Enhanced electrical properties in sub-10-nm WO3 nanoflakes prepared via a two-step sol-gel-exfoliation method
© Zhuiykov and Kats; licensee Springer. 2014
Received: 9 May 2014
Accepted: 6 July 2014
Published: 18 August 2014
The morphology and electrical properties of orthorhombic β-WO3 nanoflakes with thickness of ~7 to 9 nm were investigated at the nanoscale with a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), current sensing force spectroscopy atomic force microscopy (CSFS-AFM, or PeakForce TUNA™), Fourier transform infra-red absorption spectroscopy (FTIR), linear sweep voltammetry (LSV) and Raman spectroscopy techniques. CSFS-AFM analysis established good correlation between the topography of the developed nanostructures and various features of WO3 nanoflakes synthesized via a two-step sol-gel-exfoliation method. It was determined that β-WO3 nanoflakes annealed at 550°C possess distinguished and exceptional thickness-dependent properties in comparison with the bulk, micro and nanostructured WO3 synthesized at alternative temperatures.
The layered transitional quasi-two-dimensional (Q2D) semiconductor oxides MO3 (M = Mo, W), have recently attracted significant interest because they demonstrate quantum confinement effects at the few-layer limit [1, 2]. Among them, tungsten trioxide (WO3) is an n-type semiconductor in an indirect bandgap of 2.6 to 2.9 eV  with excellent electrochromic and gasochromic properties . It has electron Hall mobility of ~12 cm2V-1 s-1 at room temperature and responsive to the blue end of the visible spectrum (λ < 470 nm) . Extrinsic n-doping is therefore not required for WO3 to exhibit significant conductivity. Similar to graphene, WO3 can be mechanically or chemically exfoliated to provide fundamental layers. However, unlike graphene, which does not have bandgap, Q2D WO3 has rather large bandgap, making Q2D WO3 nanoflakes more versatile as candidates for thin, flexible devices and potential applications in catalysis , optical switches  displays and smart windows , solar cells  optical recording devices  and various gas sensors . It has become one of the most investigated functional semiconductor metal oxides impacting many research fields ranging from condensed-matter physics to solid-state chemistry .
However, despite great interest of the research and industrial communities to the bulk and microstructured WO3, nanoscaled Q2D WO3 with thickness less than ~10 nm has received relatively little attention so far compared to its microstructured counterparts and to Q2D transitional metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te). In addition, last year's reports on alternative transitional semiconductor oxide Q2D MoO3 have exhibited exceptional thickness-dependent properties and the substantial increased of the charge carriers mobility (up to 1,100 cm2 V-1 s-1) in Q2D MoO3[2, 12]. It was also recently proven for MoSe2 that the reduction of bandgap can be achieved through decreasing the thickness of Q2D nanoflakes down to monolayer . Therefore, realization of WO3 in its Q2D form can further engineer the materials' electrical properties, as quantum confinement effects in 2D form will significantly influence charge transport, electronic band structure and electrochemical properties . More importantly, nanostructuring of WO3 can enhance the performance of this functional Q2D material revealing unique properties that do not exist in its bulk form .
The development of Q2D materials is generally a two-step process, the synthesis of the layered bulk material followed by the exfoliation process . Although there is a wide range of controlled methods of synthesis available to produce different morphologies of WO3 nanostructures, such as microwave-assisted hydrothermal , vapour-phase deposition , sol-gel , electron-beam  and arc-discharge , synthesis of Q2D WO3 is a topic that is yet to be widely explored. For instance, in a recent report, it was demonstrated that one possible way of bandgap reduction in bulk WO3 is to increase its sintering temperature . However, what is the most favourable sintering temperature for exfoliation Q2D WO3 nanoflakes remains largely unexplored.
In this work, we present for the first time new distinguishing thickness-dependent electrical properties of Q2D β-WO3 obtained for nanoflakes with thickness below ~10 nm developed via two-step sol-gel-exfoliation method. These properties were mapped without damaging the sample by carefully controlling the sample-tip force. This is performed by using current sensing force spectroscopy atomic force microscopy (CSFS-AFM), also known as PeakForce TUNA™ , which allowed simultaneous measurements of the topography and the current flowing between the tip and the sample from the real-time analysis of force-distance curves measured for a tip oscillating in the kilohertz regime, far below the resonance frequency of the cantilever . This technique also provided direct control of the force applied between tip and sample, thus avoiding any damage to the sample or misleading interpretation owing to tip contamination. In addition, new thickness-dependent electrochemical properties of Q2D β-WO3 nanoflakes were obtained and compared to the similar properties of the commercially available WO3. The electro-catalytic properties of Q2D β-WO3 were obtained by investigation samples for hydrogen evolution reaction (HER) from water by linear sweep voltammetry (LSV) and a Tafel plot. The obtained results indicate that Q2D β-WO3 nanoflakes are promising electro-catalyst for the HER [6, 23, 24].
