Parikshit
Sahatiya
and
Sushmee
Badhulika
*
Indian Institute of Technology Hyderabad, Hyderabad 502205, India. E-mail: sbadh@iith.ac.in; Fax: +91-40-23016032; Tel: +91-40-23018443
First published on 16th September 2015
Here, we report a simple, one-step method for the in situ synthesis and alignment of a single graphene-doped zinc oxide (Gr–ZnO) composite nanofiber fabricated by electrospinning across electrodes and its subsequent use for ultraviolet (UV) detection. The study involves the optimization of the calcination temperature and time-dependent electrospinning for alignment of the Gr–ZnO composite nanofiber across gold electrodes. SEM analysis and Raman studies revealed the presence of ZnO embedded within uniformly distributed Gr flakes throughout the surface of the composite fiber. XRD analysis of Gr–ZnO confirmed a highly crystalline ZnO structure with amorphous graphene. Different weight percentages of graphene flakes were used to synthesize the composite to determine the optimum composition that enabled the synergistic contribution of both ZnO and graphene for UV sensing. I–V measurements under UV illumination showed that the composite to exhibit superior UV sensing performance with a ∼1892-fold increase in conductance for 0.5 wt% of graphene. The underlying mechanism of charge transfer of photogenerated electrons and holes from ZnO to graphene under UV illumination was studied. The method presented in this study provides a simple, effective and economically viable strategy for the alignment of various composite fibers and can therefore be used in a wide range of sensing applications.
In this study, we report the one-step in situ synthesis and alignment of a single graphene-doped ZnO nanofiber by the use of electrospinning. Graphene, which is a two-dimensional carbon monolayer, has gained much attention in electronics and optoelectronics because of its excellent charge carrier transport mobility, high electrical conductivity, high optical transmittance and high mechanical stability.19 As well as these properties, its work function plays an important role in fabricating a UV sensor when used in conjunction with ZnO, as the fact that graphene's work function is lower than the conduction band of ZnO allows the transfer of photogenerated electrons to graphene. We demonstrate that time, which in this case is defined as the duration of operation of electrospinning, is a vital parameter for the alignment of a single nanofiber between pre-patterned electrodes. Our key strategy involves the optimization of the operating time for fabricating a single aligned Gr–ZnO nanofiber device in a controllable and reproducible manner. By varying the operating time, the number of fibers deposited between the electrodes could be controlled. The device was then evaluated for UV detection. Unlike conventional ZnO-based UV sensors, which suffer from issues related to the recombination of charge carriers, this approach utilizes the excellent transport properties and high optical transmittance of graphene to enhance the performance of the UV sensor. To the best of our knowledge, this is the first time that a detailed study of the effect of the electrospinning time on the density of ZnO-based fibers and subsequent in situ alignment of a single graphene–ZnO nanofiber composite for UV sensing has been reported.
Gr–ZnO nanofiber between the gold electrodes. Thereafter, the devices were calcined at various temperatures such as 400 °C, 500 °C and 600 °C to remove organic contents and residues. The complete process, which includes the fabrication of the gold electrode by photolithography and electrospinning, is pictorially shown in Fig. 1.
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Fig. 1 (a) Schematic of the microfabrication of gold electrodes; (b) electrospinning with collector as pre-patterned electrode; (c) UV sensing with single aligned Gr–ZnO nanofiber device. |
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Fig. 3 Graph demonstrating the near-linear relationship between no. of fibers and operating time of electrospinning for 0.5 wt% graphene. |
Composite nanofibers that were synthesized at 4 seconds electrospinning time were calcined at different temperatures to study the morphology of the fibers with increasing temperature. Fig. 4a and b show high-magnification SEM images of pure ZnO and Gr–ZnO nanofiber, respectively, both calcined at 400 °C.
A pure ZnO nanofiber exhibits a smooth surface morphology, whereas the composite displays flake-like structures uniformly all over the surface of a ZnO nanofiber. The morphology of the composite, as seen in Fig. 4b, is a ZnO nanofiber embedded in graphene flakes. Furthermore, as can be clearly seen, the flakes are transparent, which is an inherent property of graphene. The diameter and morphology of the composite fibers changed based on the composition of the solution used for electrospinning. Upon the addition of graphene, the conductivity of the solution changed. As the synthesis of the composite was achieved by electrospinning, the conductivity of the solution is an important parameter in determining the diameter of nanofibers. In electrospinning, stretching of the polymer solution is caused by repulsion of charges on its surface. Hence, when the conductivity increases more charges are carried by the electrospinning jet, which causes the polymer solution to stretch more, thereby decreasing the diameter. In our work, upon the addition of graphene the conductivity of the solution increased and this in turn decreased the overall diameter and morphology of the composite. This observation is in agreement with various studies that are reported on electrospinning.21,22 In addition, the initial dispersion of graphene is an important parameter that affects the diameter and morphology of the nanofiber.23–26 It is evident from Fig. 4a and b that the initial graphene concentration in the electrospinning solution affected both the diameter and the morphology of the nanofiber. As can be seen in Fig. 4b, graphene is uniformly dispersed all over the surface of ZnO, which proves that 0.5 wt% graphene is sufficient to form a uniform dispersion. The dispersion of higher wt% graphene was not studied, as a UV sensor requires dark current to be low. An increase in the graphene content increases the dark current, thereby affecting the sensitivity of the sensor. This is explained in the latter part of the discussion, where optimization studies to select the optimum Gr/ZnO composition for UV sensing are discussed.
