ZnO nanoneedle/H2O solid-liquid heterojunction-based self-powered ultraviolet detector
© Li et al.; licensee Springer. 2013
Received: 3 July 2013
Accepted: 23 August 2013
Published: 8 October 2013
ZnO nanoneedle arrays were grown vertically on a fluorine-doped tin oxide-coated glass by hydrothermal method at a relatively low temperature. A self-powered photoelectrochemical cell-type UV detector was fabricated using the ZnO nanoneedles as the active photoanode and H2O as the electrolyte. This solid-liquid heterojunction offers an enlarged ZnO/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. High photosensitivity, excellent spectral selectivity, and fast photoresponse at zero bias are observed in this UV detector. The self-powered behavior can be well explained by the formation of a space charge layer near the interface of the solid-liquid heterojunction, which results in a built-in potential and makes the solid-liquid heterojunction work in photovoltaic mode.
Ultraviolet (UV) detectors play an essential role in a wide range of civil and military applications including UV astronomy, environmental monitoring, flame sensing, secure space-to-space communications, and chemical/biological analysis[1–3]. As a wide bandgap material, ZnO has emerged as one of the most promising materials for UV detectors due to its exceptional photosensitivity and high radiation hardness[4–6]. ZnO has a direct wide bandgap of 3.37 eV, eliminating the need for costly filters to achieve visible-blind operation as that in traditional photomultipliers and silicon photodetectors. Its bandgap can be tuned in a wide range simply by doping with a small mole fraction of Al, Mg, or Cd, which enables ZnO to be used in different detection ranges. In the past, most ZnO-based photodetectors were fabricated in planar type based on ZnO thin films grown by sputtering, pulsed laser deposition, or molecular beam epitaxy. Different kinds of UV detectors based on ZnO have been investigated with metal-semiconductor-metal[7–10], p-i-n[4, 11, 12], p-n junction[5, 13, 14], or Schottky barrier-type[15–17] structures. However, factors such as high cost, difficulty of integrating with Si substrate, and complicated fabrication process have drawn back the potential application of planar-type ZnO photodetectors.
Recently, there is a growing interest in UV detectors based on one-dimensional (1D) nanostructures of ZnO like nanowires[18–20] or nanobelts due to the highly susceptible photoelectric properties by means of electron-hole generation or recombination under UV illumination. ZnO nanowire-based UV sensors exhibit a high on/off ratio between photoresponse current and dark current because of the large surface-to-volume ratio and the high crystal quality. Additionally, characteristics such as fast response and recovery time, visible light blindness, and potential for flexible electronics[22, 23] further contribute to 1D UV detectors' competence. However, the very low photoresponse current due to the small size of individual nanowires is an essential hindrance to single ZnO nanowire-based UV detectors[18, 20, 24]. Efficient routes like integrating multiple nanomaterials or assembling nanoarrays often lead to a complicated, time-consuming, and uneconomic device fabrication process[24–26]. On the other hand, 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, large-scale use of batteries as a power source will lead to environmental pollution[27–29].
In this letter, we introduce a self-powered UV detector based on a ZnO nanoneedle/water solid-liquid heterojunction structure. ZnO nanoneedle arrays were grown on a fluorine-doped tin oxide (FTO)-coated glass substrate by spin coating and subsequent hydrothermal method without any costly epitaxial process. X-ray diffraction (XRD) and scanning electron microscope (SEM) results proved a high-quality, vertically aligned ZnO nanoneedle array structure. A self-powered photoelectrochemical cell-type UV detector was assembled using the ZnO nanoneedles as the active photoanode and H2O as the electrolyte, which has almost the same structure as that of a conventional dye-sensitized solar cell but without dye adsorption. The solid-liquid heterojunction owes an inherent built-in potential across the interface which behaves in a Schottky barrier manner. The built-in potential acts as the driving force to separate the electron-hole pairs from recombination and generate photocurrent[28–30]. Hence, this ZnO/water heterojunction-based UV detector operates in photovoltaic mode, eliminating the need for external electric bias, which demonstrates a great potential in realizing self-powered UV detection and a self-driven integrated nanopower-nanodevice system.
