tiny. The width of nano rod was closed to ~20 nm with the length variation of 50−200 nm. Nano rods and nano composites in the Figure 2 tended to aggregate due to the nature of nanoparticles. To have a better understanding of nano composite microstructures, high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM) were used to examine the lattice fringes, selected area electron diffraction patterns (SAED), and element mappings as shown in Figure 3. Figure 3a shows a localized FE-SEM image of Ag2S-decorated (AgIn)xZn2(1-x)S2 nano rod. It showed Ag2S nano particles were deposited on nano rod of (AgIn)xZn2(1-x)S2, which was confirmed by element mappings of Zn, S, and Ag from STEM. The element mapping images showed the nano rod was mostly composed of ZnS with Ag2S nanoparticles of 5–20 nm in diameter decorated on nano rod surfaces. It is worth noting that Ag2S nanoparticles are very stable and do not break off even when treated by ultrasonic



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probe. As shown in Figure 3a, the Ag2S nanoparticles were tightly deposited on nano rod surfaces to form (AgIn)xZn2(1-x)S2/Ag2S nano heterojunctions. These heterojunction is propitious to electron transfer between two phases and thus promoting photo reactions. The Ag and In nano particles on nano rods could not be observed by element mappings from STEM due to the small amounts of Ag and In which were doped into ZnS lattice to form the solid solution. The typical amounts of Ag and In nanoparticles were 9.57 at% and 1.48 at%, respectively, confirmed by energy dispersive spectroscopy (EDS) on FE-SEM, as shown in Figure 3a. The amount of Ag was much more than that of In, because EDS detector obtained the Ag signals majorly contributed from Ag2S nanoparticles on nano rod surfaces. Because of the equal molarity of Ag and In being added in preparation and the concern of charge neutrality for forming the stable (AgIn)xZn2(1-x)S2, it can assume that the same molarity of Ag and In remained in nano rods. Therefore, the excessive 8.09 at% Ag was contributed from the Ag2S. The calculated amount of typical Ag2S deposited on nano rod surfaces was ~4 at%. Figure 3b shows the interplanar distance of 3.08 Å, which well agrees with the lattice spacing of (111) plane of monoclinic Ag2S. The interplanar spacing of 1.91 Å corresponding to the (220) plane of Sphalerite ZnS was also observed. These results indicated that both of the (AgIn)xZn2(1-x)S2 nano rod and Ag2S nano particles were well crystallized in nano size, as supported by diffraction spots of (111) for Ag2S and (220) for (AgIn)xZn2(1-x)S2 solid solution in Figure 3c. The Zn element mapping in Figure 3a obviously showed that the positions of Zn and Ag elements overlapped each other with more amounts of Zn elements at the interfaces between Ag2S and (AgIn)xZn2(1-x)S2 nano rod. This result indicates that the nano particle-assembled (AgIn)xZn2(1-x)S2 nano rods reacted with the surface-attached Ag+ ions to form Ag2S due to its lower formation energy. The reactions of forming Ag2S involve


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diffusion mechanisms50. The proposed reaction kinetics of depositing Ag2S nanoparticle on nano rod is shown in Figure 4. The high Zn-mapping content at the interface between (AgIn)xZn2(1-x)S2 and Ag2S indicates that the formation of Ag2S involves the contribution of (AgIn)xZn2(1-x)S2 nano rod. As the AgNO3 solution was added into (AgIn)xZn2(1-x)S2 nano rod-dispersed solution, Ag+ ions would deposit on the surfaces of (AgIn)xZn2(1-x)S2 nano rod or attach to (AgIn)xZn2(1-x)S2-rich surfaces as shown in Figure 4a. The formation of Ag2S nuclei by reacting the surface-attached Ag+ and the S2- source from (AgIn)xZn2(1-x)S2 nano rod caused the Zn2+ ions (together with small amount of Ag+ and In3+) in nano rod were dissolved and diffused out of the nano rods to cover the Ag2S nanoparticles as shown in Figures 4b and 4c. When Na2S solution was added into the solution, the Ag+ ions from the added AgNO3 reacted with the S2- ions from Na2S to form Ag2S on the Ag2S nucleation and the nuclei grew bigger to form the Ag2S nanoparticles. During the growth stage of Ag2S, those dissolved and diffused Zn2+ ions (together with small amount of Ag+ and In3+) also reacted with Na2S to have the highest Zn-mapping concentration located at the nano rod/Ag2S interface (Figure 3a) and the diffused concentration of (AgIn)xZn2(1-x)S2 on Ag2S

