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electrolyte heterojunctions
Journal of Alloys and Compounds 675 (2016) 325e330
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Self-powered UVevisible photodetectors based on ZnO/Cu2O nanowire/electrolyte heterojunctions Zhiming Bai, Yinghua Zhang* School of Civil and Environmental Eengineering, University of Science and Technology Beijing, Beijing 10083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2015 Received in revised form 1 March 2016 Accepted 9 March 2016 Available online 10 March 2016
ZnO/Cu2O branched heterojunction arrays (BHAs) were synthesized via hydrothermal and chemical bath deposition process for the application in photoelectrochemical type self-powered UVevisible photodetectors. At zero bias, the photodetectors based on the ZnO/Cu2O nanowire/electrolyte heterojunctions exhibited high responsivities of 19.3 mA W1 and 8.2 mA W1 for UV and visible light, respectively, with a rise time of 0.14 s and a decay time of 0.36 s. The spectral response measurements carried out on the self-powered photodetectors based on ZnO/Cu2O BHAs demonstrate a significant increase of responsivity in the visible region compared to ZnO nanowire arrays (NWAs). The excellent photodetecting performance, low cost, non-toxic, and facile fabrication process make the photoelectrochemical type selfpowered photodetectors based on metal oxide semiconductors promising candidates for nextgeneration UVevisible photosensing applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: ZnO Cu2O Self-powered Photodetector UVevisible
1. Introduction Self-powered nanodevices are taking more and more attention in applications such as environment monitoring, implantable biology-detectors, and portable electronics [1]. Based on photovoltaic effect, self-powered photodetectors can convert optical signal to electrical signal via the photoelectron excitation process [2,3]. In other words, they can be powered by the incident light without consuming external power. Compared to traditional photodetectors, the self-powered ones can operate without using batteries, which greatly enhance their mobility and adaptability. Also, they have faster response time, lower dark current, higher sensitivity, and more simple fabrication process [4]. According to interfacial characterization, the self-powered photodetectors can be divided into three categories: pen junction type [5e7], Schottky junction type [8,9], and photoelectrochemical type [10,11]. Among them, the photoelectrochemical type has a better application prospect, because it has low cost and simple fabrication process, and can be built based on earth-abundant and inexpensive materials. For a photoelectrochemical type self-powered photodetector, a built-in potential at the solid/liquid interface acts as a driving force
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.03.051 0925-8388/© 2016 Elsevier B.V. All rights reserved.
to separate the photogenerated electronehole pairs and generate photocurrent. Thus, the photodetectors based on solid/liquid heterojunctions can operate in photovoltaic mode without any external bias [11]. In the past few years, wide band-gap metal oxide semiconductors, such as TiO2 [12e14], ZnO [15e20], and SnO2 [21], have been widely used in self-powered UV photodetectors. As a direct band-gap semiconductor, ZnO has similar energy band structure as TiO2, and the typical electron mobility in ZnO is much higher than that in TiO2 [22,23]. One-dimensional ZnO NWAs are preferred over their bulk counterparts because of their massive surface-to-volume ratio, low reflectivity, and high charge carrier collection ability [24]. Nevertheless, pure ZnO NWAs have weak intrinsic sensitivity to visible illumination, which limits their applications in UVevisible fingerprint recognition and bispectral image detection [25]. Therefore, a band-gap engineering device composed of ZnO NWAs and narrow band-gap semiconductors is highly desirable to get good response in UVevisible region. As an attractive direct band-gap semiconductor (2.0 eV), Cu2O is a promising candidate material for solar cells and solar water splitting due to its suitable absorbance in the visible region, low cost, and non-toxicity [26e31]. The type-II band alignment between ZnO and Cu2O is beneficial for the separation of photogenerated electronehole pairs. In addition, incorporation of Cu2O onto ZnO NWAs can extend the spectral response to the visible region [28]. However, to the best of our knowledge, there are few reports about ZnO/Cu2O heterojunction NWAs for PEC type self-
