- Email: [email protected]
A high-sensitive ultraviolet photodetector composed of double-layered TiO2 nanostructure and Au nanoparticles film based on Schottky junction
Materials Chemistry and Physics 194 (2017) 42e48
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A high-sensitive ultraviolet photodetector composed of doublelayered TiO2 nanostructure and Au nanoparticles film based on Schottky junction Huan Wang a, Pei Qin a, Guobin Yi a, *, Xihong Zu a, Li Zhang b, **, Wei Hong c, Xudong Chen c a b c
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, PR China School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, PR China School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel double-layered TiO2 nanostructure was synthesized by a simple method. An UV photodetector composed of TiO2 and Au was designed and fabricated. The preparation method of TiO2/Au UV photodetector was simple and convenient. The UV photodetector based on TiO2/ Au showed excellent sensitivity to UV light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2016 Received in revised form 28 February 2017 Accepted 14 March 2017
In this study, a Schottky-type ultraviolet (UV) photodetector based on double-layered nanostructured TiO2/Au films was fabricated. Double-layered titanium dioxide (TiO2) nanostructures composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods on fluorine-doped tin oxide (FTO) pre-coated glass substrates were synthesized via a convenient hydrothermal method using titanium butoxide and hydrochloric acid as the starting precursor, without involving the use of any other surfactants and catalysts. A granular-shaped thin-layer of Au film using vacuum sputter coating technique was subsequently deposited on TiO2 for the formation of Schottky-type photodetector. The as-fabricated Schottky device showed various photocurrent responses when irradiated with different wavelength of UV light. This suggests that the newly-developed photodetectors have promising potential for identifying different UV light wavelengths. © 2017 Elsevier B.V. All rights reserved.
Keywords: Double-layered TiO2 Au nanoparticles film Hydrothermal method UV photodetector Schottky junction
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Yi), [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.matchemphys.2017.03.019 0254-0584/© 2017 Elsevier B.V. All rights reserved.
As a member of the optoelectronic device family, Ultraviolet (UV) photodetector is becoming increasingly important in a wide
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
range of fields including solar UV monitoring, flame alarm, and biological detection [1e5]. So far, UV detectors based on wide bandgap semiconductor materials such as SiC, GaN, ZnS, ZnO, and TiO2 have attracted significant interest owing to their excellent wavelength selectivity (“visible-blindness”) and a large possibility of room-temperature operation [6e18]. As a well-known wide band-gap semiconductor, TiO2 has shown a lot of potential technical applications in a number of fields such as photocatalysts, gas sensors, dye-sensitized solar cells, and so on [19e23]. Furthermore, the wide band-gap (3.2 eV for anatase and 3.0 eV for rutile) make it very suitable for the UV light detection without any filter required to block visible or infrared light. Thus, UV photodetectors based on TiO2 have been widely investigated owing to excellent physical, chemical, and optical properties associated with TiO2 in the past decades [24e27]. In particular, the metal-semiconductor Schottky-type UV detector has been paid more attention [28e30]. Zhang et al. [28] reported a Schottky diode ultraviolet detector based on TiO2 nanowire array with Ag nanoparticles. The device of TiO2/Ag nanocomposites showed a good UV photoelectric property. It showed that the semiconductor TiO2 nanostructure combined with metal had potential application in UV detection and sensing. However, some photodetectors demonstrated good device performance and operation stability, but these devices suffered from the complexity and high cost of the fabrication process, hindering from the commercialization of these devices. Therefore, it is very important to develop an efficient and convenient method to fabricate metal-TiO2 nanostructure device for the application in UV detection. The fundamental necessities in designing UV detectors are high sensitivity, speed, high-responsivity and low-noise. And the nanostructure has great influence on the performance of the photovoltaic device. In this work, a novel double-layered TiO2 nanostructure composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods were grown on fluorine-doped tin oxide (FTO) pre-coated glass substrates via a simple and economic hydrothermal method and Schottky-type photodetector based on double-layered TiO2 nanostructure with Au nanoparticles was fabricated. The preparation method of the double-layered TiO2 and TiO2/Au heterostructures is very simple and convenient, and we can easily make it large-scale production. The as-prepared UV photodetector generated a very high photocurrent, which is much higher than that of the similar UV detectors reported by others [6,18]. The high UV photocurrent generated at low bias is favorable for its practical applications. On the other hand, the as-fabricated Schottky-type UV photodetector showed high-responsivity and various photocurrent responses under different wavelength of UV light illumination, this suggests that the newly-developed device is a potential candidate as UV photodetector for identifying different wavelengths UV light.
43
FTO substrate by a simple hydrothermal method. Typically, 25 mL deionized water and 25 mL concentrated hydrochloric acid were mixed with stirring for 1 min. Subsequently, 0.8 mL titanium butoxide was added dropwise to the above solution. After stirring for another 3 min, two pieces of FTO substrates (ultrasonically cleaned for 30 min in acetone, ethyl alcohol and deionized water respectively) were placed at an angle against the wall of the Teflon-liner stainless steel autoclave with the conducting side facing up. Afterwards, the autoclave was sealed and the hydrothermal synthesis was conducted at 140 C in an electric oven. After 18 h, the autoclave was taken out from the oven and it was cooled to room temperature under flowing water. Finally, the double-layered TiO2 nanostructure were obtained after being rinsed with deionized water and dried in ambient air. The granular-shaped thin-layer of Au film electrode was coated to the top surface of the as-prepared double-layered TiO2 nanostructures by vacuum sputter coating (SBC-12, KYKY Technology, China). The UV photodetector composed of the double-layered TiO2 nanostructures and Au electrode based on Schottky contact was obtained. The total active area of UV photodetector composed of double-layered TiO2 and Au was ~0.36 cm2. 2.3. Characterization techniques The morphology and structure of the samples was examined using SEM (JEOL, JSM-7001F, Rigaku Tokyo, Japan). The crystal structures of the samples were characterized by XRD (X’pert Powder, PANalytical, Holland) with fixed Cu-Ka line (l ¼ 0.15405 nm) in the Bragg angle ranging from 20 to 75 . X-ray tube voltage and current were set at 40 kV and 40 mA. UVeVis absorption spectra of the samples were recorded in the range of 220e800 nm using a UVeVis spectrophotometer (UV 2450, Shimadzu). Photoluminescence (PL) spectra were used for studying the optical properties of the TiO2 nanostructures. The photoelectric properties of the TiO2/Au device were measured by a CHI660D electrochemical workstation. The UV source was provided by a portable UV lamp (~8W, ENF-280C, USA). 3. Results and discussion 3.1. Morphology and structure of the prepared TiO2 and TiO2/Au Fig. 1 shows the typical SEM images of the double-layered TiO2
2. Materials and experimental 2.1. Materials Titanium butoxide ([CH3(CH2)3O]4Ti, 98%) was purchased from Tianjin Fuchen Chemical Reagents Factory and it was used without any other treatment. Hydrochloric acid (36.5%e38% by weight, analytical grade) was purchased from Guangzhou Chemical Reagent No. 2 Factory and it was used without any other treatment. FTO substrate (F: SnO2, 14 U/sq, glass thickness: 1.1 mm) was purchased from Nippon Sheet Glass (Japan). 2.2. Preparation of the TiO2 and TiO2/Au The double-layered TiO2 nanostructures were growing on an
Fig. 1. The SEM images of the TiO2 growth on FTO substrate, (a) top view, (b) and (c) show the corresponding morphology at higher resolution, (d) show the sectional view.
