# Influence of Zn in the CdS Crystal Lattice and Its Impact over the Catalytic Activity in the H2 Production

Authors: Luis Fernando Morelos-Medina; Rufino Nava-Mendoza; Carlos Martín Cortés-Romero; Rodrigo Rafael Velázquez-Castillo; J. Santos-Cruz; María de los Ángeles Cuán-Hernandez
Journal: Journal of Chemistry
 (2022)

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## Abstract

Solid solutions ZnxCd1−xS were compared by coprecipitation and sonochemistry methods, where x=0.2. This rate is the one that presents the best photocatalytic activity and the best performance for the production of hydrogen from a water source, according to our previous results. The influence of the synthesis method on the morphology and its electronic properties of the CdS and Zn0.2Cd0.8S systems was investigated, as well as the effect of the incorporation of Zn in the crystal lattice and its effect on the production of hydrogen from the division of water. X-ray diffraction characterization confirmed the formation of a solid solution corresponding to a face-centered cubic crystalline phase with preferential growth of (111), (220), and (311) planes and hierarchical morphology according to SEM and TEM microscopy, formed by aggregated nanoparticles of ZnxCd1−xS compound. The crystallite size decreased with the Zn incorporation in the crystal lattice from 8.61 nm by a coprecipitation method to 4.37 nm when the synthesis was assisted with sonochemistry, and the crystallite sizes range from 7.92 nm to 3.80 nm for the case of CdS pure. Other characterization techniques were also used, such as photoluminescence (PL), Raman microscopy, and N2 adsorption and desorption. The PL results show tunability of band edge emission as a function of zinc concentration in the Zn0.2Cd0.8S nanoparticles. For the water splitting to hydrogen production, the separation of the charge carriers promoted by the incorporation of Zn in addition to the shallow trap emissions generated defects in the catalyst structure; such processes are important factors found which increased with the sonochemical method, as well as the potential matched positions.


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## Body

## 1. Introduction

The production of H2 from the photolysis of water is one of the current alternatives under development, as it is a way to get cleaner energy. This process has been found to have an acceptable cost with a sustainable environmental impact. Its production requires water and releases oxygen, while its use requires oxygen and releases water vapor, which means a cycle that, in principle, does not compromise the environment [1, 2]. There are different methods for obtaining H2 which are electrolysis, natural gas, oil, and coal, these being very costly and generate high levels of contaminants with low efficiency in the production of hydrogen [3]. The production of H2 using the water photolysis is an efficient method that does not generate pollutants and has a low cost; it is done under a controlled environment with a low energy consumption [4–6].Derived from the need and the current energy demand, it is necessary to propose alternatives that are less harmful to the environment. In this sense, the semiconductor materials designed at the nanometric level turn out to be an alternative [7]. The success of these materials is that with small quantities, adequate and feasible yield production of the desired compound can be generated. It is known that semiconductors have the capacity to generate the pair hollow-electron that provides special and specific characteristics at electronic level. The quantization of energy at subatomic levels allows the manipulation of matter in a reaction pathway. In particular, the solid solution of Zn0.2Cd0.8S is a semiconductor material of direct band, as well as the CdS, being it attractive as photocatalyst for the activation of water for the production of H2 [8, 9]. The advantage of incorporating Zn into the CdS crystal lattice is that it is activated by the irradiation of visible light and does not corrode as easily as pure CdS, in addition to observing a better performance in the water splitting reaction. The proposed mechanism is related to the promotion of the electron to empty levels of energy to generate the hollow-electron pair. Whereas if water is present in the system, instead of the electron returning to the basal state by recombination of the hollow-electron pair, the electron will interact with the electronic states of water, because this process is thermodynamically favorable, thereby reducing the water molecule to produce H2 [10–12]. Furthermore, the types and concentrations of dopants play a key role in the efficiency of photocatalysts and influence their practical applications. Therefore, it is important to investigate how the concentration of impurities (Zn) in the photocatalyst affects the optical properties from the point of view of basic physics and its applications; therefore, the application of the photoluminescence technique provides valuable information in this regard which can be related to tramp states formation [13–17].Based on the aforementioned and according to the studies carried out previously by the group [18, 19], it has been obtained that the relation x=0.2 is the one that presents the best photocatalytic activity for the production of hydrogen such being the reason why we focused on this rate. The current investigation concentrates in the on understanding of whether the assisted synthesis with sonochemistry improves the catalytic activity of the material, as well as to get an insight about relevant aspects, structural, and electronics that promote a better performance due to the presence of Zn within the crystalline lattice of the CdS.


## 2. Methodology

### 2.1. Materials

In order to synthesize nanostructured photocatalysts of CdS and Zn0.2Cd0.8S, the following materials were used as precursors: zinc acetate dehydrate at 98% (Zn (CH3COO)2 • 2 H2O) cadmium acetate at 99% (Cd (CH3COO)2 • 2H2O), sodium sulfide 99% (Na2S • 9 H2O), and ethanol obtained from Nice Chemical Company. All the glassware used in this experimental work were acid washed. The chemical reagents used were analytical reagent grade without further purification. Ultrapure water was used for all dilution and sample preparation.The CdS and Zn0.2Cd0.8S solid solution (x=0.20 wt.% of Zn) were synthesized by a coprecipitation method and also assisted by ultrasonic radiation. This method of synthesis consists in the dissolution of the reagents in an aqueous (deionized water) medium. CdS and ZnxCd1−xS were prepared using Cd (CH3COO)2 • 2 H2O and Zn(CH3COO)2 • 2 H2O (Fluka, analytical grade) at room temperature using 3.3 g of Na2S • 9H2O (analytical grade) as a precipitating agent. The quantity of each precursor was dissolved in 20 mL of water with constant stirring for 15 min at 40°C. For the preparation of the photocatalysts, the C4H6O4Zn solution was added directly into the C4H6O4Cd solution, creating a homogeneous mixture. Afterwards, the ultrasonic radiation was applied, and the Na2S was added dropwise and slowly. A sonotrode used a 27 kHz frequency and a 50% intensity; then, five applications of ultrasonic radiation were done in 90 s intervals. The solution was cooled for 5 min between each application to avoid overheating of the material. The same procedure was used to obtain the pure CdS, namely, CdS-SQ, and the different Zn0.2Cd0.8S solid solutions, namely, Zn0.2Cd0.8S-SQ. Finally, the materials obtained were washed and dried several times. Subsequently, a thermal treatment was given for 18 h in a muffle at 80°C, and the heating was slowly brought from room temperature to 80°C.


### 2.2. Characterization Methods

The X-ray diffraction (XRD) patterns of the powdered samples were recorded using an Advance D8 diffractometer with a Cu Kα radiation (λ=1.54060 Å). Volume-averaged crystallite sizes were determined by applying the Williamson–Hall method [20] because it turns out to be more accurate for compounds with low crystallinity. The surface morphology of the synthetized samples was investigated by scanning electron microscopy (SEM) using a JEOL JSM-6060 LV microscope. A micro-Raman spectrometer DRX II Thermo Scientific model was used, employing a light wavelength of 455 nm (blue laser) and a time exposure of 6 s, with an average of 16 samplings. The spectra were recorded at room temperature within the wavelength range of 50–3500 cm−1. The UV-Vis DR spectra in the 190–800 nm range of the samples were recorded at room temperature using an Ocean Optics Inc. spectrometer (Mini-DT2) provided with a diffuse reflectance accessory. The optical energy bandwidth of the different semiconductors systems was calculated by using the Kubelka-Munk (K-M) function [21] and Tauc’s plot method [22–24] in which the linear part of the curve is extrapolated to find its intersection with the horizontal axis.For the photoluminescence spectra (PL), a He-Cd (Omnichrome–Series 56) laser emitting at 325 nm with an optical excitation power of ~15 mW at room temperature was used. The radiative emission from the sample was focalized to the entrance slit of an HRD-100 Jobin-Yvon double monochromator with a resolution better than 0.05 nm and detected with an Ag-Cs-O Hamamatsu photomultiplier with a spectral response in the range of 350-1000 nm.


### 2.3. Photocatalytic Activity

The hydrogen production from aqueous solutions of Na2S and Na2SO3 over CdS and Zn0.2Cd0.8S photocatalysts was measured under visible light irradiation in a closed quartz reaction cell designed for withdrawal of the gas samples at desired intervals. To minimize small UV emissions from the lamp, the photocatalysts were irradiated under visible light for 5 h with a 350 W Xe lamp equipped with a cutoff filter (l–420 nm). The powder samples (0.2 g) were kept in suspension under magnetic stirring in an aqueous solution (200 mL) containing 0.35 M Na2S+0.25 M Na2SO3 as sacrificial reagents. These salts were added to suppress the photocorrosion of the sulfide-based photocatalysts. Using this mixed solution, the photocatalytic reaction should proceed as reported in [24], where the amount of H2 evolved was detected with an online thermal conductivity detector (TCD) gas chromatography (NaX zeolite column, nitrogen as a carrier gas). Blank experiments showed that hydrogen was not produced without a catalyst being added or light irradiation.


