Relationships of surface oxygen vacancies with photoluminescence and photocatalytic performance of ZnO nanoparticles

Transcription

1 Science in China Ser. B Chemistry 2005 Vol.48 No Relationships of surface oxygen vacancies with photoluminescence and photocatalytic performance of ZnO nanoparticles JING Liqiang 1,2, YUAN Fulong 1,HOUHaige 1,XINBaifu 1,CAIWeimin 2 & FU Honggang 1 1. The Key Laboratory of Physical Chemistry, School of Chemistry and Material Sciences, Heilongjiang University, Harbin , China; 2. Department of Environmental Sciences and Engineering, Harbin Institute of Technology, Harbin , China Correspondence should be addressed to Fu Honggang ( jlqiang@sohu.com) Received June 16, 2004 Abstract The ZnO nanoparticles are prepared by a precipitation process, and also are characterized by means of the modern testing techniques such as XPS, ESR, SPS and PL. The activity of the as-prepared ZnO is evaluated in the photocatalytic oxidation of gas phase n-c 7 H 16. The relationships of surface oxygen vacancies (SOV) with photoluminescence (PL) and photocatalytic performance are discussed in details. The results show that the smaller the particle size, the larger the SOV content, the stronger the PL signal, the higher the photocatalytic activity, indicating that the SOV, PL and photocatalytic activity have inherent relationships. This was because of the reasons that the PL signal is attributed to the free and binding excitons resulting from the SOV, while the SOV is favorable for a photocatalytic oxidation reaction since the SOV can easily capture the photoinduced electrons, and the captured electrons had strong interactions with the adsorbed oxygen. In addition, the surface states of ZnO nanoparticles, arising from the SOV and oxygen species, are very abundant. Keywords: ZnO, nanoparticle, oxygen vacancy, photoluminescence, photocatalysis. DOI: /03yb0191 ZnO is one of important semiconductor materials, applied widely in the fields such as the cerams, piezoelectric sensors, catalysts and luminescence apparatus. ZnO nanoparticles not only are ideal materials to prepare newly electronic apparatus [1], but also are further expanded for application in some important fields like photoelectric conversion [2], photocatalyst [3,4], chemical sensor [5] and luminescence materials [6]. In those applied fields, the functional performances all are closely related to the surface characteristics like SOV of the paticles [7]. Some testing techniques such as electron spin resonance (ESR), surface photovoltage spectrum (SPS) and photoluminescence spectroscopy (PL) can disclose some information about surface characteristics (like SOV and defects), surface states and the separation and/or recombination of photoinduced electron-hole pairs of nanosized semiconductor materials so that it is very useful to prepare functional semiconductor materials with higher activity by providing a powerful basis in the theory [8 15]. In this paper, ZnO nanoparticles with three kinds of particle size are prepared, and also are characterized by XPS, SPS, ESR and PL spectra. The relationships of surface structure with PL and photocatalytic activity Copyright by Science in China Press 2005

2 26 Science in China Ser. B Chemistry of ZnO nanoparticles are principally discussed, and the inherent factors influencing mainly the material function are revealed in essence. 1 Experimental The ZnO nanoparticles are prepared by a precipitation method as follows [16]. Firstly, the precursor, Zn 5 (CO 3 ) 2 (OH) 6, highly dispersed and easily decomposed, is synthesized, then the precursor is calcined to obtain ZnO nanoparticles. The ZnO samples calcined at 320, 430 and 550k for1harereferredtoasa,b and c, respectively. The samples are analyzed by XRD using a Rigaku D/MAX-rA powder diffractometer with a nickel-filtered Cu K α radiation source, the phase structure and crystallite size are determined from the X-ray diffraction patterns. The samples are observed with a HITACHI H-8100 transmission electron microscope (TEM), the size and morphology are obtained from the TEM micrograph. The surface compositions of the samples are examined by a VG ES- CALAB MKII X-ray photoelectron spectrometer (XPS) using a monochromatic aluminum X-ray source. The electron spin state and structure on the surface of the samples are measured by Bruker ER 200D- SRC electron paramagnetic resonance spectrometry (ESR). The surface photovoltage spectra of the samples are obtained with the surface photovoltage spectrometer made by the Department of Chemistry, Jilin University [17,18]. The photoluminescence (PL) spectra of the samples are recorded with a PE LS 55 spectrofluorophotometer. The experiment to evaluate the photocatalytic activity for degrading the gas phase n-c 7 H 16 is described in the literature [19]. 2 Results and discussion 2.1 The crystal structure, particle size and morphology of ZnO nanoparticles The TEM photomicrographs show that ZnO nanoparticles, calcined at 320, 430 and 550k, allare very small global grain with a narrow size range, their average particle sizes are about 14, 19 and 26 nm [12], respectively. In addition, the crystallite size D of nanoparticles can be estimated from the width of the lines in the X-ray diffraction spectrum with the aid of Scherrer formula: D =Kλ/(β cosθ ), where λ is the wavelength of the X-ray used, β is the width of the line at the half-maximum intensity, and K is a constant [16]. The estimated values D of ZnO nanoparticles calcined at 320, 430 and 550k are 13.5, 19.3 and 26.1 nm, respectively, which is in good agreement with the TEM results, indicating that ZnO nanoparticles can easily be dispersed. Moreover, XRD measurements show that the samples are ZnO polycrystalline powders. Their number of XRD PDF is , with a wurtzite structure attached to hexagonal system. 2.2 The surface composition and structure of ZnO nanoparticles According to the different characterizing binding energy of different elements, the element composition on the material surface can be examined. The Zn 2p XPS peaks of different ZnO nanoparticles all are sharp, and their binding energies are nearly the same, all about at ev, demonstrating that Zn element exists mainly as the form of Zn 2+ chemical state on the surface of different ZnO nanoparticles. It can be seen that the O 1S XPS of ZnO nanoparticles are wide and asymmetric (showed in fig. 1), indicating that at least two kinds of oxygen species are present, i.e. the crystal lattice ( ev) and adsorbed oxygen ( ev), and the latter including chemically and physically adsorbed oxygen [12]. Table 1 shows the XPS data of O and Zn elements on the surface of ZnO nanoparticles. It can be found that the atomic number ratios of Zn to O (total) of different ZnO nanoparticles almost keep unchanged, demonstrating that compared with Zn, the O amount does not change at all. However, the percentage of the O (crystal lattice) to the O (total) increases as the calcining temperature increases, meanwhile the atomic number ratios of Zn to O (crystal lattice) decreases, indicating that the amount of O (crystal lattice) increases, while that of O (adsorbed) decreases. In addition, it should be pointed out that the atomic number ratios of Zn to O (crystal lattice) are much larger than 1, which demonstrates that there are lots of oxygen vacancies on the surface of ZnO

