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Nanostructures

High-Performance Photoelectrochemical-Type SelfPowered UV Photodetector Using Epitaxial TiO2/SnO2 Branched Heterojunction Nanostructure Xiaodong Li,* Caitian Gao, Huigao Duan, Bingan Lu, Youqing Wang, Lulu Chen, Zhenxing Zhang, Xiaojun Pan, and Erqing Xie*

TiO2/SnO2 branched heterojunction nanostructure with TiO2 branches on electrospun SnO2 nanofiber (B-SnO2 NF) networks serves as a model architecture for efficient self-powered UV photodetector based on a photoelectrochemical cell (PECC). The nanostructure simultaneously offers a low degree of charge recombination and a direct pathway for electron transport. Without correcting 64.5% loss of incident photons through light absorption and scattering by the F-doped tin oxide (FTO) glass, the incident power conversion efficiency reaches 14.7% at 330 nm, more than twice as large as the nanocrystalline TiO2 (TiO2 NC, 6.4%)-film based PECC. By connecting a PECC to an ammeter, the intensity of UV light is quantified using the output short-circuit photocurrent density (Jsc) without a power source. Under UV irradiation, the self-powered UV photodetector exhibits a high responsivity of 0.6 A/W, a high on/off ratio of 4550, a rise time of 0.03 s and a decay time of 0.01 s for Jsc signal. The excellent performance of the B-SnO2 NF-based PECC type selfpowered photodetector will enable significant advancements for next-generation photodetection and photosensing applications.

1. Introduction Sensing of ultraviolet (UV) light is critical for a variety of industrial and scientific applications, including communications, remote control, environmental monitoring, binary switches in imaging techniques, chemical/biological sensing, as

Dr. X. Li,[+] Dr. C. Gao,[+] Y. Wang, L. Chen, Dr. Z. Zhang, Dr. X. Pan, Prof. E. Xie School of Physical Science and Technology Lanzhou University Lanzhou 730000, Gansu, PR China E-mail: [email protected]; [email protected] Prof. H. Duan, Dr. B. Lu Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education State Key Laboratory for Chemo/Biosensing and Chemometrics Hunan University Changsha 410000, Hunan, PR China [+]These authors contributed equally to this work. DOI: 10.1002/smll.201202408 small 2013, DOI: 10.1002/smll.201202408

well as in future memory storage and optoelectronic circuits.[1] One-dimensional (1D) nanostructure UV photodetectors based on photoconductivity have been demonstrated to be excellent candidates for applications.[2,3] However, this kind of photodetectors exhibit a surroundings-dependent behavior[4,5] and a long recovery time due to the presence of a carrierdepletion layer at the nanomaterial surface caused by surface trap states.[2,6] Moreover, such photodetectors needs to be driven by electric power, which is usually provided by batteries or other energy storage/supply systems. As the development of sensor network that contain huge amounts of small sensors, energy supply for such sensor systems is one of the main challenges to human beings in the 21st century. Recently, self-powered nanodevices and nanosystems which can function well without an external power source have attracted increasing attentions.[7,8] Self-powered UV photodetectors have been reported in terms of their dispense with external power sources, fast time response, low dark current and high sensitivity in a variety of detector structures such as Schottky[9] and p-n junction type[6,10] and photoelectrochemical cell (PECC) type.[8,11] Of the various