Zn 2p 1023.1 eV

5000 4000 3000 2000

Ag 3d 368.6 eV Ag 3d 374.3 eV In 3d 446.7 eV In 3d 454.1 eV

6000

S 2p 163.6 eV S 2p 164.7 eV

(a)

Zn 2p 1046.5 eV

nano particles as shown in Figures 4d and 4e.

Intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000 0

0

200

400

600

800

1000

1200

Binding energy (eV)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. (a) XPS full scan spectrum of (AgIn)xZn2(1-x)S2/Ag2S nano composite and (b) XPS spectra of Ag (3d), In (3d), S (2p) and Zn (2p) peaks present in (AgIn)xZn2(1-x)S2/Ag2S nano composite. XPS Analysis. XPS analysis was carried out to determine the chemical state of each element in (AgIn)xZn2(1-x)S2/Ag2S nano composite. Figure 5 indicates (a) the presence of elements such as Ag, In, Zn, and S in (AgIn)xZn2(1-x)S2/Ag2S nano composite and (b) the binding energy values of those elements. The binding energy values of Ag (3d5/2) and Ag (3d3/2) were 368.6 eV and 374.3eV, respectively. Our observed binding energy values of Ag were in good agreement with literature data51 of 368.3 eV and 374.3 eV. The chemical state of In in nano composite has been observed from the binding energy values of 446.7 eV and 454.1 eV, which were assigned to In (3d5/2) and In (3d3/2) orbitals, respectively. The observed peaks of In were consistent with literature data51. The ranges of In binding energy values


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were 446−447 eV and 453.54–454.54 eV for In (3d5/2) and (3d3/2), respectively. The observed binding energy values of S (2p3/2) and S (2p1/2) were 163.6 eV and 164.7 eV, respectively, which were consistent with literature data51 (164 eV and 165.18 eV). The binding energy values of Zn (2p3/2) and Zn (2p1/2) were also noticed at 1023.1 eV and 1046 eV, respectively. The observed values were closed to literature data51 of 1021.8 eV and 1044.77 eV.

0.6 0.5

(ii) Eg = 2.81 eV (i) Eg = 1.92 eV

3.00E-037 2.50E-037 2.00E-037

2

0.4

4.00E-037 3.50E-037

-1 2

0.7

(b) (α hυ ) (eV.cm )

(iv) (iii) (ii) (i)

(a) 0.8 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2

1.00E-037 5.00E-038

0.1 0.0

1.50E-037

0.00E+000

300

400

500

600

700

800

1.5

Wavelength (nm)

2.0

2.5

3.0

3.5

4.0

hυ (eV)

Figure 6. (a) Diffuse spectra reflectance of (i) Ag2S, (ii) (AgIn)xZn2(1-x)S2, and 20% Ag2Sloaded (AgIn)xZn2(1-x)S2/Ag2S nano composite catalysts (iii) before and (iv) after photocatalytic testing, and (b) the Ag2S and (AgIn)xZn2(1-x)S2 bandgap energy determination by absorbance spectrum fitting.

Diffuse Reflectance Spectra of Ag2S, (AgIn)xZn2(1-x)S2, and (AgIn)xZn2(1-x)S2/Ag2S Before and After Photocatalytic Testing. Figure 6 shows the diffuse reflectance spectra of Ag2S, (AgIn)xZn2(1-x)S2, and (AgIn)xZn2(1-x)S2/Ag2S before and after photocatalytic testing and absorbance spectrum fitting for pure Ag2S and solid solution of (AgIn)xZn2(1-x)S2. Figure 6a showed Ag2S nano particles had a strong visible light absorbance with the calculated optical bandgap of 1.92 eV. The bandgap of Ag2S could be tailored between 1.64–2.51 eV



