Visible Light Reductive Photocatalysis of Azo-Dyes with n–n Junctions Based on Chemically Deposited CdS

Abstract

New composite photocatalysts have been obtained by chemical bath deposition of CdS on top of either nanostructured crystalline ZrO2 or TiO2 films previously deposited on conductive glass FTO. Their morphological, photoelectrochemical and photochemical properties have been investigated and compared. Time resolved spectroscopic, techniques show that in FTO/TiO2/CdS films the radiative recombination of charges, separated by visible illumination of CdS, is faster than in FTO/ZrO2/CdS, evidencing that carrier dynamics in the two systems is different. Photoelectrochemical investigation evidence a suppression of electron collection in ZrO2/CdS network, whereas electron injection from CdS to TiO2 is very efficient since trap states of TiO2 act as a reservoir for long lived electrons storage. This ability of FTO/TiO2/CdS films is used in the reductive cleavage of N=N bonds of some azo-dyes by visible light irradiation, with formation and accumulation of reduced aminic intermediates, identified by ESI-MS analysis. Needed protons are provided by sodium formate, a good hole scavenger that leaves no residue upon oxidation. FTO/TiO2/CdS has an approximately 100 meV driving force larger than FTO/ZrO2/CdS under illumination for azo-dye reduction and it is always about 10% more active than the seconds. The films showed very high stability and recyclability, ease of handling and recovering.

Advances in Hybrid Composites for Photocatalytic Applications: A Review

“…Within the organic chemistry panorama, polymers can guarantee very high versatility in terms of structural and optical response with high potential in several applications [ 78 , 79 , 80 ], but their efficiency is hampered by the low thermal and short-time structural stability of the compounds ( Figure 7 ).…”

Composite CdS/TiO2 Powders for the Selective Reduction of 4-Nitrobenzaldehyde by Visible Light: Relation between Preparation, Morphology and Photocatalytic Activity

“…In Table 2 we report the rate values for each CdS/TiO 2 -(1-4) calculated after 30 min of irradiation. The evidence of the photoconversion rate being independent of the concentration of the dye is probably motivated by the fact that the reduction of the azo compound is nearly isoergonic with the occupied electron states in the semiconductor [9,44] and only the highest lying filled electronic states may actually be active in promoting the desired reductive process. We note that the mechanism of this reduction reaction must be complex, since it involves a multielectronic charge transfer possibly accompanied by proton transfer as a charge compensating mechanism.…”

“…Fe 2 O 3 [4], Cu 2 O [5] and WO 3 [6] have been used, but cadmium sulfide (CdS) is one of the best, both because of its band gap value (2.4 eV) and appropriate band edge position for photocatalytic redox reactions [7]. Spectral response of CdS/TiO 2 systems can be extended to the visible light region and the appropriate matching of band edges allows the injection of electrons generated from CdS into TiO 2 , effectively increasing the lifetimes of separated charges [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Moreover, some negative aspects typical of CdS itself, such as easy agglomeration and poor photostability, can be improved in the combined photocatalyst.…”

“…For these reasons, CdS/TiO 2 composite systems attract intensive studies and have been employed in oxidation and reduction processes. Most of them refer to crystalline or nanostructured materials and have been investigated in the photoelectrocatalytic cells for hydrogen evolution [7,16,23,24], in the photocatalytic degradation of organic pollutants [19,20,25] and dyes [9,[11][12][13][14][15]24], and in some reductive transformations [13,14,26]. When UV-vis illumination is employed, an improvement of photoactivity, with respect to TiO 2 alone, is usually observed due to the presence of CdS [21,22].…”

A Novel Hydrothermal CdS with Enhanced Photocatalytic Activity and Photostability for Visible Light Hydrogenation of Azo Bond: Synthesis and Characterization

“…More generally, the location of the conduction band at negative potentials makes CdS a photocatalyst of interest for reductive transformations that take advantage of electrons promoted in the conduction band [ 23 ]. For example, we recently demonstrated that FTO/TiO 2 /CdS slides, in which CdS was deposited by chemical bath deposition, were an efficient and stable photocatalytic system for the reductive cleavage of N=N bond in azodyes with visible light [ 24 ].…”