Enhanced Optical and Sensing Properties of One-Step Synthesized Pt

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Enhanced Optical and Sensing Properties of One-Step Synthesized Pt-ZnO Nanoflowers Xin-Yu Xue,*,† Zhao-Hui Chen,† Li-Li Xing,† Chun-Hua Ma,† Yu-Jin Chen,‡ and Tai-Hong Wang§ College of Sciences, Northeastern UniVersity, Shenyang 110004, China, College of Sciences, Harbin Engineering UniVersity, Harbin 150001, China, and Micro-Nano Technologies Research Center, Hunan UniVersity, Changsha 410082, China ReceiVed: July 27, 2010; ReVised Manuscript ReceiVed: September 7, 2010

Pt-ZnO nanoflowers are prepared via a novel one-step hydrothermal route, and Pt nanoparticles are uniformly loaded on the whole surface of the nanoflowers. The growth mechanism of Pt-ZnO nanoflowers is proposed to be a four-stage process. With the help of Raman scattering, photoluminescence, and gas sensing measurements, it has been demonstrated that the optical and sensing properties of Pt-ZnO nanoflowers are greatly enhanced. The surface defects decrease, the concentration of bound excitons under UV illumination increases, and the surface adsorption is enhanced and accelerated. These probably arise from the chemical and electrical effect of Pt. Our results could provoke a promising direction to achieve higher optical and sensing properties of ZnO one-dimensional nanostructures. Introduction Recently, ZnO one-dimensional (1D) nanostructures have attracted significant attention due to their unique physical and chemical properties, such as high chemical activity, wide direct band gap, good thermal stability, and high electron mobility.1-5 Among various morphologies of 1D nanostructures, nanoflowers with random branches are very helpful for avoiding agglomeration and offering good electron transportation.6,7 In some experimental and theoretical works, ZnO nanoflowers have been reported to have high performance in photocatalysts,8 chemical sensors,9,10 solar cells,3,11 etc. Such behaviors are attributed to the surface states, including surface defects (surface oxygen vacancies), bound excitons, and surface adsorption. It is widely accepted that solar conversion efficiency and UV-vis emission of ZnO nanoflowers are dominated by surface oxygen vacancies and bound excitons.12 In addition, surface defects and surface adsorption are responsible for gas sensing of ZnO nanoflowers.13,14 Nowadays, it is believed that surface states can be greatly improved by catalytic noble metals, such as Pt.15-20 In order to achieve higher optical and sensing properties and establish the direct relationship between Pt doping and surface states on ZnO nanoflowers, we synthesized Pt-ZnO nanoflowers via a novel one-step hydrothermal route (all precursors are added at the same time, and the growth and modification processes take place in the same conditions) and carried out optical and gas sensing measurements on Pt-ZnO nanoflowers. Compared to traditional two-step methods,18-20 this is the most simple and yet effective strategy to prepare uniformly catalyst-modified ZnO 1D nanostructures. With the help of Raman scattering, photoluminescence (PL), and gas sensing experimental data, we illustrated that surface defects, bound excitons, and surface adsorption are greatly improved by the chemical and electrical effect of Pt nanoparticles and one-step modification with Pt nanoparticles * To whom correspondence should be addressed. Phone: 86-02483687658. E-mail: [email protected]. † Northeastern University. ‡ Harbin Engineering University. § Hunan University.