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due to the lattice distortion caused by coexisting of monoclinic Ag2S and cubic Ag phases in a quantum dot52. ZnS had a strong absorbance only in UV region with the calculated optical bandgap of 3.68 eV which was consistent with previous works of Borah et al.53 and Üzar et al.54. After forming solid solution, the visible light absorbance of (AgIn)xZn2(1-x)S2 increased and its absorbance edge was shifted to a lower bandgap value. The calculated bandgap of (AgIn)xZn2(1-x)S2 was about 2.81 eV, as shown in Figure 6b. The narrower bandgap energy of (AgIn)xZn2(1-x)S2 was related to the orbital hybridization of Ag 4d and In 5s+5p that respectively contributed to the acceptor and donor levels in the band diagram.32,33 The introducing of Ag and In into the lattices of ZnS respectively caused the valence band became more negative and conduction band became more positive. Therefore, bandgap energy between valence and conduction bands was much narrower. After being used for photocatalytic testing, the nano composite showed a continuous visible light absorption due to the coverage of Cr(OH)3 nanoparticles on the nano composite surfaces, as confirmed from XPS measurement in Figure 8c.

Figure 7. Photocatalytic reduction of hexavalent chromium in the presence of (a) commercial ZnS, (b) Ag2S, (c) (AgIn)xZn2(1-x)S2, and (d) 10%, (e) 20%, (f) 30% Ag2S-loaded 20 ACS Paragon Plus Environment

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(AgIn)xZn2(1-x)S2 nano composite under visible light illumination. (g) 20% Ag2S-loaded (AgIn)xZn2(1-x)S2 nano composite tested in dark condition. The inset (h) with no color degradation is related to results of (g) and the inset (i) with color degradation is related to results of (d).

Photocatalytic Reduction of (AgIn)xZn2(1-x)S2/Ag2S Nano Composite. To test the photocatalytic reduction of (AgIn)xZn2(1-x)S2/Ag2S nano composite, six kinds of catalyst systems including commercial ZnS, Ag2S, (AgIn)xZn2(1-x)S2, and 10%, 20%, and 30% Ag2Sloaded (AgIn)xZn2(1-x)S2/Ag2S nano composites were investigated to reduce the 20 ppm potassium dichromate solution of 100 mL, as shown in Figure 7. To test the photocatalytic reduction of 20 ppm potassium dichromate solution, 20 mg of each catalyst system was used to be dispersed in the solution by treating in sonication bath followed by vigorous stirring for 30 min. The commercial ZnS could adsorb about 8% Cr(VI) at t = 0 min and degrade Cr(VI) by 33% in 90 min. Ag2S nano particles could reduce only 20% of Cr(VI) in 90 min, while the solid solution of (AgIn)xZn2(1-x)S2 could reduce 40% of Cr(VI) in 90 min. The photocatalytic reduction of (AgIn)xZn2(1-x)S2 solid solution was better than that of Ag2S nano particles, because (AgIn)xZn2(1-x)S2 was n-type semiconductor material as confirmed by Hall measurement with an average carrier concentration value of (4.14 ± 0.98) ×1016 cm-3. n-type semiconductor had the higher photocatalytic reduction capability, since electrons were the majority carriers, which were primarily responsible for current transport in (AgIn)xZn2(1-x)S2 solid solution. Ag2S was a p-type semiconductor with a lower bandgap as confirmed by Bose et al.55 and El-Nahass et al.56 Photocatalytic activities of (AgIn)xZn2(1-x)S2 solid solution was better than those of ZnS due to the narrow bandgap energy of (AgIn)xZn2(1-x)S2 after forming solid solution. The deposition of p-type Ag2S nano particles on n-type (AgIn)xZn2(1-x)S2 nano rods give a strong synergistic effect to photocatalytic reduction of Cr(VI), as shown in Figure 21 ACS Paragon Plus Environment


