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J. Phys. Chem. C 2008, 112, 16754–16758
Monodisperse, Mesoporous ZnxCd1-xS Nanoparticles as Stable Visible-Light-Driven Photocatalysts Wenzhong Wang,* Wei Zhu, and Haolan Xu State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: August 21, 2008
Monodisperse, mesoporous ZnxCd1-xS nanoparticles are successfully realized as visible-light-driven photocatalyst via a facile self-assembly route in which sodium dodecylsulfate is utilized as the surfactant. Despite the analogous size and configuration, the alloys with different Zn content exhibit composition-dependent absorption properties in the visible zone resulting from the distinguishing band gap. Under visible light irradiation (λ g 400 nm), correspondingly, these photocatalysts show composition-dependent photocatalytic efficiencies for the degradation of rhodamine B. The performance increases evidently with the decrease in the Zn content in the alloys. In addition, they are stable enough to be recycled multiple times and demonstrate higher stability than binary CdS sample synthesized through the same process. This finding is attractive for presenting new possibilities for chalcogenide semiconductors as single-phase, visible-light-sensitive catalysts in photocatalytic applications, especially in the field of environmental remediation. 1. Introduction Exploration of photochemically active semiconductors, especially with high photocatalytic activity under visible light irradiation, is one of the most challenging topics in the global interdisciplinary problems, energy and environment.1 The remarkable progress of photocatalysis in the past decade has been focused on TiO2-based materials due to their superior properties (e.g., stability, high response to UV light).2 From a viewpoint of a solar light energy conversion, attempts have been made to extend the absorption edge into the visible light region; for example, by doping N or C into TiO2.3 Notably, some chalcogenide semiconductors (e.g., CdS)4 have excellent properties in that the band gap (Eg) corresponds well to the spectrum of sunlight, and the conduction band edge is more negative than the H2O/H2 redox potential. Although successful photocatalytic systems have yet to be achieved (primarily due to inherent instability of the materials in the photoinduced reaction, such as photocorrosion5), these semiconductors are attractive as visible-light-driven catalysts once the stability is improved. As a typical alloyed chalcogenide semiconductor, recently, ZnxCd1-xS has been intensively studied because of its highdensity optical recording, blue or even UV laser diodes.6 These studies have revealed that ZnxCd1-xS has good and tunable absorption properties in the visible zone of the solar spectrum resulting from the regulable Eg between CdS and ZnS materials. As a response to their sufficiently negative flat band potentials,7 ZnxCd1-xS alloys are excellent candidates as visible-light-driven photocatalysts, particularly in the potential application of environmental remediation. Along with the development of nanoscience and technology, on the other hand, it has been great anticipated that nanosized photocatalysts could introduce manifold evolution of the activity in photochemical redox processes due to size effects,8 such as an increase in the surface area and surface concentration of catalytically active sites, acceleration of the diffusion of * To whom correspondence should be addressed. Phone: +86-21-52415295. Fax: +86-21-5241-3122. E-mail:
[email protected].
photogenerated charge carriers from the interior to the surface of the nanocrystal (NC), etc. In principle, the smaller the size, the higher the activity. However, a catalyst with a smaller dimension always accompanies a stronger tendency of aggregation, coupled with the tremendous difficulties in separation, recovery, and recycling in industrial applications. How to make the most of the advantages depends primarily on the manipulation of the nanoscale configuration. For an ideal nanosized photocatalyst, the structural factors, involving high surface-tovolume ratio, appropriate dimensions, and a narrow size distribution, are all important. Furthermore, a carefully designed architecture has been demonstrated to be advantageous to improve the stability of the target photocatalyst.4b Herein, for the first time, we present a facile, self-assembly approach for mass production of monodisperse ZnxCd1-xS mesoporous nanoparticle (MPNP) photocatalysts composed of building blocks of tiny NCs. In view of the homogeneous size and porous structure, these MPNPs yield a high surface-tovolume ratio with effective prevention of further aggregation, and the relatively larger dimensions make it more convenient to separate and recycle them than common NCs. The photodegradation of rhodamine B (RhB) was employed to evaluate the photocatalytic activities of the alloys with different Zn content under visible light illumination (λ g 400 nm). It is demonstrated that these samples exhibit excellent and composition-dependent performance, and they show high stability in the photocatalytic reaction. 