is an effective way to achieve higher optical and sensing properties of ZnO 1D nanostructures. Experimental Section All chemicals were analytical-grade reagents and used as purchased without further purification. Zn(CH3COO)2 · 2H2O, NaOH, and H2PtCl6 · 6H2O (Sinopharm Chemical Reagent Co. Ltd.) were used as precursors. Pt-ZnO nanoflowers were synthesized via a novel simple one-step hydrothermal route. A 0.070 g amount of Zn(CH3COO)2 · 2H2O and 0.400 g of NaOH were dissolved into 39 mL of distilled water. After the mixture was magnetically stirred for 20 min, 1 mL of H2PtCl6 · 6H2O (0.05M) aqueous solution was introduced, and this solution was transferred into a 50 mL Teflonlined stainless steel autoclave. It was then sealed and maintained at 180 °C for 2 h. After slowly being cooled to room temperature, gray-colored solid powders were collected by centrifugation and washed with distilled water and absolute ethanol. The powders were finally dried at 60 °C for 12 h. Results and Discussion The crystal phase of Pt-ZnO nanoflowers is characterized by X-ray powder diffraction (XRD; D/max 2550 V, Cu KR radiation). As shown in Figure 1, the sharp diffraction peaks indicate the good crystalline quality of Pt-ZnO nanoflowers. The XRD pattern shows 11 peaks at 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 66.4°, 68.0°, 69.1°, 72.6°, and 77.0°, indexed to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of the ZnO crystal given by the standard data file (JCPDS file No. 36-1451), respectively. ZnO nanoflowers are of wurtzite structure (lattice constants a ) 3.249, c ) 5.206 Å; space group P63mc (186)). There are no other clear sharp peaks coincident with those peaks of other impurities. The diffraction peaks of Pt can not be observed in this XRD pattern, which can be explained by considering that the quantity of Pt is much smaller than that of ZnO. The morphology of Pt-ZnO nanoflowers is characterized by scanning electron microscopy (SEM; JEOL JSM-6700F). Figure 2a is a typical SEM image of Pt-ZnO nanoflowers. A large

10.1021/jp1070067  2010 American Chemical Society Published on Web 10/12/2010

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Figure 1. XRD patterns of Pt-ZnO nanoflowers.

Figure 2. (a) SEM image of Pt-ZnO nanoflowers. (b) SEM image of one single Pt-ZnO nanoflower; (inset) high-resolution SEM image of the tip region.

amount of ZnO nanoflowers with uniformly sized multiple branches are observed, and the agglomeration of Pt nanoparticles is not observed. Rod-like structures of each branch have lengths of 1.5-2.5 µm and diameters of 150-250 nm with an aspect ratio of 10. Figure 2b is a zoomed-in version of one single Pt-ZnO nanoflower, which reveals a hexagonal axel supporting protruding rod-like petal nanostructures. It should be noted that the surface is not clean, coated with many nanoparticles. A highresolution SEM image of the tip region is inserted on the right top corner of Figure 2b, and it can be observed that the whole surface of ZnO nanoflowers is uniformly coated with a large amount of Pt nanoparticles. The microstructure and component of Pt-ZnO nanoflowers are analyzed by transmission electron microscopy (TEM; JEOL JEM-2010) with an energy-dispersive X-ray spectrometer (EDS). Figure 3a is a TEM image of Pt-ZnO nanoflowers,

Xue et al. clearly showing their flower-like morphology. Each branch is observed to be tapered at the apex. Figure 3b is a TEM image of the tip of the nanoflowers, showing that Pt nanoparticles are uniformly distributed on the tips. Figure 2c is a TEM image of the limb surface, showing that Pt nanoparticles are also uniformly distributed on the limbs. Thus, it can be concluded that Pt nanoparticles are uniformly loaded on the whole surface of the nanoflowers. Figure 2d is a high-resolution TEM (HRTEM) image showing the interface between Pt and ZnO. The contrast difference between ZnO and Pt is apparent, and both ZnO and Pt are of single-crystal structures. The interplanar distance of 0.24 nm corresponds to (101) planes of ZnO, and that of 0.22 nm in the supported nanoparticles corresponds to Pt (111) planes.12,14,21 It also can be seen that Pt nanoparticles have diameters of 6-8 nm. The component of Pt-ZnO nanoflowers is examined by using EDS attached to TEM. The EDS spectrum of Pt-ZnO nanoflowers is shown in Figure 3e. The presence of the Cu peaks in the EDS spectrum comes from the copper grids used as a support of the samples in TEM observations. It can be seen from this spectrum that the three elements (Pt, Zn, and O) exist at this region. Similar EDS results have been obtained at 10 other different areas, which further confirm that the Pt nanoparticles are uniformly distributed in the whole system. Figure 3f contains the corresponding electron diffraction patterns of Pt-ZnO nanoflowers in TEM apparatus, showing two different natures of crystals. One set of diffraction patterns can be indexed to single-crystal ZnO, and the other irregular diffraction spots (marked with white arrows) arise from the small-sized Pt nanoparticles.22,23 These results confirm that ZnO nanoflowers are uniformly coated with Pt nanoparticles and that both of them are of single-crystal structures. The schematic illustration of growth mechanism for Pt-ZnO nanoflowers under hydrothermal condition is shown in Scheme 1. First, ZnO nuclei form from dehydration of Zn(OH)42- ions in alkaline environment,24,25 and the reactions are as follows

