A self-powered UV photodetector based on TiO2 nanorod arrays
© Xie et al.; licensee Springer. 2013
Received: 11 March 2013
Accepted: 7 April 2013
Published: 23 April 2013
Large-area vertical rutile TiO2 nanorod arrays (TNAs) were grown on F/SnO2 conductive glass using a hydrothermal method at low temperature. A self-powered ultraviolet (UV) photodetector based on TiO2 nanorod/water solid–liquid heterojunction is designed and fabricated. These nanorods offer an enlarged TiO2/water contact area and a direct pathway for electron transport simultaneously. By connecting this UV photodetector to an ammeter, the intensity of UV light can be quantified using the output short-circuit photocurrent without a power source. A photosensitivity of 0.025 A/W and a quick response time were observed. At the same time, a high photosensitivity in a wide range of wavelength was also demonstrated. This TNA/water UV detector can be a particularly suitable candidate for practical applications for its high photosensitivity, fast response, excellent spectral selectivity, uncomplicated low-cost fabrication process, and environment-friendly feature.
Ultraviolet (UV) photodetector has been a popular research issue for its potential applications in a wide range of fields, such as remote control, chemical analysis, water purification, flame detection, early missile plume detection, and secure space-to-space communications . To avoid the use of filters and achieve visible-blind operation, wide bandgap semiconductors, such as GaN, SiC, ZnO, and TiO2[2–8], have been studied during the last decade for wide-spreading usage in photodetection, especially in the ultraviolet region. Among conventional available UV photodetectors, quite many kinds of structures have been fabricated, which in most cases are based on epitaxial growth process and various solid-state junction structures. Typical examples are photodetectors based on p-n junction, p-i-n photodiodes, Schottky barrier (SB), metal–semiconductor-metal, and metal-insulator-semiconductor structures [9–15]. These photodetectors typically require an external bias as the driving force to prevent the recombination of photogenerated electron–hole pairs. For large-area two-dimensional arrays that contain huge amounts of small UV sensors, energy supply will be one of the main challenges for such sensor systems.
Recently, self-powered nanodevices and nanosystems have attracted lots of attention due to their various advantages. Xu et al. fabricated a nanowire pH sensor and a nanowire UV sensor powered by a piezoelectric nanogenerator equipped with a capacitor, demonstrating a self-powered system composed entirely of nanowires . Yang et al. reported a self-powered ultraviolet photodetector based on a single Sb-doped ZnO nanobelt bridging an ohmic contact and a Schottky contact, in which high photoresponse sensitivity and short response time were observed . Bai et al. reported a ZnO nanowire array ultraviolet photodetector with self-powered properties, in which a high sensitivity of 475 without external bias is found . Although n-type semiconducting ZnO is a significant material for optoelectronic applications, it is unstable under both acidic and alkaline conditions. Also, the photoresponse of ZnO-based UV detector is sensitive to the surrounding atmosphere and can be easily affected by oxygen as well as water molecules. On the other hand, TiO2 nanostructures have also emerged as very promising materials for optoelectronic devices due to their excellent physical and chemical properties, such as high melting point, chemical inertness, physical stability, direct bandgap (rutile 3.0 eV), high photoconversion efficiency, and photostability. Self-powered UV photodetectors based on a photochemical cell have been fabricated using a liquid I-/I3- redox couple electrolyte and a nanocrystalline TiO2 film  or a multilayer TiO2 nanorod-assembled cloth/nanorod array-based electrode . Impressive performances were observed in these UV detectors. However, liquid I-/I3- redox couple electrolyte is not ideal for long-term operation: it is highly corrosive, volatile, and photoreactive, interacting with common metallic components and sealing materials. From this point, water-based electrolytes may be the safest, most stable, and most environment-friendly electrolyte. Lee et al. reported a UV detector based on TiO2/water solid–liquid heterojunction . This self-powered UV photodetector behaves similar to a Schottky diode and works in photovoltaic mode. Moreover, TiO2/water solid–liquid heterojunction UV detector exhibits high photosensitivity, excellent spectral selectivity, linear variations in photocurrent, and fast response. Cao et al. reported the photocurrent response of TiO2 nanorod arrays under UV illumination using a 0.5 M Na2SO4 aqueous electrolyte , in which TiO2 nanostructures can harvest more incident light photons compared to a flat thin-film active layer because of the markedly enlarged TiO2/electrolyte contact area. However, they did not report its photosensitivity and spectral response. All of these reported results indicate that self-powered UV detectors based on TiO2 nanostructures show great potential as excellent candidates for commercial UV photodetectors. Further advancements for TiO2-based self-powered UV detectors demand a deeper understanding of the main parameters determining the photoelectric behavior, which also requires additional research and insight into the electrical transporting process in these nanostructured devices.
