pH-induced wurtzite-zinc blende heterogeneous phase formation, optical properties tuning and thermal stability improvement of green synthesized CdS nanoparticles

Abstract

This is the first paper to report on the pH response to heterogeneous wurtzite/zinc blende phase transformation, optical tunability and thermal stability advancement of the CdS nanoparticles synthesized via co-precipitation, followed by subsequent thermal treatment at a desired annealing temperature of 320 °C, while the solution pH was varied during CdS synthesis by adjusting the ammonium salt concentration. The surface morphology, crystalline structure, functional groups, optical properties and thermal stability of CdS were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), UV–visible spectrophotometer, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that a dominant sharp band occurs in the FTIR spectra, which authenticates the presence of Cd–S bonds. XRD results reveal that as the pH declines, CdS in the initial cubic phase has gradually transformed into a heterogeneous phase with the coexistence of cubic and hexagonal structures. As observed from the SEM images, the CdS nanoparticles display a homogeneous, smooth and spherically shaped morphology. Optical absorption characterized by UV–visible spectrophotometry denotes that the band gap decreases proportionally with pH, which could be attributed to the formation of larger grain sizes from the aggregation of many small nanocrystallites. TGA and DSC analyses demonstrate an improvement in the thermal stability of CdS with increasing pH values. Consequently, the present findings dictate that pH tunability could be a valuable approach to procuring the desired properties for the respective applications of CdS in diverse fields.

1. Introduction

Recently, semiconductor nanomaterial synthesis has gained enormous attention from the research community due to the formation of very small particle sizes in the nanoscale range between 1 and 100 nm. The semiconductor nanomaterials also exhibit unique properties that are different from bulk materials ascribed to the quantum confinement effect, which contributes to the improvement of electromagnetic radiation, optical properties, mechanical strength, thermal stability, energy storage, photocatalytic degradation, etc [[1], [2], [3], [4], [5]]. Cadmium sulfide (CdS) belongs to the II-VI group chalcogenide binary semiconductor family with a wide direct band gap of around 2.42 eV. Nowadays, CdS has emerged as a promising material due to its great potential for diverse functionalities to be used in a wide variety of fields, such as photocatalysis, solar energy storage, lasers, diodes, sensors, detectors, photovoltaics and transistors. Besides, it has its own advantages of low preparation cost, tunable band gaps, strong optical absorption, and excellent electrical and thermal stability [[6], [7], [8], [9], [10], [11]].

Antecedently, CdS nanoparticles have been experimentally collected through different synthetic methods, such as sol-gel [12], laser ablation [13], hydrothermal [14], solvothermal [15], green biosynthesis [16] and precipitation methods [17]. From these mentioned techniques, various morphologies have been discovered, for instance, nanoparticles, nanospheres, nanowires, nanorods, nanotubes, etc. are among the favorite nanostructures produced, as reported previously in the literature [[15], [16], [17], [18], [19]]. As we know in common, CdS occurs naturally in three different phase structures: cubic (zinc blende), hexagonal (wurtzite) and also a heterogeneous phase with the coexistence of cubic and hexagonal mixtures, which is particularly dependent on the preparation method [7,20,21]. Nevertheless, the heterogeneous phase of an II-VI group semiconductor nanostructure might become the subject of research interest in the near future, since such a structure might introduce a novel and unique physicochemical property that leads to advanced applications, especially in the fields of photocatalysis and solar energy storage.

Despite plentiful works devoted to the physical properties of CdS nanoparticles, the investigation of the pH variation on thermally co-precipitated CdS nanoparticles in a heterogeneous phase has never been reported. By comparing the previous works in the literature, Ren et al. [22] reported that the CdS nanoparticles that exist in the cubic structure were prepared by the hydrothermal method using NaOH and NH₄OH as pH modifiers. Mohanraj et al. [23] performed the synthesis of hexagonal-phase CdS nanoparticles with pH value variation using ultrasonic wave irradiation. Uchil et al. [24] investigated the effect of pH on the size of CdS nanoparticles in cubic phase structures synthesized by chemical diffusion across a biological membrane. Barman et al. [25] reported the effect of pH on the hexagonal-CdS nanocrystalline thin films synthesized in the polymer matrix through an ion exchange reaction. In this work, the novel CdS nanoparticles in a heterogeneous phase of cubic and hexagonal were synthesized at different solution pH values (i.e., 12.90, 11.28 and 10.00) via a co-precipitation method between the reactions of cadmium chloride (CdCl₂) and thiourea precursors, in which ammonium nitrate was utilized as a surfactant and pH modifier. The CdS was then subjected to a critical annealing treatment at 320 °C to initiate grain growth and crystalline phase transformation, at which the heating temperature is double compared to that reported by Alrababah et al. [21]. Ammonium nitrate can be considered a vital sustainable green synthetic material because it contains non-toxic nitrogen, hydrogen and oxygen elements, whereas this material is usually used as artificial fertilizers, antibiotics and also as nutrients for yeast. Consequently, the surface morphology, crystalline structure, functional groups, optical properties and thermal stability of the CdS were thoroughly characterized by SEM, XRD, FTIR, UV–visible spectrophotometer, DSC and TGA.

