Structural, optical, and electronic characteristics of non-stoichiometric nanocadmium sulfide

Samples of non-stoichiometric nano-CdS1−x were synthesized by a simple thermolysis method by lowering the ratio of thiourea relative to cadmium acetate as starting precursors: Cd(Ac):(1 − x) thiourea (x = 0.0, 0.03, 0.05, and 0.1). X’pert HighScore Plus program manifested biphasic CdS (cubic and hexagonal), and the Rietveld analysis was utilized to match the structural and microstructure parameters of the formed samples. The possibility of formed CdS1−xOx alloy due to the sulfur deficiency is also examined using the Rietveld method. A high-resolution transmission electron microscope imaging exhibited nanosize particles with homogeneous morphology. Fourier transform infrared spectrometer was utilized to confirm the existence of O2 in CdS1−x matrix. The band-gap energies for CdS1−x are reduced below the values of energy gaps of CdS and CdO upon increasing the parameter (x) forming a band-gap “bowing.” The photoluminescence (PL) emitted visible colors depending on the amount of sulfur deficiency and excitation wavelength used. The maximum PL intensity observed in CdS0.9 sample confirmed the presents of oxygen inside the matrix. The influence of oxygen substitution or vacancies of sulfur on the electronic structure and optical features of CdS was also investigated applying density function calculations.


Introduction
In the last years, inorganic semiconductor materials such as ZnS, ZnO, CdS, SnS 2 , CuS, or CdSe [1][2][3][4][5][6] have been broadly considered due to their uses in the people's daily life. Furthermore, the nanostructures of the semiconductor materials reveal an interesting linear absorption, photoluminescence emission, and nonlinear characteristics as compared with bulk form [1][2][3][4][5][6]. Cadmium sulfide (CdS) is a familiar semiconductor photocatalytic material that possesses a wide band gap &2.4 eV [3], and it exhibited a rapid generation of photo-induced electron-hole pairs [7]. Unfortunately, CdS has rapid recombination of carriers and photocorrosion which limited its uses in photocatalytic activity [8]. Furthermore, CdS needed high temperatures and expensive inert environments during preparation with traditional methods [9]. Several methods were used to prepare CdS in different sizes, shapes, and morphologies to enhance its stability and photocatalytic activity under visible light [10]. CdS can crystallize in one or more phases (cubic zinc blend, hexagonal wurtzite, rock-salt phase) [11]. CdS can be found with the hexagonal phase under ambient conditions, while CdS with the cubic phase can exist when the crystallite size reduced. Finally, under applying high pressure, CdS with the rock-salt structure can be obtained [12]. It was found that the number of vacancies (V cd ), cadmium interstitial (I cd ), and stacking faults defects affect the structure transformation of CdS from one phase to another [13]. CdS with mixed hexagonal and cubic phases can be obtained as it has a crystallize size of 4-5 nm, while it is formed either in cubic or hexagonal only when the crystallite size is below or above this crystallite size value, respectively [14]. On the other hand, changing the ratio of S to Cd in CdS samples to be non-stoichiometric can transfer the structure of CdS from hexagonal to cubic configuration [15] or produce CdS with two crystal structures (cubic and hexagonal); all these factors affect the physical features of the material [16]. CdS formed as a core (hexagonal phase) and shell (cubic phase) exhibited a higher photocatalytic H 2 production than CdS nanorods with a hexagonal structure with photocatalytic stability over 400 h [17]. Shen et al. varied the Cd to S ration during preparation CdS by the solvothermal method to control the zinc-blende and wurtzite structures inside the formed material to investigate their photocatalyst performance [18]. Vaquero et al. prepared CdS also by solvothermal method but they changed the annealing temperature and water/thiourea ratio during the preparation of CdS sample, and they found that all formed samples exhibited a hexagonal structure only [19]. Pandya et al. found the energy gap of CdS thin film, regardless of the Cd amount with respect to the S amount [20]. On the other hand, CdO is an n-type semiconductor with a direct optical band gap 2.2-2.5 eV and high transparency over a wide spectral range and higher electron mobility compared with CdS [21]. It was found that the photocatalytic effect of CdO-CdS to produce hydrogen from water composites is better than CdO or CdS nanoparticles alone [22]. Very few reports have been published on the synthesis and characteristics of Cd(S/O) films [23]. Oxygen incorporated in nanocrystalline CdS films to form CdS 1-x O x alloy improved solar cell performance [23]. Moreover, the optical band gap of Cd(S/O) films reduced from 2.34 eV for CdS to 2.26 eV according to the CdO amount in the film [24]. Non-stoichiometric CdS can adjust the band gap with new optical properties. The current study is aimed to explore the effect of changing the Cd:S ratio in the starting precursors on the characteristics of the formed CdS. As mentioned above, there are a lot of different procedures were used to produce the nanomaterials, and among of them, the thermolysis technique does not need a solvent during the reaction and produces a large mass scale amount. Non-stoichiometric CdS nanomaterials were formed applying a thermolysis process; cadmium acetate: ( 1-x ) thiourea, x = 0.0, 0.03, 0.05, and 0.1. X-ray diffraction phase investigation was performed to determine the phases developed in the CdS 1-x samples and to study the possibility of oxygen incorporate in the formed system. The structural, microstructural, and optical properties of the formed samples have been investigated applying X-ray diffraction, high-resolution transmission electron microscope (HRTEM), Fourier transform infrared spectrometer, photoluminescence, and UV-Vis devices. DFT calculation was applied to study the electronic and optical features of CdS, in the case of existence of some sulfur vacancy in CdS matrix (CdS 0.9 ) or substituted of some sulfur by oxygen in CdS matrix (CdS 0.9 O 0.1 ).

