Efficient ultraviolet photodetectors based on TiO₂ nanotube arrays with tailored structures

Abstract

Recently, ultraviolet (UV) photodetectors based on TiO₂ semiconductors have attracted intensive attention, due to their wide applications in environmental and biological research, optical communication, astronomical investigations and missile launch detection. However, there still remain material- and fabrication-related obstacles in realizing highly efficient UV photodetectors. Here, we reported the exploration of the efficient UV photodetectors based on the highly ordered TiO₂ nanotube arrays (TNAs). The TNAs were prepared by a two-step anodic oxidation with tailored tube lengths and wall thicknesses, and then transplanted to a transparent FTO substrate to construct a front-illuminated photodetector. The as-assembled photodetectors exhibit a satisfactory stability and wavelength selectivity with a high photocurrent, photo-to-dark current ratio and responsivity up to 1395 μA, 10 730 and 176.3 A W⁻¹ under the UV illumination of 350 nm (45 μW cm⁻¹) at a given bias of 2 V with TiO₂ tube length of 14.7 μm, respectively, suggesting their promising applications in efficient UV photodetectors.

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1. Introduction

Recently, ultraviolet (UV) photodetectors have attracted intensive attention, due to their wide applications in environmental and biological research, gas sensors, optical communication, astronomical studies, missile launch detection, and so forth.^1–5 To obtain the high sensitivity of the UV detection, the characteristics, such as high photogain, excellent wavelength selectivity and fast electron transportation, are highly desired. In this regard, one-dimensional (1D) wide bandgap semiconductor nanostructures (e.g., TiO₂, SiC, GaN and ZnO)^3,6–12 are considered as the excellent candidates for the exploration of efficient UV photodetectors. Among these wide bandgap semiconductor families, TiO₂ nanotube arrays (TNAs), especially for the highly ordered TNAs anodized from the Ti foils, arouse considerable interest in the opto/electronic nanodevices,^13–18 owing to their facile fabrication, low cost and superior performances,^19–21 e.g., excellent light-trapping, large specific surface area, well-defined charge carriers transport highway and so on.^7,20,22

To date, some works have been devoted to the exploration of the TNA-based UV photodetectors based on the anodized TNAs. Liu et al.²³ reported the photodetectors by transplanting the anodized TNAs to a Si substrate with a gold electrode, suggesting their UV photodetection responsivity up to 30 A W⁻¹. However, the electrons transportation mainly occurred in the lateral direction between the neighbored tubes, which did not take the advantage of the high pathway of the radial tube walls effectively. To overcome this issue, the photodetectors were realized with metal–semiconductor–metal structure by depositing the metal electrode on the top of the anodic TNAs based on Ti substrate.^24,25 In such configuration, the light could only be introduced from the metal electrode direction, which brought a significant loss of the light flux caused by the shade of the opaque electrode. Thereby, a front-illuminated TNA-based device is urgently required, which might be constructed by transplanting the TNAs to FTO/TCO transparent substrate to prevent the light loss. Moreover, the thicknesses and lengths of the tube walls, which determine the specific surface area and charge carrier transportation,^20,26 exert a profound influence on the responsivity of photodetectors. However, to the best of our knowledge, little attention has been paid to this point. Herein, there still remain material- and fabrication-related obstacles in realizing highly efficient UV photodetectors.

Here, we report the exploration of the efficient photodetectors based on the highly ordered TNAs fabricated by a two-step anodization. The growth of the TNAs with tailored tube lengths and wall thicknesses has been accomplished by optimizing the process of the anodic oxidation. Different to the photodetectors constructed mainly on the opaque Si and Ti substrates, the TNAs were detached from the opaque Ti substrate without damage, and then transplanted to a transparent FTO substrate as the functional units of the photodetectors. The as-constructed front-illuminated photodetectors exhibit a satisfied stability and wavelength selectivity with a high photocurrent, photo-to-dark current ratio and responsivity up to 1395 μA, 10 730 and 176.3 A W⁻¹ under the UV illumination of 350 nm (45 μW cm⁻¹) at a given bias of 2 V with TiO₂ tube length of 14.7 μm, respectively, showing the highly efficient activities and their very promising application as efficient UV photodetectors.

