Journal of Colloid and Interface Science 422 (2014) 30–37
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Facile synthesis and enhanced visible-light photocatalytic activity of Ag2S nanocrystal-sensitized Ag8W4O16 nanorods Xuefei Wang a, Sha Zhan a, Yan Wang a, Ping Wang a, Huogen Yu a,⇑, Jiaguo Yu b, Changzheng Hu c a
Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China c Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials, Guangxi Zhuang Autonomous Region, Guilin University of Technology, Guilin 541004, PR China b
a r t i c l e
i n f o
Article history: Received 28 November 2013 Accepted 9 February 2014 Available online 20 February 2014 Keywords: Ag2S Ag8W4O16 Ion-exchange Nanocrystals Sensitization Semiconductor Photocatalyst
a b s t r a c t Narrow band-gap (NBG) Ag2S nanocrystals (NCs) attaching on the surface of wide band-gap (WBG) Ag8W4O16 nanorods were prepared by employing a facile in situ anion exchange method with the reaction between S2 and WO2 4 , and the photocatalytic activity was evaluated by the photocatalytic decolorization of methyl orange solution under visible-light irradiation. It was found that in situ anion exchange could uniformly deposit Ag2S NCs on the surface of Ag8W4O16 nanorods, controllably adjust the size, distribution and amount of Ag2S NCs, and solidly connect Ag2S NCs to the Ag8W4O16 nanorods via the replacement of S2 in the solution with lattice WO2 4 on the Ag8W4O16 surface. The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4O16 composite photocatalysts exhibited obviously higher activity compared with the pure Ag8W4O16 and N-TiO2 photocatalysts. On the basis of band structures of Ag2S and Ag8W4O16 semiconductors and the quantum size effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O16 nanorods was proposed to account for the effective visible-light photocatalytic activities. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Heterogeneous semiconductor photocatalysis has been considered to be a potential and promising approach toward solving worldwide environmental and energy-related issues [1–5]. However, the traditional wide band-gap (WBG) semiconductor photocatalysts such as TiO2 still cannot be widely used in practical applications due to their limited visible-light absorption and low photocatalytic efficiency [6]. Thus, a lot of scientific research in the field of photocatalysis has been focused on the modification of the traditional photocatalysts to make them sensitive to visible light and/or to improve the photocatalytic efficiency. To date, various modification methods have been deeply explored, including various anions and/or cations doping [7–11], sensitization of semiconductor photocatalysts with absorbed molecules [12,13], coupling with narrow band-gap semiconductors [14–16], and grafting transition metal ions such as Fe(III) and Cu(II) (or their oxides) [17–22]. Among them, narrow band-gap (NBG) semiconductor ⇑ Corresponding author. Fax: +86 27 87879468. E-mail address: [email protected] (H. Yu). http://dx.doi.org/10.1016/j.jcis.2014.02.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
nanocrystals (NCs) coupled with WBG semiconductor photocatalysts have become one of the most important strategies owing to their special advantages: (i) possessing flexible tunable optical band gaps with wide light absorption by modulating the sizes and compositions of NCs; (ii) a high photocatalytic efficiency by the utilization of hot electrons or the generation of multiple charge carriers with a single photon by the NCs; (iii) the higher photo-stability for the NCs compared with the organic dyes [23–26]. Among the big family of NBG semiconductor NCs, metal chalcogenides (CdS, CdSe, PbS, Bi2S3, etc.) have received a significant interest due to their suitable band gap, easy fabrication, low cost and high performance [27–30]. As an important metal chalcogenide, Ag2S is a direct, narrow band-gap (1.1 eV) semiconductor and has also been extensively used in the fields of photocatalysis such as photocatalytic decomposition of organic pollutants [31,32], photocatalytic H2 production [33] and solar cell [34], owing to its good chemical stability and excellent optical limiting properties [35,36]. To improve the interfacial charge transfer and photocatalytic efficiency, it is highly desirable for uniform distribution and solid connection of the Ag2S NCs on the WBG semiconductor photocatalysts without surfactant agents. However, the
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
reported methods in documents for attaching Ag2S NCs to WBG semiconductor materials (such as TiO2 and ZnO) usually are complex, multi-steps and even inevitably bring about severe aggregation, poor dispersity and loose connection [33,35,37–40]. On the other hand, although surfactant agents adsorbed on the interface between Ag2S NCs and WBG semiconductor materials can alleviate the above problems, they are usually harmful to the photocatalytic activities. Thus, it is crucial and also remains a challenge to develop a new and facile route for the preparation of Ag2S NCs coupling with WBG semiconductor materials without surfactant agents. In our previous studies, WBG Ag8W4O16 nanorods with a diameter of 30–100 nm and a length of ca. 1 lm could be easily prepared and the resulting Ag8W4O16 nanorods showed a cotton-like structure in the aqueous solutions [6]. As a consequence, there are many advantages for Ag8W4O16 nanorods as precursors to prepare NBG nanocrystal-sensitized Ag8W4O16 nanorod photocatalysts via anion exchange reaction because of their high specific surface area and suspensible property. In this study, NBG Ag2S nanocrystal-sensitized WBG Ag8W4O16 nanorod photocatalysts were prepared by a facile in situ anion exchange reaction between S2 and WO2 4 without the addition of any surfactants. It was found that Ag2S NCs could be uniformly and solidly dispersed on the surface of Ag8W4O16 nanorods via the replacement of S2 to WO2 4 , and the sizes and amount of Ag2S NCs could be easily controlled by adjusting the S2 concentration. On the basis of quantum effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O16 nanorods was proposed to account for the enhanced visible-light photocatalytic activities. To the best of our knowledge, this is the first report about the enhanced visible-light photocatalytic activity of Ag2S nanocrystalsensitized Ag8W4O16 nanorod photocatalysts via the quantum effect mechanism. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts. 2. Experimental details All reagents are analytical grade supplied by Shanghai Chemical Reagent Ltd. (PR China) and used as received without further purification. 2.1. Preparation of WBG Ag8W4O16 nanorods The preparation of Ag8W4O16 nanorods was described in our previous study [41,42]. Briefly, the starting aqueous solutions of AgNO3 (0.01 mol L1) and Na2WO4 (0.005 mol L1) were first prepared. The synthesis of Ag8W4O16 nanorods was achieved by a simple precipitation reaction between Ag+ and WO2 4 ions in distilled water. In a typical synthesis, 50 mL of AgNO3 aqueous solution was poured into 50 ml of Na2WO4 aqueous without stirring and then a white suspension was formed immediately and the precipitate showed a cotton-like product. After the reaction solution was incubated at room temperature for 12 h in the dark, the white precipitate was collected, rinsed with distilled water, and dried at room temperature to obtain Ag8W4O16 nanorods. 2.2. Preparation of Ag2S nanocrystal-sensitized Ag8W4O16 nanorods The synthesis of Ag2S NCs attaching on Ag8W4O16 nanorods was achieved by an in situ anion-exchange reaction of Ag8W4O16 nanorods in Na2S aqueous solution in a dark condition. Initially, 0.1 g of Ag8W4O16 nanorods was added into a 10 mL Na2S solution without stirring. After the reaction solution was incubated at room temperature for 6 h, the precipitate was collected, rinsed with distilled water, and dried at 60 °C to obtain Ag2S nanocrystal-sensitized
31
Ag8W4O16 nanorods. To simplify the sample name, the as-prepared samples are referred to as Ag2S/Ag8W4O16 (x), where x representing the amount of Ag2S nanoparticles. When the Na2S concentration was controlled to be 0.05, 0.1, 0.5, 1, and 5 mM, the resulting amount of Ag2S can be calculated to be 0.12, 0.25, 1.25, 2.53, and 13.78 wt%, respectively. For comparison, a pure Ag2S material was also prepared by a direct precipitation reaction of AgNO3 (0.1 M) and Na2S (0.05 M) solutions in the dark.
2.3. Preparation of N-doped TiO2 N-doped TiO2 (N-TiO2) was also prepared according to our previous study [8,43]. Briefly, 17 mL of tetrabutylorthotitanate was added into an NH3H2O solution (NH3 = 0–10 wt%) under stirring. After stirring for another 1 h, the suspension solution was aged at room temperature (25 °C) for 24 h. The resulted suspension was filtrated, washed with distilled water for 4 times and dried at 60 °C for 6 h, and then was calcined at 500 °C for 2 h to obtain N-TiO2. It was found that when the concentration of NH3 was controlled to be 1 wt%, the obtained N-TiO2 showed the highest photocatalytic activity under the visible light irradiation. Thus, in this study, the N-TiO2 prepared from the 1 wt% of NH3 solution was used as the reference in this study.