Zn2+ + 4OH- f Zn(OH)24

(1)

Zn(OH)24 f ZnO + H2O + 2OH

(2)

As NaOH can provide hydroxide anions very quickly due to its strong basicity, a large amount of growth units (Zn(OH)42complexes) can be generated.24-26 Therefore, more growth units around the ZnO nuclei may lead to a faster growth kinetics, which causes sheet defects on initial ZnO crystal.24,25,27,28 The initial ZnO crystal has many polar (0001) surfaces due to many deviations from the idealized fourling structures of ZnO.24,27,28 Second, ZnO crystals grow preferentially along the [0001] direction and form a flower-like shape due to the higher growth rate along the [0001] direction.24,25,27-30 Also, formation of flower-shaped ZnO can be attributed to the experimental conditions (pH value, Zn2+ concentration, time, and temperature).25,31-33 Third, the free hydroxyl groups (provided by NaOH) attached at the surface of ZnO nanoflowers are protonated and thus conjugate with PtCl62- via electrostatic attraction.7 This stage is the key for formation of Pt nanoparticles along the surface of ZnO nanoflowers. At this stage, it should be noted that growth of ZnO and reduction of PtCl62- take place at the same time, which may probably induce the doping process of Pt in the nanoflowers. Finally, PtCl62- ions are reduced, and Pt is nucleated and located on the surface of ZnO nanoflowers.7 Raman scattering measurements are performed on Pt-ZnO nanoflowers and bare ZnO nanoflowers (synthesized under the

One-Step Synthesized Pt-ZnO Nanoflowers

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Figure 3. (a) TEM image of one single Pt-ZnO nanoflower. (b) Enlarged view of the tip region of the nanoflowers. (c) Enlarged view of the limb region of the nanoflowers. (d) HRTEM image of Pt-ZnO nanoflowers. (e) EDS spectrum of Pt-ZnO nanoflowers. (f) Corresponding electron diffraction patterns of Pt-ZnO nanoflowers.

same condition) in air at room temperature by using the 488 nm line of an Ar+ laser as excitation. Their Raman scattering patterns are shown in Figure 4. For ZnO materials, A1 and E1 modes are polar and split into transverse-optical (A1T and E1T modes) and longitudinal-optical (A1L and E1L modes) phonons and the E2 mode, consisting of two modes of low- and highfrequency phonons (E2L and E2H modes), is Raman active. Raman scattering peaks at about 332, 382, and 438 cm-1 in bare ZnO nanoflowers (curve 1) correspond to the acoustic overtone (A1, A1T, and E2H modes).34-38 A very weak shoulder around 410 cm-1 in bare ZnO nanoflowers corresponds to the mode of E1T.39 The peak at 206 cm-1 in bare ZnO nanoflowers is attributed to multiphonon processes.40,41 The peak at 576 cm-1 in bare ZnO nanoflowers can be assigned to a vibration mode of E1L symmetry, which is most sensitive to the presence of doping elements.41 From curve 2 of Figure 4 it can be seen that the E1L mode intensity of Pt-ZnO nanoflowers is enhanced to 51.6, much higher than that of bare ZnO nanoflowers (36.4),

SCHEME 1: Schematic Illustration of the Growth Mechanism for Pt-ZnO Nanoflowers under Hydrothermal Condition

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Figure 4. Raman scattering spectra of bare ZnO nanoflowers (curve 1) and Pt-ZnO nanoflowers (curve 2) in air at room temperature after normalizing both the Raman scattering intensities.

