J. Phys. Chem. C 2010, 114, 10725–10729
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Ultraviolet Photodetectors Based on Anodic TiO2 Nanotube Arrays Jianping Zou,† Qing Zhang,*,† Kai Huang,† and Nicola Marzari‡ Microelectronics Center, School of Electrical & Electronic Engineering, Nanyang Technological UniVersity, 639798, Singapore, and Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 ReceiVed: February 5, 2010; ReVised Manuscript ReceiVed: May 10, 2010
Anodic TiO2 nanotube arrays prepared by electrochemical anodization were used to fabricate ultraviolet (UV) photodetectors. The devices annealed at 450 °C exhibit the highest UV-sensitive photoconductance due to the pure anatase phase of the TiO2. The large surface area and one-dimensional nanostructure of the TiO2 nanotubes lead to great photosensitivity (more than 4 orders of magnitude) and fast response with rise time and decay time of 0.5 and 0.7 s, respectively. High responsivity of 13 A/W is found under 1.06 mW/cm2 UV (λ ) 312 nm) illumination at 2.5 V bias, which is much higher than those of commercial UV photodetectors. The high responsivity mainly comes from the internal gain induced by the desorption of oxygen from the nanotube surfaces and the reduction of the Schottky barrier at TiO2/Ag contact under UV illumination. The devices are promising for large-area UV photodetctor applications. Introduction In recent years, ultraviolet (UV) photodetectors have drawn much attention because of civilian and military applications in environmental and biological research, flame sensors, optical communication, astronomical studies, missile launch detection, and so forth.1-4 As the most common devices for UV photodetection, silicon photodiodes exhibit some intrinsic limitations, such as difficulty of blocking out visible and infrared photons and degradation of devices under UV irradiation. UV photodetectors based on wide bandgap semiconductors such as ZnO, SiC, and GaN show excellent wavelength selectivity (“visibleblindness”) and possibility of room-temperature operation.5-8 However, the disadvantages of these UV photodetectors are obvious, such as high cost and complicated fabrication processes. TiO2 is a wide bandgap (anatase 3.2 eV and rutile 3.0 eV) n-type semiconductor and has been studied in many fields, such as photocatalysis, dye-sensitized solar cells, and gas sensors because of its outstanding physical, chemical, and optical properties.9-12 The distinctive UV absorption characteristics make TiO2 very suitable for UV detection against the background of infrared and/or visible light. In particular, onedimensional TiO2 such as nanotube is more attractive due to its large specific surface area and well-defined charge carriers transport path. Anodic oxidation is a powerful and efficient technique for fabricating highly ordered TiO2 nanotube arrays.13,14 The dimensions of the TiO2 nanotubes can be precisely controlled during the anodization process. Although a UV photodetector based on TiO2 nanocrystalline film has been reported,15 little attention has been paid to UV photo-to-current conversion of the TiO2 nanotube arrays. In comparison with conventional wide bandgap semiconductor devices, the UV photodetectors based on the anodic TiO2 nanotube arrays show many advantages such as ease of fabrication, large device area, and, most importantly, low cost. In the present work, the UV photoelectric properties of the TiO2 nanotube array devices were studied systematically. The particular nanotubular structure * Corresponding author. E-mail:
[email protected]. † Nanyang Technological University. ‡ Massachusetts Institute of Technology.