## 2.1. Materials

In order to synthesize nanostructured photocatalysts of CdS and Zn0.2Cd0.8S, the following materials were used as precursors: zinc acetate dehydrate at 98% (Zn (CH3COO)2 • 2 H2O) cadmium acetate at 99% (Cd (CH3COO)2 • 2H2O), sodium sulfide 99% (Na2S • 9 H2O), and ethanol obtained from Nice Chemical Company. All the glassware used in this experimental work were acid washed. The chemical reagents used were analytical reagent grade without further purification. Ultrapure water was used for all dilution and sample preparation.The CdS and Zn0.2Cd0.8S solid solution (x=0.20 wt.% of Zn) were synthesized by a coprecipitation method and also assisted by ultrasonic radiation. This method of synthesis consists in the dissolution of the reagents in an aqueous (deionized water) medium. CdS and ZnxCd1−xS were prepared using Cd (CH3COO)2 • 2 H2O and Zn(CH3COO)2 • 2 H2O (Fluka, analytical grade) at room temperature using 3.3 g of Na2S • 9H2O (analytical grade) as a precipitating agent. The quantity of each precursor was dissolved in 20 mL of water with constant stirring for 15 min at 40°C. For the preparation of the photocatalysts, the C4H6O4Zn solution was added directly into the C4H6O4Cd solution, creating a homogeneous mixture. Afterwards, the ultrasonic radiation was applied, and the Na2S was added dropwise and slowly. A sonotrode used a 27 kHz frequency and a 50% intensity; then, five applications of ultrasonic radiation were done in 90 s intervals. The solution was cooled for 5 min between each application to avoid overheating of the material. The same procedure was used to obtain the pure CdS, namely, CdS-SQ, and the different Zn0.2Cd0.8S solid solutions, namely, Zn0.2Cd0.8S-SQ. Finally, the materials obtained were washed and dried several times. Subsequently, a thermal treatment was given for 18 h in a muffle at 80°C, and the heating was slowly brought from room temperature to 80°C.


## 2.2. Characterization Methods

The X-ray diffraction (XRD) patterns of the powdered samples were recorded using an Advance D8 diffractometer with a Cu Kα radiation (λ=1.54060 Å). Volume-averaged crystallite sizes were determined by applying the Williamson–Hall method [20] because it turns out to be more accurate for compounds with low crystallinity. The surface morphology of the synthetized samples was investigated by scanning electron microscopy (SEM) using a JEOL JSM-6060 LV microscope. A micro-Raman spectrometer DRX II Thermo Scientific model was used, employing a light wavelength of 455 nm (blue laser) and a time exposure of 6 s, with an average of 16 samplings. The spectra were recorded at room temperature within the wavelength range of 50–3500 cm−1. The UV-Vis DR spectra in the 190–800 nm range of the samples were recorded at room temperature using an Ocean Optics Inc. spectrometer (Mini-DT2) provided with a diffuse reflectance accessory. The optical energy bandwidth of the different semiconductors systems was calculated by using the Kubelka-Munk (K-M) function [21] and Tauc’s plot method [22–24] in which the linear part of the curve is extrapolated to find its intersection with the horizontal axis.For the photoluminescence spectra (PL), a He-Cd (Omnichrome–Series 56) laser emitting at 325 nm with an optical excitation power of ~15 mW at room temperature was used. The radiative emission from the sample was focalized to the entrance slit of an HRD-100 Jobin-Yvon double monochromator with a resolution better than 0.05 nm and detected with an Ag-Cs-O Hamamatsu photomultiplier with a spectral response in the range of 350-1000 nm.


## 2.3. Photocatalytic Activity

The hydrogen production from aqueous solutions of Na2S and Na2SO3 over CdS and Zn0.2Cd0.8S photocatalysts was measured under visible light irradiation in a closed quartz reaction cell designed for withdrawal of the gas samples at desired intervals. To minimize small UV emissions from the lamp, the photocatalysts were irradiated under visible light for 5 h with a 350 W Xe lamp equipped with a cutoff filter (l–420 nm). The powder samples (0.2 g) were kept in suspension under magnetic stirring in an aqueous solution (200 mL) containing 0.35 M Na2S+0.25 M Na2SO3 as sacrificial reagents. These salts were added to suppress the photocorrosion of the sulfide-based photocatalysts. Using this mixed solution, the photocatalytic reaction should proceed as reported in [24], where the amount of H2 evolved was detected with an online thermal conductivity detector (TCD) gas chromatography (NaX zeolite column, nitrogen as a carrier gas). Blank experiments showed that hydrogen was not produced without a catalyst being added or light irradiation.


## 3. Results and Discussion

### 3.1. Physicochemical Characterization

#### 3.1.1. XRD Analysis

The XRD patterns of the CdS and Zn0.2Cd0.8S photocatalysts are shown in Figure 1. According to the diffractogram lines obtained for pure CdS and the solid solutions Zn0.2Cd0.8S synthesized for both methods, coprecipitation and method assisted by sonochemistry and X-ray diffractograms show patterns in the form low crystallinity nanoparticles.Figure 1
(a) and (b) Diffractograms of CdS and CdS-SQ by coprecipitation and sonochemistry. (b) and (d) Diffractograms of Zn0.2Cd0.8S and Zn0.2Cd0.8S-SQ by coprecipitation and sonochemistry.For the case of pure CdS, X-ray diffractograms have three characteristic Bragg diffractions at positions 26.45°, 43.88°, and 51.97°, corresponding to the crystallographic planes (111), (220), and (311) of the face-centered cubic crystalline phase which confirm the Hawleyite phase (from International Centre for Diffraction Data, the powder diffraction file (PDF) corresponds to 03-065-2887). All the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ samples showed similar X-ray diffraction profiles confirming the similarity between the reflection sets and the Hawleyite crystalline phase, maintaining even with the incorporation of Zn in the solid solution. However, the Bragg’s reflections, for all ZnxCd1−xS solid solutions and the pure CdS synthetized with both methods, are constituted by wide and poorly defined peaks, thus indicating low crystallinity and size [25, 26]. The crystallite size was assessed through the Williamson–Hall method as displayed in Table 1. The crystallite sizes decreased almost a half range when the synthesis was assisted by sonochemistry; therefore, for the CdS by coprecipitation, it is 8.61 nm and 4.37 nm for the assisted by sonochemistry, CdS-SQ, while for Zn0.2Cd0.8S is 7.92 and for Zn0.2Cd0.8S-SQ ranges in about 3.80 nm.Table 1
Crystallite size (from XRD), wavelength (λ), and energy band gap estimation (ΔEbg) of pure CdS and Zn0.2Cd0.8S solid solution obtained by coprecipitation and assisted by ultrasonic radiation.
CatalystCrystal size (nm)λ (nm)ΔEbgeVCdS8.615642.20CdS-SQ4.375622.21Zn0.2Cd0.8S7.925482.26Zn0.2Cd0.8S-SQ3.805302.34It is also worth noting that the incorporation of Zn into the CdS decreases the size of the crystallite regardless of the synthesis method. This is related to a lattice compression of the crystal lattice, probably due to the size of the metal ions, Cd+2 and Zn+2 (0.97 Ǻ against 0.74 Ǻ [27]), where a slight shift to larger angles is observed in the XRD patterns for Zn0.2Cd0.8S solid solution with respect to CdS (see Figure 1). The XRD reflections shift towards the diffraction angles of ZnS (in the face-centered cubic crystalline phase (PDF) 01-005-0566 [28]) that seems to indicate the formation of a solid solution of Zn0.2Cd0.8S, which is in line with the reported literature [29–31].


#### 3.1.2. BET Analysis

In order to obtain the pore structure and specific surface area of the CdS and Zn0.2Cd0.8S photocatalysts prepared with the different techniques, Brunauer-Emmett-Teller (BET) measurements were performed using a BET analyzer (NOVA 1000e, Surface area and pore size analyzer), and the BET recorded data is presented in Table 2. Figure 2 shows the N2 absorption-desorption isotherms of the samples which matched well with type II isotherm classified by IUPAC; its hysteresis loop was associated with materials that are nonporous or microporous [32]. As can be observed in the table, the pore diameter of CdS (by coprecipitation having a surface area of 105.269 m2/g and pore diameter of 7.808 nm) is slightly greater than CdS-SQ composite (112.173 m2/g, pore diameter 3.418 nm), but smaller in terms of surface area. In the case of the other samples where Zn is present the surface area ranges similar to the CdS sample, the pore diameter resembles the CdS-SQ. It seems that the incorporation of Zn into the CdS crystal lattice and the synthesis assisted by sonochemistry promotes a slight increase in the specific surface area and the total pore volume, which could be related to extra accessible adsorption sites, most likely generated due to defects in the structure, as observed in the micrographs obtained by TEM and corroborated with the photoluminescence analysis carried out (shown later). Also, the BET-specific surface and the reducibility of the catalysts are important factors to be evaluated, because they are related to the good catalytic performance, as previously reported [33].Table 2
Specific surface analysis of synthesized photocatalysts.
SamplesSurface areaPore volumePore diameterCdS105.269 m2/g0.280 cm3/g7.808 nmZn0.2Cd0.8S108.673 m2/g0.318 cm3/g3.415 nmCdS-SQ112.173 m2/g0.322 cm3/g3.418 nmZn0.2Cd0.8S-SQ106.604 m2/g0.309 cm3/g3.414 nmFigure 2
Typical N2 absorption-desorption isotherms of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained with coprecipitation method and assisted with sonochemical prepared samples.


