The Effect of Ultraviolet Radiation on the Chemical Bath Deposition of Bis(thiourea) Cadmium Chloride Crystals and the Subsequent CdS Obtention

Summary

This article presents a protocol for the synthesis of bis(thiourea) cadmium chloride crystals by chemical bath deposition. Two experiments are described: one aided by ultraviolet light compared to one without ultraviolet light.

Abstract

In this work, the effects on the preparation of bis(thiourea) cadmium chloride crystals when illuminated with ultraviolet (UV) light at a wavelength of 367 nm using the chemical bath deposition technique are studied comparatively. Two experiments are performed to make a comparison: one without UV light and the other with the aid of UV light. Both experiments are performed under equal conditions, at a temperature of 343 K and with a pH of 3.2. The precursors used are cadmium chloride (CdCl₂) and thiourea [CS(NH₂)₂], which are dissolved in 50 mL of deionized water with an acidic pH. In this experiment, the interaction of electromagnetic radiation is sought at the moment the chemical reaction is carried out. The results demonstrate the existence of an interaction between the crystals and the UV light; the UV light assistance causes crystal growths in an acicular shape. Also, the final product obtained is cadmium sulfide and shows no evident difference when synthesized with or without the use of UV light.

Introduction

An important area of research is single crystals; their growth is aimed at different applications. These can be used as non-linear optical materials applied in the areas of laser technology, in the field of optoelectronics, and for the storage of information¹, which provides an area of opportunity for their investigation. Bis(thiourea) cadmium chloride is a metal-organic material and can be synthesized from two precursors, thiourea and cadmium chloride, obeying the following chemical formula: 2CS(NH₂)₂ + CdCl₂ CdCl₂-[CS(NH₂)₂]₂. This metal-organic material has been prepared under different reaction conditions, such as temperature and pH, but never with the assistance of ultraviolet (UV) light.

The influence of pH on the structure of the crystal has been reported; at a pH < 6, it is possible to obtain the formation of monocrystals. These, in turn, are modified depending on the pH range. At an interval of 6 to 4, it is possible to obtain hexagonal structures, for if pH is < 4, an orthorhombic crystalline structure is obtained². The ion dissociation is promoted by the acidic pH Cd²⁺ and Cl^– since it prevents cadmium hydroxide formation [Cd(OH)₂]. This stabilizes the cadmium: a cadmium atom joins with two sulfur-free radicals and two chlorines.

Here, the synthesis is carried out using the chemical bath deposition technique (CBD), controlling the different conditions that intervene at the time of the chemical reaction³. In CBD, the factors that control the chemical reaction are the following: the solution temperature, the precursor ions, the solution pH, the number of reagents, and the agitation speed, to name a few. On the other hand, the compared technique used here is called photochemical bath deposition (PCBD) because it uses UV light assistance. There have been reports in which UV light assistance has been used to synthesize films of CuS_x^4,5, ZnS⁶, CdS⁷, and InS⁸, among others. Ichimura and Gunasekaran⁹ present in their work that sulfate solutions have an absorption edge close to 300 nm. Due to this absorption range, ultraviolet radiation is applied, which results in a similar emission range to that of the absorbed solutions.

Another property of bis(thiourea) cadmium chloride is its degradation when heated. It exhibits an initial decomposition at temperatures of 512 K and above, forming cadmium sulfide (CdS). The degradation reaction is as follows: [Cd(CS[NH₂])₂]Cl₂ → Δ CdS + HNCS + NH₃ + NH₄SCN. This degradation generates thiocyanuric acid and various thiocyanates^10,11. Also, in the research group, some effects caused by the UV radiation were studied¹². Last, in this work, a comparative synthesis procedure for bis(thiourea) cadmium chloride crystals is described, as well as the effects of UV light.

Protocol

CAUTION: The chemicals used in this protocol are toxic and carcinogenic; thus, the safety recommendations and procedures must be followed carefully. Wear proper protective equipment and consult any relevant materials safety data sheet (MSDS). 1. Synthesis of Bis(thiourea) Cadmium Chloride Preparation of the precursor solution Pour 500 mL of deionized water into a 1 L beaker with constant agitation; add 0.3 mL of hydrochloric acid at a 36.5% concentratio…

Representative Results

The UV-Vis diffusion's reflectance absorption spectra in both precursor solutions, A and B, show the existence of a bis(thiourea) cadmium chloride complex—CdCl2-(CS(NH2)2)2. This is evidenced by a broad absorption band within the range of 250 – 500 nm in Figure 2c. In turn, Figure 2c is the combination of the main absorption bands of the isolated CdCl2 and CS(NH<sub…

Discussion

The discussion presented in this section focuses only on the protocol and not on the results already shown in the representative results.

One of the most critical parts of the protocol is the preparation of the precursor solution. It is fundamental to maintain an acidic pH to avoid the Cd(OH)₂ formation. If the pH is not acidic, it leads to the direct formation of CdS due to the thiourea dissociation and the Cd(OH)₂ formation.
The second most important step is step …

Materials

Reagents
Cadmium chloride Anh. ACS, 99.4 %	Fermont	PQ24291	Highly toxic
Thiourea technical grade, 99.9 %	Reasol	R5913	Toxic
Hydrochloric acid, 36.5 – 38.0 %	J.T.Baker	MFCD00011324	Highly corrosive liquid
Material
Filter paper	Whatman	1440 125	40, Ashless, Circles, 125 mm
Beaker	Kimax	1400	100 mL
Volumetric Flask	Kimax	28012-100	Class A 100 mL
Glass Funnel	Kimax	28980-150	Addition Funnel, Long Stem, 60° Angle, Wide Top. Type I, Class B.
Watch glasses	Pyrex	9985-150	Corning, 150 mm
Crucibles	Fisherbrand	FB-965-D	High-Form Porcelain
Equipment
Furnace	Briteg Instrumentos Cientificos S.A. de C.V.	1010
Fume Hood	Fisher Alders, S.A. de C.V.	F1124
Light surce	Philips	PL-S 9W UV-A/2P 1CT/6X 10 CC
pH meter	OAKTON	WD-35419-10
Hotplate whit magnetic stirrer	Cole-Parmer	JZ-04660-75