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self-powered UV photodetectors, the PECC-type is more promising due to its key advantages:[8] low-cost, simple manufacturing process, and composed of abundant and nontoxic raw materials. It is demonstrated that the PECC-type self-powered UV photodetector based on a nanocrystalline TiO2 film[8] exhibits a fast decay time of 0.03 s for Jsc signal, which is much superior to the photoconductivity-based UV photodetectors made by 1D nanostructures.[2,4,12] However, it is still a challenge to further increase the performance of the PECC-based photodetectors due to the nature drawbacks of TiO2 nanocrystalline films,[13] such as numerous grain boundaries existing in the nanoparticle film and the slower electron mobility of TiO2 as compared with other materials such as SnO2, as shown in Figure 1a. To overcome these limitations and achieve overall practical advantages in PECC-based photodetectors, here we report a TiO2/SnO2 branched heterojunction nanostructure (B-SnO2 NF) with TiO2 branches epitaxially grown on an electrospun SnO2 nanofibers network that serves as a working electrode for efficient PECC-type photodetectors. Schematic device configuration and energetics of operation of the PECC are illustrated in Figure 1b and c, respectively. As shown in Figure 1a and b, the B-SnO2 NF outperforms the TiO2 NC film mainly for three reasons: (1) SnO2 is an excellent metal oxide semiconductor with higher electron mobility (∼100 to 200 cm2 V−1 S−1[14]) than TiO2 (∼0.1 to 1.0 cm2 V−1 S−1[15]),

indicating a faster diffusion transport of photoinduced electrons in SnO2 than in TiO2; (2) the epitaxial growth of TiO2 branches on the SnO2 trunk forms a core-shell structure, which yields a surface dipole layer toward SnO2,[13,16] shifting the conduction band of SnO2 to a more negative value and suppressing interfacial charge recombination; and (3) compared with other methods to prepare 1D nanostructures,[17] electrospinning offers the most straightforward and costeffective approach to generate 1D nanostructures with large surface area as well as provide direct path ways for electron transport,[16,18] which are desired advantageous features for the PECC-based photodetectors.

2. Results and Discussion 2.1. Morphology, Structure, and Formation of the B-SnO2 NF Photoanodes

Electrospinning of the precursor solution containing PVP and SnCl2 followed by sintering of the as-spun nanofibers generates SnO2 nanofibers with a uniform average diameter of ∼52 nm (as illustrated in Figure S1, Supporting Information). Figure 2a is a scanning electron microscopy (SEM) image of the SnO2 NF electrode fabricated by drop-drying method. The continuous fibers were cut into nanorods with an average length of ∼1 μm and an average aspect ratio of 17 upon ultrasonic dispersion. From the SEM image shown in inset of Figure 2a, it can be seen that the SnO2 NFs formed from numerous primary nanoparticles ∼12.5 nm in diameter. The SnO2 NFs reported here, compared to NFs from previous reports,[16,19] have smaller diameters, leading to larger surface to volume ratios, a desired feature for PECC applications. The branches were synthesized through a solution heteroepitaxial growth method by directly dipping the SnO2 film into an aqueous solution consisting of DI water (10 mL), HCl (0.1 mL), and TiCl3 solution (0.1 mL, 20 wt.% of TiCl3 in H2O and HCl solution). Figure 2b shows the typical morphology of the B-SnO2 NF electrode, where short needle-shaped branches were grown uniformly on the entire surface of the SnO2 NF trunks. The morphology and crystal structure of the B-SnO2 NF were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The TEM image in Figure 1. Diagram for self-powered UV photodetector based on photoelectrochemical cells Figure 2c shows that the branches densely with TiO2 NC (a) and B-SnO2 NF (b) as photoanodes. (c) Energetics of operation of the B-SnO2 and uniformly cover the surface of the NF UV photodetector. The photons with energy larger than band gap of SnO2 and TiO2 promote NFs. The branches possess a cone shape electrons from the valence band (VB) to conduction band (CB), leaving behind a hole. The with an average length of 26 nm and a base hole migrates to the TiO2 | electrolyte interface, where it oxides an electron donor in electrolyte diameter around 5 nm. Further insight into (I−). The resulting oxidized species (I3−) are then reduced by electrons from external circuit at the counter electrode, completing the circuit. The open-circuit voltage (Voc) corresponds to the the structural information was obtained difference between the quasi-Fermi level of TiO2 under illumination and the electrochemical by HRTEM taken from the SnO2/TiO2 interface, Figure 2d. The lattice spacings at potential of electrolyte.