Growth of ZnO nanoneedle arrays by hydrothermal process
ZnO nanoneedle arrays were grown using solution deposition method on FTO glass covered with a ZnO seed layer. Zinc acetate dehydrate was dissolved in the mixed solution of ethanolamine and 2-methoxyethanol to yield a homogeneous and stable colloid solution, which served as the seed solution. The ZnO seed layer was formed by spin coating the colloid solution at 3,000 rpm followed by annealing in a furnace at 400°C for 1 h. The following hydrothermal growth was carried out at 90°C for 6 h in a Teflon bottle by placing the seeded substrates vertically in aqueous growth solutions, which contain 20 mM zinc nitrate, 20 mM hexamethylenetetramine, and 125 mM 1,3-diaminopropane. Then the FTO glass with ZnO nanoneedle arrays was rinsed with deionized water thoroughly and annealed at 500°C for 1 h to remove any residual organics and to improve the crystalline structure.
Assembly of the solid-liquid heterojunction-based UV detector
Characterization of ZnO nanoneedle arrays and the UV photodetector
The crystal structure of the ZnO nanoneedle arrays was analyzed by XRD (XD-3, PG Instruments Ltd., Beijing, China) with Cu Kα line radiation (λ = 0.15406 nm). The surface morphology was characterized using a scanning electron microscope (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., Beijing, China). The photoresponse characteristics of the UV detector under illumination were recorded with a programmable voltage-current sourcemeter (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 the light source. For the photoresponse switching behavior measurement, photocurrent was measured by an electrochemical workstation (RST5200, Zhengzhou Shirusi Instrument Technology Co. Ltd, Zhengzhou, China).
Results and discussion
When incident light travels through FTO glass and reaches the active layer of ZnO nanoneedle arrays, photons with energy exceeding that of the ZnO bandgap will be absorbed and electron-hole pairs will be generated thereafter. The built-in potential across the interface works as the driving force to separate the electron-hole pairs. Negative charge moves along the ZnO nanoneedle and gets collected by the FTO electrode and poured into the external circuit easily since the work function of FTO matches with the conduction band of ZnO. The positive holes are driven to the surface and got captured by the reduced form of the redox molecule (h+ + OH- → OH·). Fast removal of holes can be expected across the heterojunction due to the large surface area. The oxidized form of the redox molecule is reduced back to the reduced form OH- at the counter electrode (Pt/FTO) by the electrons that re-entered into the UV detector from the external circuit (e- + OH· → OH-). The circuit was completed in this manner, demonstrating a self-powered UV detection property.
Overall, the ZnO nanoneedle array/water solid-liquid heterojunction is one type of regenerative UV detector. Considering the tunability of the absorption edge of ZnO by simply changing the concentration of the doping element like Al[33, 34] or Mg[35, 36] and excellent spectral selectivity of this system, we suggest that the spectral response should be tailored by elemental doping in a relatively wide range, which presents a promising versatile potential. In addition, the photoresponsivity and time performance of the solid-liquid heterojunction can also be improved by seeking for the optimized electrolyte solution. The simple fabrication technique, low cost, and environmental friendliness (nontoxic composition) further add to the solid-liquid UV detector's commercial application.
In conclusion, c-axis-preferred ZnO nanoneedle arrays have been successfully prepared on a transparent conductive FTO substrate via a simple hydrothermal method. A new type of self-powered UV detector based on a ZnO nanoneedle array/water solid-liquid heterojunction structure is fabricated, which exhibits a prominent performance for UV light detection. The photocurrent responds rapidly with UV light on-off switching irradiation under ambient environment. The mechanism of the device is suggested to be associated with the inherent built-in potential across the solid-liquid interface which works in a Schottky barrier manner that separates the electron-hole pairs generated under UV irradiation. The large relative surface and high crystal quality further promote the photoresponse. This new type of self-powered solid-liquid heterojunction-based 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.
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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