2. Materials and methods

All raw materials used for sample preparation were of analytical reagent grade and were directly consumed without any specific refinement. Thiourea (CH₄N₂S), cadmium chloride (CdCl₂), ammonium nitrate (NH₄NO₃) and potassium hydroxide (KOH) were provided by Merck Ltd. Deionized water was used as the common solvent to prepare all the aqueous solutions. A facile co-precipitation method followed by an annealing treatment was adopted for the synthesis of CdS nanoparticles in the heterogeneous phase. In the present work, for the synthesis of CdS nanoparticles, CdCl₂ and thiourea precursors were first used, respectively, as sources to yield the Cd⁺ and S⁻ ions in the KOH solution. NH₄NO₃ could play the roles of capping agent and pH adjuster in order to obtain the preferable properties of CdS nanoparticles. The preparation procedure is described as follows: in the beginning, a mixture solution was acquired by adding CdCl₂ (0.02 M), thiourea (0.2 M), KOH (0.5 M) and NH₄NO₃ in deionized water to form an aqueous solution. Concentrations of NH₄NO₃ were varied for pH modification, therefore, three different pH values were subjected to sample synthesis, i.e., 12.90, 11.28 and 10.00. The mixture solution was then stirred in the water bath under consistent magnetic stirring at a temperature of 80 °C for nearly 120 min. The color of the initial mixture solution gradually transformed from a colorless solution to a yellowish suspension, which implies the formation of CdS precipitates. Afterwards, the yellow precipitates were filtrated and washed repeatedly with distilled water to remove unreacted impurities, then further dried at room temperature for 24 h. Lastly, the precipitates underwent thermal annealing treatment in a furnace at 320 °C for 3 h in order to procure high-quality CdS nanoparticles. After annealing, the powders were removed from the furnace and allowed to cool naturally by air at room temperature prior to characterization.

The properties were characterized by XRD, FTIR, SEM, TGA, DSC and UV–visible spectrophotometer. The structural phase and crystallinity of the CdS nanoparticles were scrutinized by a Rigaku Miniflex-TM II X-ray diffractometer (XRD) utilizing CuKα radiation (λ = 1.541 Å) that operated at a voltage and current of 40 kV and 40 mA, respectively. The XRD diffractograms were scanned in the 2θ angle range from 20° to 80° with a speed of 0.02^o per second. The mean crystallite size of the nanoparticles was determined from XRD analysis using the Debye-Scherrer method. The surface morphology and grain size of the samples were characterized using a JSM JEOL-6360 scanning electron microscope (SEM) system at an acceleration voltage of 15 kV with a magnification of × 20,000. An FTIR spectrometer (Thermo Nicolet Alvatar 380 FTIR) was used to conduct the functional group identification, in which the IR spectra were recorded in the spectral region of 400 to 400 cm⁻¹ with a wavenumber resolution of 2 cm⁻¹. A Shimadzu 1601 PC UV–visible spectrophotometer interfaced with a computer was used to acquire the optical absorption spectra of the samples at room temperature in the wavelength range of 300–700 nm. TGA and DSC measurements were carried out at a heating rate of 20 °C min⁻¹ by applying a thermal analyzer of the Mettler Toledo TGA/DSC1 Star System in the heating temperature range of 30 °C–900 °C, operating under the flow of O₂.