2. Experimental details

The highly ordered TNAs were prepared via a two-step anodic oxidation at a bias DC voltage of 50 V in a two-electrode electrochemical cell,²⁷ in which the graphite sheets and Ti foils were used as the cathode and anode, respectively. The Ti foils (250 μm in thickness, 99.7% in purity, Sigma Aldrich) were degreased by ultrasonication successively in acetone, ethanol and deionized (DI) water for 10 min during each step, and followed by rinsing with DI water. The resulted Ti foils were immersed into a mixture of hydrofluoric acid (HF), nitric acid (HNO₃) and DI water (HF : HNO₃ : DI = 1 : 4 : 5, volume percent) for 30 s to smooth the surface. Rinsed by DI water and blow-dried by a nitrogen gun sequentially, the foils were anodic oxidized for 2 h in ethylene glycol containing 0.4 wt% NH₄F and 2 vol% H₂O at room temperature. The as-grown TNAs were removed under a strong blowing N₂ after rinsing with DI water, leaving alone the substrate to guide the second anodic oxidation, which was performed at the similar anodization recipes. To modulate the structures of the TNAs, various anodic times from 20 to 150 min were applied. The resultant samples were then annealed at 450 °C for 2 h with a heating rate of 1 °C min⁻¹.

To fabricate the front-illuminated photodetectors, the as-annealed TNAs were firstly detached from the Ti substrate by anodizing again,²⁸ which was performed at the same anodic condition for ∼5 min, and subsequently immersed into hydrogen peroxide to detach the TNAs from the Ti substrate. The resultant free-standing TNAs were then rinsed with DI water and ethanol before air drying naturally. Afterwards, the free-standing TNAs were vertically transplanted to a FTO substrate, on which was previously spin-coated a thin layer of TiO₂ nanoparticles paste as an adhesive layer. These products were annealed at 450 °C for 30 min to immobilize the TNAs on FTO to make a good contact between the TNAs and FTO. Finally, Ag suspension (05001-AB, SPI Supplies, USA) was doctoral-bladed onto the top face of the TNAs, for constructing the TNA-based photodetectors.

The morphology and structure of the as-fabricated samples were characterized by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan). The phase composition of the TNAs was identified by X-ray diffraction (XRD, D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406). Optical absorption of the resultant TNAs was examined with a UV-vis spectrophotometer (U-3900, Hitachi, Japan). The photoelectric performance was analyzed with a program-controlled semiconductor characterization system (4200-SCS, Keithley, USA). A 500 W xenon arc lamp and a monochromator with a series of band pass filters were used to produce the monochromatic light, in which the light intensity of the output was calibrated with a stand light power meter (FZ-A Radiometer, Photoelectric Instrument Factory of Beijing Normal University, China). All the measurements were carried out at room temperature in ambient conditions.

