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Photoswitches of One-Dimensional Ag2MO4 (M ) Cr, Mo, and W) Liang Cheng, Qi Shao, Mingwang Shao,* Xianwen Wei, and Zhengcui Wu* Anhui Key Laboratory of Functional Molecular Solids, and College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, P. R. China ReceiVed: October 8, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008
Large-scale, high-purity, and uniform one-dimensional Ag2MO4 (M ) Cr, Mo, and W) were obtained by a facile hydrothermal method. The as-prepared Ag2MO4 materials exhibited linear current-voltage (I-V) characteristics and excellent photoresponse. As the light source was switched on and off, the currents could be reversibly switched between high and low value at the voltage of 0.1 V. Thus, the results suggested that the light- to dark-conductivity ratios of these compounds were correlated with the ionic potential of the metal. The extension of the photoresponses to silver silicate and silver vanadate also showed similar exciting results, indicating their potential applications in photoswitch devices in the future. 1. Introduction Over the past decade, one-dimensional (1D) semiconductors with well-controlled size, morphology, and chemical composition have been the focus of scientific research due to their unique chemical/physical properties and potential applications in electrons, photonics, chemical sensing, and biological imaging.1 These materials2 have been successfully synthesized, which offers great opportunities to investigate their novel electronic and optical properties for deep fundamental insights into materials science. Metal chromate/molybdate/tungstate is an important family of inorganic materials, which have wide potential applications in various fields,3 such as photoluminescence, optical fibers, scintillator materials, humidity sensors, magnetic properties, microwave applications, and catalysis. A few reports are related with the synthesis of metal chromate/molybdate/tungstate through hydrothermal methods. For example, Yu et al. have successfully prepared Ag2MoO4, Ag6Mo10O33, and Ag2Mo2O7.4 Li et al.5 and Shen et al.1e have synthesized Ag2CrO4 and AgVO3, respectively. These works are all labeled as important and valuable. It is essential to investigate the optical properties of 1D material and explore their practical applications. Herein, Ag2MoO4, Ag2WO4, and Ag2CrO4 with uniform morphologies were prepared through a facile hydrothermal method without any surfactant or template. A photoconduction device was fabricated based on Ag2MO4, and the photoconductive properties were researched. The current-voltage (I-V) characteristics of the as-prepared products exhibited a unique rectifying behavior with the fast and reversible photoswitching response under on/ off light exposure conditions. The results implied that there might be a relation between the values of the photo- to darkconductivity ratio (PDC ratio) of the Ag2MO4 and the corresponding metal ionic potentials. This conclusion was interesting and might be found potential application in light-controlled devices in the future. * To whom correspondence should be addressed. Tel: +86-553-3869303. Fax: +86-553-3869303. E-mail:
[email protected] (M.W.S.);
[email protected] (Z.C.W.).
Figure 1. XRD patterns: (a) Ag2MoO4, (b) Ag2WO4, and (c) Ag2CrO4. No impurity is detected, indicating that the products have high purity.
2. Experimental Section 2.1. Preparation of Ag2MO4 (M ) Mo, W, and Cr). All of the chemical regents used in our experiments were of analytical grade and were used without further purification. Aqueous solutions were prepared using distilled water. In a typical procedure, 0.5 mmol of AgNO3 and 0.25 mmol of Na2MoO4 · 2H2O (or Na2WO4 · 2H2O or Na2CrO4 · 2H2O) were dissolved in 25 mL of distilled water, respectively. Then the AgNO3 solution was added into the Na2MoO4 solution slowly under magnetic stirring to form a mixture at room temperature, which was adjusted to pH 2 using HNO3 solution. The resulting mixture was transferred into a Teflon-lined autoclave of 60 mL capacity, heated to 150 °C for 12 h, and then cooled to room temperature naturally. The resultant was collected and washed several times with absolute ethanol and distilled water and dried under vacuum at 50 °C for 5 h. 2.2. Characterization. The phase and crystallography of the products were characterized by a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu KR radiation (λ ) 0.15406 nm). A scanning rate of 0.05 deg s-1 was applied to record the pattern in the 2θ range of 10-80°. The morphologies of the products were analyzed with a field emitting scanning electron
10.1021/jp808907e CCC: $40.75 2009 American Chemical Society Published on Web 01/13/2009
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Figure 2. SEM image of Ag2MoO4 with wire-like morphology: (a) at low magnification and (b) at high magnification.
