Sonochemical Synthesis, Optical Properties, and Electrical Properties

Jan 29, 2005 - Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100080, People's Republic of China. Chem. ... Ultrasonic irra...
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Chem. Mater. 2005, 17, 887-892

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Sonochemical Synthesis, Optical Properties, and Electrical Properties of Core/Shell-Type ZnO Nanorod/CdS Nanoparticle Composites Tao Gao,* Qiuhong Li, and Taihong Wang Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100080, People’s Republic of China ReceiVed August 31, 2004. ReVised Manuscript ReceiVed NoVember 4, 2004

A simple sonochemical route for the surface synthesis of CdS nanoparticles on ZnO nanorods is reported. Ultrasonic irradiation of a mixture of ZnO nanorods, cadmium chloride, and thiourea in an aqueous medium for 2 h yields ZnO nanorod/CdS nanoparticle (ZnO/CdS) composites with core/shell-type geometry. The powder X-ray diffraction of the ZnO/CdS composites shows additional diffraction peaks corresponding to the hexagonally structured CdS, apart from the signals from the ZnO nanorod cores. Transmission electron microscopy images of the ZnO/CdS composites reveal that the ZnO nanorods are coated with CdS nanoparticles with typical diameters of about 5-10 nm. The room temperature photoluminescence spectrum of the ZnO/CdS composites has two emission bands: an ultraviolet emission peak at 376 nm and a green emission around 523 nm. The conductance of the ZnO/CdS composites shows an enhancement compared with that of the uncoated ZnO nanorods. The appealing application of the ZnO/CdS composites as ethanol sensors is presented and a possible sensing mechanism is discussed.

Introduction The engineering of materials in nanometer scale has become an emerging interdisciplinary field based on physics, chemistry, biology, and materials science. It is known that many fundamental properties of materials (optical, electrical, mechanical, etc.) can be expressed as a function of their size, composition, and structural order. Consequently, the design and controlled fabrication of nanomaterials with functional properties are required to meet the ever-increasing demands (e.g., structural and compositional complexity) placed on materials science and performance by nanotechnology.1 Although there has already been much progress in the synthesis and assembly of nanoscale materials such as nanoparticles, nanotubes, nanowires, nanorods, and nanobelts,2 effective strategies to produce tailored nanostructured materials reliably and predictably are still required. Surface coating (or surface modification) has been recognized as one of the most advanced and intriguing methods to build tailored nanomaterials.1,3 Materials are coated for a number of reasons: Coatings can alter the charge, functionality, and reactivity of the surface, and enhance a material’s thermal, mechanical, or chemical stability.1,3 Typically, surface coating involves tailoring the surface properties of the nanomaterials, often accomplished by coating or encapsulating them within a shell of a preferred material. This procedure has previously been exploited to synthesize core/ shell-type nanoparticles such as CdS/PbS,4 SnO2/TiO2,5 Au/ * Corresponding author. E-mail: [email protected].

(1) (2) (3) (4)

Caruso, F. AdV. Mater. 2001, 13, 11. Rao, C. N. R.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. Zhou, H. S.; Sasahara, H.; Honma, I.; Komiyama, H.; Haus, J. W. Chem. Mater. 1994, 6, 1534. (5) Bedja, I.; Kamat, P. V. J. Phys. Chem. 1995, 99, 9182.