3 Relationships of surface oxygen vacancies with photoluminescence and photocatalytic performance of ZnO nanoparticles 27 nanoparticles, and the lower the calcining temperature, the more the SOV content. The oxygen vacancies are very active species, easily combined with other groups to become stable, which is responsible for the existence of a certain amount of adsorbed oxygen on the nanoparticle surfaces [16,20]. and the interactions are unequal, that is to say, the bigger the adsorbed oxygen amount, the more remarkable the hyperfine structure. Thus, the magnetic nucleuses can be attributed to the adsorbed 17 O species [21,22]. Fig. 2. ESR spectra of ZnO at liquid nitrogen temperature. Fig. 1. XPS of O 1s on the surface of ZnO (a). The ZnO nanoparticles all can exhibit a stable and strong ESR signal with the g factor of at room temperature. It can be concluded that the ESR signal results from the SOVs, which can capture and bind the electrons so that the paramagnetic resonance source with S = 1/2 is produced [20]. The ESR intensity rapidly decreases as the particle size of ZnO nanoparticles increases, which corresponds to the decrease inthe content of the SOVs. However, the ESR spectrum of ZnO nanoparticles has an obviously hyperfine structure with 6 lines of unequal intensity at liquid nitrogen temperature as showed in fig. 2, and the coincidence constant is G, which demonstrates that the electrons captured by the SOVs have strong interactions with ambient magnetic nucleuses of I =5/2, Therefore, the action essences of the oxygen vacancies on the surface of ZnO nanoparticles are revealed by means of the ESR method mentioned above, in other words, easily capturing and binding electrons, and there are strong interactions between the captured electrons and adsorbed oxygens, which favor the PL and photocatalytic oxidation reaction. 2.3 The surface photovoltage and PL characteristics The direction that the photoinduced charge carriers move to can change in the presence of an external electric field, and the electronic transitions related to the surface states can also contribute to the SPS signal. Fig. 3 shows the electric field induced surface photovoltage (EFISPS) of ZnO nanoparticles (a). It can be seen that the SPS response is weakened when a positive external electric field is added. On the contrary, if a negative external electric field is employed, the SPS Table 1 XPS data of O and Zn elements on the surface of ZnO nanoparticles ZnO samples Binding energy of Zn2p/eV Binding energy of two kinds of O1s/eV Percentage of that O to total O (%) Atomic ratio of Zn/O(L) Atomic ratio of Zn/O(T) a (320k) , , b (430k) , , c (550k) , , O(L): crystal lattice oxygen; O(T): total oxygen.