#### 3.1.3. SEM and TEM Analysis

SEM images are shown in Figure3, for the photocatalysts of CdS and Zn0.2Cd0.8S. In the images, it can be observed that the materials are constituted by agglomerates of particles with almost spherical and irregular morphology, forming interconnected open network, mainly mesoporous. In addition, large cavities (macroporosity) can be observed, which is in line with the results obtained by BET analysis. It should be noted that photocatalysts that have a low Zn content (ZnCdS) present a higher microporosity compared to pure CdS in which mesoporosities of uniform size are mainly observed.Figure 3
Micrographics of the synthesized materials with both methods: (a) CdS by coprecipitation; (b) CdS-SQ assisted with sonochemistry; (c) Zn0.2Cd0.8S by coprecipitation; and (d) Zn0.2 Cd0.8S-SQ assisted with sonochemistry. Scale of 1 μm.
(a)(b)(c)(d)TEM and HRTEM techniques were conducted in order to analyze the photocatalyst morphology, interplanar spacing, shape, and growth orientation of crystallites.HRTEM images in Figures4 and 5 show different interatomic planes of the crystalline network formed during the growth of the synthesis of the pure CdS (Figure 4(a1–a3)), CdS-SQ (Figure 4(b1–b3)), Zn0.2Cd0.8S (Figure 5(c1-c3)), and Zn0.2Cd0.8S-SQ (Figure 5(d1-d3)) samples, synthetized for both methods, coprecipitation and assisted with sonochemistry. A small portion of the micrographs obtained with the TEM were analyzed to get insight of the growth of the crystal lattice in a little more detail; these are shown in Figures 4(a1), 4(b1), 5(c1), and 5(d1). Also, a selected portion was processed by the DFT, obtaining the interference pattern in the reciprocal space, which chose a diffuse band in a circular shape formed by points in different positions for pure CdS and Zn0.2Cd0.8S synthetized with both methods (Figures 4(a2), 4(b2), 5(c2), and 5(d2)). The first transformation confirmed the polycrystalline nature of the samples by the different orientations shown in the reciprocal space.Figure 4
High-resolution transmission electron microscopy (HRTEM) patterns of pure CdS. (a1) Synthetized by coprecipitation and (b1) synthesis assisted with sonochemistry, the growth patterns (randomly oriented polycrystalline material); (a2) and (b2) the selected portion shows the Fourier transformed from (a1) and (b1); and (a3) and (b3) show the interplanar spacing between planes.Figure 5
HRTEM pattern of Zn0.2Cd0.8S solid solutions in (c1) is synthetized by coprecipitation and in (d1) is assisted with sonochemistry. (c2) and (d2) The selected portion shows the Fourier transformed from (c1) and (d1); and (c3) and (d3) show the interplanar spacing between planes.Afterwards, two points of the circle were selected to reconstruct the lattice of the underlying crystalline structure. So, with the aid of a virtual mask and a virtual filter included in the software (SAD) by means, an inverse Fourier transform was executed, whereby it allowed us obtain the image back to its almost original appearance, yet with a single orientation of planes and a better definition, sharpness, and contrast. As a result, the distances between the mountains and valleys of these line patterns were obtained, which can be seen in Figures4(a-3), 4(b-3), 5(c-3), and 5(d-3), where all the indexes obtained (hkl) of the diffracted rings agreed well with the JCPDS data 01-089-0440 for the cubic phase of the CdS. The patterns showed that the materials consisted of randomly oriented polycrystalline material in which the (111) plane is the most intense; these are consistent with such obtained in the DRX diffraction patterns, regardless of the sample content Zn; these results indicate that the cubic phase is mainly present. In the micrographs, see Figure 6; it can also be observed that the assisted by sonochemistry method has more defects in the crystal lattice than unassisted one, which can also influence the electronic properties of the material, as will be discussed later.Figure 6
Selected region from micrographs showed in Figures4 and 5, to see a typical HRTEM plan-view magnification of (a) CdS, (b) CdS-SQ, (c) Zn0.2Cd0.8S, and (d) Zn0.2Cd0.8S-SQ solid solutions.
(a)(b)(c)(d)


#### 3.1.4. Raman Spectroscopy Analysis

Figure7 shows the Raman spectra for the pure CdS, CdS-SQ, Zn0.2 Cd0.8S, and Zn0.2 Cd0.8S-SQ solid solutions. A blue laser (455 nm) was used to obtain the longitudinal vibrational modes of the crystal CdS lattice. From the set of spectra shown in Figure 7, a very prominent band around 300 cm−1 corresponds to the first longitudinal mode of optical vibration (LO) of CdS and other less intense bands around 600 cm−1 and 900 cm-1 correspond to the second 2LO and 3LO vibrational modes of cubic CdS [34]. The excitation of three longitudinal order optical phonon modes in the Raman scattering spectrum of nanoparticles of CdS could be attributed to resonance effects; also, the changes observed in the intensity of the Raman peaks could be attributed to the damage and disorder induced by Zn incorporation. For instance, in Table 3, the shift values of the 1LO phonon by the Zn incorporation in the lattice network are shown, in which the values change from ~292 to 297 cm-1 for coprecipitation method and ~287 to 292 cm-1 for the assisted sonochemistry method. It should be noted that the values obtained are slightly lower than some of those reported or expected ~300 cm-1 [35]. This can be attributed to the confinement effect of optical phonons, which is most prominent for the smallest quantum dots (QDs) and also the low crystallinity. The second 2LO vibrational mode for the case of coprecipitation method was registered in 600 cm-1 as expected, both for the assisted sonochemistry method were lowered, ~586 and 590 cm-1 (for pure CdS-SQ and Zn0.2 Cd0.8S-SQ solid solutions, respectively). It should also be noted that the shapes of the signals are broadened in the spectra even though they are not so well defined, that is, that they have a lot of noise, so a marked difference in the exciton-phonon coupling strength cannot be noticed in all samples, but that it can attribute a decrease in the interaction strength, which decreases when the size of the QD also decreases as also has been reported by Seong et al. [36]. The 3LO resonance mode is not observed in the present samples.Figure 7
Raman spectra recorded with the 455 nm laser line for pure CdS and Zn0.2Cd0.8S solid solutions: (a) pure CdS (synthetized by coprecipitation method); (b) pure CdS-SQ (synthetized by assisted sonochemical method); (c) Zn0.2Cd0.8S; (synthetized by coprecipitation method); and (d) Zn0.2Cd0.8S-SQ (synthetized by assisted sonochemical method).Table 3
Photoluminescence wavelength (λ) positions and its corresponding emission energies for the synthetized photocatalysts.
SampleWavelengthλ (nm)Emission energy (eV)CdS431-5282.88-2.34CdS-SQ532, 5952.32, 2.08Zn0.2Cd0.8S623, 6521.98, 1.83Zn0.2Cd0.8S-SQ412, 508, 6433.01, 2.4, 1.92


#### 3.1.5. UV-Vis Diffuse Reflectance Spectra (UV-Vis DRS)

In order to obtain the electronic properties of the synthesized materials, the study of UV-Vis diffuse reflectance spectroscopy was carried out. The semiconductor nature of CdS and ZnS elicits a response under light stimulation in the visible region for CdS (about 600 nm) and in the mild or intermediate UV range for ZnS (200–340 nm). All the photocatalytic samples show a similar absorption edge with differences in the position of the absorption edge as a function of the Zn concentration in the Zn0.2Cd0.8S solid solution. From the UV-Vis spectra shown in Figure 8(a), it is possible to observe that the absorption edges of the Zn0.2Cd0.8S photocatalysts gradually shift from 570 nm to 540 nm, as a result of the incorporation of Zn in the CdS crystal lattice. All visible radiation found after 600 nm is reflected by the material, providing the characteristic color of these compounds, from orange to yellow (Figure 8(c)) which is in line with what was obtained in the UV-Vis spectra. In the case of the Zn0.2Cd0.8S and pure CdS samples synthesized in this work, it is expected that the band gap energy widths will be less than 2.4 eV (reported for bulk CdS [37]) due to the crystalline quality and the small size of the crystallite which promotes changes in the energy gap, as will be discussed below.Figure 8
Diffuse reflectance (DR) UV-Vis spectra of pure CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ solid solutions (a), Tauc’s plots (b), and sample colorings (c).
(a)(b)(c)The optical energy bandwidth of the studied systems CdS and Zn0.2Cd0.8S was obtained using the Kubelka-Munk function [21] and the Tauc graph method [22–24] as mentioned in the characterization methods by using the equation
(1)ahv=Ahv‐Ebgnwhere α is the absorption coefficient, h is Planck’s constant, υ is the photon frequency, A is the probability parameter for the transition, Ebg is the optical band gap, and n=2 for direct allowed transition, as it is the case of these semiconductor solid solutions.Figure8(b) shows the ahυ2 values versus hυ for synthetized photocatalysts. The energy bandwidth Ebg of the pure CdS and Zn0.2Cd0.8S samples, which were calculated from the slope of the Tauc’s plot, are listed in Table 1. From the table, notice that the energy band gap increases by Zn presence. The changes in the gap of Zn0.2Cd0.8S solid solution depend of the crystallite size and indirectly of the synthesis method, because in this case, the assisted sonochemistry method promotes smaller crystallite size of the samples, no matter if it is Zn is or not in the crystalline network, from 8.61 nm for the CdS to 4.37 nm for CdS-SQ and from 7.92 for Zn0.2Cd0.8S to 3.80 nm for the Zn0.2Cd0.8S-SQ. Because the crystallite size is smaller, the band gap energy increases for Zn0.2Cd0.8S-SQ, synthetized by sonochemistry, rather than Zn0.2Cd0.8S synthetized by coprecipitation.


