Mechanism of Electrochemical Deposition and Coloration of Electrochromic V2O5 Nano Thin Films: an In Situ X-Ray Spectroscopy Study
© Lu et al. 2015
Received: 22 August 2015
Accepted: 27 September 2015
Published: 5 October 2015
Electrochromic switching devices have elicited considerable attention because these thin films are among the most promising materials for energy-saving applications. The vanadium oxide system is simple and inexpensive because only a single-layer film of this material is sufficient for coloration. Vanadium dioxide thin films are fabricated by electrochemical deposition and cyclic voltammetry. Chronoamperometric analyses have indicated that the thin V2O5 film demonstrates faster intercalation and deintercalation of lithium ions than those of the thick V2O5 film, benefiting the coloration rate. Despite substantial research on the synthesis of vanadium oxides, the monitoring of electronic and atomic structures during growth and coloration of such material has not been thoroughly examined. In the present study, in situ X-ray absorption spectroscopy (XAS) is employed to determine the electronic and atomic structures of V2O5 thin films during electrochemical growth and then electrochromic coloration. In situ XAS results demonstrate the growth mechanism of the electrodeposited V2O5 thin film and suggest that its electrochromic performance strongly depends on the local atomic structure. This study improves our understanding of the electronic and atomic properties of the vanadium oxide system grown by electrochemical deposition and enhances the design of electrochromic materials for potential energy-saving applications.
KeywordsIn situ X-ray spectroscopy study Electrochromism V2O5 thin films
The recent years have witnessed growing environmental concerns and increasing energy demands . In the USA, up to 40 % energy is used for primary energy consumption of buildings, which contribute over 30 % CO2 emissions . Thus, the effective use of energy has increasingly become an important issue. Smart windows can change optical properties by using an applied electric field or current, thereby avoiding excessive solar heating while taking advantage of heating mechanisms when necessary. Vanadium oxide systems comprise many oxide phases, including VO, V2O3, VO2, V6O13, V3O7, and V2O5. In particular, vanadium pentoxide is the most stable oxide in such systems. V2O5 exhibits an energy gap of approximately 2.2 eV and undergoes semiconductor–metal transition at around 250 °C. V2O5 materials demonstrate a color change when lithium ions are injected into or extracted from the layer spaces. Hence, V2O5 materials provide significant potential for applications in energy-saving devices, such as catalysts, sensors, electronic materials, and battery electrodes [3–5]. V2O5 films have been fabricated by various methods, such as reactive DC magnetron sputtering , vacuum-evaporated deposition , chemical vapor deposition , sol–gel method , spray pyrolysis technique , and electrodeposition . Electrochemical method is greatly advantageous over other methods in terms of economics and flexibility; typically, this method can also be used to fabricate mesostructured thin films . The high surface area of the mesoporous structure of V2O5 thin films provides porous channels, which facilitate fast ion diffusion and effective strain relaxation upon cycling of Li ion intercalation and deintercalation. Numerous studies on the application of V2O5 in energy-saving (smart windows ) or energy storage devices (batteries/supercapacitors ) have reported various synthesis methods , electrochemical properties, and photocatalytic performances . Several studies have also demonstrated the importance of electronic structure characterization [17–19]. Eyert et al.  indicated that octahedral distortions enhance the optical band gap. Willinger et al.  compared the basic structural VO5 units in α-V2O5 and γ-V2O5 with regard to the differences in their geometric and electronic structures. The relation between geometric and electronic structures is critical because the basic structural unit is also common to industrial vanadium phosphorus oxide (VPO) catalysts . However, the detail determination of the local atomic/electronic structure during growth of such electrochemically deposited film and during electrochromic coloration has not been thoroughly examined because of the lack of proper characterization tools. In the current study, an in situ electrochemical liquid cell was built, and in situ synchrotron hard X-ray absorption spectroscopy (XAS) was performed to monitor the electronic and atomic structures during the electrochemical deposition of V2O5 nano thin films. Changes in atomic/electronic structure upon coloration were determined. This approach is critical in determining the electrochemical growth and coloration mechanism, providing a great opportunity to further understand and improve the electrochemical properties of electrochromic materials.
Characterization and Measurement of Electrochemical Properties
Scanning electron microscopy (SEM, model PHILIPS S-4300) was used to examine the surface morphology. Electrodeposition was performed on a potentiostat/galvanostat (Princeton Applied Research, Versa STAT 4). A conventional three-electrode cell was utilized with an ITO substrate. V2O5 thin films were electrodeposited to a 1:1 mixture of deionized water and ethanol, which contained 1 M VOSO4·xH2O. Electrodepositions were performed at −0.7 V against the reference electrode for 20, 40, and 60 s. The lithium-ion intercalation/deintercalation properties of the V2O5 material were investigated in 1 M LiClO4 solution containing propylene carbonate. Cyclic voltammetry was performed between −1 and +1.25 V (versus Pt) at a scanning rate of 0.025 V s−1 using a potentiostat/galvanostat (Princeton Applied Research, Versa STAT 4). Chronoamperometric measurements were obtained at a constant voltage of +0.5 V, and the current change was monitored for 45 s at room temperature.
Measurement of Electronic Structure
Results and Discussion
Electrochromic switching devices have been widely investigated because such switching can control the throughput of visible light and solar radiation into buildings by applying electrical voltage, thereby imparting energy efficiency. Electrochemical in situ XAS studies indicated that the electrodeposition of V2O5 under constant 0.7 V conditions produced V2O5 thin films with different local electronic and atomic structures along with variation of film thickness. Given the increasing film thickness, the local geometrical symmetry of V2O5 thin films varies from V5+ with octahedral symmetry to V5+ with pyramid symmetry. In situ XAS was performed in the present study to monitor the effects of delithiation/lithiation on the vanadium oxidation states and the local atomic structures of the V2O5 thin films. Color switching upon the intercalation of hydrogen is caused by the valence change of the cations. Such effect is accompanied by structural rearrangement. This in situ XAS electrochemical cell setup allows real-time monitoring of the element-specific electronic structural changes in a system at all stages of electrochemical reaction. It is anticipated that using this technique, the growth parameters of V2O5 thin films can be fine-tuned, achieving optimization.
This study was financially supported by the National Science Council of Taiwan under contract nos. MOST 104-2112-M-032-008-MY3 and 102-2112-M-001-004-MY3.
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