SnO2,6 SiO2/ZnS,7 SiO2/Au,8 and Fe/Au9 with novel optical, electrical, and magnetic properties that are usually superior to their individual components. Moreover, much effort has recently been invested to create new class of nanomaterials through surface coating. For example, carbon nanotubes coated with SnO2,10 CdS,11 ZnS,12 and silica,13 core/shelltype TiO2/CdS nanowires,14 Cu2S/Au nanowires,15 Zn/ZnO nanobelts,16 and SnO2/CdS nanobelts17 have already been reported in the literature. Investigations have largely been spurred by the scientific interest for these core/shell-type nanomaterials in modern materials science, and by their technological importance: Core/shell-type nanomaterials are potentially useful in optics, optoelectronics, catalysis, chemical engineering, biology, and so forth.1 There are many deposition approaches that are widely used for the surface coating of a nanoscale object such as solgel coating, self-assembly, and sonochemical processing.1,3 Sonochemical synthesis has been proven to be a useful technique to generate core/shell-type nanomaterials, as recent works have shown with SiO2/ZnS,7 SiO2/Au,8 SnO2/CdS,17 and SiO2/CdS.18 Ultrasound effects chemical changes due (6) Oldfield, G.; Ung, T.; Mulvaney, P. AdV. Mater. 2000, 12, 1519. (7) Dhas, N. A.; Zaban, A.; Gedanken, A. Chem. Mater. 1999, 11, 806. (8) Pol, V. G.; Gedanken, A.; Calderon-Moreno, J. Chem. Mater. 2003, 15, 1111. (9) Lin, J.; Zhou, W.; Kumbhar, A.; Wiemann, J.; Fang, J.; Carpenter, E. E.; O’Connor, C. J. J. Solid State Chem. 2001, 159, 26. (10) Han, W. Q.; Zettl, A. Nano Lett. 2003, 3, 681. (11) Cao, J.; Sun, J. Z.; Hong, J.; Li, H. Y.; Chen, H. Z.; Wang, M. AdV. Mater. 2004, 16, 84. (12) Zhao, L.; Gao, L. J. Mater. Chem. 2004, 14, 1001. (13) Colorado, R., Jr.; Barron, A. R. Chem. Mater. 2004, 16, 2691. (14) Cao, J.; Sun, J. Z.; Li, H. Y.; Hong, J.; Wang, M. J. Mater. Chem. 2004, 14, 1203. (15) Wen, X.; Yang, S. Nano Lett. 2002, 2, 451. (16) Ding, Y.; Kong, X. Y.; Wang, Z. L. J. Appl. Phys. 2004, 95, 306. (17) Gao, T.; Wang, T. H. Chem. Commun. 2004, 2558.

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to cavitation phenomena involving the formation, growth, and implosive collapse of bubbles in liquid, which generates localized hot spots having a temperature of roughly 5000 °C, pressures of about 500 atm, and a lifetime of a few microseconds.19 These extreme conditions can drive chemical reactions such as oxidation, reduction, dissolution, and decomposition, which have been exploited to prepare a variety of metal, oxide, sulfide, and carbide nanoparticles.8 Sonication of the precursor in the presence of support materials provides an alternative means of trapping the produced nanoparticles.7,17,18 In the present work, we report the surface synthesis of CdS nanoparticles on ZnO nanorods with the aid of ultrasound. ZnO nanorods have been chosen as the core materials for the following two reasons: (a) ZnO is an important multifunctional semiconductor with unique electrical, optoelectronic, and luminescent properties, and many important practical applications,20 and (b) ZnO one-dimensional (1D) nanomaterials such as nanowires,20 nanorods,21 nanotubes,22 and nanobelts23 represent a broad class of nanoscale building blocks that have been used to assemble functional devices such as lasers,24 photodetectors,25 field emitters,26 and gas sensors.27 It is expected that the surface coating of the ZnO 1D nanomaterials with a preferred material (CdS has been chosen as the coating material owing to its suitable band gap and appealing optoelectronic properties) enables us to construct ZnO-based core/shell-type composites with novel optical and electrical properties. In this paper, core/shelltype ZnO nanorod/CdS nanoparticle (ZnO/CdS) composites are successfully achieved by ultrasonic irradiation of a mixture of single crystalline ZnO nanorods, cadmium chloride, and thiourea in an aqueous medium. Optical and electrical properties of the obtained ZnO/CdS composites have been investigated to show the interaction between the ZnO nanorod cores and the external CdS nanoparticle shells. Moreover, applications of the ZnO/CdS composites as ethanol sensors have been presented. Experimental Section Materials. Analytically pure Zn powders, cadmium chloride (CdCl2‚2.5H2O), and thiourea (H2NCSNH2) were purchased from (18) Dhas, N. A.; Gedanken, A. Appl. Phys. Lett. 1998, 72, 2514. (19) Suslick, K. S. Science 1990, 247, 1439. (20) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H.-J.AdV. Funct. Mater. 2002, 12, 319. (21) (a) Wu, J. J.; Liu, S. C. AdV. Mater. 2002, 14, 215. (b) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2004, 84, 3603. (22) (a) Hu, J. Q.; Bando, Y. Appl. Phys. Lett. 2003, 82, 1401. (b) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395. (23) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Li, Y. B.; Bando, Y.; Sato, T.; Kurashima, K. Appl. Phys. Lett. 2002, 81, 144. (c) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81, 757. (24) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 18971899. (25) Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. AdV. Mater. 2002, 14, 158. (26) (a) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. (b) Wan, Q.; Yu, K.; Wang, T. H.; Lin, C. L. Appl. Phys. Lett. 2003, 83, 2253. (27) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654.