#### 3.1.6. Photoluminescence Measurements

Figure9 displays the room temperature photoluminescence (PL) emission spectra of CdS and Zn0.2Cd0.8S solid solution synthesized by coprecipitation and assisted by ultrasonic radiation following the laser emitting at 325 nm with an optical excitation power of ~15 mW at room temperature. The PL spectra for all the synthesized samples show a wide band of excitation energy with different intensities. Thus, for the pure CdS obtained by coprecipitation, the band ranges from 2 eV to approximately 3.25 eV, presenting a lower intensity than for instance the CdS-SQ, where it is the one with the highest excitonic transition of all.Figure 9
PL spectra of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained by coprecipitation and sonochemistry methods.The emission range for the CdS-SQ ranges from 1.7 eV to 2.7 eV approximately, presenting a maximum peak at 2.1 eV. While for the solid solution of Zn0.2Cd0.8S by coprecipitation, the PL spectrum ranges from approximately 1.7 to 2.5 eV, with a maximum peak at 2.0 eV, showing an intensity similar to that of CdS obtained by coprecipitation. Zn0.2Cd0.8S-SQ displays also a wide spectrum, showing three bands of energy with maximum values in 1.9, 2.4, and 3.0 eV and of major intensity than the Zn0.2Cd0.8S obtained by coprecipitation, being the medium band with a maximum peak in 2.4 eV the most intense. It can be seen that for each sample, there is always a PL energy band lower than its corresponding forbidden band (ΔEbg), except for CdS synthesized by coprecipitation (see Table 1); these PL bands are associated with shallow trap emissions in the system. Therefore, this suggests that the radiative transition occurs from both, the surface states and the excitonic transition. The broad PL peak exhibited a trend of shifting towards higher energies by Zn2+ present in the network for the solid solutions Zn0.2Cd0.8S-SQ powder providing evidence for the formation of nanometric Zn0.2Cd0.8S-SQ crystallites. As all the samples show poor crystalline quality and increased structural and morphological disorders according to XRD, HRTEM micrographics, and micro-Raman results, which is the reason that it generates the greater broadening and asymmetry in the PL signals. The decrease in the crystallite sizes of CdS-SQ and Zn0.2Cd0.8S-SQ solid solutions increase the separation of both inter- and intraband levels, which causes the high intensity emission. The intraband separation increases the trapped luminescence efficiency and inhibits the excitation emission via nonradiative surface recombination.Table3 exhibits the wavelengths and their energies corresponding to the photoluminescence emissions for the synthesized photocatalysts. It can be associated with trap states that are promoted by native defects produced in the nanocrystals due to very small size. Then, the emission bands observed around 595 nm for CdS-SQ, 652 nm for Zn0.2Cd0.8S, and 643 nm for Zn0.2Cd0.8S-SQ correspond to shallow trap emissions from defects in the structure related to the Cd atom, and the emission bands observed around 675 nm corresponds to defects of S atom, also known as vacancies, namely, Vcd and VS; these emissions have also been observed in a previous report but for films of this semiconductor [38]. It is worth mentioning that because the VCd and Vs emissions are very close to each other, it is not possible to observe them as separate signals in the PL spectra and also by mathematical treatment cannot separate them, so they have been placed together as a one signal (Figure 10). While the emission bands around 508 nm can be related to the transition from the shallow trap level, the band around 532 nm radiative transition from the deep trap states promoted by the sulfur vacancy has been also reported. [39]. In the case of the emission band at ~412 nm, it can be attributed to a high-level transition in CdS semiconductor crystallites. Such is generally related to the transition of electrons from the edge of the conduction band to holes, trapped at the interstitial sites of Cd2+ [40]. Also, as it can be observed in Figure 6, there are an increase in the internal defects resulting from increased lattice strain due replacement of Cd2+ ions by Zn2+ ions in ZnxCd1-xS, as it was reported [41]. A blue shift of the excitonic emission is observed in the nanoparticles of Zn0.2Cd0.8S; this is attributed to the incorporation of Zn into the crystal lattice of CdS.Figure 10
Charge carriers’ separation schematic diagram for the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ systems.


### 3.2. Photocatalytic Activity

The photocatalysts of pure CdS and Zn0.2Cd0.8S photocatalysts were evaluated for hydrogen production by a water separation reaction under visible light irradiation (λ=420 nm). The reaction was performed at room temperature in an aqueous solution containing Na2SO3 and Na2S as sacrifice reagents. In the control experiments, in pure Na2S/Na2SO3 solutions, there was a lack of hydrogen detachment observed in the absence of visible light radiation or photocatalysts. The rate of hydrogen detachment based on irradiation time is shown in Figure 11. As can be seen, the pure CdS photocatalyst shows low photoactivity for H2 production and has an increase when Zn is present in the sample. According to previous studies of the group, the maximum amount of hydrogen production is when the concentration of Zn in solid solutions is 0.2% by weight. In Table 3, the H2 production at 5 hr is presented. Since it is possible to see in the table the production of H2 when Zn is present, in the photocatalyst, ZnCdS is higher than for pure CdS, regardless of the synthesis technique used. It is also possible to observe that when the synthesis is assisted with sonochemistry, the production of H2 is slightly improved. This can be related to several aspects which are discussed below.Figure 11
Performance in hydrogen production as a function of time.According to the reported data, it was demonstrated that the CdS efficiently captures the energy of light through the excitation of electrons through its direct band gap. However, it manifests a low photocatalytic activity for hydrogen production, which is due to the rapid recombination of the hollow-electron pair, while with the incorporation of Zn in the network of the catalyst, the recombination process is slower; CdS is absorbed in the visible region of light, while ZnS is absorbed in the ultraviolet region; the incorporation of Zn in the CdS catalyst network showed a slight shift towards blue. These observations are due to the overall size and potential-well effects, where the potential-well is deeper for ZnCdS than CdS NPs. It should also be mentioned that the incorporation of Zn in the network generates a stress, which can result in an asymmetric internal electric field across the interface that could change both the electronic states and the absorption properties of the photocatalyst.As it is known in a catalytic reduction-oxidation (redox) reaction, the duration of the hollow pair of electrons together with the selection of the semiconductors with the matched potential positions is very important, related to this in Figure11; an estimate of these was made for the different synthesized systems. The band gap (ΔEbg), valence band (VB), and the conduction band (CB) are shown in the figure, as well as the shallow trap emissions from defects in the structure related to the Cd and S atom (VCd and VS) that were observed by photoluminescence have been also incorporated. This shallow trap emissions can act as a reservoir of electrons promoting a higher production of H2, according to the position of the energy levels estimated shown in Figure 10, while the shallow trap levels are at edge of the conduction band and the deep trap states can no longer be activated with visible light. The energetic position of the trap states together with the band conduction (for the CdS-SQ, the ZnCdS, and the ZnCdS-SQ that have a negative potential [40] in regard to the potential of H+/H2 which is set at 0 eV) is associated to the increment in the hydrogen production (see Table 4).Table 4
Comparison of some characteristics of unsupported and supported Zn0.2Cd0.8S photocatalysts tested in water-splitting reaction in the same conditionsa.
Bulk Zn0.2Cd0.8SBulk Zn0.2Cd0.8SBulk CdSBulk CdSPreparation methodSonochemicalCoprecipitationSonochemicalCoprecipitationMorphologyHierarchical 3D morphologyHierarchical 3D morphologyHierarchical 3D morphologyHierarchical 3D morphologyH2 production at 5 h (μmol)1044981308243ΔEbg (eV)2.342.262.212.20aReaction conditions were Wcat.=0.2 g, Vsolution=200 mL, 350 W Xe, visible light irradiation time of 5 h, and 0.35 M Na2S/0.25 M Na2SO3 sacrificial agents.From the table, the hydrogen production that can be observed is considerably higher for ZnCdS and ZnCdS-SQ since, in addition to the aforementioned, a greater separation of the charge carriers was obtained due to the presence of Zn in the semiconductor material, which lengthens more the duration of the hole-electron pair. In the case of CdS-SQ, a slight increase in hydrogen production was observed with respect to the CdS synthesized by coprecipitation. This can be attributed to the fact that it was possible to reduce the size of the crystallite and the presence of shallow trap emissions, but the rapid recombination of the hollow-electron pair occurs, which has a significant impact in the performance of the photocatalyst for the hydrogen production.


## 3.1. Physicochemical Characterization

### 3.1.1. XRD Analysis

The XRD patterns of the CdS and Zn0.2Cd0.8S photocatalysts are shown in Figure 1. According to the diffractogram lines obtained for pure CdS and the solid solutions Zn0.2Cd0.8S synthesized for both methods, coprecipitation and method assisted by sonochemistry and X-ray diffractograms show patterns in the form low crystallinity nanoparticles.Figure 1
(a) and (b) Diffractograms of CdS and CdS-SQ by coprecipitation and sonochemistry. (b) and (d) Diffractograms of Zn0.2Cd0.8S and Zn0.2Cd0.8S-SQ by coprecipitation and sonochemistry.For the case of pure CdS, X-ray diffractograms have three characteristic Bragg diffractions at positions 26.45°, 43.88°, and 51.97°, corresponding to the crystallographic planes (111), (220), and (311) of the face-centered cubic crystalline phase which confirm the Hawleyite phase (from International Centre for Diffraction Data, the powder diffraction file (PDF) corresponds to 03-065-2887). All the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ samples showed similar X-ray diffraction profiles confirming the similarity between the reflection sets and the Hawleyite crystalline phase, maintaining even with the incorporation of Zn in the solid solution. However, the Bragg’s reflections, for all ZnxCd1−xS solid solutions and the pure CdS synthetized with both methods, are constituted by wide and poorly defined peaks, thus indicating low crystallinity and size [25, 26]. The crystallite size was assessed through the Williamson–Hall method as displayed in Table 1. The crystallite sizes decreased almost a half range when the synthesis was assisted by sonochemistry; therefore, for the CdS by coprecipitation, it is 8.61 nm and 4.37 nm for the assisted by sonochemistry, CdS-SQ, while for Zn0.2Cd0.8S is 7.92 and for Zn0.2Cd0.8S-SQ ranges in about 3.80 nm.Table 1
Crystallite size (from XRD), wavelength (λ), and energy band gap estimation (ΔEbg) of pure CdS and Zn0.2Cd0.8S solid solution obtained by coprecipitation and assisted by ultrasonic radiation.
CatalystCrystal size (nm)λ (nm)ΔEbgeVCdS8.615642.20CdS-SQ4.375622.21Zn0.2Cd0.8S7.925482.26Zn0.2Cd0.8S-SQ3.805302.34It is also worth noting that the incorporation of Zn into the CdS decreases the size of the crystallite regardless of the synthesis method. This is related to a lattice compression of the crystal lattice, probably due to the size of the metal ions, Cd+2 and Zn+2 (0.97 Ǻ against 0.74 Ǻ [27]), where a slight shift to larger angles is observed in the XRD patterns for Zn0.2Cd0.8S solid solution with respect to CdS (see Figure 1). The XRD reflections shift towards the diffraction angles of ZnS (in the face-centered cubic crystalline phase (PDF) 01-005-0566 [28]) that seems to indicate the formation of a solid solution of Zn0.2Cd0.8S, which is in line with the reported literature [29–31].