44
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
nanostructures produced by a convenient hydrothermal method. As clearly seen in Fig. 1a, the obtained TiO2 sample is composed of one layer of TiO2 nanoflowers on a layer of TiO2 nanorods array film with the average diameter of ~160 nm for the nanorods and the average height of ~2.5 mm for the arrays (Fig. 1b and d). What is more, each TiO2 nanorod consists of many TiO2 nanowires (Fig. 1c) while each TiO2 nanoflower was composed of many nanorods. The layer of TiO2 nanorods array film could be grown on FTO substrate owing that the rutile TiO2 and FTO have a small lattice mismatch [31]. Meanwhile, there were many TiO2 crystal nucleuses in the reactant solution owing to the hydrolysis of titanium butoxide during the growth of TiO2 nanorods array. Some TiO2 nanoflowers crystal nucleuses also will grow when the TiO2 nanorods array were growing. The initial reactant concentration has a significant influence on the shape and morphology of the TiO2 nanostructures [32]. The double-layered TiO2 nanostructures have bigger surface area compared with the single layered TiO2 nanorods array, and it has the larger receiving area to absorb UV light when it is applied to a UV photodetector. Fig. 2a-d shows the SEM images of the fabricated Schottky device which composed of one layer of granular-shaped Au film on the top surface of the double-layered TiO2 nanostructure. As shown in Fig. 2(b and c), a granular-shaped thin layer of Au nanoparticles was coated on the surface of both the TiO2 nanoflower and the TiO2 nanorods. Furthermore, it showed that the Au contact well with the TiO2 nanoflowers and nanorods, which contributed to the formation of Schottky junction and the transportation of the electrons. The energy dispersive X-ray (EDX) analysis (Fig. 2e) reveals the presence of titanium (Ti), oxygen (O) and aurum (Au) signals, which come from TiO2 and Au. Fig. 2f shows the fabrication process of the Schottky device with the Au nanoparticles as the metal electrode and the double-layered TiO2 nanostructures as the n-type semiconductor. X-ray diffraction (XRD) analysis provides the information about crystalline quality and orientation of the nanostructure. Fig. 3 shows the XRD patterns of FTO substrate and double-layered TiO2 nanostructure and TiO2/Au nano-composite on FTO substrate. The diffraction peaks of the double-layered TiO2 can be assigned well to tetragonal rutile crystalline phase of TiO2 with a reference pattern (JCPDS file no. 21-1276). The peaks at 2q ¼ 28.1, 37.0 , 41.9 , 56.8 , 63.4 , 69.2 and 70.2 can be indexed to (110), (101), (111), (220), (002), (301) and (112) crystal planes of rutile TiO2. For the TiO2/Au nanocomposite, the characteristic peaks of Au (JCPDS file no. 040784) were also clearly presented in addition to the diffraction peaks of TiO2, it showed that the TiO2/Au nanocomposites composed of double-layered TiO2 and Au nanoparticles was successfully prepared.
Fig. 2. The SEM images (a, b, c, d) of the TiO2/Au nanocomposite nanostructures, (b) and (c) show the corresponding morphology at higher resolution, (d) show the sectional vies. (e) EDX spectrum of the TiO2/Au nanocomposite, (f) fabrication process of the Schottky device composed of Au and TiO2.
Fig. 3. XRD patterns of the double-layered TiO2 nanostructure and TiO2/Au nanocomposite on FTO substrate.
3.2. Optical properties of the TiO2/Au device Fig. 4a shows the absorption spectrum of the double-layered TiO2 nanostructure. It is noted that clearly, the double-layered TiO2 has a strong absorption in the UV band, implying that the as-synthesized TiO2 nanostructures have a strong sensitivity to the UV light. The optical band gap energy of the double-layered TiO2 nanostructure is calculated by Tauc plot shown in Fig. 4b. The absorption band gap energy can be determined by the following equation [33,34]. (ahn)n ¼ В(hn Eg) where hn is the photon energy, a is the absorption coefficient, В is a constant relative to the material and n is a value that depends on the nature of transition (n ¼ 2 for direct band gap, 2/3 for direct forbidden gap and 1/2 for indirect band gap). On extrapolating the line from linear portion of the absorption edge, it gives the optical band gap energy. That is to say, (ahn)1/2 versus hn extrapolate to a ¼ 0 gives the optical band gap energy. The calculated band gap energy of the double-layered TiO2 nanostructure is 3.0 eV, which also implies that the double-layered TiO2 is rutile (3.0 eV) phase. Fig. 5 shows the room temperature photoluminescence (PL) features of the as-synthesized double-layered TiO2 with different excitation wavelengths. As is known, the PL spectrum is obtained as a result of the competition among electron-hole separations, electron-phonon scattering and electron-hole recombination. TiO2 has a direct band-gap but is subjected to dipole-forbidden transition [35]. PL of TiO2 has three kinds of physical origins: self-trapped excitons, oxygen vacancies and surface states defects [36e38]. Most of the surface states defects are oxygen vacancies or the Ti4þ ions adjacent to oxygen vacancies [39]. In the present case, the stronger peaks at 413 nm should be attributed to the emission from the band edge recombination process of TiO2. The peak at 425 nm should be corresponding to inter-band recombination [36]. The other prominent peak near at 465 nm was due to oxygen vacancy [39]. What’s more, Fig. 5 exhibits that the double-layered TiO2 has an excellent optical quality and the intensity of photoluminescence increased with the increasing of excitation wavelength, which implies that the longer wavelength of UV light irradiated on the TiO2 sample can generate more photoelectrons and hole. In order to examine the UV photosensitivity of the TiO2/Au device, three UV
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
45
Fig. 4. (a) UVeVis absorption spectrum of the double-layered TiO2 nanostructure and (b) the Tauc plot of (ahn)1/2 as a function of photon energy.
Fig. 5. Photoluminescence spectra of the double-layered TiO2 nanostructures excited with different wavelengths.
Fig. 6. (a) I-V curves of the UV detector measured without UV and with different UV illumination, (b) I-t curves of the device under different UV illumination at 1 V forward-bias voltage. (c) Schematic energy band diagram of the TiO2 and Au interface.
general equations: sources with different excitation wavelength including 254 nm from ultraviolet radiation c (UVC) and 312 nm from ultraviolet radiation b (UVB) and 365 nm from ultraviolet radiation a (UVA) were chosen to test the I-V and I-t curves of the TiO2/Au device.
qV 1 I ¼ I0 exp hkT
(1)
3.3. UV photo-detection properties of the TiO2/Au device
qFB I0 ¼ A ST2 exp kT
(2)
Fig. 6a shows the I-V curves of the TiO2/Au device measured without and with different wavelengths UV illumination, which revealed that the TiO2/Au device possesses a rectifier response. This result indicates the presence of a non-Ohmic contact in our device, which can be treated as the Schottky contact [11]. Fig. 6c shows the schematic energy diagram of the interface of Au and TiO2. As is known, once the metal that has a higher work function than that of the n-type semiconductor contacted with each other, the electrics in the n-type semiconductor will transfer into the metal to maintain the flattening of the Fermi level. Thus, a depletion region will form in the interface of the metal and n-type semiconductor [29,40,41]. The symbols of “þ” and “-” in Fig. 6c present the positive ions ion in TiO2 and negative electrons in the interface, respectively. The performance of the UV photodetectors is dependent on the quality of the Schottky contacts. Therefore, it is necessary to measure the actual Schottky barrier height and the ideality factor of the device. The dark current transport over the Schottky barrier could be explained by the thermionic emission theory using the following
Here I0 is the reverse saturation current, S is the active area of the device, q is the electronic charge, V is the bias voltage, k is the Boltzmann’s constant, T is the absolute temperature, A* is the effective Richardson constant, which has a theoretical value of 1200 A cm2K2, h is the ideality factor, FB is the effective Schottky barrier height. According to the I-V dark curve near 0 V, the I0 is estimated to be 3.1 nA, the effective Schottky barrier height and ideality factor were determined to be 0.61 eV and 1.6. The value of FB was approximately equal to the theoretical value between the work function of Au (5.1 eV) and rutile TiO2 (4.1e4.6 eV) [28]. This indicates good quality of the Schottky contact between TiO2 and Au, it confirms a very limited Fermi level pinning effect at the TiO2/Au interface. Such a high barrier can cause a wide depletion region and the direction of the inner electric field in the depletion region is from TiO2 to Au, as shown in Fig. 8(a and b). On the other hand, the photocurrent varies with different wavelengths UV light illumination. When the UV irradiation wavelength is longer, the photocurrent is much higher. This result
46
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
Fig. 7. Spectral responsivity characteristic of the UV detector under 1 V bias.