microscopy (FESEM S-4800), equipped with an energy dispersive X-ray (EDX) spectroscope. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) pattern, and high-resolution transmission electron microscope (HRTEM) were captured with a JEOL-2010 transmission electron microscope, using an accelerating voltage of 200 kV. The photo images were taken from an Olympus optical microscope. 2.3. Measurements of Photoconductive Properties. All of the photoconductivity measurements were tracked with a CHI 620B electrochemical workstation. Light was supplied with incandescence lamp (12 V, 10 W). 3. Results and Discussion Figure 1a displays the XRD pattern of the Ag2MoO4 obtained at pH 2, and the diffraction peaks may be indexed as the cubic phase of Ag2MoO4 with the calculated lattice constant of a ) 0.9252 ( 0.0089 nm, which is consistent with the reported value, a ) 0.9260 nm (JCPDS card No.76-1747). No impurity is detected, indicating that the products have high purity. Figure 2a displays a panoramic SEM image of the as-prepared product, which shows its large-scale uniform wire-like morphology with lengths up to several hundreds of micrometers. The high magnification SEM image shown in Figure 2b further clearly confirms the products with a uniform diameter of 80 nm. The TEM image (Figure 3) exhibits a Ag2MoO4 wire with a diameter of 80 nm. When the wires were exposed to electron beams, a lot of silver nanoparticles appeared on their surface. It is because the Ag2MoO4 is sensitive to the electron beam irradiation.4a High-resolution transmission electron microscopy (HRTEM) is employed to confirm the crystal structure of the Ag2MoO4. The HRTEM image (Figure 3, low inset) illustrates that the wire has lattice planes with spacings of 0.535 and 0.330 nm, corresponding to the d spacings of the (111) and (2-20) planes of the cubic phase of Ag2MoO4 respectively, indicating the growth direction is [1-10]. The SAED (Figure 3, upper inset) taken from this area displays the crystalline structure of Ag2MoO4, whose bright diffraction spots may be indexed as (111) and (2-20) respectively. Figure 1b shows the XRD pattern of the Ag2WO4 obtained at pH ) 7, the diffraction peaks may be assigned to orthorhombic phase of Ag2WO4 with the calculated lattice constants of a ) 1.0919 ( 0.0080 nm, b ) 1.1977 ( 0.0085 nm, and c ) 0.5902 ( 0.0022 nm, which are in agreement with the reported values, a ) 1.082 nm, b ) 1.201 nm, and c ) 0.590 nm (JCPDS card No.34-0061). The morphology of the as-prepared Ag2WO4 sample is examined by SEM image (Figure 4a), which exhibits that the
Figure 3. TEM image of the Ag2MoO4 with a diameter of 80 nm; HRTEM image of Ag2MoO4 (low inset) showing clear crystal lattice, which may be indexed as (111) and (2-20) crystal planes; and the SAED pattern (upper inset) suggesting single crystallinity of the Ag2MoO4 wire.
