Optical Study of Redox Behavior of Silicon Nanoparticles Induced by

Mar 26, 2009 - Syed Hamad , G. Krishna Podagatlapalli , V. S. Vendamani , S. V. S. Nageswara Rao , A. P. Pathak , Surya P. Tewari , and S. Venugopal R...
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J. Phys. Chem. C 2009, 113, 6480–6484

Optical Study of Redox Behavior of Silicon Nanoparticles Induced by Laser Ablation in Liquid Shikuan Yang, Weiping Cai,* Guangqiang Liu, Haibo Zeng, and Peisheng Liu Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: February 10, 2009

The redox behavior of silicon nanoparticles (Si-nps), produced by laser ablation in liquid, was studied based on the optical measurements of Si colloidal solution after addition of noble metal salts. It was found that such prepared Si-nps can reduce Au3+ ions quickly, in contrast to the conventional microsized Si powders which are very stable in the noble metal ion solutions. Also, the Si-nps can reduce Ag+ ions, which should not occur according to the overall redox potentials of the ion solutions and bulk Si. Such reductive behaviors were attributed to the fresh surface of the Si-nps prepared by laser ablation in liquid and size-induced positive redox potentials for the system containing silicon and metal salts. This study could be beneficial to deepen the understanding of physical/chemical behaviors of Si-nps and important in the control of the chemical stability of the particles. Importantly, due to the nontoxic properties of Si and Si oxide, the reductive Si-nps have demonstrated the potential applications in pollution remediation of heavy metal ions, like reduction of Cr6+ or Hg2+, in wastewater or soil, in addition to the optical and electronic industry. 1. Introduction It is well-known that silicon nanoparticles (Si-nps) have recently attracted much attention in both scientific research and application areas, due to their unique optical, electrical, and chemical properties.1,2 Because of the quantum confinement effect, small Si-nps can emit bright blue light,3,4 which is important in biomedical tagging, red-green-blue silicon-based full color display, and flash memories.5-7 With a size decrease of the Si particles, the activity will increase and hence the chemical stability of the particles will decrease. Although it has been found that the stability of Si-nps could be improved by surface modification, such as alkyl passivation,8-10 there has been very limited reporting about the redox properties of Sinps so far, to our best knowledge. Here, we present an optical study of the reductive behavior of the Si-nps, dispersed in ethanol, in situ produced by laser ablation in liquid.11 It has been found that in contrast to the normal Si powders, the Sinps show strong reductive properties to noble metal ions. This study could be beneficial to deepen the understanding of physical/chemical behaviors of Si-nps and important in control of the chemical stability of the particles. Furthermore, this reductive property of Si-nps could also be useful in the fabrication of new nanomaterials with composite structure, such as Si/noble metals or SiO2/noble metal nanoarchitecture. Importantly, due to the nontoxic properties of Si and Si oxide, such reductive Si-nps have exhibited potential applications in pollution remediation of the heavy metal ions, like reduction of Cr6+ and Hg2+, in wastewater or soil, which are from industrial discharge and are toxic to bacteria, plants, and human.12-14 The details are reported in this article. 2. Experimental Section A colloidal solution of Si-nps was first prepared by laser ablation of a silicon wafer immersed in a liquid, as previously * To whom all correspondence should be addressed: E-mail: [email protected].