indicates that the prepared device can be used as an UV photodetector which could identify different wavelengths UV light. Fig. 6b shows the I-t curves of the TiO2/Au device when irradiated with 254 nm, 312 nm and 365 nm UV light, with the forward-bias voltage of 1 V. The I-t curves in Fig. 6b showed the response of photocurrent with the different UV light irradiation. Clearly, the photocurrent immediately had a jump when the UV source was turned on, and the photocurrent significantly decreased when the UV source was turned off. Furthermore, the photocurrent of the TiO2/Au device increased with increasing the wavelength of UV light. The possible reason lies in that the longer UV light can generate more photoelectrons and holes in the TiO2 layer of this TiO2/Au device. The results agree with the above mentioned PL spectra shown in Fig. 5. On the other hand, the photocurrent of the photodetector irradiated with 365 nm was as high as ~0.17 mA under 1 V bias, which was much higher than that UV photodetectors based on single TiO2 nanorods array film were reported by others. The reasons may be that the double-layered TiO2 had the bigger received area to absorb the UV light to produce higher photocurrent. The wavelength selectivity of the TiO2/Au UV detector was measured in the range of 220e500 nm at 1 V bias, the result is shown in Fig. 7. It is clearly seen that the TiO2/Au device showed excellent UV light detection selectivity in a spectral range between 270 and 410 nm. It means that the device can be used as photodetector for UV light, especially for the UV-A (320e400 nm). The maximum UV responsivity is about 138.5 A/W at the wavelength of 360 nm. The UV responsivity to the short wavelength (220e260) is much lower than that of the long wavelength UV light, it dues to weak penetration of the short UV light. The edge of 410 nm is attributed to the absorption edge of the TiO2 layer.
Fig. 8. Diagrammatic sketch of TiO2/Au Schottky junction in different voltage biases with UV illumination; (a) in forward-bias, (b) in reverse-bias.
The high sensitivity and photocurrent of the Au/TiO2 device are promising for large-area UV detector applications. Table 1 is a comparison of the sensitivity and photocurrent to other UV detectors related to this work. It shows that our device had the higher photocurrent and responsivity than most of other UV detectors. Although the structures of the UV detectors were different, it means that the prepared Au/TiO2 device showed the advantages of high photocurrent and responsivity than others. Fig. 8(a and b) give the diagrammatic sketch of TiO2/Au Schottky device in different biases with UV irradiation. Without UV illumination, when the Au electrode was contacted with the positive terminal and TiO2 was contacted with the negative terminal of external circuit, the electrons in Au moved out to the positive terminal of the external circuit and the electrons come out from the
Table 1 Comparison of the sensitivity to other UV detectors related to this work. Photodetector
Bias
Photocurrent (mA)
Photo-responsivity (A/W)
On/off ratio
Ref.
GR/Au/GaN Au/ZnO/GR SrTiO3/TiO2 TiO2 Ag/TiO2 Ag/TiO2 Au/TiO2
1V 1V 5V 0V 5 V 6.5 V 1V
128.1 50.5 114 13.2 10.2 0.75 163.6
25 ~178 ~90.1 4.33 103 3.1 0.18 136.4
3.1 ~2.2 104 3.5 103 12.1 ~2.1 102 8.5 ~5.5 104
[8] [18] [22] [23] [28] [30] This work
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
negative terminal of the external circuit come into the TiO2 of the TiO2/Au device. The direction of the inner electric field in the depletion region was from TiO2 to Au, thus, the electrons of the external circuit could come to the positive terminal of the inner electric field, and the current of the TiO2/Au device increased when the voltage of the external circuit increased. On the other hand, when the Au electrode was contacted with the negative terminal and TiO2 was contacted with the positive terminal of the external circuit, the electrons come out from the negative terminal of the external circuit and enter the Au of the device. In this case, the direction of the inner electric field in the depletion region was from TiO2 to Au, the electrons of the external circuit could not pass through the inner electric field, thus, the TiO2/Au device had a relatively low dark current in reverse-bias shown in Fig. 6a. When the TiO2/Au device was irradiated with UV light, the electron-hole pairs were generated in TiO2 as shown in Fig. 8a-b. When the Au was contacted with the positive terminal of the external circuit, the photo-generated electrons moved to the depletion region and decreased the intensity of the inner electric field, which could increase the current of the TiO2/Au device shown in Fig. 6(aeb). Furthermore, the photo-generated electrons can also increase the electrical conductivity of the device. So the current also increased with the UV illumination when the TiO2/Au device is in the negative terminal. On the other hand, a widely accepted photoconduction mechanism of TiO2 nanorods functions as follows. There were so many oxygen vacancies in TiO2 nanostructure which was proved by the results of PL spectra shown in Fig. 5. In a dark atmosphere, oxygen molecules would be absorbed into the surface of TiO2 and capture the free electrons (O2 þ e/ O 2 ), and it would form a low-conductivity depletion layer near the surface of the TiO2. The electron-hole pairs would be photogenerated in TiO2 when the UV light was illumination, and some of the photogenerated holes would migrate to the surface to discharge the negatively charged absorbed oxygen ions (hþ þ O 2 / O2), oxygen is photo-desorbed from the surface, and eliminate the chances to recombine the electron-hole pairs, which could increase the conductivity of the device and gain a higher photocurrent as the results shown in Fig. 6. 4. Conclusions In summary, double-layered TiO2 nanostructure composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods was successfully grown on FTO substrate by a simple and economic hydrothermal method. The as-obtained double-layered TiO2 exhibited excellent optical properties and crystalline quality as the results shown in PL spectra and XRD patterns. The fabricated Schottky-type UV photodetector based on double-layered TiO2 nanostructure with Au nanoparticles film showed high sensitivity to UV light. Furthermore, the UV photodetector showed different photocurrent when irradiated with different wavelength of UV light. It showed that the as-prepared TiO2/Au heterostructure had potential applications for UV detector and sensor. This work will provide opportunities for developing efficient UV photodetectors that can recognize different wavelengths of UV light. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51233008, 51273048, 51203025). References [1] Y.N. Hou, Z.X. Mei, H.L. Liang, D.Q. Ye, C.Z. Gu, X.L. Du, Dual-band MgZnO ultraviolet photodetector integrated with Si, Appl. Phys. Lett. 102 (2013)
47
153510. [2] L. Mandal, N.S. Chaudhari, S. Ogale, Self-Powered UV-vis photodetector based on ZnIn2S4/hydrogel interface, ACS Appl. Mater. Interfaces 5 (2013) 9141e9147. [3] B.C. Cheng, J. Xu, Z.Y. Ouyang, C.C. Xie, X.H. Su, Y.H. Xiao, S.J. Lei, Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity, Opt. Express 11 (2013) 29719e29730. [4] Q. Li, T. Qiu, H.X. Hao, H. Zhou, T.Z. Wang, Y. Zhang, X. Li, G.L. Huang, J. Cheng, Rapid and on-site analysis of illegal drugs on the nanoemicroscale using a deep ultraviolet-visible reflected optical fiber sensor, Analyst 137 (2012) 1596e1603. [5] Y.R. Xie, L. Wei, G.D. Wei, Q.H. Li, D. Wang, Y.X. Chen, S.S. Yan, G.L. Liu, L.M. Mei, J. Jiao, A self-powered UV photodetector based on TiO2 nanorod arrays, Nanoscale Res. Lett. 8 (2013) 188. [6] S.I. Inamdar, K.Y. Rajpure, High-performance metal-semiconductor-metal UV photodetector based on spray deposited ZnO thin films, J. Alloys Compd. 595 (2014) 55e59. [7] Y.Z. Jin, J.P. Wang, B.Q. Sun, J.C. Blakesley, N.C. Greenham, Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles, Nano Lett. 8 (2008) 1649e1653. [8] A.V. Babichev, H. Zhang, P. Lavenus, F.H. Julien, A.Y. Egorov, Y.T. Lin, L.W. Tu, M. Tchernycheva, GaN nanowire ultraviolet photodetector with a graphene transparent contact, Appl. Phys. Lett. 103 (2013) 201103. [9] X.S. Fang, Y. Bando, M.Y. Liao, U.K. Gautam, C.Y. Zhi, B. Dierre, B.D. Liu, T.Y. Zhai, T. Sekiguchi, Y. Koide, D. Golberg, Single-crystalline ZnS nanobelts as ultraviolet-light sensors, Adv. Mater. 