product was also wire-like with a length up to several micrometers. A typical TEM image in Figure 4b indicates that the Ag2WO4 are straight and uniform wires with diameters of 115 nm. The electron diffraction pattern (Figure 4b, upper inset) indicated that the wire is prefect single crystal. The HRTEM image (Figure 4b, low inset) shows a high degree of crystallinity with a clear crystal lattice, which could be indexed as (002) and (440) planes, indicating that the Ag2WO4 is a single crystal grown in the direction of [001]. Figure 1c exhibits the XRD pattern of the Ag2CrO4 obtained at pH 7, all the diffraction peaks may be indexed to orthorhombic phase of Ag2CrO4 with the calculated lattice constants of a ) 0.7014 ( 0.0031 nm, b ) 0.1005 ( 0.0053 nm, and c ) 0.5538 ( 0.0018 nm, which are accordant to the reported values, a ) 0.7022 nm, b ) 0.1006 nm, and c ) 0.5538 nm (Ag2CrO4 JCPDS card No.26-0952). The SEM image (Figure 5a) reveals that the sample takes the shape of a wire, which is smooth with a length of several micrometers. Figure 5b depicts the TEM image of the Ag2CrO4 with an average diameter of 80 nm. And upper inset is the corresponding SAED pattern, which reveals that the wire is single crystalline with the bright diffraction spots indexed as
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Figure 4. (a) SEM image showing aligned Ag2WO4 with wire-like morphology and (b) TEM image of Ag2WO4 with the diameter 115 nm, HRTEM image of Ag2WO4 showing clear crystal lattice with (002) and (440) crystal planes (low inset), and the SAED pattern (upper inset) suggesting single crystallinity of the Ag2WO4 wire.
Figure 5. (a) SEM image showing aligned Ag2CrO4 with wire-like morphology and (b) TEM image of the Ag2CrO4 with the diameter 80 nm, HRTEM image of Ag2CrO4 showing clear crystal lattice with (200) and (020) crystal planes (low inset), and the SAED pattern (upper inset) suggesting single crystallinity of the Ag2CrO4 wire.
(200) and (020) respectively. The HRTEM shown in Figure 5b (low inset) illustrates that it grows along the [010] orientation. The morphologies of Ag2MO4 depend heavily on the experimental conditions. A series of contrastive experiments were done to research the key factors that affected the morphology of the products. Here, Ag2MoO4 was selected as a model to demonstrate the performance of pH value and reaction time. When the pH is higher than 7, only irregular Ag2MoO4 particles with large size are obtained (Figure 6a). While the pH is decreased to 5, the morphology of the sample consists of wire-like and particle-like structures (Figure 6b). As the pH is further decreased to 2, large scale Ag2MoO4 wires are obtained, as shown in Figure 2b. The reaction time also has a significant influence on the formation of wires. Figure 6c shows the SEM image of the Ag2MoO4 wires prepared in the reaction time of 5 h; the surfaces of the product were relatively rough, and the wires assembled to a bundle. When the reaction time was increased to 8 h, the wires began to grow from the center of the bundle, as shown in Figure 6d. There are also several other factors that influence the crystallinity and the morphology of Ag2MoO4 wires, such as temperature and concentration. From a series of experiments, the optimal reaction conditions to synthesize the high quality Ag2MoO4 wires are at pH 2 and 150 °C for 12 h. Similarly, the optimal reaction conditions to synthesize large scale Ag2WO4 and Ag2CrO4 wires are at pH 7 and 180 °C for 20 h.
Photoswitches of the Ag2MO4. In order to measure the current signals through the Ag2MoO4, indium tin oxide (ITO) coated glass with an electrode gap of 50 µm was employed as the substrate. A bundle of Ag2MoO4 was dispersed and bridged over the electrodes with effective length of 60 µm, as shown in Figure 7a (inset). Gold gap electrodes were fabricated on the substrate by thermal evaporation with a micrometer-sized Au wire as the mask; by slightly moving the Au-wire mask, Au-Au gap electrodes were deposited. The distance between the wires and light source was 10 cm. Figure 7a shows the I-V curves measured in dark (curve a) and under illumination (curve b) for comparison. Both of them exhibit good linear behavior, which proves a fine ohmic contact between the Ag2MoO4 wires and Au electrodes. The conductivity of the wires rapidly increased under illuminating with an incandescence lamp. In these cases, the energy from the light excites the electrons in the semiconductor Ag2MoO4 from the valence band into the conduction band, increasing the charge carrier concentration via direct electron-hole pair creation and thus enhancing the conductivity of the wires. Figure 7b shows the reversible photoconductive characteristics of the Ag2MoO4. A voltage of 0.1 V was applied across the two electrodes and the current recorded while the light was alternatively on and off at 10 s intervals. Obviously, the current through the Ag2MoO4 promptly increased and decreased according to the illumination on and off, which proves the device
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Figure 6. SEM images of Ag2MoO4 with wire-like morphology: the pH dependent growth process with (a) pH 7 and (b) pH 5, and the reaction time dependent growth process with (c) t ) 5 h and (d) t ) 8 h.