3. Results and discussion

3.1. Functional groups

The FTIR spectroscopy was executed to distinguish different types of functional groups present in the studied samples. Fig. 1 exhibits the recorded FTIR spectra of the annealed CdS nanoparticles synthesized at different pH values. From the figure, we can observe that several characteristic absorption bands appear in the spectra. The signature absorption band that occurred in the lower wavenumber range of 400–700 cm⁻¹ is attributable to the vibrational mode of CdS stretching [26,27]. The related band intensity becomes stronger and shifts to a lower wavenumber with increasing pH values. This effect could be due to the formation of a high-order crystalline structure, which remarkably improves the transmission efficiency of the sample and results in a high-intensity spectrum. On the other hand, the moderate and broad band located around 3350 cm⁻¹ belongs to the O–H stretching vibration of H₂O molecules attached to the sample surface [28,29]. This band intensity increases with pH value, indicating that there is a stronger binding affinity of water molecules to the particles in a more alkaline medium, as the hydroxide ion concentration increases. Moreover, a weak peak is observed at 1625 cm⁻¹, which corresponds to C–O stretching vibration [30,31]. The appearance of such a peak may be related to the uptake of CO₂ molecules from the annealed atmosphere during the annealing process, which ultimately attached themselves to the surfaces of molecules. Nevertheless, the related peak intensity experiences a moderate deterioration at higher pH values, which can be attributed to the surface area reduction of smaller particle sizes, as authenticated by SEM analysis. Furthermore, a moderate peak detected at 1093 cm⁻¹ is assigned to the S–O stretching vibration of the sulphate group [30,32]. This peak is due to the presence of trace contaminants in very small quantities, produced from different chemical reactions between the precursors and solvent. However, this peak becomes negligible when the pH value declines, which is mainly affected by the adaptability of the compounds in different solution mediums. Interestingly, a new peak emerged at 2008 cm⁻¹ for the sample synthesized at the lower pH value. This peak might correspond to the C≡N stretching vibration or isothiocyanate group of –N=C=S [5], which is contributed by the thiourea residue after the hydrolysis process. Nevertheless, a prominent band observed around 1441 cm⁻¹ can be assigned to the CH₂ deformation vibration [33], which appeared only at lower pH values due to a sufficient supply of H⁺ ions to initiate the interaction between the precursors used in this study.

Fig. 1.

FTIR spectra of the CdS nanoparticles synthesized at different pH values.

3.2. Surface morphology

SEM is a powerful imaging tool used to examine the surface morphology of nanomaterials at high magnifications. The SEM micrographs of the present CdS nanoparticles prepared with different pH values are displayed in Fig. 2(a–c). From the figure, it can be seen that all the CdS nanoparticles have a uniform and spherical-shaped morphology, with an average diameter ranging from 60 to 85 nm. Also, each particle possesses a well-defined grain boundary [Fig. 2(a)]. Besides, each nanoparticle can be constructed from the accumulation of an infinite number of small nanocrystallites with a size of around 3–4 nm, according to the estimation from the XRD data analysis. Moreover, the sample exhibits a morphology evolution with solution alkalinity, in which, as the pH decreases, it can be observed that the nanocrystallites and small grains tend to agglomerate to formulate a larger particle size with considerable outgrowth in a regular manner [Fig. 2(b)]. The average nanoparticle diameter is estimated to be ∼62 nm, ∼79 nm, and ∼84 nm, respectively, for the samples prepared at pH values of 12.90, 11.28 and 10.00. Neighboring tiny-sized nanocrystallites with high surface energy often adhere to each other through weak forces in order to diminish the total surface energy, thereby yielding the agglomeration of small crystallites to form larger particles and grain sizes [Fig. 2(c)]. Moreover, this finding also indicates that the ammonium nitrate used tends to adhere the particles together to form a chain-like structure, especially at lower pH values when the hexagonal phases begin to appear. The ionic concentration of cadmium (Cd²⁺) and sulphide (S²⁻) mainly reliant on the dissociation rate of cadmium chloride and thiourea [34]. In the alkaline medium, the base catalysts thermally decompose the thiourea by releasing the sulphide ions to facilitate the CdS formation when the pH exceeds 7. Commonly, the hydroxide (OH⁻) ions arise from the increased addition of ammonium nitrate to the solution, which assists in the release of sulphide ions. Therefore, at higher pH values, more sulphide ions are released from the thiourea hydrolysis due to an increase in hydroxide concentration [20]. Consequently, this has prevented the agglomeration and, in turn, increased the amount of finer CdS, which is produced in a smaller size and network-like structure. Otherwise, larger CdS particles are formed as the sulphide concentration decreases at lower pH values.

Fig. 2.

SEM micrographs of the present CdS nanoparticles prepared with different pH values: (a) (a) 12.90; (b) 11.28 and (c) 10.00.