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Figure 2. SEM images of (a) SnO2 NF and (b) B-SnO2 NF film on FTO substrate, (c) TEM image of a single B-SnO2 NF. (d) HRTEM image taken from the SnO2/TiO2 interface.

the interface are measured to be 0.34, 0.27 and 0.23 nm, which are consistent with the spacings of [110] and [101] planes of rutile SnO2 and [100] planes of rutile TiO2, respectively. Moreover, the HRTEM image displays a perfect interfacial lattice relationship between the branches and the trunks, indicating that the solution heteroepitaxial growth method is a powerful technique to synthesize TiO2/SnO2 branched heterojunction nanostructures. To further investigate the crystal structure of TiO2/SnO2 heterojunction nanostructure, X-ray diffraction (XRD) and Raman spectroscopy were carried out on SnO2 NF and B-SnO2 NF at room temperature, as shown in Figure 3. Figure 3a compares the XRD spectra of SnO2 NF with that of B-SnO2 NF. All diffraction peaks of SnO2 NFs can be well indexed to a rutile structure of SnO2 (JCPDS no. 41-1445). In the case of B-SnO2 NF film, two weak additional XRD peaks characteristic of rutile TiO2 appeared at 2θ = 27.45 and 36.09o (JCPDS no. 34-0180). Figure 3b shows Raman scattering spectra of SnO2 NF and B-SnO2 NF. Pure SnO2 NF shows no identifiable peak, whereas the B-SnO2 NF shows three distinct peaks at 239, 438 and 604 cm−1, which should be assigned accordingly to B1g, Eg and A1g mode of rutile TiO2, respectively.[20] It is worth mentioning that both XRD and Raman scattering peaks for anatase TiO2 were absent in acquired XRD and Raman scattering spectra, again demonstrating the growth of rutile TiO2 branches on SnO2 NFs. The anisotropic growth of TiO2 nanoneedles can be understood from shape-control chemistry.[21,22] Firstly, the SnO2 nanocrystallites on the surface of the NFs act as nucleation centers for the growth of TiO2 nanoneedle structure because small 2013, DOI: 10.1002/smll.201202408

of the close match in lattice constant between rutile SnO2 (a = b = 0.474 nm)[23] and rutile TiO2 (a = b = 0.459 nm).[24] Driven by a minimization of the lattice mismatch, the rutile phase TiO2 nanocrystallites (TiO2 formed in the early stage of the heteroepitaxial growth) are preferentially oriented along the [001] direction on the facets of SnO2 nanocrystallites with minimum lattice mismatch, as is the case for most tetragonal systems.[22,25] Herein, the random distribution of crystal orientations in the SnO2 NFs provide numerous nucleation centers for heteroepitaxial growth of TiO2 branches, resulting in densely and uniformly covering of the entire surface of the SnO2 NF trunks, which is a requisite feature in the formation of SnO2-TiO2 core-shell structure. In contrast, in a similar solution heteroepitaxial growth process, TiO2 nanorods branched out only from the two side facets of the single crystal SnO2 nanowire trunks,[26] indicating that nucleation and growth require epitaxy on the facets of SnO2 nanowire with minimum lattice mismatch, which in turn demonstrate that the SnO2 nanocrystallites on the surface of the NFs really act as nucleation centers for the growth of TiO2 nanoneedles. Then under the synthetic condition we used, the Cl− ions

Figure 3. XRD (a) and Raman scattering (b) spectra of SnO2 NF and B-SnO2 NF.

