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Notes Synthesis of Selenium Nanoparticle and Its Photocatalytic Application for Decolorization of Methylene Blue under UV Irradiation Sudip Nath,† Sujit Kumar Ghosh,† Sudipa Panigahi,† Thomas Thundat,‡ and Tarasankar Pal*,† Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6123 Received March 16, 2004. In Final Form: June 20, 2004
Introduction The emerging field of nanoparticles has stimulated much interest in recent years because of their characteristic properties, which are strikingly different from those of the bulk. Nanoparticles hold promise as innovative materials with new electronic, magnetic, and catalytic properties.1-6 Among all the nanomaterials, semiconductor nanoparticles are proved to be very important for technological application in electronic gadgets. The semiconductor nanoparticles housed in a suitable optically transparent host have exhibited excellent nonlinear properties, saturable absorption, and optical bistability.7 Among all the semiconductors, selenium is the most important one in view of its various applications. It is used as rectifiers, solar cells, photographic exposure meters, and xerographs.8 It also exhibits excellent glassforming tendencies.9 Linear and nonlinear optical properties of Se and its attraction toward Cd, Zn, etc.10 altogether make this an important field of study. A few reports have been published regarding the preparation of selenium nanoparticles. Among them physical vapor deposition, vapor phase diffusion, and wet chemical methods are significant and popular so far.8,11,12 However, a reproducible but simple method of preparation of stable selenium nanoparticles with good catalytic activity is still a challenge. In this study, we have described the synthesis of selenium nanoparticles through the reduction of aqueous selenious acid solution by sodium borohydride. To prevent aggregation of the particles and to offer stability, a nonionic * To whom correspondence may be addressed. E-mail: tpal@ chem.iitkgp.ernet.in. † Indian Institute of Technology. ‡ Oak Ridge National Laboratory. (1) Yee, C. K.; Ulman, A.; Ruiz, J. D.; Parikh, A.; White, H.; Rafailovich, M. Langmuir 2003, 19, 9450. (2) Jana, N. R.; Pal, T. Langmuir 1999, 15, 3458. (3) Kamat P. V. Chem. Rev. 1993, 93, 267. (4) Pradhan, N.; Pal, A.; Pal, T. Langmuir 2001, 17, 1800. (5) Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Langmuir 2001, 17, 578. (6) Kaifer, A. E.; Strimbu, L.; Liu, J. Langmuir 2003, 19, 483. (7) Rajalakshmi, M.; Arora, A. K. Solid State Commun. 1999, 110, 75. (8) Chen, Y.; Sun, Q.; Li, H. Chem. Lett. 2003, 32, 448. (9) Nagels P.; Sleecks, E.; Callaerts, R.; Tichy, L. Solid State Commun. 1995, 94, 49. (10) Quinlan, F. T.; Kuther, J.; Tremel, W.; Knoll, W.; Risbud, S.; Stroeve, P. Langmuir 2000, 16, 4049. (11) Gates B.; Yin, Y.; Xia, Y. J. Am. Chem. Soc. 2000, 122, 12582. (12) Mayers B.; Jiang, X.; Sunderland, D.; Cattle, B. Xia, Y. J. Am. Chem. Soc. 2003, 125, 13364.