3.3. Structural properties

XRD has been used to scrutinize the crystallographic properties and phase orientations of the studied materials. Fig. 3 depicts the XRD pattern of the thermally treated CdS nanoparticles prepared at different pH solution values. From the XRD pattern, the occurrence of strong and multiple peaks represents a good polycrystalline nature of CdS, which is in well accordance with the previously published works [[5], [6], [7], [8]]. Commonly, it is a widely recognized fact that CdS might exist in a stable wurtzite hexagonal phase, a metastable zinc blende cubic phase, or a mixture of both [7,35]. The diffraction peaks located at 2θ = 26.59°, 43.97°, 51.87° and 51.87° are respectively oriented along the (111), (220), (311) and (331) planes, which suit the characteristics of a CdS cubic structure according to JPDS card file 75–1546. Additionally, at a higher pH of 12.90, it is discovered that the sample possesses a distinctive crystalline quality of cubic phase with the (111) plane as the preferential orientation, in which the observed high intensity and sharp diffracted peak with a small FWHM denote a uniform size distribution of nanoparticles [27]. However, as the pH reduces to 11.28, some of the nanoparticles in the cubic phase slightly transform into the hexagonal phase, resulting in the coexistence of cubic and hexagonal phase structures. Inherently, the appearance of small diffraction peaks positioned at 2θ of 23.43°, 26.65°, 30.33°, 36.33°, 43.81°, 47.91° and 52.03° are sequentially assigned to (100), (002), (101), (102), (110), (103) and (112) planes that belong to a hexagonal structure of CdS (JPDS card file 77–2306). The diffraction peaks that represent the cubic and hexagonal structures are indistinguishable at 2θ = 26.59°, 43.97° and 51.87°, which are due to the overlapping of these phases. Interestingly, under this circumstance, the cubic oriented structures are still predominant and have a higher stability than the hexagonal phases, which become a crucial feature for solar cell applications. Henceforward, the relative peak intensity and sharpness of the cubic and hexagonal phases demonstrate a slight decay when the pH declines to 10.00, accompanied with a slight bandwidth narrowing. This feature indicates that a lower pH still tends to convert the cubic CdS into a hexagonal phase with a smaller crystallite size. Additionally, there are no other diffraction peaks regarding impurities or unwanted compounds found in the samples, revealing the formation of high-purity, single-phase CdS nanoparticles. Besides, the color of the sample changes from the original pure yellow to an orange-yellow as the pH value decreases. On the other hand, the mean lattice constants of a = 0.4136 nm, c = 0.6713 nm are obtained for the hexagonal CdS, while a = 0.582 nm for the cubic CdS, respectively, which are in accordance with the previous findings [18,36]. Meanwhile, the FWHM increases as the pH value decreases. The observed diffraction peak broadening indicates the reduction of crystallinity and crystallite size [15]. For further clarification, the average crystallite size was subsequently computed by applying Scherrer’s expression: D = 0.9 λ/β cos θ [7,9,37], where D represents the crystallite size, β is the FWHM of the peak in radians, θ denotes the related Bragg diffraction angle and λ designates the X-ray wavelength in nm. Afterwards, the mean crystallite size is ascertained to be 3.99 nm, 3.79 nm, and 3.44 nm at pH values of 12.90, 11.28, and 10.00, respectively, which further elucidates a decrease in crystallite size as the pH value reduces. Advantageously, CdS with a small crystallite size and a larger surface area can serve as an efficient photocatalyst to facilitate the movement of electrons or ions towards the particle's surface as active centers for photocatalytic activity enhancement. Besides, dislocation density (δ) defines the lattice or structural disorder in a material, which can be determined by Williamson and Smallman’s relation (δ = n/D²) [38,39]. Herein, the mean dislocation density (δ) of the present samples was obtained by substituting the computed D values into the equation. The obtained D and δ values are summarized in Table 1. Henceforward, it is worthwhile to remark that the mean dislocation density arises when finer crystallite sizes are formed, implying an increase in the lattice disorder probabilities.

Fig. 3.

XRD diffractogram of the CdS nanoparticles prepared at different pH values.

Table 1.

