Langmuir 2001, 17, 4129-4130
Trap State Emission from TiO2 Nanoparticles in Microemulsion Solutions Hirendra Nath Ghosh* and Soumyakanti Adhikari Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Received January 10, 2001. In Final Form: April 5, 2001
Introduction The optical and electronic properties of semiconductor nanoclusters have recently been of great interest.1 This is largely due to the phenomenon of quantum confinement, whereby in sufficiently small particles, the optical and electronic properties depend strongly on the particle size. In addition to quantum confinement effects, semiconductor nanoclusters show properties that are strongly affected by their large surface-to-volume ratios. A significant fraction of the atoms reside on the nanocluster surface. These surface atoms having “dangling bonds” may act as electron and hole traps that can dominate electron/hole recombination and other processes. It is often possible (and desirable) to passivate the surface traps. Several studies have shown that passivation of surface traps has large effects on the nanocluster photophysics.2-6 Band-edge and trap state emissions are very common for direct band gap semiconductors7 like CdS, GaAs, and ZnO, but for indirect semiconductors7 like TiO2 and GaP, these types of emission are very difficult to observe. Photoand electroluminescence spectra were detected by Nakato et al.8 from n-TiO2 and transition metal doped n-TiO2 on the electrode surface. In situ photoluminescence of commercially available TiO2 particles at 77 K was reported by Anpo et al.9 Emission from specially synthesized nanosize TiO2 particles (d ) ∼3 nm) was also observed by Claus et al.,10 but emission from widely used anatase TiO2 nanoparticles in solution phase is not reported in the literature. In the present investigation, we have observed the emission from TiO2 nanoparticles after dispersing the particles in the water pool of a microemulsion. The microemulsion contains NaLS (sodium lauryl sulfate)/ water/cyclohexane/1-butanol with w0 ) 14 (w0 ) [H2O]/ [surfactant]). The diameters of the water pools are around 4-5 nm. Time-resolved emission measurements were carried out to monitor the emission observed from the nanoparticles. From lifetime decay analysis, we get an * Corresponding author. E-mail:
[email protected]. (1) Semiconductor Nanoclusters - Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier: Amsterdam, 1997. (2) Spahel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (3) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (4) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946. (5) Kortan, A. R.; Hull, R.; Opila, R. L.; Bawendi, M. G.; Steigerwald, M. L.; Carroll, P. J.; Brus, L. E. J. Am. Chem. Soc. 1990, 112, 1327. (6) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927. (7) Pankove, J. I. Optical Processes in Semiconductors; Dover Publications, Inc.: New York, 1971. (8) Nakato, Y.; Tsumura, A.; Tsubomura, H. Chem. Phys. Lett. 1982, 85, 387. (9) Anpo, M.; Tomonari, M.; Fox, M. A. J. Phys. Chem. 1989, 93, 7300. (10) Liu, Y.; Claus, R. O. J. Am. Chem. Soc. 1997, 119, 5273.
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idea of multiexponential recombination dynamics of the photogenerated electrons and holes in the nanoparticles. Experimental Section (a) Materials. NaLS (sodium lauryl sulfate) was obtained from Fluka and purified from ether. Titanium(IV) tetraisopropoxide {Ti[OCH(CH3)2]4} (Aldrich, 97%), cyclohexane (Aldrich), 1-butanol (Aldrich), and isopropyl alcohol (Aldrich) were purified by distillation. Nanopure water was used for making aqueous solutions. Nanometer-size TiO2 was prepared by controlled hydrolysis of titanium(IV) tetraisopropoxide following standard procedure as described elsewhere.11-13 (b) Preparation of Microemulsion. The microemulsion was prepared using the following composition (2 g sodium lauryl sulfate (NaLS)/1.8 cm3 water/ 46.5 cm3 cyclohexane/9.5 cm3 1-butanol). The diameters of the water pools are around 4-5 nm. Pileni et al.14 have shown an empirical relation between w0 and r as r ) 1.5w0 (where r is the radius of the water pool for the microemulsion). According to Adhikari et al.,15 the water droplet size is ∼4.2 nm in the present microemulsion studied. TiO2 nanoparticles in this study can be incorporated into the water pool. For making the experimental solution, first we dissolve NaLS in cyclohexane and then add 1-butanol. Dry TiO2 powder was dissolved separately in water, and then a certain volume of this solution was added to the surfactant solution to get the desired w0. After adding TiO2/water, the solution was shaken vigorously for 10-15 min to obtain a transparent microemulsion solution. This part is very crucial because emission can be obtained only when TiO2 particles just fit in the water pool, though a slight swelling of the reverse micelle is expected.14 The experiments were also carried out at other w0 values (e.g., w0 ) 12, 16, and 19).
Results and Discussion UV-vis spectroscopy (Shimadzu 160A spectrophotometer) is used to characterize the optical absorption of the nanoparticles in the microemulsion as shown in Figure 1 (inset). It has been observed that the optical absorbance of the nanoparticles does not change by dispersing the nanoparticles in the microemulsion. The absorption and the corresponding band gap energy of TiO2 are λOS ) 385 nm and Eg ) 3.2 eV and match well with the literature value of the anatase TiO2 particles.16 The emission from the TiO2 nanoparticles in microemulsion at room temperature was observed by excitation of the sample with 350 nm light using a fluorescence spectrometer (Hitachi model F-4010) and is shown in Figure 1. Emission spectra were recorded exciting at different wavelengths from 300 to 360 nm at 10 nm intervals to confirm that the band (400-600 nm) is coming from the emission of TiO2 nanoparticles rather than scattering (not shown in the figure). At each excitation wavelength, the emission band was the same with a peak at 445 nm and a shoulder at 550 nm. The emission quantum yield (φ) was found to be 0.002. All the experiments were carried out in nitrogen purged conditions. The emission band matched well with photoluminescence (11) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L. J. Phys. Chem. 1984, 88, 709. (12) Ghosh, H. N.; Asbury, J. B.; Lian, T. J. Phys. Chem. B 1998, 102, 6482. (13) Ghosh, H. N J. Phys. Chem. B 1999, 103, 10382. (14) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985, 118, 414. (15) Adhikari, S.; Joshi, R.; Gopinathan, C. J. Colloid Interface Sci. 1997, 191, 268. (16) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1994, 92, 5196.
