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We report on a solution-based method to fabricate a multidimensional heterojunction composed of one-dimensional zinc oxide nanorods (ZnO NRs) decorated with zero-dimensional cupric oxide nanoparticles (CuO NPs). The ZnO NRs were vertically grown on a reduced graphene oxide (rGO) thin film, and the sensing properties of the synthesized heterojunction were investigated under ultraviolet (UV) irradiation at room temperature to assess their practical application. The CuO NPs decorated on the ZnO NRs play an essential role in creating numerous p-n heterojunctions at the interface and remediating oxygen-related defects in the ZnO NRs. Relative to the ZnO NR structures without CuO NPs, the CuO/ZnO multidimensional heterostructures show higher sensitivity and faster response, demonstrating their potential use as UV sensors.
© 2015 Optical Society of America
1. Introduction
Zinc oxide (ZnO)-based semiconductors are considered promising photonic materials in the ultraviolet (UV) region as a result of their n-type direct band gap energy of 3.37 eV and large exciton binding energy of 60 meV [1,2 ]. When compared to bulk ZnO and ZnO thin films, ZnO nanostructures exhibit novel properties in terms of their optics, electrical transport and photoconductivity due to quantum confinement effects and an increased surface area. These properties lead to various applications in electronics [3] and optoelectronics [4,5 ] UV detection is one of important applications for nanostructural ZnO devices due to its wide band gap and thermal stability. However, p-type doping of ZnO remains a challenge [6], so ZnO homojunction or heterojunction structures are still limited in their ability to detect UV radiation. Interestingly, heterostructures can be formed with components that have different functionalities but favorable band alignment, and this can lead to the functional integration of the properties of both the materials and can also result in novel interface effects and phenomena [7]. Therefore, extensive efforts have been made to increase the separation efficiency of the photo generated electron-hole pairs within the ZnO nanocatalysts. ZnO-based heterostructural nanocatalysts, such as ZnO-CuO, ZnO-ZnS, ZnO-SnO2, and ZnO-NiO [8–10 ] may improve photocatalytic activity by extending the photo response range and increasing the electron-hole pair separation efficiency. Among these materials, ZnO-CuO nanocomposites (NCs) have been identified as promising candidates for use during hydrogen production, gas sensing, and photodegradation [11]. CuO is one of few p-type direct band gap (~1.7 eV) metal oxide-based semiconductors. It has the additional advantages of being non-toxic, low cost and abundant as a starting material. The ZnO-CuO NCs heterostructure is expected to possess improved abilities, such as faster electron-hole separation and the suppression of recombination by the mutual transfer of the photo-generated electrons or holes in the heterojunctions.
Recently, two-dimensional (2-D) graphene and graphene oxide (GO), which have remarkable physical and chemical properties, have emerged and are being considered for a variety of applications. This study investigates the integration of multidimensional nanostructures consisting of zero-dimensional (0-D) CuO nanoparticles (NPs), one-dimensional (1-D) ZnO nanorods (NRs) and 2-D materials to pave the way for new applications because such structures have unique, promising properties that are distinct from those of the individual components. Among various synthesis methods, low-temperature solution-based process has become the most popular and suitable approach to fabricate multidimensional heterojunction nanostructures because it enables the synthesis of different nanomaterials with conventional and unconventional substrates while maintaining compatibility with several coating techniques. In this letter, a heterojunction composed of materials of different dimensions, including 0-D CuO NPs, 1-D ZnO NRs, 2-D rGO and graphene is produced using a solution-based process and is tested under UV irradiation to assess its use toward practical applications.
2. Experiments
Figure 1 shows a schematic diagram of the solution process that was used to fabricate the multidimensional heterojunction structure for UV sensing. The process begins with spray-coating the GO on an ITO/coated glass substrate to provide an even surface for the ZnO NR growth. This step is followed by the hydrothermal growth of the ZnO NR arrays. The solution process to grow the vertically aligned ZnO NRs is described in detail elsewhere [12]. The ZnO NRs grown on the substrate are preheated for 5 min in an air atmosphere at 300 °C. Afterward, aqueous solution-synthesized CuO NPs are sprayed onto the ZnO NRs/reduced GO. Then, graphene films grown via chemical vapor deposition are transferred onto the surface of the ZnO NR arrays. A 100-nm thick Au layer is deposited as the top electrode using electron beam evaporation. The morphologies and structures of the ZnO NR arrays were characterized via atomic force microscopy (AFM, Veeco 3100), scanning electron microscopy (SEM, S-4700, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) with energy-dispersive spectrometry (EDS). The optical properties of the heterojunction structures were investigated using photoluminescence spectra with a He-Cd excited source (325 nm), and the current-voltage (I-V) characteristics were measured under exposure to a UV lamp using a semiconductor characterization system (Keithley 4200-MSTech).