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powered UVevisible photodetection study. In this work, ZnO NWAs were grown on a fluorine-doped tin oxide (FTO) glass substrate by a low-temperature hydrothermal method. The ZnO/Cu2O BHAs were prepared through a chemical bath deposition (CBD) process and subsequent heat treatment. The ZnO/Cu2O BHAs showed fast response speed and high photosensitivity in both UV and visible light region at zero bias. By introducing the branched Cu2O nanowires, the heterojunction NWAs supply more photogenerated carriers upon visible light illumination. Also, the enlarged solid/liquid contact area and the enhanced charge transport properties contribute to the improvement in the photoresponse in the visible region. The feasible fabrication process and the excellent detecting performance reveal that this selfpowered photodetector has promising applications in the UVevisible photosensing field. 2. Experimental section 2.1. Fabrication of ZnO/Cu2O BHAs The ZnO NWAs were prepared on a FTO conductive glass substrate by a hydrothermal method detailed in a previous paper [32]. The branched Cu2O nanowires were deposited onto the surface of the as-grown ZnO NWAs by a CBD method followed by a thermal reduction process. The ZnO NWAs were first soaked in 0.1 mol L1 Cu(CH3COO)2 aqueous solution for 30 s to deposit Cu2þ on the nanowire surface, and then rinsed by deionized water to remove CH3COO. The sample was successively put in 0.2 mol L1 NaOH aqueous solution and maintained at 60 C for 30 s to form ZnO/ Cu(OH)2 BHAs. Then, the sample was washed again by deionized water to get rid of Naþ. The coating cycles were repeated for 10 times, and the samples were dried in air. Finally, the as-prepared ZnO/Cu(OH)2 BHAs were baked at 500 C for 2 h in argon atmosphere, leading to the production of the ZnO/Cu2O BHAs. 2.2. Assembling of PEC type self-powered UVevisible photodetectors The counter electrodes were prepared by depositing a 50-nmthick Pt film on the FTO conductive glass substrates using DC sputtering technology. Then, the ZnO/Cu2O BHAs photoanodes were adhered to the Pt/FTO electrodes using a thermoplast hotmelt sealing foil (Suryln 1702, 60 mm). The space between the electrodes was filled with 0.5 mol L1 Na2SO4 aqueous solution as electrolyte. The active area of the self-powered photodetectors was fixed at 0.3 cm2. 2.3. Characterizations Morphology and composition of the samples were characterized by a field emission scanning electron microscope (FESEM, FEI QUANTA 3D FESEM) equipped with an energy dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) patterns were conducted on a Rigaku DMAX-RB using Cu Ka X-ray radiation source to analyze the crystal structure of the as-prepared samples. The UVevisible absorption spectra were obtained on a UVeviseNIR spectrophotometer (Varian Cary 5000). The photoelectrical properties of the fabricated self-powered photodetectors were characterized using a semiconductor characterization system (Keithley 4200-SCS). A 355 nm laser was used as the UV light source. The visible light (l > 425 nm) was provided by a solar simulator (Oriel, 91159 A, 70 mW cm2) equipped with a visible band pass filter. A 500 W xenon lamp (ChangTuo, Inc., China) in combination with a monochromator (AnHe, Inc., China) was used as a monochromatic light source for the measurements of spectral
responsivity. The electrochemical impedance spectra (EIS) measurements were carried out using an electrochemical workstation (Solartron SI1287/SI 1260) in the frequency range of 0.1 Hze100 kHz at a sinusoidal perturbation with 10 mV amplitude. 3. Results and discussion Fig. 1(a) and (b) show the top-view and cross-sectional SEM images of the as-prepared ZnO NWAs. The ZnO nanowires are vertically aligned on the FTO substrate with an average diameter of 120e200 nm and a mean length of about 3.8 mm. The individual nanowire is hexagonal-prism-like and has smooth surface. After the CBD and thermal reduction process, the pine-like Cu2O nanobranches were homogeneously grew on the top of the ZnO NWAs, as shown in Fig. 1(c). The diameter of ZnO nanowires became slightly smaller, indicating that they were corroded by NaOH aqueous solution during the CBD process. It can be seen from Fig. 1(d) that the other part of Cu2O nanocrystals with a size of about 80 nm grew on the side wall of the ZnO nanowires. During the heat treatment process, the Cu(OH)2 was dehydrated to produce CuO, and the CuO was subsequently reduced to Cu2O at high temperature [33]. The length of ZnO nanowires was reduced to about 3.4 mm due to the alkali corrosion. The chemical composition and crystal structure of ZnO/Cu2O BHAs were investigated using EDX and XRD techniques. The EDX spectrum indicates the sample is composed of O, Zn, Cu and Au (the signals of Au originate from the gold spraying process), as shown in Fig. 2(a). The strong diffraction peak of 34.4 corresponds to (002) planes of ZnO, indicating that the ZnO nanowires are highly c-axis oriented (Fig. 2(b)). The peaks centered at 47.5 and 62.9 correspond to the (102) and (103) planes of hexagonal wurtzite ZnO [34]. There are three characteristic peaks with 2q values of 29.5 , 36.3 , and 61.6 , corresponding to the (110), (111), and (220) crystal planes of cubic Cu2O (JCPDS card no. 65-3288) [35]. UVevisible absorption spectra were used to analyze the optical properties of the prepared samples (Fig. 3). It is clear that a sharp absorption edge appears at about 380 nm for the pure ZnO NWAs, and they have very weak absorption in the range of 400e700 nm, which