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a faster interfacial electron recombination and lower trapping density.[27] The synthesis of the TiO2 branches on the SnO2 trunk forms a core-shell structure, which creates a surface dipole layer toward SnO2 due to the higher isoelectric points of TiO2 (∼5-5.5) than that of SnO2 (∼4-4.4).[13,16] Such a dipole layer might shift the conduction band of SnO2 to a more negative value, causing an increase in Voc and Jsc. Moreover, as shown in Figure 4b, the higher light absorption of B-SnO2 NF than that of SnO2 NF can also contribute to the enhancement of photovoltaic performance of the B-SnO2 NF PECC. The distinct photovoltaic behavior of the B-SnO2 NF is its large Jsc of 60.1 μA cm−2 compared with only 34.7 μA cm−2 for TiO2 NC, increased by 73%. Three factors may be responsible for this enhanced photocurrent: better light absorption, slower electron recombination or faster electron transport. With regard to the light absorption, as shown in Figure 4b, Figure 4. (a) J–V characteristics of PECCs with B-SnO2 NF, TiO2 NC and SnO2 NF film as − 2 photoanodes under 100 μW cm irradiance at 330 nm. (b) Absorption of the three films the B-SnO2 NF and TiO2 NC film on FTO fabricated on FTO. Inset compares the absorption of the three films in the wavelength range glass have the same absorptions in the from 250 nm to 450 nm. (c) Bode phase plots of B-SnO2 NF, TiO2 NC and SnO2 NF PECC wavelength range from 300 nm to 340 nm. under 100 μW cm−2 irradiance at 330 nm. (d) Charge-collection time (τc) calculated from Therefore, the absorption should not be photocurrent decay curves of SnO2 NF, B-SnO2 NF and TiO2 NC PECC as a function of Jsc. the reason of the enhanced photocurrent. For the second factor, we used electrocan selectively adsorb onto the [100] crystal plane and reduce chemical impedance spectroscopy (EIS) to compare the rates the surface energy of this plane, resulting in anisotropic of interfacial recombination of electrons from conduction growth in the [001] direction (Figure 2d). band of the semiconductor to oxidative species in electrolyte. The bode phase plots (frequency vs. phase angle) of the EIS results for the above two films are shown in Figure 4c. The 2.2. Enhanced Photovoltaic Performance of the B-SnO2 lifetime of the electrons in the oxide film (τn) can be estiNF PECC mated according to the relation τn = 1/2πfmax, where fmax is the maximum frequency of the mid-frequency peak in the The B-SnO2 NF photoelectrodes were tested under a bode phase plots. As shown in Figure 4c, the fmax value for 100 μW cm−2 UV irradiance (λ = 330 nm) and compared to the B-SnO2 NF is ∼0.299 Hz, much smaller than the values those with the photoelectrodes of SnO2 NF and nanocrystalline for TiO2 NC (∼2.27 Hz), indicating a longer electron lifetime TiO2 film (TiO2 NC). Here, the optimized film thickness was in B-SnO2 NF (τn = 0.53 s) film than in TiO2 NC film (τn = 2.6 μm for B-SnO2 NF electrode (as illustrated in Figure S2, 0.070 s). Thirdly, we measured the charge-collection time (τc) Supporting Information). As shown in Figure 4a, short-circuit to compare the charge transport property in the three films. current density (Jsc) and open-circuit voltage (Voc) of the SnO2 The τc was obtained by first-order decay kinetics fitting of NF PECC increased slightly from 10.7 μA cm−2 and 0.195 V the transient photocurrent profile in a short-circuit condition in dark to 15.8 μA cm−2 and 0.201 V under 100 μW cm−2 UV according to the equation[28] irradiance (λ = 330 nm), respectively, indicating a poor photoJsc = J 0 + C exp(−t/τ c ) voltaic performance. While for the B-SnO2 NF PECC, the Jsc and Voc increases sharply from 8.75 μA cm−2 and 0.342 V in where C, t and J0 are positive constant, time and initial shortdark to 60.1 μA cm−2 and 0.378 V under 100 μW cm−2 UV circuit current density, respectively. With the same method, irradiance, respectively. Given a fill factor of 0.650 and a light the τc of the PECCs based on SnO2 NF, B-SnO2 NF and TiO2 intensity of 100 μW cm−2, the power conversion efficiency NC were obtained as a function of Jsc (Figure 4d). The Jsc was (PCE) is up to 14.7%, this value reaches 41.4% when it is adjusted by varying the intensity of the bias light impinging corrected for 64.5% loss of incident photons through light on the device. As shown in Figure 4d, the collection time absorption and scattering by the F-doped tin oxide (FTO) of B-SnO2 NF is 1 to 17 times faster than that of TiO2 NC glass (as illustrated in Figure S3, Supporting Information). films, the difference becoming larger as the Jsc decreases. In These results are expected because the conduction-band edge addition to the higher electron mobility of SnO2 than that of of SnO2 is 300 mV more positive than that of TiO2, leading to TiO2 as mentioned above, the novel 1D structure of SnO2 NF