Average crystallite size and dislocation density of CdS nanoparticles annealed at different pH values.

pH value	Average crystallite size, D (nm)	Mean Dislocation Density, δ (1/nm2) × 10−3	Crystalline phase
12.90	3.99	62.81	Cubic
11.28	3.79	69.62	Cubic and Hexagonal
10.00	3.44	84.51	Cubic and Hexagonal

3.4. Optical properties and band gap

Optical absorption studies are crucial in identifying an appropriate photocatalyst for light-driven pollutant degradation, which requires an adequate number of photons to be captured by the activity center [[40], [41], [42], [43], [44], [45], [46]]. UV–Vis spectrophotometry was utilized to determine the optical absorption of the present samples. Essentially, the obtained absorbance spectra represent the bandgap transition of electrons between the multi-level valence and conduction bands. The narrower bandgap means that only a small amount of energy is needed to induce electron-hole recombination between the bands, which would become a vital indication for improving photocatalytic activity [41]. The optical measurement was carried out at room temperature in the ultraviolet and visible spectral regions between 300 and 700 nm, as depicted in Fig. 4. This is purposefully done in order to evaluate the susceptibility of the CdS nanoparticles to serve as efficient photocatalysts and solar energy storage devices. From the figure, it could be observed that the spectra appeared to be smooth and continuous in the UV–visible region, implying excellent optical absorption of CdS with a uniform and narrow size distribution, as also evidenced from SEM analysis. Additionally, the highest absorbance is observed in the ultraviolet region, then decreases slowly with wavelength, and lastly becomes flatter in the visible region. Furthermore, a small and sharp absorption peak could be traced in the UV region at around 332 nm, indicating that the UV light at this wavelength could be efficiently absorbed by the sample to induce and further predominate the photocatalytic activity. Henceforward, it is interesting to point out that there is an increase in the absorbance intensity of CdS in the UV–visible region when the pH value decreases. This trend is mainly caused by the increased amount of molecular absorbing center ascribed to the presence of larger crystalline grain sizes with less surface imperfection [17]. The absorbance intensity significantly increases at a higher pH value of up to 10.00, which is mainly contributed by the structure reconstruction in the heterogeneous phase that possesses the strongest light absorption. The intensified absorption can enhance the capture of more photons to excite conceivable charge carrier transitions during photocatalytic degradation activity [30]. Additionally, the fundamental absorption edge of CdS nanoparticles shifts slightly towards lower wavelengths (redshift) with increasing pH values. This phenomenon implies the growth of larger particle sizes with a narrower band gap [6], since the bandgap energy of semiconductor compounds may vary with size and particle condition [7]. Subsequently, for further elucidation, the corresponding band gap was determined from the optical absorbance data, according to a well-known Tauc’s relationship, as given by the following equation [38]:

(αhυ) = A (hυ − E_g)ⁿ (1)

where α represents the optical absorption coefficient, A is a typical constant, h is the Planck’s constant, υ is the light frequency, hυ denotes the photon energy, and E_g is the band gap energy. Meanwhile, the value of exponent n relies mainly on the transition type for the effective absorption in a semiconductor, in which n would take the values of 1/2, 3/2, 2, and 3, responsible for direct allowed, direct forbidden, indirect allowed, and indirect forbidden electronic transitions, respectively [42,43]. Generally, CdS complies with the selection rule of direct-allowed transition, hence, n was assigned the value of 1/2 for further data evaluation [44,45]. Fig. 5(a, b and c) depicts the Tauc’s plots of (αhν)² versus photon energy (hν) for the CdS prepared at various pH values, in which the observed fitted linear portion in the higher energy region confirms the direct optical transition. The band gap energy is then ascertained from the interception of the photon energy axis at hν = 0 by the extrapolated linear curve segment. The band gap of CdS nanoparticles is determined to range from 3.37 eV to 3.41 eV, which is much higher than bulk CdS (2.40 eV). In detail, the band gap is identified to be 3.41 eV, 3.40 eV and 3.37 eV for the samples that grow at pH 12.90, 11.28 and 10.00, respectively, which indicates a slight decrease in the band gap as the pH decreases. This reduction may correspond to the formation of larger grain sizes as a result of agglomeration. A narrower band gap is useful for photocatalytic applications since less photon energy is required to photoexcite the charge carrier transition for instantaneous degradation activity on organic molecules. The acquired absorption edge, band gap and mean grain size of CdS nanoparticles are displayed in Table 2 for comparison.

Fig. 4.

UV–visible absorption spectra of CdS nanoparticles synthesized with different pH values of: (a) 12.90; (b) 11.28 and (c) 10.00.

Fig. 5.

Tauc’s plots of (αhν)² versus photon energy (hν) for the CdS prepared at different pH values: (a) 12.90, (b) 11.28 and (c) 10.00.

Table 2.