Xue et al.

Figure 6. Response and recovery of Pt-ZnO nanoflowers upon exposure to 10, 100, and 200 ppm ethanol at a work temperature of 300 °C.

TABLE 1: Sensitivity of Pt-ZnO Nanoflowers upon Exposure to 10, 100, and 200 ppm Ethanol at Different Work Temperatures

Figure 5. PL spectra of bare ZnO nanoflowers (curve 1) and Pt-ZnO nanoflowers (curve 2) in air at room temperature; (inset) enlarged view of PL spectra at the range of the intrinsic absorption edge and bound excitons.

which partly confirms that the Pt element is effectively doped in the whole system. Also, this enhancement may be probably attributed to the better crystallinity of Pt-ZnO than ZnO and the surface-located Pt nanoparticles.41 A new peak at 628 cm-1 has been observed in Pt-ZnO nanoflowers, which have never been observed in ZnO or Pt nanostructures. The interaction between Pt and ZnO may be the origin. Compared with bare ZnO nanoflowers, blue shifts have been observed in almost all of the Raman scattering peaks of Pt-ZnO nanoflowers. The peaks, corresponding to A1, A1T, E2H, and E1L modes, are blue shifted to 334, 386, 440, and 582 cm-1, respectively. By considering the fact that the blue shift of the E2H mode peak is usually ascribed to fewer surface defects (surface oxygen vacancies) of ZnO,42 the surface oxygen vacancies of Pt-ZnO nanoflowers are fewer than that of bare ZnO nanoflowers. The reduced surface defects are probably attributed to the chemical and electrical effect of Pt.18,43-46 Due to the catalysis effect of Pt nanoparticles (Pt is a far better oxygen diffusion catalyst than ZnO), the coverage of chemisorbed oxygen on Pt-ZnO nanoflowers surface is larger than that on bare ZnO nanoflowers. Also, adsorbed oxygen can diffuse faster to surface defects of Pt-ZnO nanoflowers. Thus, the amount of surface oxygen vacancies of Pt-ZnO nanoflowers is smaller than that of bare ZnO nanoflowers. The other possibility is that the Pt nanoparticles formation process has cleared many surface defects by electron abstraction from the defects through the work function of Pt (5.65 eV) being larger than that of ZnO (5.3 eV). Figure 5 shows PL spectra for Pt-ZnO nanoflowers and bare ZnO nanoflowers in air at room temperature following excitation with a He-Cd laser (λ ) 325 nm). The broad peak over the green luminescence range, usually due to surface defects,12,47-51

sensitivity temperature concentration

130 °C

170 °C

210 °C

250 °C

300 °C

10 ppm 100 ppm 200 ppm

2.6 3.2 4.3

2.8 3.7 5.9

3.3 5.9 7.1

4.1 6.0 14.9

4.5 33.1 87.1

is suppressed dramatically in Pt-ZnO nanoflowers, which further confirms that surface oxygen vacancies of Pt-ZnO nanoflowers are fewer than those of bare ZnO nanoflowers. The inset of Figure 5 is an enlarged view of PL spectra at the range of the intrinsic absorption edge (peak I) and bound excitons (peak II). A red shift of the intrinsic absorption edge is clearly observed in Pt-ZnO nanoflowers, which arises from Pt element doping (the quantum confinement effect should be neglected).12,52,53 It also can be seen that the intensity of peak II is greatly increased in Pt-ZnO nanoflowers. The Pt-ZnO nanoflowers growth process renders internal defects states, which lead to a higher concentration of bound excitons under optical excitation. These results suggest that Pt-ZnO nanoflowers have potential applications in solar cells. The gas sensing properties of Pt-ZnO nanoflowers were investigated by characterizing the sensors fabricated from them by our previous method.54 The sensitivity S is usually defined as S ) Ra/Rg, where Ra is the sensor resistance in air and Rg is the sensor resistance in test gas. The response and recovery of Pt-ZnO nanoflower sensors upon exposure to ethanol at the work temperature of 300 °C is shown in Figure 6. Five periods are examined at each fixed concentration of ethanol, and very good repeatability and stability is achieved. The sensitivity of Pt-ZnO nanoflowers against 10, 100, and 200 ppm ethanol is 4.5, 33.1, and 87.1 (as shown in Table 1), respectively, higher than that of bare ZnO nanoflowers (sensitivity is 3.5, 16.2, and 23.2, respectively). The sensitivity of Pt-ZnO nanoflowers is also higher than that of various ZnO 1D nanostructures.10,55-57 Both the response and the recovery time of Pt-ZnO nanoflowers are very short (2 and 20 s, respectively). Good stability, high sensitivity, fast response and recovery of Pt-ZnO nanoflowers support their promising applications in gas sensor at the industrial level. Table 1 shows the sensitivity of Pt-ZnO nanoflowers upon exposure to 10, 100, and 200 ppm ethanol at 130, 170, 210, 250, and 300 °C. The work temperature can be lowered down, which can further facilitate their applications in complex conditions. The excellent gas sensing of Pt-ZnO nanoflowers can be attributed to the modulation of surface