3. Results and discussion

To accomplish the tailored growth of the TNAs with different wall thicknesses and tube lengths, various times are applied during the anodic oxidation with a DC bias voltage of 50 V. Fig. 1 shows the typical SEM images of the as-fabricated TNAs by varying the anodizing times ranged from 20 to 150 min. It seems that the TNAs could be formed once the anodization lasted for 20 min.^29–31 The tube length is averagely sized in ∼3.2 μm with a typical inner diameter and wall thickness of ∼59 and 50 nm, respectively (shown as the inset in Fig. 1(a)). With the anodization duration up to 40 min, the representative length of the TNA wall is accordingly increased up to ∼6.3 μm (Fig. 1(b)) with a typical inner diameter and wall thickness of ∼94 and ∼15 nm, respectively (shown as the inset in Fig. 1(b)). Fig. 1(c–e) show the typical cross-section SEM images of the as-prepared TNAs with anodic times up to 60, 80 and 120 min, respectively, leading to the growth of the TNAs with the tube lengths of ∼9.5, 11.7 and 14.7 μm, respectively, implying that the lengths of the TNAs are increased with extending anodic oxidation. Accordingly, the thicknesses of the tube walls decrease from 14 to 12 nm. It is worth noting that, when the anodic time is fixed up to 120 min, the uniform and highly-ordered growth of the TNAs in large scale with debris-free and few-crack surface could be accomplished (Fig. 1(f)). The as-grown tubes are close packed with a very uniform diameter size distribution (averaged sized in ∼100 nm, Fig. 1(g) and (h)). Further extending the anodic time up to 150 min, the growth of the TNAs begins to collapse to some extent, in which some debris, cracks and disorder nanosheets are randomly produced on the top surface of the TNAs (the inset in Fig. 1(i)). This could be ascribed to the fact that, with the anodic time extended long enough, the over-etching and disintegration of the tube walls will happen inevitably, making the formation of the “nanograss” with numerous debris and cracks on the top of the TNAs.³² It suggests that, in current case, the proper anodic oxidation time should be limited within 120 min for the growth of high-qualified TNAs.

Fig. 1 (a–e) Typical cross-section SEM images of the TNAs with the anodization times of 20, 40, 60, 80 and 120 min under low magnifications. (f–h) The typical SEM images of the as-fabricated TNAs with a anodization time of 120 min via various view angles under high magnifications. (i) A typical cross-section SEM images of the TNAs with the anodization time of 150 min under a low magnification. The insets are the corresponding top-view images of the as-fabricated TNAs with the scale bar of 100 nm.

The as-anodized TiO₂ nanotubes are usually amorphous, which should be crystallized under the high temperatures prior to be applied in devices. Fig. 2(a) shows the XRD patterns of the as-annealed TNAs with an anodic oxidation time of 120 min at 450 °C in a tube furnace. The peaks, centered at 2θ = 25.3°, 37.8° and 48.0°, can be indexed to the diffractions of (101), (004) and (200) planes of anatase TiO₂ (JCPDS Card no. 21-1272), respectively. All other diffractions can also be indentified to the anatase TiO₂, apart from some signals from the Ti substrate at 2θ = 35.1°, 40.2° and 53.0°, which correspond to diffractions of (100), (101) and (102) crystal planes of the metal Ti, respectively (JCPDS Card no. 44-1294). The narrow and sharp diffractions mean that the as-fabricated TNAs are highly crystallized after the annealing treatment. Further characterizations of the as-annealed TNAs are examined by TEM. Highly ordered tubular structures can be observed clearly in the low-magnification TEM images (Fig. 2(b)). Fig. 2(c) shows the typical selected area electron diffraction (SAED) pattern of the TNAs recorded from the marked area of A in Fig. 1(b). The diffraction spot rings could be sequentially indexed to (101), (004), (200) and (105) crystal planes of anatase TiO₂ (JCPDS, Card no. 21-1272), confirming that the resultant TNAs are of pure anatase TiO₂ phase. Fig. 2(d) shows the corresponding HRTEM images of the TNAs recorded from the marked area of B in Fig. 1(b), disclosing the single-crystalline nature of a single nanotube. The lattice spacing of the interplane is ∼0.35 nm, responding to the distance of (101) crystal plane faces of anatase TiO₂.

Fig. 2 (a) A typical XRD pattern of the TNAs with a anodization time of 120 min. (b) The representative TEM image of the TNAs under a low magnification. (c and d) The corresponding SAED pattern and HRTEM image of the TNAs recorded from the marked area of A and B in (b).

