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

1. Preparation of the precursor solution
   1. 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% concentration, making sure that the pH of the solution is as close as possible to 3, using a pH meter.
      CAUTION: To avoid any health effects, performing this action inside a fume hood is highly recommended.
   2. Pour 50 mL of the precursor solution prepared as indicated in step 1.1.1 into each of two 100 mL beakers (hereafter, these beakers will be named A and B).
      NOTE: Two experiments (A and B, dependent on the beaker used) are carried out simultaneously. Only experiment B will be exposed to UV light.
2. Weigh 2.29 g of CdCl₂ for each beaker (A and B).
   CAUTION: Perform this action inside the fume hood as the material used is identified as hazardous. Cadmium is highly toxic and identified as carcinogenic when inhaled; it must be handled carefully.
3. Weigh 1.33 g of CS(NH₂)₂ for each beaker (A and B).
   CAUTION: Perform this action inside the fume hood as the material used is dangerously unhealthy. Thiourea is highly toxic when inhaled and must be handled carefully.
4. Add 2.29 g of CdCl₂ and 1.33 g of CS(NH₂)₂ to each beaker (A and B) containing 50 mL of the precursor solution.

2. Comparative Synthesis of Bis(thiourea) Cadmium Chloride

1. Experimental arrangement without UV light (A).
   1. Place the beaker (A) on the stirring hot plate and heat it up to 343 K. Set the plate on a moderate stirring speed.
      CAUTION: Perform this action inside the fume hood.
   2. Keep the solution in the beaker (A) moderately stirring for 2 h on the plate at 343 K.
2. Experimental arrangement with UV light (B).
   1. Place the beaker B on the plate and heat it up to 343 K. Set the stirring speed on a moderate level and switch on the UV light source.
      NOTE: The experimental arrangement is shown in Figure 1.
      CAUTION: Perform this action inside the fume hood.
   2. Keep the temperature and stirring conditions as described in step 2.2.1 for 2 h.

3. Obtaining Bis(thiourea) Cadmium Chloride Crystals

1. Mount 2 glass funnels with filter paper (No. 40, Ø = 125 mm), each one over a 100 mL volumetric flask, for A and B. To avoid the crystal formation at this stage; do not let the solution cool before filtering it.
2. Filter the solutions A and B through the paper, each into their own 100 mL flask.
   CAUTION: Perform this action inside the fume hood.
3. Let both solutions inside the volumetric flasks cool to room temperature.
   NOTE: The crystals begin to grow in the first minutes inside the volumetric flasks.
4. Prepare the filtering assembly again (step 3.1) with a new filter paper.
5. Filter the solutions with the crystals through the filter paper into separate volumetric flasks.
   CAUTION: Perform this action inside the fume hood.
6. Transfer the crystals on the filter paper to the respective watch glass.
   NOTE: At this point, there are 2 watch glasses, 1 for A and 1 for B.
7. Confirm the presence of bis(thiourea) cadmium chloride crystals by powder X-ray diffraction (XRD) and Raman spectroscopy, as reported in Trujillo et al.¹².

4. Calcination of the Crystals to Obtain CdS

1. Place the crystals obtained in step 3.7 into 2 different crucibles, 1 for A and 1 for B.
2. Preheat an electric laboratory furnace and stabilize its temperature at 773 K or higher.
3. Place the crucibles of step 4.1 inside the preheated furnace.
   CAUTION: The released vapors during calcination are toxic. Make sure that the furnace is in a fume hood due to the toxic fumes that it will be exhausted.
4. Let the material stand inside the furnace for 1 h at 773 K. Then, turn off the furnace and let it cool to room temperature. After that, remove the crucibles from the oven.

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(NH₂)<...

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

List of materials used in this article

Name	Company	Catalog Number	Comments
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