described.4 Briefly, a silicon wafer was supersonically rinsed in deionized water and then ethanol for 1 h. The cleaned silicon wafer was immersed in a vessel filled with about 8 mL of ethanol. The irradiation on the wafer was conducted by the first harmonic (1064 nm) Nd:YAG laser, operated at 100 mJ/pulse with pulse duration of 10 ns and frequency of 10 Hz. The laser beam was focused on the Si wafer with a spot size about 2 mm in diameter by using a lens with a focal length of 150 mm. The solution was continuously stirred during irradiation. After irradiation for 30 min, a brown yellow silicon colloidal solution was obtained. The yield of Si-nps in our condition can be estimated to be about 50 µg/min by the laser shot-induced mass loss of the silicon wafer. So, 30 min of irradiation produces the colloidal solution containing about 0.2 g/L of Si. The as-prepared 8 mL colloidal solution was subsequently diluted to 40 mL with deionized water (or forming a colloidal aqueous solution with 20% ethanol) and then added with different amounts of 0.3 mM HAuCl4 aqueous solution during magnetic stirring. The reaction occurred immediately and finished in a few minutes after addition. Optical absorption measurements were performed on a Cary 5E UV-vis-IR spectrometer from 200 to 800 nm in wavelength at room temperature. After addition of HAuCl4 and complete reaction, some of the colloidal solutions were centrifuged at 12 000 rounds/min and the upper transparent part was removed, then the distilled water was added. This process was repeated 3 times to wipe off the remaining ions, before drying at 40 °C to obtain powder products. Some of the as-prepared irradiated solution (parallel sample) without addition of HAuCl4 was dried at 40 °C to obtain pure powders for reference. X-ray diffraction (XRD) was measured on an X-ray diffractometer (the Philips X’Pert) with a Cu KR line (0.15419 nm). Transmission electron microscopic (TEM) examination was conducted on JEM-200CX operated with 200 kV by dispersion of these powders in ethanol and dropping the solution on the carbon-coated Cu mesh.

10.1021/jp810787d CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

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Figure 3. Microstructural examination of the sample shown in Figure 2b: (a) TEM image and (b) high-resolution TEM image of panel a. Figure 1. Morphology of the original silicon nanoparticles prepared by laser ablation of Si wafer in ethanol: (a) TEM image [insets are the corresponding SAED (upper) and XRD (lower)]; (b) high-resolution TEM image of panel a.

Figure 4. Raman spectra of the as-prepared Si-nps (curve a) and the powders from the sample shown in curve h of Figure 2a (curve b).

Figure 2. (a) Optical absorption spectral evolution of the Si colloidal solution with the added amounts of HAuCl4 (0.3 mM) (see the text): curve a, the Si colloidal solution without addition; curves b-h, after addition of HAuCl4 for 4, 14, 24, 28, 34, 40, and 50 mL, respectively; curve I, for HAuCl4 aqueous solution (0.5 mM). (b) The XRD corresponding to the sample shown in curve h of panel a.

3. Results 3.1. Si Nanoparticles. The as-prepared colloidal solution by laser ablation in liquid is very stable and no precipitation occurs within two months. The lower inset in Figure 1a shows the XRD pattern of the as-prepared powders (the reference sample). The products consist of a silicon phase with a diamond structure. TEM examination shows that the products (or Si) are nearly spherical particles, as illustrated in Figure 1a. Most of them are about 6 nm in size, and a few are much larger (about 15 nm). The selected area electron diffraction (SAED) pattern corresponds to the diffraction of Si [see the top inset of Figure 1a], in good agreement with XRD data. Further examination in high-resolution TEM has revealed that many of the Si-nps are aggregates consisting of ultrafine crystals with a mean size of about 4 nm, as seen in Figure 1b. These results are similar to those in our previous report.4 3.2. Optical Measurements. The optical absorption spectrum for the as-prepared silicon colloidal aqueous solution is shown as curve a in Figure 2a. There only exists an absorption edge

below 400 nm. After addition of HAuCl4, the colloidal solution changes from pale yellow to pink, depending on the added amount. Figure 2a shows the corresponding spectral evolution of the optical absorption. A small addition of HAuCl4 solution leads to the red-shift of the absorption edge (see curve b in Figure 2a). When adding 14 mL of HAuCl4 solution, a shoulder can be seen around 520 nm superimposed on the background of the red-shifted absorption edge (curve c in Figure 2a). If more HAuCl4 solution is added, the shoulder becomes an obvious absorption peak around 520 nm, as illustrated in curves d and e of Figure 2a (corresponding to addition of 24 and 28 mL). Further addition leads to significant enhancement of the peak accompanied by a red-shift from 520 nm to 540 nm corresponding to an addition of 50 mL (curve h in Figure 2a). For comparison, the optical absorption spectrum of HAuCl4 solution was also measured, as indicated in curve I of Figure 2a. There is an absorption peak around 310 nm. This peak directly originates from the d-d transition of the AuCl4- ions, which is a unique “fingerprint” for the AuCl4- ions and has been reported previously.15,16 The optical absorption shoulder or the peak around 520-540 nm should be attributed to the surface plasmon resonance (SPR) of Au nanoparticles, which is well-known and extensively reported.17,18 This means that Si-nps can induce reduction of AuCl4- ions and formation of Au nanoparticles, which has been confirmed by the following experiments. XRD measurement has indicated the formation of metal Au, but no Si crystal is detected, for the corresponding powders from the sample shown in curve h of Figure 2a, as illustrated in Figure 2b. The TEM observation has demonstrated the existence of nearly spherical nanoparticles smaller than 20 nm in mean size (see Figure 3a). However, the corresponding selected area electronic diffraction shows the coexistence of Au and Si crystals. The high-resolution TEM examination has revealed that there are a few Si-nps but with