3. Results and discussion
Figures 2(a) and 2(b) respectively show typical SEM images for ZnO NRs grown on ITO films and GO films. While ZnO NRs grown on the ITO films are distributed non-uniformly as a result of the rough surface of the ITO, those grown on the rGO films are uniformly aligned and have a length of about 1 µm with a diameter of around 80 nm. AFM measurements were employed to investigate the surface morphologies of the ITO and rGO films. Figure 2(c) shows that many pores are located along the ITO grains and the surface roughness was found to be 3.8 nm. These grains disturb the formation of the ZnO seed layer that results in non-uniform ZnO NRs arrays. To minimize the grain effect, the GO with two-dimensional sheets are spray-coated onto the ITO surface to form an even seed layer. In fact, the surface roughness significantly reduces from 3.8 nm to 2.9 nm after spray-coating of the GO film. Subsequently, a Ga-doped ZnO solution is spin-coated on the substrate at 3000 rpm for 20 s. Ga-doping concentration in the buffer layer could facilitate the size control of ZnO NRs. As Ga is incorporated into ZnO, a Zn host atom is replaced by a Ga atom at the lattice point of the ZnO crystal, resulting in decreasing the crystal grain size. An increase in Ga-dopiong concentration in the buffer layer reduced the size of the grain in the buffer layer, resulting in longer and thinner ZnO nanorods.12 The Ga-doped ZnO-coated substrate is then heated in air at 300 °C for 1 h to form a seed layer to grow the ZnO NRs. This process can also reduce the oxygen functional group of GO [13] and can improve the crystallinity of the ZnO NRs. The surface of the reduced GO (rGO) is flat, as shown in Fig. 2(d). The seed layers are not coated uniformly until the rGO films are covered on the ITO surface, and as a result, well-aligned ZnO NRs are grown on the rGO films.
Fig. 2 The birds-eye view of the SEM micrographs of (a) the ZnO NRs grown on the ITO substrate and (b) grown on GO films coated on the ITO substrate. The AFM images of the ITO substrate (c) before and (d) after spray-coating the GO films.
Figure 3(a) shows a TEM image of CuO NPs decorated on smooth and straight ZnO NRs with a diameter of ~80 nm. The heterojunction interface between the CuO NPs and ZnO NRs obviously formed, as can be seen in Fig. 3(b). The EDS analysis from different position of a single ZnO NRs decorated with CuO NPs were shown in Figs. 3(c)-3(e) to further confirm the atomic concentration of O, Cu and Zn varying according to the location. Our results exhibited that Cu concentrations at the top-, middle- and bottom-ZnO NRs are 69.7%, 26.4% and 17%, respectively. It means that CuO NPs with an average diameter of 5 nm are unlikely to distribute evenly over a ZnO NRs; it was high at the top and low at the bottom of the ZnO NRs. A possible explanation for this phenomenon is that the top surface of ZnO NRs has a higher chance to form a junction with CuO NPs than its bottom during spray-coated process.
Fig. 3 TEM images and EDS analyses for the atomic concentration of the CuO NPs coated on the ZnO NRs.--
The room-temperature PL spectra of the ZnO NRs and the ZnO-CuO NCs are shown in Fig. 4 for comparison. The PL spectra are composed of a UV centered at around 360 nm and two visible emission peaks located at 570 nm (green) and 700 nm (red), respectively. The sharp narrow UV emission peak revealed that a well-defined material with a bandgap of ZnO was achieved because this peak is originated from the recombination of excitons [14]. Whereas, two separate emission peaks in the visible range related to the deep levels defects of the synthesized ZnO NRs. The green emission might be due to the recombination of electrons at the conduction band with holes trapped in oxygen-related defects; the red emission peak can be attributed to interstitial oxygen defect. An intensities ratio between UV emission (IUV) and the visible (VIS) emission (IVIS) were often used to evaluate the deep levels defects of the produced ZnO NRs. It is clear to see that IUV/IVIS ratio of CuO NPs coated ZnO NRs is significantly higher than that of bare ZnO NRs sample, which implied that lower defects were induced after coating CuO NPs. The effect of the ZnO-CuO NCs on the photoluminescence properties needs to be further investigated since it has been seldom reported in the literature. Generally, defects in ZnO are either oxygen vacancy or formation of interstitial Zn. A possible explanation for this observation is that a formation of the heterojunction between CuO NPs and ZnO NRs might produce sufficient supplies of O, which facilitates the reduction of oxygen vacancy defects and surface defects at the origin of the interband radiative recombination. In addition, the appearance of two emission peaks in visible range is quite different from the common spectra in literature for ZnO NRs in which a broad defect- related peak is often observed. It is extensively reported that this broad band is a superposition of different defect bands emitting in different wavelengths. This is probably one reason for debate, since different samples have different defect configurations due to different growth methods and growth conditions. It should be noted that in this work ZnO NRs were grown on rGO with the use of Ga-doped ZnO seeded-layer. Thus, the use of rGO as well as Ga-doped ZnO seeded-layer may have synergistic effect on varying defect configuration of synthesized ZnO NRs, leading to a different broad emission peak in visible range.
Figure 5(a) shows the typical current-voltage (I-V) characteristics of the ZnO NRs without and with CuO NPs in the dark and under illumination with UV at a wavelength of 370 nm. The I-V curves are linear at around a zero applied bias, which indicates a good ohmic behavior of the Au contacts with the ITO/rGO/ZnO NRs/graphene structures. The dark current for the bare ZnO NRs and the ZnO-CuO NCs is respectively 10.1 mA and 6.5 mA at 1 V, with the lower dark current being desirable for better sensors.