2.4. Characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima III X-ray diffractometer (Japan) using Cu Ka radiation. Morphological analysis was performed by an S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) and JEM-2100F transmission electron microscopy (TEM, JEOL, Japan). UV–vis absorption spectra were obtained using a UV–visible spectrophotometer (UV-2550, SHIMADZU, Japan). X-ray photoelectron spectroscopy (XPS) measurements were done on a KRATOA XSAM800 XPS system with Mg Ka source. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Nitrogen adsorption–desorption isotherms were obtained on an ASAP 2020 (Micromeritics Instruments, USA) nitrogen adsorption apparatus. The sample was degassed at 60 °C prior to BET measurements. The Brunauer–Emmett–Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.25.
2.5. Photocatalytic activity The evaluation of photocatalytic activity of the prepared samples for the photocatalytic decolorization of methyl orange (MO) aqueous solution was performed at ambient temperature. 0.05 g of the prepared sample was dispersed into 10 ml of MO solution (15 mg/L) in a culture dish with a diameter of ca. 5 cm. A 55 W fluorescence lamp was used at a light source. The average light intensity striking the surface of the reaction solution was about 14,700 lx, as measured by a luxmeter (ZDS-10, Shanghai). The concentration of MO was determined by an UV–visible spectrophotometer (UV-1240, SHIMADZU, Japan). After visible-light irradiation for some time, the reaction solution was centrifuged to measure the concentration of MO. As for the MO aqueous solution with low concentration, its photocatalytic decolorization is a pseudo-first order reaction and its kinetics may be expressed as ln(c0/c) = kt, where k is the apparent rate constant, and c0 and c are the MO concentrations at initial state and after irradiation for t min, respectively [42,43].
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3. Results and discussion 3.1. Microstructures of the Ag8W4O16 nanorods Fig. 1a is the SEM image of the as-prepared Ag8W4O16 sample and clearly shows that the sample is composed of many nanorods with a diameter of 30–100 nm and a length of ca. 1 lm. Fig. 1b is a higher magnification TEM image of the as-prepared Ag8W4O16 sample and reveals the nearly same morphology information as the above SEM image. According to XRD result (Fig. 3a), these nanorods can be attributed to Ag8W4O16 (JCPDS no. 70-1719) with an orthorhombic structure [44]. 3.2. Strategy for the synthesis of the Ag2S/Ag8W4O16 nanorods
3.3. Morphology and microstructures of the Ag2S/Ag8W4O16 (x) nanorods When the Ag8W4O16 nanorods are added into the Na2S solution, the white cotton-like product gradually becomes brown, indicating the formation of Ag2S. Fig. 4a–f shows the SEM images of the Ag2S NCs attaching on the Ag8W4O16 nanorods. Compared with the smooth surface of Ag8W4O16 nanorods shown in Fig. 1, some nanoparticles are loaded over the surface of Ag8W4O16 nanorods in Fig. 4a–f. More specifically, when the Na2S concentration was very low (0.1 mM), some sporadic nanoparticles with a size of 5–10 nm (an average size of ca. 6 nm) were formed on the surface
Fig. 2. Schematic illustration of the formation process for Ag2S/Ag8W4O16 nanorods by ion-exchange route.
Ag2S (JCPDS NO. 14-0072) ♦ ♦♦
-112
e
•: Ag8W4O16
-121 121 -103
♦
Relative intensity (a.u.)
The above prepared Ag8W4O16 nanorods were used as the precursor for the synthesis of Ag2S/Ag8W4O16 nanorods, and the corresponding schematic illustration for the formation process was shown in Fig. 2. When the Ag8W4O16 nanorods are dispersed into the Na2S solution, Ag2S NCs can be rapidly formed on the surface of Ag8W4O16 nanorods via an in situ anion exchange between S2 in the solution and the lattice WO2 4 in the Ag8W4O16 due to a lower solubility of Ag2S (Ksp = 8 1051) than that of the Ag8W4O16 (Ksp = 5.5 1012). If the S2 in the solution reacts directly with dissolved Ag+ to form Ag2S phase, it is possible for us to observe many individual Ag2S particles in the solution. In this study, however, almost no individual Ag2S aggregates can be found in addition to the Ag2S/Ag8W4O16 composite. Therefore, the above results clearly suggest that the Ag2S NCs are possibly in situ and solidly formed on the surface of Ag8W4O16 nanorods by an ion-exchange reaction of S2 in the solution with WO2 4 in the Ag8W4O16 nanorods. In addition, owing to a suspensible property of the cotton-like Ag8W4O16 nanorods in the Na2S solution, it is expected that the Ag2S NCs can uniformly formed on the surface of Ag8W4O16 nanorods (shown below in Fig. 4). As a consequence, a well-defined Ag2S NCs coupling with Ag8W4O16 nanorods is successfully constructed by the present facile route.
x3
♦ :Ag2S
d c b •
a
20
• •
•
30
• • ••
•
• 40
50
• 60
70
2θ (degree) Fig. 3. XRD patterns of different samples: (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.25), (c) Ag2S/Ag8W4O16 (1.25), (d) Ag2S/Ag8W4O16 (13.78) and (e) Ag2S.
of Ag8W4O16 nanorods (Fig. 4a and b, and Fig. S1). With increasing Na2S concentration from 0.1 to 0.5 mM, more nanoparticles with a larger size of 5–25 nm (an average size of ca. 12 nm) were uniformly loaded on the surface of Ag8W4O16 nanorods (Fig. 4c and d, and Fig. S1). When the Na2S concentration was further increased to 5 mM, nanoparticles with a size of 15–30 nm (an average size of ca. 22 nm) were more densely coated on the surface of Ag8W4O16 nanorods (Fig. 4e and f, and Fig. S1). The X-ray energy dispersion spectra (EDS) inserted in Fig. 4a, c and e, reveal that all the above samples include Ag, W, O and S elements, suggesting that these nanoparticles on the surface of Ag8W4O16 nanorods are Ag2S nanoparticles. The weight ratios of Ag2S nanoparticles in these samples Ag2S/Ag8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25), and Ag2S/Ag8W4O16 (13.78) are 0.23, 1.46, and 12.87 wt%, respectively, according to the EDS data. However, in contrast, the sizes of Ag2S sample
Fig. 1. SEM and TEM images of the as-prepared Ag8W4O16 nanorods.
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
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Fig. 4. SEM images of different samples: (a and b) Ag2S/Ag8W4O16 (0.25), (c and d) Ag2S/Ag8W4O16 (1.25), (e and f) Ag2S/Ag8W4O16 (13.78) and (g and h) Ag2S. The inserts in Fig. 4a, c and e are the EDS spectra of Ag2SAg8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25) and Ag2S/Ag8W4O16 (13.78), respectively.
(Fig. 4g and h) prepared from the precipitation reaction of AgNO3 and Na2S solution arrange from 100 to 200 nm due to the extremely small Ksp of Ag2S. The above results strongly suggest that the size of Ag2S nanoparticles can be easily controlled by adjusting the Na2S concentration to control the formation rate of Ag2S phase. More importantly, no individual Ag2S aggregates can be found in the corresponding SEM images of the samples, which indicate that the Ag2S nanoparticles should be in situ and solidly formed on the surface of Ag8W4O16 nanorods by the ion-exchange reaction of S2 with WO2 4 . Furthermore, the BET specific surface area of the samples Ag8W4O16, Ag2S/Ag8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25) and Ag2S/Ag8W4O16 (13.78) are 7.1, 7.3, 8.1 and 8.7 m2/g, respectively, which also indicate the increase about the amount of Ag2S NCs over the surface of Ag8W4O16 nanorods with increasing Na2S concentration.