Absorption edge, mean grain size and band gap energy of CdS nanoparticles annealed at different pH values.

pH value	Band gap energy (eV)	Absorption edge (nm)	Absorption band (nm)	Mean grain size (nm)
12.90	3.41	387	332	62.3
11.28	3.40	390	333	79.2
10.00	3.37	394	341	84.0

3.5. Thermal stability

The thermal stability of the present samples was investigated through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). These methods may also divulge the existence of any attached molecules at the surface of CdS nanoparticles [21]. Fig. 6 displays the TGA curves of the CdS nanoparticles prepared at different pH values. From the figure, a minor weight loss is initially observed in the range from 20 °C to 50 °C, which could occasionally be ascribed to evaporation of absorbed water molecules from the particle surface. Afterwards, there is no further weight loss for a short temperature range after 50 °C, and the sample becomes thermally stable up to around 100 °C, whereby this stable range extends to a higher temperature from 83 °C towards 121 °C as the pH declines from 12.90 to 10.00, respectively. After this short constant range, the weight exhibits a major continuous decay with temperatures up to 450 °C. Essentially, this apparent weight loss indicates particularly the decomposition of the organic molecules bonded to the CdS, as validated by FTIR analysis. Meanwhile, it could also be contributed by the breakage of some weak bonds resided between Cd and S in the CdS molecules. As observed from the figure, the weight loss decreases much faster at higher pH value with a much steeper curve gradient observed when compared to the others. Henceforward, the decomposition rate was computed from the gradient of the TGA curve, and the weight loss and decomposition rate for the present samples are tabulated in Table 3. Obviously, the weight loss and decomposition rate reduce with decreasing pH values, indicating the improved thermal stability of the CdS nanoparticles, which are beneficial as additives for coatings used in painting and commercial plastics [30]. Further, the improvement in thermal stability could be attributed to the coexistence of cubic and hexagonal structural phases with smaller nanocrystallite size produced, resulting in a stronger interaction force between heterogeneous CdS nanoparticles. Eventually, there is no weight loss determined after the temperature exceeds 450 °C, which implies that the samples have dried up and become stable after this temperature. Differential Scanning Calorimetry (DSC) measurement is commonly used to examine the amount of heat emitted or absorbed by the substances at higher or lower temperatures at a controlled homogeneous rate. Fig. 7 shows the DSC curve for the CdS nanoparticles prepared with different pH values. From the figure, an intense endothermic peak is observed on each DSC curve in the 400–570 °C temperature range, which is consistent with the TGA results. This peak might be related to the decomposition of organic compounds and the desorption of related gas molecules from the surface of the nanoparticles. Consequently, the present findings manifest that CdS possesses high thermal stability, as evidenced by a high thermal decomposition temperature without the presence of unidentified intermediates or contaminants.

Fig. 6.

TGA curve for the CdS nanoparticles synthesized with different pH values of: (a) 12.90; (b) 11.28 and (c) 10.00.

Table 3.

The weight loss, decomposition rate and endothermic peak for the CdS prepared at different pH values.

pH value	Weight Loss Δm (%)	Decomposition rate (mg/°C)	Endothermic peak position (oC)
12.90	15.62	0.00404	496
11.28	15.50	0.00170	462
10.00	14.48	0.00159	465

Fig. 7.

DSC curve for the CdS nanoparticles formulated at different pH values of: (a) 12.90; (b) 11.28 and (c) 10.00.

4. Conclusion

In conclusion, the heterogeneous structure of CdS was successfully produced through the sustainable greenish precipitation method, followed by high-temperature annealing at 320 °C. As shown in the XRD pattern, the heterogeneous structure that consists of a mixture of cubic and hexagonal phases becomes more noticeable and intense when the pH value decreases. The mean crystallite sizes are ascertained to be 3.99 nm, 3.79 nm, and 3.44 nm at pH values of 12.90, 11.28, and 10.00, respectively. The CdS nanoparticles have a spherical and uniform morphology at nanosized scales, with an average diameter ranging between 60 and 85 nm. A dominant peak in the lower wavenumber range of 400–700 cm⁻¹ represents the Cd–S molecular bond as observed in the FTIR spectra. The bandgap energy of CdS is obtained between 3.37 and 3.41 eV and decreases with increasing grain size as the pH value decreases, which shows the suitability of this material to be used as a photocatalyst. Also, thermal stability increases with the existence of heterogeneous structures.

Author contribution statement

Chan Kok Sheng: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Yousef Mohammad Alrababah: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Data availability statement

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper