Three-dimensional ZnO porous films for self-cleaning ultraviolet photodetectors

Abstract

Three-dimensional (3D) ZnO porous films composed of an interconnected skeleton were fabricated successfully through atomic layer deposition method using carbon nanoparticles as template. After surface modification, they showed an excellent superhydrophobic property with a contact angle larger than 160° and a sliding angle less than 1°. Based on the superhydrophobic 3D ZnO porous films, self-cleaning ultraviolet photodetector devices were fabricated. The devices exhibited a rise time of 42.03 s, a recovery time of 5.84 s and a responsivity of 9 mA W⁻¹ at a 5 V bias under a low light illumination of 14.38 μW cm⁻². The mechanism for the enhanced ultraviolet photoresponse from ZnO porous films is discussed.

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Introduction

Due to the wide range of both civilian and military applications such as environmental monitoring, high temperature flame detection and secure space-to-space communications, ultraviolet (UV) photodetectors have been becoming more and more important and have attracted a great deal of attention in recent years.^1–3 Although the traditional silicon based photodetectors are sensitive in the UV region, they are not solar blind so that costly filters are necessary to stop the low energy photons (visible and infrared light). With the use of wide bandgap materials such as ZnO, diamond, SiC, GaN and ZnS, the need for these filters could be eliminated. By virtue of the direct bandgap of 3.37 eV, high exciton binding energy and high thermal stability, ZnO has been seemed to be a good candidate for photon detect devices operating in harsh environments.^4,5

In the past few years, there were intensive studies on the UV response of one dimensional (1D) ZnO due to their large surface-to-volume ratio, reduced dimensionality of the active area, and optoelectronic property, which not only can prolong the photo-generated carrier lifetime but also shorten the carrier transit time.^6,7 Despite the good UV photoresponse, problems such as the integration of 1D ZnO into working devices, conformability, and large-scale fabrication are far from perfect due to the elaborate fabrication procedures including hydrothermal method, photolithography, electron beam lithography and focused ion beam.^8–11 In this context, the ZnO porous films an industry-degree fabrication of UV sensing materials is highly desirable seem to be a more attractive option for developing integrated optoelectronics devices over 1D nanostructure-based ones in terms of ease in fabrication, specific surface area, and most importantly, low cost.^12,13

Porous materials have drawn increasing attention owing to their unique structure and attractive applications in field emitters and optoelectronic devices.^14–16 Most of reported fabrication for porous materials need complex processes involving but with low throughput. Herein, we design a convenient procedure to fabricate the three-dimensional (3D) ZnO porous films composed of interconnected skeleton through atomic layer deposition (ALD) method using carbon nanoparticles (CNPs) networks template. The CNPs networks were synthesized via a simple and low-cost flame synthesis process.

In most cases, UV photodetectors are applied in some special environment, thereby the ZnO nanostructure applications for UV photodetectors are hindered due to their hydrophilic characteristics. If the ZnO nanostructure can be rendered superhydrophobic property, one could make them applicable for the diverse and growing practical applications. So far numerous techniques have been developed to synthesize superhydrophobic surface.^17–20 Among these schemes, it is worth noting that one of the convenient and efficient methods is coating hydrophobic materials on porous materials surface. Deng et al. obtained a superhydrophobic and superoleophobic porous candle soot by coating a 25 nanometer-thick silica shell on the surface.¹⁹ In this paper, we synthesize three-dimensional (3D) ZnO porous films through atomic layer deposition (ALD) method using carbon nanoparticles (CNPs) as template. By further surface hydrophobic modification, a contact angle (CA) up to 160.6° while the sliding angle (SA) less than 1° were obtained on the ZnO porous films. The ZnO porous films based UV photodetectors show a rise time of 42.03 s and a responsivity of 9 mA W⁻¹ at a 5 V bias voltage under low light illumination intensity of 14.38 μW cm⁻².

Experimental

CNPs synthesize

To fabricate the ZnO porous films, we first synthesized CNPs template using a common alcohol burner as tool in a space without apparent cross-ventilation. The ignition of the wick was keep without moving for at least 15 min to obtain a steady flame. After that, about 6 cm above the wick, a quartz slide was held above the flame of an alcohol burner to grow CNPs. The growth process lasted for 3 min, and then the glass slide was taken out of the flame, cooled to room temperature.

ZnO porous film synthesize

ZnO with several nanometers in thickness were grown on the CNPs films in a homemade ALD system with a process pressure of ∼200 mTorr. ZnO was deposited from zinc diethyl (97%, Aldrich) and distilled water at 200 °C under Ar atmosphere, using 0.05 s precursor pulses, a reaction time of 10 s and a purge time of 30 s. According to the TEM observation, the average growth rate of ZnO was about 1.3 Å per cycle. To obtain the ZnO porous films and to increase their degree of crystallinity, the composite film was calcined at 500 °C in air for 1 h.

