The enhancement of a self-powered UV photodetector based on vertically aligned Ag-modified ZnO nanowires

Abstract

A self-powered photoelectrochemical cell-type UV detector has been fabricated using Ag nanoparticle-modified ZnO NWs as an active photoanode and H₂O as the electrolyte. The fabricated device shows a high responsivity (0.37 A W⁻¹). The enhancement is attributed to the modification of the structure which enables more effective separation and directional transfer.

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Nanostructured materials are currently attracting extensive attention to reduce the cost and/or enhance the efficiency of electronic devices such as solar cells, photodetectors, etc. Due to the wide direct band gap (E_g ∼ 3.3 eV at 300 K) and large exciton binding energy (∼60 meV), there has been much research interest in ZnO due to its prospects in optoelectronic applications.^1,2 Especially, ZnO nanowires (NWs), with a high aspect ratio, provide not only a large surface area for charge transfer but also a long pathway for light absorption in terms of the optoelectronic applications in the near UV-wavelength regions, which makes it a promising material for UV detectors.^3–7 Up to now, various UV detectors based on ZnO have been reported such as metal–semiconductor–metal, p–i–n, p–n junction, or Schottky barrier planar-type structures.^5,8–11 However, in order to fulfil high photosensitivity, rigid epitaxial processes and single-crystal substrates are needed, which lead to high production cost.⁴

Recently, an emerging self-powered UV detector based on a ZnO/water solid–liquid heterojunction (SLHJ) exhibits a practical UV detecting performance, which is constructed without complicated epitaxial process.^12,13 This sandwich-like structure resembles a conventional dye-sensitized solar cell but without dye absorption. It is featured by an inherent built-in electrical field generated by Schottky barrier which acts as the driving force to separate the electron–hole pairs from recombination. This structure eliminates the needs for an external applied voltage, which presents a great deal of potential in self-powered UV detection. This self-powered photodetector demonstrates fast photoresponse speed (both decay and rise times are less than 0.5 s).^12,14 However, the conducted photocurrent and photoresponsivity are still low. In order to further improve the performance of ZnO based self-powered UV detectors, more research work should be conducted to optimize the structure of the ZnO photoanode and get a deep insight into the electric transport process.

Usually, a metal and a semiconductor form Schottky barriers at their interfaces as a result of their different work function and band alignment, therefore, contributing to a facilitative separation of photoexcited charges.^10,15 More recently, deriving from the benefit of the localized surface plasmon resonance (LSPR), noble metals have drawn an intensive interest.^16–18 To further propel the development of such UV-detectors, it is rational to deposit noble metal nanoparticles onto the surface of ZnO NWs for enhancing the performances of the corresponding devices. Metals like Au have electron-storage properties which improve charge separation efficiency in semiconductor–metal composite systems.¹⁷ These properties have been investigated in the fields of biomedicine, nanodevices and photo-catalysts. Compared with other metal–semiconductor heterostructures, Ag/ZnO with different nanostructures can be more easily prepared by a series of simple methods. It has been reported that Ag/ZnO composite system exhibits enhanced photocatalytic property, which mainly results from the inhibited recombination of the photoinduced electron–hole pairs.^19,20 Generally, the current Ag–ZnO heterostructures were obtained by randomly and uncontrollably loading Ag nanoparticles onto the surfaces of ZnO NWs.^19–21 In this work, to avoid the time-consuming and uncontrollable growth process, metallic Ag nanoparticles are attached on the ZnO NWs using a facile and practical strategy, i.e., photo-deposition reduction method, which is based on the redox reactions of aqueous chemical species.²² The size and amount of Ag nanoparticles can be tailored by adjusting the reaction parameters. The as-prepared Ag nanoparticles attached ZnO NWs serve as the active photoanode and H₂O as the electrolyte. By the attachment of Ag nanoparticles, the responsivity and photocurrent of the device increase markedly compared to the bare ZnO NWs based one.