4 28 Science in China Ser. B Chemistry nanoparticles decreases with the increasing particle size. Fig. 3. SPS and EFISPS of ZnO nanoparticles (a). response becomes more intense, which is a common characteristic of n-type semiconductors [23,24]. When the experiment is under a negative bias, the SPS response mainly exhibits two obvious features. One is that the SPS response increases, due to the same direction of the external and built-in electric field. The other is that the strong SPS response is broadened, which is possibly attributed to the surface states. Thus, it can be concluded according to the above results of XPS and ESR that the surface states mainly result from the oxygen vacancies and adsorbed oxygen. In addition, it can also demonstrate from several small SPSpeaksinfig.3(a ) that the surface states are abundant on ZnO nanoparticles. Fig. 4 shows the PL spectra of ZnO nanoparticles with different exciting wavelengths. It can be seen that ZnO samples exhibit a strong and wide PL signal in the range from 400 to 550 nm, with two obvious PL peaks at about 420 and 480 nm, attributed to band edge free and binding excitons, respectively [9,25,26]. There are lots of oxygen vacancies on the nanoparticle surface, and the particle size is very fine so that the average distance the electrons can move freely is very short, which makes the oxygen vacancies bind easily electrons to form excitons. Thus, the exciton energy level near the bottom of the conduction band can come into being, and the PL band of the exciton can also occur [27],asshowedinfig.5.Ingeneral,thesmaller the particle size, the larger the SOV content, the higher the probability of exciton occurrence, and the stronger the PL signal. Therefore, the PL intensity of ZnO Fig. 4. PL spectra of ZnO nanoparticles with different exciting wavelengths. The schematic diagram of exciton PL band of ZnO nanoparti- Fig. 5. cles. In addition, although the used excitation energies both are higher than the bandgap energy, the PL spectra are not completely the same, in other words, there is at least difference in the intensity. However, the PL peak positions do not change, indicating that there are

5 Relationships of surface oxygen vacancies with photoluminescence and photocatalytic performance of ZnO nanoparticles 29 a few fixed exciton energy levels on the surface of ZnO nanoparticles, which are possibly related to the surface states resulting from the oxygen vacancies and adsorbed oxygen. Moreover, it can also be proved from several small PL peaks that the surface states are abundant on ZnO nanoparticles, which is in good agreement with the results of EFISPS above. 2.4 The photocatalytic activity of ZnO nanoparticles The SOVs and defects can become the centers to capture photoinduced electrons during the process of photocatalytic reactions so that the recombination of photoinduced charge carriers can be effectively inhibited. Moreover, the oxygen vacancies can promote the adsorption of O 2, and there is strong interaction between the photoinduced electrons bound by oxygen vacancies and adsorbed O 2, indicating that the binding for photoinduced electrons of the oxygen vacancies can favor the capture for photoinduced electrons of adsorbed O 2,andCO 2 free group, a very active group to promote the oxidation of organic substances [28],is further produced at the same time. This can overcome to a certain degree the key shortcoming, the slow transfer step of the photoinduced electrons to the adsorbed oxygen. Therefore, the SOVs and defects can be well in favor for the photocatalytic reactions. Fig. 6 shows the photocatalytic degradation curves of gas phase n-c 7 H 16 on different ZnO nanoparticles. It can be found that the orders of the activity of ZnO samples are as follows: ZnO (a) > ZnO (b) > ZnO (c), in other words, the activity goes down with decreasing the Fig. 6. The photocatalytic degradation curves of n-c 7H 16 on different ZnO nanoparticles. SOV content, which is in good agreement with the characterization results discussed above, indicating that the SOVs can play an active role in the photocatalytic reactions. 3 Conclusions In this paper, the relationships of the surface structure with the photoluminescence and photocatalytic activity of ZnO nanoparticles are mainly investigated, to reveal principally the internal factors influencing the material functional performance in essence. The results show that the smaller the particle size, the more the SOV content, the higher the ESR intensity, the stronger the PL signal, and the higher the photocatalytic activity, of ZnO nanoparticles. This is because that the ESR and PL signals result mainly from the oxygen vacancies, which is in favor for the photocatalytic reactions, indicating that the SOVs play an important role in exhibiting the material functional performance. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos & ), the Natural Science Foundation of Heilongjiang Province of China (Grant No. B0305), the Science Foundation for Excellent Youth of Heilongjiang Province of China (2002), the Supporting Plan of Education Bureau of Heilongjiang Province (Grant No. 1054G035), the Science Foundation of Chinese Postdoctor (Grant No ), and the Science Foundation for Excellent Youth of Heilongjiang University of China (2003). References 1. Weller, H., Quantized semiconductor particles A novel state of matter for materials science, Advanced Materials, 1993, 5(2): [DOI] 2. Vogel, R., Hoyer, P., Weller, H., Quantum-sized PbS, CdS, Ag 2S, Sb 2S 3,andBi 2S 3 particles as sensitizers for various nanoporous wide-bandgap semiconductors, J. Phys. Chem., 1994, 98(12): Hoffmann, M. R., Martin, S. T., Choi, W. et al., Environmental applications of semiconductor photocatalysis, Chem. Rev., 1995, 95(1): Litter, M. I., Heterogeneous photocatalysis and transition metal ions in photocatalytic systems, Appl. Catal. B, 1999, 23(1): [DOI] 5. Sakohara,S.,Tickanen,L.D.,Anderson,M.A.,Luminescence properties of thin zinc oxide membranes prepared by the sol-gel technique: change in visible luminescence during firing, J. Phys. Chem., 1992, 96(26): Dijken, A. V., Meulenkamp, E. A., Vanmaekelbergh, D. et al., The kinetics of the radiative and nonradiative processes in nanocrystalline ZnO particles upon photoexcitation, J. Phys. Chem B., 2000, 104: [DOI]

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