10.1021/la010062i CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001
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Langmuir, Vol. 17, No. 13, 2001
Figure 1. Emission spectrum (curve d) of TiO2 nanoparticles in microemulsion (sodium lauryl sulfate/water/cyclohexane/1 butanol with w0 ) 14, where w0 ) [water]/[surfactant]) exciting at 350 nm light at 298 °C. [TiO2] ) 0.10 g/L (φ ) 0.002). [Inset: optical absorbance spectra of microemulsion (a), TiO2/water (b), and TiO2/microemulsion (c)].
spectra of anatase TiO2 powder in air.17 To the best of our knowledge, we are reporting the emission from widely used anatase TiO2 nanoparticles in the solution phase for the first time. In the present investigation, the band gap emission could not be seen and emission was observed only in the microemulsion with w0 ) 14. A dramatic enhancement of emission from surfacemodified CdS colloidal nanoparticles has been observed earlier.3 A simple hypothesis had been proposed that the kinetically stable semiconductor colloids have a high density of surface defect sites. These sites cover a broad range of energies and structures. Many such defect sites exist at mid-band gap energies. These sites are involved in trapping initially produced electron-hole pairs, as evident from the fact that emission (resulting from recombination of electron-hole pairs) is significantly redshifted relative to the band gap absorption. The existence of different trap sites provides multiple pathways for radiative and nonradiative recombination. The molecules, which modify the surface of nanoparticles, bind to lower energy trap sites, which are directly involved in nonradiative decay. Such binding sites can act as efficient traps and can increase the quantum yield of emission. In the present study, the TiO2 nanoparticles are dispersed in the water pool of a microemulsion. After dissolving the nanoparticles in water, the pH of the colloidal solution become ∼2.5-3. At this lower pH, the surface of TiO2 nanoparticles becomes positively charged. The headgroup of the surfactant molecules (NaLS) can be attached to the surface of the nanoparticles. The sulfonic acid group can act as a surface modifier to remove the lower lying surface defects. Such interactions can promote the radiative recombination pathway and enhance the emission intensity. The emission is associated with transitions of electrons from the conduction band edge to holes trapped at interstitial Ti3+ sites.17,18 The lifetime analysis of the observed emission from a time-resolved fluorescence instrument which works on the principle of time-correlated single photon counting (model 199, Edinburgh Instruments) gave multiexponential decays with time constants (17) Poznyak, S. K.; Sviridov, V. V.; Kulak, A. I.; Samtsov, M. P. J. Electroanal. Chem. 1992, 340, 73. (18) Smandek, B.; Gerischer, H. Electrochim. Acta 1989, 34, 1411.
Notes
Figure 2. Emission decay kinetics of TiO2 nanoparticles in microemulsion (sodium lauryl sulfate/water/cyclohexane/1 butanol with w0 ) 14) exciting at 350 nm light at 298 °C. [TiO2] ) 0.10 g/L. The decay kinetics is fitted multiexponentially with τ1 ) 250 ps (22%), τ2 ) 2.53 ns (38%), and τ3 ) 7.7 ns (40%). L is the lamp profile.
of 250 ps (22.2%), 2.53 ns (37.74%), and 7.7 ns (40.06%) shown in Figure 2. From the lifetime analysis, we get an idea of recombination dynamics of photogenerated electrons and holes, which in this case is multiexponential. It has been reported earlier by Rothenberger et al.19 that electron trapping is much faster than hole trapping. Trapped electrons can recombine with holes in the valence band. As the trapping time for the electron is faster than that for the hole, the initial recombination reaction will take place between electrons in the shallow trap and holes in the valence band. So, the faster component in the observed decay kinetics can be assigned to recombination of trapped electrons and holes in the valence band. As the time progresses, the holes will also move to the shallow trap and electrons will move to the deeper trap. Hence, the slower components can be assigned to the recombination of trapped electrons and trapped holes of different trap depth. The wavelength dependence studies on emission kinetics can provide more information on the recombination dynamics of photogenerated electrons and holes in the TiO2 nanoparticles of different trap depths. Further, experiments are being carried out by changing the excitation and emission wavelengths, which will provide the true picture of recombination dynamics of trapped electrons and trapped holes. We are also carrying out experiments by varying the water pool size and composition of the microemulsion. The change in the water pool size and the nature of the surfactant should vary the density of trap states of the TiO2. Measurements in this direction will provide a clear picture of the trap state dynamics of TiO2 nanoparticles. Acknowledgment. We are thankful to Dr. A. V. Sapre for many fruitful discussions. We are also thankful to Dr. T. Mukherjee, Head, RC & CD Division, and Dr. J. P. Mittal, Director, Chemistry & Isotope Group, for their encouragement. LA010062I (19) Rothenberger, G.; Moser, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. Soc 1985, 107, 8054.