Ultra-thin sub-10-nm Q2D WO3 nanoflakes were obtained via two-step sol-gel-exfoliation process. All of the following precursors including sodium tungstate dehydrate (Na2WO4.2H2O), hydrogen peroxide (H2O2, 30%), ethanol, polyethylene glycol (PEG, MW: 20,000), nitric acid (HNO3, 65%) and perchloric acid (HClO4) were used. Initially, 1 g of Na2WO4.2H2O precursor dissolved in 10 ml de-ionized (DI) water. Then, 6 ml of HNO3 was added drop wise to the solution to obtain a greenish yellow precipitation (H2WO4). After washing with DI water for several times, the remained H2WO4 was dissolved in 2 ml H2O2 and stirred at room temperature for 2 h. The procedure was followed by addition of known amount of PEG to obtain a viscous sol and as a result, adherence and homogeneity of the final transparent films can be improved. Then, 30 ml ethanol was added and the sol was stirred for another 2 h. After 1 day of ageing, the prepared sol was deposited on the Au- and Cr-coated Si substrates by using spin-coating instrument (RC8 Spin coater, Karl Suss, Garching, Germany).
The obtained sol-containing thin films were placed in oven at 80°C for a week to achieve the complete gelation. The dried films were subsequently sintered at 550, 650, 700, 750 and 800°C, respectively, for 1 h at the heating rate of 1°C min-1. The selection of these temperatures for sintering nanostructured WO3 was based on the fact that orthorhombic β-WO3 phase can be obtained at various annealing temperatures up to 740°C . Another reason was to investigate at which sintering temperatures mechanical exfoliation is possible and at which particular annealing temperature exfoliation provides the best results. After the samples were sintered and removed from the oven, they were conditioned at room temperature for 7 days. Reproducibility of all sol-gel WO3 samples was high. The last phase of the process was to apply mechanical exfoliation in order to obtain extremely thin layers for all further analysis. In the mechanical exfoliation method, scotch-tape was used and the similar procedure applied as per exfoliation of the graphene nanosheets. The reproducibility of exfoliated 2D WO3 nanoflakes depended upon the mechanical force applied on the scotch-tape during exfoliation process. Therefore, some of the exfoliated Q2D WO3 nanoflakes were thicker than others.
Structural and physical-chemical characterization
The crystallinity of the sol-gel-developed WO3 was characterized by RINT 2100VLR/PC, Rigaku X-ray diffractometer (Shibuya-Ku, Tokyo, Japan) with CuKα radiation (α = 0.1542 Å) at angle step of 1° min-1. XRD intensities and records were collected using a scintillation detector, and each sample was scanned over the 2-theta range 10° to 80°. Spectral analyses were carried out using Bruker ZRD search match programme, EVA™ (Billerica, MA, USA), and crystalline phases were analysed using the ICDD-JCPDS powder diffraction database. Both the surface morphology and structural configuration of Q2D WO3 nanoflakes were evaluated by a Philips XL30 field emission scanning electron microscopy (SEM). Iridium coating was also applied to the sample to improve the quality of the imaging. All the measurements were completed at room temperature. Meanwhile, the local chemical homogeneity of the WO3 nanoflakes were conducted by Type N energy dispersive X-ray spectrometer (EDX) (Hitachi Science Systems Ltd., Japan) equipped with JOEL-JSM 5600 LV SEM. Fourier transform infra-red absorption spectroscopy (FTIR) measurements were performed in air at room temperature by using Nicolet 6700 FTIR Spectrometer (Thermo Fisher Scientific, Breda, The Netherlands). Background gas for this examination was N2. FTIR spectrometer had the following working parameters during the analysis: IR polarization, zero/no polarization; angle of incidence, 90° perpendicular to the sample; analysing material, KBr and type of detector, MCT detector. During each measurement, the background spectrum was registered and consequently subtracted from the sample spectrum captured to obtain the final spectra. These were studied by employing Omnic Spectroscopy Software Suite. All the spectra were acquired in the following range: 4,000 to 400 cm-1. Before experiments, WO3 nanostructures were preheated to 200°C for removal of adsorbed moisture and CO2 and then cooled down to room temperature. For FTIR measurements, Q2D WO3 nanoflakes were also prepared on Au/Si substrate. Ultra-high clean N2 was selected as a background gas. It was flowing through the cell containing WO3 for 10 min with speed 100 ml min-1. After that, WO3 nanostructures were exposed to the air for 10 min before any measurements commenced. At each experiment and evaluation, the background spectrum was recorded and subtracted from the sample spectrum obtained .