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Figure 5. Optical absorption spectrum of the sample after addition of 10 mL of 40 mM AgNO3 aqueous solution to the 8 mL as-prepared Si colloidal ethanol solution and stirring for 24 h.

much smaller size than those of the as-prepared sample (see the circles in Figure 3b). The existence of Si-nps after complete reduction of HAuCl4 is further verified by Raman shift spectra, as illustrated in Figure 4. Addition of HAuCl4 has not induced the disappearance of the Raman peak of Si-nps (see curve b in Figure 4).19 The peak shifts to red compared with that before HAuCl4 addition (see curve a in Figure 4), indicating reduction of the silicon grain size. If AgNO3, instead of HAuCl4 solution, was added to the asprepared Si colloidal solution, the reduction of Ag+ ions will also take place. Figure 5 presents the optical absorption spectrum for the sample after addition of 10 mL of 40 mM AgNO3 aqueous solution to 8 mL of as-prepared Si colloidal ethanol solution and stirring for 24 h. We can observe one broad absorption peak located around 440 nm, which is the wellknown SPR peak of silver nanoparticles.17,18 However, the Sinps colloidal solution cannot reduce Cu2+ ions without other additives, meaning that the reductive potential of such-prepared Si-nps is that between Cu and Ag. Finally, it should be mentioned that if HAuCl4 solution (the same for the AgNO3 solution) was added with the normal commercial microsized silicon powders (about 5 µm in size and much more than that of the Si-nps), no change was observed. The optical absorption spectrum is the same as that of HAuCl4 solution (similar to curve I in Figure 2a), indicating no reaction. 4. Discussion Although the fundamental mechanism regarding formation of nanostructures by laser ablation in liquid is still not fully understood, according to previous studies of laser ablation at the liquid-solid interface, interaction between pulsed laser light and the target can produce instant local high-temperature and high-pressure plasma plumes above the target surface.20-24 In our case, the hightemperature and high-pressure Si plasma will be formed on the solid-liquid interface quickly after the interaction between the pulsed laser and the silicon wafer. Subsequently, such hightemperature and high-pressure Si plasma will ultrasonically and adiabatically expand, leading to cooling of the silicon plume and hence to formation of silicon clusters. With the extinguishment of the plasma, the formed adjacent silicon clusters aggregate quickly into Si-nanoparticles.25 4.1. Redox Properties of Si-nps. Whether the reduction reaction can take place spontaneously depends on the overall redox potentials of ion solutions and Si-nps. The standard redox potential of gold ions is +1.002 V for the reaction26

Figure 6. Optical absorption spectral evolution with the time in the dark during magnetic stirring for the mixed solution prepared by adding 5 mL of Si colloidal ethanol solution (containing Si about 0.2 g/L) into 20 mL of K2Cr2O7 aqueous solution (0.05 mM). Curve a is the spectrum of the mixed solution without addition of Si. Curves b, c, and d correspond to the optical absorption spectra of the Si-nps contained mixed solution after remaining in the dark for 12, 48, and 120 h, respectively, and subsequent centrifuging at 12000 round/min for 1 h to remove the suspended nanoparticles in the solution.

and that of bulk Si is -0.84 V for the reaction27

Si f Si4+ + 4e-

4Au3+ + 3Si f 4Auo + 3Si4+

(1)

(3)