The representative TEM images of the Ag2S/Ag8W4O16 (1.25) sample are shown in Fig. 5. It can be clearly seen that the morphology of the sample is consistent with the SEM images shown in Fig. 4c and d, which presents a nanorod structure with the average diameter of 30 nm and a length of ca. 1 lm. In addition, the surface of nanorods is decorated with small Ag2S nanoparticles, which have an average size of 8 nm. The further HRTEM image in Fig. 5b indicates that the spacing between adjacent lattice fringes of the nanoparticles is 0.239 nm, which is close to the d spacing of the (2 2 0) plane of Ag2S. Fig. 3 shows the XRD patterns of Ag2S/Ag8W4O16 (x) nanorods. It can be clearly seen that the intensity of the diffraction peaks of the Ag8W4O16 nanorods gradually decreases with increasing Na2S concentration (Fig. 3b–d), indicating the gradual transformation of Ag8W4O16 phase to Ag2S. However, the characteristic peaks associ-
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Fig. 5. The representative TEM images of the Ag2S/Ag8W4O16 (1.25) sample.
ated with Ag2S are not obviously detected in Fig. 3b–d. For comparison, the XRD pattern of pure Ag2S sample is also shown in Fig. 3e, which can match well with the standard monoclinic Ag2S crystal structure (JCPDS no. 14-0072). However, the peak intensity of Ag2S is obviously weaker compared to that of Ag8W4O16. This is a possible reason to explain why Ag2S phase cannot be detected obviously in the Ag2S/Ag8W4O16 (x) nanorods. Another possible reason can be attributed to the small size, limited amount and high dispersion of Ag2S nanoparticles on the surface of Ag8W4O16 nanorods [39]. The surface elemental compositions and chemical status of the Ag2S/Ag8W4O16 nanorods are further analyzed by XPS. Fig. 6A presents a comparison of the XPS spectra of the Ag8W4O16 and Ag2S/ Ag8W4O16 (1.25) samples. For the Ag8W4O16 sample, the Ag, W, O and C elements are observed and the atomic ratio of Ag:W:O is nearly about 2.1:1:4, which indicates the existence for small amounts of noble metal Ag except for Ag8W4O16 owing to the photosensitive property of Ag8W4O16. The photoelectron peak of C element is due to the adventitious hydrocarbon from XPS instrument itself. Compared with the Ag8W4O16 sample, a new S element with
a binding energy at 162 eV (S2p) was found in the Ag2S/Ag8W4O16 (1.25) sample in addition to the Ag, W, O and C elements based on the XPS survey spectra. Fig. 6B presents the high-resolution XPS spectrum of S2p for Ag2S/Ag8W4O16 (1.25). The binding energies corresponding to S2p3/2 and S2p1/2 are 161.5 and 162.7 eV, respectively, indicating the presence of S2 ion, which further demonstrates the formation of Ag2S on the Ag8W4O16 nanorods [45]. Fig. 6C shows the high-resolution Ag3d XPS spectrum of Ag2S/Ag8W4O16 (1.25). The two peaks at approximately 368.6 and 374.5 eV can be ascribed to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively [43,46]. The Ag 3d5/2 and Ag 3d3/2 peaks can be further divided into four different bands, that is, 368.4, 369.8, 374.3, and 375.7 eV, respectively. The banding energies at 368.4 and 374.3 eV are attributed to Ag+ ion, and those at 369.8 and 375.7 eV are attributed to Ag0 species [47]. Fig. 7A shows the UV–vis spectra of the Ag2S/Ag8W4O16 (x) nanorods. As for the Ag8W4O16, an absorption shoulder at 400–500 nm is also found in addition to the sharp band gap absorption at about 400 nm. The visible-light absorption can be attributed to the localized surface plasmon resonance of Ag
B
200
400
600
800
S 2p
S 1/2p
Relative intensity (a.u.)
OKLL
O1s
Ag3p
W4d C1s
W4f
0
W4p
Ag3d
b a
AgMN
S2p
Relative intensity (a.u.)
A
S 3/2p
1000
156
159
Binding energy (eV)
162
165
Binding energy (eV)
C
+
Relative intensity (a.u.)
Ag
Ag 3d +
Ag
0
0
Ag
365
370
Ag
375
Binding energy (eV) Fig. 6. (A) XPS survey spectra of (a) Ag8W4O16 and (b) Ag2S/Ag8W4O16 (1.25); high-resolution of (B) S2p region and (C) Ag3d region for Ag2S/Ag8W4O16 (1.25).
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X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
B 15
A 1.2
12
c d b a
1/2
e d c b a
0.6
0.3
(αhv)
A
0.9 9 6 3
3.15 eV 0.0 300
400
500
600
700
800
0 2.5
3.0
Wavelength / nm
3.5
4.0
Photon energy (eV)
Fig. 7. (A) UV–vis spectra and (B) the plots of the (ahv)1/2 versus photon energy of (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.25), (c) Ag2S/Ag8W4O16 (1.25), (d) Ag2S/Ag8W4O16 (13.78) and (e) Ag2S.
3.4. Photocatalytic activities and mechanism analysis of the Ag2S/ Ag8W4O16 (x) nanorods The photocatalytic performance of the Ag2S/Ag8W4O16 nanorods was evaluated by the photocatalytic decolorization of MO aqueous solution. For comparison, the photocatalytic activity of N-TiO2, Ag8W4O16, and Ag2S, was also tested under the identical experimental conditions. The controlled experimental results suggest that without photocatalyst, the photobleaching of MO under visible light irradiation is negligible (not shown here). Fig. 8 shows the photocatalytic performance of the Ag2S/Ag8W4O16 (x) nanorods. It can be clearly observed that the Ag8W4O16 nanorods show a very low photocatalytic activity with a k value of 0.95 103 min1. When a small amount of Ag2S nanoparticles are loaded on the Ag8W4O16 nanorods, the photocatalytic activity of the Ag2S/ Ag8W4O16 (0.13) is slightly improved and its corresponding k is 2.6 103 min1. With further increase in Ag2S nanoparticles, the resulting Ag2S/Ag8W4O16 (x) sample shows a remarkably enhanced photocatalytic activity. Especially, the resulting Ag2S/Ag8W4O16 (1.25) shows the highest photocatalytic performance with a k of 20.8 103 min1, a value higher than that of Ag8W4O16 by more than 20 times and that of N-TiO2 by 10 times. However,
24 20
3 -1 k 10 / min
nanoparticles coated on the surface of Ag8W4O16 nanorods. For the Ag2S, it shows wide visible-light absorption from 400 to 800 nm due to its narrow band gap of 1.1 eV. When the Ag2S nanoparticles are loaded to the surface of Ag8W4O16 nanorods, the resulting Ag2S/Ag8W4O16 (x) samples show enhanced visible-light absorption from 400 to 800 nm. To further explore the band structures of the Ag2S/Ag8W4O16 (x) samples, the band gap energies can be estimated from a plot of (ahv)1/2 versus photo energy (hv) and the intercept of the tangent to the plot will give a good approximation of the indirect band gap energies of the samples (Fig. 7B). The band gap energy for the Ag8W4O16 nanorods was estimated to be 3.15 eV, which is identical with the previous result [48]. After the surface loading by Ag2S nanoparticles, it is interesting to find that the band gap of the resulting Ag2S/Ag8W4O16 (x) samples show no change compared with that of the Ag8W4O16. In general, the band structure of a semiconductor is mainly determined by its crystal structure. When the S2 ions are doped into the crystal lattices of Ag8W4O16 phase, the electronic structure of Ag8W4O16 phase can be changed, which makes the absorption edge of Ag8W4O16 shift to a longer wavelength. In the present work, it is clear that the Ag2S/Ag8W4O16(x) samples show a same value of the band gap energy as the pure Ag8W4O16 phase (Fig. 7B). Therefore, it was suggested that no S2 ions were incorporated into the crystal lattices of Ag8W4O16 phase and only Ag2S NCs were formed on the surface of Ag8W4O16 nanorods.
16 12 8 4 0 a
b
c
d
e
f
g
h
Fig. 8. The corresponding rate constant (k) for photocatalytic decolorization of MO solution over various samples under visible-light irradiation: (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.13), (c) Ag2S/Ag8W4O16 (0.25), (d) Ag2S/Ag8W4O16 (1.25), (e) Ag2S/Ag8W4O16 (2.5), (f) Ag2S/Ag8W4O16 (13.78), (g) Ag2S and (h) N-TiO2.
further increase in the Ag2S amount causes a rapid reduction of the photocatalytic activity. For the Ag2S/Ag8W4O16 (13.78) sample, its k value decreases to 5.13 103 min1, which is about one fourth of the Ag2S/Ag8W4O16 (1.25). To clarify the potential mechanism for the above photocatalytic results, the band structures of semiconductor photocatalysts are investigated considering their important roles in the photocatalytic reactions. Ag8W4O16 is an indirect band gap semiconductor with a wide band gap of 3.15 eV according to its UV–vis spectrum [48]. Therefore, it only can absorb UV light and do not show visible-light photocatalytic activity. The low visible-light photocatalytic activity of Ag8W4O16 nanorods in Fig. 8 can be attributed to the plasmon resonance absorption of metallic Ag nanoparticles. As for the Ag2S photocatalyst, it shows no visible-light photocatalytic activity though it can strongly absorb visible light (Fig. 7) [49]. This can be well explained by the following band structure analysis of Ag2S. The conduction band (CB) energy level of Ag2S can be calculated according to the equation of Ec = v + 0.5Eg, where Ec, v and Eg are the CB energy level, absolute electronegativity and band gap of semiconductor, respectively. Therefore, the CB and valence band (VB) energy levels of Ag2S are calculated to be ca. +0 V and 1.1 V (vs. SHE), respectively. It is clear that compared with the single-electron reduction of oxygen (O2 + e + H+ ? HO2 (aq), 0.046 V vs SHE) [44,50], the CB electrons of Ag2S show a poor reduction power owing to its more positive potential, thus resulting in a negligible photocatalytic performance. To further explore the photocatalytic mechanism of the
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Fig. 9. Schematic diagrams for the possible photocatalytic mechanism of the Ag2S NCs attaching over Ag8W4O16 nanorods under visible-light irradiation.