### 3.1.2. BET Analysis

In order to obtain the pore structure and specific surface area of the CdS and Zn0.2Cd0.8S photocatalysts prepared with the different techniques, Brunauer-Emmett-Teller (BET) measurements were performed using a BET analyzer (NOVA 1000e, Surface area and pore size analyzer), and the BET recorded data is presented in Table 2. Figure 2 shows the N2 absorption-desorption isotherms of the samples which matched well with type II isotherm classified by IUPAC; its hysteresis loop was associated with materials that are nonporous or microporous [32]. As can be observed in the table, the pore diameter of CdS (by coprecipitation having a surface area of 105.269 m2/g and pore diameter of 7.808 nm) is slightly greater than CdS-SQ composite (112.173 m2/g, pore diameter 3.418 nm), but smaller in terms of surface area. In the case of the other samples where Zn is present the surface area ranges similar to the CdS sample, the pore diameter resembles the CdS-SQ. It seems that the incorporation of Zn into the CdS crystal lattice and the synthesis assisted by sonochemistry promotes a slight increase in the specific surface area and the total pore volume, which could be related to extra accessible adsorption sites, most likely generated due to defects in the structure, as observed in the micrographs obtained by TEM and corroborated with the photoluminescence analysis carried out (shown later). Also, the BET-specific surface and the reducibility of the catalysts are important factors to be evaluated, because they are related to the good catalytic performance, as previously reported [33].Table 2
Specific surface analysis of synthesized photocatalysts.
SamplesSurface areaPore volumePore diameterCdS105.269 m2/g0.280 cm3/g7.808 nmZn0.2Cd0.8S108.673 m2/g0.318 cm3/g3.415 nmCdS-SQ112.173 m2/g0.322 cm3/g3.418 nmZn0.2Cd0.8S-SQ106.604 m2/g0.309 cm3/g3.414 nmFigure 2
Typical N2 absorption-desorption isotherms of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained with coprecipitation method and assisted with sonochemical prepared samples.


### 3.1.3. SEM and TEM Analysis

SEM images are shown in Figure3, for the photocatalysts of CdS and Zn0.2Cd0.8S. In the images, it can be observed that the materials are constituted by agglomerates of particles with almost spherical and irregular morphology, forming interconnected open network, mainly mesoporous. In addition, large cavities (macroporosity) can be observed, which is in line with the results obtained by BET analysis. It should be noted that photocatalysts that have a low Zn content (ZnCdS) present a higher microporosity compared to pure CdS in which mesoporosities of uniform size are mainly observed.Figure 3
Micrographics of the synthesized materials with both methods: (a) CdS by coprecipitation; (b) CdS-SQ assisted with sonochemistry; (c) Zn0.2Cd0.8S by coprecipitation; and (d) Zn0.2 Cd0.8S-SQ assisted with sonochemistry. Scale of 1 μm.
(a)(b)(c)(d)TEM and HRTEM techniques were conducted in order to analyze the photocatalyst morphology, interplanar spacing, shape, and growth orientation of crystallites.HRTEM images in Figures4 and 5 show different interatomic planes of the crystalline network formed during the growth of the synthesis of the pure CdS (Figure 4(a1–a3)), CdS-SQ (Figure 4(b1–b3)), Zn0.2Cd0.8S (Figure 5(c1-c3)), and Zn0.2Cd0.8S-SQ (Figure 5(d1-d3)) samples, synthetized for both methods, coprecipitation and assisted with sonochemistry. A small portion of the micrographs obtained with the TEM were analyzed to get insight of the growth of the crystal lattice in a little more detail; these are shown in Figures 4(a1), 4(b1), 5(c1), and 5(d1). Also, a selected portion was processed by the DFT, obtaining the interference pattern in the reciprocal space, which chose a diffuse band in a circular shape formed by points in different positions for pure CdS and Zn0.2Cd0.8S synthetized with both methods (Figures 4(a2), 4(b2), 5(c2), and 5(d2)). The first transformation confirmed the polycrystalline nature of the samples by the different orientations shown in the reciprocal space.Figure 4
High-resolution transmission electron microscopy (HRTEM) patterns of pure CdS. (a1) Synthetized by coprecipitation and (b1) synthesis assisted with sonochemistry, the growth patterns (randomly oriented polycrystalline material); (a2) and (b2) the selected portion shows the Fourier transformed from (a1) and (b1); and (a3) and (b3) show the interplanar spacing between planes.Figure 5
HRTEM pattern of Zn0.2Cd0.8S solid solutions in (c1) is synthetized by coprecipitation and in (d1) is assisted with sonochemistry. (c2) and (d2) The selected portion shows the Fourier transformed from (c1) and (d1); and (c3) and (d3) show the interplanar spacing between planes.Afterwards, two points of the circle were selected to reconstruct the lattice of the underlying crystalline structure. So, with the aid of a virtual mask and a virtual filter included in the software (SAD) by means, an inverse Fourier transform was executed, whereby it allowed us obtain the image back to its almost original appearance, yet with a single orientation of planes and a better definition, sharpness, and contrast. As a result, the distances between the mountains and valleys of these line patterns were obtained, which can be seen in Figures4(a-3), 4(b-3), 5(c-3), and 5(d-3), where all the indexes obtained (hkl) of the diffracted rings agreed well with the JCPDS data 01-089-0440 for the cubic phase of the CdS. The patterns showed that the materials consisted of randomly oriented polycrystalline material in which the (111) plane is the most intense; these are consistent with such obtained in the DRX diffraction patterns, regardless of the sample content Zn; these results indicate that the cubic phase is mainly present. In the micrographs, see Figure 6; it can also be observed that the assisted by sonochemistry method has more defects in the crystal lattice than unassisted one, which can also influence the electronic properties of the material, as will be discussed later.Figure 6
Selected region from micrographs showed in Figures4 and 5, to see a typical HRTEM plan-view magnification of (a) CdS, (b) CdS-SQ, (c) Zn0.2Cd0.8S, and (d) Zn0.2Cd0.8S-SQ solid solutions.
(a)(b)(c)(d)


### 3.1.4. Raman Spectroscopy Analysis

Figure7 shows the Raman spectra for the pure CdS, CdS-SQ, Zn0.2 Cd0.8S, and Zn0.2 Cd0.8S-SQ solid solutions. A blue laser (455 nm) was used to obtain the longitudinal vibrational modes of the crystal CdS lattice. From the set of spectra shown in Figure 7, a very prominent band around 300 cm−1 corresponds to the first longitudinal mode of optical vibration (LO) of CdS and other less intense bands around 600 cm−1 and 900 cm-1 correspond to the second 2LO and 3LO vibrational modes of cubic CdS [34]. The excitation of three longitudinal order optical phonon modes in the Raman scattering spectrum of nanoparticles of CdS could be attributed to resonance effects; also, the changes observed in the intensity of the Raman peaks could be attributed to the damage and disorder induced by Zn incorporation. For instance, in Table 3, the shift values of the 1LO phonon by the Zn incorporation in the lattice network are shown, in which the values change from ~292 to 297 cm-1 for coprecipitation method and ~287 to 292 cm-1 for the assisted sonochemistry method. It should be noted that the values obtained are slightly lower than some of those reported or expected ~300 cm-1 [35]. This can be attributed to the confinement effect of optical phonons, which is most prominent for the smallest quantum dots (QDs) and also the low crystallinity. The second 2LO vibrational mode for the case of coprecipitation method was registered in 600 cm-1 as expected, both for the assisted sonochemistry method were lowered, ~586 and 590 cm-1 (for pure CdS-SQ and Zn0.2 Cd0.8S-SQ solid solutions, respectively). It should also be noted that the shapes of the signals are broadened in the spectra even though they are not so well defined, that is, that they have a lot of noise, so a marked difference in the exciton-phonon coupling strength cannot be noticed in all samples, but that it can attribute a decrease in the interaction strength, which decreases when the size of the QD also decreases as also has been reported by Seong et al. [36]. The 3LO resonance mode is not observed in the present samples.Figure 7
Raman spectra recorded with the 455 nm laser line for pure CdS and Zn0.2Cd0.8S solid solutions: (a) pure CdS (synthetized by coprecipitation method); (b) pure CdS-SQ (synthetized by assisted sonochemical method); (c) Zn0.2Cd0.8S; (synthetized by coprecipitation method); and (d) Zn0.2Cd0.8S-SQ (synthetized by assisted sonochemical method).Table 3
Photoluminescence wavelength (λ) positions and its corresponding emission energies for the synthetized photocatalysts.
SampleWavelengthλ (nm)Emission energy (eV)CdS431-5282.88-2.34CdS-SQ532, 5952.32, 2.08Zn0.2Cd0.8S623, 6521.98, 1.83Zn0.2Cd0.8S-SQ412, 508, 6433.01, 2.4, 1.92


