Goran Dražič

Ex situ and In situ Infrared Spectroelectrochemical Investigations of V 2 O 5 Crystalline Films

Vanadium oxide (V 2 O 5 ) films were prepared by dip-coating from V-oxoisopropoxide sols and heating at 300°C for 1 h. Transmission electron microscopy combined with small area electron and X-ray diffraction revealed that the films consist of disordered V 2 O 5 grains with an orthorombic structure (P mmn ). Electromotor force (emf), cyclic voltammetric (CV), and chronocoulometric measurements, combined with in situ ultraviolet-visible (UV-vis) spectroelectrochemical measurements, revealed similar electrochromic and electrochemical properties with other sputtered and sol-gel derived V 2 O 5 films. IR absorbance and reflection-absorption spectra, performed at near-grazing incidence angle (NGIA) conditions (80°, P-polarized), allowed us to compare the observed transverse optical (TO) and longitudinal optical (LO) frequencies of films with the TO and LO mode frequencies obtained from the dispersion analyses of V 2 O 5 crystalline reflection IR spectra. TO and LO spectra of charged films show that Li + …O interactions modify the terminal V-O A (vanadyl), bridging V-O B -V, and edge-sharing V-O C stretching frequencies, suggesting that these interactions take place between the oxygens bordering the cavities in which Li + ions are accommodated during charging. The red-frequency shift of the V-O A stretchings [1016 cm - 1 (TO), 1035 cm -1 (LO)] and the disappearance of the bridging V-O B -V stretching mode [795 cm -1 (TO), 895 cm -1 (LO)] can be used as a diagnostic tool to differentiate between films cycled in the safe and unsafe potential regions. A polaron absorption was observed above 2000 cm -1 in the ex situ TO and in situ NGIA reflection-absorption (LO) spectra of films charged in the safe potential region.

Gasochromic behavior of sol-gel derived Pd doped peroxopolytungstic acid (W-PTA) nano-composite films

The behavior of sol-gel prepared thin films exhibiting a gasochromic effect; i.e., a reversibly change in colour from transparent when in air to blue when in H2, has been studied. The films were prepared from a Pd (PdCl2) doped peroxopolytungstic acid sol using a dip-coating technique. Transmission electron microscopy together with selected area electron diffractrometry revealed that the films consist of monoclinic and hexagonal nanocrystalline grains (∼2–5 nm) embedded in an amorphous phase. This amorphous hydrated phase was established using Infrared (IR) and Raman spectroscopy. Characteristic vibrations observed in the in-situ IR spectra of the coloured and bleached states revealed the presence and the importance of terminal W=O and W—OH2 groups in the colouring of the films. Colouring/bleaching changes of Pd doped W-PTA films observed using in-situ UV-Vis spectroscopy are described in terms of Pd concentration, and the number of reducing/oxidising cycles. The rate of colouring/bleaching is greater for films containing a higher concentration of Pd but the change in the optical density does not increase, i.e., shows saturation.

Electrochromic and structural investigation of InVO4 and some other vanadia-based oxide films

We extended our previous studies of M 3+ VO 4 orthovanadate (M 3+ = Ce, Fe) and M 3+ 2 V 4 O 13 (Fe 2 V 4 O 13 ) films to include InVO 4 films because their photopic transmittance T vis(initial) is above 0.885. Structural studies (TEM and XRD) revealed that films prepared from In(NO 3 ) 3 .5H 2 O and V-oxoisopropoxide sols annealed at 500°C consist of the mixed monoclinic (InVO 4 -I) and orthorhombic (InVO 4 -III) phases while the addition of acetylacetone (acac) to the sol results in amorphisation. The charge capacity of InVO 4 /acac films is between - 30 and - 40 mC cm -2 (single dipped films) and their electrochemical stability is more than 1000 cycles. In situ UV visible spectroelectrochemical measurements show that charging to - 35 mC cm 2 decreases the photopic transmittance from T vis(initial) = 0.885 to T vis(ins) = 0.722, which contrasts the higher optical passiveness of previously investigated CeVO 4 films (T vis(ins) = 0.90). The increase in the broad absorption below 600 cm -1 in IR spectra of charged discharged InVO 4 films shows that films irreversibly uptake lithium ions. Charging up to -20 mC cm -2 does not affect the IR spectra while charging up to -40 mC cm -2 results in the transformation of the film structure, which becomes similar to that of the amorphous InVO 4 films obtained at 300°C and to certain transition vanadate glasses. IR spectra of other vanadia-based films, i.e. FeVO 4 , Fe 2 V 4 O 13 , CeVO 4 , and V 2 O 5 films in different state of lithiation are discussed.