Ag2S/Ag8W4O16 samples, the band structures of Ag8W4O16 are also investigated according to the above-mentioned formula, and the corresponding CB and VB energy levels of Ag8W4O16 can be calculated to be ca. 0.1 V and 3.05 V (vs. SHE), respectively. In this case, the Ag2S/Ag8W4O16 composite materials should exhibit no or very low visible-light photocatalytic activity, as the CB position of Ag2S (+0 V vs. SHE) is obviously more positive than that of Ag8W4O16 (0.1 V vs. SHE) and oxygen reduction. However, the experimental results clearly indicated that the Ag2S/Ag8W4O16 nanorods exhibit excellent photocatalytic performance under visible light irradiation (Fig. 8). As a consequence, a photocatalytic mechanism about the quantum effect of Ag2S nanoparticles was proposed and shown in Fig. 9. It is well known that the quantum effect of nanoparticles enables the absorption edge of a semiconductor blue shift, and the conductor position can shift to a more negative potential with the decreasing size. On the basis of the effective mass approximation model, the blue shift of Ag2S NCs relative to the bulk is dominated by the confinement of electrons and holes, as described by the following equation [51]:
2
DEg ðRÞ ¼
h
8m0 R
2
1 1 þ me mh
where DEg(R) is the band gap shift for the crystal radius R, h the Planck’s constant, m0 the electron mass, while me and mh are the effective masses of electrons and holes, respectively. According to the equation, it is clear that the smaller are the radius, the wider are the band gap of Ag2S NCs. Considering the small electron effective mass ðme ¼ 0:13m0 Þ versus the significantly larger hole mass ðmh ¼ 1:14m0 Þ, most of the band-gap increase can be regarded as a shift of the CB energy level to a more negative potential (vs NHE). Thus, Ag2S NCs with a more negative potential are expected to transfer photogenerated electrons from the CB of Ag2S to Ag8W4-
O16 surface to reduce oxygen, while the photogenerated holes are retained on the VB of Ag2S to oxidize organic substances effectively, resulting in a high photocatalytic performance (Fig. 9). However, when the size of Ag2S nanoparticles further increases, the corresponding quantization effect of Ag2S would disappear gradually and the resultant Ag2S/Ag8W4O16 samples show a decreasing photocatalytic performance (such as Ag2S/Ag8W4O16 (13.78)). Similar results of the quantization effect about the Ag2S NCs are also obtained in the previous report [52]. It is well-known that an obvious drawback for metal chalcogenides as a photocatalyst (such as CdS) is its instability, as CdS is easily eroded by the photo-generated holes, which can cause a decreasing photocatalytic performance [53]. Therefore, the cycle performance for the Ag2S/Ag8W4O16 (x) samples is necessary in terms of the stability of photocatalytic materials. In this case, the Ag2S/Ag8W4O16 (1.25) sample with the highest photocatalytic activity is chosen for the cycle performance of the photocatalytic tests. The results (Fig. 10A) indicate that no obvious deactivation can be observed even after five cycles. More specially, the SEM image of the Ag2S/Ag8W4O16 (1.25) sample after five cycles shows that no additional Ag2S nanoparticles can be found except for the Ag2S nanoparticles loaded on the Ag8W4O16 surface, which further indicates that the Ag2S nanoparticles are solidly grafted on the Ag8W4O16 nanorods (Fig. 10B). In addition, the Ag2S/Ag8W4O16 (1.25) sample after five cycles showed a similar XPS spectrum (Fig. S2) and UV–vis spectra (not shown here) compared with the as-prepared Ag2S/Ag8W4O16 sample, which suggests that the Ag2S/Ag8W4O16 composite is a stable photocatalytic material under visible-light irradiation. Moreover, the strong combination of Ag2S with the Ag8W4O16 nanorods can also contribute to the rapid transfer of photogenerated electrons from Ag2S CB to the Ag8W4O16 via their well interface, resulting in an enhanced photocatalytic activity and stability. 4. Conclusion In summary, Ag2S NCs attaching on the surface of WBG Ag8W4O16 nanorods were prepared by employing a facile anion exchange method with the reaction between S2 and WO2 4 . It was found that anion exchange can in situ and uniformly deposit Ag2S on the surface of Ag8W4O16, controllably adjust the size and amount of Ag2S NCs by adjusting the S2 concentration, and solidly connect Ag2S NCs to the Ag8W4O16 surface via the replacement of S2 to WO2 4 . The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4O16 composite photocatalysts with 2.16 wt% Ag2S NCs exhibited the highest activity with a k value of 20.8 103 min1 for the discoloration of MO under visible light illumination, which exceeds that of pure Ag8W4O16 by more than 20 times and that of N-TiO2 by 10 times, respectively. By analyzing the band structure of
-3 -1 Rate constant k 10 / min
A 24 20 16 12 8 4 0
1st
2nd
3th
4th
5th
Fig. 10. (A) Five cycles of photocatalytic decolorization of MO solution for Ag2S/Ag8W4O16 (1.25) under visible-light irradiation; (B) SEM image of Ag2S/Ag8W4O16 (1.25) after the fifth cycle.
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
Ag2S and Ag8W4O16 semiconductors, it was found that the photocatalytic process intensively depended on the Ag2S NCs quantum size effect. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts and narrow band gap semiconductor NCs with turntable band gaps also open up new horizons for harvesting solar light in the visible and infrared region. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (51208396, 21277107, and 61274129), the Program for NCET (NCET-13-0944) and 973 Program (2013CB632402). This work was also financially supported by the Fundamental Research Funds for the Central Universities (Grant 2013-1a-036, 2013-1a-039), the Self-determined and Innovative Research Funds of WUT (Grant 20131049714001) and MinistryProvince Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-Ferrous Metal and featured Materials, Guangxi Zhuang Autonomous Region. We thank FGM group at Wuhan University of Technology for assistance with XRD measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.02.009. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [2] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [3] S.W. Liu, J.G. Yu, B. Cheng, M. Jaroniec, Adv. Colloid Interface Sci. 173 (2012) 35–53. [4] P. Wang, J. Wang, X. Wang, H. Yu, J. Yu, M. Lei, Y. Wang, Appl. Catal. B: Environ. 132–133 (2013) 452–459. [5] P. Wang, J. Wang, T. Ming, X. Wang, H. Yu, J. Yu, Y. Wang, M. Lei, ACS Appl. Mater. Interfaces 5 (2013) 2924–2929. [6] T. Froschl, U. Hormann, P. Kubiak, G. Kucerova, M. Pfanzelt, C.K. Weiss, R.J. Behm, N. Husing, U. Kaiser, K. Landfester, M. Wohlfahrt-Mehrens, Chem. Soc. Rev. 41 (2012) 5313–5360. [7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [8] R. Liu, P. Wang, X. Wang, H. Yu, J. Yu, J. Phys. Chem. C 116 (2012) 17721–17728. [9] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M. Chi, Y. Yin, Angew. Chem. Int. Ed. 50 (2011) 7088–7092. [10] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483–5486. [11] M. Iwasaki, M. Hara, H. Kawada, H. Tada, S. Ito, J. Colloid Interface Sci. 224 (2000) 202–204. [12] M. Gratzel, J. Photochem. Photobiol., C 4 (2003) 145–153. [13] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Coord. Chem. Rev. 248 (2004) 1363–1379. [14] D. Wu, M. Long, ACS Appl. Mater. Interfaces 3 (2011) 4770–4774.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile synthesis and enhanced visible-light photocatalytic activity of Ag2S nanocrystal-sensitized Ag8W4O16 nanorods Xuefei Wang a, Sha Zhan a, Yan Wang a, Ping Wang a, Huogen Yu a,⇑, Jiaguo Yu b, Changzheng Hu c a
Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, PR China State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China c Ministry-Province Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-Ferrous Metal and Featured Materials, Guangxi Zhuang Autonomous Region, Guilin University of Technology, Guilin 541004, PR China b
a r t i c l e
i n f o
Article history: Received 28 November 2013 Accepted 9 February 2014 Available online 20 February 2014 Keywords: Ag2S Ag8W4O16 Ion-exchange Nanocrystals Sensitization Semiconductor Photocatalyst
