Abstract

Vanadium dioxide has been of interest for thermochromic smart windows to control the near infrared radiation (NIR), but suffers limitations due to high transition temperature, low luminous transmittance and low solar modulation ability. In this paper, we report on low switching temperature and improved balance between luminous transmittance and solar modulation ability of VO₂ derived from thermal reduction of sol-gel deposited vanadium pentoxide (V₂O₅) on graphene. Thermal reduction of vanadium pentoxide (V₂O₅) was done in an innert atmosphere of flowing argon gas under normal atmospheric pressure. The optical properties of thermally reduced films were investigated using UV-VIS-NIR spectrophotometer while transition temperature was determined by measuring sheet resistance as a function of temperature. The films revealed semiconductor to metal transition after investigation. An analysis of transmittance showed an improved NIR switching efficiency of 46.86% for VO₂ on graphene compared to 41.79 for VO₂ on bare glass. Resistance drops corresponding to 2 and 3 orders of magnitude for VO₂/glass and VO₂/graphene /glass respectively were realized. VO₂ film on graphene/glass substrate had a transition temperature of 55.4 ^oC lower than 61.2^oC for VO₂ on bare glass substrate. The low Semiconductor-Metal Transition (SMT) temperature and the enhanced trade-off between luminous transmittance and solar modulation ability are noted to emanate from the increased crystallinity and nano-scale crystallites of VO₂ films enhanced by graphene matrix. The results reveal that graphene enhanced solar modulation ability and lowered transition temperature through structural ordering and nanoscale reduction of VO₂ crystallites.

1. Introduction

VO₂ undergoes a fully reversible semiconductor-metal transition (SMT) at a temperature of about 68°C, coupled with a structural phase transition (SPT) between monoclinic VO₂ (M) phase and rutile VO₂ (R) phase ^{1, 2}. To take advantage of interesting functions due to temperature–dependent switching, VO₂ has been used in various applications for sensing and in optical communications such as in IR bolometers, photonic crystals and its application as a switchable window glazing for energy efficient application has also been discussed for decades ^{3, 4}. However the practical use of VO₂ for energy efficient windows has long been hampered by low luminous transmittance, low solar modulation ability and high transition temperature ⁵. Again growth of VO₂ films on amorphous substrates like glass does not easily enhance crystallinity at relatively low annealing temperatures ⁶. The microstructure and properties of VO₂ films exhibit strong dependence upon the substrates as well as the buffer layers used. By varying the substrates used, the crystallization behavior (amorphous, polycrystalline or epitaxial film), transition temperature and hysteresis width of VO₂ films can be regulated. To date, buffer layers of SiO₂, ZnO, SiNx, RuO₂, TiO₂ and doping metals (V, W, Fe and Ni) have been used ^{4, 7}. Graphene, which is recognized as a zero-gap semiconductor (namely, a semimetal), is a two dimensional (2D) hexagonal honeycomb structure and possesses excellent carrier mobility ≈ 200000 cm² V^-1 S^-1 and thermal conductivity ≈ 5.0 × 10³ W m^-1 K^{-1 8}. Based on its unique properties, one can expect that graphene has a potential to act as a functional buffer layer. However, it is still a mystery how a graphene layer will influence the microstructure and properties of VO₂ films, which needs to be investigated to elucidate its effects as a buffer layer. Graphene has an advantage of high optical transmission, thermal stability and mechanical flexibility. Graphene serves to aid the formation of densely interlinked VO₂ nanocrystals ⁹. A higher density of grain boundaries (smaller grain size) provides a greater number of nucleating defects which in turn reduces transition temperature. Similarly a higher number of grain boundaries reduce hysteresis width, that is, the difference between transition temperature in heating and cooling ^{5, 9, 10}. Moreover, the defects in VO₂ nanoparticles can act as nucleation sites of the phase transition ¹⁰. It has been reported that, sharp SMT accompanied by narrow hysteresis width is aided by stoichiometric VO₂ ¹¹. As a result of epitaxial growth, stoichiometry for graphene supported VO₂ is expected to be universal across the VO₂ film thickness. It has also been reported that, the magnitude of change in resistance and the narrowness of the hysteresis are indicators of how close the stoichiometry is to 1: 2 for VO₂ ^{10, 11}. Graphene aids the reduction of VO₂ crystals size to nanoscale which then makes the phase transition temperature to reduce. Depression of phase transition temperature is also attributed to high thermal conductivity of graphene which ensures efficient transfer of heat from graphene to VO₂ ⁹. By taking advantage of a tunable localized plasmon resonance in metallic state VO₂ nanoparticles, the optical absorption at τ ˃ τ_c can be shifted to a shorter wavelength in the near infrared range where the solar intensity is high, thereby improving the modulation of solar transmittance (∆Tsol) through put. It has been reported that nanothermochromics works very well indeed, and the luminous transmittance and solar transmittance modulation can be significantly improved ^{5, 10}. In this study, VO₂ thin film is coated on bare glass and again on graphene supported by glass substrate. The films were analyzed in terms of thermochromic, electrical and optical properties using sheet resistance and UV-VIS-NIR spectrophotometry. This work is geared towards investigation of effect of graphene on thermochromic properties of VO₂ derived from thermally reduced sol gel deposited V₂O₅ on graphene.