is consistent with the wide band-gap of ZnO (Eg ¼ 3.37 eV) [36]. After coating the narrow band-gap Cu2O, the light harvesting capacity of ZnO/Cu2O BHAs was improved, especially in the visible light region, and the absorption edge in the UV region was apparently red-shifted. The 3-demensional (3D) configuration of Cu2O nanobranches increases the surface roughness and contributes to elevating the light absorption. The results indicate that the ZnO NWAs coupling with Cu2O nanocrystals will have enhanced photoresponse in the visible range. Fig. 4(a) and (b) show the schematic diagram and the energy band profile of the self-powered UVevisible photodetector based on ZnO/Cu2O BHAs, which has a structure similar to dye-sensitized solar cells. When UV light illuminates on the self-powered photodetectors, the electronehole pairs are generated in Cu2O and ZnO. The photogenerated electrons in the conductive band of Cu2O drift toward the conductive band of ZnO driven by the internal electric field at the ZnO/Cu2O interface. At the same time, the photogenerated holes in ZnO migrate to the valence band of Cu2O. In the effect of the built-in electric field at the Cu2O/electrolyte interface, the photoinduced holes in Cu2O flow into the electrolyte, and subsequently capture electrons from OH (hþ þ OH ¼ OH). The oxidized species are then reduced at the interface of electrolyte/Pt by the photoinduced electrons from the external circuit. Throughout the entire process, the migration of charge carriers is driven by the space electric field at the ZnO/Cu2O interface and the Cu2O/electrolyte interface. In other words, this kind of photodetectors can operate at zero bias. Upon visible light illumination, the
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Fig. 1. (a) The top-view SEM image and (b) the cross-sectional SEM image of ZnO NWAs. (c) The top-view SEM image and (d) the cross-sectional SEM image of ZnO/Cu2O BHAs.
Fig. 2. (a) The EDX spectrum and (b) the XRD pattern of ZnO/Cu2O BHAs.
photogenerated electronehole pairs are mainly generated in Cu2O. The electrons migrate to the Pt counter electrodes through ZnO, FTO and the external circuit, and recombine with the holes via redox reactions in electrolyte. Fig. 4(c) depicts the J-V characteristics of the self-powered photodetectors under UV light illumination (6 mW cm2, l ¼ 355 nm). It can be seen clearly that the J-V curves of ZnO NWAs and ZnO/Cu2O BHAs in the dark both show rectification characteristics, indicating the formation of the semiconductor/liquid heterojunctions. In addition, compared with the ZnO NWAs, the ZnO/Cu2O BHAs show an enlarged photocurrent density, indicating better conductivity. Upon light illumination, the photodetectors have much better photoresponse at reverse bias than that at forward bias. This is because that the strength of space electric field is
enhanced as the reverse bias increase, resulting in more efficient separation of the photogenerated carriers. These photodetectors also can operate in photovoltaic mode. The ZnO NWAs show a higher short-circuit photocurrent density (Jsc) of 296 mA cm2 than that of 107 mA cm2 for the ZnO/Cu2O BHAs. The photoresponse switching behaviors of the photodetectors are shown in Fig. 4(d). The periodical switch of Jsc signals from “On” to “Off” demonstrates the devices have fine stability and reproducibility. The sensitivities of ZnO NWAs and ZnO/Cu2O BHAs to the 6 mW cm2 UV light (l ¼ 355 nm) are 1439 and 525, respectively. Under 25 mW cm2 visible light (l > 425 nm) illumination, ZnO/ Cu2O BHAs have a much higher on/off ratio of 1945 than that of 39 for ZnO NWAs. Upon light illumination, a spike in photoresponse can be seen due to the transient effect in light excitation, and then
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the photocurrent quickly returns to a steady state because of the recombination of carriers through interface and surface states [37,38]. The rise time (defined as the time from the dark current to 1e1/e (63%) of the maximum photocurrent) and the decay time (defined as the time from 100% to 37% of the steady photocurrent) are about 0.14 s and 0.36 s, respectively. Table 1 compares the selfpowered photoresponse performance of ZnO/Cu2O BHAs with the previous researches of others. Our ZnO/Cu2O BHAs photodetectors exhibit enhanced Jsc and longer rise time. The longer response time is caused by the interface and surface states of ZnO/Cu2O BHAs. The spectral response was measured to investigate the wavelength selectivity of the prepared samples, as shown in Fig. 5. The peak responsivity of ZnO NWAs is 42.1 mA W1 at 370 nm, which is 118% higher than that of ZnO/Cu2O BHAs (19.3 mA W1). It should be noted that the photodetectors were illuminated from the Pt side (see Fig. 4(a)). The Cu2O out layers hinder the UV absorption of the core ZnO nanowires, resulting in less photogenerated carrier concentration. The overpotential of Cu2O for water oxidation is small [27], and the photogenerated holes from the valence band of ZnO Fig. 3. The UVeVis absorption spectra of ZnO NWAs and ZnO/Cu2O BHAs.