2. Experimental Sections 2.1. Synthesis of the Samples. Typically, 40 mL of 0.125 mol/L sodium dodecylsulfate (SDS, A.R.) aqueous solution was transferred into a 100 mL, three-neck flask in the ambient environment under stirring, and then zinc acetate (A.R.) and cadmium acetate (A.R.) with a specified x value in ZnxCd1-xS (x ) 0, 0.05, 0.2, 0.28, 0.78 and 0.9) and a total molar amount of 1 mmol were added into the flask. The as-generated solution was placed in a laboratory ultrasonic bath (160 W and 59 KHz, Kudos SK3300HP, Shanghai) for 10 min treatment, followed
10.1021/jp805359r CCC: $40.75 2008 American Chemical Society Published on Web 10/03/2008
ZnxCd1-xS Nanoparticles by heating to 80 °C in a reflux system. Subsequently, 10 mL of 0.1 mol/L thioacetamide (A.R.) aqueous solution was injected quickly into the hot solution. The reaction was allowed to proceed at 80 °C for 3 h. As a response to the increasing value x in ZnxCd1-xS, the as-produced suspension changed from jacinth to yellowy. The resulting colloidal samples were readily collected by centrifugation, washed with deionized water and ethanol several times, and dried in air. 2.2. Characterizations. The X-ray diffraction (XRD) patterns were recorded on a Japan Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu KR radiation over the range of 20° e 2θ e 60°. The scanning electron microscope (SEM) characterizations were performed on a JEOL JSM-6700F field emission scanning electron microscope. The transmission electron microscope (TEM) analyses were carried out by a JEOL JEM-2100F field emission electron microscope. A Hitachi U-3010 UV-vis spectrophotometer was used to record the UV-vis spectra of various samples. Nitrogen adsorptiondesorption measurements were conducted at 77.35 K on a MicromeriticsTristar3000analyzer.TheBrunauer-Emmett-Teller (BET) surface area was estimated using adsorption data. 2.3. Photocatalytic Activity Measurement. Under visible light irradiation, the photocatalytic activities of the ZnxCd1-xS alloys were evaluated by the photodegradation of organic contaminants, and rhodamine B (C28H31ClN2O3, RhB) was used as the model pollutant. A cylindrical pyrex flask (capacity ∼100 mL) was used as the photoreactor vessel. Typically, the reaction system containing RhB (aqueous solution, 1 × 10-5 M, 50 mL) and alloyed ZnxCd1-xS nanoparticle photocatalysts (50 mg) was magnetically stirred in the dark for 30 min to reach the adsorption equilibrium of RhB with the catalyst and was then exposed to light from a 500 W Xe lamp equipped with a UV cutoff filter (λ g 400 nm). At specific time intervals, 3 mL of the reactive solution was withdrawn by a syringe. The solution was centrifuged to remove the ZnxCd1-xS powder before being analyzed by the UV-vis absorption spectra. To investigate the stability of ZnxCd1-xS photocatalysts, three cycles of photocatalytic measurements were employed by using Zn0.2Cd0.8S as a representative sample, using binary CdS as the contrast. 3. Results and Discussion 3.1. Preparation and Characterization of ZnxCd1-xS Samples. ZnxCd1-xS is a ternary sulfide semiconductor with regulable composition. In the surfactant-assistant synthesis, the Zn content in the alloys is easily tuned from 0.05 to 0.9 by changing the initial concentration of the reactants,6b,9 which offers an efficient access to control the subsequent photocatalytic activity. Figure 1 reveals the typical XRD patterns of the products. Despite the different composition, all the samples display the same wurtzite phase. However, the diffraction peaks are gradually shifted to larger angles with the increase in x. The successive shifts of the XRD patterns indicate that the crystals obtained are not a mixture of ZnS and CdS, but ZnxCd1-xS solid solution.10 The broad diffraction peaks of the ZnxCd1-xS imply the small crystal size of the product. The average size is in the range of 3.2-4.6 nm with the increase in x, as roughly estimated by the Debye-Scherrer formula. Figure 2 illustrates the morphological and structural properties of the alloys for which Zn0.78Cd0.22S as a typical example. As shown in both SEM and TEM images (Figure 2a and b), the product is composed of a large quantity of monodisperse spherical nanoparticles with an average diameter 85 ( 15 nm which is in good agreement with the result measured by dynamic light scattering (Figure 2d). Due to the occurrence of many spots with clear contrast difference (inset of Figure 2b), these
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Figure 1. Powder XRD patterns of the monodisperse ZnxCd1-xS mesoporous nanoparticles synthesized in the self-assembly route.
