Since white light emitting diode (LED) has two major disadvantages, including blue-light-emission and glare, it is harmful to the human health. This project was focused on the synthesis of bismuth oxides using ultrasound-assisted hydrothermal and spray pyrolysis technology. The surface on LED module was deposited with bismuth oxide and UV chip was utilized to excite RGB phosphors to improve the color rendering index, color temperature, luminous efficiency, and blue-ray-emission and glare. In addition, bismuth oxide photocatalyst was used to degrade the hazardous indoor air pollutant, such as formaldehyde, to achieve the worth of white LED involving health, safe, and environmental benignity.
Two methods were carried out in this project to synthesize bismuth oxides. In the first method, the addition of polymer (PVP) surfactant was beneficial to the size distribution ranging from 100 nm to 1 μm by using ultrasound-assisted hydrothermal preparation. After 3 hours of reaction course, the size of bismuth oxide was the smallest and the monoclinic α-Bi2O3 was thus formed. Under different thermal decomposition temperatures used such as 600, 650, 700, and 800℃ by spray pyrolysis, spherical structure and tetragonal β-Bi2O3 was substantially observed. According to the UV-ray inhibition tests, it showed that the UV inhibition effects for 4.72 wt% Bi2O3/PVP-3 h and 9.04 wt% SP700 could be 95.65% and 97.71%, respectively. This result has demonstrated that bismuth oxides as synthesized have excellent inhibition of UV light from LEDs.。
The spectral distribution of RGB phosphor self-mixed in the research was relatively complete in the wavelength from 420 to 780 nm and 20 mA of operational current could produce white light. In the regards, 6474K of the correlated color temperature represented cold white LEDs and the color rendering index was calculated more than 85 CRI which might meet the requirement of high color resolution. When bismuth oxide prepared by different routes was directly deposited on the surface of LED chip, the normal spectra distribution of white light was still figured out. If web printing technique was adopted to coat bismuth oxide film on the substrate surface of LED module, the increase in bismuth oxide contents would significantly reduce the intensity of emitting light within the wavelength of 400 to 500 nm. The use of SP700-2:8 could make the color temperature fall in 4186k that prefers a soft white light. In addition, deposition of bismuth oxide using SP700-1:15 exhibited the best efficiency on reducing glare light compared with the other processes.
The removal rates of formaldehyde were 18% (Bi2O3/PVP-3 h) and 64% (SP700), respectively, using bismuth oxide catalyst directly coated on the LED chip of RGB phosphor after 48 hours of reaction course. Using web printing to deposit bismuth oxide on the substrate surface, it was found that formaldehyde was degraded about 24% (Bi2O3/PVP-3 h) and 70% (SP700), respectively. Based on these results, the white LEDs coated with bismuth oxide catalyst synthesized either by ultrasound or spray pyrolysis has promising efficiency on UV light inhibition and the reduction of LED glare as well. Nevertheless, the removal of indoor formaldehyde still needs further assessment and improvement for achieving better functions that could meet the novelty of next-generation lighting system.
