# Nanomaterials in Photocatalysis: An In-Depth Analysis of Their Role in Enhancing Indoor Air Quality
## 8 CdS-Based Nanomaterials  
Cadmium sulfide (CdS) is a semiconductor material that has been studied since the late 19 th century, primarily for its optical properties, such as its distinct yellow color. In the early 20 th century, CdS was known for its use in photoconductors and light-emitting devices. The photocatalytic properties of CdS began to be explored more actively in the 1970s. The recognition that semiconductors could drive photocatalytic reactions, particularly the decomposition of water into hydrogen and oxygen under UV light, led to increased interest in CdS as a photocatalyst for environmental applications. In the early 1990s, researchers began examining CdS for its ability to degrade organic pollutants and reduce carbon dioxide. As a semiconductor with a band gap of approximately 2.4 eV, CdS can absorb visible light effectively [198]. The conduction band of CdS is less positive than those of other typical semiconductors, such as TiO\({}_{2}\) and ZnO, enabling heterojunction formation with these semiconductors to facilitate efficient transfer of photogenerated electrons across their electronic bands. However, CdS is susceptible to photo-corrosion, degrading into sulfur and cadmium ions, which diminishes its photocatalytic performance [199]. To mitigate this issue, CdS is often combined with other semiconductors to reduce the degradation [199,200]. In one study [201], CdS-TiO\({}_{2}\) photocatalysts supported on fiberglass cloth (FGC) were evaluated for benzene degradation, with varying CdS nanoparticle concentrations. UV-Vis absorption spectra revealed that higher CdS concentrations increased the adsorption capacity of CdS-TiO\({}_{2}\) photocatalysts. Photocatalytic benzene removal was tested under both UV-Vis and visible light. Under UV-Vis radiation (Figure 27a), increased CdS nanoparticle content initially enhanced photocatalytic activity, peaking at 0.005 M with a degradation efficiency of 92.8%. Beyond this concentration, activity declined. Under visible light (Figure 27b), all CdS-TiO\({}_{2}\) samples outperformed TiO\({}_{2}\) alone, with optimal CdS concentration at 0.005 M, achieving a maximum degradation efficiency of 32.7%. The activity was lower under visible light as only CdS could photogenerate electrons in this range. Since the conduction band of CdS is positioned below that of TiO\({}_{2}\), electron transfer from CdS to TiO\({}_{2}\) is facilitated, while holes accumulate at the CdS valence band, enhancing charge separation and photocatalytic efficiency. Excessive CdS, however, can create recombination centers, reducing the activity [201] (Figure 27). Another investigation synthesized TiO\({}_{2}\) shells on CdS nanorods (CdS@TiO\({}_{2}\)) to study phenol degradation under visible light [202]. All CdS@TiO\({}_{2}\) samples exhibited superior photocatalytic efficiency compared to untreated CdS, with the optimal configuration using 2 mL of titanium tetrabutoxide as the Ti precursor. The enhanced efficiency was attributed to the effective heterojunction between CdS and TiO\({}_{2}\), promoting charge separation. Photoluminescence measurements confirmed this by showing reduced intensity in CdS@TiO\({}_{2}\) samples, indicative of a lower recombination rate for charge carriers [202]. Compared to other high-performance photocatalysts like platinum or other noble metal-based materials, CdS is relatively inexpensive to synthesize, but cadmium is a toxic heavy metal, and CdS-based photocatalysts could pose environmental and health risks if not handled or disposed of properly. CdS tends to undergo photocorrosion under UV irradiation, which limits its long-term stability and photocatalytic efficiency. This degradation occurs due to the dissolution of Cd\({}^{2+}\) ions from the material's surface.