Gao et al. Beijing Chemical Reagents Company, China, and used without further purification. Preparation of ZnO Nanorods. ZnO nanorods were obtained by oxidizing ZnO/Zn particles in air at high temperature, which was reported previously by Kitano et al.28 This oxidized layer suppresses the rapid vaporization of the inner liquid Zn during the growth process, resulting large and uniform tetrapod-like ZnO nanorods.28 We have followed their procedure. In brief, 10 g of Zn powder was added to 200 mL of doubly distilled water. After vigorous stirring for 10 min, the suspension was filtered and washed with doubly distilled water. Then the obtained paste was put into a glass beaker without covering and dried at 70 °C in air for 5 h to obtain ZnO/Zn particles. For the growth of ZnO nanorods, 3 g of ZnO/Zn particles was placed in an alumina crucible. The crucible was covered by an alumina plate and placed inside a quartz tube at the center of an electronic resistance furnace. The furnace was heated to 920 °C at a rate of 100 °C/min, held at this temperature for 1 h, and then cooled naturally to room temperature. Mass production of the ZnO nanorods was achieved successfully via such a smoldering reaction.28 Sonochemical Synthesis of CdS Nanoparticles on ZnO Nanorods. Our synthetic approach follows the sonochemical synthesis of CdS nanoparticles in an aqueous solution, in which the ZnO nanorod carriers are presented.17,18 In a typical case, 100 mg of as-synthesized ZnO nanorods, 500 mg of cadmium chloride, and 250 mg of thiourea were dissolved in 100 mL of doubly distilled water to obtain a reaction mixture. Before ultrasonic irradiation, high purity (99.99%) argon gas was bubbled through the reaction mixture for 30 min to expel dissolved oxygen.7,8 The reaction mixture was then irradiated with high-intensity ultrasound (100 W, 40 kHz) at room temperature in ambient air for 2 h. During the irradiation, the temperature of the reaction mixture rose to ∼80 °C, and the colorless reaction mixture turned light yellow, indicating the generation of CdS. After irradiation the excess CdS nanoparticles were separated from the reaction mixture by centrifugation (at 6000 rpm). Then the resulting powders were washed thoroughly (three times) with doubly distilled water and ethanol, centrifuged at 10 000 rpm, and allowed to dry naturally in air before characterization. We also prepared CdS nanoparticles by using the above-mentioned procedures, without the presence of ZnO nanorod carriers. Structural Characterization. The X-ray diffraction patterns of the as-synthesized ZnO/CdS composites were measured with a D/Max-2400 powder X-ray diffractometer (Cu KR irradiation, λ ) 0.1542 nm). The chemical composition analysis was achieved by energy-dispersive X-ray spectroscopy (EDS; HITACHI S-4200 scanning electron microscope). Morphology and microstructures of the as-synthesized ZnO/CdS composites were studied by scanning electron microscopy (SEM; XL30 S-FEG) and transmission electron microscopy (TEM; JEM-2010). Specimens for TEM investigations were prepared by placing a small drop of sample suspension in ethanol on a carbon-coated TEM grid, followed by air-drying to remove the solvent. Property Characterization. Photoluminescence (PL) measurements were carried out on a RPM 2000 vis-UV spectrophotometer using the He-Cd laser line of 325 nm as an excitation source at room temperature. Samples for PL measurement were prepared by placing the as-prepared ZnO/CdS composites on a 3-in. silicon wafer, and then pressing them into a sheet with a thickness of about 0.1 mm. The silicon wafer had been characterized, and no PL emission was observed from 330 to 800 nm. For comparison, (28) Kitano, M.; Hamabe, T.; Maeda, S. Okabe, T. J. Cryst. Growth 1990, 102, 965. (29) Ahn, S. E.; Lee, J. S.; Kim, H.; Kim, S.; Kang, B. H.; Kim, K. H.; Kim, G. T. Appl. Phys. Lett. 2004, 84, 5022.