a b s t r a c t Narrow band-gap (NBG) Ag2S nanocrystals (NCs) attaching on the surface of wide band-gap (WBG) Ag8W4O16 nanorods were prepared by employing a facile in situ anion exchange method with the reaction between S2 and WO2 4 , and the photocatalytic activity was evaluated by the photocatalytic decolorization of methyl orange solution under visible-light irradiation. It was found that in situ anion exchange could uniformly deposit Ag2S NCs on the surface of Ag8W4O16 nanorods, controllably adjust the size, distribution and amount of Ag2S NCs, and solidly connect Ag2S NCs to the Ag8W4O16 nanorods via the replacement of S2 in the solution with lattice WO2 4 on the Ag8W4O16 surface. The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4O16 composite photocatalysts exhibited obviously higher activity compared with the pure Ag8W4O16 and N-TiO2 photocatalysts. On the basis of band structures of Ag2S and Ag8W4O16 semiconductors and the quantum size effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O16 nanorods was proposed to account for the effective visible-light photocatalytic activities. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Heterogeneous semiconductor photocatalysis has been considered to be a potential and promising approach toward solving worldwide environmental and energy-related issues [1–5]. However, the traditional wide band-gap (WBG) semiconductor photocatalysts such as TiO2 still cannot be widely used in practical applications due to their limited visible-light absorption and low photocatalytic efficiency [6]. Thus, a lot of scientific research in the field of photocatalysis has been focused on the modification of the traditional photocatalysts to make them sensitive to visible light and/or to improve the photocatalytic efficiency. To date, various modification methods have been deeply explored, including various anions and/or cations doping [7–11], sensitization of semiconductor photocatalysts with absorbed molecules [12,13], coupling with narrow band-gap semiconductors [14–16], and grafting transition metal ions such as Fe(III) and Cu(II) (or their oxides) [17–22]. Among them, narrow band-gap (NBG) semiconductor ⇑ Corresponding author. Fax: +86 27 87879468. E-mail address: [email protected] (H. Yu). http://dx.doi.org/10.1016/j.jcis.2014.02.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
nanocrystals (NCs) coupled with WBG semiconductor photocatalysts have become one of the most important strategies owing to their special advantages: (i) possessing flexible tunable optical band gaps with wide light absorption by modulating the sizes and compositions of NCs; (ii) a high photocatalytic efficiency by the utilization of hot electrons or the generation of multiple charge carriers with a single photon by the NCs; (iii) the higher photo-stability for the NCs compared with the organic dyes [23–26]. Among the big family of NBG semiconductor NCs, metal chalcogenides (CdS, CdSe, PbS, Bi2S3, etc.) have received a significant interest due to their suitable band gap, easy fabrication, low cost and high performance [27–30]. As an important metal chalcogenide, Ag2S is a direct, narrow band-gap (1.1 eV) semiconductor and has also been extensively used in the fields of photocatalysis such as photocatalytic decomposition of organic pollutants [31,32], photocatalytic H2 production [33] and solar cell [34], owing to its good chemical stability and excellent optical limiting properties [35,36]. To improve the interfacial charge transfer and photocatalytic efficiency, it is highly desirable for uniform distribution and solid connection of the Ag2S NCs on the WBG semiconductor photocatalysts without surfactant agents. However, the
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
reported methods in documents for attaching Ag2S NCs to WBG semiconductor materials (such as TiO2 and ZnO) usually are complex, multi-steps and even inevitably bring about severe aggregation, poor dispersity and loose connection [33,35,37–40]. On the other hand, although surfactant agents adsorbed on the interface between Ag2S NCs and WBG semiconductor materials can alleviate the above problems, they are usually harmful to the photocatalytic activities. Thus, it is crucial and also remains a challenge to develop a new and facile route for the preparation of Ag2S NCs coupling with WBG semiconductor materials without surfactant agents. In our previous studies, WBG Ag8W4O16 nanorods with a diameter of 30–100 nm and a length of ca. 1 lm could be easily prepared and the resulting Ag8W4O16 nanorods showed a cotton-like structure in the aqueous solutions [6]. As a consequence, there are many advantages for Ag8W4O16 nanorods as precursors to prepare NBG nanocrystal-sensitized Ag8W4O16 nanorod photocatalysts via anion exchange reaction because of their high specific surface area and suspensible property. In this study, NBG Ag2S nanocrystal-sensitized WBG Ag8W4O16 nanorod photocatalysts were prepared by a facile in situ anion exchange reaction between S2 and WO2 4 without the addition of any surfactants. It was found that Ag2S NCs could be uniformly and solidly dispersed on the surface of Ag8W4O16 nanorods via the replacement of S2 to WO2 4 , and the sizes and amount of Ag2S NCs could be easily controlled by adjusting the S2 concentration. On the basis of quantum effect of Ag2S NCs, a possible photocatalytic mechanism about the Ag2S nanocrystal-sensitized Ag8W4O16 nanorods was proposed to account for the enhanced visible-light photocatalytic activities. To the best of our knowledge, this is the first report about the enhanced visible-light photocatalytic activity of Ag2S nanocrystalsensitized Ag8W4O16 nanorod photocatalysts via the quantum effect mechanism. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts. 2. Experimental details All reagents are analytical grade supplied by Shanghai Chemical Reagent Ltd. (PR China) and used as received without further purification. 2.1. Preparation of WBG Ag8W4O16 nanorods The preparation of Ag8W4O16 nanorods was described in our previous study [41,42]. Briefly, the starting aqueous solutions of AgNO3 (0.01 mol L1) and Na2WO4 (0.005 mol L1) were first prepared. The synthesis of Ag8W4O16 nanorods was achieved by a simple precipitation reaction between Ag+ and WO2 4 ions in distilled water. In a typical synthesis, 50 mL of AgNO3 aqueous solution was poured into 50 ml of Na2WO4 aqueous without stirring and then a white suspension was formed immediately and the precipitate showed a cotton-like product. After the reaction solution was incubated at room temperature for 12 h in the dark, the white precipitate was collected, rinsed with distilled water, and dried at room temperature to obtain Ag8W4O16 nanorods. 2.2. Preparation of Ag2S nanocrystal-sensitized Ag8W4O16 nanorods The synthesis of Ag2S NCs attaching on Ag8W4O16 nanorods was achieved by an in situ anion-exchange reaction of Ag8W4O16 nanorods in Na2S aqueous solution in a dark condition. Initially, 0.1 g of Ag8W4O16 nanorods was added into a 10 mL Na2S solution without stirring. After the reaction solution was incubated at room temperature for 6 h, the precipitate was collected, rinsed with distilled water, and dried at 60 °C to obtain Ag2S nanocrystal-sensitized
31
Ag8W4O16 nanorods. To simplify the sample name, the as-prepared samples are referred to as Ag2S/Ag8W4O16 (x), where x representing the amount of Ag2S nanoparticles. When the Na2S concentration was controlled to be 0.05, 0.1, 0.5, 1, and 5 mM, the resulting amount of Ag2S can be calculated to be 0.12, 0.25, 1.25, 2.53, and 13.78 wt%, respectively. For comparison, a pure Ag2S material was also prepared by a direct precipitation reaction of AgNO3 (0.1 M) and Na2S (0.05 M) solutions in the dark.
2.3. Preparation of N-doped TiO2 N-doped TiO2 (N-TiO2) was also prepared according to our previous study [8,43]. Briefly, 17 mL of tetrabutylorthotitanate was added into an NH3H2O solution (NH3 = 0–10 wt%) under stirring. After stirring for another 1 h, the suspension solution was aged at room temperature (25 °C) for 24 h. The resulted suspension was filtrated, washed with distilled water for 4 times and dried at 60 °C for 6 h, and then was calcined at 500 °C for 2 h to obtain N-TiO2. It was found that when the concentration of NH3 was controlled to be 1 wt%, the obtained N-TiO2 showed the highest photocatalytic activity under the visible light irradiation. Thus, in this study, the N-TiO2 prepared from the 1 wt% of NH3 solution was used as the reference in this study.