nanoparticles are demonstrated as an assembled structure consisting of tiny NCs ∼3-5 nm in size, which agrees well with the value estimated from the XRD pattern. The HRTEM image (Figure 2c) shows the lattice fringes of NCs with an interplanar spacing of 3.23 Å, matching well with the (002) plane separation of wurtzite Zn0.78Cd0.22S. Figure 2e is the corresponding electron diffraction pattern, which shows the polycrystalline texture. Arising from the spaces among the tiny NCs and the MPNPs, furthermore, the particles are characteristic of mesoporous materials (Figure 2f, the typical type IV isotherms). The pore-size distribution reveals a number of pores of less than 2.2 nm in the sample, and the BET specific surface area is calculated to be around 41.15 m2 g-1, approximately that of Degussa P25 powder (∼45 m2 g-1).11 Self-assembly of NCs in the presence of surfactants provides a class of facile and efficient methods for generating homogeneous nanoparticles in large quantities. The assembled structure is normally related to a high surface-to-volume ratio, facility in separation and recycle, and stability in the suspension, which are all attractive for the photocatalysis application. In our system, the anionic surfactant, sodium dodecylsulfate, easily attracts the cation (Cd2+ and Zn2+) in the solution to produce metastable M2+-SDS molecules. Then the short term of the ultrasound treatment stimulates the self-aggregation of M2+-SDS molecules to form spherical clusters with a uniform size, which provides nucleation domains for the subsequent reaction between M2+ and S2- to produce ZnxCd1-xS NCs. A monodisperse, porous structure is finally achieved. Similar to Zn0.78Cd0.22S, as shown in Figure 3, an analogous morphology is observed in the other samples with different x. However, the difference is reflected by the facts that, with the increase in the Zn content, the colors of the samples change gradually from jacinth to yellowy, and the band gaps increase. Figure 4 shows the absorption spectra of the as-prepared ZnxCd1-xS MPNPs. As x increases, the absorption edge becomes blue-shifted, leading to a weaker absorption in the visible zone. Correspondingly, the Eg value increases by degrees from 2.36 to 3.17 eV, estimated using the following equation a(V) ) A(h′V/2 - Eg)m/2,12 where the value of m is 1, because the ZnxCd1-xS is a semiconductor with a direct ban gap. 3.2. Photocatalytic Performance of ZnxCd1-xS Samples. To evaluate the photocatalytic activity of the MPNPs under visible light (λ g 400 nm), we investigated the degradation of rhodamine B (RhB) over the different MPNP photocatalysts. For the photodegradation of RhB, it is notable that two competitive processes occur, including N-demethylation and the
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Figure 2. (a) SEM and (b) TEM image of Zn0.78Cd0.22S MPNPs; the inset is a magnified individual; (c) representative HRTEM image; (d) dynamic light-scattering analysis, indicating the narrow size distribution with an average diameter of 85 ( 15 nm; (e) electron diffraction pattern; and the (f) nitrogen adsorption/desorption isotherm and Barrett-Joyner-Halenda pore size distribution plot (inset) of Zn0.78Cd0.22S MPNPs.
destruction of the conjugated structure.1d,13 De-ethylation of the fully N,N,N′,N′-tetraethylated rhodamine molecule results in the stepwise transformation to other rhodamine-based derivates (e.g., N,N,N′-triethylated rhodamine, N,N′-diethylated rhodamine, N-ethylated rhodamine, and rhodamine), accompanied by the wavelength position of its major absorption’s moving from 552 to 498 nm, and the destruction of the conjugated structure leads to the decrease in the total amount of all rhodamine species. In the present cases, the RhB concentration was analyzed by checking the characteristic absorption at 552 nm, and the total amount of all rhodamine species was approximately determined by the maximum absorption measurement, since the molar extinction coefficients (max) of different rhodamine species are in a narrow range.1d,13 First, the adsorption percentages of RhB over ZnxCd1-xS samples in adsorption equilibrium were measured and are listed in Table 1. It clearly reveals that, resulting from the similar monodisperse and mesoporous configuration, these samples exhibit excellent and analogous adsorption percentages. During the following photodegradation process, generally, visible light
irradiation of the aqueous RhB/ZnxCd1-xS dispersion leads to an apparent decrease in absorption with a concomitant shift to shorter wavelengths (Figure S1 of the Supporting Information). As a function of irradiation time, the plots of C/C0 of both RhB and total rhodamine species are summarized in Figure 5. Apparently, the photocatalytic activity of the MPNPs is strongly dependent on the composition, descending gradually with the increase in the Zn content. Because x e 0.20, the characteristic absorption peaks of RhB (552 nm) disappear completely within 20 min. The prolonged exposure time results in a gradual decrease of the rhodamine species concentration. This behavior is also observed as x ) 0.28, except that RhB disappears after about 40 min. As x ) 0.78, two competitive photodegradation processes are always in concomitance. As x ) 0.90, although the amount of RhB diminishes gradually accompanying the increase in the exposure time, the change in the amount of rhodamine species is not appreciable, indicating the RhB decomposition occurs mainly via the N-demethylation process. For the separate activities of the alloys, the difference of the intrinsic Eg should be the dominating reason that results in
ZnxCd1-xS Nanoparticles
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Figure 3. SEM images of the other samples with different values of x, revealing the overall availability of the self-assembly route in the production of monodisperse ZnxCd1-xS nanoparticles. x ) (a) 0.05, (b) 0.2, (c) 0.28, and (d) 0.9.