This means that silicon atoms can reduce Au3+ ions without other additives thermodynamically. The Si-nps in this work was in situ produced in ethanol without exposure to oxygen and should keep fresh silicon surfaces. When HAuCl4 was added to the as-prepared Si-nps colloidal solution, Au3+ ions react with silicon atoms on the Si-nps’ surface, leading to reduction of Au3+ ions and formation of Au nanoparticles, which also induces dissolution of Si atoms and the decrease of Si-nps’ size. However, the normal commercial Si powders cannot reduce the Au3+ ions. This can be attributed to the surface oxidation during exposure to air. X-ray photoelectron spectroscopic measurement has confirmed the existence of a silica layer on the microsized Si particles. To confirm that the reduction originates from the fresh silicon surfaces, we prepared Si-nps in deionized water instead of ethanol. Obviously, the Si-nps in the water cannot be free from native oxide. It has been shown that the Si-nps prepared in the water exhibit much slower reductive speed of Au3+ ions, compared with those in ethanol. Further, if adding the Si-nps prepared in ethanol and aged for at least one month, the color of the Au3+solution changes much slower than that after addition of the freshly prepared Si-nps in ethanol. For the latter, the color changes immediately from brown yellow to pink (within a few seconds). So, it is reasonable to believe that the Si-nps prepared in ethanol are of a fresher surface. For the silver case, however, it is quite different from the aforementioned. The standard redox potential of Ag is 0.7996 V for the reaction27

Ag+ + e- f Ag +

AuCl4- + 3e- f Auo + 4Cl-

(2)

The potential of these two half-cells is a positive value (0.16 V). According to the mixed potential theory (MPT),28,29 such positive overall potential should lead to a spontaneous occurrence of the following reaction.

(4)

For our case, Ag concentration is about 22 mM. The corresponding redox potential should be about 0.71 V according to the Nernest equation.30 The overall potential is negative (-0.13

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Figure 7. The Cr6+ normalization concentration (from the optical absorbance measurement at 370 nm in Figure 6) in the mixed solution versus the time. The starting Cr6+ concentration C0 is 0.4 mM. The line is to aid the eye.

V) for the Ag/Si system. On the basis of the MPT, bulk Si is thermodynamically unable to reduce Ag+ without the other agents. This is obviously contrary to the results of our Si-nps, which can reduce Ag+ ions. Such abnormal reduction could be attributed to the size effect of the Si-nps. It is well-known that nanoparticles are of higher surface activity due to the high surface-to-volume ratio than bulk ones.5 This means that the redox potential of Si-nps should be higher that the standard value of -0.84 V and even high enough for the Ag+ ions to be reduced, or the reaction

4Ag+ + Si f 4Ag + Si4+

(5)

can occur. The finding that small Si-nps have an increased redox potential is important in understanding the physical/chemical properties of Si-nps. Calculations concerning such variation of redox potential as a function of particle size are in progress. Such findings probably supply an easy way to determine the size of particles based on the redox potential detection. As for the case that the Si-nps cannot reduce the Cu2+ ions, the potential increment induced by the reduced size of the Sinps is insufficient to cover the gap between them in redox potentials. 4.2. Applications in Pollution Remediation. Since the standard redox potentials for Cr6+ to Cr3+ and Hg2+ to Hg are +1.33 and +0.851 V, respectively,27 both of which are higher than that of bulk Si, it can be expected that our Si colloidal solution could be used for remediation of the Cr6+ or Hg2+ ion pollution in wastewater or soil,31 which has been confirmed by our further experiments. Here we only show the reduction of Cr6+ to Cr3+ ions based on optical measurements to demonstrate the validity of pollution remediation by the Si colloidal solution prepared in this study. We added the as-prepared Si colloidal ethanol solution (5 mL, containing about 0.2 g/L of Si) into K2Cr2O7 aqueous solution (20 mL, 0.05 mM) and kept the mixture in the dark during magnetic stirring. There are two absorption peaks around 270 and 370 nm for the mixture solution without Si, as seen in curve a of Figure 6. The peak at 370 nm directly originates from the intrinsic absorption of Cr6+ ions, or is a unique “fingerprint” for the Cr6+ ions, and its intensity is often used as the concentration label of Cr6+ ions.32 After addition of our Sinps, the peak at 370 nm decreases with time, indicating reduction of Cr6+ concentration in the solution, as illustrated in curves c and d of Figure 6, corresponding to the optical absorption spectra of the Si-nps containing mixed solution after remaining in the dark for different times and subsequent centrifuging at 12 000 rounds/min for 1 h to remove the suspended nanoparticles in