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7. The systems of 10%, 20%, and 30% Ag2S-loaded (AgIn)xZn2(1-x)S2/Ag2S nano composites could adsorb Cr(VI) by 23%, 30% and 22% at t = 0 min, respectively. All the nano composite systems almost had the same performances in reducing 99% Cr(VI) in 90 min under visible light illumination. The reduction of Cr(VI) was confirmed by the color degradation of aliquots after adding diphenyl carbazide and sulfuric acid to the Cr(VI) solution. Figure 7i showed the color degradation from pink to colorless, that indicated the reduction of Cr(VI) in the presence of (AgIn)xZn2(1-x)S2/20% Ag2S nano composites under visible light illumination. To ensure the elimination of Cr(VI) was caused by the photo reduction instead of adsorption, the nano composite system with 20% Ag2S loading was also tested by dispersing it in the Cr(VI) solution under vigorous stirring for 90 min without light illumination. The result shown in Figure 7h exhibited no color degradation, indicated the reduction of Cr(VI) was originated only from photo redox reactions on photocatalyst under visible light illumination. The kinetics of photodegradation reaction was related to the pseudo-first-order kinetics, in which ln(Co/C) = kt with k a constant and t the illumination time. Based on the pseudofirst-order kinetic calculation from the data in Figure 7, the calculated k values for catalyst systems of (AgIn)xZn2(1-x)S2 nano rods, commercial ZnS, Ag2S nano particles, and 10%, 20%, and 30% Ag2S-loaded (AgIn)xZn2(1-x)S2/Ag2S nano composites under visible light illumination and (AgIn)xZn2(1-x)S2/20%Ag2S nano composite without light illumination were 0.00509 min-1, 0.00315 min-1, 0.00236 min-1, 0.05401 min-1, 0.04198 min-1, 0.03379 min-1 and 0.00157 min-1, respectively. The k value of n-type (AgIn)xZn2(1-x)S2 nano rods was 2.16 times higher than that of p-type Ag2S and 1.62 times higher than that of commercial ZnS. With deposition of Ag2S to form (AgIn)xZn2(1-x)S2/Ag2S nano composites, k values were


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more than 10 times higher than that of the (AgIn)xZn2(1-x)S2 single phase but decreased with the increase in the Ag2S-loaded amount. This phenomenon indicates that p-type Ag2S nano particles should not be too much on n-type (AgIn)xZn2(1-x)S2 nano rods, otherwise the Ag2S nano particles would cover most of the (AgIn)xZn2(1-x)S2 nano rod surfaces, which can limit the interaction between Cr(VI) and (AgIn)xZn2(1-x)S2 nano rod. After forming the p-n nano heterojunction, the k value of 10% Ag2S-loaded (AgIn)xZn2(1-x)S2/Ag2S nano composites had a 23-times increase, as compared to that of bare Ag2S nano particles. The result reveals that the photocatalytic activity of nano composite has been greatly improved due to the formation of the p-n nano heterojunction between Ag2S and (AgIn)xZn2(1-x)S2. Furthermore, by comparing the k values of (AgIn)xZn2(1-x)S2/20% Ag2S nano composites with and without the visible light illumination, the photocatalytic activity of the catalyst system with visible light illumination was 27 times higher than that without light illumination. To show the photocatalytic reduction capability of our catalyst, several research works with a relatively equivalent condition were listed in Table 1 for the comparative purpose. Table 1. Comparisons of several photo catalyst systems used to reduce Cr(VI). No

Catalyst system

Dosage of catalyst

Hole scavenger agents

pH value

Light source

Reference

1.

(AgIn)xZn2(1-x)S2 /Ag2S

0.2 g/L

No hole scavenger agents

Visible light

Present work

2. 3. 4. 5.