micelle, Triton X-100, has been introduced into the reaction medium.13 The particles were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopic (XPS) studies. We have also addressed the catalytic behavior of selenium nanoparticles without any other chemicals for the reaction. There are several published reports14-16 of dye decolorization by different reagents. However, this is the first fruitful attempt where the selenium nanoparticles have been exploited as a catalyst for such type of dye degradation in a clean photochemical pathway. Experimental Section All the reagents used were of AR grade. Double distilled water was used throughout the experiment to prepare the solutions. Selenious acid was purchased from Merck. Poly(oxyethylene) isooctylphenyl ether, Triton X-100 (simply TX-100, Aldrich), was used as received. Sodium borohydride (NaBH4) was purchased from Sigma, and aqueous solutions were prepared freshly in ice-cold water when required. Methylene blue (MB) (S. D. Fine Chemicals, India) was used as received. Atomic force microscopy (AFM) techniques were used to characterize the solution phase selenium nanoparticles. Solid selenium particles were characterized by X-ray photoelectron spectroscopic (XPS) studies. An atomic force microscope (Digital Instruments, USA) with Nanoscope III was used to study the Se nanoparticles under consideration. XPS studies were carried out using a VG Scientific instrument (ESCA LAB, MK II, UK). In the case of dye decolorization the experimental solution was taken in a quartz cuvette of 1 cm path length and photoirradiated in a photoreactor fitted with ordinary germicidal lamps (Philips, India) of wavelength ∼365 nm. The photoreactor can generate a variable flux of 100-850 lx. The flux was monitored using a digital lux meter (model LX 101), Taiwan. The light intensity inside the photoreactor was calibrated with an Ophir power meter (NOVA display and 30-A-SH sensor). The average temperature inside the photoreactor was 30 ( 2 °C. UV-visible spectra of each solution were measured in a Shimadzu UV-160 digital spectrophotometer (Kyoto, Japan) keeping the solution in the cuvette. Cyclic voltammetry was studied in a CH Instrument electrochemical analyzer (model 600A). Pt wires were used as both working and auxiliary electrodes, whereas calomel was used as reference electrode and KCl was used as a supporting electrolyte for this purpose. Selenium nanoparticles were prepared by the reduction of aqueous selenious acid with freshly prepared ice-cold NaBH4. In practice, 100 µL of 10-2 M selenious acid was treated with 1 mL of 10-2 M NaBH4, and the final volume of the solution was made to 3 mL. The color of the solution changes from colorless to yellow immediately after the addition of borohydride. When the solution is allowed to stand (∼15 min), the particles agglomerate to form a red precipitate. The red color of the precipitate then transformed into black while standing for a long time. To avoid agglomeration, TX-100 micelles (100 µL, 10-2 M) were introduced into the reaction mixture from outside within 5 min of the NaBH4 reaction, and thus the orange colored sol remained stable for months. Then the dye decolorization was studied employing preformed selenium nanoparticles (100 µL, 3.2 × 10-4 M) with aqueous solution of the dye (10-4 M, 3 mL), and the solution was irradiated (13) Nath, S.; Ghosh, S. K.; Pal, T. Chem. Commun. 2004, 966. (14) Stathatos, E.; Petrova, T.; Lianos, P. Langmuir 2001, 17, 5025. (15) Hallock, A. J.; Berman, E. S. F.; Zare, R. N. J. Am. Chem. Soc. 2003, 125, 1158. (16) Chu, W.; Ma, C. W. Chemosphore 1998, 37, 961.
10.1021/la049318l CCC: $27.50 © 2004 American Chemical Society Published on Web 07/30/2004
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Chart 1. Schematic Representation of Stabilization of Se Nanoparticles
with UV inside the photoreactor. The successive decrease in absorbance of the reaction mixture was monitored at regular time intervals by the UV-vis spectrophotometer.
Figure 1. AFM images of selenium nanoparticle stabilized in TX-100 micelle.
Results and Discussion The solution phase evolution of selenium nanoparticles by NaBH4 is primarily authenticated from UV-vis spectrophotometry with the evolution of featureless absorbance of the yellow colored solution, which monotonically increases toward higher energy. Selenious acid is reduced as follows:
2H2SeO3 + BH4- f 2Se + H2BO3- + 3H2O This observation corroborated the literature report of the yellow color due to the presence of selenium nanoparticles in solution.17 The color implies the formation of amorphous selenium (a-Se) existed in the form of spherical colloids. On standing, the amorphous particles slowly agglomerate and turn into black trigonal selenium (t-Se). To prevent such an agglomeration process, nonionic micelle (TX-100, 10-2 M) was introduced into the solution within the first 5 min of Se nanoparticles evolution.13 Other common cationic and anionic micelles (aqueous 10-2 M) like CTAB and SDS have a similar stabilization effect for the Se particles under consideration. The stabilization of Se nanoparticles in the presence of TX-100 is shown in Chart 1. In Figure 1 the AFM image of the selenium nanoparticles is shown. AFM imaging was done for visualization of the particles dispersed in the surfactant medium. From an AFM image, the actual size of the particles was found to be ∼25 ( 4 nm. The tapping mode technique was employed to image the particles under ordinary laboratory conditions. In this mode, the AFM tip is oscillating near its resonance frequency and only touches the sample periodically. This enables minimum sample damaging and provides better images for the sensitive “soft” samples as in case of Se particles. All images were recorded at a very slow scan rate to avoid sample damaging. Due to this precaution, no impact on the image was observed. Images were recorded in height, amplitude, and phase modes simultaneously. In Figure 2 the diameter histogram of the particle obtained from AFM microscopic technique is shown. The histogram of the particle diameter is obtained by analyzing 50 particles, imaging from several regions of the disk. In Figure 3, a wide range XPS pattern of the solid Se particles is presented. The XPS study shows that the spectrum corresponds to selenium particles. One thing is to be mentioned that to have the XPS, the sample was not stabilized in micelle whereas the particles are allowed to agglomerate to form precipitate so that they can be (17) Johnson, J. A.; Saboungi, M.; Thiyagarajan, P.; Csencsits, R.; Meisel, D. J. Phys. Chem. B 1999, 103, 59.