Fig. 4. (a) The schematic diagram and (b) the energy band profile of the self-powered UVeVis ZnO/Cu2O BHA photodetectors. (c) J-V characteristics of photodetectors with ZnO NWA and ZnO/Cu2O BHA photoanodes under 6 mW cm2 illumination at 355 nm. (d) Time responses of photocurrent densities of the photodetectors upon 6 mW cm2 UV illumination (l ¼ 355 nm) and 25 mW cm2 Vis illumination (l > 425 nm) at 0 V bias.
Table 1 Comparison of photoresponse performance of the photoelectrochemical type self-powered photodetectors based ZnO nanomaterials. Materials
Jsc(mA cm2)
Rise time (ms)
Light wavelength (nm)
Light intensity (mW cm2)
References
ZnO nanoneedles ZnO nanowires ZnO/Cu2O BHAs
0.8 4 107
100 100 140
365 Solar light 355
1.25 100 6
[15] [39] Our work
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3D branched architecture of ZnO/Cu2O BHAs possesses enlarged surface area, which contributes to speeding up the carrier transfer at interfaces and reducing interfacial reaction resistance.
4. Conclusions
Fig. 5. Spectral responsivity characteristics of the photodetectors with irradiance wavelength ranging from 300 nm to 700 nm under 0 V bias.
We have successfully synthesized ZnO/Cu2O heterojunction arrays for the self-powered photodetecting applications. Based on the photovoltaic effect, the photodetectors with ZnO/Cu2O BHAs photoanodes show excellent photosensing performance both in the UV and visible region at zero bias. They have high on/off ratios of 525 and 1945 for UV light (6 mW cm2, l ¼ 355 nm) and visible light (25 mW cm2, l > 425 nm), respectively, with a fast rise time of 0.14 s and a decay time of 0.36 s. The spectral response studies of ZnO/Cu2O BHAs show a large enhancement in photoresponse in the visible region. Our results indicate the photoelectrochemical type self-powered photodetectors based on metal oxides have great prospect in high-sensitivity and high-speed UVevisible light detecting applications.
Fig. 6. EIS measurements of ZnO NWA photoanodes and ZnO/Cu2O BHA photoanodes (a) in the dark and (b) under AM 1.5G illumination (70 mW cm2).
are prone to aggregate in Cu2O, increasing the carrier recombination. In addition, during the preparation process, the alkaline corrosion to the ZnO nanowires introduced unexpected defect states at the ZnO/Cu2O interface, which may act as recombination centers. The ZnO/Cu2O BHAs exhibited substantially greater photoactivity in the visible region from 400 nm to 500 nm than that of pure ZnO NWAs, mainly due to the enhancement in the light absorption caused by the Cu2O nanobranches. The visible responsivity peak located at a wavelength of 400 nm is proximately 8.2 mA W1. The results demonstrate that the Cu2O can increase the visible absorption, and the photogenerated electrons in Cu2O can migrate to the core ZnO nanowires. In order to get insight into the intrinsic electric and charge transfer properties of the samples, EIS plots were carried out both in the dark and under illumination, as shown in Fig. 6. It is generally known that the radii of the semicircles in the EIS plots reflect the charge transfer ability. After coating Cu2O, the radius was reduced compared to ZnO NWAs, suggesting more excellent electrical conductivity. This is consistent with the J-V curves measured in the dark in Fig. 4(c). Upon illumination, the arc radii were nearly two orders of magnitude smaller than those in the dark, because of the increased photogenerated carrier concentration. The ZnO/Cu2O BHAs have smaller radius, indicating higher photoirradiated carrier generation rate and faster interfacial charge transfer velocity. The
Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC) (51474017), the Fundamental Research Funds for the Central Universities (FRF-TP-15-107A1) and the China Postdoctoral Science Foundation (2015M580979).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Self-powered UVevisible photodetectors based on ZnO/Cu2O nanowire/electrolyte heterojunctions Zhiming Bai, Yinghua Zhang* School of Civil and Environmental Eengineering, University of Science and Technology Beijing, Beijing 10083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2015 Received in revised form 1 March 2016 Accepted 9 March 2016 Available online 10 March 2016
ZnO/Cu2O branched heterojunction arrays (BHAs) were synthesized via hydrothermal and chemical bath deposition process for the application in photoelectrochemical type self-powered UVevisible photodetectors. At zero bias, the photodetectors based on the ZnO/Cu2O nanowire/electrolyte heterojunctions exhibited high responsivities of 19.3 mA W1 and 8.2 mA W1 for UV and visible light, respectively, with a rise time of 0.14 s and a decay time of 0.36 s. The spectral response measurements carried out on the self-powered photodetectors based on ZnO/Cu2O BHAs demonstrate a significant increase of responsivity in the visible region compared to ZnO nanowire arrays (NWAs). The excellent photodetecting performance, low cost, non-toxic, and facile fabrication process make the photoelectrochemical type selfpowered photodetectors based on metal oxide semiconductors promising candidates for nextgeneration UVevisible photosensing applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: ZnO Cu2O Self-powered Photodetector UVevisible
1. Introduction Self-powered nanodevices are taking more and more attention in applications such as environment monitoring, implantable biology-detectors, and portable electronics [1]. Based on photovoltaic effect, self-powered photodetectors can convert optical signal to electrical signal via the photoelectron excitation process [2,3]. In other words, they can be powered by the incident light without consuming external power. Compared to traditional photodetectors, the self-powered ones can operate without using batteries, which greatly enhance their mobility and adaptability. Also, they have faster response time, lower dark current, higher sensitivity, and more simple fabrication process [4]. According to interfacial characterization, the self-powered photodetectors can be divided into three categories: pen junction type [5e7], Schottky junction type [8,9], and photoelectrochemical type [10,11]. Among them, the photoelectrochemical type has a better application prospect, because it has low cost and simple fabrication process, and can be built based on earth-abundant and inexpensive materials. For a photoelectrochemical type self-powered photodetector, a built-in potential at the solid/liquid interface acts as a driving force
* Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.03.051 0925-8388/© 2016 Elsevier B.V. All rights reserved.
to separate the photogenerated electronehole pairs and generate photocurrent. Thus, the photodetectors based on solid/liquid heterojunctions can operate in photovoltaic mode without any external bias [11]. In the past few years, wide band-gap metal oxide semiconductors, such as TiO2 [12e14], ZnO [15e20], and SnO2 [21], have been widely used in self-powered UV photodetectors. As a direct band-gap semiconductor, ZnO has similar energy band structure as TiO2, and the typical electron mobility in ZnO is much higher than that in TiO2 [22,23]. One-dimensional ZnO NWAs are preferred over their bulk counterparts because of their massive surface-to-volume ratio, low reflectivity, and high charge carrier collection ability [24]. Nevertheless, pure ZnO NWAs have weak intrinsic sensitivity to visible illumination, which limits their applications in UVevisible fingerprint recognition and bispectral image detection [25]. Therefore, a band-gap engineering device composed of ZnO NWAs and narrow band-gap semiconductors is highly desirable to get good response in UVevisible region. As an attractive direct band-gap semiconductor (2.0 eV), Cu2O is a promising candidate material for solar cells and solar water splitting due to its suitable absorbance in the visible region, low cost, and non-toxicity [26e31]. The type-II band alignment between ZnO and Cu2O is beneficial for the separation of photogenerated electronehole pairs. In addition, incorporation of Cu2O onto ZnO NWAs can extend the spectral response to the visible region [28]. However, to the best of our knowledge, there are few reports about ZnO/Cu2O heterojunction NWAs for PEC type self-
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powered UVevisible photodetection study. In this work, ZnO NWAs were grown on a fluorine-doped tin oxide (FTO) glass substrate by a low-temperature hydrothermal method. The ZnO/Cu2O BHAs were prepared through a chemical bath deposition (CBD) process and subsequent heat treatment. The ZnO/Cu2O BHAs showed fast response speed and high photosensitivity in both UV and visible light region at zero bias. By introducing the branched Cu2O nanowires, the heterojunction NWAs supply more photogenerated carriers upon visible light illumination. Also, the enlarged solid/liquid contact area and the enhanced charge transport properties contribute to the improvement in the photoresponse in the visible region. The feasible fabrication process and the excellent detecting performance reveal that this selfpowered photodetector has promising applications in the UVevisible photosensing field. 