Ag/TiO2 TiO2 ZnO/kaolin TiO2

1 g/L 2 g/L 1 g/L 2 g/L

Citric acid Citric acid + H2O2 Citric acid Formic acid + Fe(III)

No pH adjustment (pH = 7) pH = 2 pH = 6−8.5 pH = 4 pH = 2.5

UV UV UV UV

(5) (28) (57) (44)

6 7

N-F-codoped TiO2 Polymer-sensitized TiO2 Dye-sensitized ZnO

0.6 g/L 1 g/L

Benzoic acid Phenol

pH = 4 pH = 3

(58) (59)

1 g/L

Alizarin Red S

pH = 7

UV Visible light Visible light

8

(60)



Table 1 obviously shows that the nano composite in the present work can reduce Cr(VI) without the need of hole scavenger agents in its mechanism. This result indicates n-type (AgIn)xZn2(1-x)S2/p-type Ag2S nano composite is efficient in separating photo-induced electron and hole pairs to support photo reactions. Many research works asserted the photo reduction reaction of Cr(VI) was much retarded without involving hole scavenger agents (organic compounds),5,28,44,57-60 however (AgIn)xZn2(1-x)S2/Ag2S nano composite with p-n heterojunction and acceptor-donor level involving in the solid solution of (AgIn)xZn2(1-x)S2 showed a good photo reduction of Cr(VI) without adding any hole scavenger agent and pH adjustment.

(a)

st

1.0

1 run nd 2 run rd 3 run th 4 run

0.8

C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0.0 -30 -15

0

15

30

45

60

75

90

Time (min)

(b)

KeV


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(c)

Figure 8. (a) Reusability of 20% Ag2S-loaded (AgIn)xZn2(1-x)S2/Ag2S nano composite in detoxification of hexavalent chromium under visible light illumination. (b) EDS analysis of (AgIn)xZn2(1-x)S2/Ag2S nano composite was used to identify the existence of chromium element after being used for photocatalytic reduction experiment. (c) XPS spectrum of Cr(III), which was deposited on the surfaces of (AgIn)xZn2(1-x)S2/Ag2S nano composite after being used for photocatalytic reduction experiment.

Reusability of (AgIn)xZn2(1-x)S2/Ag2S Nano Composite. Reusability of (AgIn)xZn2(1x)S2/Ag2S

nano composite was done by repeatedly using the used photocatalyst to reduce the

fresh Cr(VI) solution under visible light illumination. The reusability was conducted for four times with 20 mg of (AgIn)xZn2(1-x)S2/20%Ag2S nano composite to reduce 100 ml of 20 ppm potassium dichromate. Each time after being used, the nano composite was collected from solution by centrifugation followed by washing with alcohol three times and drying in vacuum oven at 80 ⁰C for 12 h before being used for the next run. Figure 8a shows the performances of (AgIn)xZn2(1-x)S2/20%Ag2S nano composite in reducing Cr(VI) for four consecutive runs. The catalyst adsorbed 39.1%, 29.9%, 30.4%, and 30.1% Cr(VI) after sequential tests. The photo reduction performance for the four runs had achieved 99.8%,



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87.4%, 81.4%, and 80.1% of Cr(VI) degradation in a duration period of 90 min. After the 4th utilization of (AgIn)xZn2(1-x)S2/Ag2S nano composites, 4.3 at% chromium element on nano composite surfaces was detected by EDS analysis (Figure 8b). The chromium element was originated from the precipitation of Cr(III) in the form of Cr(OH)3 after photoreduction experiment. The reduction capability of nano composite on Cr(VI) after first run was constantly decreased because of the Cr(OH)3 deposition on nano composite surface as confirmed by XPS spectra in Figure 8c. The XPS result shows the characteristic peaks of Cr(III). The binding energy values of Cr (2p3/2) and Cr (2p1/2) were 577.4 eV and 585.7 eV, respectively, which were consistent with literature data for Cr(III) in Cr(OH)350,61,62. The coverage of Cr(OH)3 on nano composite surfaces limited the interaction between Cr(VI) in solution and catalyst surfaces. The more reusability tests the nano composite catalyst had, the more amounts of Cr(OH)3 covered on the catalyst surfaces and leading to continuous degradation of photoredox ability. The precipitation of Cr(OH)3 after being reduced from Cr(VI) was also confirmed by previous work of J.A. Navio, et al.63 Although the potential oxidation of Cr(III) was more negative than that of water, Cr(OH)3 could not be oxidized back to Cr(VI) after precipitated on (AgIn)xZn2(1-x)S2/Ag2S nano composite surface. As a result, water would be oxidized to oxygen by photogenerated holes during photocatalytic reduction of Cr(VI). Photocatalytic Oxidation of Water by (AgIn)xZn2(1-x)S2/Ag2S Nano Composite. To ensure the role of photogenerated holes of nano composite in oxidizing water to oxygen, the photoreduction of Cr(VI) was done in a 500 mL quartz reactor with a connection to a gas chromatography (GC) system in order to detect the generated oxygen gas during photocatalytic experiment. In this experiment, 160 mg (AgIn)xZn2(1-x)S2/Ag2S nano