results in highly UV-sensitive photoconductance and fast response characteristics of the devices. The internal photoconductive gain mechanism corresponding to high responsivity was also discussed. Experimental Methods Titanium (Ti) foils (99.7% purity), 0.25 mm thick, were degreased by ultrasonication in acetone and then isopropanol, respectiviely, for about 30 min, followed by rinse with deionized (DI) water, and finally dried in the air before used. Highly ordered TiO2 nanotube arrays over large areas were prepared by a potentiostatic anodization in a two-electrode electrochemical cell with a platinum (Pt) sheet as counter electrode. All anodization experiments were carried out at room temperature. The Ti foils were anodized in a 0.25 wt % NH4F (98+% ACS reagent) and ethylene glycol (99.8% anhydrous) solution. The as-anodized samples were ultrasonically cleaned in DI water for 15-30 s to remove surface debris. Afterward, the samples were rinsed with DI water and dried in the air. For comparison, compact TiO2 layers were also fabricated by anodization of Ti foils in 1 M H2SO4 electrolyte. For UV photoconductance studies, the prepared anodic TiO2 nanotube arrays were annealed at different temperatures for 2 h in ambient atmosphere. The morphology of the TiO2 nanotube arrays was characterized with a scanning electron microscope (SEM; JEOL JSM-5910LV), and the crystallographic structures were characterized by X-ray diffractometry (XRD; Siemens D5005). All UV photoconductance and dark conductance measurements were performed at room temperature. A Spectroline E-series Ultraviolet hand lamp (λ ) 312 nm) was used as light source. The photocurrent and dark current were recorded by an Agilent B1500A system. Results and Discussion Figure 1a shows the SEM top- and cross-sectional view (inset) of the TiO2 nanotube array anodized at 50 V for 5 h. It is evident that the highly ordered nanotube array consists of very regular tubes with an average inner diameter of 80 nm. These nanotubes
10.1021/jp1011236 2010 American Chemical Society Published on Web 06/02/2010
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Figure 1. SEM images of (a) top view and cross-sectional view (inset) of the as-anodized TiO2 nanotube arrays and (b) top view of the 750 °C annealed TiO2 nanotube arrays (the inset shows a top view with low magnification).
are overall 7 µm thick and have very smooth walls typical of nanotubes grown in organic electrolytes.16 The as-anodized TiO2 nanotube arrays are typically amorphous, and can be crystallized by annealing in air.17 The evolution of the surface morphology as a result of annealing shows that the regular tubular structure remains stable until around 650 °C. There is no significant change in the pore diameter or wall thickness even after annealing for 2 h. However, when the annealing temperature is increased to 750 °C, the tubular structure completely collapses leaving many deep and large cracks,18 as shown in Figure 1b. The photoelectrochemical properties of the TiO2 nanotube arrays depend on the crystallinity and isomorph type (anatase phase and rutile phase).17 Figure 2a shows the XRD patterns of the TiO2 nanotube arrays annealed at different temperatures for 2 h in ambient atmosphere. It can be seen that the annealing temperature could significantly affect the crystalline phase of the TiO2 nanotubes. The as-anodized sample typically has an amorphous structure (only reflections from the Ti foil can be seen). As the sample is annealed at 350 °C, the anatase phase occurs with a low and wide characteristic peak corresponding to the anatase (101) plane. The transition from amorphous to anatase has begun. After annealing at 450 °C, the sample is pure anatase phase and the characteristic diffraction peak becomes stronger and sharper, indicating an improvement in crystallinity. According to the Scherrer equation,19 the average crystallite size varies from 8.3 nm (T ) 350 °C) to 13.13 nm (T ) 450 °C). For the sample annealed at 550 °C, in addition to obvious anatase phase, weak rutile phase ((110) plane) can also be observed, suggesting that the anatase-to-rutile phase transition has taken place. When the temperature is beyond 550 °C, the rutile (110) peak grows whereas the anatase (101) peak diminishes gradually. As the temperature is increased to 750 °C, the anatase phase disappears completely, only leaving the rutile phase. As a wide bandgap semiconductor, the anodic TiO2 nanotube arrays have remarkable UV absorption property.17 Thus, the TiO2 nanotube arrays should show an intensive UV photoconductance response with intrinsic “visible-blindness”. The schematic device structure is depicted in Figure 2b. The top surface of the TiO2 nanotube arrays was tightly connected to a thin copper wire with a small silver paste pad (area ) 3.14 mm2). The back-electrode is the Ti foil underneath the nanotube arrays. The illuminated area of UV light is about 7 mm2 for all measurements. Figure 2c shows the current of the TiO2 nanotube arrays annealed at different temperatures both in the dark and upon 1.8 mW/cm2 UV light illumination (λ ) 312 nm). For all
temperatures, the dark current is around 10-9 A magnitude at 0.5 V bias. Obvious photoresponses can be seen when the UV light is irradiated on the devices. The photoconductance properties of the TiO2 nanotube arrays significantly depend on the annealing temperatures. When the temperature is increased from 350 to 750 °C, the photocurrent initially increases gradually and reaches the maximum at 450 °C, and then decreases gradually. The low annealing temperature results in a lower crystallinity. Many defects such as oxygen vacancies in the nanotube arrays greatly block the transport of the photogenerated carriers, and even become the recombination centers of the electron-hole pairs. As the temperature is increased, the crystallinity of the nanotube arrays is improved gradually, and the amount of the defects is reduced. Therefore, the photocurrent begins to increase with temperature and reaches the maximum at 450 °C. With the further increase in temperature, the TiO2 nanotube arrays begin to transform from anatase with higher photoelectric activity into rutile with lower photoelectric