The as-annealed TNAs were then detached from the Ti substrate and transferred onto a FTO glass substrate, on which a thin layer of TiO₂ nanoparticle paste is previously spin-coated as the adhesive layer. Fig. 3(a) shows the typical cross-section of the device configuration, which is consisted of TNAs, adhesive layer and FTO substrate from the top to the bottom, respectively. The used TiO₂ adhesive layer with the subsequent annealing treatment guarantees the self-standing TNAs to tightly contact the FTO for ensuring the collection and transport of the charge carriers. To construct the photodetector device, the doctor-bladed Ag paste is used as the top and bottom electrodes, which is schematically shown in Fig. 3(b). Differently from most of the reported works, the UV light in current case is introduced from the transparent FTO substrate into the photodetector to avoid the shading of the electrode, which allows the formation of front-illuminated photodetectors. To investigate the length-dependent photodetection properties, three types of TNAs with the typical lengths of 6.3, 11.7 and 14.7 μm are chosen to be utilized as the functional units. Fig. 3(c) shows the time-dependent photocurrent responses of these three photodetectors illuminated by a 350 nm wavelength pulse UV light with an intensity of 45 μW cm⁻². The tests with five repeated cycles are carried out at a 2 V bias under periodically off/on UV light with an internal of 250 s. It presents the following information: (i) the photocurrents are stable in peak value and can be readily recovered from the final to the initial states of each circle for all samples with the UV light off/on. The time-dependent photocurrent response of the typical device with the TNA tube length of 14.7 μm for more than 20 repeated cycles also show the nearly identical photodetecting activities (the experimental set up not shown here), suggesting the satisfied stability and reproducibility of the TNA-based photodetectors; (ii) the excited photocurrents are ∼86, 645 and 1395 μA corresponding to the photodetectors with the TNA lengths of 6.3, 11.7 and 14.7 μm, respectively, suggesting that the tube length played a significant effect on their responsibilities; (iii) the detected dark- and photo-currents are ∼0.13 and 1395 μA, respectively, demonstrating that the ca. photo-to-dark current ratio is up to 10 370. These experiments as mentioned above verify that the as-constructed TNA UV photodetectors with tailored structures are highly efficient. Fig. 3(d) shows the response and recovery times (defined as the times rise to 90% of the maximum photocurrent and fall to 1/e (37%) of the stable one, respectively) of these three TNA-based photodetectors. For the photodetectors with 6.3, 11.7 and 14.7 μm TNA lengths, the response and recovery times are 41 and 2 s, 45 and 11 s, 82 and 14 s, respectively, disclosing that they increase with the increase of the TNA lengths. This phenomenon might be ascribed to following points: (i) the different mean velocity of the scattering electrons induced by the various TNA lengths. At a fixed bias voltage, the intensities of the electric fields should be reduced with the raise of the tube lengths. (ii) The different surface areas of the TNA with various lengths. The longer length of the TNA could bring a higher surface area, which would prolong the response/recovery time due to the slower oxygen adsorption/desorption processes.

Fig. 3 (a) The representative SEM images of the constructed photodetector by transplanting the TNAs onto the FTO substrate. (b) Schematic illustration for the assembled front-illuminated photodetector configuration. (c) The typical time dependent photoresponses of the photodetectors with different TNA lengths under a pulse UV light with intensity of 45 μW cm⁻². (d) The typical responses and recovery times of the photodetectors with different TNA lengths. (e) The typical spectral responsivities of the photodetectors with different TNA lengths. (f) The representative current–voltage characteristics of the photodetector with the TNA length of 14.7 μm in the dark and upon the UV light illumination.