the solution. Figure 7 presents the normalized Cr6+ concentration (C/C0, C0 is the starting Cr6+ concentration) versus time from the time-dependent change of the peak intensity at 370 nm in Figure 6. If we added the normal commercial Si powders to the Cr6+ aqueous solution, however, the optical spectrum is nearly unchanged within several weeks, almost the same as curve a in Figure 6. This indicates that normal commercial Si powders cannot reduce the Cr6+, in contrast to the Si-nps prepared in this study. This could also be attributed to the surface oxide layer of the Si particles. Laser ablation in liquid can produce the Si-nps in liquid without oxidation. Such prepared Si-colloidal solution could be a good candidate to be used for pollution remediation, such as reduction of heavy metal ions in wastewater or soil. 5. Conclusions In summary, the redox properties of Si-nps, produced by laser ablation in ethanol, were studied based on the optical measurements of Si colloidal solution after addition of noble metal salts. The Si-nps in situ produced in the ethanol can keep fresh particle surfaces and thus exhibit reductive properties to Au3+ ions due to the overall positive redox potential for the metal ions/Si-nps system, in contrast to the conventional Si microsized powders which have no such reductive behavior because of the inevitable oxide layer on the surface of particles. Also, the Si-nps can reduce Ag+ ions, which should not occur according to the overall redox potentials of ion solutions and bulk Si, due to size-induced increase in the redox potential of Si-nps. Such prepared Si colloidal solution can be used for pollution remediation like the reduction of heavy metal ions Cr6+ or Hg2+ in wastewater or soil due to nontoxic properties of Si and SiO2. Furthermore, this study could also be useful in the decoration of Si-nps to acquire multifunctionalized products,33 with magnetic properties, surface enhanced Raman properties, and unique blue light emission together. Acknowledgement. . This work is financially supported by the Natural Science Foundation of China (Grant Nos. 50671100 and 10604055), the National Basic Research Program of China (973 Program) (Grant No. 2007CB936604), and the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KJCX2-SW-W31). References and Notes (1) Hirschman, K. D.; Tsybeskov, L.; Duttagupta, S. P.; Fauchet, P. M. Nature (London) 1996, 384, 338. (2) Bruchez, M. Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (3) Sˇvrcˇek, V.; Sasaki, T.; Shimizu, Y.; Koshizaki, N. Appl. Phys. Lett. 2006, 89, 213113. (4) Yang, S. K.; Cai, W. P.; Zeng, H. B.; Li, Z. G. J. Appl. Phys. 2008, 104, 023516. (5) Ding, Z. F.; Quinn, B. M.; Haram, S. K.; Pell, L. E.; Korgel, B. A.; Bard, A. J. Science 2002, 296, 1293. (6) Larson, D. R.; Zipfel, W. R.; Williams, R. M.; Clark, S. W.; Bruchez, M. P.; Wise, F. W.; Webb, W. W. Science 2003, 300, 1434. (7) Wang, L.; Reipa, V.; Blasic, J. Bioconj. Chem. 2004, 15, 409. (8) Zou, J.; Baldwin, R. K.; Pettigrew, K. A.; Kauzlarich, S. M. Nano Lett. 2004, 4, 1181. (9) Neiner, D.; Chiu, H. W.; Kauzlarich, S. M. J. Am. Chem. Soc. 2006, 128, 11016. (10) Neyfeh, M. H.; Barry, N.; Therrien, J.; Akcakir, O.; Gratton, E.; Belomoin, G. Appl. Phys. Lett. 2001, 78, 1131. (11) (a) Zeng, H. B.; Cai, W. P.; Li, Y.; Hu, J. L.; Liu, P. S. J. Phys. Chem. B 2005, 109, 18260. (b) Kabashin, A. V.; Meunier, M.; Kingston, C.; Luong, J. H. T. J. Phys. Chem. B 2003, 107, 4527. (c) Pyatenko, A.; Shimokawa, K.; Yamaguchi, M.; Nishimura, O.; Suzuki, M. Appl. Phys.

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