The final products were prepared through two steps: the fabrication of ZnO NWs and the synthesis of modified ZnO NWs with Ag nanoparticles grafted on the NWs. All the chemicals used in this study were of analytical grade and used directly without any further purification. The ZnO NWs were synthesized through a modified hydrothermal method as reported before. In detail, zinc acetate dihydrate was dissolved in the mixed solution of ethanolamine and 2-methoxyethanol. The concentrations of both zinc acetate dihydrate and ethanolamine in the resulting solution are 0.75 M. The resulting mixture was then agitated and stirred at 60 °C for 120 min to yield a homogeneous and stable colloid solution. After stirring colloid solution at room temperature for 12 h, the final product was synthesized, which serves as coating solution.²³ Before the growth of ZnO NWs, a 30 nm ZnO seed layer was spin-coated on a fluorine-doped tin oxide (FTO) coated glass substrate. The seed layer was formed by spin coating the colloid solution at 3000 rpm followed by annealing in a furnace at 400 °C for 1 h. Subsequently, the seeded substrates with the coating side upside-down were immersed in a 100 mL growth solution containing 20 mM zinc acetate (Zn(CH₃COO)₂) and hexamethylenetetramine (C₆H₁₂N₄, HMTA) loaded in a Teflon liner stainless-steel autoclave at 90 °C for 6 h. After the reaction was completed, the grown ZnO NWs were carefully rinsed with deionized water (18.3 MΩ cm resistivity) and ethanol several times followed by a drying step in an oven at 60 °C. The Ag nanoparticles modified ZnO NWs were prepared by a simple photo-reduction method. Silver nitrate (AgNO₃) was dissolved in an aqueous solution consisted of 4 mL ethanol and 6 mL deionized water at various concentrations (0.001–0.004 mM) by an ultrasonic treatment step. Afterwards, as-prepared ZnO NWs substrates were immersed in the aqueous solution above and illuminated under a beam of 100 μW cm⁻² of ultraviolet light (λ = 365 nm) for 1–2 minutes. In the following step, the as-prepared modified Ag–ZnO NWs substrates were rinsed with deionized water and ethanol several times and dried at 80 °C in air. The structure of this kind of UV detector is the same as dye-sensitized solar cells (DSSCs) except for the content of dye molecular. In brief, the obtained Ag nanoparticles-modified ZnO NWs vertically grown on the FTO substrates were used as the active electrode. A 30 nm-thick Pt film was deposited on FTO glass by the electron beam evaporation technique and formed the counter electrode. A 60 μm-thick sealing material was sandwiched between these two electrodes to glue them together by heating in an oven at 110 °C for 2 minutes. Finally, some deionized water used as an electrolyte was injected into the space between the two electrodes. Then, a self-powered UV detector was fabricated. More specifically, all the ZnO NWs were grown under the same experimental conditions. Among them, some were used to fabricate ZnO NWs based devices directly and the rest were decorated by Ag nanoparticles. The exposed area of both bare ZnO NWs based device and Ag nanoparticles modified device are 3.2 cm², respectively. And all the testing processes were under the same testing condition.

The morphology and structure of the products were characterized by using a field emission scanning electron microscope (FE-SEM Hitachi S-4800) equipped with an energy-dispersive X-ray spectrometer (EDX). The current–voltage (I–V) characteristics were measured using an Agilent E5270B parameter analyser under ambient conditions. The temporal response of the UV detector was measured by illuminating the devices with a UVA-LED. The UV-vis absorption spectra were recorded with a spectrophotometer (UV-3600).

The typical SEM images of ZnO NWs and Ag nanoparticles-modified ZnO NWs are shown in Fig. 1(a) and (b). As shown in the images, the ZnO nanowires are grown almost perpendicularly on the FTO substrates with an average length of ∼2 μm and a diameter from ∼80 to 120 nm. It can be seen gaps are well distributed between the NWs so that the space are available to the penetration of electrolyte. When ZnO NWs were dipped into an aqueous AgNO₃ solution containing mixture of ethanol and deionized water under illumination treatment, the color of the surface of as-prepared ZnO NWs substrates turned to faint yellow indicating the attachment of Ag nanoparticles occurred immediately. The solutions with various concentrations of AgNO₃ were prepared. We find out the optimized concentration of the solution is around 0.001 mM. Ag nanoparticles would be collided to shell-like slices with big size which fully covered the top of the ZnO NWs once the concentration go beyond the given value. As shown in Fig. 1(b), nanoparticles are successfully and uniformly attached on the NWs. EDX mapping images were collected by scanning the whole area of Fig. 1(b). Fig. 1(c)–(e) show the image of each element corresponding to Zn, Ag, and O, respectively. These EDX mapping images confirm the successful deposition of Ag nanoparticles on ZnO NWs.