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gives another explanation to the faster τc. As for the case of TiO2 nanocrystalline film, it has been generally accepted that the electron transport in a nanocrystalline film is interrupted by a series of a trapping and detrapping process, which is particularly serious in the case of nanocrystalline film because of numerous grain boundaries existing in the nanocrystalline film.[13,29] Moreover, the smaller particle size and higher space charge region in the nanocrystalline film lead to complete depletion of macroscopic electric fields, and therefore, the conduction band remains flat throughout the particle.[29] Whereas the SnO2 NF in the present study consists of a dense packing of grains along length and diameter of the NFs, as can be seen from TEM image of a single SnO2 NF (as illustrated in Figure S4, Supporting Information). Such densely packed structure can support a small electric field due to a partially depleted space charge region within its volume, which is thought to accelerate the electrons in the random 1D-nanostructure film.[29] On the other hand, the τc of SnO2 NF is nearly equal to that of B-SnO2 NF across the whole Jsc range, indicating that the growth of TiO2 branches makes no difference to the electron transport property of the SnO2 NF network. Therefore, the slower electron recombination and faster electron transport in the B-SnO2 NF photoelectrode contributed to the 73% increase in photocurrent generation, which is a key advantageous feature in sensing of UV light.

2.3. Sensing of UV Light The advancements for UV photodetectors should include visible-blind, high responsivity, fast time response and good photosensitivity linearity in large light intensity range. The wavelength-related photoelectrical response was measured in terms of the current signal in short-circuit condition using a Xe lamp with a monochromator. The relationship between the responsivity (defined as photocurrent per unit of incident optical power) and the incident light wavelength is shown in Figure 5. These spectral responses exhibit that the B-SnO2 NF PECC has more excellent UV photoresponse, 12 times higher than SnO2 NF and 56% higher than TiO2 NC PECC for the peak values at 330 nm. The peak responsivity of B-SnO2 NF PECC is approximately 0.6 A/W. For comparison, the responsivity of most commercial UV photodetectors is in the range of 0.1–0.2 A/W.[6] The responsivity during the visible region for the B-SnO2 NF PECC drops ∼411 times as compared with the peak, indicating that the device can be used as visible-blind UV photodetector. It should be noted that there are also week humps centered at 680 nm for the three devices. The details of the responsivities in the visible region are shown in inset of Figure 5. These responses small 2013, DOI: 10.1002/smll.201202408

Figure 5. Spectral responses of Jsc signals of B-SnO2 NF, TiO2 NC and SnO2 NF PECCs measured under the light intensity of 50 μW cm−2. Inset shows the details of the responses in visible range, which was measured under the light intensity of 300 μW cm−2.

should be attributed to the trap-to-trap electron transport due to the existence of trap states in TiO2 and SnO2, as demonstrated in the previous reports.[8,30] In order to get clear information about the stability and responsiveness of the self-power UV photodetector, we measured the photoresponse switching behavior of the device. As shown in Figure 6a, the Jsc signal can be reproducibly

Figure 6. (a) Time responses of Jsc of B-SnO2 NF PECC upon 40 mW cm−2 UV light illumination (λ = 365 nm) measured for light-on and -off states. (b) Rising time (τr) and decay time (τd) of B-SnO2 NF and TiO2 NC PECC. (c) Jsc as a function of the incident UV (λ = 365 nm) intensity in a wide range from 10 μW cm−2 to 40 mW cm−2. (d) shows photosensitivity linearity and photoresponse switching behaviors under low light intensity from 10 μW cm−2 to 100 μW cm−2.