Fig. 3(e) shows the spectral responses of the as-assemble photodetectors to the UV lights at a given bias of 5 V. The responses of these three photodetectors, with the tube lengths of 6.3, 11.7 and 14.7 μm, exhibit good consistent in the range from 300 to 450 nm. It also suggests that their responsivities are highly sensitive to the wavelengths of the incident lights, which are consistently peaked at ∼370 nm. Notably, the responsivities of the devices are gradually increased with the increase of the tube lengths. For the photodetector with a 6.3 μm tube length, the responsivities are ∼2.45 and 7.68 A W⁻¹ at 330 nm and 340 nm, respectively, followed by a remarkable enhancement at 370 nm with a peak value of 36.6 A W⁻¹. Interestingly, in regard to the photodetector with a 14.7 μm tube length, the peak responsivity is remarkably improved to 176.3 A W⁻¹. The UV light responsivity at 370 nm is ∼66 times to that at 400 nm, implying the excellent wavelength selectivity of current photodetector devices. The current–voltage properties of the photodetector (exampled by the TNAs with 14.7 μm length) in the dark and upon the UV illumination at 350 nm, are presented in a logarithmic scale in Fig. 3(f). The current–voltage characteristics both in dark and upon UV radiation exhibit the similar non-linear behaviors and nearly symmetric curves at the negative and positive biases, suggesting that the as-fabricated TNA photodetectors should be a Schottky contact.^1,33

To explore the possible mechanism of the tube-length dependent responsivities of the TNA-based photodetectors, following analyses are conducted. The responsivity of the device is defined as the excited photocurrent per unit of the incident light power, which is generally given as:¹¹

where R is the responsivity, I is the photocurrent, A is the active area of the device, and E is the per unit of the incident light power, η is the quantum efficiency, h is Planck's constant, c is the velocity of the light, λ is the wavelength of incident light and, g is the photoconductive gain, respectively. It implies that, at a fixed wavelength of the incident UV light, the responsivity should be proportional to the η and g. Fig. 4(a) shows the UV-vis absorption of the TNA-based devices with the lengths of 6.3, 11.7 and 14.7 μm, respectively. The spectra show the nearly identical abrupt absorption edges at ∼390 nm, which is in accordance to the optical absorption gap of the anatase TiO₂ (∼3.2 eV) and the cut-off wavelength of the spectra response as shown in Fig. 3(e). There is almost no difference in the absorptions from 350 to 390 nm, suggesting that the η of the devices is independent on the tube lengths. That is to say, the contribution of η to the significant improvements in the photocurrent response (Fig. 3(c)) and responsivity (Fig. 3(e)) could be excluded, which nevertheless should be mainly ascribed to the enhancement of g with the increase of the TNA lengths. This could be supported by the fact that the g at the peak responsivities of the photodetectors, with the tube lengths of 6.3, 11.7 and 14.7 μm, are ca. 122.8, 194.3 and 591.6, respectively. The higher g might be attributed to the variation of the conductivity (Δσ) of the TNAs, due to the desorption of oxygen molecules on the surface of the TiO₂ nanotubes.^34,35 Under the dark circumstance, it is commonly considered that the oxygen molecules are apt to be absorbed onto the large surface of the nanotubes and formed numerous negative charge at the surface of the tube-wall by trapping free electrons from the n-type TiO₂ semiconductor.^23,25 At the same time, various positive charge at the counterpart of the site donating electrons and accordingly, a low-conductivity depletion region could be formed near the surfaces of the nanotubes. Upon the UV light illumination, the electron–hole pairs would be generated within the body of the nanotubes. The photogenerated holes would migrate to the tube surfaces to discharge the adsorbed oxygen ion and lead to the desorption of the oxygen molecules from the tube surfaces, as schematically illustrated in Fig. 4(b). Thereby, the recombination of the electrons during transportation are greatly limited, making an enhancement of the conductivity, which is related to the variations of the photogenerated carriers (Δn) and the carrier mobility (Δμ). For the TiO₂ nanotube, Δn is much higher than Δμ, and the photocurrent can be given by:^24,34,36,37

