(Invited) Aspects of Vibrational Spectroscopy in Investigation of Low-Temperature Pigmented V-Oxide Films
Authors: Surca Angelja Kjara , Dražić Goran , Mihelčič Mohor 
Published: February 2020

Vanadium(V) oxide (V2O5) has attracted attention mainly due to its use in lithium secondary batteries. Consequently, various studies of its lithiated phases were made, leading to their identification as α, ε, δ, γ [1]. When prepared in the form of thin films V2O5 can be applied in a battery-type electrochromic (EC) devices as counter electrode, recognized for its multicolor electrochromism, electrochemical cycling stability in a specified potential range, good ion-storage capacity and relatively high transparency in the bleached state [2]. However, as for other transition metal oxides a temperature of above 300 °C is demanded for V2O5 formation, which rules out the possibility of its deposition on plastic substrates. With the aim to overwhelm this circumstance, a two-step procedure according to paint formulation procedures was developed in our laboratory. This preparation procedure has already been proven to be effective for Ni1-xO, TiO2, WO3 pigmented films [3,4]. During the first step, an oxide crystalline powder is prepared via sol-gel route and thermal treatment at elevated temperatures. As a second step, the milling process is performed in an appropriate solvent and with the use of dispersants to prevent the agglomeration of so-formed nanoparticles. The final dispersion can be used for spin-coating deposition of films on polymeric substrates, followed by thermal treatment at 150 °C.

The first aim of this work is to investigate the possibility of the application of the described two-step procedure for the preparation of V-oxide pigmented films. The sol for the preparation of crystalline powder in the first step is made of vanadium(V) oxoisopropoxide diluted in 2-propanol. Other routes as hydrothermal synthesis will be investigated as well. In the second step, the size distribution of nanoparticles in dispersion is a factor that crucially determines the deposition possibilities of films. Therefore, the relation between crystalline particles and dispersant is studied via transmission electron microscopy (TEM). The structural and surface characterization of pigmented films is further made using infrared (IR) and Raman spectroscopies, scanning electron (SEM) microscopy, contact angles and surface energy. In situ UV-visible absorbance in a custom-made trielectrode cell is used to detect optical changes during intercalation/deintercalation processes. The possibilities of formation of EC devices with appropriate pigmented films will be considered and the performance compared to already existing EC systems.

The second aim of this work is the study of possibilities of combined analytical tools for the investigation of intercalation/deintercalation in EC films and the developed V-oxide pigmented films will be used as an example. Especially spectroelectrochemistry, with a wide range of spectroscopic techniques can be advantageous. The above mentioned in situ UV-visible absorbance spectroelectrochemistry is widely used in investigation of EC films and devices for detection of optical modulation. More deep insight into the structural changes that occur in films can be performed using vibrational spectroscopies. Different techniques are characterized by certain advantages and disadvantages.

For example, IR reflection-absorption spectroscopy can be coupled with electrochemical technique ex situ or in situ. While ex situ approach lack due to uncertainty during the transfer from the electrochemical cell to the spectrometer, the disadvantage of in situ cell is absorption of the electrolyte. Consequently, a cell with a very thin layer of electrolyte between the silicon window and EC film on reflective substrate should be designed. While ex situ technique offers usual IR RA spectra with regard to the uncovered background, only changes that occur in the structure during intercalation (or deintercalation) are detected in in situ spectra, which makes the identification much more difficult.

IR absorbance spectroelectrochemistry can be performed ex situ after deposition of films on partly IR transparent silicon wafers. However, despite galvanostatic charging large overpotentials of silicon wafers may blur the direct comparison with electrochemical measurements of films on conductive substrates.

With regard to IR spectroscopy, Raman spectroscopy is more complementary than competitive. As reported, the structural changes of polycrystalline V2O5 films can be significantly different than that of bulk materials [1]. From this reason, the ranking of the Raman spectra of novel V-oxide pigmented films is welcomed. With this aim there is constructed a new custom-made three-electrode in situ Raman cell and the V-oxide pigmented films studied for structural changes during coloration/bleaching processes.

[1] Baddour-Hadjean et al., Chem. Rev. 2010, 110, 1278–1319.

[2] Surca et al., J. Electrochem. Soc. 1999, 146, 232-242.

[3] Mihelčič et al., Sol. Energy Mater. Sol. Cells 2012, 107, 175-187.

[4] Mihelčič et al., Sol. Energy Mater. Solar Cells 2014, 120, 116-130.