Figure 2. Diameter histogram of the selenium particles.
Figure 3. Wide range XPS pattern of solid selenium particles.
collected from aqueous solution. The wide range XPS spectra of the sample shows three distinct peaks for Se 3d, 3p, and 3s orbitals at 55.3, 171.2, and 234.2 eV, implying the valance state of Se is zero in the sample. Among the three XPS signals the Se 3d is the most interesting probe to provide a fine XPS characterization of the surface. Due to higher binding energy of Se 2p, 2s, and 1s electrons, they do not show any signal in that range. The other two peaks appeared due to “Auger” lines of Se. In this case the spectra show a slight higher binding energy than that expected from the theoretical values. This might be explained taking the idea that when an insulating sample is charged, all the electrons leaving the surface will appear with reduced kinetic energy and therefore experience apparently a higher binding energy, because they mostly overcome the additional attraction of the positively charged surface.
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Figure 4. UV-vis spectra for the photodecolorization of methylene blue in water. The time interval between successive measurements is 5 min. Conditions: [MB] ) 10-4 M, [Se] ) 1.1 × 10-5 M, flux of UV light ) 340 lx.
To study the catalytic activity of the selenium nanoparticles, methylene blue was chosen as a model compound, which is very useful to study redox reactions.18 Moreover, the selection of the dye was made because of the fact that the dye becomes completely colorless under UV irradiation and thus the color bleaching can be monitored easily using a visible spectrophotometer. Previously Houas et al.19 reported that the degradation of methylene blue leads to complete mineralization due to CO2, SO42-, NO3-, and NH4+ formation under UV irradiation. Under anaerobic conditions, chemical reduction of the dye leads to temporary color bleaching (usual reduction to leuco methylene blue) and hence dye color can be regenerated easily.20 However, decolorization results due to the irreversible degradation (aerobic) of the dye skeleton causing the permanent color bleaching and hence oxidants or oxygen/air cannot regenerate the color of the dye.
2C16H18N3SCl + 25O2 ) 2HCl + 2H2SO4 + 6HNO3 + 32CO2 + 12H2O To study the photodegradation process, aqueous solutions of methylene blue were employed along with TX-100 stabilized Se nanoparticles for UV irradiation. The progress of the dye decolorization was monitored by the decrease in absorbance of the peak due to methylene blue at 662 nm. In Figure 4 the absorption spectra of successive decolorization of the methylene blue is shown. It has been found that the reaction follows an overall first-order kinetics and evolution of nitrates and sulfate was confirmed qualitatively during the photodegradation process. The UV light leads to the degradation of the dye structure, which is again catalyzed to a great extent in the presence of selenium nanoparticles. In the absence of Se under the same experimental condition, a very slow rate of degradation was observed in the experimental time scale. It is, therefore, obvious that selenium catalyzes the reaction efficiently. It is interesting to note that the reaction is catalyzed to a negligible extent with commercially available Se and agglomerated black Se particles. A comparison of reaction parameter is shown in Figure 5 where the catalytic activity of TX-100 stabilized Se nanoparticles is found to play an extremely important role. Such an effect (18) Ghosh, S. K.; Kundu, S.; Mandal, M.; Pal, T. Langmuir 2002, 18, 8756. (19) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J. Appl. Catal., B 2001, 31, 145. (20) Pal, T.; De, S.; Jana, N. R.; Pradhan, N.; Mandal, R.; Pal, A.; Beezer, A. E.; Mitchell, J. C.; Langmuir 1998, 14, 4724.