I_photo = ΔσEA = qμΔn_eff(d)EA (2)

where d is the diameter of the nanotube, E is the electric field, A is the effective area and Δn_eff(d) is the variation of the effective carrier densities, which are usually equal to the sum of the photogenerated carriers (Δn) and accumulated electrons concentration (Δn_acc(d)) owing to the oxygen absorption/desorption. Fig. 4(c) schematically demonstrates the typical structure of the TiO₂ nanotube derived from the anodic oxidation, which usually exhibits a V-shaped sidewall profile.^19,20 Upon the UV illumination, the population of the photogenerated electron–hole pairs at a tube cross section per unit length is proportional to (d_o²) − (d_i²), and the variation of the electrons per unit length, due to the discharged oxygen ions and surface trapped holes, is proportional to (d_o + d_i) (d_o and d_i are the outer and inner tube diameters of the TiO₂ nanotubes, respectively). Herein, the Δn_acc(d) can be expressed as:^23,34,38

Fig. 4 (a) The typical normalized absorption spectra of the TNAs with the lengths of 6.3 μm, 11.7 μm and 14.7 μm. (b) Schematic illustration of the oxygen absorption on the surface of the TNA surface in the dark, and desorption induced by the UV light illumination. (c) Schematic illustration for the structures of the as-grown single TiO₂ nanotube. (d) The relationship between the TNA lengths and anodic times.

It means that the Δn_acc(d) is inversely proportional to the thickness of the tube wall. Based on the discussion as mentioned above, it can be briefly concluded that the responsivity R could be inversely proportional to the value of d_o − d_i, which is shown as bellow:

According to the observations under SEM (Fig. 1), the as-fabricated TiO₂ nanotube possesses a thinner thickness with a longer anodized time, thus leading to an enhancement of the responsivities. It is also noted that different lengths of the TNAs would result in a various surface areas, which can be calculated by (l_o is tube length). Accordingly, with the increase of the tube lengths caused by the raise of the anodic time, the surface areas should be increased. A larger surface area can facilitate the oxygen adsorption/desorption, making the significant enhancement on the photodetecting activities. Current work suggests that the highly efficient photodetector could be achieved by using longer TNAs with thinner tube walls, which could be accomplished by just prolonging the anodic oxidation times. We also attempt to use the even longer TNAs (e.g., 150 min anodic time, 15 μm length, as shown in Fig. 4(d)) as the functional unit for obtaining further enhanced performance. However, it shows a slightly degradation of the responsivity compared to that of the TNA-based photodetectors with a 14.7 μm length. This could be mainly attributed to the increased traps and recombination centers induced by collapse of the nanotubes with numerous debris, cracks and “nanograss” on the top surface of the TNAs (shown as the inset in Fig. 1(i)).

4. Conclusions

In conclusion, we have demonstrated the tailored growth of highly ordered TNAs. The as-fabricated TNAs exhibit an uniform diameter size distribution in large scale, and their tube lengths as well as the wall thicknesses have been modulated by adjusting the anodic oxidation times at a bias DC voltage of 50 V in a two-electrode electrochemical cell. The front-illuminated photodetector based on Ag/TiO₂ nanotubes/FTO configuration has been realized by transplanting the TNAs from the Ti substrate to a transparent FTO glass. Illuminated by the UV light with a wavelength of 350 nm (45 μWcm⁻¹) at a bias of 2 V, the as-assembled photodetector with TiO₂ nanotube length of 14.7 μm exhibits the best photodetecting activities. The photocurrent can be readily recovered from the final to the initial state for each cycle with the UV light off/on, suggesting the satisfied stability of the TNA-based photodetector devices. The photodetectors exhibit good wavelength selectivity with a high photocurrent, photo-to-dark current ratio and responsivity up to 1395 μA, 10 730 and 176.3 A W⁻¹, respectively, suggesting their potential applications in efficient UV photodetectors.

Acknowledgements

The work was supported by National Natural Science Foundation of China (NSFC, Grant no. 51372122, 11401328 and 51372123), National-level College Students' Innovative Entrepreneurial Training Plan Program (Grant no. 201411058013), and W. M. Wang Entrepreneurial Foundation (Grant no. 2014001).

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