2. Experimental Procedure

1 g of V₂O₅ powder was dissolved in hydrogen peroxide (H₂O_2, 30%, 100 ml) at room temperature in a glass beaker while stirring to ensure uniform solubility. Since the reaction is exothermic, H₂O₂ was added at a rate of 10 cm³ every 30 s while stirring. The stirring continued while oxygen gas was released until a clear orange-yellow solution was formed. After the orange-yellow color appeared gelation started and lasted until a red-brown gel was formed (V₂O₅.2H₂O). The gel was allowed to age in the open glass beaker under ambient temperature to form homogeneous dark red gel.

Before deposition, glass samples measuring 2cm by 2cm were cleaned successively in detergent, acetone, ethanol and distilled water. The glass substrates were rinsed using distilled water then dried with pressurized warm air before dehydrating them on a hot plate. Dip coating method was used to coat the samples by dipping the substrate in the gel at a constant speed of 10 mm/min to allow and ensure homogeneous wetting of the surface. After dipping was over, the substrate was held inside the gel for 5 min to allow sufficient interaction of the gel with substrate surface for complete wetting. The substrates were withdrawn at a constant speed of 5 mm/min to entrain a uniform thickness thin film. The deposited films were allowed to dry in air at ambient temperature for 1 hour before subjecting them to annealing at a temperature of 500°C.

After the Samples were placed in a horizontal tube furnace, argon gas was pumped into the furnace at a rate of 300 sccm for 10 min to drive out air and then lowered to 100 sccm during heat treatment. After annealing, the furnace was allowed to cool naturally where the cooling rate was controlled at 5°C/minute until the temperature reached 100°C. The furnace was then allowed to cool naturally to room temperature after which the samples were retrieved from the furnace for analysis.

To determine optical properties, transmittance spectrum was measured using UV-VIS-NIR spectrophotometer (JascoV-530) equipped with a custom-built heating unit. The measurements were performed in the wavelength range 250-2500 nm at sample temperature below and above critical temperature (transition temperature). Optical performance was done using simulated solar irradiation of 1000W/m² (AM 1.5) for the UV, luminous light and IR regions individually. The integrated luminous transmittance and solar transmittance were calculated using equation 1while the solar modulation ability of the films was calculated using equation 2. The measured parameters in the optical characterization were also used to evaluate thermochromic performance of the films.

(1)

Where T(λ)denotes transmittance at wavelength λ, i denotes lum or sol for calculations, φ_lum is standard luminous efficiency function for the photonic vision and φ_sol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon.

The corresponding domains of integration were 280-400 nm, 400-780 nm and 750-2500 nm for UV, luminous light and IR regions respectively.

(2)

Where T_sol refers to solar transmittance, τ is temperature and τ_c is transition temperature.

Electrical properties were analyzed through sheet resistance (R_s). Four point probe technique was used to detect the switching of the VO₂ film from semiconductor state to metallic state as the temperature of the sample was varied from below to above critical temperature (τ_c) at a rate of 2°C/min. The sheet resistance was used to determine the SMT temperature (τ_c) and the hysteresis width as well as judging the crystallinity and stoichiometry of the films. Determination of magnitude of change in sheet resistance (R_s) between the temperature driven states of VO₂ was a good indicator of the achievement of crystalline VO₂.

3. Results and Discussion

The transmittance spectrum for VO₂ on bare glass and VO₂ on graphene buffer layer as a function of wavelength (250-2500nm) above and below transition temperature are as shown in Figure 1. The impact of the composition and structure on the optical properties was very clear from the UV-VIS-NIR spectrophotometry measurements. The transmittance at 25°C increases steeply towards long wavelengths whereas it decreases at 90°C. This is attributed to the increase in IR-reflecting behavior of the high temperature metallic phase.