Fig. 1 SEM images of the ZnO NWs and Ag nanoparticles-modified ZnO NWs. (a) SEM image of ZnO NWs; (b) cross-sectional image of Ag nanoparticles-modified ZnO NWs; (c–e) EDX images of each element in the whole area showed in (b).

Typical I–V characteristics curves of the two UV detectors based on bare ZnO NWs and Ag nanoparticles-modified ZnO NWs under dark and 365 nm UV illumination of 60 μW cm⁻² are shown in Fig. 2(a) and (b), respectively. A diode behaviour of the photodetector is demonstrated from the dark I–V characteristics of the devices.

Fig. 2 (a) I–V characteristics of typical ZnO NWs/water UV detector in darkness and under the illumination of 60 μW cm⁻² of UV light (λ = 365 nm); (b) I–V characteristics of typical Ag–ZnO NWs/water UV detector in darkness and under the illumination of 60 μW cm⁻² of UV light (λ = 365 nm).

These devices can operate at photovoltaic mode without any external bias. It can be clearly seen that dark current is quite low for both detectors compared with the photocurrent at 0 V bias. More specifically, compared with bare ZnO NWs based device, the photocurrent of Ag nanoparticles-modified ZnO NWs based device has been greatly enhanced, achieving 22.3 μA cm⁻². This value is much higher than bare ZnO NWs based device of 6.75 μA cm⁻². Therefore, the Ag nanoparticles-modified ZnO NWs can enhance the photoelectric activity significantly.

High responsivity and fast time response are necessary parameters for UV detectors. The responsivity is calculated by the equation

where R is the responsivity, I represents the photocurrent, A is the active area of the UV device, and S is the irradiance of the UV light source which is measured by a standard UV power meter.²⁴ All parameters of the two devices are collected in Table 1. The responsivity of the bare ZnO NWs based device is 0.11 A W⁻¹, considerably higher than the corresponding values reported by other groups,^12,14,25,26 which are demonstrated in Table 2.

Table 1 The parameters of the UV detectors based on bare ZnO NWs and Ag nanoparticles-modified ZnO NWs^a

Samples	Idark (μA cm^−2)	Iphoto (μA cm^−2)	τg (s)	τd (s)	Responsivity (A W^−1)
a τg, τd: growth time constant; decay time constant.
ZnO	0.14	6.75	0.15	0.50	0.11
Ag–ZnO	0.69	22.3	0.14	0.52	0.37

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Table 2 Comparison of the photoelectrochemical cell-structured UV detectors

Samples	Photocurrent density (μA cm^−2)	τg (s)	τd (s)	Responsivity (A W^−1)
ZnO	6.75	0.15	0.50	0.11
Ag–ZnO	22.3	0.14	0.52	0.37
ZnO ref. 12	2.50	∼0.1	∼0.1	0.02
TiO2 ref. 14	3.13	0.15	0.05	0.02
TiO2 ref. 14	—	∼0.5	∼0.5	0.07

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Inspiringly, the responsivity of Ag nanoparticles-modified one reaches 0.37 A W⁻¹. The photocurrent time response of the devices was measured at 0 V bias under an intermittent irradiation of a 365 nm UV LED with a power of 60 μW cm⁻². The incident radiation is switched with an on/off interval of 10 s. Five repeated cycles are displayed in Fig. 3. As illustrated in Fig. 3, the photocurrent without external bias of Ag nanoparticles-modified ZnO NWs based device is considerably higher than the bare ZnO NWs based one and the photocurrent is observed to be repeatable. The rise time and the decay time of both devices are nearly the same (∼0.15 s and 0.50 s), implying a rapid photoresponse characteristic.