CSFS-AFM measurements were performed to construct and identify the surface profile and simultaneously obtain typical topographical, tunnelling and current/voltage properties of the developed and exfoliated Q2D WO3 nanoflakes [26–29]. This procedure was completed using Bruker MultiMode-8 Atomic Force System with installed Peak Force TUNA module (model: MM8-PFTUNA for MultiMode8 AFM system, Rheinstetten, Germany) and the data was analysed by employing NanoScope Analysis software. Raman spectroscopy was used to determine and identify the vibration and rotation information regarding the chemical bonds . μSense-L-532-B Laboratory Raman Analyser (Warsash Scientific Pty Ltd, St, Redfern NSW, Australia) was employed for this purpose. During the testing, CCD detector was cooled down to -60°C. The spectra obtained were studied by RamanReader-M Software (Enwave Optronics Inc, Irvine, CA, USA). Impedance measurements were conducted using a frequency response analyser (AUTOLAB-PGSTAT30, Echo-Chemie, Utrecht, The Netherlands) in the 0.1 M H2SO4 solution at a room temperature. Lastly, the HER with Q2D WO3 nanoflake as the catalyst was measured using standard three-electrode electrochemical configuration in 1.0 M H2SO4 electrolyte de-aired with Ar, where saturated calomel electrode (Pine Research Instrumentation) and graphite rod (Sigma Aldrich, St. Louis, MO, USA) have been used as reference and counter electrodes, respectively. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE) using Pt wires as working and counter electrodes. In 1.0 M H2SO4, ERHE = ESCE + 0.256 V. Potential sweeps were taken with a 5 mV s-1 scan rate. Electrodes were cycled at least 30 cycles prior to any measurements.
Results and discussion
Characterization of properties of Q2D WO3 nanoflakes
Comprehensive information in relation to the developed ultra-thin Q2D WO3 and their electrochemical properties, such as chemical structure, oxidation states, adsorption properties etc., must be obtained and optimized in order to achieve their best analytical performance in various applications. For this purpose, CSFS-AFM, FTIR and Raman spectroscopy techniques were used.
Considering that WO3 contains cations in the highest degree of oxidation (+6), CO molecules do not adsorb on its surface because of full coordination. The frequency values obtained in spectra of CO adsorbed on Q2D WO3 nanoflakes shifted to the lower values compared to the assignments represented for microstructured WO3. This is connected with the fact that in the analysed Q2D WO3 nanoflakes, the degree of oxidation on some parts of the WO3 surface has been changed and few WO3-x sites appeared on the surface of nanoflakes causing CO adsorption. It should be noted that some residual hydrated WO3 is most likely present in the sample because hydrated WO3 is formed in the sol-gel process and then converted to β-WO3 during sintering [37, 39]. In the obtained spectra, the peaks in the fingerprint region, namely, at 1,048 and 1,161 cm-1, are assigned to stretching mode of W-O, whereas the stretch at 984 cm-1 is due to W = O vibrations. The W-O stretching modes are less intense, and changes in the low-frequency modes may indicate some modifications in the tungsten-oxide framework. This is possibly owing to the fact that the surface of exfoliated Q2D WO3 itself contains various defects. In general, the majority of experimental phenomena discussed above were associated to adsorption on expected sites of oxide nanoflake surface (co-ordinatively unsaturated cations, hydroxyls and their pair). However, the appearance of the most active surface centres suggests a connection with defects in nanoflakes [38, 40]. The other factors influencing properties of the ‘real’ oxide surfaces are (i) the presence of different lattice defects in the surface layer of nanoflake and (ii) their chemical composition, which in many cases, may differ from that in the microstructured material. There was also one stretch observed at 1,265 cm-1 (Si), which directly relates to the substrate platform. The WO3 FTIR spectra also indicated that there were no impurities present in the prepared and exfoliated samples.