Hydrophobic functionalization

Firstly, 1 ml 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17), 1 ml acetic acid (pH = 3) and 100 ml alcohol were mixed to form the modification solution. After that, the glass slide coated with the ZnO porous film was immersed into the above solution for 20 h at room temperature. Finally, the glass slide was washed with alcohol and dried by N₂, and then the glass slide was baked at 100 °C in air to completely remove the alcohol for 30 min.

Device fabrication

Two silver electrodes, which distance is ∼0.5 mm from each other, were printed on the top of the as-prepared superhydrophobic ZnO porous film. The silver paste was used a little excessive to ensure that it can fully penetrate into the porous structure, and accordingly to guarantee the enough contact between ZnO and silver paste, as a result, a good ohmic contact between the silver electrode and ZnO was obtained.

Structural characterization

The morphologies of the samples were characterized using field emission scanning electron microscope (SEM, FEI Nova NanoSem450) and transmission electron microscope (TEM, FEI Titan Probe Corrected microscopy). X-ray diffraction (XRD) data was obtained on a Bruker D8 Advanced diffractometer (Cu K_α radiation). The content of the functional groups was characterized by Fourier transform infrared absorption spectroscopy (FTIR, VERTEX 70) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, ESCALab250). The nitrogen adsorption–desorption isotherms were measured at −196 °C using a Micromeritics tristar II 3020 system. The specific surface area of the ZnO porous structures was determined by the nitrogen adsorption data in the relative pressure range from 0.05 to 0.2 according to the Brunauer–Emmett–Teller (BET) equation. By means of researching the adsorption branches of the nitrogen isotherms, the Barrett–Joyner–Halenda (BJH) method was introduced to determine the pore-size distribution curve. A SL200B contact angle meter (Kino Industry, USA) was employed to measure the surface wettability of the device.

UV response measurements

Before testing the performance of the as-prepared self-cleaning UV detector, spectral transmittance (or absorption) of the device ranging from 200 to 800 nm was measured using a spectrophotometer (UV-2550), thereby, it can guarantee a strong response to the UV irradiation from a UV lamp (365 nm); on the other hand, the incident power density of the UV lamp was determined as well.

Next, we determine the metal–semiconductor contact type of the device is ohmic contact whether by mapping out the I–V curve in the dark and the UV light irradiation condition (365 nm), respectively.

After completion of the above work, the performance of the device determined using a semiconductor performance analysis system: the device was first placed in dark environment for some time until the dark current was stable, after that, the UV lamp was placed about 1 cm above the device quickly to start the test. When mapping the I–V curve, the voltage range of −5 V – 5 V, and the voltage change in steps of 0.1 V; when mapping the I–T curve, the applied voltage is 5 V with the sampling time interval of 0.1 s.

Results and discussion

Fabrication process of ZnO porous film is illustrated in Fig. 1a. Porous CNPs film was firstly deposited on the quartz glass by a simple flame synthesis method.²¹ Next, a ZnO thin layer was grown on the surface of the template through ALD method, the as-prepared composite was then calcined at 500 °C in air for 1 h to combust the CNPs template. Thus a pure crystalline ZnO porous film was obtained. The top-view of scanning electron microscope (SEM) images of the CNPs film, the ZnO/CNPs composite and the ZnO porous film are described in Fig. 1b–d, respectively. As can be seen, the CNPs are homogeneous with a uniform size of 20–30 nm, forming a fractal-like network. The thickness of ZnO shell was about 8 nm (Fig. 1g). After coated by ZnO shell, comparing with the CNPs template, the ZnO/CNPs composite are only slightly larger in particle size, while the overall morphology without significant change (Fig. 1c). The porous structure can still be maintained after the CNPs template was eliminated and pure ZnO film was obtained (Fig. 1d and e). The surface morphology of the fabricated ZnO porous film was probed through atomic force microscope, which showed that the arithmetical mean deviation of the profile (R_a) and the point height of irregularities (R_z) was about 6.03 and 105.3 nm (Fig. S1†), respectively. This result was similar to CNPs-templated films reported in the literature.^14,20 This morphology can be further approved by low- and high-resolution TEM images. In Fig. 1f, the dendrite-like morphology of ZnO can be seen and the nanoparticles were curved and twisted with each other, which were composed of the network-like structures. The high-resolution TEM image in Fig. 1h reveals that the ZnO nanoparticles have good crystallinity with ∼0.26 nm uniform lattice spacing. The corresponding selected area electron diffraction (SAED) pattern of ZnO nanoparticles (inset in Fig. 1h) can be indexed to the Wurtzite structure of hexagonal ZnO. This is in good agreement with the XRD analysis (Fig. S2†). The result indicates that CNPs have been oxidized, leaving ZnO only after annealing at 500 °C for 1 h in air. The nitrogen adsorption–desorption isotherm at 77 K for ZnO porous films demonstrates that its specific surface area is up to 21.97 m² g⁻¹ in accordance with the standard BET method (Fig. S3†). The porous structure and large surface area of ZnO porous film could contribute to larger light absorption, leading to enhanced photoelectrical response.