### 3.1.5. UV-Vis Diffuse Reflectance Spectra (UV-Vis DRS)

In order to obtain the electronic properties of the synthesized materials, the study of UV-Vis diffuse reflectance spectroscopy was carried out. The semiconductor nature of CdS and ZnS elicits a response under light stimulation in the visible region for CdS (about 600 nm) and in the mild or intermediate UV range for ZnS (200–340 nm). All the photocatalytic samples show a similar absorption edge with differences in the position of the absorption edge as a function of the Zn concentration in the Zn0.2Cd0.8S solid solution. From the UV-Vis spectra shown in Figure 8(a), it is possible to observe that the absorption edges of the Zn0.2Cd0.8S photocatalysts gradually shift from 570 nm to 540 nm, as a result of the incorporation of Zn in the CdS crystal lattice. All visible radiation found after 600 nm is reflected by the material, providing the characteristic color of these compounds, from orange to yellow (Figure 8(c)) which is in line with what was obtained in the UV-Vis spectra. In the case of the Zn0.2Cd0.8S and pure CdS samples synthesized in this work, it is expected that the band gap energy widths will be less than 2.4 eV (reported for bulk CdS [37]) due to the crystalline quality and the small size of the crystallite which promotes changes in the energy gap, as will be discussed below.Figure 8
Diffuse reflectance (DR) UV-Vis spectra of pure CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ solid solutions (a), Tauc’s plots (b), and sample colorings (c).
(a)(b)(c)The optical energy bandwidth of the studied systems CdS and Zn0.2Cd0.8S was obtained using the Kubelka-Munk function [21] and the Tauc graph method [22–24] as mentioned in the characterization methods by using the equation
(1)ahv=Ahv‐Ebgnwhere α is the absorption coefficient, h is Planck’s constant, υ is the photon frequency, A is the probability parameter for the transition, Ebg is the optical band gap, and n=2 for direct allowed transition, as it is the case of these semiconductor solid solutions.Figure8(b) shows the ahυ2 values versus hυ for synthetized photocatalysts. The energy bandwidth Ebg of the pure CdS and Zn0.2Cd0.8S samples, which were calculated from the slope of the Tauc’s plot, are listed in Table 1. From the table, notice that the energy band gap increases by Zn presence. The changes in the gap of Zn0.2Cd0.8S solid solution depend of the crystallite size and indirectly of the synthesis method, because in this case, the assisted sonochemistry method promotes smaller crystallite size of the samples, no matter if it is Zn is or not in the crystalline network, from 8.61 nm for the CdS to 4.37 nm for CdS-SQ and from 7.92 for Zn0.2Cd0.8S to 3.80 nm for the Zn0.2Cd0.8S-SQ. Because the crystallite size is smaller, the band gap energy increases for Zn0.2Cd0.8S-SQ, synthetized by sonochemistry, rather than Zn0.2Cd0.8S synthetized by coprecipitation.


### 3.1.6. Photoluminescence Measurements

Figure9 displays the room temperature photoluminescence (PL) emission spectra of CdS and Zn0.2Cd0.8S solid solution synthesized by coprecipitation and assisted by ultrasonic radiation following the laser emitting at 325 nm with an optical excitation power of ~15 mW at room temperature. The PL spectra for all the synthesized samples show a wide band of excitation energy with different intensities. Thus, for the pure CdS obtained by coprecipitation, the band ranges from 2 eV to approximately 3.25 eV, presenting a lower intensity than for instance the CdS-SQ, where it is the one with the highest excitonic transition of all.Figure 9
PL spectra of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained by coprecipitation and sonochemistry methods.The emission range for the CdS-SQ ranges from 1.7 eV to 2.7 eV approximately, presenting a maximum peak at 2.1 eV. While for the solid solution of Zn0.2Cd0.8S by coprecipitation, the PL spectrum ranges from approximately 1.7 to 2.5 eV, with a maximum peak at 2.0 eV, showing an intensity similar to that of CdS obtained by coprecipitation. Zn0.2Cd0.8S-SQ displays also a wide spectrum, showing three bands of energy with maximum values in 1.9, 2.4, and 3.0 eV and of major intensity than the Zn0.2Cd0.8S obtained by coprecipitation, being the medium band with a maximum peak in 2.4 eV the most intense. It can be seen that for each sample, there is always a PL energy band lower than its corresponding forbidden band (ΔEbg), except for CdS synthesized by coprecipitation (see Table 1); these PL bands are associated with shallow trap emissions in the system. Therefore, this suggests that the radiative transition occurs from both, the surface states and the excitonic transition. The broad PL peak exhibited a trend of shifting towards higher energies by Zn2+ present in the network for the solid solutions Zn0.2Cd0.8S-SQ powder providing evidence for the formation of nanometric Zn0.2Cd0.8S-SQ crystallites. As all the samples show poor crystalline quality and increased structural and morphological disorders according to XRD, HRTEM micrographics, and micro-Raman results, which is the reason that it generates the greater broadening and asymmetry in the PL signals. The decrease in the crystallite sizes of CdS-SQ and Zn0.2Cd0.8S-SQ solid solutions increase the separation of both inter- and intraband levels, which causes the high intensity emission. The intraband separation increases the trapped luminescence efficiency and inhibits the excitation emission via nonradiative surface recombination.Table3 exhibits the wavelengths and their energies corresponding to the photoluminescence emissions for the synthesized photocatalysts. It can be associated with trap states that are promoted by native defects produced in the nanocrystals due to very small size. Then, the emission bands observed around 595 nm for CdS-SQ, 652 nm for Zn0.2Cd0.8S, and 643 nm for Zn0.2Cd0.8S-SQ correspond to shallow trap emissions from defects in the structure related to the Cd atom, and the emission bands observed around 675 nm corresponds to defects of S atom, also known as vacancies, namely, Vcd and VS; these emissions have also been observed in a previous report but for films of this semiconductor [38]. It is worth mentioning that because the VCd and Vs emissions are very close to each other, it is not possible to observe them as separate signals in the PL spectra and also by mathematical treatment cannot separate them, so they have been placed together as a one signal (Figure 10). While the emission bands around 508 nm can be related to the transition from the shallow trap level, the band around 532 nm radiative transition from the deep trap states promoted by the sulfur vacancy has been also reported. [39]. In the case of the emission band at ~412 nm, it can be attributed to a high-level transition in CdS semiconductor crystallites. Such is generally related to the transition of electrons from the edge of the conduction band to holes, trapped at the interstitial sites of Cd2+ [40]. Also, as it can be observed in Figure 6, there are an increase in the internal defects resulting from increased lattice strain due replacement of Cd2+ ions by Zn2+ ions in ZnxCd1-xS, as it was reported [41]. A blue shift of the excitonic emission is observed in the nanoparticles of Zn0.2Cd0.8S; this is attributed to the incorporation of Zn into the crystal lattice of CdS.Figure 10
Charge carriers’ separation schematic diagram for the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ systems.


## 3.1.1. XRD Analysis

The XRD patterns of the CdS and Zn0.2Cd0.8S photocatalysts are shown in Figure 1. According to the diffractogram lines obtained for pure CdS and the solid solutions Zn0.2Cd0.8S synthesized for both methods, coprecipitation and method assisted by sonochemistry and X-ray diffractograms show patterns in the form low crystallinity nanoparticles.Figure 1
(a) and (b) Diffractograms of CdS and CdS-SQ by coprecipitation and sonochemistry. (b) and (d) Diffractograms of Zn0.2Cd0.8S and Zn0.2Cd0.8S-SQ by coprecipitation and sonochemistry.For the case of pure CdS, X-ray diffractograms have three characteristic Bragg diffractions at positions 26.45°, 43.88°, and 51.97°, corresponding to the crystallographic planes (111), (220), and (311) of the face-centered cubic crystalline phase which confirm the Hawleyite phase (from International Centre for Diffraction Data, the powder diffraction file (PDF) corresponds to 03-065-2887). All the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ samples showed similar X-ray diffraction profiles confirming the similarity between the reflection sets and the Hawleyite crystalline phase, maintaining even with the incorporation of Zn in the solid solution. However, the Bragg’s reflections, for all ZnxCd1−xS solid solutions and the pure CdS synthetized with both methods, are constituted by wide and poorly defined peaks, thus indicating low crystallinity and size [25, 26]. The crystallite size was assessed through the Williamson–Hall method as displayed in Table 1. The crystallite sizes decreased almost a half range when the synthesis was assisted by sonochemistry; therefore, for the CdS by coprecipitation, it is 8.61 nm and 4.37 nm for the assisted by sonochemistry, CdS-SQ, while for Zn0.2Cd0.8S is 7.92 and for Zn0.2Cd0.8S-SQ ranges in about 3.80 nm.Table 1
Crystallite size (from XRD), wavelength (λ), and energy band gap estimation (ΔEbg) of pure CdS and Zn0.2Cd0.8S solid solution obtained by coprecipitation and assisted by ultrasonic radiation.
CatalystCrystal size (nm)λ (nm)ΔEbgeVCdS8.615642.20CdS-SQ4.375622.21Zn0.2Cd0.8S7.925482.26Zn0.2Cd0.8S-SQ3.805302.34It is also worth noting that the incorporation of Zn into the CdS decreases the size of the crystallite regardless of the synthesis method. This is related to a lattice compression of the crystal lattice, probably due to the size of the metal ions, Cd+2 and Zn+2 (0.97 Ǻ against 0.74 Ǻ [27]), where a slight shift to larger angles is observed in the XRD patterns for Zn0.2Cd0.8S solid solution with respect to CdS (see Figure 1). The XRD reflections shift towards the diffraction angles of ZnS (in the face-centered cubic crystalline phase (PDF) 01-005-0566 [28]) that seems to indicate the formation of a solid solution of Zn0.2Cd0.8S, which is in line with the reported literature [29–31].


## 3.1.2. BET Analysis

In order to obtain the pore structure and specific surface area of the CdS and Zn0.2Cd0.8S photocatalysts prepared with the different techniques, Brunauer-Emmett-Teller (BET) measurements were performed using a BET analyzer (NOVA 1000e, Surface area and pore size analyzer), and the BET recorded data is presented in Table 2. Figure 2 shows the N2 absorption-desorption isotherms of the samples which matched well with type II isotherm classified by IUPAC; its hysteresis loop was associated with materials that are nonporous or microporous [32]. As can be observed in the table, the pore diameter of CdS (by coprecipitation having a surface area of 105.269 m2/g and pore diameter of 7.808 nm) is slightly greater than CdS-SQ composite (112.173 m2/g, pore diameter 3.418 nm), but smaller in terms of surface area. In the case of the other samples where Zn is present the surface area ranges similar to the CdS sample, the pore diameter resembles the CdS-SQ. It seems that the incorporation of Zn into the CdS crystal lattice and the synthesis assisted by sonochemistry promotes a slight increase in the specific surface area and the total pore volume, which could be related to extra accessible adsorption sites, most likely generated due to defects in the structure, as observed in the micrographs obtained by TEM and corroborated with the photoluminescence analysis carried out (shown later). Also, the BET-specific surface and the reducibility of the catalysts are important factors to be evaluated, because they are related to the good catalytic performance, as previously reported [33].Table 2
Specific surface analysis of synthesized photocatalysts.
SamplesSurface areaPore volumePore diameterCdS105.269 m2/g0.280 cm3/g7.808 nmZn0.2Cd0.8S108.673 m2/g0.318 cm3/g3.415 nmCdS-SQ112.173 m2/g0.322 cm3/g3.418 nmZn0.2Cd0.8S-SQ106.604 m2/g0.309 cm3/g3.414 nmFigure 2
Typical N2 absorption-desorption isotherms of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained with coprecipitation method and assisted with sonochemical prepared samples.