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switched from the “ON’’ state to the ‘‘OFF’’ state by periodically turning the UV light on and off with a power density of 40 mW cm−2 and the wavelength of 365 nm. The on/off ratio is as high as 4550. From the rising and recovering edge of the current response in Figure 6a, the rise time (τr) and decay time (τd, defined as time to recovery to 1/e (37%) of the maximum photocurrent) of the B-SnO2 NF PECC are around 0.03 and 0.01 s for Jsc, respectively, faster than 0.08 and 0.02 s for TiO2 NC PECC, indicating rapid photoresponse characteristics. Because electron diffusion within a nanoparticle network film is determined by trapping/detrapping events, the time responses of photocurrent of such PECC usually depends on light intensity.[16,31] As shown in Figure 6b, both of τr and τd are faster for B-SnO2 NF than for TiO2 NC films, the difference becoming larger as the Jsc decreases. The τr and τd for B-SnO2 NF keeps constant at ∼0.03 s and ∼0.01 s when the light intensity is larger than 1.8 mW cm−2 and 630 μW cm−2, respectively, followed by gradually increasing to 90% transmittance, 14 ohm per square, Nippon, Japan) by drop-drying method. The films were then sintered at 500 °C for 30 min. For the synthesis of the branches, the annealed SnO2 NF films were immersed in a brown glass bottle with an aqueous solution[25] consisting of DI water (10 mL), HCl (0.1 mL), and TiCl3 solution (0.1 mL, 20 wt% of TiCl3 in H2O and HCl solution, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China), and then kept at 80 °C for 35 min in an oven. At last, the samples were washed with deionized water and subsequently annealed at 450 °C for 1 h in air. Assembling of Self-Powered UV Photodetectors: Preparation of the nanocrystalline TiO2 film and assembling of self-powered UV photodetectors were described in our previous work.[8,30] In brief, the sintered B-SnO2 NF and TiO2 NC electrodes and the platinized counter electrodes were assembled into sandwich-type cells. The platinized electrodes were prepared by spin-coating a 4.5 mM isopropanol solution of H2PtCl6·6H2O, and then heated at 400 °C for 20 min. The interelectrode space was filled with a liquid electrolyte consisting of LiI (0.1 M), 1, 2-dimethyl-3-propylimidazolium iodide (0.6 M), I2 (0.05 M) and 4-tert-butylpyridine (0.5 M) in acetonitrile. Characterizations: The morphology and structure of the samples were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, FEI Tecnai F30). X-ray diffraction (XRD, Philips, X’pert pro, Cu Kα, 0.154056 nm) was employed to characterize the structural properties of the samples. Raman scattering spectra were carried out on a Jobin-Yvon LabRam HR80 spectrometer (with a 532 nm line of Torus 50 mW diode-pumped solid-state laser) under the backscattering geometry at room temprature. Photovoltaic performance and electrochemical impedance spectroscopy (EIS) of the devices were obtained on an Electrochemical Workstation (RST5200, Zhengzhou Shiruisi Instrument Technology Co., Ltd, China). EIS measurements were carried out in the frequency range of 0.01 Hz to 100 kHz at open-circuit voltage with a potential pulse of 10 mV in amplitude. A UV detector (ST-513, UVAB, SENTRY OPTRONICS CORP., Taiwan) was used to quantify UV irradiance. A 500 W Xenon lamp equipped with a monochromator and an ultraviolet band pass filter (365 nm) was used as light source.

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was financially supported by the National Science Foundation of China (No. 61176058), partially by Natural Science Foundation of Gansu Province (1107RJYA280) and Fundamental Research Funds for the Central Universities (No. lzujbky-2012-34). H. D. thanks the sponsorship from National Science Foundation of China (Grant Nos. 11274107 and 61204109). The authors also thank Tikang Hou at Hunan University, China for useful discussions and his generous help with the light absorption measurements.

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