activity.20 And the content of rutile increases with the increasing temperature (see Figure 2a), leading to a gradual decrease in the photocurrent. The incident UV light intensity dependence of the photocurrent in the nanotube arrays device annealed at 450 °C is shown in Figure 2d. The photocurrent, Iph, follows a fitted power-law relationship with the intensity, P: Iph∝P1.31. To study the effect of the TiO2 nanotubular structure on the photoconductance, compact TiO2 layers and TiO2 nanotube arrays with the same thickness (∼250 nm) were fabricated and annealed at 450 °C. The UV photoresponses of the compact TiO2 layer and the TiO2 nanotube array are shown in Figure 3, panels a and b, respectively. It can be seen that the ratio of photocurrent-to-dark current of the TiO2 nanotube array is more than 4 orders of magnitude, larger than that of the compact TiO2 layer. The higher photocurrent of the TiO2 nanotube array can be attributed to the larger surface area of the nanotubular structure which can more effectively absorb the UV light. Figure 3, panels a and b, also reveals the time response characteristics of these two devices under the UV illumination. The TiO2 nanotube array shows a very fast response with a rise time of 0.5 s and a decay time of 0.7 s. However, a slow response with a rise time of 90 s and a decay time of 5 s is observed in the case of the compact TiO2 layer. Such a great difference in time response arises from the different structures of the compact layer and the nanotube array. The one-dimensional structure of nanotube provides a well-defined transport path for the photogenerated carriers and thus reduces losses incurred by charge hopping across nanograin boundaries in the compact layer. Each
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Figure 2. (a) XRD spectra of the TiO2 nanotube arrays annealed at different temperatures for 2 h in air. (b) Schematic of a TiO2 nanotube arrays device structure. (c) The annealed temperature dependence of the current for the TiO2 nanotube arrays devices both in the dark and upon 1.8 mW/cm2 UV light illumination (λ ) 312 nm) at 0.5 V bias. (d) Photocurrent of the 450 °C annealed device as a function of the incident UV (λ ) 312 nm) intensity at 0.5 V bias.
nanotube makes it more difficult for an electron to jump outside the nanostructure than to stay within the structure during transport. Therefore, the TiO2 nanotube array could possess higher electron collection efficiencies which in turn shows faster time response compared with the compact TiO2 layer. In the present work, the top electrode of the devices is a small silver pad, indicating a very small effective charge collection region. If a large area transparent conductive film (FTO or ITO) is applied as the top electrode instead of the silver pad, we believe the photo-to-current conversion efficiency of the TiO2 nanotube arrays could be improved further, leading to higher photocurrent and faster response. Figure 3c shows the response of the TiO2 nanotube array device under a continuous UV light rectangle pulse. The repeatable response of the photocurrent suggests the excellent stability and repeatability of the device. Figure 3d shows typical I-V characteristics of the TiO2 nanotube array device both in the dark and upon 1.06 mW/cm2 UV light illumination (λ ) 312 nm). We can see that the photocurrent could reach to 249.68 µA at 2.5 V bias. Figure 3f shows the spectral response of the TiO2 nanotube array device at the bias of 2.5 V. The responsivity of the device is defined as photocurrent per unit of incident optical power. This spectral response exhibits that the device has excellent UV photoresponse. The peak responsivity is approximately 13 A/W. For comparison, the responsivity of
most commercial UV photodetectors is in the range of 0.1-0.2 A/W.2 If the quantum efficiency (number of electron-hole pairs generated per incident photon) is assumed to be unity, the peak responsivity corresponds to a internal photoconductive gain (number of carriers detected per photogenerated electron-hole pair) of 52. We note that the photoconductive gain in our devices is much larger than unity, so in addition to the photocurrent simply produced by the photogenerated charges, there must be an internal gain mechanism which causes further carriers to be injected and transported through the device. It is generally accepted that oxygen molecules are adsorbed onto n-type TiO2 surfaces by trapping free electrons from its conduction band
Ο2 (g) + e- f Ο2- (ad) which leads to a low-conductivity depletion region near the surface.21,22 The oxygen adsorption is particularly prominent in the nanotubular structure, where the surface area is large and the depletion regions may extend to a few tens of nanometers which is comparable to the wall thickness of the TiO2 nanotubes.23 Upon UV illumination, the photogenerated holes move to the surface along the potential gradient produced by bandbending and desorb oxygen from the surface
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h+ + Ο2- (ad) f Ο2 (g) resulting in an increase in the free carrier concentration and a decrease in the width of the depletion layer. This leads to an enhancement of carrier injection, producing a persistent photocurrent. In order to verify the effect of the oxygen adsorption and desorption on the photocurrent, the photoresponse of the TiO2 nanotube arrays in vacuum was measured and shown in Figure 4. Upon UV illumination, in 15 min the photocurrent increases up to ∼1 mA, much higher than that of the photocurrent in air (see Figure
Zou et al. 3b) under the same illumination conditions. This is because in vacuum more oxygen ions can be photodesorbed from the surface of the TiO2 nanotubes so that the depletion region is reduced, leading to a higher photocurrent than that in air. When the UV light is turned off, within 1.6 h, the current only decreases from 1 to 0.17 mA, less than 1 order of magnitude. This very slow current decay process can be attributed to the suppression of oxygen readsorption in vacuum. However, when the air is pumped into the vacuum chamber, due to the readsorption of oxygen molecules the current shows a fast decay process and reaches its initial state.