21 (2009) 2034e2039. [10] S.N. Lu, J.J. Qi, S. Liu, Z. Zhang, Z.Z. Wang, P. Lin, Q.L. Liao, Q.J. Liang, Y. Zhang, Piezotronic interface engineering on ZnO/Au-based Schottky junction for enhanced photoresponse of a flexible self-powered UV detector, ACS Appl. Mater. Interfaces 6 (2014) 14116e14122. [11] H. Wang, G.B. Yi, X.H. Zu, X.M. Jiang, Z. Zhang, H.S. Luo, A highly sensitive and self-powered ultraviolet photodetector composed of titanium dioxide nanorods and polyaniline nanowires, Mater. Lett. 138 (2015) 204e207. [12] Y.J. Hwang, C. Hahn, B. Liu, P.D. Yang, Photoelectrochemical properties of TiO2 nanowire arrays: a study of the dependence on length and atomic layer deposition coating, ACS Nano 6 (2012) 5060e5069. [13] H.T. Ding, J.Y. Shao, Y.C. Ding, W.Y. Liu, H.M. Tian, X.M. Li, One-dimensional Au-ZnO heteronanostructures for ultraviolet light detectors by a two-step dielectrophoretic assembly method, ACS Appl. Mater. Interfaces 7 (2015) 12713e12718. [14] D.Y. Guo, Z.P. Wu, Y.H. An, X.C. Guo, X.L. Chu, C.L. Sun, L.H. Li, P.G. Li, W.H. Tang, Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in b-Ga2O3 solar-blind ultraviolet photodetectors, Appl. Phys. Lett. 105 (2014) 023507. [15] I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009e2013. [16] B. Liang, H.T. Huang, Z. Liu, G. Chen, G. Yu, T. Luo, L. Liao, D. Chen, G.Z. Shen, Ladder-like metal oxide nanowires: synthesis, electrical transport, and enhanced light absorption properties, Nano Res. 7 (2014) 272e283. [17] Y.F. Hu, J. Zhou, P.H. Yeh, Z. Li, T.Y. Wei, Z.L. Wang, Supersensitive, fastresponse nanowire sensors by using Schottky contacts, Adv. Mater. 22 (2010) 3327e3332. [18] Q. Xu, Q.J. Cheng, J.X. Zhong, W.W. Cai, Z.F. Zhang, Z.Y. Wu, F.Y. Zhang, A metal-semiconductor-metal detector based on ZnO nanowires grown on a graphene layer, Nanotechnology 25 (2014) 055501. [19] Q.Y. Wang, Y.B. Xu, S.H. Zhang, S.M. Gao, Q.D. Zheng, Q.Q. Yuan, W.Q. Rong, C.L. Yin, J.F. Wang, M.J. Wang, Self-organized TiO2 nanotube arrays sensitized by uniform Cd0.8Zn0.2S nanoparticles for highly effective photoelectrochemical performance, Mater. Des. 92 (2016) 102e106. [20] K.S. Chen, K. Xie, X.R. Feng, S.F. Wang, R. Hu, H.S. Gu, Y. Li, An excellent roomtemperature hydrogen sensor based on titania nanotube-arrays, Int. J. Hydrogen Energy 37 (2012) 13602e13609. [21] Z.J. Zhou, J.Q. Fan, X. Wang, W.H. Zhou, Z.L. Du, S.X. Wu, Effect of highly ordered single-crystalline TiO2 nanowire length on the photovoltaic performance of dye-sensitized solar cells, ACS Appl. Mater. Interfaces 3 (2011) 4349e4353. [22] F.Y. Jing, D.Z. Zhang, F. Li, J.R. Zhou, D.M. Sun, S.P. Ruan, High performance ultraviolet detector based on SrTiO3/TiO2 heterostructure fabricated by two steps in situ hydrothermal method, J. Alloys Compd. 650 (2015) 97e101. [23] C.L. Cao, C.G. Hu, X. Wang, S.X. Wang, Y.S. Tian, H.L. Zhang, UV sensor based on TiO2 nanorod arrays on FTO thin film, Sens. Actuat. B 156 (2011) 114e119. [24] D.Z. Wang, W.J. Zhou, P.G. Hu, Y. Guan, L.M. Chen, J.H. Li, G.C. Wang, H. Liu, J.Y. Wang, G.Z. Cao, H.D. Jiang, High ethanol sensitivity of palladium/TiO2 nanobelt surface heterostructures dominated by enlarged surface area and nano-schottky junctions, J. Colloid Interface Sci. 388 (2012) 144e150. [25] S.X. Yang, S. Tongay, S.S. Li, J.B. Xia, J.Q. Wu, J.B. Li, Environmentally stable/selfpowered ultraviolet photodetectors with high sensitivity, Appl. Phys. Lett. 103 (2013) 143503. [26] M.J. Yang, J.L. Zhu, W. Liu, J.L. Sun, Novel photodetectors based on doublewalled carbon nanotube film/TiO2 nanotube array heterodimensional contacts, Nano Res. 4 (2011) 901e907. [27] H.L. Huang, Y.N. Xie, Z.F. Zhang, F. Zhang, Q. Xu, Z.Y. Wu, Growth and fabrication of sputtered TiO2 based ultraviolet detectors, Appl. Surf. Sci. 293 (2014)
48
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
248e254. [28] H.F. Zhang, S.P. Ruan, H.L. Li, M. Zhang, K.B. Lv, C.H. Feng, W.Y. Chen, Schottky diode ultraviolet detector based on TiO2 nanowire array, IEEE Electron Device Lett. 33 (2012) 83e85. [29] K.B. Lv, M. Zhang, C.X. Liu, G.H. Liu, H.C. Li, S.P. Wen, Y. Chen, S.P. Ruan, TiO2 ultraviolet detector based on LaAlO3 substrate with low dark current, J. Alloys Compd. 580 (2013) 614e617. [30] P. Chinnamuthu, J.C. Dhar, A. Mondal, A. Bhattacharyya, N.K. Singh, Ultraviolet detection using TiO2 nanowirearray with Ag Schottky contact, J. Phys. D. Appl. Phys. 45 (2012) 135102. [31] B. Liu, E.S. Aydil, Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells, J. Am. Chem. Soc. 131 (2009) 3985e3990. [32] H. Wang, G.B. Yi, M. Tan, X.H. Zu, H.S. Luo, X.M. Jiang, Initial reactant controlled synthesis of double-layered TiO2 nanostructures and characterization of its spectra of absorption and photoluminescence, Mater. Lett. 148 (2015) 5e8. [33] A.M. Salem, S.M. Selim, Structure and optical properties of chemically deposited Sb2S3 thin films, J. Phys. D. Appl. Phys. 34 (2001) 12e17. [34] M.K. Singh, M.C. Mathpal, A. Agarwal, Optical properties of SnO2 quantum dots synthesized by laser ablation in liquid, Chem. Phys. Lett. 536 (2012)
87e91. [35] B.S. Zhou, L.Z. Xiao, T.J. Li, J.L. Zhao, Z.Y. Lai, S.W. Gu, Absorption red shift in TiO2 ultrafine particles with surfacial dipole layer, Appl. Phys. Lett. 59 (1991) 591826. [36] H. Tang, H. Berger, P.E. Schmid, F. Levy, Photoluminescence in TiO2 anatase single crystals, Solid State Commun. 87 (1993) 847e850. [37] L.V. Saraf, S.I. Patil, S.B. Ogale, Synthesis of nanophase TiO2 byionbeam sputtering and cold condensation technique, Int. J. Mod. Phys. B 25 (1998) 2635e2647. [38] N. Serpone, D. Lawless, R. Khairutdinov, Size effects on the photophysical properties of colloidal anatase Ti02 particles: size quantization or direct transitions in this indirect semiconductor, J. Phys. Chem. 99 (1995) 16646e16654. [39] A. Maurya, P. Chauhan, S.K. Mishra, R.K. Srivastava, Structural, optical and charge transport study of rutile TiO2 nanocrystals at two calcination temperatures, J. Alloys Compd. 509 (2011) 8433e8440. [40] Raymond T. Tung, The physics and chemistry of the Schottky barrier height, Appl. Phys. Rev. 1 (2014) 011304. [41] B.L. Liu, L. Chen, G. Liu, A.N. Abbas, M. Fathi, C.W. Zhou, High-performance chemical sensing using Schottky-contacted chemical vapor deposition grown monolayer MoS2 transistors, ACS Nano 8 (2014) 5304e5314.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A high-sensitive ultraviolet photodetector composed of doublelayered TiO2 nanostructure and Au nanoparticles film based on Schottky junction Huan Wang a, Pei Qin a, Guobin Yi a, *, Xihong Zu a, Li Zhang b, **, Wei Hong c, Xudong Chen c a b c
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510006, PR China School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, PR China School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A novel double-layered TiO2 nanostructure was synthesized by a simple method. An UV photodetector composed of TiO2 and Au was designed and fabricated. The preparation method of TiO2/Au UV photodetector was simple and convenient. The UV photodetector based on TiO2/ Au showed excellent sensitivity to UV light.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2016 Received in revised form 28 February 2017 Accepted 14 March 2017
In this study, a Schottky-type ultraviolet (UV) photodetector based on double-layered nanostructured TiO2/Au films was fabricated. Double-layered titanium dioxide (TiO2) nanostructures composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods on fluorine-doped tin oxide (FTO) pre-coated glass substrates were synthesized via a convenient hydrothermal method using titanium butoxide and hydrochloric acid as the starting precursor, without involving the use of any other surfactants and catalysts. A granular-shaped thin-layer of Au film using vacuum sputter coating technique was subsequently deposited on TiO2 for the formation of Schottky-type photodetector. The as-fabricated Schottky device showed various photocurrent responses when irradiated with different wavelength of UV light. This suggests that the newly-developed photodetectors have promising potential for identifying different UV light wavelengths. © 2017 Elsevier B.V. All rights reserved.