2.4. Characterization X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima III X-ray diffractometer (Japan) using Cu Ka radiation. Morphological analysis was performed by an S-4800 field emission scanning electron microscope (FE-SEM, Hitachi, Japan) and JEM-2100F transmission electron microscopy (TEM, JEOL, Japan). UV–vis absorption spectra were obtained using a UV–visible spectrophotometer (UV-2550, SHIMADZU, Japan). X-ray photoelectron spectroscopy (XPS) measurements were done on a KRATOA XSAM800 XPS system with Mg Ka source. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Nitrogen adsorption–desorption isotherms were obtained on an ASAP 2020 (Micromeritics Instruments, USA) nitrogen adsorption apparatus. The sample was degassed at 60 °C prior to BET measurements. The Brunauer–Emmett–Teller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.25.
2.5. Photocatalytic activity The evaluation of photocatalytic activity of the prepared samples for the photocatalytic decolorization of methyl orange (MO) aqueous solution was performed at ambient temperature. 0.05 g of the prepared sample was dispersed into 10 ml of MO solution (15 mg/L) in a culture dish with a diameter of ca. 5 cm. A 55 W fluorescence lamp was used at a light source. The average light intensity striking the surface of the reaction solution was about 14,700 lx, as measured by a luxmeter (ZDS-10, Shanghai). The concentration of MO was determined by an UV–visible spectrophotometer (UV-1240, SHIMADZU, Japan). After visible-light irradiation for some time, the reaction solution was centrifuged to measure the concentration of MO. As for the MO aqueous solution with low concentration, its photocatalytic decolorization is a pseudo-first order reaction and its kinetics may be expressed as ln(c0/c) = kt, where k is the apparent rate constant, and c0 and c are the MO concentrations at initial state and after irradiation for t min, respectively [42,43].
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3. Results and discussion 3.1. Microstructures of the Ag8W4O16 nanorods Fig. 1a is the SEM image of the as-prepared Ag8W4O16 sample and clearly shows that the sample is composed of many nanorods with a diameter of 30–100 nm and a length of ca. 1 lm. Fig. 1b is a higher magnification TEM image of the as-prepared Ag8W4O16 sample and reveals the nearly same morphology information as the above SEM image. According to XRD result (Fig. 3a), these nanorods can be attributed to Ag8W4O16 (JCPDS no. 70-1719) with an orthorhombic structure [44]. 3.2. Strategy for the synthesis of the Ag2S/Ag8W4O16 nanorods
3.3. Morphology and microstructures of the Ag2S/Ag8W4O16 (x) nanorods When the Ag8W4O16 nanorods are added into the Na2S solution, the white cotton-like product gradually becomes brown, indicating the formation of Ag2S. Fig. 4a–f shows the SEM images of the Ag2S NCs attaching on the Ag8W4O16 nanorods. Compared with the smooth surface of Ag8W4O16 nanorods shown in Fig. 1, some nanoparticles are loaded over the surface of Ag8W4O16 nanorods in Fig. 4a–f. More specifically, when the Na2S concentration was very low (0.1 mM), some sporadic nanoparticles with a size of 5–10 nm (an average size of ca. 6 nm) were formed on the surface
Fig. 2. Schematic illustration of the formation process for Ag2S/Ag8W4O16 nanorods by ion-exchange route.
Ag2S (JCPDS NO. 14-0072) ♦ ♦♦
-112
e
•: Ag8W4O16
-121 121 -103
♦
Relative intensity (a.u.)
The above prepared Ag8W4O16 nanorods were used as the precursor for the synthesis of Ag2S/Ag8W4O16 nanorods, and the corresponding schematic illustration for the formation process was shown in Fig. 2. When the Ag8W4O16 nanorods are dispersed into the Na2S solution, Ag2S NCs can be rapidly formed on the surface of Ag8W4O16 nanorods via an in situ anion exchange between S2 in the solution and the lattice WO2 4 in the Ag8W4O16 due to a lower solubility of Ag2S (Ksp = 8 1051) than that of the Ag8W4O16 (Ksp = 5.5 1012). If the S2 in the solution reacts directly with dissolved Ag+ to form Ag2S phase, it is possible for us to observe many individual Ag2S particles in the solution. In this study, however, almost no individual Ag2S aggregates can be found in addition to the Ag2S/Ag8W4O16 composite. Therefore, the above results clearly suggest that the Ag2S NCs are possibly in situ and solidly formed on the surface of Ag8W4O16 nanorods by an ion-exchange reaction of S2 in the solution with WO2 4 in the Ag8W4O16 nanorods. In addition, owing to a suspensible property of the cotton-like Ag8W4O16 nanorods in the Na2S solution, it is expected that the Ag2S NCs can uniformly formed on the surface of Ag8W4O16 nanorods (shown below in Fig. 4). As a consequence, a well-defined Ag2S NCs coupling with Ag8W4O16 nanorods is successfully constructed by the present facile route.
x3
♦ :Ag2S
d c b •
a
20
• •
•
30
• • ••
•
• 40
50
• 60
70
2θ (degree) Fig. 3. XRD patterns of different samples: (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.25), (c) Ag2S/Ag8W4O16 (1.25), (d) Ag2S/Ag8W4O16 (13.78) and (e) Ag2S.
of Ag8W4O16 nanorods (Fig. 4a and b, and Fig. S1). With increasing Na2S concentration from 0.1 to 0.5 mM, more nanoparticles with a larger size of 5–25 nm (an average size of ca. 12 nm) were uniformly loaded on the surface of Ag8W4O16 nanorods (Fig. 4c and d, and Fig. S1). When the Na2S concentration was further increased to 5 mM, nanoparticles with a size of 15–30 nm (an average size of ca. 22 nm) were more densely coated on the surface of Ag8W4O16 nanorods (Fig. 4e and f, and Fig. S1). The X-ray energy dispersion spectra (EDS) inserted in Fig. 4a, c and e, reveal that all the above samples include Ag, W, O and S elements, suggesting that these nanoparticles on the surface of Ag8W4O16 nanorods are Ag2S nanoparticles. The weight ratios of Ag2S nanoparticles in these samples Ag2S/Ag8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25), and Ag2S/Ag8W4O16 (13.78) are 0.23, 1.46, and 12.87 wt%, respectively, according to the EDS data. However, in contrast, the sizes of Ag2S sample
Fig. 1. SEM and TEM images of the as-prepared Ag8W4O16 nanorods.
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
33
Fig. 4. SEM images of different samples: (a and b) Ag2S/Ag8W4O16 (0.25), (c and d) Ag2S/Ag8W4O16 (1.25), (e and f) Ag2S/Ag8W4O16 (13.78) and (g and h) Ag2S. The inserts in Fig. 4a, c and e are the EDS spectra of Ag2SAg8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25) and Ag2S/Ag8W4O16 (13.78), respectively.
(Fig. 4g and h) prepared from the precipitation reaction of AgNO3 and Na2S solution arrange from 100 to 200 nm due to the extremely small Ksp of Ag2S. The above results strongly suggest that the size of Ag2S nanoparticles can be easily controlled by adjusting the Na2S concentration to control the formation rate of Ag2S phase. More importantly, no individual Ag2S aggregates can be found in the corresponding SEM images of the samples, which indicate that the Ag2S nanoparticles should be in situ and solidly formed on the surface of Ag8W4O16 nanorods by the ion-exchange reaction of S2 with WO2 4 . Furthermore, the BET specific surface area of the samples Ag8W4O16, Ag2S/Ag8W4O16 (0.25), Ag2S/Ag8W4O16 (1.25) and Ag2S/Ag8W4O16 (13.78) are 7.1, 7.3, 8.1 and 8.7 m2/g, respectively, which also indicate the increase about the amount of Ag2S NCs over the surface of Ag8W4O16 nanorods with increasing Na2S concentration.