Notes
Figure 5. Absorbance as a function of time in the presence of (b) no catalyst (selenium) particles, (f) commercially available a-Se, (2) agglomerated black a-Se, (9) TX-100 stabilized Se nanoparticles. Conditions: [MB] ) 10-4 M, [Se] ) 1.1 × 10-5 M, flux of UV light ) 340 lx.
Figure 6. The variation of rate with different concentrations of selenium particles in aqueous medium. Conditions: [MB] ) 10-4 M, flux of UV light ) 340 lx.
Figure 7. Absorbance vs time plot for the photodecolorization of methylene blue in the presence of TX-100 stabilized selenium nanoparticles in aqueous medium with variable flux of UV light. Conditions: [MB] ) 10-4 M, [Se] ) 1.1 × 10-5 M.
is schematically presented in Scheme 1. It has also been shown that the decolorization of the dye depends linearly with the concentration of the catalyst particles used. The linear variation in rate (the rate has been calculated, considering the first order kinetics of the degradation process) with the change in concentration of nanoparticles in the reaction mixture is shown in Figure 6. Interestingly, the rate of decolorization has been found to have a bearing
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Scheme 1. Schematic Representation of Degradation of Methylene Blue in H2O, Black Se, and TX-100 Stabilized Se Nanoparticles
on the flux of UV light. So the detail study has been carried out for the dye decolorization with variable flux of UV light keeping all other parameters unaltered. Figure 7 depicts a series of results where the absorbance values have been presented as a function of time with respect to different flux of UV light. Cyclic voltametry with a glassy carbon electrode for the reaction mixtures of both the UVexposed and unexposed solutions confirmed the degradation of the dye skeleton. The reversible redox cycle of the dye gets lost completely; i.e., the dye becomes redox inactive after the UV irradiation. It is to be mentioned that the nonionic micelle TX-100 not only imparts stability to the nanoparticles but also interacts with the dye molecules (hydrophobic interaction). Authentication of the dye-micelle interaction was revealed from the observed red shift (though small) in the λmax value of the dye in the micellar environment. The dye shows the λmax at 662 nm in water, which is shifted to 664 nm in TX-100 medium. It has already been mentioned that the black agglomerated solid Se particles has very little catalytic activity with respect to TX-100 stabilized Se particles. The decolorization of the dye with freshly prepared Se (i.e., just after its preparation by borohydride) was not studied immediately because of the obvious interferences from the unreacted borohydride in the reaction medium. Therefore, after the preparation of Se particles, the solution along with borohydride was reserved overnight to decompose all the unreacted borohydride. After the decomposition of unreacted borohydride, the solution left the precipitated Se particles at the bottom of the container. To avoid such aggregation and/or precipitation, TX-100 micelle was introduced (as mentioned above) into the reaction medium within 5 min of NaBH4 addition. In a separate set of experiments, unreacted borohydride was
decomposed by warming the freshly prepared Se solution on a water bath. Here, the rate of photodegradation of the dye involving fresh Se particles (where unreacted NaBH4 was decomposed on water bath) was same as that of the solution containing aged (kept overnight) Se particles. The micelle leads to an interaction with dye as well as endows stability to the nanoparticles. Therefore the UVassisted degradation process becomes vulnerable in the presence of TX-100 stabilized selenium nanoparticle, and the reaction reaches completion with the total degradation of dye skeleton. Conclusion The paper discusses the synthesis and especially the stabilization of selenium nanoparticles in micellar environment with a relatively simple procedure that may be preferred to existing preparation methods. The nanoparticles were characterized in both solution and solid phases to ensure its structure. The characteristic catalytic behavior of the Se particles is established by studying the decolorization of methylene blue in the presence of UV light. It has been authenticated from the study that the nanoparticles afforded a complete mineralization process and the rate of dye decolorization varies linearly with the nanoparticle concentration. The simple, reproducible, and less-time-consuming preparation of stable selenium nanoparticles with an effective catalytic efficiency becomes the key aspect of the article. Acknowledgment. The authors are grateful to Council of Scientific and Industrial Research (CSIR), Department of Science and Technology (DST), New Delhi, and Inter University Consortium (IUC), Mumbai, for financial assistance. LA049318L