To study the effect of the calcination temperature on the composite, fibers were calcined at elevated temperatures of 500 °C and 600 °C. Fig. 4c and d show high-magnification images of the Gr–ZnO composite calcined at 500 °C and 600 °C, respectively. As can be clearly seen, there is a decrease in the fiber diameter (∼250 nm) and also the flake size. The decrease in the nanofiber diameter can be attributed to the decomposition and evaporation of organic components and residues. The decrease in the flake size of graphene was caused by the burning of carbon content at high temperatures, which is evident from the Raman spectra of the composite nanofiber calcined at 600 °C (Fig. S1 in ESI†), which further decreased the overall diameter of the nanofiber.27 Hence, the optimized composite nanofiber calcined at 400 °C was chosen for UV sensing applications because of its higher graphene content.
Fig. 5 shows a high-magnification SEM image of the contact of a gold electrode and a Gr–ZnO composite nanofiber. The image was taken at a 45° tilt to ascertain that uniform contact had been established between the fiber and the metal electrode, which minimized the contact resistance between the two, thereby enhancing the transfer of electrons from the composite nanofiber to the gold contact and vice versa. This was achieved by calcination after in situ alignment of the fiber, which stabilized the contact. I–V curves that were recorded further confirmed the near-ohmic contact. The in situ alignment of fibers is advantageous in forming a near-ohmic contact, which involves heating both the electrode and the nanofiber simultaneously, thus avoiding the additional heating steps that are often required when the electrodes are fabricated after the electrospinning process.
Fig. 6 shows X-ray diffraction (XRD) patterns of graphene, pure ZnO nanofibers and the Gr–ZnO nanofiber composite calcined at 400 °C in the range of 20° < 2θ < 60°. The 2θ peaks at 31.8° (100), 34.4° (002), 36.3° (101), 47.5° (102) and 56.6° (110) indicate the hexagonal wurtzite crystal structure of ZnO nanofibers. The diffraction peaks at 26.1° and 54.71°, which are denoted by G (002) and G (004), respectively, indicate the crystal planes of graphene. The broad graphene peak at 26.1° in the Gr–ZnO XRD pattern indicates amorphous carbon, which is due to defects that were induced by the addition of ZnO. Also, the low intensity of the peak can be attributed to the low concentration of graphene in the composite. The XRD data thus confirm the high crystallinity of Gr–ZnO composite nanofibers.
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Fig. 6 XRD patterns of graphene, pure ZnO nanofibers and 0.5 wt% Gr–ZnO nanofiber composite calcined at 400 °C. |
Detailed chemical/molecular analysis was performed in terms of Raman spectroscopy to confirm the formation of the composite. Fig. 7 presents the Raman spectrum of the Gr–ZnO composite with 0.5 wt% graphene flakes. The well-known G peak (∼1574 cm−1), which is characteristic of an sp2 hybridized carbon peak, is evident. The other peak, which is at ∼1359 cm−1 and is denoted as the D band, which originates from structural imperfections, is also prominent. The intensity of the D peak is greater than that of the G peak, which also confirms the formation of the composite due to an increase in disorder caused by the addition of ZnO. Also, the addition of ZnO to graphene made it amorphous, which caused the intensity of the G peak to be low and not uniform.13 With 0.5 wt% graphene in the composite, the typical Raman peaks of ZnO disappear. This is due to the large number of graphene flakes that were embedded in the ZnO nanofiber, which concealed the Raman signals of ZnO.