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composites were well dispersed in 450 mL of 20 ppm potassium dichromate solution and irradiated by a 150 W halogen lamp under vigorous stirring. To easily observe the oxygen evolution in this experiment, large amounts of catalyst and solution were used. The sampling of the generated gases in reactor was performed by flowing argon gas with the purity of 99.99% as a gas carrier into the GC system in the time interval of 30 min. Before starting the experiment, the reactor containing (AgIn)xZn2(1-x)S2/Ag2S-dispersed K2Cr2O7 aqueous solution was purged for 2 h, then the air from reactor was checked by flowing it into GC system using Ar gas to ensure all oxygen remaining in reactor had been removed. The oxygen gas generated from the photo reaction was detected by TCD (Thermal Conductivity Detector) in GC system. The experimental results of photocatalytic oxidation of water were shown in Figure 9. As can be seen in Figure 9, the maximum TCD potential achieved at t = 2 h and decreased afterwards. This data indicated that most of the Cr(VI) ions in solution had been reduced during the first two hours and, at the same time, water oxidation also achieved the maximum amount of oxygen at 57.2 μmole/g·h. Therefore, our photocatalytic reaction of (AgIn)xZn2(1-x)S2/Ag2S nano composite performs the reduction of Cr(VI) to Cr(OH)3 and the oxidation of water to O2. The result also confirms that our (AgIn)xZn2(1-x)S2/Ag2S nano composite has the ability in prolong the lifetime of the photogenerated charge carriers.



Oxygen evolution (µmole/g.h)

1.0

60 0.8

50 40

0.6

30 0.4

20 10

0.2

0 0

1

2

3

4

5

6

7

8

9

Maximum TCD potential (mVolt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 10

Time (h)

Figure 9. Oxygen evolution and the maximum TCD potential as a result of water oxidation during photocatalytic reduction of Cr(VI) in the presence of (AgIn)xZn2(1-x)S2/Ag2S nano composites under visible light illumination.

Figure 10. Photo reduction mechanisms of (AgIn)xZn2(1-x)S2/Ag2S nano composite catalyst.


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ZnS (AgIn)xZn2(1-x)S2

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(AgIn)xZn2(1-x)S2/Ag2S

450

500

550

600

650

700

750

Wavelength (nm) Figure 11. Photoluminescence emission spectra of ZnS nanoparticles, (AgIn)xZn2(1-x)S2 solid solution, and (AgIn)xZn2(1-x)S2/Ag2S nano composite.

Mechanisms of (AgIn)xZn2(1-x)S2/Ag2S Photocatalytic Activity. To understand the mechanisms of (AgIn)xZn2(1-x)S2/Ag2S nano composite in reducing Cr(VI), the concepts of (AgIn)xZn2(1-x)S2 solid solution in decreasing bandgap energy and the effectiveness of p-n heterojunction in decreasing the recombination rate of photo carriers need to be elucidated. The conduction and valence bands of ZnS are contributed from hybridized orbitals of Zn 4s and 4p and S 3p because of the covalent bonding between them.64-67 After introducing Ag and In to the lattice of ZnS to form a solid solution, the bandgap energy was reduced from 3.63 eV to 2.81 eV, as shown in Figure 6b. The narrowing in bandgap is related to the broadening of the conduction and valence bands. The band broadening is caused by the orbital hybridization induced by Ag and In, as shown in Figure 10. The valence band moves to higher energy due to the hybridization of Ag 4d and S 3p orbitals,31 while the conduction band moves to lower energy due to the hybridization of Zn 4s and In 5s and 5p orbitals.32 For