One-Step Synthesized Pt-ZnO Nanoflowers

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defects and surface adsorption of Pt-ZnO nanoflowers by the chemical and electrical effect of Pt nanoparticles.18,43-46 In air, as discussed above, oxygen molecules can be more easily adsorbed and dispersed on the surface of Pt-ZnO nanoflowers by the assistance of Pt nanoparticles. Compared with bare ZnO nanoflowers, more adsorbed oxygen can diffuse faster to surface defects and capture free electrons from the conduction band to become oxygen ions, which results in stronger electron depletion on the ZnO surface. At the same time, the Schottky contacts between the ZnO nanoflowers and the Pt nanoparticles lead to an additional depletion layer at the interface by electron abstraction from surface defects. The Pt-ZnO nanoflower sensor resistance is very large in air. However, on the other hand, such increased resistance of the sensors in air limits their application at room temperature. In ethanol, the rate of the reaction between oxygen ions and ethanol molecules is greatly accelerated by Pt catalytic activity. The reaction is as follows Pt

C2H5OH + 6O- 98 2CO2 + 3H2O + 6e-

(3)

Electrons are more easily released back to the conduction band through the reaction, which greatly decreases the sensor resistance in ethanol. Conclusions In summary, Pt-ZnO nanoflowers with high uniformity were prepared via a novel one-step hydrothermal route. A four-stage growth mechanism of Pt-ZnO nanoflowers was proposed. Raman scattering, PL, and gas sensing measurements were performed on Pt-ZnO nanoflowers, and enhanced optical and sensing properties by the chemical and electrical effect of Pt nanoparticles have been demonstrated. The present results implied that one-step modification with Pt nanoparticles was an effective way to achieve higher optical and sensing properties of ZnO 1D nanostructures. Acknowledgment. This work was partly supported from the Fundamental Research Funds for the Central Universities (N090405017), Liaoning Natural Science Foundation (20091027), Specialized Research Fund for the Doctoral Program of Higher Education of China (20090042120025), and the National Natural Science Foundation of China (Nos. 51072038, 50772025). References and Notes (1) Huang, M. H.; Mao, S.; Feick, H. N.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Vayssieres, L. AdV. Mater. 2003, 15, 464. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (4) Huang, M. H.; Wu, Y. Y.; Feick, H. N.; Tran, N.; Weber, E.; Yang, P. D. AdV. Mater. 2001, 13, 113. (5) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. AdV. Funct. Mater. 2002, 12, 323. (6) Umetsu, M.; Mizuta, M.; Tsumoto, K.; Ohara, S.; Takami, S.; Watanabe, H.; Kumagai, I.; Adschiri, T. AdV. Mater. 2005, 17, 2571. (7) Umar, A.; Lee, S.; Im, Y. H.; Hahn, Y. B. Nanotechnology 2005, 16, 2462. (8) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2009, 131, 4397. (9) Wang, X. D.; Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (10) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (11) Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114. (12) Dhas, V.; Muduli, S.; Lee, W.; Han, S. H.; Ogale, S. Appl. Phys. Lett. 2008, 93, 243108.

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