Keywords: Double-layered TiO2 Au nanoparticles film Hydrothermal method UV photodetector Schottky junction
1. Introduction * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Yi), [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.matchemphys.2017.03.019 0254-0584/© 2017 Elsevier B.V. All rights reserved.
As a member of the optoelectronic device family, Ultraviolet (UV) photodetector is becoming increasingly important in a wide
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
range of fields including solar UV monitoring, flame alarm, and biological detection [1e5]. So far, UV detectors based on wide bandgap semiconductor materials such as SiC, GaN, ZnS, ZnO, and TiO2 have attracted significant interest owing to their excellent wavelength selectivity (“visible-blindness”) and a large possibility of room-temperature operation [6e18]. As a well-known wide band-gap semiconductor, TiO2 has shown a lot of potential technical applications in a number of fields such as photocatalysts, gas sensors, dye-sensitized solar cells, and so on [19e23]. Furthermore, the wide band-gap (3.2 eV for anatase and 3.0 eV for rutile) make it very suitable for the UV light detection without any filter required to block visible or infrared light. Thus, UV photodetectors based on TiO2 have been widely investigated owing to excellent physical, chemical, and optical properties associated with TiO2 in the past decades [24e27]. In particular, the metal-semiconductor Schottky-type UV detector has been paid more attention [28e30]. Zhang et al. [28] reported a Schottky diode ultraviolet detector based on TiO2 nanowire array with Ag nanoparticles. The device of TiO2/Ag nanocomposites showed a good UV photoelectric property. It showed that the semiconductor TiO2 nanostructure combined with metal had potential application in UV detection and sensing. However, some photodetectors demonstrated good device performance and operation stability, but these devices suffered from the complexity and high cost of the fabrication process, hindering from the commercialization of these devices. Therefore, it is very important to develop an efficient and convenient method to fabricate metal-TiO2 nanostructure device for the application in UV detection. The fundamental necessities in designing UV detectors are high sensitivity, speed, high-responsivity and low-noise. And the nanostructure has great influence on the performance of the photovoltaic device. In this work, a novel double-layered TiO2 nanostructure composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods were grown on fluorine-doped tin oxide (FTO) pre-coated glass substrates via a simple and economic hydrothermal method and Schottky-type photodetector based on double-layered TiO2 nanostructure with Au nanoparticles was fabricated. The preparation method of the double-layered TiO2 and TiO2/Au heterostructures is very simple and convenient, and we can easily make it large-scale production. The as-prepared UV photodetector generated a very high photocurrent, which is much higher than that of the similar UV detectors reported by others [6,18]. The high UV photocurrent generated at low bias is favorable for its practical applications. On the other hand, the as-fabricated Schottky-type UV photodetector showed high-responsivity and various photocurrent responses under different wavelength of UV light illumination, this suggests that the newly-developed device is a potential candidate as UV photodetector for identifying different wavelengths UV light.
43
FTO substrate by a simple hydrothermal method. Typically, 25 mL deionized water and 25 mL concentrated hydrochloric acid were mixed with stirring for 1 min. Subsequently, 0.8 mL titanium butoxide was added dropwise to the above solution. After stirring for another 3 min, two pieces of FTO substrates (ultrasonically cleaned for 30 min in acetone, ethyl alcohol and deionized water respectively) were placed at an angle against the wall of the Teflon-liner stainless steel autoclave with the conducting side facing up. Afterwards, the autoclave was sealed and the hydrothermal synthesis was conducted at 140 C in an electric oven. After 18 h, the autoclave was taken out from the oven and it was cooled to room temperature under flowing water. Finally, the double-layered TiO2 nanostructure were obtained after being rinsed with deionized water and dried in ambient air. The granular-shaped thin-layer of Au film electrode was coated to the top surface of the as-prepared double-layered TiO2 nanostructures by vacuum sputter coating (SBC-12, KYKY Technology, China). The UV photodetector composed of the double-layered TiO2 nanostructures and Au electrode based on Schottky contact was obtained. The total active area of UV photodetector composed of double-layered TiO2 and Au was ~0.36 cm2. 2.3. Characterization techniques The morphology and structure of the samples was examined using SEM (JEOL, JSM-7001F, Rigaku Tokyo, Japan). The crystal structures of the samples were characterized by XRD (X’pert Powder, PANalytical, Holland) with fixed Cu-Ka line (l ¼ 0.15405 nm) in the Bragg angle ranging from 20 to 75 . X-ray tube voltage and current were set at 40 kV and 40 mA. UVeVis absorption spectra of the samples were recorded in the range of 220e800 nm using a UVeVis spectrophotometer (UV 2450, Shimadzu). Photoluminescence (PL) spectra were used for studying the optical properties of the TiO2 nanostructures. The photoelectric properties of the TiO2/Au device were measured by a CHI660D electrochemical workstation. The UV source was provided by a portable UV lamp (~8W, ENF-280C, USA). 3. Results and discussion 3.1. Morphology and structure of the prepared TiO2 and TiO2/Au Fig. 1 shows the typical SEM images of the double-layered TiO2
2. Materials and experimental 2.1. Materials Titanium butoxide ([CH3(CH2)3O]4Ti, 98%) was purchased from Tianjin Fuchen Chemical Reagents Factory and it was used without any other treatment. Hydrochloric acid (36.5%e38% by weight, analytical grade) was purchased from Guangzhou Chemical Reagent No. 2 Factory and it was used without any other treatment. FTO substrate (F: SnO2, 14 U/sq, glass thickness: 1.1 mm) was purchased from Nippon Sheet Glass (Japan). 2.2. Preparation of the TiO2 and TiO2/Au The double-layered TiO2 nanostructures were growing on an
Fig. 1. The SEM images of the TiO2 growth on FTO substrate, (a) top view, (b) and (c) show the corresponding morphology at higher resolution, (d) show the sectional view.
