ABSTRACT
Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO2:TiO2(P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO2:TiO2, and Pt:TiO2) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO2 layers on TiO2 nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si-OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO2(P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO2:TiO2(P25) nanopowder exceeded the dye degradation performance of both the individual SiO2:TiO2(P25) (1.5 fold) and Pt:TiO2(P25) (4-fold) catalysts by 6-fold as compared to TiO2(P25). This approach thus shows that by understanding the individual materials' behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.
ABSTRACT
To modify the luminescence properties of Ce3+-doped Y3Al5O12 (YAG) phosphors, they have been coated with a carbon layer by chemical vapor deposition and subsequently heat-treated at high temperature under N2 atmosphere. Luminescence of the carbon coated YAG:Ce3+ phosphors has been investigated as a function of heat-treatment at 1500 and 1650 °C. The 540 nm emission intensity of C@YAG:Ce3+ is the highest when heated at 1650 °C, while a blue emission at 400-420 nm is observed when heated at 1500 °C but not at 1650 °C. It is verified by X-ray diffraction (XRD) that the intriguing luminescence changes are induced by the formation of new phases in C@YAG:Ce3+-1500 °C, which disappear in C@YAG:Ce3+-1650 °C. In order to understand the mechanisms responsible for the enhancement of YAG:Ce3+ emission and the presence of the blue emission observed for C@YAG:Ce3+-1500 °C, the samples have been investigated by a combination of several electron microscopy techniques, such as HRTEM, SEM-CL, and SEM-EDS. This local and cross-sectional analysis clearly reveals a gradual transformation of phase and morphology in heated C@YAG:Ce3+ phosphors, which is related to a reaction between C and YAG:Ce3+ in N2 atmosphere. Through reaction between the carbon layer and YAG host materials, the emission colour of the phosphors can be modified from yellow, white, and then back to yellow under UV excitation as a function of heat-treatment in N2 atmosphere.
ABSTRACT
The orange-red emitting phosphors based on M(2)Si(5)N(8):Eu (M = Sr, Ba) are widely utilized in white light-emitting diodes (WLEDs) because of their improvement of the color rendering index (CRI), which is brilliant for warm white light emission. Nitride-based phosphors are adopted in high-performance applications because of their excellent thermal and chemical stabilities. A series of nitridosilicate phosphor compounds, M(2-x)Si(5)N(8):Eu(x) (M = Sr, Ba), were prepared by solid-state reaction. The thermal degradation in air was only observed in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10, but it did not appear in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.02 and Ba analogue with x = 0.10. This is an unprecedented investigation to study this phenomenon in the stable nitrides. The crystal structural variation upon heating treatment of these compounds was carried out using the in situ XRD measurements. The valence of Eu ions in these compounds was determined by electron spectroscopy for chemical analysis (ESCA) and X-ray absorption near-edge structure (XANES) spectroscopy. The morphology of these materials was examined by transmission electron microscopy (TEM). Combining all results, it is concluded that the origin of the thermal degradation in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10 is due to the formation of an amorphous layer on the surface of the nitride phosphor grain during oxidative heating treatment, which results in the oxidation of Eu ions from divalent to trivalent. This study provides a new perspective for the impact of the degradation problem as a consequence of heating processes in luminescent materials.