Fig. 1 The fabrication approach and morphology of the 3D ZnO porous films. (a) Schematic of approach for the CNPs-templated mesoporous 3D ZnO porous film, SEM image of (b) CNPs, (c) ZnO/CNPs and (d) ZnO porous film, the inset is the corresponding high-resolution SEM image, (e) the cross-section view of the ZnO porous film, (g) the TEM image of the ZnO/CNPs, (f) low- and (h) high-resolution TEM images of ZnO interconnected nanoparticles, the inset in (h) is the corresponding SEAD.

For applications on various conditions such as environmental monitoring and missile launching detection, the UV photodetector needs to be not only thermal and chemical stable, but also superhydrophobic. For a material surface, once the CA larger than 150° and the sliding angle less than 10°, it can be considered as superhydrophobic material, which could guarantee the surface remain clean despite the surroundings.^22–25 Based on the above definition, the so-called self-cleaning function can be defined as following: water droplets can roll off easily on a superhydrophobic surface, taking the dirt away at the meanwhile.^26,27 Generally, the superhydrophobic property result from both of the very rough surfaces and low-surface-energy. In our experiment, to reduce the surface energy of the device, the hydrophilic ZnO porous films were coated with FAS-17 through a simple wet chemical method. Meanwhile, the light transmittance performance of the functionalized ZnO porous films just drop a little compared with it without modification (Fig. S4†), which provides the possibility for the application in photoelectric devices.

The high-resolution TEM image of the as-fabricated functionalized ZnO porous films is displayed in Fig. 2a. It can be seen that FAS-17 has been successfully coated on the surface of ZnO particles uniformly with a thickness of ∼2 nm. FTIR and XPS were also introduced to characterize the functionalized ZnO porous films to evaluate the modification effect. Fig. 2b shows the FTIR spectrum of ZnO porous films before and after functionalized by FAS-17. According to the FTIR spectrum, there are some peaks both present on the two samples: the strong and broad absorption peak located on 3437 cm⁻¹ can be assigned to the stretching vibrations of O–H, while the absorption peak located on around 1630 cm⁻¹ is result from the –OH bending vibration.²⁸ In addition, the absorption band in the range of ∼430 cm⁻¹ for the samples can be assigned to the stretching vibration of Zn–O.

Fig. 2 (a) High resolution TEM image of ZnO interconnected nanoparticles after FAS-17 modification, (b) FTIR spectrum and (c) XPS spectrum of the 3D ZnO porous film before and after FAS-17 modification.

Whereas there are some new peaks appear in the fingerprint region after modification: the absorption peak located on 1065 cm⁻¹ can be assigned to the asymmetric stretching vibration Si–O–C, peaks located on about 802 cm⁻¹ and 1151 cm⁻¹ are the characteristic absorption peaks of Si–O–Si and C–F, respectively. These fingerprint-region peaks confirm the successful modification on the ZnO surface of FAS-17. What's more, XPS spectra (Fig. 2c) further confirms the existence of the functional silicon and fluorine groups on the ZnO surface which peaks positioned at 151.6 eV and 685.5 eV, 832.0 eV, 856.9 eV respectively. The above TEM and FTIR characterizations indicated that the ZnO porous films have been modified by FAS-17 successfully.

To quantify the surface wettability of the as-prepared device, a SL200B contact angle meter (Kino Industry, USA) was employed to measure the apparent CA and SA. For the functionalized ZnO porous films, a water droplet gently deposited on the surface shows an apparent CA above 160° (Fig. 3a) and rolls off easily, which is much larger than that data (8.2°) without modification (Fig. S5†), demonstrating the excellent surface superhydrophobicity.²⁹ The functionalized ZnO porous film shows a very small slide angle less than 1°. This phenomenon shows that the droplet on the film is stay in the Cassie state, where the water droplet upon the rough structure thereby the air is trapped under the water droplet (Fig. 3b), and a small contact area between water droplet and solid surface is caused, leading to a large CA and a small SA. Fig. 3c–f displayed the time-resolved images of the rolling off process of the water droplet on the functionalized ZnO surface. Owing to the extremely low adhesion of the coating to water, the water droplet rolled off immediately. This result shows that the devices remain dry without considering the surroundings.^22–24 In addition, the functionalized ZnO surface could maintain its structure after water impact (Fig. S6 and Video S1†), and showed oleophobic property (Fig. S7†).