Development of TiO2 pastes modified with Pechini sol–gel method for high efficiency dye-sensitized solar cell

A titanium oxide layer used for a dye-sensitized solar cell (DSSC) has to meet two opponent properties to assure a high efficiency DSSC: good connection between TiO2 grains and a large inner surface area. Three different paste formulations based on commercial nanocrystalline TiO2 powder (Degussa P25) are studied. Results confirm that modification of the TiO2 paste with the Pechini sol–gel method increases the surface area of the TiO2 layer while maintaining good connections between the nanocrystalline grains, consequently the efficiency of the DSSC increases from 1.8% to 5.3%. The structure and morphology of the TiO2 layers are described by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD).

Structural and IR spectroscopic analysis of sol-gel processed CuFeMnO4 spinel and CuFeMnO4/silica films for solar absorbers

Black colored CuFeMnO4 spinel powders and films were prepared using sol-gel process from Mn-acetate and Fe- and Cu-chloride precursors. Films were deposited by dip-coating technique and heat-treated at 500°C. For CuFeMnO4/silica films 3-aminopropyl-triethoxysilane (3-APTES) or tetraethoxysilane (TEOS) were used in molar proportion (Mn : Cu : Fe) : silica = 1 : 1. Films and powders were prepared by heating at 500°C. IR spectroscopic measurements were employed to follow the hydrolysis-condensation reactions in (Mn : Cu : Fe)/3-APTES sols hydrolysed with water, and (Mn : Cu : Fe)/TEOS sols hydrolysed with (NH3)aq (Stöber processing). The resulting coatings were examined with transmission electron microscopy (TEM) combined with electron dif-fraction analyses, Rutherford back scattering (RBS) and proton induced X-ray emission (PIXE) techniques. Results revealed that (Mn : Cu : Fe)/3-APTES films had a composite structure consisting of the upper Cu1.4Mn1.6O4 spinel and the lower amorphous SiO2 layer. RBS measurements confirmed the composite structure, showing also that the composition of the film was Mn : Cu : Fe = 1 : 0.96 : 0.29, i.e. close to the precursors ratio Mn : Cu : Fe = 3 : 3 : 1. (Mn : Cu : Fe)/TEOS films prepared from sols which were catalysed with (NH3)aq consisted of amorphous monodispersed spherical SiO2 particles with a size of about 400–420 nm. Solar absorbance (as) and thermal emittance (eT) values of CuFeMnO4 (500°C) and (Mn : Cu : Fe)/TEOS films (500°C) showed that CuFeMnO4 films could be used as potential selective coatings for solar absorbers in solar collector systems.