44
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
nanostructures produced by a convenient hydrothermal method. As clearly seen in Fig. 1a, the obtained TiO2 sample is composed of one layer of TiO2 nanoflowers on a layer of TiO2 nanorods array film with the average diameter of ~160 nm for the nanorods and the average height of ~2.5 mm for the arrays (Fig. 1b and d). What is more, each TiO2 nanorod consists of many TiO2 nanowires (Fig. 1c) while each TiO2 nanoflower was composed of many nanorods. The layer of TiO2 nanorods array film could be grown on FTO substrate owing that the rutile TiO2 and FTO have a small lattice mismatch [31]. Meanwhile, there were many TiO2 crystal nucleuses in the reactant solution owing to the hydrolysis of titanium butoxide during the growth of TiO2 nanorods array. Some TiO2 nanoflowers crystal nucleuses also will grow when the TiO2 nanorods array were growing. The initial reactant concentration has a significant influence on the shape and morphology of the TiO2 nanostructures [32]. The double-layered TiO2 nanostructures have bigger surface area compared with the single layered TiO2 nanorods array, and it has the larger receiving area to absorb UV light when it is applied to a UV photodetector. Fig. 2a-d shows the SEM images of the fabricated Schottky device which composed of one layer of granular-shaped Au film on the top surface of the double-layered TiO2 nanostructure. As shown in Fig. 2(b and c), a granular-shaped thin layer of Au nanoparticles was coated on the surface of both the TiO2 nanoflower and the TiO2 nanorods. Furthermore, it showed that the Au contact well with the TiO2 nanoflowers and nanorods, which contributed to the formation of Schottky junction and the transportation of the electrons. The energy dispersive X-ray (EDX) analysis (Fig. 2e) reveals the presence of titanium (Ti), oxygen (O) and aurum (Au) signals, which come from TiO2 and Au. Fig. 2f shows the fabrication process of the Schottky device with the Au nanoparticles as the metal electrode and the double-layered TiO2 nanostructures as the n-type semiconductor. X-ray diffraction (XRD) analysis provides the information about crystalline quality and orientation of the nanostructure. Fig. 3 shows the XRD patterns of FTO substrate and double-layered TiO2 nanostructure and TiO2/Au nano-composite on FTO substrate. The diffraction peaks of the double-layered TiO2 can be assigned well to tetragonal rutile crystalline phase of TiO2 with a reference pattern (JCPDS file no. 21-1276). The peaks at 2q ¼ 28.1, 37.0 , 41.9 , 56.8 , 63.4 , 69.2 and 70.2 can be indexed to (110), (101), (111), (220), (002), (301) and (112) crystal planes of rutile TiO2. For the TiO2/Au nanocomposite, the characteristic peaks of Au (JCPDS file no. 040784) were also clearly presented in addition to the diffraction peaks of TiO2, it showed that the TiO2/Au nanocomposites composed of double-layered TiO2 and Au nanoparticles was successfully prepared.
Fig. 2. The SEM images (a, b, c, d) of the TiO2/Au nanocomposite nanostructures, (b) and (c) show the corresponding morphology at higher resolution, (d) show the sectional vies. (e) EDX spectrum of the TiO2/Au nanocomposite, (f) fabrication process of the Schottky device composed of Au and TiO2.
Fig. 3. XRD patterns of the double-layered TiO2 nanostructure and TiO2/Au nanocomposite on FTO substrate.
3.2. Optical properties of the TiO2/Au device Fig. 4a shows the absorption spectrum of the double-layered TiO2 nanostructure. It is noted that clearly, the double-layered TiO2 has a strong absorption in the UV band, implying that the as-synthesized TiO2 nanostructures have a strong sensitivity to the UV light. The optical band gap energy of the double-layered TiO2 nanostructure is calculated by Tauc plot shown in Fig. 4b. The absorption band gap energy can be determined by the following equation [33,34]. (ahn)n ¼ В(hn Eg) where hn is the photon energy, a is the absorption coefficient, В is a constant relative to the material and n is a value that depends on the nature of transition (n ¼ 2 for direct band gap, 2/3 for direct forbidden gap and 1/2 for indirect band gap). On extrapolating the line from linear portion of the absorption edge, it gives the optical band gap energy. That is to say, (ahn)1/2 versus hn extrapolate to a ¼ 0 gives the optical band gap energy. The calculated band gap energy of the double-layered TiO2 nanostructure is 3.0 eV, which also implies that the double-layered TiO2 is rutile (3.0 eV) phase. Fig. 5 shows the room temperature photoluminescence (PL) features of the as-synthesized double-layered TiO2 with different excitation wavelengths. As is known, the PL spectrum is obtained as a result of the competition among electron-hole separations, electron-phonon scattering and electron-hole recombination. TiO2 has a direct band-gap but is subjected to dipole-forbidden transition [35]. PL of TiO2 has three kinds of physical origins: self-trapped excitons, oxygen vacancies and surface states defects [36e38]. Most of the surface states defects are oxygen vacancies or the Ti4þ ions adjacent to oxygen vacancies [39]. In the present case, the stronger peaks at 413 nm should be attributed to the emission from the band edge recombination process of TiO2. The peak at 425 nm should be corresponding to inter-band recombination [36]. The other prominent peak near at 465 nm was due to oxygen vacancy [39]. What’s more, Fig. 5 exhibits that the double-layered TiO2 has an excellent optical quality and the intensity of photoluminescence increased with the increasing of excitation wavelength, which implies that the longer wavelength of UV light irradiated on the TiO2 sample can generate more photoelectrons and hole. In order to examine the UV photosensitivity of the TiO2/Au device, three UV
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
45
Fig. 4. (a) UVeVis absorption spectrum of the double-layered TiO2 nanostructure and (b) the Tauc plot of (ahn)1/2 as a function of photon energy.
Fig. 5. Photoluminescence spectra of the double-layered TiO2 nanostructures excited with different wavelengths.
Fig. 6. (a) I-V curves of the UV detector measured without UV and with different UV illumination, (b) I-t curves of the device under different UV illumination at 1 V forward-bias voltage. (c) Schematic energy band diagram of the TiO2 and Au interface.
general equations: sources with different excitation wavelength including 254 nm from ultraviolet radiation c (UVC) and 312 nm from ultraviolet radiation b (UVB) and 365 nm from ultraviolet radiation a (UVA) were chosen to test the I-V and I-t curves of the TiO2/Au device.
qV 1 I ¼ I0 exp hkT
(1)
3.3. UV photo-detection properties of the TiO2/Au device
qFB I0 ¼ A ST2 exp kT
(2)
Fig. 6a shows the I-V curves of the TiO2/Au device measured without and with different wavelengths UV illumination, which revealed that the TiO2/Au device possesses a rectifier response. This result indicates the presence of a non-Ohmic contact in our device, which can be treated as the Schottky contact [11]. Fig. 6c shows the schematic energy diagram of the interface of Au and TiO2. As is known, once the metal that has a higher work function than that of the n-type semiconductor contacted with each other, the electrics in the n-type semiconductor will transfer into the metal to maintain the flattening of the Fermi level. Thus, a depletion region will form in the interface of the metal and n-type semiconductor [29,40,41]. The symbols of “þ” and “-” in Fig. 6c present the positive ions ion in TiO2 and negative electrons in the interface, respectively. The performance of the UV photodetectors is dependent on the quality of the Schottky contacts. Therefore, it is necessary to measure the actual Schottky barrier height and the ideality factor of the device. The dark current transport over the Schottky barrier could be explained by the thermionic emission theory using the following
Here I0 is the reverse saturation current, S is the active area of the device, q is the electronic charge, V is the bias voltage, k is the Boltzmann’s constant, T is the absolute temperature, A* is the effective Richardson constant, which has a theoretical value of 1200 A cm2K2, h is the ideality factor, FB is the effective Schottky barrier height. According to the I-V dark curve near 0 V, the I0 is estimated to be 3.1 nA, the effective Schottky barrier height and ideality factor were determined to be 0.61 eV and 1.6. The value of FB was approximately equal to the theoretical value between the work function of Au (5.1 eV) and rutile TiO2 (4.1e4.6 eV) [28]. This indicates good quality of the Schottky contact between TiO2 and Au, it confirms a very limited Fermi level pinning effect at the TiO2/Au interface. Such a high barrier can cause a wide depletion region and the direction of the inner electric field in the depletion region is from TiO2 to Au, as shown in Fig. 8(a and b). On the other hand, the photocurrent varies with different wavelengths UV light illumination. When the UV irradiation wavelength is longer, the photocurrent is much higher. This result
46
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
Fig. 7. Spectral responsivity characteristic of the UV detector under 1 V bias.
indicates that the prepared device can be used as an UV photodetector which could identify different wavelengths UV light. Fig. 6b shows the I-t curves of the TiO2/Au device when irradiated with 254 nm, 312 nm and 365 nm UV light, with the forward-bias voltage of 1 V. The I-t curves in Fig. 6b showed the response of photocurrent with the different UV light irradiation. Clearly, the photocurrent immediately had a jump when the UV source was turned on, and the photocurrent significantly decreased when the UV source was turned off. Furthermore, the photocurrent of the TiO2/Au device increased with increasing the wavelength of UV light. The possible reason lies in that the longer UV light can generate more photoelectrons and holes in the TiO2 layer of this TiO2/Au device. The results agree with the above mentioned PL spectra shown in Fig. 5. On the other hand, the photocurrent of the photodetector irradiated with 365 nm was as high as ~0.17 mA under 1 V bias, which was much higher than that UV photodetectors based on single TiO2 nanorods array film were reported by others. The reasons may be that the double-layered TiO2 had the bigger received area to absorb the UV light to produce higher photocurrent. The wavelength selectivity of the TiO2/Au UV detector was measured in the range of 220e500 nm at 1 V bias, the result is shown in Fig. 7. It is clearly seen that the TiO2/Au device showed excellent UV light detection selectivity in a spectral range between 270 and 410 nm. It means that the device can be used as photodetector for UV light, especially for the UV-A (320e400 nm). The maximum UV responsivity is about 138.5 A/W at the wavelength of 360 nm. The UV responsivity to the short wavelength (220e260) is much lower than that of the long wavelength UV light, it dues to weak penetration of the short UV light. The edge of 410 nm is attributed to the absorption edge of the TiO2 layer.