The representative TEM images of the Ag2S/Ag8W4O16 (1.25) sample are shown in Fig. 5. It can be clearly seen that the morphology of the sample is consistent with the SEM images shown in Fig. 4c and d, which presents a nanorod structure with the average diameter of 30 nm and a length of ca. 1 lm. In addition, the surface of nanorods is decorated with small Ag2S nanoparticles, which have an average size of 8 nm. The further HRTEM image in Fig. 5b indicates that the spacing between adjacent lattice fringes of the nanoparticles is 0.239 nm, which is close to the d spacing of the (2 2 0) plane of Ag2S. Fig. 3 shows the XRD patterns of Ag2S/Ag8W4O16 (x) nanorods. It can be clearly seen that the intensity of the diffraction peaks of the Ag8W4O16 nanorods gradually decreases with increasing Na2S concentration (Fig. 3b–d), indicating the gradual transformation of Ag8W4O16 phase to Ag2S. However, the characteristic peaks associ-
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X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
Fig. 5. The representative TEM images of the Ag2S/Ag8W4O16 (1.25) sample.
ated with Ag2S are not obviously detected in Fig. 3b–d. For comparison, the XRD pattern of pure Ag2S sample is also shown in Fig. 3e, which can match well with the standard monoclinic Ag2S crystal structure (JCPDS no. 14-0072). However, the peak intensity of Ag2S is obviously weaker compared to that of Ag8W4O16. This is a possible reason to explain why Ag2S phase cannot be detected obviously in the Ag2S/Ag8W4O16 (x) nanorods. Another possible reason can be attributed to the small size, limited amount and high dispersion of Ag2S nanoparticles on the surface of Ag8W4O16 nanorods [39]. The surface elemental compositions and chemical status of the Ag2S/Ag8W4O16 nanorods are further analyzed by XPS. Fig. 6A presents a comparison of the XPS spectra of the Ag8W4O16 and Ag2S/ Ag8W4O16 (1.25) samples. For the Ag8W4O16 sample, the Ag, W, O and C elements are observed and the atomic ratio of Ag:W:O is nearly about 2.1:1:4, which indicates the existence for small amounts of noble metal Ag except for Ag8W4O16 owing to the photosensitive property of Ag8W4O16. The photoelectron peak of C element is due to the adventitious hydrocarbon from XPS instrument itself. Compared with the Ag8W4O16 sample, a new S element with
a binding energy at 162 eV (S2p) was found in the Ag2S/Ag8W4O16 (1.25) sample in addition to the Ag, W, O and C elements based on the XPS survey spectra. Fig. 6B presents the high-resolution XPS spectrum of S2p for Ag2S/Ag8W4O16 (1.25). The binding energies corresponding to S2p3/2 and S2p1/2 are 161.5 and 162.7 eV, respectively, indicating the presence of S2 ion, which further demonstrates the formation of Ag2S on the Ag8W4O16 nanorods [45]. Fig. 6C shows the high-resolution Ag3d XPS spectrum of Ag2S/Ag8W4O16 (1.25). The two peaks at approximately 368.6 and 374.5 eV can be ascribed to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively [43,46]. The Ag 3d5/2 and Ag 3d3/2 peaks can be further divided into four different bands, that is, 368.4, 369.8, 374.3, and 375.7 eV, respectively. The banding energies at 368.4 and 374.3 eV are attributed to Ag+ ion, and those at 369.8 and 375.7 eV are attributed to Ag0 species [47]. Fig. 7A shows the UV–vis spectra of the Ag2S/Ag8W4O16 (x) nanorods. As for the Ag8W4O16, an absorption shoulder at 400–500 nm is also found in addition to the sharp band gap absorption at about 400 nm. The visible-light absorption can be attributed to the localized surface plasmon resonance of Ag
B
200
400
600
800
S 2p
S 1/2p
Relative intensity (a.u.)
OKLL
O1s
Ag3p
W4d C1s
W4f
0
W4p
Ag3d
b a
AgMN
S2p
Relative intensity (a.u.)
A
S 3/2p
1000
156
159
Binding energy (eV)
162
165
Binding energy (eV)
C
+
Relative intensity (a.u.)
Ag
Ag 3d +
Ag
0
0
Ag
365
370
Ag
375
Binding energy (eV) Fig. 6. (A) XPS survey spectra of (a) Ag8W4O16 and (b) Ag2S/Ag8W4O16 (1.25); high-resolution of (B) S2p region and (C) Ag3d region for Ag2S/Ag8W4O16 (1.25).
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X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
B 15
A 1.2
12
c d b a
1/2
e d c b a
0.6
0.3
(αhv)
A
0.9 9 6 3
3.15 eV 0.0 300
400
500
600
700
800
0 2.5
3.0
Wavelength / nm
3.5
4.0
Photon energy (eV)
Fig. 7. (A) UV–vis spectra and (B) the plots of the (ahv)1/2 versus photon energy of (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.25), (c) Ag2S/Ag8W4O16 (1.25), (d) Ag2S/Ag8W4O16 (13.78) and (e) Ag2S.
3.4. Photocatalytic activities and mechanism analysis of the Ag2S/ Ag8W4O16 (x) nanorods The photocatalytic performance of the Ag2S/Ag8W4O16 nanorods was evaluated by the photocatalytic decolorization of MO aqueous solution. For comparison, the photocatalytic activity of N-TiO2, Ag8W4O16, and Ag2S, was also tested under the identical experimental conditions. The controlled experimental results suggest that without photocatalyst, the photobleaching of MO under visible light irradiation is negligible (not shown here). Fig. 8 shows the photocatalytic performance of the Ag2S/Ag8W4O16 (x) nanorods. It can be clearly observed that the Ag8W4O16 nanorods show a very low photocatalytic activity with a k value of 0.95 103 min1. When a small amount of Ag2S nanoparticles are loaded on the Ag8W4O16 nanorods, the photocatalytic activity of the Ag2S/ Ag8W4O16 (0.13) is slightly improved and its corresponding k is 2.6 103 min1. With further increase in Ag2S nanoparticles, the resulting Ag2S/Ag8W4O16 (x) sample shows a remarkably enhanced photocatalytic activity. Especially, the resulting Ag2S/Ag8W4O16 (1.25) shows the highest photocatalytic performance with a k of 20.8 103 min1, a value higher than that of Ag8W4O16 by more than 20 times and that of N-TiO2 by 10 times. However,
24 20
3 -1 k 10 / min
nanoparticles coated on the surface of Ag8W4O16 nanorods. For the Ag2S, it shows wide visible-light absorption from 400 to 800 nm due to its narrow band gap of 1.1 eV. When the Ag2S nanoparticles are loaded to the surface of Ag8W4O16 nanorods, the resulting Ag2S/Ag8W4O16 (x) samples show enhanced visible-light absorption from 400 to 800 nm. To further explore the band structures of the Ag2S/Ag8W4O16 (x) samples, the band gap energies can be estimated from a plot of (ahv)1/2 versus photo energy (hv) and the intercept of the tangent to the plot will give a good approximation of the indirect band gap energies of the samples (Fig. 7B). The band gap energy for the Ag8W4O16 nanorods was estimated to be 3.15 eV, which is identical with the previous result [48]. After the surface loading by Ag2S nanoparticles, it is interesting to find that the band gap of the resulting Ag2S/Ag8W4O16 (x) samples show no change compared with that of the Ag8W4O16. In general, the band structure of a semiconductor is mainly determined by its crystal structure. When the S2 ions are doped into the crystal lattices of Ag8W4O16 phase, the electronic structure of Ag8W4O16 phase can be changed, which makes the absorption edge of Ag8W4O16 shift to a longer wavelength. In the present work, it is clear that the Ag2S/Ag8W4O16(x) samples show a same value of the band gap energy as the pure Ag8W4O16 phase (Fig. 7B). Therefore, it was suggested that no S2 ions were incorporated into the crystal lattices of Ag8W4O16 phase and only Ag2S NCs were formed on the surface of Ag8W4O16 nanorods.
16 12 8 4 0 a
b
c
d
e
f
g
h
Fig. 8. The corresponding rate constant (k) for photocatalytic decolorization of MO solution over various samples under visible-light irradiation: (a) Ag8W4O16, (b) Ag2S/Ag8W4O16 (0.13), (c) Ag2S/Ag8W4O16 (0.25), (d) Ag2S/Ag8W4O16 (1.25), (e) Ag2S/Ag8W4O16 (2.5), (f) Ag2S/Ag8W4O16 (13.78), (g) Ag2S and (h) N-TiO2.
further increase in the Ag2S amount causes a rapid reduction of the photocatalytic activity. For the Ag2S/Ag8W4O16 (13.78) sample, its k value decreases to 5.13 103 min1, which is about one fourth of the Ag2S/Ag8W4O16 (1.25). To clarify the potential mechanism for the above photocatalytic results, the band structures of semiconductor photocatalysts are investigated considering their important roles in the photocatalytic reactions. Ag8W4O16 is an indirect band gap semiconductor with a wide band gap of 3.15 eV according to its UV–vis spectrum [48]. Therefore, it only can absorb UV light and do not show visible-light photocatalytic activity. The low visible-light photocatalytic activity of Ag8W4O16 nanorods in Fig. 8 can be attributed to the plasmon resonance absorption of metallic Ag nanoparticles. As for the Ag2S photocatalyst, it shows no visible-light photocatalytic activity though it can strongly absorb visible light (Fig. 7) [49]. This can be well explained by the following band structure analysis of Ag2S. The conduction band (CB) energy level of Ag2S can be calculated according to the equation of Ec = v + 0.5Eg, where Ec, v and Eg are the CB energy level, absolute electronegativity and band gap of semiconductor, respectively. Therefore, the CB and valence band (VB) energy levels of Ag2S are calculated to be ca. +0 V and 1.1 V (vs. SHE), respectively. It is clear that compared with the single-electron reduction of oxygen (O2 + e + H+ ? HO2 (aq), 0.046 V vs SHE) [44,50], the CB electrons of Ag2S show a poor reduction power owing to its more positive potential, thus resulting in a negligible photocatalytic performance. To further explore the photocatalytic mechanism of the
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Fig. 9. Schematic diagrams for the possible photocatalytic mechanism of the Ag2S NCs attaching over Ag8W4O16 nanorods under visible-light irradiation.