Electrochromic and Structural Studies of Nanocrystalline Fe/V (1:2) Oxide and Crystalline Fe2 V 4 O 13 Films

The sol-gel route was used for the preparation of Fc/V oxide films with a molar ratio Fe:V = 1:2. Transmission electron microscopy (TEM) measurements of films prepared at 300°C showed that the films consist of a FeVO 4 -II phase with the grains (1 nm) embedded in an amorphous phase. TEM measurements of films heated at 400°C agreed with the X-ray diffraction results, confirming Fe 2 V 4 O 13 grains with dimensions up to 50 nm. Charge capacities per unit thickness (Q ins d -1 /Q ext d -1 ) were determined in a I M LiClO 4 /propylene carbonate (PC) electrolyte using cyclic voltammetry (1.5 to -1.5 V vs. Ag/AgCl, 4.8 to 1.8 V vs. Li). The IR spectroscopic result is the assignment of the bands between 1000 and 600 cm -1 to V-O (terminal) and V-O-V/V-O-Fe (bridging) stretching modes of VO 4 tetrahedra. Ex situ IR spectra of Fe 2 V 4 O 13 films (400°C) charged/discharged to various extents showed that at low charging (Q ins d - (-0.13 mC cm -2 2 nm -1 ) terminal and bridging modes are affected by lithium insertion. Higher chargings (Q ins d -1 (-0.33 mC cm 2 nm -1 ) result in the amorphization of the films and in the appearance of a broad absorption below 600 nm ascribed to the Li + -O modes. The optical response of films is evaluated in terms of photopic transmittance and chromaticity coordinates Results confirmed that films are suitable for use in electrochromic transmissive (smart) windows.

UV-visible and IR spectroelectrochemical studies of FeVO4 sol-gel films for electrochromic applications

The sol-gel synthesis route, in combination with dip-coating deposition, was used for the preparation of FeVO4 films. TEM measurements of Fe/V (1 : 1)-oxide films heated at 400°C reveal that the films consist of a triclinic FeVO4-I and an orthorhombic FeVO4-II phases with a grain size of up to 50 nm. The electrochromic properties of the films were tested in 1 M LiClO4/propylene carbonate (PC) using various electrochemical techniques and in-situ UV-visible spectroelectrochemical measurements. The best compromise between the charge capacity per film thickness (Qd−1 = −0.14 mC cm−2 nm−1), electrochemical stability (>1000 cycles) and optical modulation (ΔTvis = 0.15) was achieved in the potential range of 4.80 to 1.80 V vs. Li, which suggests that FeVO4 films can be used as counter-electrodes in electrochromic devices.Extensive IR-spectroscopy studies of FeVO4 films in charged/discharged states revealed the following spectra changes: (i) small charging (−0.01 mC cm−2 nm−1) leads to a variation in the intensity of all the vibrational bands without shifting their frequencies, (ii) higher chargings bring about the intensity and frequency changes of bridging V—O···Fe and V···O···Fe stretchings showing that vanadium, and probably also iron, are involved in the insertion/extraction processes, (iii) below 500 cm−1 broad absorption appears due to the Li+—O modes, which also remained in the IR spectra of discharged (bleached) states revealing the irreversible lithiation, and (iv) charging to −0.30 and −0.50 mC cm−2 nm−1 leads to the amorphisation of the film structure.

Electrical and IR spectroscopic studies of peroxopolytungstic acid/organic–inorganic hybrid gels

Abstract We investigated the electrical and structural properties of an organic–inorganic hybrid (ICS-PPG), composed of 3-isocyanatopropyl-triethoxysilane (ICS) and poly(propylene glycol)bis-(2-amino-propyl ether) (2-APPG), mixed with peroxopolytungstic acid (W-PTA), (W-PTA/ICS-PPG for short). Impedance spectroscopic measurements done on ICS-PPG gels hydrolysed with 4 mol.% HCl revealed that proton conductivity is about 10 −5 S/cm and increases two times when temperature is increased from room temperature to 80°C. ICS-PPG with the added W-PTA exhibits room temperature proton conductivities from 1.86×10 −6 S/cm (10 mol.% W-PTA) to 1.45×10 −3 S/cm (99.18 mol.% W-PTA) while the proton conductivity of W-PTA gels is 2.54×10 −3 S/cm. All samples are characterised with low electronic conductivity (∼10 −8 –10 −9 S/cm). Results revealed that the addition of ICS-PPG to W-PTA increases thermal stability of the composites (