It is noteworthy that the intensity of the peaks for the exfoliated Q2D WO3 nanoflakes sintered at 550°C was about two times higher than that the strength of peaks for the same sol-gel-developed WO3. At the same time, the magnitude of the peaks for exfoliated Q2D WO3 nanoflakes sintered at 650°C was just ~0.3 to 0.5 times higher compared to the intensity of peaks for their sol-gel-developed WO3 counterparts. This finding has confirmed the thickness-dependent properties of ultra-thin Q2D WO3. Following sintering at 550°C, there is a reduction in the spectral line width consistent with greater crystalline phase formation. Well-defined bands 712 and 802 cm-1 modes exhibit significant changes, with the mode at 712 cm-1 being particularly sensitive to the cation intercalation . Consequently, these results and observations open up a possibility for the future potential use of 2D WO3 as suitable nanomaterial for various sensing platforms [1, 10, 46] and reaffirmed that the sintering temperature of 550°C more suitable for synthesis of 2D WO3 than 650°C aiming their further exfoliation and cation intercalation.
The Tafel plots (Figure 9B) were constructed from the LSV voltammograms in the voltage region of -0.02 to -0.20 V. The Tafel slopes for commercial WO3, Q2D WO3 nanoflakes and hexagonal WO3 nanowires are -157, -112 and -116 mV decade-1, respectively . The lower Tafel slope obtained from Q2D WO3 nanoflakes indicates that it is a superior material as a hydrogen production electrode of HER compared to hexagonal WO3 nanowires  and commercial WO3. This could be attributed to the enhanced electrons transfer kinetics in ultra-thin Q2D nanoflakes, which can play a decisive role as a driving force to reduction of the electrochemical resistance . These results demonstrate that Q2D β-WO3 nanoflakes developed via two-step sol-gel-exfoliation method can be effective electrode materials with improved HER activity.
Orthorhombic Q2D β-WO3 nanoflakes, typically with lengths and widths of the order of 50 to 100 nm and thickness of 7 to 9 nm were produced by a two-step sol-gel-exfoliation method. It was experimentally determined that exfoliation of the ultra-thin Q2D β-WO3 nanoflakes was only possible at nanostructures sintered at 550 and 650°C. Spectral evidence for β-WO3 phase exists in the Raman measurements. This is also consistent with the absence of other crystalline phases in the XRD measurements of this material. CSFS-AFM, FTIR, Raman and electrochemical measurements further confirmed that the annealing temperature of 550°C is the most acceptable sintering temperature for WO3, if ultra-thin Q2D β-WO3 nanoflakes with thickness of ~7 to 9 nm have to be obtained. The results of the conducted research have reassured the thickness-dependent properties of the ultra-thin Q2D WO3 nanoflakes and demonstrated that Q2D WO3 nanoflakes can be excellent electro-catalytic material for HER with high activity and stability in water. The present study also illustrates the fundamental role the nanostructure of WO3 on the catalytic performance. The high surface-to-volume ratio of Q2D WO3 nanoflakes, controllable deposition and compatibility with existing semiconductor fabrication infrastructure suggest that the reported Q2D β-WO3 nanostructures can be utilized in new generation of low-cost oxide semiconductor functional devices including solar cells and various sensing platforms. Moreover, both the fabrication process and its framework have great compatibility with other emerging Q2D semiconductors and conductors such as graphene.
S.Z. obtained his Ph.D. in Materials Science and Engineering in 1991. He has combined experience as Research Scientist working at the different universities in Australia, Japan and Europe and industrial environments for more than 23 years. He is a Principal Research Scientist at Materials Science and Engineering Division of CSIRO. His research interests lie in the area of the development, design and evaluation of new functional nanomaterials for state-of-the-art functional devices. He is also Chairman of FP-011-02 Technical Committee of Standards Australia International and a Head of the Australian delegation in International Standards Organization: ISO TC21/SC8 Technical Committee since 2005. He has published 2 monographs, 6 chapters to books and more than 170 peer-reviewed scientific publications. He is a recipient of the 2007, 2011 and 2013 Australian Academy of Science/Japan Society for Promotion of Science and 2010 Australian Government Endeavour Executive Awards for his work on nanostructured materials.
E.K. was awarded a BSc (Applied Chemistry) from the University of RMIT, Victoria, Australia (1997). From 1998 until 2004, Eugene worked as a Research Project Officer at Scientific Services Laboratory, Melbourne, Australia. During this period, he was responsible for both technical and management components of Sample and Compliance testing of fire equipment, including detection equipment. Eugene has joined CSIRO Materials Science and Engineering Division in 2004. His current research involves development of nanostructured semiconductor materials for various functional devices.
The work was supported by the Research and Development Program of both CSIRO Sensors and Sensor Networks Transformational Capability Platform (SSN TCP) and CSIRO Materials Science and Engineering Division.
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