Fig. 8. Diagrammatic sketch of TiO2/Au Schottky junction in different voltage biases with UV illumination; (a) in forward-bias, (b) in reverse-bias.
The high sensitivity and photocurrent of the Au/TiO2 device are promising for large-area UV detector applications. Table 1 is a comparison of the sensitivity and photocurrent to other UV detectors related to this work. It shows that our device had the higher photocurrent and responsivity than most of other UV detectors. Although the structures of the UV detectors were different, it means that the prepared Au/TiO2 device showed the advantages of high photocurrent and responsivity than others. Fig. 8(a and b) give the diagrammatic sketch of TiO2/Au Schottky device in different biases with UV irradiation. Without UV illumination, when the Au electrode was contacted with the positive terminal and TiO2 was contacted with the negative terminal of external circuit, the electrons in Au moved out to the positive terminal of the external circuit and the electrons come out from the
Table 1 Comparison of the sensitivity to other UV detectors related to this work. Photodetector
Bias
Photocurrent (mA)
Photo-responsivity (A/W)
On/off ratio
Ref.
GR/Au/GaN Au/ZnO/GR SrTiO3/TiO2 TiO2 Ag/TiO2 Ag/TiO2 Au/TiO2
1V 1V 5V 0V 5 V 6.5 V 1V
128.1 50.5 114 13.2 10.2 0.75 163.6
25 ~178 ~90.1 4.33 103 3.1 0.18 136.4
3.1 ~2.2 104 3.5 103 12.1 ~2.1 102 8.5 ~5.5 104
[8] [18] [22] [23] [28] [30] This work
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
negative terminal of the external circuit come into the TiO2 of the TiO2/Au device. The direction of the inner electric field in the depletion region was from TiO2 to Au, thus, the electrons of the external circuit could come to the positive terminal of the inner electric field, and the current of the TiO2/Au device increased when the voltage of the external circuit increased. On the other hand, when the Au electrode was contacted with the negative terminal and TiO2 was contacted with the positive terminal of the external circuit, the electrons come out from the negative terminal of the external circuit and enter the Au of the device. In this case, the direction of the inner electric field in the depletion region was from TiO2 to Au, the electrons of the external circuit could not pass through the inner electric field, thus, the TiO2/Au device had a relatively low dark current in reverse-bias shown in Fig. 6a. When the TiO2/Au device was irradiated with UV light, the electron-hole pairs were generated in TiO2 as shown in Fig. 8a-b. When the Au was contacted with the positive terminal of the external circuit, the photo-generated electrons moved to the depletion region and decreased the intensity of the inner electric field, which could increase the current of the TiO2/Au device shown in Fig. 6(aeb). Furthermore, the photo-generated electrons can also increase the electrical conductivity of the device. So the current also increased with the UV illumination when the TiO2/Au device is in the negative terminal. On the other hand, a widely accepted photoconduction mechanism of TiO2 nanorods functions as follows. There were so many oxygen vacancies in TiO2 nanostructure which was proved by the results of PL spectra shown in Fig. 5. In a dark atmosphere, oxygen molecules would be absorbed into the surface of TiO2 and capture the free electrons (O2 þ e/ O 2 ), and it would form a low-conductivity depletion layer near the surface of the TiO2. The electron-hole pairs would be photogenerated in TiO2 when the UV light was illumination, and some of the photogenerated holes would migrate to the surface to discharge the negatively charged absorbed oxygen ions (hþ þ O 2 / O2), oxygen is photo-desorbed from the surface, and eliminate the chances to recombine the electron-hole pairs, which could increase the conductivity of the device and gain a higher photocurrent as the results shown in Fig. 6. 4. Conclusions In summary, double-layered TiO2 nanostructure composed of one layer of TiO2 nano-flowers on one layer of TiO2 nanorods was successfully grown on FTO substrate by a simple and economic hydrothermal method. The as-obtained double-layered TiO2 exhibited excellent optical properties and crystalline quality as the results shown in PL spectra and XRD patterns. The fabricated Schottky-type UV photodetector based on double-layered TiO2 nanostructure with Au nanoparticles film showed high sensitivity to UV light. Furthermore, the UV photodetector showed different photocurrent when irradiated with different wavelength of UV light. It showed that the as-prepared TiO2/Au heterostructure had potential applications for UV detector and sensor. This work will provide opportunities for developing efficient UV photodetectors that can recognize different wavelengths of UV light. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51233008, 51273048, 51203025). References [1] Y.N. Hou, Z.X. Mei, H.L. Liang, D.Q. Ye, C.Z. Gu, X.L. Du, Dual-band MgZnO ultraviolet photodetector integrated with Si, Appl. Phys. Lett. 102 (2013)
47
153510. [2] L. Mandal, N.S. Chaudhari, S. Ogale, Self-Powered UV-vis photodetector based on ZnIn2S4/hydrogel interface, ACS Appl. Mater. Interfaces 5 (2013) 9141e9147. [3] B.C. Cheng, J. Xu, Z.Y. Ouyang, C.C. Xie, X.H. Su, Y.H. Xiao, S.J. Lei, Individual ZnO nanowires for photodetectors with wide response range from solar-blind ultraviolet to near-infrared modulated by bias voltage and illumination intensity, Opt. Express 11 (2013) 29719e29730. [4] Q. Li, T. Qiu, H.X. Hao, H. Zhou, T.Z. Wang, Y. Zhang, X. Li, G.L. Huang, J. Cheng, Rapid and on-site analysis of illegal drugs on the nanoemicroscale using a deep ultraviolet-visible reflected optical fiber sensor, Analyst 137 (2012) 1596e1603. [5] Y.R. Xie, L. Wei, G.D. Wei, Q.H. Li, D. Wang, Y.X. Chen, S.S. Yan, G.L. Liu, L.M. Mei, J. Jiao, A self-powered UV photodetector based on TiO2 nanorod arrays, Nanoscale Res. Lett. 8 (2013) 188. [6] S.I. Inamdar, K.Y. Rajpure, High-performance metal-semiconductor-metal UV photodetector based on spray deposited ZnO thin films, J. Alloys Compd. 595 (2014) 55e59. [7] Y.Z. Jin, J.P. Wang, B.Q. Sun, J.C. Blakesley, N.C. Greenham, Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles, Nano Lett. 8 (2008) 1649e1653. [8] A.V. Babichev, H. Zhang, P. Lavenus, F.H. Julien, A.Y. Egorov, Y.T. Lin, L.W. Tu, M. Tchernycheva, GaN nanowire ultraviolet photodetector with a graphene transparent contact, Appl. Phys. Lett. 103 (2013) 201103. [9] X.S. Fang, Y. Bando, M.Y. Liao, U.K. Gautam, C.Y. Zhi, B. Dierre, B.D. Liu, T.Y. Zhai, T. Sekiguchi, Y. Koide, D. Golberg, Single-crystalline ZnS nanobelts as ultraviolet-light sensors, Adv. Mater. 