Fig. 3 Superhydrophobicity of the device surface. (a) A 6 μL water droplet was deposited on the surface of 3D ZnO porous film after superhydrophobic modification which possesses a static contact angle of 160.6°, (b) the Cartoon of a water drop deposited on the fractal-like interface, (c)–(f) time-resolved images of the rolling off process of the water droplet on the modified ZnO surface.

The photoelectric response performance of the fabricated UV photodetector device based on ZnO porous films was measured and the schematic diagram of the device was displayed in the inset of Fig. 4a. A mercury lamp (365 nm, 14.38 μW cm⁻²) was used as UV light source to illuminate the device and the photocurrent was recorded at a constant bias of 5 V. The symmetrical and linear characteristics of the photocurrent–voltage (I–V) curve, which measured under dark and illumination conditions, respectively, confirm the good ohmic contact between the Ag electrodes and ZnO porous films, as shown in Fig. 4a.³⁰ The typical photocurrent versus time (I–T) characteristic under UV illumination was shown in Fig. 4b. The device shows a sharp response with a high photocurrent switching ratio of 6.1. The response and recovery time after the FAS-17 coating were 42.03 s and 5.84 s, respectively. The responsivity is 9 mA W⁻¹ at a 5 V bias under low light illumination of 14.38 μW cm⁻². The response time value of the self-cleaning device is much better than those of pure ZnO thin films upon UV light illumination with a similar condition in air at room temperature.^30–32

Fig. 4 Typical (a) I–V curve and (b) I–T curve for the ZnO porous films based device after modification under UV illumination (14.38 μW cm⁻²), the inset in (a) is the structure of UV photodetector device.

The enhanced UV photoelectrical response could be attributed to the excellent photoelectrical characteristics and novel structure of the fabricated devices. Due to their high specific surface area, the optoelectronic response performance of ZnO nanomaterials was mainly decided by their surface state, which would induce upward band bending near the surface and trap holes.³³ In dark, oxygen molecules would absorb on the surface of ZnO nanomaterials and deplete electrons, therein, thus a thin depletion layer with low electrical conductivity would be created. Electron-hole pairs would be generated upon UV illumination, and the holes would migrate to the surface of ZnO due to the band bending and discharge the adsorbed oxygen molecules, result in an accumulation in electron concentration and higher electrical conductivity.^34,35 In our experiment, benefitting from the unique 3D porous structure, the 3D ZnO porous film could largely elongate the average length of the path before light escape from it, thus the light absorption would increase. In addition, the high specific surface area makes light act on ZnO surface adequately, leading to a fast response upon UV illumination. The last but not the least, the coated fluorine silane had the similar effect as the oxygen (capture the free electrons in dark and trap holes under UV illumination), which could increase the lifetime of photo-generated carriers and further improve the optoelectronic response performance of the ZnO porous films.

Conclusions

In summary, we have demonstrated a simple procedure to assemble the self-cleaning UV photodetectors based on 3D ZnO porous films. After functionalized by FAS-17, the devices showed a superhydrophobic property with a contact angle of 160.6° and a sliding angle less than 1°. The UV photodetector exhibits a rise time and recovery time of 42.03 s and 5.84 s, respectively, and a notable photocurrent improvement of 6.1 times under the low power density of 14.38 μW cm⁻². Both the large specific surface area and the novel structure of ZnO porous films contributed to the enhanced UV photoelectrical response performance. Considering the advantages of fabrication procedure and the superhydrophobic property, the devices have potential for use in large-area UV photodetector applications.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (51322210, 61434001, 51202077), Director Fund of WNLO, China Postdoctoral Science Foundation (2013M531691), and Open Project of Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology (KLEISEAM201302). The authors thanks to the facility support of the Center for Nanoscale Characterization & Devices (CNCD), WNLO of HUST, and the Analysis and Testing Center of HUST.

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Footnote

† Electronic supplementary information (ESI) available: AFM images, XRD pattern, adsorption–desorption curve and UV-vis spectra of the 3D ZnO porous film, etc. See DOI: 10.1039/c5ra13372f

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