According to the transmittance spectra, optical properties are significantly different from each sample. Thermochromic property of VO₂ are more pronounced in graphene supported sample than in graphene free sample, where NIR modulation was revealed in the two samples when the transmittance of the films where measured at 25°C and 90°C. The switching efficiencies were 41.79% and 46.86% for VO₂/glass sample and VO₂/graphene/glass sample respectively. Higher switching efficiency of VO₂ on graphene is attributed to nanocrystalline structure of VO₂ enhanced by graphene matrix ⁹. Coarse structure with large crystals of VO₂ on bare glass may have caused scattering of light in the visible region resulting to lower transmittance ¹⁰. In addition, reduction of crystal size to nano-scale resulted to increase in band-gap. Increased band gap caused less changes in the free carriers transitions (low absorption) resulting to higher transmittance of the semiconductor phase of VO₂ on graphene ¹¹. Graphene serves to aid formation of highly interlinked VO₂ nanocrystals ⁹. By taking advantage of a tunable localized plasmon resonance in metallic state VO₂ nanoparticles, the optical absorption (at τ > τ_c) can be shifted to a shorter wavelength in the near infrared range where the solar intensity is high, thereby improving the modulation of solar transmittance (∆T_sol) through put ⁵. It has been reported that nanothermochromics works very well indeed, and the luminous transmittance and solar transmittance modulation can be significantly improved ^{5, 12}.

The results showing variation of sheet resistance with temperature for VO₂ thin films on glass and VO₂ thin films on mono-layer graphene measured using four point probe are demonstrated in Figure 2. A two-step decrease in sheet resistance (two points of inflexion) was observed in the heating loop for both samples, as reported earlier ¹⁴. VO₂/graphene/glass sample exhibited a sharp resistance drop from 1.20 × 10⁵ to 1.2 × 10² Ω/sq. compared to VO₂/glass sample which dropped from 1.25 × 10⁵ to 1.2 × 10³ Ω/sq. Such resistance drops corresponds to about 2 and 3 orders of magnitude for VO₂/glass and VO₂/graphene /glass respectively. Less sharp resistance drop for VO₂ on bare glass could be as a result of presence of large number of microscopic crystals each characterized by its own hysteresis loop, which resulted to blurred and broadened switching ¹⁵. From the literature, this can be attributed to lack of enhancement towards better crystallinity because of the amorphous nature of glass and common soda lime glass that may have diffused into the VO₂ film resulting to Na_xV₂O₅ material that lacks the thermochromic properties that makes VO₂ an attractive material for smart windows and other applications ¹². This characterization elucidates that the average (between cooling and heating) which gives the transition temperature were 61.2°C and 55.4°C for VO₂/glass and VO₂/graphene/glass respectively. Hysteresis for VO₂/glass and VO₂/ graphene followed the same trend at 6.2°C and 5.0°C respectively as shown in Figure 2.

Under wider perspective, depression of phase transition and hysteresis width have several attributes; structural ordering, strain, doping with other transition metals and scaling to nano-scale dimensions ^{6, 7, 9, 16}. Factors such as grain size, residual stress, percent crystallinity and impurities can affect the transition temperature substantially ¹². Since the VO₂ preparation described here does not use any transition metal, we attribute reduction of the phase transition temperature to the effect of graphene buffer layer on reduction of VO₂ crystal to nanoscale.

The graphene support behaves as an electron donor and provides electrons for VO₂, which can thus increase the electron density in the VO₂ film and modify the electronic structure of VO₂ ^{17, 18}. Thus, such an increase in electron density in the VO₂ film, especially upon light irradiation, will consequentially distabilize the semiconductor phase of the thermochromic VO₂ film on the basis of the theory of Zylbersztejn and Mott, in which the SMT of the VO₂ is shown to be a Mott-Hubbard transition and driven by the increase in electron density. Therefore, once the electron density reaches a critical value, the SMT can be triggered and a metal phase can be generated ^{17, 19}. Based on these analyses, the SMT of the thermochromic VO₂/graphene/glass film may be triggered at a relatively lower temperature than that of VO₂/Glass. In addition, with the high thermal conductivity reported for graphene, the efficient heat transfer from graphene to VO₂ may account for depression of SMT temperature.

4. Conclusion

The transition temperature of VO₂ thin films has been reduced using graphene as a buffer layer. This was revealed by sheet resistance measurements where VO₂ film on graphene/glass substrate had a transition temperature of 55.4°C lower than 61.2°C for VO₂ on bare glass substrate. The trade-off between the luminous transmittance and solar modulation ability improved substantially to 40% and 6.3% respectively with a remarkable improvement on ∆T_sol by 21.1% for VO₂/graphene films. This improvement is attributed to epitaxial growth of VO₂ film on graphene. Thus, graphene demonstrated a potential of being used as a buffer layer to enhance thermochromic and optical properties of VO₂ for applications such as in sensors, electrical switches, energy saving windows and other nano technology devices.

References