In this paper, self-powered UV detectors were fabricated based on single-crystalline rutile TiO2 nanorod arrays (TNAs), which were grown directly on fluorine-doped tin oxide (FTO) glass by a low-temperature hydrothermal method. This UV photodetector establishes a built-in potential due to its Schottky barrier-like behavior. The built-in potential separates the electron–hole pairs generated by UV light and makes the photodetector generate photocurrent without any external bias. A considerable photocurrent response was observed under UV light illumination. Also, this self-powered photodetector demonstrates fast photoresponse speed, high photosensitivity, excellent spectral selectivity, uncomplicated low-cost fabrication process, and environment-friendly feature.
Growth of TiO2 nanorod arrays by hydrothermal process
The single-crystalline rutile TNAs used for this study were grown vertically on FTO glass using the following hydrothermal methods: a diluted hydrochloric solution was prepared by mixing 50 mL of deionized water with 40 mL of concentrated hydrochloric acid and was stirred at ambient temperature for 5 min, and then 400 μL of titanium tetrachloride was added to the mixture. After being stirred for another 10 min, the mixture was injected into a stainless steel autoclave with a Teflon container cartridge. The FTO substrates were ultrasonically cleaned and were placed at an angle against the Teflon container wall with the conducting side facing down. Hydrothermal synthesis was conducted at 180°C for 2 h. After synthesis, the autoclave was cooled to room temperature under flowing water, and the FTO substrates were taken out, rinsed thoroughly with deionized water, and annealed at 500°C for 1 h to improve the crystalline structure.
Assemble of TNA/water solid–liquid heterojunction
Characterization of the TNA samples and the UV photodetector
The crystal structure of the TNA samples were examined by X-ray diffraction (XRD; XD-3, PG Instruments Ltd., Beijing, China) with Cu Kα radiation (λ = 0.154 nm) at a scan rate of 2°/min. The surface morphology was characterized using a scanning electron microscope (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan). The optical transmittance was measured using a UV-visible dual-beam spectrophotometer (TU-1900, PG Instruments, Ltd.). The photoresponse characteristics of the self-powered UV detector in the dark and under illumination were recorded with a programmable voltage–current source (2400, Keithley Instruments Inc., Cleveland, OH, USA). A 500-W xenon lamp (7ILX500, 7Star Optical Instruments Co., Beijing, China) equipped with a monochromator (7ISW30, 7Star Optical Instruments Co.) was used as light source for spectral response characterization. For the photoresponse switching behavior measurement, a UV LED (NCSU033B(T), Nichia Co., Japan) with a wavelength of 365 nm was used as light source, and the photocurrent was obtained by an electrochemical workstation (RST5200, Zhengzhou Shirusi Instrument Technology Co. Ltd, Zhengzhou, China).
Results and discussion
The working principle of the device is discussed simply in the following. When UV light (310 ~ 420 nm) shines on the TNA/water UV detector, the incident photons that pass through the FTO glass into the TNAs and electrons in TiO2 are excited from the valence band to the conduction band and then generate electron–hole pairs in the TNAs. The built-in potential produced by solid–liquid heterojunction separates the UV light-generated electron–hole pairs. The separated holes move from the valence band of the TNAs into the interface of TNA/water, subsequently seizing the electrons from the water OH- anions (h+ + OH- → HO·). Considering the quite large TNA/water surface area, the small diameter of the nanorods, and the built-in interface potential, a fast removal of holes from the surface can be expected. On other hand, the separated electrons transport into the TNA conduction band and are collected easily by the FTO contact as the work function of FTO matches the conduction band of TiO2. These electrons move into the external circuit and then come back to the Pt layer of the detector, thereupon returning the electrons to HO· radicals (e- + HO· → OH-) at the interface of water/Pt. In this way, the built-in potential makes the UV detector generate photocurrent without any external bias. Even though zero bias is applied, the UV detector exhibits high photosensitivity [21, 24].
In conclusion, a photoelectrochemical cell-structured self-powered UV photodetector was developed using water as the electrolyte and a rutile TiO2 nanorod array as the active photoelectrode. This device exhibits a prominent performance for UV light detection. Under ambient environment, the photocurrent responses rapidly with UV light on/off switching irradiation. Also, this self-powered TNA/water UV detector demonstrates high photosensitivity and excellent spectral selectivity. All of these results indicate that this novel UV detector can be a promising candidate as a low-cost UV photodetector for commercially integrated photoelectronic applications.
This work was supported by the National Key Basic Research Program of China (2013CB922303, 2010CB833103), the National Natural Science Foundation of China (60976073, 11274201, 51231007), the 111 Project (B13029), and the Foundation for Outstanding Young Scientist in Shandong Province (BS2010CL036).
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