As a part of optimization studies to select the optimum Gr/ZnO composition, 3 different weight percentages of graphene flakes, i.e., 0.2 wt%, 0.5 wt% and 0.8 wt%, were used to synthesize the composites and subsequent UV sensing was performed. It was found that for the sample containing 0.2 wt% graphene, the value of the dark current that was recorded was the lowest; however, upon UV exposure the resulting current became saturated after 30 minutes, thus yielding a ∼586-fold change in conductance as opposed to a ∼1892-fold change for the sample with 0.5 wt% graphene. For 0.8 wt% graphene, the dark current that was recorded was high, making it unsuitable for UV sensing applications. Hence, in order to achieve better sensitivity, a 0.5 wt% graphene composition was selected to synthesize the composite. Figures that show values of dark resistance measured as a function of different compositions and the I–V response of a typical device with 0.2 wt% graphene can be found in the ESI (Fig. S2 and S3, respectively†). Fig. 8 shows the current–voltage (I–V) response of a typical single aligned Gr–ZnO composite nanofiber device with 0.5 wt% Gr calcined at 400 °C. The reason for choosing the sample calcined at 400 °C was the greater visibility of the flake size in the SEM images, which indicates the presence of more graphene, which helps enhance the transport properties of the photogenerated electrons. Moreover, graphene decomposes above 500 °C, which is evident from Raman spectra, as discussed earlier in this section. The I–V response under UV illumination was measured at 5, 15, 30, and 45 minutes to study the response of current with time. Under UV illumination, electron–hole pairs are generated when the energy of the illumination is greater than or equal to the band gap of ZnO. For pure ZnO, photogenerated electrons tend to recombine, which decreases the carrier lifetime and hence the current. Hence, the need for graphene as a transport material arises for capturing photogenerated electrons, thereby preventing them from recombining and ultimately increasing the carrier lifetime. In the case of the Gr–ZnO nanofiber composite, as the work function of graphene is lower than the conduction band of ZnO28 photogenerated electrons are readily transferred to graphene and the higher electron mobility of graphene enables faster collection of charge carriers at the gold electrodes, thereby increasing the carrier lifetime of the photogenerated electrons. For pure ZnO nanofibers, in the absence of UV light a depletion layer is formed due to the oxidation of n-type ZnO nanofibers by adsorption of atmospheric oxygen molecules, thereby decreasing the conductivity.29 Before starting to measure the current under UV illumination, samples were kept in the dark for 12 hours so as to stabilize them. Under exposure to UV light for 5 minutes there was a 69% decrease in resistance for 0.5 wt% graphene, which indicates a quick response to UV illumination for both devices. After 15 minutes, a 98.67% decrease in resistance for 0.5 wt% graphene was observed. This increase in conductance can be attributed to the faster collection of photogenerated electrons by graphene. As the time of illumination was increased to 30 minutes and finally 45 minutes, 99.8% and 99.9% decreases in resistance were observed, respectively. As time increased, the amount of photogenerated electrons that were transferred to graphene increased, thereby increasing the current. After 30 minutes of UV exposure, the decrease in the resistance was less compared to the decrease in 15–30 minutes for 0.5 wt% graphene and it became saturated after 40 to 45 minutes of UV illumination. The above phenomena can be attributed to the decomposition of graphene due to photogenerated holes and electrons, which react with atmospheric oxygen and moisture, thereby forming reactive oxygen species that oxidize graphene.30
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Fig. 8 I–V curves of a typical Gr–ZnO device with 0.5 wt% Gr with and without UV illumination at various time instants. |
Fig. 9 provides a schematic representation of the process. Photogenerated electrons tend to be transported towards the gold electrode due to the very high charge transport mobility of graphene, but few electrons react with atmospheric oxygen to form O2 radicals. Also, photogenerated holes react with atmospheric oxygen and form hydroxyl (OH) radicals. These reactive oxygen species that are generated on the surface of graphene are responsible for the oxidation of graphene, thereby reducing the carbon content and increasing the oxygen content and hence leading to saturated conductivity. There have been few studies that reported the use of ZnO/graphene composites for UV sensing. Dang et al.31 fabricated a phototransistor with a channel based on vertical ZnO nanorods and graphene. Boruah et al.32 demonstrated a UV sensor based on ZnO nanowires grown on graphene foam. Fu et al.33 reported a high-performance single ZnO nanowire produced using chemical vapor deposition and sandwiched between graphene sheets with an on–off ratio of 800.
Liang et al.34 demonstrated the one-step synthesis of a SnO2/graphene nanocomposite by a solvothermal method for application in Li-ion batteries, but in our approach we demonstrated in a single step not only the in situ synthesis of a ZnO/graphene nanofiber but also the alignment of a single composite fiber across the electrodes, thus integrating it into a device. This controlled approach to the synthesis and precise positioning of the fiber can be performed simultaneously across multiple electrodes on a wafer, thus making this technique suitable for large-scale commercial applications.
Zhang et al.35 synthesized a ZnO nanowire using an electrodeposition method with a transparent graphene contact. Chang et al.36 demonstrated the use of a ZnO nanorod/graphene composite produced by a solution growth method, which was then drop-cast between metal electrodes. Wang et al.37 fabricated a flexible UV sensor with a reduced graphene oxide/ZnO composite synthesized by a hydrothermal method. Unlike most of the above-mentioned works, which involve working with high-density nanofibers in the form of mats or nanorods, or complex fabrication techniques such as defining electrodes using e-beam lithography, a transfer process or drop-casting, which even in ideal conditions will lead to device-to-device variation, our work involves a simple, reproducible approach for the one-step in situ alignment of graphene–ZnO using electrospinning. This method is advantageous as it allows precise manipulation of the resistance of a sensor upon UV exposure by aligning a single composite nanofiber across the electrodes. In addition, the current work employs a two-terminal device configuration, which can be easily integrated further for an all-electronic readout signal for real-time applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15351d |
This journal is © The Royal Society of Chemistry 2015 |