ZnO Nanorod/CdS Nanoparticle Composites

Figure 1. XRD patterns of (a) uncoated ZnO nanorods and (b) ZnO/CdS composites.

sonochemically generated CdS nanoparticles and uncoated ZnO nanorods, having the same weight as those of the ZnO/CdS composite samples, were also characterized by using the abovementioned procedures. The samples for the measurement of current-voltage (I-V) curves were prepared by the following procedures.29 A thin copper wire (0.5 mm diameter) was put on a glass substrate. After thermal evaporation of a thin layer (∼100 nm) of Au successively, the copper wire was removed. Then a two-probe configuration of the electrodes was created on the glass substrate. The as-synthesized ZnO/CdS composites were ultrasonically dispersed in ethanol to obtain a paste. Then the paste was coated on the two electrodes and allowed to dry naturally in air to form a thin film with typical thickness and length about 0.1 and 5 mm, respectively. The devices were put into a vacuum chamber (pumped by a mechanical pump) and a dc source was connected between the two gold electrodes. The samples used for the gas sensing studies were prepared as follows:17 After ultrasonic dispersion in ethanol, the ZnO/CdS composites were coated on an alumina tube (outer diameter 1 mm; inner diameter 0.6 mm), on which a pair of Au electrodes had been printed at the two ends of the alumina tube. The distance between the two Au electrodes was 3 mm. After the paste was dried naturally in air, the devices were put into a chamber (5 L in volume) to test their gas-sensing properties, in which the target gas such as ethanol vapor had been introduced. A Ni-Cr alloy meander heater (outer diameter 0.5 mm) was placed inside the alumina tube to produce an optimized operation temperature.

Results and Discussion Structural Characteristics of ZnO/CdS Composites. Figure 1a shows the X-ray diffraction (XRD) patterns of the uncoated ZnO nanorods. All the diffraction peaks can be indexed to the hexagonally structured ZnO with cell constants

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of a ) 0.324 nm and c ) 0.519 nm, which are consistent with the standard values for bulk ZnO (JCPDS Card No. 36-1451), revealing that the ZnO nanorods are crystalline with a hexagonal structure. No diffraction peaks from the other crystalline forms were detected. After the ZnO nanorods were sonochemically treated in an aqueous solution of cadmium chloride and thiourea, some additional diffraction peaks corresponding to the hexagonally structured CdS phase (JCPDS Card No. 41-1049) appear, revealing that hexagonally structured CdS is produced in the reaction mixture during the ultrasound irradiation. The broad nature of the CdS XRD peaks shows that the sizes of the CdS nanoparticles are very small. The diameters of the sonochemically generated CdS powders are about 8 nm (estimated from the Scherrer formula). The coating level of CdS in the ZnO/ CdS composites is 6% (weight percent) calculated from the XRD spectrum (Figure 1b). In Figure 2a, we present TEM images of the uncoated ZnO nanorods. It can be seen that the needlelike ZnO nanorods are smooth on the surface, usually 20-40 nm in diameter, and several microns in length. It is also found that the ZnO nanorods are single crystalline with hexagonal structures as determined by the selected-area electron diffraction (SAED) pattern (inset of Figure 2a). The TEM micrograph of the sonochemically generated CdS (Figure 2b) shows the presence of nanosized particles. The diameter distribution of the CdS nanoparticles is obtained using the statistical results of 20 particle sizes obtained during the TEM observation. It shows that the diameters of the CdS nanoparticles varied from 5 to 12 nm, with an average of 8.5 nm, in good agreement with the XRD results (∼8 nm). However, due to a strong tendency of aggregation,18 many of the individual CdS nanoparticles seem to be attached to each other to form particles with bigger sizes (20-50 nm), as shown in Figure 2b. The inset of Figure 2b shows the corresponding SAED pattern of the CdS nanoparticles. All the diffraction rings can be indexed to a hexagonally structured CdS phase, which is in agreement with the XRD results. Figure 3 shows typical TEM images of the ZnO/CdS composites. In comparison with Figure 2a, it can be seen clearly that ZnO nanorods have been coated with CdS nanoparticles with the aid of the ultrasound irradiation. The CdS nanoparticles are nearly spherical in shapes and show some aggregation. From Figure 3a-c, one can see that the individual ZnO nanorods have been coated with different amounts of CdS nanoparticles, from several particles (Figure

Figure 2. TEM images of (a) uncoated ZnO nanorods (inset, corresponding SAED pattern) and (b) sonochemically generated CdS powders (inset, corresponding SAED pattern).

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Figure 4. Room temperature PL spectra of (a) sonochemically generated CdS nanoparticles, (b) uncoated ZnO nanorods, and (c) ZnO/CdS composites.

Figure 3. Typical TEM images of ZnO/CdS composites. (d) ZnO-CdS heterojunction interface.

3a) to a beplastered layer (Figure 3c). A high-resolution TEM image (Figure 3d) shows an individual CdS nanoparticle closely attached on the ZnO nanorod core. The measured spacing of the crystallographic plane is 0.316 nm, which corresponds to the {101} lattice plane of a hexagonal CdS crystal. This states that the sonochemical growth of CdS nanoparticles is along the 〈101〉 direction. Moreover, it can also be observed that the ZnO-CdS heterostructure possesses a very clean interface, revealing that the CdS nanoparticles are grown directly from the ZnO nanorod surface. No buffer layers, crystalline or amorphous, are observed in the interface regions. Various groups have employed a range of sonochemical approaches to synthesize metal sulfate nanoparticles in aqueous solution. For example, Wang et al.30 have reported the sonochemical synthesis of CdS nanoparticles by irradiation of a mixture of cadmium chloride, sodium thiosulfate, and 2-propanol. Dhas et al.18 have reported the surface synthesis of CdS nanoparticles on silica microspheres by using cadmium sulfate and thiourea as precursors. For the sonochemical growth of ZnO/CdS core/shell-type composites, there are four primary stages:7,18,30 H2O f H• + OH• 2H• + RS f H2S + R•

(RS ) H2NCSNH2)

(1) (2)

S2- + Cd2+ f CdS

(3)

n(CdS) + ZnO f ZnO/CdS

(4)

Reaction 1 is the formation of radicals (H• and OH•) from the ultrasound-initiated dissociation of water.19 It is known that the H• radical can act as reducing species; hence, it can trigger the decomposition of thiourea18 to generate S2- in solution via reaction 2. Moreover, the generation of S2- in (30) Wang, G. Z.; Wang, Y. W.; Chen, W.; Liang, C. H.; Li, G. H.; Zhang, L. D. Mater. Lett. 2001, 48, 269.

solution from the ultrasound-induced decomposition of S-containing precursors such as sodium thiosulfate30 and thioacetamide7 have also been reported. The produced S2reacts with Cd2+ in solution to form CdS clusters, as shown in reaction 3. When there is a supporter such as ZnO nanorod, the sonochemically generated CdS clusters would be attached on its surface to form a composite nanomaterial with core/ shell-type geometry.17,18 It should be pointed out that ultrasound-induced cavitation19 also plays an important role in the activation and cleanness of ZnO nanorod surfaces for the adhesion of the resulting S2- and CdS species, which is necessary to form a core/shell-type nanostructure with clean interfaces, as shown in Figure 3d. Photoluminescence of ZnO/CdS Composites. Figure 4 shows the room temperature photoluminescence (PL) spectra of sonochemically generated CdS nanoparticles, ZnO nanorods, and ZnO/CdS composites for comparison. The PL spectrum of the CdS nanoparticles (Figure 4a) show a broad emission band centered at 550 nm, which can be attributed to the emission from the defect states, such as cadmium interstitials or sulfur vacancies in CdS nanoparticles.18 The PL spectrum of ZnO nanorods (Figure 4b) presents two emission bands. One is an UV emission peak at 382 nm, which corresponds to the near band edge emission of ZnO. The other is a broad visible emission band from 450 to 600 nm with a peak at 500 nm, which has commonly been attributed to the oxygen vacancies of ZnO.20,21 The PL spectrum of the ZnO/CdS composites (Figure 4c) is similar to that of the uncoated ZnO nanorods: a UV emission centered at 376 nm and a broad green emission peaked around 523 nm. In comparison with the uncoated ZnO nanorods, the ZnO/CdS composites show a small (∼6 nm) blue shift in the UV region and a large (∼23 nm) red shift in the visible emission. According to the structural characteristics of the ZnO/CdS composites, both the blue shift and the red shift in the PL spectrum could be attributed to the interaction between the two semiconductors of ZnO and CdS. In defect chemistry, ZnO is a well-known n-type ionic semiconductor originating from the intrinsic defects of oxygen vacancies. It is generally accepted that the chemical properties of oxygen and sulfur are similar to each other; consequently, the sulfur anions generated in the solution

ZnO Nanorod/CdS Nanoparticle Composites

Figure 5. I-V curves of uncoated ZnO nanorods (a) and ZnO/CdS composites (b) measured in air at room temperature.

would have a tendency to be captured by the oxygen vacancies located mainly on the surface of ZnO nanorods.20,21 Absorption of sulfur anions on the ZnO nanorods provides the initial nucleation sites for the surface growth of CdS nanoparticles, which is consistent with the TEM observation (see Figure 3d). The sulfur atoms can diffuse in further from the surface to the inner part of the ZnO nanorods, resulting a sulfur-doping effect in ZnO. An impurity atom in a semiconductor makes the choice of either a shallow donor in a substitutional site or a deep level via a lattice distortion. In the present case, it is reasonable that the sulfur dopants can act as singly charged donors in ZnO.31-33 Due to the small density of states of ZnO near the conduction band minimum, the conduction band edge is filled by excessive carriers donated by the impurities, leading to a blue shift of optical band-to-band transitions known as the Burstein-Moss (BM) effect. Such a BM blue shift has previously been observed for Al-,31 and In-,32 and S-doped ZnO.33 We suggest that a similar mechanism would be applicable to the blue shift in the UV region of ZnO/CdS composites in comparison with that of the uncoated ZnO nanorods. Moreover, it is understandable that the capturing sulfur atoms or CdS nanoparticles can greatly change the surface structures of the ZnO nanorods. For example, the oxygen vacancies in the surface area of ZnO nanorods will be annihilated with the absorption of sulfur atoms or CdS nanoparticles, which decreases significantly the 500-nm emission of the ZnO nanorods, as shown in Figure 4c. With the absorption of sulfur atoms or CdS nanoparticles, formation of new defect or surface states such as Zn-Cd-S, or Cd-Zn-O in the interface region of the ZnO-CdS heterojunction,34 is expected to play an important role in the red shift in the visible region of the ZnO/CdS composites. However, details of these newly formed defect or surface states in the ZnO/CdS composites are not very clear at this stage. Electrical Transport Properties of ZnO/CdS Composites. Figure 5 shows the typical current-voltage (I-V) curves of the ZnO nanorods and the ZnO/CdS composites for (31) Hur, T. B.; Hwang, Y. H.; Kim, H. K. J. Appl. Phys. 2004, 96, 1507. (32) Kim, K. J.; Park, Y. R. Appl. Phys. Lett. 2001, 78, 475. (33) Geng, B. Y.; Wang, G. Z.; Jiang, Z.; Xie, T.; Sun, S. H.; Meng, G. W.; Zhang, L. D. Appl. Phys. Lett. 2003, 82, 4791. (34) Hotchandani, S.; Kamat, P. V. J. Phys. Chem. 1992, 96, 6834.

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comparison. The current increases linearly with the bias as the voltage is scanned from -5.0 to 5.0 V, revealing that Ohmic contact between the electrodes and the materials is obtained. The value of the measured resistance of the ZnO/ CdS composites is about 5.5 GΩ, which is smaller than that of the uncoated ZnO nanorods, 33 GΩ. One can see that the ZnO/CdS composites have an improved conductance in comparison with that of the uncoated ZnO nanorods. This conclusion has also been supported by our other samples (not shown here). Such an enhancement in the conductance of the ZnO/CdS composites can be attributed to the interaction between the two semiconductors of ZnO and CdS. Two factors should be considered for such conductance enhancement. One is the sulfur doping as mentioned above. The excess carriers supplied by the sulfur impurities to the conduction band contribute to the increase of the electrical conductance of the ZnO nanorods.31-33 Another is an additional electron injection from the CdS nanoparticle shell into the ZnO nanorod core. It is known that ZnO is a wide band gap (Eg ) 3.37 eV, at 300 K) semiconductor while CdS is a smaller one (Eg ) 2.42 eV, at 300 K). Due to their favorable energetics, when ZnO couples with CdS, a ZnOCdS heterojunction with a staggered gap forms. To achieve thermal equilibrium, electrons from CdS will flow into ZnO, forming an accumulation layer of electrons in the potential well adjacent to the interface of the ZnO/CdS heterojunction.34 Due to the additional electron injection from CdS (also with high mobility), the conductance of the ZnO nanorod will be enhanced. It has been well documented that using two semiconductors in contact having different redox energy levels of their corresponding conduction and valence bands has been actually considered as one of the most promising methods for the development of photoelectrochemical cells.34,35 Therefore, the core/shell-type ZnO/CdS composites show also appealing application in such optoelectronic devices. Ethanol-Sensing Properties of the ZnO/CdS Composites. Figure 6a shows the response curve of ZnO/CdS composites to 10 ppm ethanol vapors in air at an operating temperature of 300 °C. For comparison, the response curve of uncoated ZnO nanorods measured at the same conditions is also presented. The sensitivity (S ) Ggas/Gair) of these devices is defined as the conductance variation in air (Gair) and in target gases (Ggas). It is found that, at any concentration of ethanol vapors, the ZnO/CdS composites have a higher sensitivity than the uncoated ZnO nanorods, as shown in Figure 6b. Clearly, the ZnO/CdS composites feature an improved ethanol-sensing performance such as high sensitivity compared to the uncoated ZnO nanorods. The sensing mechanism of the ZnO-based gas sensors has been discussed in much literature.27,36 The most widely accepted model is based on the modulation of the depletion layer by oxygen absorption. Oxygen from the ambient adsorbs on the exposed surface of ZnO and, extracting an electron from the ZnO conduction band, ionizes to O- or O2-; O- is believed to be dominant.36 Consequently, depletion layers are formed in the surface area of ZnO, causing (35) Nasr, C.; Hotchandani, S.; Kim, W. Y.; Schmehl, R. H.; Kamat, P. V. J. Phys. Chem. B 1997, 101, 7480. (36) Windischmann, H.; Mark, P. J. Electrochem. Soc. 1979, 126, 627.

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nanoparticles.37,38 When the electrons in the conduction band of CdS are mostly trapped in its depletion layer, the amount of electrons injected from CdS into ZnO is small. In a reducing atmosphere, the amount of free electrons in the conduction band of CdS will increase greatly due to the oxygen desorption; consequently, the amount of injection electrons from CdS into ZnO increases simultaneously to achieve new thermal equilibrium at the interface of the ZnOCdS heterojunction. This reveals that electron injection from CdS nanoparticle shells into ZnO nanorod cores at a reducing atmosphere is more efficient than that in air. These additional electrons can enhance further the conductance of ZnO, resulting in an improvement of the gas sensitivity of ZnO/ CdS composites in comparison with that of the uncoated ZnO nanorods. Conclusions

Figure 6. (a) Response curves of uncoated ZnO nanorods and ZnO/CdS composites to 10 ppm ethanol vapor in air. (b) Ethanol-sensing properties of uncoated ZnO nanorods and ZnO/CdS composites at various concentrations of ethanol vapors in air. Working temperature 300 °C.

the carrier concentration and electron mobility (due to scattering) to decrease. When exposed to such reducing gases as ethanol, the ethanol molecules will react with the adsorbed O-, releasing the trapped electron back to the conduction band, and then both the carrier concentration and carrier mobility of ZnO increase. Such a variation in the conductance of ZnO can be used to detect target gases, for both reducing and oxidizing.27 One can see that the gas sensitivity of a ZnO sensor is dominated by its conductance variations; therefore, the factors that can improve such variations can enhance its gas sensitivity. It should be pointed out that the absorption-desorption process of oxygen occurs also on the surfaces of CdS

We have prepared ZnO nanorod/CdS nanoparticle (ZnO/ CdS) composites with core/shell-type geometry by ultrasonically irradiating a mixture of single crystalline ZnO nanorods, cadmium chloride, and thiourea in an aqueous medium for 2 h. It is found that the interaction between the ZnO nanorod and the CdS nanoparticle greatly influences the optical and electrical properties of the obtained ZnO/CdS composites. The method used in this paper can be exploited to other materials systems, which will offer promising opportunities for the design and fabrication of new optoelectronic devices such as highly sensitive gas sensors. Acknowledgment. This work is supported by the Special Funds for Major State Basic Research Project (No.G2001CB3095) and the National Natural Science Foundation of China (No. 69925410 and 60236010). T.G. thanks the China Postdoctoral Science Foundation for financial support. Supporting Information Available: TEM images of sonochemically generated CdS nanoparticles, and EDS patterns of ZnO/CdS composites (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0485456 (37) Lantto, V.; Golovanov, V. Sens. Actuators, B 1995, 24-25, 614. (38) The gas sensors based on the sonochemically generated CdS powders showed no response to the 10 ppm ethanol vapor, but did show response to ethanol vapor with concentrations higher than 1000 ppm.