Figure 7. (a) I-V curves of a bundle of Ag2MoO4 measured in the dark (curve a) and under illumination by using an incandescence lamp (12 V, 10 W) (curve b) and the image of a bundle of Ag2MoO4 bridging over ITO electrodes from an optical microscope (inset) and (b) photoconductive characteristics of the device during light switching on/off. A voltage of 0.01 V was applied across the Au-Au electrodes, and the current was recorded during the light alternatively on and off at 10 s intervals.
has high sensitivity and repeatable behavior. The measurements were performed for ten periods and proved that the photoconduction behavior was reproducibility. Similarly, Figure 8, panels a and b, shows the photoresponse as a function of time when the incandescence lamp was switched on and off, which indicates that the Ag2CrO4 and Ag2WO4 wires could be reversibly switched for many times between low and high currents, the photoconductivity characteristics suggest that the Ag2MO4 are good candidates for photoelectronic switches. Mechanism of Photoconductivity. The photoconductivity is ascribed to the fact that the desorption of oxygen narrows the barrier width. In the dark, oxygen molecules adsorb near the grain boundary, trap free electrons from the conduction band, and become oxygen ions (O-, O2-, or O2-).6 This windens the depletion layer, induces the band bending around the grain boundary, and creates the potential barrier.7 Under illumination, the light excites the electrons jumping from valence band to conduction band, leaving holes in valence band. At the same time, the electrons captured by the oxygen ions are photoexcited back into the conduction band, which lead to the desorption of oxygen,8 and the depletion layer is narrowed. Accordingly, the
barrier is effectively lowered and narrowed by light illumination, which results in the increase of conductance upon exposure to light. It will be clear from the above paragraph that oxygen ions play an important role in the photoconductivity. Because positive ions with high charges and small radii have a strong attraction for the electrons of adjacent ions, such as O-, O2-, or O2-, we tentatively put forward that there might exist relationship between the PDC ratio and ionic potential Z2/r, where Z is the cationic charge and r is the ionic radius. The PDC ratios of one-dimensional AgMO (M ) Cr, Mo, W, V, and Si) and corresponding metal ionic potentials were shown in Figure 9. As the data show: the bigger the metal ionic potentials, the larger the PDC ratios, which implies that the PDC ratios were found to be positively related to the ionic potentials of the corresponding metal of AgMO. The results might be explained as follows: the metal ions with big ionic potentials attract oxygen ions strongly and favor the absorbed oxygen molecules to trap more free electrons from conduction band, which may decrease the dark-conductivity and accordingly
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Figure 8. Photoconductive characteristics as the incandescence lamp switching on and off: (a) Ag2WO4 and (b) Ag2CrO4, a voltage of 0.01 V was applied and the current was recorded during the light alternatively on and off at 10 s intervals.
Supporting Information Available: Some experiment data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 9. PDC ratios of one-dimensional AgMO (M ) Cr, Mo, W, V, and Si) vs corresponding metal ionic potentials.
enhance the PDC ratio as it depends on more the darkconductivity rather than the photoconductivity. It should be noted that this study has examined only via experiments. The possible relationship needs to be simulated in further theoretical work. In summary, uniform and high-purity Ag2MO4 were successfully synthesized via a facile hydrothermal approach, without any template or surfactant. The photoswitchable conductivity of a bundle of Ag2MO4 (M ) Cr, Mo, and W) exhibited the unique photoswitching response, which was fast and reversible under on/off light exposure conditions. There was a positive correlation between PDC ratios of these compounds and the ionic potential of the corresponding metal, indicating possible application in photosensitive devices in the future. Acknowledgment. This project was supported by the National Natural Foundation of China (20571001), the National Basic Research Program of China (973 Program) (Grant No. 2006CB933000), and Anhui Provincial Natural Science Foundation (070414185).
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