2. Experimental section 2.1. Fabrication of ZnO/Cu2O BHAs The ZnO NWAs were prepared on a FTO conductive glass substrate by a hydrothermal method detailed in a previous paper [32]. The branched Cu2O nanowires were deposited onto the surface of the as-grown ZnO NWAs by a CBD method followed by a thermal reduction process. The ZnO NWAs were first soaked in 0.1 mol L1 Cu(CH3COO)2 aqueous solution for 30 s to deposit Cu2þ on the nanowire surface, and then rinsed by deionized water to remove CH3COO. The sample was successively put in 0.2 mol L1 NaOH aqueous solution and maintained at 60 C for 30 s to form ZnO/ Cu(OH)2 BHAs. Then, the sample was washed again by deionized water to get rid of Naþ. The coating cycles were repeated for 10 times, and the samples were dried in air. Finally, the as-prepared ZnO/Cu(OH)2 BHAs were baked at 500 C for 2 h in argon atmosphere, leading to the production of the ZnO/Cu2O BHAs. 2.2. Assembling of PEC type self-powered UVevisible photodetectors The counter electrodes were prepared by depositing a 50-nmthick Pt film on the FTO conductive glass substrates using DC sputtering technology. Then, the ZnO/Cu2O BHAs photoanodes were adhered to the Pt/FTO electrodes using a thermoplast hotmelt sealing foil (Suryln 1702, 60 mm). The space between the electrodes was filled with 0.5 mol L1 Na2SO4 aqueous solution as electrolyte. The active area of the self-powered photodetectors was fixed at 0.3 cm2. 2.3. Characterizations Morphology and composition of the samples were characterized by a field emission scanning electron microscope (FESEM, FEI QUANTA 3D FESEM) equipped with an energy dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) patterns were conducted on a Rigaku DMAX-RB using Cu Ka X-ray radiation source to analyze the crystal structure of the as-prepared samples. The UVevisible absorption spectra were obtained on a UVeviseNIR spectrophotometer (Varian Cary 5000). The photoelectrical properties of the fabricated self-powered photodetectors were characterized using a semiconductor characterization system (Keithley 4200-SCS). A 355 nm laser was used as the UV light source. The visible light (l > 425 nm) was provided by a solar simulator (Oriel, 91159 A, 70 mW cm2) equipped with a visible band pass filter. A 500 W xenon lamp (ChangTuo, Inc., China) in combination with a monochromator (AnHe, Inc., China) was used as a monochromatic light source for the measurements of spectral
responsivity. The electrochemical impedance spectra (EIS) measurements were carried out using an electrochemical workstation (Solartron SI1287/SI 1260) in the frequency range of 0.1 Hze100 kHz at a sinusoidal perturbation with 10 mV amplitude. 3. Results and discussion Fig. 1(a) and (b) show the top-view and cross-sectional SEM images of the as-prepared ZnO NWAs. The ZnO nanowires are vertically aligned on the FTO substrate with an average diameter of 120e200 nm and a mean length of about 3.8 mm. The individual nanowire is hexagonal-prism-like and has smooth surface. After the CBD and thermal reduction process, the pine-like Cu2O nanobranches were homogeneously grew on the top of the ZnO NWAs, as shown in Fig. 1(c). The diameter of ZnO nanowires became slightly smaller, indicating that they were corroded by NaOH aqueous solution during the CBD process. It can be seen from Fig. 1(d) that the other part of Cu2O nanocrystals with a size of about 80 nm grew on the side wall of the ZnO nanowires. During the heat treatment process, the Cu(OH)2 was dehydrated to produce CuO, and the CuO was subsequently reduced to Cu2O at high temperature [33]. The length of ZnO nanowires was reduced to about 3.4 mm due to the alkali corrosion. The chemical composition and crystal structure of ZnO/Cu2O BHAs were investigated using EDX and XRD techniques. The EDX spectrum indicates the sample is composed of O, Zn, Cu and Au (the signals of Au originate from the gold spraying process), as shown in Fig. 2(a). The strong diffraction peak of 34.4 corresponds to (002) planes of ZnO, indicating that the ZnO nanowires are highly c-axis oriented (Fig. 2(b)). The peaks centered at 47.5 and 62.9 correspond to the (102) and (103) planes of hexagonal wurtzite ZnO [34]. There are three characteristic peaks with 2q values of 29.5 , 36.3 , and 61.6 , corresponding to the (110), (111), and (220) crystal planes of cubic Cu2O (JCPDS card no. 65-3288) [35]. UVevisible absorption spectra were used to analyze the optical properties of the prepared samples (Fig. 3). It is clear that a sharp absorption edge appears at about 380 nm for the pure ZnO NWAs, and they have very weak absorption in the range of 400e700 nm, which is consistent with the wide band-gap of ZnO (Eg ¼ 3.37 eV) [36]. After coating the narrow band-gap Cu2O, the light harvesting capacity of ZnO/Cu2O BHAs was improved, especially in the visible light region, and the absorption edge in the UV region was apparently red-shifted. The 3-demensional (3D) configuration of Cu2O nanobranches increases the surface