Ag2S/Ag8W4O16 samples, the band structures of Ag8W4O16 are also investigated according to the above-mentioned formula, and the corresponding CB and VB energy levels of Ag8W4O16 can be calculated to be ca. 0.1 V and 3.05 V (vs. SHE), respectively. In this case, the Ag2S/Ag8W4O16 composite materials should exhibit no or very low visible-light photocatalytic activity, as the CB position of Ag2S (+0 V vs. SHE) is obviously more positive than that of Ag8W4O16 (0.1 V vs. SHE) and oxygen reduction. However, the experimental results clearly indicated that the Ag2S/Ag8W4O16 nanorods exhibit excellent photocatalytic performance under visible light irradiation (Fig. 8). As a consequence, a photocatalytic mechanism about the quantum effect of Ag2S nanoparticles was proposed and shown in Fig. 9. It is well known that the quantum effect of nanoparticles enables the absorption edge of a semiconductor blue shift, and the conductor position can shift to a more negative potential with the decreasing size. On the basis of the effective mass approximation model, the blue shift of Ag2S NCs relative to the bulk is dominated by the confinement of electrons and holes, as described by the following equation [51]:
2
DEg ðRÞ ¼
h
8m0 R
2
1 1 þ me mh
where DEg(R) is the band gap shift for the crystal radius R, h the Planck’s constant, m0 the electron mass, while me and mh are the effective masses of electrons and holes, respectively. According to the equation, it is clear that the smaller are the radius, the wider are the band gap of Ag2S NCs. Considering the small electron effective mass ðme ¼ 0:13m0 Þ versus the significantly larger hole mass ðmh ¼ 1:14m0 Þ, most of the band-gap increase can be regarded as a shift of the CB energy level to a more negative potential (vs NHE). Thus, Ag2S NCs with a more negative potential are expected to transfer photogenerated electrons from the CB of Ag2S to Ag8W4-
O16 surface to reduce oxygen, while the photogenerated holes are retained on the VB of Ag2S to oxidize organic substances effectively, resulting in a high photocatalytic performance (Fig. 9). However, when the size of Ag2S nanoparticles further increases, the corresponding quantization effect of Ag2S would disappear gradually and the resultant Ag2S/Ag8W4O16 samples show a decreasing photocatalytic performance (such as Ag2S/Ag8W4O16 (13.78)). Similar results of the quantization effect about the Ag2S NCs are also obtained in the previous report [52]. It is well-known that an obvious drawback for metal chalcogenides as a photocatalyst (such as CdS) is its instability, as CdS is easily eroded by the photo-generated holes, which can cause a decreasing photocatalytic performance [53]. Therefore, the cycle performance for the Ag2S/Ag8W4O16 (x) samples is necessary in terms of the stability of photocatalytic materials. In this case, the Ag2S/Ag8W4O16 (1.25) sample with the highest photocatalytic activity is chosen for the cycle performance of the photocatalytic tests. The results (Fig. 10A) indicate that no obvious deactivation can be observed even after five cycles. More specially, the SEM image of the Ag2S/Ag8W4O16 (1.25) sample after five cycles shows that no additional Ag2S nanoparticles can be found except for the Ag2S nanoparticles loaded on the Ag8W4O16 surface, which further indicates that the Ag2S nanoparticles are solidly grafted on the Ag8W4O16 nanorods (Fig. 10B). In addition, the Ag2S/Ag8W4O16 (1.25) sample after five cycles showed a similar XPS spectrum (Fig. S2) and UV–vis spectra (not shown here) compared with the as-prepared Ag2S/Ag8W4O16 sample, which suggests that the Ag2S/Ag8W4O16 composite is a stable photocatalytic material under visible-light irradiation. Moreover, the strong combination of Ag2S with the Ag8W4O16 nanorods can also contribute to the rapid transfer of photogenerated electrons from Ag2S CB to the Ag8W4O16 via their well interface, resulting in an enhanced photocatalytic activity and stability. 4. Conclusion In summary, Ag2S NCs attaching on the surface of WBG Ag8W4O16 nanorods were prepared by employing a facile anion exchange method with the reaction between S2 and WO2 4 . It was found that anion exchange can in situ and uniformly deposit Ag2S on the surface of Ag8W4O16, controllably adjust the size and amount of Ag2S NCs by adjusting the S2 concentration, and solidly connect Ag2S NCs to the Ag8W4O16 surface via the replacement of S2 to WO2 4 . The photocatalytic results indicated that the as-prepared Ag2S/Ag8W4O16 composite photocatalysts with 2.16 wt% Ag2S NCs exhibited the highest activity with a k value of 20.8 103 min1 for the discoloration of MO under visible light illumination, which exceeds that of pure Ag8W4O16 by more than 20 times and that of N-TiO2 by 10 times, respectively. By analyzing the band structure of
-3 -1 Rate constant k 10 / min
A 24 20 16 12 8 4 0
1st
2nd
3th
4th
5th
Fig. 10. (A) Five cycles of photocatalytic decolorization of MO solution for Ag2S/Ag8W4O16 (1.25) under visible-light irradiation; (B) SEM image of Ag2S/Ag8W4O16 (1.25) after the fifth cycle.
X. Wang et al. / Journal of Colloid and Interface Science 422 (2014) 30–37
Ag2S and Ag8W4O16 semiconductors, it was found that the photocatalytic process intensively depended on the Ag2S NCs quantum size effect. This present work may provide some insight into the design of novel and high-efficiency NBG semiconductor NCs coupled with WBG semiconductor composite photocatalysts and narrow band gap semiconductor NCs with turntable band gaps also open up new horizons for harvesting solar light in the visible and infrared region. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (51208396, 21277107, and 61274129), the Program for NCET (NCET-13-0944) and 973 Program (2013CB632402). This work was also financially supported by the Fundamental Research Funds for the Central Universities (Grant 2013-1a-036, 2013-1a-039), the Self-determined and Innovative Research Funds of WUT (Grant 20131049714001) and MinistryProvince Jointly-Constructed Cultivation Base for State Key Laboratory of Processing for Non-Ferrous Metal and featured Materials, Guangxi Zhuang Autonomous Region. We thank FGM group at Wuhan University of Technology for assistance with XRD measurements. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.02.009. References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [2] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253–278. [3] S.W. Liu, J.G. Yu, B. Cheng, M. Jaroniec, Adv. Colloid Interface Sci. 173 (2012) 35–53. [4] P. Wang, J. Wang, X. Wang, H. Yu, J. Yu, M. Lei, Y. Wang, Appl. Catal. B: Environ. 132–133 (2013) 452–459. [5] P. Wang, J. Wang, T. Ming, X. Wang, H. Yu, J. Yu, Y. Wang, M. Lei, ACS Appl. Mater. Interfaces 5 (2013) 2924–2929. [6] T. Froschl, U. Hormann, P. Kubiak, G. Kucerova, M. Pfanzelt, C.K. Weiss, R.J. Behm, N. Husing, U. Kaiser, K. Landfester, M. Wohlfahrt-Mehrens, Chem. Soc. Rev. 41 (2012) 5313–5360. [7] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [8] R. Liu, P. Wang, X. Wang, H. Yu, J. Yu, J. Phys. Chem. C 116 (2012) 17721–17728. [9] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M. Chi, Y. Yin, Angew. Chem. Int. Ed. 50 (2011) 7088–7092. [10] H. Irie, Y. Watanabe, K. Hashimoto, J. Phys. Chem. B 107 (2003) 5483–5486. [11] M. Iwasaki, M. Hara, H. Kawada, H. Tada, S. Ito, J. Colloid Interface Sci. 224 (2000) 202–204. [12] M. Gratzel, J. Photochem. Photobiol., C 4 (2003) 145–153. [13] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Coord. Chem. Rev. 248 (2004) 1363–1379. [14] D. Wu, M. Long, ACS Appl. Mater. Interfaces 3 (2011) 4770–4774.
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