## 3.1.3. SEM and TEM Analysis

SEM images are shown in Figure3, for the photocatalysts of CdS and Zn0.2Cd0.8S. In the images, it can be observed that the materials are constituted by agglomerates of particles with almost spherical and irregular morphology, forming interconnected open network, mainly mesoporous. In addition, large cavities (macroporosity) can be observed, which is in line with the results obtained by BET analysis. It should be noted that photocatalysts that have a low Zn content (ZnCdS) present a higher microporosity compared to pure CdS in which mesoporosities of uniform size are mainly observed.Figure 3
Micrographics of the synthesized materials with both methods: (a) CdS by coprecipitation; (b) CdS-SQ assisted with sonochemistry; (c) Zn0.2Cd0.8S by coprecipitation; and (d) Zn0.2 Cd0.8S-SQ assisted with sonochemistry. Scale of 1 μm.
(a)(b)(c)(d)TEM and HRTEM techniques were conducted in order to analyze the photocatalyst morphology, interplanar spacing, shape, and growth orientation of crystallites.HRTEM images in Figures4 and 5 show different interatomic planes of the crystalline network formed during the growth of the synthesis of the pure CdS (Figure 4(a1–a3)), CdS-SQ (Figure 4(b1–b3)), Zn0.2Cd0.8S (Figure 5(c1-c3)), and Zn0.2Cd0.8S-SQ (Figure 5(d1-d3)) samples, synthetized for both methods, coprecipitation and assisted with sonochemistry. A small portion of the micrographs obtained with the TEM were analyzed to get insight of the growth of the crystal lattice in a little more detail; these are shown in Figures 4(a1), 4(b1), 5(c1), and 5(d1). Also, a selected portion was processed by the DFT, obtaining the interference pattern in the reciprocal space, which chose a diffuse band in a circular shape formed by points in different positions for pure CdS and Zn0.2Cd0.8S synthetized with both methods (Figures 4(a2), 4(b2), 5(c2), and 5(d2)). The first transformation confirmed the polycrystalline nature of the samples by the different orientations shown in the reciprocal space.Figure 4
High-resolution transmission electron microscopy (HRTEM) patterns of pure CdS. (a1) Synthetized by coprecipitation and (b1) synthesis assisted with sonochemistry, the growth patterns (randomly oriented polycrystalline material); (a2) and (b2) the selected portion shows the Fourier transformed from (a1) and (b1); and (a3) and (b3) show the interplanar spacing between planes.Figure 5
HRTEM pattern of Zn0.2Cd0.8S solid solutions in (c1) is synthetized by coprecipitation and in (d1) is assisted with sonochemistry. (c2) and (d2) The selected portion shows the Fourier transformed from (c1) and (d1); and (c3) and (d3) show the interplanar spacing between planes.Afterwards, two points of the circle were selected to reconstruct the lattice of the underlying crystalline structure. So, with the aid of a virtual mask and a virtual filter included in the software (SAD) by means, an inverse Fourier transform was executed, whereby it allowed us obtain the image back to its almost original appearance, yet with a single orientation of planes and a better definition, sharpness, and contrast. As a result, the distances between the mountains and valleys of these line patterns were obtained, which can be seen in Figures4(a-3), 4(b-3), 5(c-3), and 5(d-3), where all the indexes obtained (hkl) of the diffracted rings agreed well with the JCPDS data 01-089-0440 for the cubic phase of the CdS. The patterns showed that the materials consisted of randomly oriented polycrystalline material in which the (111) plane is the most intense; these are consistent with such obtained in the DRX diffraction patterns, regardless of the sample content Zn; these results indicate that the cubic phase is mainly present. In the micrographs, see Figure 6; it can also be observed that the assisted by sonochemistry method has more defects in the crystal lattice than unassisted one, which can also influence the electronic properties of the material, as will be discussed later.Figure 6
Selected region from micrographs showed in Figures4 and 5, to see a typical HRTEM plan-view magnification of (a) CdS, (b) CdS-SQ, (c) Zn0.2Cd0.8S, and (d) Zn0.2Cd0.8S-SQ solid solutions.
(a)(b)(c)(d)


## 3.1.4. Raman Spectroscopy Analysis

Figure7 shows the Raman spectra for the pure CdS, CdS-SQ, Zn0.2 Cd0.8S, and Zn0.2 Cd0.8S-SQ solid solutions. A blue laser (455 nm) was used to obtain the longitudinal vibrational modes of the crystal CdS lattice. From the set of spectra shown in Figure 7, a very prominent band around 300 cm−1 corresponds to the first longitudinal mode of optical vibration (LO) of CdS and other less intense bands around 600 cm−1 and 900 cm-1 correspond to the second 2LO and 3LO vibrational modes of cubic CdS [34]. The excitation of three longitudinal order optical phonon modes in the Raman scattering spectrum of nanoparticles of CdS could be attributed to resonance effects; also, the changes observed in the intensity of the Raman peaks could be attributed to the damage and disorder induced by Zn incorporation. For instance, in Table 3, the shift values of the 1LO phonon by the Zn incorporation in the lattice network are shown, in which the values change from ~292 to 297 cm-1 for coprecipitation method and ~287 to 292 cm-1 for the assisted sonochemistry method. It should be noted that the values obtained are slightly lower than some of those reported or expected ~300 cm-1 [35]. This can be attributed to the confinement effect of optical phonons, which is most prominent for the smallest quantum dots (QDs) and also the low crystallinity. The second 2LO vibrational mode for the case of coprecipitation method was registered in 600 cm-1 as expected, both for the assisted sonochemistry method were lowered, ~586 and 590 cm-1 (for pure CdS-SQ and Zn0.2 Cd0.8S-SQ solid solutions, respectively). It should also be noted that the shapes of the signals are broadened in the spectra even though they are not so well defined, that is, that they have a lot of noise, so a marked difference in the exciton-phonon coupling strength cannot be noticed in all samples, but that it can attribute a decrease in the interaction strength, which decreases when the size of the QD also decreases as also has been reported by Seong et al. [36]. The 3LO resonance mode is not observed in the present samples.Figure 7
Raman spectra recorded with the 455 nm laser line for pure CdS and Zn0.2Cd0.8S solid solutions: (a) pure CdS (synthetized by coprecipitation method); (b) pure CdS-SQ (synthetized by assisted sonochemical method); (c) Zn0.2Cd0.8S; (synthetized by coprecipitation method); and (d) Zn0.2Cd0.8S-SQ (synthetized by assisted sonochemical method).Table 3
Photoluminescence wavelength (λ) positions and its corresponding emission energies for the synthetized photocatalysts.
SampleWavelengthλ (nm)Emission energy (eV)CdS431-5282.88-2.34CdS-SQ532, 5952.32, 2.08Zn0.2Cd0.8S623, 6521.98, 1.83Zn0.2Cd0.8S-SQ412, 508, 6433.01, 2.4, 1.92


## 3.1.5. UV-Vis Diffuse Reflectance Spectra (UV-Vis DRS)

In order to obtain the electronic properties of the synthesized materials, the study of UV-Vis diffuse reflectance spectroscopy was carried out. The semiconductor nature of CdS and ZnS elicits a response under light stimulation in the visible region for CdS (about 600 nm) and in the mild or intermediate UV range for ZnS (200–340 nm). All the photocatalytic samples show a similar absorption edge with differences in the position of the absorption edge as a function of the Zn concentration in the Zn0.2Cd0.8S solid solution. From the UV-Vis spectra shown in Figure 8(a), it is possible to observe that the absorption edges of the Zn0.2Cd0.8S photocatalysts gradually shift from 570 nm to 540 nm, as a result of the incorporation of Zn in the CdS crystal lattice. All visible radiation found after 600 nm is reflected by the material, providing the characteristic color of these compounds, from orange to yellow (Figure 8(c)) which is in line with what was obtained in the UV-Vis spectra. In the case of the Zn0.2Cd0.8S and pure CdS samples synthesized in this work, it is expected that the band gap energy widths will be less than 2.4 eV (reported for bulk CdS [37]) due to the crystalline quality and the small size of the crystallite which promotes changes in the energy gap, as will be discussed below.Figure 8
Diffuse reflectance (DR) UV-Vis spectra of pure CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ solid solutions (a), Tauc’s plots (b), and sample colorings (c).
(a)(b)(c)The optical energy bandwidth of the studied systems CdS and Zn0.2Cd0.8S was obtained using the Kubelka-Munk function [21] and the Tauc graph method [22–24] as mentioned in the characterization methods by using the equation
(1)ahv=Ahv‐Ebgnwhere α is the absorption coefficient, h is Planck’s constant, υ is the photon frequency, A is the probability parameter for the transition, Ebg is the optical band gap, and n=2 for direct allowed transition, as it is the case of these semiconductor solid solutions.Figure8(b) shows the ahυ2 values versus hυ for synthetized photocatalysts. The energy bandwidth Ebg of the pure CdS and Zn0.2Cd0.8S samples, which were calculated from the slope of the Tauc’s plot, are listed in Table 1. From the table, notice that the energy band gap increases by Zn presence. The changes in the gap of Zn0.2Cd0.8S solid solution depend of the crystallite size and indirectly of the synthesis method, because in this case, the assisted sonochemistry method promotes smaller crystallite size of the samples, no matter if it is Zn is or not in the crystalline network, from 8.61 nm for the CdS to 4.37 nm for CdS-SQ and from 7.92 for Zn0.2Cd0.8S to 3.80 nm for the Zn0.2Cd0.8S-SQ. Because the crystallite size is smaller, the band gap energy increases for Zn0.2Cd0.8S-SQ, synthetized by sonochemistry, rather than Zn0.2Cd0.8S synthetized by coprecipitation.