roughness and contributes to elevating the light absorption. The results indicate that the ZnO NWAs coupling with Cu2O nanocrystals will have enhanced photoresponse in the visible range. Fig. 4(a) and (b) show the schematic diagram and the energy band profile of the self-powered UVevisible photodetector based on ZnO/Cu2O BHAs, which has a structure similar to dye-sensitized solar cells. When UV light illuminates on the self-powered photodetectors, the electronehole pairs are generated in Cu2O and ZnO. The photogenerated electrons in the conductive band of Cu2O drift toward the conductive band of ZnO driven by the internal electric field at the ZnO/Cu2O interface. At the same time, the photogenerated holes in ZnO migrate to the valence band of Cu2O. In the effect of the built-in electric field at the Cu2O/electrolyte interface, the photoinduced holes in Cu2O flow into the electrolyte, and subsequently capture electrons from OH (hþ þ OH ¼ OH). The oxidized species are then reduced at the interface of electrolyte/Pt by the photoinduced electrons from the external circuit. Throughout the entire process, the migration of charge carriers is driven by the space electric field at the ZnO/Cu2O interface and the Cu2O/electrolyte interface. In other words, this kind of photodetectors can operate at zero bias. Upon visible light illumination, the
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Fig. 1. (a) The top-view SEM image and (b) the cross-sectional SEM image of ZnO NWAs. (c) The top-view SEM image and (d) the cross-sectional SEM image of ZnO/Cu2O BHAs.
Fig. 2. (a) The EDX spectrum and (b) the XRD pattern of ZnO/Cu2O BHAs.
photogenerated electronehole pairs are mainly generated in Cu2O. The electrons migrate to the Pt counter electrodes through ZnO, FTO and the external circuit, and recombine with the holes via redox reactions in electrolyte. Fig. 4(c) depicts the J-V characteristics of the self-powered photodetectors under UV light illumination (6 mW cm2, l ¼ 355 nm). It can be seen clearly that the J-V curves of ZnO NWAs and ZnO/Cu2O BHAs in the dark both show rectification characteristics, indicating the formation of the semiconductor/liquid heterojunctions. In addition, compared with the ZnO NWAs, the ZnO/Cu2O BHAs show an enlarged photocurrent density, indicating better conductivity. Upon light illumination, the photodetectors have much better photoresponse at reverse bias than that at forward bias. This is because that the strength of space electric field is
enhanced as the reverse bias increase, resulting in more efficient separation of the photogenerated carriers. These photodetectors also can operate in photovoltaic mode. The ZnO NWAs show a higher short-circuit photocurrent density (Jsc) of 296 mA cm2 than that of 107 mA cm2 for the ZnO/Cu2O BHAs. The photoresponse switching behaviors of the photodetectors are shown in Fig. 4(d). The periodical switch of Jsc signals from “On” to “Off” demonstrates the devices have fine stability and reproducibility. The sensitivities of ZnO NWAs and ZnO/Cu2O BHAs to the 6 mW cm2 UV light (l ¼ 355 nm) are 1439 and 525, respectively. Under 25 mW cm2 visible light (l > 425 nm) illumination, ZnO/ Cu2O BHAs have a much higher on/off ratio of 1945 than that of 39 for ZnO NWAs. Upon light illumination, a spike in photoresponse can be seen due to the transient effect in light excitation, and then
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the photocurrent quickly returns to a steady state because of the recombination of carriers through interface and surface states [37,38]. The rise time (defined as the time from the dark current to 1e1/e (63%) of the maximum photocurrent) and the decay time (defined as the time from 100% to 37% of the steady photocurrent) are about 0.14 s and 0.36 s, respectively. Table 1 compares the selfpowered photoresponse performance of ZnO/Cu2O BHAs with the previous researches of others. Our ZnO/Cu2O BHAs photodetectors exhibit enhanced Jsc and longer rise time. The longer response time is caused by the interface and surface states of ZnO/Cu2O BHAs. The spectral response was measured to investigate the wavelength selectivity of the prepared samples, as shown in Fig. 5. The peak responsivity of ZnO NWAs is 42.1 mA W1 at 370 nm, which is 118% higher than that of ZnO/Cu2O BHAs (19.3 mA W1). It should be noted that the photodetectors were illuminated from the Pt side (see Fig. 4(a)). The Cu2O out layers hinder the UV absorption of the core ZnO nanowires, resulting in less photogenerated carrier concentration. The overpotential of Cu2O for water oxidation is small [27], and the photogenerated holes from the valence band of ZnO Fig. 3. The UVeVis absorption spectra of ZnO NWAs and ZnO/Cu2O BHAs.