21 (2009) 2034e2039. [10] S.N. Lu, J.J. Qi, S. Liu, Z. Zhang, Z.Z. Wang, P. Lin, Q.L. Liao, Q.J. Liang, Y. Zhang, Piezotronic interface engineering on ZnO/Au-based Schottky junction for enhanced photoresponse of a flexible self-powered UV detector, ACS Appl. Mater. Interfaces 6 (2014) 14116e14122. [11] H. Wang, G.B. Yi, X.H. Zu, X.M. Jiang, Z. Zhang, H.S. Luo, A highly sensitive and self-powered ultraviolet photodetector composed of titanium dioxide nanorods and polyaniline nanowires, Mater. Lett. 138 (2015) 204e207. [12] Y.J. Hwang, C. Hahn, B. Liu, P.D. Yang, Photoelectrochemical properties of TiO2 nanowire arrays: a study of the dependence on length and atomic layer deposition coating, ACS Nano 6 (2012) 5060e5069. [13] H.T. Ding, J.Y. Shao, Y.C. Ding, W.Y. Liu, H.M. Tian, X.M. Li, One-dimensional Au-ZnO heteronanostructures for ultraviolet light detectors by a two-step dielectrophoretic assembly method, ACS Appl. Mater. Interfaces 7 (2015) 12713e12718. [14] D.Y. Guo, Z.P. Wu, Y.H. An, X.C. Guo, X.L. Chu, C.L. Sun, L.H. Li, P.G. Li, W.H. Tang, Oxygen vacancy tuned Ohmic-Schottky conversion for enhanced performance in b-Ga2O3 solar-blind ultraviolet photodetectors, Appl. Phys. Lett. 105 (2014) 023507. [15] I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009e2013. [16] B. Liang, H.T. Huang, Z. Liu, G. Chen, G. Yu, T. Luo, L. Liao, D. Chen, G.Z. Shen, Ladder-like metal oxide nanowires: synthesis, electrical transport, and enhanced light absorption properties, Nano Res. 7 (2014) 272e283. [17] Y.F. Hu, J. Zhou, P.H. Yeh, Z. Li, T.Y. Wei, Z.L. Wang, Supersensitive, fastresponse nanowire sensors by using Schottky contacts, Adv. Mater. 22 (2010) 3327e3332. [18] Q. Xu, Q.J. Cheng, J.X. Zhong, W.W. Cai, Z.F. Zhang, Z.Y. Wu, F.Y. Zhang, A metal-semiconductor-metal detector based on ZnO nanowires grown on a graphene layer, Nanotechnology 25 (2014) 055501. [19] Q.Y. Wang, Y.B. Xu, S.H. Zhang, S.M. Gao, Q.D. Zheng, Q.Q. Yuan, W.Q. Rong, C.L. Yin, J.F. Wang, M.J. Wang, Self-organized TiO2 nanotube arrays sensitized by uniform Cd0.8Zn0.2S nanoparticles for highly effective photoelectrochemical performance, Mater. Des. 92 (2016) 102e106. [20] K.S. Chen, K. Xie, X.R. Feng, S.F. Wang, R. Hu, H.S. Gu, Y. Li, An excellent roomtemperature hydrogen sensor based on titania nanotube-arrays, Int. J. Hydrogen Energy 37 (2012) 13602e13609. [21] Z.J. Zhou, J.Q. Fan, X. Wang, W.H. Zhou, Z.L. Du, S.X. Wu, Effect of highly ordered single-crystalline TiO2 nanowire length on the photovoltaic performance of dye-sensitized solar cells, ACS Appl. Mater. Interfaces 3 (2011) 4349e4353. [22] F.Y. Jing, D.Z. Zhang, F. Li, J.R. Zhou, D.M. Sun, S.P. Ruan, High performance ultraviolet detector based on SrTiO3/TiO2 heterostructure fabricated by two steps in situ hydrothermal method, J. Alloys Compd. 650 (2015) 97e101. [23] C.L. Cao, C.G. Hu, X. Wang, S.X. Wang, Y.S. Tian, H.L. Zhang, UV sensor based on TiO2 nanorod arrays on FTO thin film, Sens. Actuat. B 156 (2011) 114e119. [24] D.Z. Wang, W.J. Zhou, P.G. Hu, Y. Guan, L.M. Chen, J.H. Li, G.C. Wang, H. Liu, J.Y. Wang, G.Z. Cao, H.D. Jiang, High ethanol sensitivity of palladium/TiO2 nanobelt surface heterostructures dominated by enlarged surface area and nano-schottky junctions, J. Colloid Interface Sci. 388 (2012) 144e150. [25] S.X. Yang, S. Tongay, S.S. Li, J.B. Xia, J.Q. Wu, J.B. Li, Environmentally stable/selfpowered ultraviolet photodetectors with high sensitivity, Appl. Phys. Lett. 103 (2013) 143503. [26] M.J. Yang, J.L. Zhu, W. Liu, J.L. Sun, Novel photodetectors based on doublewalled carbon nanotube film/TiO2 nanotube array heterodimensional contacts, Nano Res. 4 (2011) 901e907. [27] H.L. Huang, Y.N. Xie, Z.F. Zhang, F. Zhang, Q. Xu, Z.Y. Wu, Growth and fabrication of sputtered TiO2 based ultraviolet detectors, Appl. Surf. Sci. 293 (2014)
48
H. Wang et al. / Materials Chemistry and Physics 194 (2017) 42e48
248e254. [28] H.F. Zhang, S.P. Ruan, H.L. Li, M. Zhang, K.B. Lv, C.H. Feng, W.Y. Chen, Schottky diode ultraviolet detector based on TiO2 nanowire array, IEEE Electron Device Lett. 33 (2012) 83e85. [29] K.B. Lv, M. Zhang, C.X. Liu, G.H. Liu, H.C. Li, S.P. Wen, Y. Chen, S.P. Ruan, TiO2 ultraviolet detector based on LaAlO3 substrate with low dark current, J. Alloys Compd. 580 (2013) 614e617. [30] P. Chinnamuthu, J.C. Dhar, A. Mondal, A. Bhattacharyya, N.K. Singh, Ultraviolet detection using TiO2 nanowirearray with Ag Schottky contact, J. Phys. D. Appl. Phys. 45 (2012) 135102. [31] B. Liu, E.S. Aydil, Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells, J. Am. Chem. Soc. 131 (2009) 3985e3990. [32] H. Wang, G.B. Yi, M. Tan, X.H. Zu, H.S. Luo, X.M. Jiang, Initial reactant controlled synthesis of double-layered TiO2 nanostructures and characterization of its spectra of absorption and photoluminescence, Mater. Lett. 148 (2015) 5e8. [33] A.M. Salem, S.M. Selim, Structure and optical properties of chemically deposited Sb2S3 thin films, J. Phys. D. Appl. Phys. 34 (2001) 12e17. [34] M.K. Singh, M.C. Mathpal, A. Agarwal, Optical properties of SnO2 quantum dots synthesized by laser ablation in liquid, Chem. Phys. Lett. 536 (2012)
87e91. [35] B.S. Zhou, L.Z. Xiao, T.J. Li, J.L. Zhao, Z.Y. Lai, S.W. Gu, Absorption red shift in TiO2 ultrafine particles with surfacial dipole layer, Appl. Phys. Lett. 59 (1991) 591826. [36] H. Tang, H. Berger, P.E. Schmid, F. Levy, Photoluminescence in TiO2 anatase single crystals, Solid State Commun. 87 (1993) 847e850. [37] L.V. Saraf, S.I. Patil, S.B. Ogale, Synthesis of nanophase TiO2 byionbeam sputtering and cold condensation technique, Int. J. Mod. Phys. B 25 (1998) 2635e2647. [38] N. Serpone, D. Lawless, R. Khairutdinov, Size effects on the photophysical properties of colloidal anatase Ti02 particles: size quantization or direct transitions in this indirect semiconductor, J. Phys. Chem. 99 (1995) 16646e16654. [39] A. Maurya, P. Chauhan, S.K. Mishra, R.K. Srivastava, Structural, optical and charge transport study of rutile TiO2 nanocrystals at two calcination temperatures, J. Alloys Compd. 509 (2011) 8433e8440. [40] Raymond T. Tung, The physics and chemistry of the Schottky barrier height, Appl. Phys. Rev. 1 (2014) 011304. [41] B.L. Liu, L. Chen, G. Liu, A.N. Abbas, M. Fathi, C.W. Zhou, High-performance chemical sensing using Schottky-contacted chemical vapor deposition grown monolayer MoS2 transistors, ACS Nano 8 (2014) 5304e5314.