## 3.1.6. Photoluminescence Measurements

Figure9 displays the room temperature photoluminescence (PL) emission spectra of CdS and Zn0.2Cd0.8S solid solution synthesized by coprecipitation and assisted by ultrasonic radiation following the laser emitting at 325 nm with an optical excitation power of ~15 mW at room temperature. The PL spectra for all the synthesized samples show a wide band of excitation energy with different intensities. Thus, for the pure CdS obtained by coprecipitation, the band ranges from 2 eV to approximately 3.25 eV, presenting a lower intensity than for instance the CdS-SQ, where it is the one with the highest excitonic transition of all.Figure 9
PL spectra of CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ obtained by coprecipitation and sonochemistry methods.The emission range for the CdS-SQ ranges from 1.7 eV to 2.7 eV approximately, presenting a maximum peak at 2.1 eV. While for the solid solution of Zn0.2Cd0.8S by coprecipitation, the PL spectrum ranges from approximately 1.7 to 2.5 eV, with a maximum peak at 2.0 eV, showing an intensity similar to that of CdS obtained by coprecipitation. Zn0.2Cd0.8S-SQ displays also a wide spectrum, showing three bands of energy with maximum values in 1.9, 2.4, and 3.0 eV and of major intensity than the Zn0.2Cd0.8S obtained by coprecipitation, being the medium band with a maximum peak in 2.4 eV the most intense. It can be seen that for each sample, there is always a PL energy band lower than its corresponding forbidden band (ΔEbg), except for CdS synthesized by coprecipitation (see Table 1); these PL bands are associated with shallow trap emissions in the system. Therefore, this suggests that the radiative transition occurs from both, the surface states and the excitonic transition. The broad PL peak exhibited a trend of shifting towards higher energies by Zn2+ present in the network for the solid solutions Zn0.2Cd0.8S-SQ powder providing evidence for the formation of nanometric Zn0.2Cd0.8S-SQ crystallites. As all the samples show poor crystalline quality and increased structural and morphological disorders according to XRD, HRTEM micrographics, and micro-Raman results, which is the reason that it generates the greater broadening and asymmetry in the PL signals. The decrease in the crystallite sizes of CdS-SQ and Zn0.2Cd0.8S-SQ solid solutions increase the separation of both inter- and intraband levels, which causes the high intensity emission. The intraband separation increases the trapped luminescence efficiency and inhibits the excitation emission via nonradiative surface recombination.Table3 exhibits the wavelengths and their energies corresponding to the photoluminescence emissions for the synthesized photocatalysts. It can be associated with trap states that are promoted by native defects produced in the nanocrystals due to very small size. Then, the emission bands observed around 595 nm for CdS-SQ, 652 nm for Zn0.2Cd0.8S, and 643 nm for Zn0.2Cd0.8S-SQ correspond to shallow trap emissions from defects in the structure related to the Cd atom, and the emission bands observed around 675 nm corresponds to defects of S atom, also known as vacancies, namely, Vcd and VS; these emissions have also been observed in a previous report but for films of this semiconductor [38]. It is worth mentioning that because the VCd and Vs emissions are very close to each other, it is not possible to observe them as separate signals in the PL spectra and also by mathematical treatment cannot separate them, so they have been placed together as a one signal (Figure 10). While the emission bands around 508 nm can be related to the transition from the shallow trap level, the band around 532 nm radiative transition from the deep trap states promoted by the sulfur vacancy has been also reported. [39]. In the case of the emission band at ~412 nm, it can be attributed to a high-level transition in CdS semiconductor crystallites. Such is generally related to the transition of electrons from the edge of the conduction band to holes, trapped at the interstitial sites of Cd2+ [40]. Also, as it can be observed in Figure 6, there are an increase in the internal defects resulting from increased lattice strain due replacement of Cd2+ ions by Zn2+ ions in ZnxCd1-xS, as it was reported [41]. A blue shift of the excitonic emission is observed in the nanoparticles of Zn0.2Cd0.8S; this is attributed to the incorporation of Zn into the crystal lattice of CdS.Figure 10
Charge carriers’ separation schematic diagram for the CdS, CdS-SQ, Zn0.2Cd0.8S, and Zn0.2Cd0.8S-SQ systems.


## 3.2. Photocatalytic Activity

The photocatalysts of pure CdS and Zn0.2Cd0.8S photocatalysts were evaluated for hydrogen production by a water separation reaction under visible light irradiation (λ=420 nm). The reaction was performed at room temperature in an aqueous solution containing Na2SO3 and Na2S as sacrifice reagents. In the control experiments, in pure Na2S/Na2SO3 solutions, there was a lack of hydrogen detachment observed in the absence of visible light radiation or photocatalysts. The rate of hydrogen detachment based on irradiation time is shown in Figure 11. As can be seen, the pure CdS photocatalyst shows low photoactivity for H2 production and has an increase when Zn is present in the sample. According to previous studies of the group, the maximum amount of hydrogen production is when the concentration of Zn in solid solutions is 0.2% by weight. In Table 3, the H2 production at 5 hr is presented. Since it is possible to see in the table the production of H2 when Zn is present, in the photocatalyst, ZnCdS is higher than for pure CdS, regardless of the synthesis technique used. It is also possible to observe that when the synthesis is assisted with sonochemistry, the production of H2 is slightly improved. This can be related to several aspects which are discussed below.Figure 11
Performance in hydrogen production as a function of time.According to the reported data, it was demonstrated that the CdS efficiently captures the energy of light through the excitation of electrons through its direct band gap. However, it manifests a low photocatalytic activity for hydrogen production, which is due to the rapid recombination of the hollow-electron pair, while with the incorporation of Zn in the network of the catalyst, the recombination process is slower; CdS is absorbed in the visible region of light, while ZnS is absorbed in the ultraviolet region; the incorporation of Zn in the CdS catalyst network showed a slight shift towards blue. These observations are due to the overall size and potential-well effects, where the potential-well is deeper for ZnCdS than CdS NPs. It should also be mentioned that the incorporation of Zn in the network generates a stress, which can result in an asymmetric internal electric field across the interface that could change both the electronic states and the absorption properties of the photocatalyst.As it is known in a catalytic reduction-oxidation (redox) reaction, the duration of the hollow pair of electrons together with the selection of the semiconductors with the matched potential positions is very important, related to this in Figure11; an estimate of these was made for the different synthesized systems. The band gap (ΔEbg), valence band (VB), and the conduction band (CB) are shown in the figure, as well as the shallow trap emissions from defects in the structure related to the Cd and S atom (VCd and VS) that were observed by photoluminescence have been also incorporated. This shallow trap emissions can act as a reservoir of electrons promoting a higher production of H2, according to the position of the energy levels estimated shown in Figure 10, while the shallow trap levels are at edge of the conduction band and the deep trap states can no longer be activated with visible light. The energetic position of the trap states together with the band conduction (for the CdS-SQ, the ZnCdS, and the ZnCdS-SQ that have a negative potential [40] in regard to the potential of H+/H2 which is set at 0 eV) is associated to the increment in the hydrogen production (see Table 4).Table 4
Comparison of some characteristics of unsupported and supported Zn0.2Cd0.8S photocatalysts tested in water-splitting reaction in the same conditionsa.
Bulk Zn0.2Cd0.8SBulk Zn0.2Cd0.8SBulk CdSBulk CdSPreparation methodSonochemicalCoprecipitationSonochemicalCoprecipitationMorphologyHierarchical 3D morphologyHierarchical 3D morphologyHierarchical 3D morphologyHierarchical 3D morphologyH2 production at 5 h (μmol)1044981308243ΔEbg (eV)2.342.262.212.20aReaction conditions were Wcat.=0.2 g, Vsolution=200 mL, 350 W Xe, visible light irradiation time of 5 h, and 0.35 M Na2S/0.25 M Na2SO3 sacrificial agents.From the table, the hydrogen production that can be observed is considerably higher for ZnCdS and ZnCdS-SQ since, in addition to the aforementioned, a greater separation of the charge carriers was obtained due to the presence of Zn in the semiconductor material, which lengthens more the duration of the hole-electron pair. In the case of CdS-SQ, a slight increase in hydrogen production was observed with respect to the CdS synthesized by coprecipitation. This can be attributed to the fact that it was possible to reduce the size of the crystallite and the presence of shallow trap emissions, but the rapid recombination of the hollow-electron pair occurs, which has a significant impact in the performance of the photocatalyst for the hydrogen production.


## 4. Conclusions

The influence of the ultrasonic irradiation promotes small crystallite sizes under the experimental conditions employed in this investigation. The crystallite size of the photocatalysts obtained for CdS an Zn0.2Cd0.8S is within the nanometric scale, being those assisted by ultrasonic radiation the smallest crystallites sizes. The energy of the forbidden band depends directly on the quantum effects that are manifested at the nanometric scale; this is related to the electronic states that can be modified by joint effects, related to crystallite size, doping, morphology, crystalline phase, and crystal quality, which can be tailored by the synthesis method and variations in temperature, pressure, and pH. The results obtained with BET, TEM, and PL techniques shown defects in the structure that promotes extra accessible adsorption sites that gives a better performance in the reducibility of the catalysts for hydrogen production, when Zn is incorporated into the CdS crystal lattice, but also by the method of synthesis, in this case the assisted by sonochemistry. PL results show tunability of band edge emission as a function of zinc concentration in the Zn0.2Cd0.8S nanoparticles.For the water splitting to hydrogen production, the charge carriers’ separation promoted by Zn incorporation besides shallow trap emissions generated by defects in the catalyst structure (which increased with the sonochemical method) and the matched potential positions were important factors for the reduction reaction.



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*Source: 5567247-2022-05-12.xml*