Figure 5. Photocatalytic degradation of RhB over ZnxCd1-xS MPNPs with different Zn content. (-b-, RhB; -O-, rhodamine species). Figure 4. UV-vis absorption spectra of the as-prepared ZnxCd1-xS samples, dispersed in ethanol solution (concentration, ∼0.2 mg/mL); the inset is bandgap energy as a function of the Zn composition for the monodisperse ZnxCd1-xS MPNPs.
TABLE 1: The RhB Adsorption Percentages over Different ZnxCd1-xS Photocatalysts in Dark x value adsorption %
0.05
0.20
0.28
0.78
0.90
68.83
70.16
67.51
63.78
74
the different absorption properties in the visible zone. With the increment of x, the position of the valence band edge of ZnxCd1-xS hardly changes, whereas the conduction band (CB) becomes more negative, which lead to the remarkable blueshift of the band gap of ZnxCd1-xS solid solutions (Figure 4). This will result in reduced adsorption of the visible light, which
further leads to a decrease in the photogenerated electron hole. Therefore, the photocatalytic activity decreases (Figure 5). In addition, the more negative CB is advantageous for the reduction reactions, which may also affect the redox processes. Manipulation of the composition in the alloys offers a powerful role in producing a photocatalyst with controllable activity. On the other hand, the additional comparative experiment demonstrates that the concentration change of RhB under exposure to visible light irradiation, without any ZnxCd1-xS photocatalysts, can be ignored (Figure S2 of the Supporting Information). The capability of reusage is one of the most pivotal guidelines for an ideal photocatalyst. Whether the solid solution photocatalysts are stable, then becomes a key performance standard for the widespread photocatalytic application. Utilizing the MPNPs with x ) 0.2 as an example, we examined the stability of the alloyed MPNP photocatalysts by investigating the photodegradation behavior of RhB for three cycles under the same conditions (Figure
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Wang et al. on. However, the performance is much lower than the expectation, and the activity in the first cycle is analogous only to Zn0.2Cd0.8S and Zn0.28Cd0.72S alloys. In view of the instability of CdS, a competitive photocorrosion effect should be involved in the photodegradation process. 4. Conclusion
Figure 6. Three cycles of photocatalytic degradation of RhB over the Zn0.2Cd0.8S sample. (-b-, RhB; -O-, rhodamine species).
In summary, monodisperse ZnxCd1-xS MPNPs are synthesized as a visible-light-driven photocatalyst with compositiondependent performance for the degradation of RhB. These photocatalysts are stable enough to be recycled multiple times. Due to their homogeneous dimensions, their aqueous suspensions are stable for days under ambient conditions while maintaining their catalytic activities. Although additional research on the mechanisms of the unique photocatalysis (e.g., higher and higher activity for the destruction of the conjugated structure with cycles on) is desired, the finding presents new possibilities for chalcogenide semiconductors in the photocatalytic applications with high activity and stability. Acknowledgment. We appreciate the financial support from the National Basic Research Program of China (973 Program, 2007CB613302) and the National Natural Science Foundation of China (No. 50672117). Supporting Information Available: Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 7. Three cycles of photocatalytic degradation of RhB over a CdS sample. (-b-, RhB; -O-, rhodamine species).
6 and Figure S3 of the Supporting Information). With cycles on, first, comparison of the three absorption spectra reveals that the adsorbance reduces evidently from 70.16, to 43.2, then to 32.5% during the adsorption equilibrium (Figure S3 of the Supporting Information). However, the degradation rate of RhB changes little. While the characteristic absorption of RhB disappears within 20 min in the first cycle, the corresponding time in the second and third cycle are both less than 40 min. More specially, with cycles going on, these MPNP photocatalysts display higher and higher activity for the destruction of the conjugated structure (rhodamine species). Although in the first cycle the rhodamine species remain 31.45% after 120 min, contrasting the initial amount, all absorption peaks disappears completely after 120 min in the second cycle and 80 min in the third cycle, indicating the overall photodegradation of the organic dye pollutant. The exact mechanism of the intriguing behavior is still not clear. However, it is proposed that the enhanced activity may result from the different absorbance on the surface of the MPNPs, as most oxidative reactions occur by the OH · route. However, the dye adsorption may compete with this path. Therefore, the lower dye adsorption may lead to the higher destruction of the rhodamine species. The enhanced activity may also ascribe to a mechanism based on a surface defect state. Moreover, the XRD patterns of the catalysts before and after the photodegradation tests are essentially the same, implying the stability of the ternary chalcogenide alloys. As a contrast, the corresponding test of a pure CdS sample synthesized via the self-assembly route is also constructed (Figure 7 and Figure S4 of the Supporting Information). The photodegradation process displays a similar result, including decreased adsorbance and enhanced efficiency with cycles going
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JP805359R