Figure 3. Time-resolved photocurrent of (a) the compact TiO2 layer and (b) the TiO2 nanotube array with the same thickness (∼250 nm) under 1.06 mW/cm2 UV light illumination (λ ) 312 nm) at 0.5 V bias. (c) The photoresponses of the 250 nm thick TiO2 nanotube array device under a continuous UV light rectangle pulse. (d) The I-V characteristics of the 250 nm thick TiO2 nanotube array device both in the dark and upon 1.06 mW/cm2 UV light illumination (λ ) 312 nm). (f) The spectral response of the TiO2 nanotube array device at the bias of 2.5 V. (All devices were annealed at 450 °C.)
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J. Phys. Chem. C, Vol. 114, No. 24, 2010 10729 for rise and decay of the photocurrent are about 0.5 and 0.7 s, respectively. The photoresponsivity of 13 A/W is found under 1.06 mW/cm2 UV (λ ) 312 nm) illumination at 2.5 V. The high responsivity mainly comes from the internal gain induced by the desorption of oxygen and the reduction of the Schottky barrier (TiO2/Ag) under UV illumination. Considering the advantages of the nanotube arrays such as unique structure, ease of fabrication, large device area, and low cost, the anodic TiO2 nanotube arrays based devices have potential for large-area UV photodetector applications. Acknowledgment. We acknowledge generous support of this work by Singapore-MIT Alliance. References and Notes
Figure 4. Photoresponse of the 250 nm thick TiO2 nanotube array device in vacuum under 1.06 mW/cm2 UV light illumination (λ ) 312 nm) at 0.5 V bias.
In addition to oxygen adsorption and desorption, the reduction of the Schottky barrier at TiO2/Ag contact under UV light irradiation may also contribute to the internal gain mechanism. In our devices, the top electrode contact is a Schottky contact composed of TiO2 and Ag. Therefore there is a significant barrier to the injection of electrons into the TiO2. However, the existence of surface states at the TiO2/Ag interface can significantly modify the injection barrier.6 A likely mechanism is that the presence or absence of adsorbed oxygen at the TiO2/ Ag interface modifies the density of defect states and alters the injection barrier. Under UV illumination, the desorption of the oxygen at the TiO2/Ag interface reduces the height of the injection barrier and hence reduces the width of the depletion region. The reduction of the Schottky barrier causes more electrons to cross the barrier and then enhances the photocurrent. This mechanism is consistent with the observed superlinear intensity dependence of the photocurrent (see Figure 2d), since the current is expected to respond superlinearly to changes in effective barrier height. Conclusions We have demonstrated the UV photoresponses of the anodic TiO2 nanotube arrays based photodetectors. The annealing temperature can remarkably influence the photoelectric properties of the devices. At 450 °C, the highest UV-sensitive photoconductance can be achieved due to the pure anatase phase with high photoelectric activity in the devices. Compared to the compact TiO2 layer devices, the TiO2 nanotube array devices show higher photocurrent and faster response due to the larger internal surface area and well-defined one-dimensional charge carriers transport path. The ratio of the photocurrent-to-dark current is over 4 orders of magnitude. The characteristic times
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