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comparison, energy bands of pure ZnS and AgInS2 are also shown in Figure 10. By increasing the x value in (AgIn)xZn2(1-x)S2, the energy gap of solid solution will decrease continuously.66 If the x value equaled to zero or one, the band gap would equal to bandgap of ZnS or AgInS2, respectively.68 Based on this concept, the bandgap of nano rods of (AgIn)xZn2(1-x)S2 decreased to 2.81 eV from the original ZnS bandgap value of 3.63 eV. The alteration of (AgIn)xZn2(1-x)S2 bandgap caused a light absorbance red shift to the visible light region. The photo electron and hole pairs would be effectively separated after coupling the ntype (AgIn)xZn2(1-x)S2 nano rods with p-type Ag2S nano particles, as supported by the results of photocatalytic experiments in Figures 7d,e,f. When Ag2S nano particles were attached to the surfaces of (AgIn)xZn2(1-x)S2 nano rods, p-n nano heterojunctions were established at the interfaces. Because of carrier concentration gradients, electrons in n-type (AgIn)xZn2(1-x)S2 would diffuse from (AgIn)xZn2(1-x)S2 nano rods to p-type Ag2S nano particles and holes in pAg2S would diffuse from Ag2S nano particles to n-type (AgIn)xZn2(1-x)S2 nano rods. In equilibrium, there is a depletion zone near by the p-n heterojunction, where p-type Ag2S nano particles had a negative charge zone while n-type (AgIn)xZn2(1-x)S2 nano rods had a positive charge zone. An electric field (ξ) was created at the p-n nano heterojunctions and called contact potential,30 as shown in Figure 10. These mechanisms obviously retarded the recombination rate of photo carriers after being excited by visible light irradiation, as confirmed by photoluminescence (PL) experiment in Figure 11. After exciting with light at 410 nm, ZnS showed a higher light emission intensity compared to that of (AgIn)xZn2(1-x)S2 solid solution in visible light region. However, the (AgIn)xZn2(1-x)S2/Ag2S nano composite showed a relative low emission in visible light region. The results of PL experiment indicated the recombination rate of photo-generated electron and holes was much suppressed for


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(AgIn)xZn2(1-x)S2/Ag2S nano composite during the photo reduction of Cr(VI). PL spectrum is related to the photogenerated electrons and holes transfer behavior and it can be used to represent the recombination of photogenerated charge carriers42,69. Therefore, this work provides an effective approach to prolong the lifetime of photo-induced electrons for executing the photo reduction reaction of Cr(VI) and photo oxidation reaction of water. Single-phase materials with bandgaps sensitive to the visible light, such as (AgIn)xZn2(1-x)S2 and Ag2S, are promising for light harvesting, but to prolong lifetime of the charge carriers is much important, which can be fulfilled by utilizing the concept of p-n diode to design the nano composites.

4. CONCLUSION In summary, n-type (AgIn)xZn2(1-x)S2/p-type Ag2S nano composites with nano rod-shaped (AgIn)xZn2(1-x)S2 solid solution and Ag2S nano particles were fabricated in simple methods and characterized including the test for detoxification of hexavalent chromium. The formations of the solid solution and the p-n nano heterojunction in nano composite photocatalyst are the fundamental design for the photo reduction mechanism of hexavalent chromium. The bandgap narrowing by introducing Ag and In into ZnS lattice causes the broadening of valence and conduction bands that increases light absorbance and induces more photo electron and hole pairs under visible light illumination. The photo-induced electron and hole pairs are then efficiently separated by the built-in electric field at the p-n nano heterojunction between p-type Ag2S and n-type (AgIn)xZn2(1-x)S2. The retardation in the recombination rate obviously provides enough time for photo reactions to fast detoxify hexavalent chromium. The present work strategies will not only contribute to the utilization



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of wide bandgap semiconductor material for visible light photocatalyst but also extends our technological viewpoints in designing highly efficient metal sulfide photocatalyst.

AUTHOR INFORMATION Corresponding Authors * Fax: +011-886-2-27303291, Email: [email protected]

ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan under Grant no. MOST 103-2218-E-011-015

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