Fig. 4. (a) The schematic diagram and (b) the energy band profile of the self-powered UVeVis ZnO/Cu2O BHA photodetectors. (c) J-V characteristics of photodetectors with ZnO NWA and ZnO/Cu2O BHA photoanodes under 6 mW cm2 illumination at 355 nm. (d) Time responses of photocurrent densities of the photodetectors upon 6 mW cm2 UV illumination (l ¼ 355 nm) and 25 mW cm2 Vis illumination (l > 425 nm) at 0 V bias.
Table 1 Comparison of photoresponse performance of the photoelectrochemical type self-powered photodetectors based ZnO nanomaterials. Materials
Jsc(mA cm2)
Rise time (ms)
Light wavelength (nm)
Light intensity (mW cm2)
References
ZnO nanoneedles ZnO nanowires ZnO/Cu2O BHAs
0.8 4 107
100 100 140
365 Solar light 355
1.25 100 6
[15] [39] Our work
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3D branched architecture of ZnO/Cu2O BHAs possesses enlarged surface area, which contributes to speeding up the carrier transfer at interfaces and reducing interfacial reaction resistance.
4. Conclusions
Fig. 5. Spectral responsivity characteristics of the photodetectors with irradiance wavelength ranging from 300 nm to 700 nm under 0 V bias.
We have successfully synthesized ZnO/Cu2O heterojunction arrays for the self-powered photodetecting applications. Based on the photovoltaic effect, the photodetectors with ZnO/Cu2O BHAs photoanodes show excellent photosensing performance both in the UV and visible region at zero bias. They have high on/off ratios of 525 and 1945 for UV light (6 mW cm2, l ¼ 355 nm) and visible light (25 mW cm2, l > 425 nm), respectively, with a fast rise time of 0.14 s and a decay time of 0.36 s. The spectral response studies of ZnO/Cu2O BHAs show a large enhancement in photoresponse in the visible region. Our results indicate the photoelectrochemical type self-powered photodetectors based on metal oxides have great prospect in high-sensitivity and high-speed UVevisible light detecting applications.
Fig. 6. EIS measurements of ZnO NWA photoanodes and ZnO/Cu2O BHA photoanodes (a) in the dark and (b) under AM 1.5G illumination (70 mW cm2).
are prone to aggregate in Cu2O, increasing the carrier recombination. In addition, during the preparation process, the alkaline corrosion to the ZnO nanowires introduced unexpected defect states at the ZnO/Cu2O interface, which may act as recombination centers. The ZnO/Cu2O BHAs exhibited substantially greater photoactivity in the visible region from 400 nm to 500 nm than that of pure ZnO NWAs, mainly due to the enhancement in the light absorption caused by the Cu2O nanobranches. The visible responsivity peak located at a wavelength of 400 nm is proximately 8.2 mA W1. The results demonstrate that the Cu2O can increase the visible absorption, and the photogenerated electrons in Cu2O can migrate to the core ZnO nanowires. In order to get insight into the intrinsic electric and charge transfer properties of the samples, EIS plots were carried out both in the dark and under illumination, as shown in Fig. 6. It is generally known that the radii of the semicircles in the EIS plots reflect the charge transfer ability. After coating Cu2O, the radius was reduced compared to ZnO NWAs, suggesting more excellent electrical conductivity. This is consistent with the J-V curves measured in the dark in Fig. 4(c). Upon illumination, the arc radii were nearly two orders of magnitude smaller than those in the dark, because of the increased photogenerated carrier concentration. The ZnO/Cu2O BHAs have smaller radius, indicating higher photoirradiated carrier generation rate and faster interfacial charge transfer velocity. The
Acknowledgments This work was supported by the Natural Science Foundation of China (NSFC) (51474017), the Fundamental Research Funds for the Central Universities (FRF-TP-15-107A1) and the China Postdoctoral Science Foundation (2015M580979).
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