Fig. 5 (a) IV characteristics of the bare ZnO NRs and CuO-ZnO NCs before and after illumination with a wavelength of 370 nm and (b) schematic diagram showing charge generation and transfer by UV light. The rise and fall time for (c) bare ZnO NRs and (d) CuO-ZnO NCs under Xenon lamp illumination.
The photoconduction mechanism in the ZnO NR photodetector occurs as follows. (1) Oxygen molecules are absorbed at the surface of the ZnO NRs when the ZnO NRs are exposed to air, and the absorbed oxygen molecules capture the free electron from the conduction band that is present in the n-type semiconductor, forming a negative space charge layer near the surface and resulting in a higher resistivity. (2) Electron-hole pairs are photogenerated under UV illumination with photon energy larger than the semiconductor band gap. (3) The holes then migrate to the surface along the potential slope form by band bending to neutralize the negatively charged adsorbed oxygen ions, and therefore, the oxygen is photodesorbed from the surface. The unpaired electrons are collected at the anode when an electric field is applied, which increases the conductivity. This hole-trapping mechanism through oxygen adsorption and desorption at the surface of the ZnO NRs increases the density of the trap states due to the dangling bonds at the surface, which consequently enhances the photoresponse. When the UV light is switched off, the oxygen molecules are reabsorbed on the surface of the ZnO NRs and revert to their initial state.
The following reaction has been proposed to take place:
The UV sensing performance is evaluated by measuring the ratio of the UV illumination current versus the dark current (IUV/Idark). At a constant voltage of 1V, the IUV/Idark ratios were 3.8 and 7.7 respectively for a device without and with CuO NPs. This result clearly indicates a twofold increase in the sensing activity when the spray-coating of CuO NP was applied. It should be noted that the dark current of the ZnO NRs decreases from 10.1 to 6.5 mA while the UV photocurrent increases from 38 to 50 mA when the CuO NPs are decorated on the surface of the ZnO NRs. These results indicate that the photoconductance (Gpc = I/V) of the ZnO-CuO NCs is higher than that of bare ZnO NRs. Note the remarkably lager effect caused by the CuO NPs covering the surface of the ZnO NRs. The increased photoconductance can be attributed to the higher charge carrier concentration associated with the ZnO/CuO heterojunction and the lower concentration of charge trapping defects at the surface of the ZnO-CuO NCs. Moreover, there is a favourable p-n junction formation which helps in the separation of electron-hole pairs in the UV light and the suppression of recombination, as shown in Fig. 5(b). Figures 5(c) and 5(d) show the photo-response that was measured from the device without and with CuO NPs at a bias of 1 V, with the UV light alternately turned on and off. When compared to a bare ZnO structure, the CuO-ZnO NCs structure can be reversibly switched between low and high conduction states with good stability and reproducibility. With respect to the rising and falling edges, the rise and decay times for the bare ZnO structures are estimated to be 22 and 15 s, respectively, which are slower than those of the CuO-ZnO NC structures (rise/decay time: 11.3/9 s). When the CuO NPs were spray-coated on the surface of the ZnO NRs, a favourable p-n junction formed, which helps in separating the electron-hole pairs that are generated in the presence of UV light and avoiding recombination [15]. Since the conduction and valence bands for CuO lie above those of ZnO, which favor the thermodynamic transfer of excited electrons and holes, the carrier separation in different semiconductors effectively inhibits the recombination of electron-hole pairs and promotes the enhancement of the photocurrent within the CuO/ZnO heterojunction. In addition, we clearly see overshooting feature in the photo response measurement for the ZnO NRs (Fig. 5(c)) when the UV light were switched from on to off state. As inferred from PL results, distinct response spectra for bare ZnO NRs and ZnO NRs coated with CuO NPs is consistent with the oxygen-related defect. The higher defect-related emission peak of the bare ZnO NRs in Fig. 4 implied that oxygen-related deep defect levels were inherently formed during hydrothermal growth. These defects can trap photo-generated carriers therefore contribute to the degradation of the photo-response of the materials when irradiated with UV light due to non-radiative recombination.4. Summary
A solution-based process was developed to produce a prototype of integrated nanomaterials of various dimensions, including 0-D CuO NPs, 1-D ZnO NRs and 2-D graphene. The ZnO NRs were synthesized on rGO films through a hydrothermal method, followed by spray-coating the CuO NPs onto the ZnO/rGO. Our results indicate that the CuO NPs play an important role in remediating the oxygen related defects of ZnO NRs and forming a p-n heterojunction at the CuO/ZnO interface with a graphene film that allows the individual ZnO NRs to connect to each other. Consequently, the optical sensing device fabricated based on the proposed heterojunction exhibits a higher sensitivity to UV light illumination with a faster response.
Acknowledgments
This work was supported by the Basic Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A1A2008104), Basic Science Research Program through the NRF funded by the Ministry of Science, ICT & Future Planning (2010-0019694) and the BK21 Plus Center for future Energy Materials and Devices.
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