Fig. 3 (a) Photocurrent density response of ZnO NWs based device under on/off radiation of 60 μW cm⁻² of UV light (λ = 365 nm); (b) photocurrent density response of Ag–ZnO NWs based device under on/off radiation of 60 μW cm⁻² of UV light (λ = 365 nm).

To further study the benefit from Ag nanoparticles, the UV-vis absorption spectra and UV-vis diffuse reflectance spectra of the as-prepared Ag nanoparticles-modified ZnO NWs and the bare ZnO NWs are measured. As shown in Fig. 4(a), in the whole range of measuring wavelength from 300 nm to 700 nm, absorption enhancements are found for the Ag nanoparticles-modified ZnO NWs. More precisely, both the two curves show a steep absorption at wavelength of ∼365 nm, which is corresponded to the bandgap of ZnO. The obvious enhancement of the absorption at the wavelength of ∼365 nm derives from the surface plasmon resonance (SPR) absorption of Ag nanoparticles.

Fig. 4 (a) UV-visible light absorption spectra of ZnO NWs and Ag–ZnO NWs; (b) UV-visible diffuse reflectance spectra of ZnO NWs and Ag–ZnO NWs.

Moreover, a possible mechanism for the enhancement of photoelectric behaviours is proposed. It seems that Ag may be acting as a sensitizer for ZnO by absorbing extra photons through SPR, or it may be scattering light to increase optical path length and thus absorption in ZnO. According to Fig. 4(b), the values of diffuse reflectance of both samples are close to zero in UV-A range so that the increase of photocurrent density is not incurred by the scattering light. Thus, combined with the analysis of Fig. 4(a), we conclude that the increase of the photocurrent density in Ag modified device stems from the absorption of extra photons through SPR. To further prove our assumption, we have tested the performance of devices modified by Ag nanoparticles with different sizes (ESI†).

The working principle of the device is clarified in the following. Since the E_f of the ZnO is higher than the redox potential of the electrolyte, electrons transfer from the conduction band of ZnO into the electrolyte until a new equilibrium is reached. Afterwards, a space charge layer is produced, owing to the difference of the charge distribution from the material. Hence, a built-in electric field is subsequently formed due to the electric potential difference across the solid–liquid interface. When the UV light irritates on the Ag–ZnO/H₂O UV detector, the photons with energy exceeding that of the ZnO bandgap are absorbed, then electrons are excited from valance band to conduction band and electron–hole pairs are generated thereafter. Subsequently, the generated electron–hole pairs are forced to separate by the built-in electric field. In the next step, both the electrons from the collections of the sinks of Ag nanoparticles and the electrons directly travelling along the ZnO NWs get captured by the FTO electrode. These electors transfer into the external circuit and return to the Pt layer of the counter electrode. On the other hand, the holes from the valance band of ZnO are driven to the surface and get captured by the OH⁻ anion in the water (h⁺ + OH⁻→ OH˙). The radical OH˙ is then reduced to OH⁻ anion at the counter electrode (Pt/FTO) by the electrons returned from the external circuit.¹² Here the Pt serves as both a catalyst for the redox reaction and conducting road for the electrons. In this process, the circuit keeps running without any external bias as long as the appropriate UV light at certain wavelength maintained illumination.

In summary, vertical ZnO NWs have been successfully grown on a transparent conductive FTO substrate through a hydrothermal method and a modified photoelectrochemical cell-structured self-powered UV detector was fabricated. A facile and efficient method is implemented to deposit the Ag nanoparticles onto the surface of ZnO NWs. The Ag nanoparticles enhance the absorption of the UV light which amounts to a larger photocurrent. With the relatively high responsivity and rapid response, the UV detecting performance of our device is much more superior to the previously reported similar structured UV detectors.

Acknowledgements

This work was supported by National Natural Science Foundation of China under Grant No. 51302244, 51172204 and 91333203, Program for Innovative Research Team in University of Ministry of Education of China under Grant No. IRT13037, Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ13E020001, and Research Funds for the Central Universities under Grant No. 2014FZA4008 and 2015FZA4007.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08187d

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