Methods and materials
Non-stoichiometric CdS nanomaterials were synthesized by applying a thermolysis process, where the thiourea amount was reduced as compared with the amount of cadmium acetate in the beginning of synthesis. Different non-stoichiometric parameter (x) was applied: cadmium acetate:(1 -x) thiourea,  [25] depended on the Rietveld profile technique [26] was carried out to examine the crystal structure and microstructure of the formed CdS 1-x system. The techniques, such as X-ray diffraction (X'pert MPD, Philips with Cu-K a source), high-resolution transmission electron microscope (HRTEM), Fourier transform infrared (FTIR, Bruker Tensor 27 FTIR spectrometer), UV-Vis (Shimadzu UV-3101PC UV-Vis-NIR), and photoluminescence (luminescence spectrophotometer (RF-1501 SHIMADZU, Ltd)) were utilized for careful investigations of all samples. Different electronic structure's parameters were obtained by applying Cambridge sequential total energy package (CASTEP) code [27] relies on first-principle calculation. The supercell of CdS with cubic structure, 211-unit cell dimension, ultra-soft pseudopotentials, GGA-PBE conditions, and energy cutoff (380 eV) [28,29], more details can be found in [16]. The 4d 10 5s 2 and 3s 2 3p 4 , electrons relevant to Cd and S atoms were employed for the pseudopotential arrangements, respectively. For comparing the DFT calculations for some sulfur vacancy in CdS (CdS 0.9 ) with those for some sulfur substituted by oxygen in CdS (CdS 0.9-O 0.1 ), the previous criteria were applied.
3 Results and discussions 3.1 XRD structural investigation X-ray diffraction patterns have been measured, Fig. 2a, with high quality (small 2h-step and linear detector) for accurate structural and microstructural analysis. Applying the program X'pert HighScore plus, all samples of CdS 1-x (0 B x B 0.1) manifested pure CdS structure. Also, two CdS phases are detected in all samples; cubic F À 4 3m and hexagonal P 6 3 m c with the cubic phase, a little bit dominant, Table 1. Structural analysis is carried out by applying the Rietveld method; Fig. 2b displays the obtained pattern fitting for x = 0.03 as example. The crystallite sizes of the two phases are quite similar with an average value around 5 nm. Figure 2c, d shows the HTEM images with different magnifications. Homogenous morphology and a uniform size (almost no size distribution) can be noticed with a very small size resembling quantum dots (around 5 nm), but the particles are highly agglomerated. The inset image of Fig. 2d illustrates a set atom planes with high degree of dislocations arising from vacant sites due to sulfur deficiency and/or oxygen incorporation into the CdS lattice. Rietveld analysis emphasized that O atoms are introduced into the CdS 1-x lattice to recompense the sulfur deficiency, producing an alloy CdS 1-x O x . Better fitting of diffraction pattern demands the incorporation of O in vacant sites of sulfur. Furthermore, during the Rietveld analysis, O ions are found to compensate S atoms substitutionally at the crystallographic site of sulfur; trails to reside O atoms in the interstitial voids in CdS lattice yield poor pattern fitting with bad reliability factors. Oxygen diffusion into ZnSe, CdSe, and CdS semiconductors upon air annealing has been detected in X-ray quantitative phase study [30]. Similar results obtained for annealed ZnTe; O atoms were diffused into the lattice before the semiconductor was oxidized [31]. The refined structural parameters and the phases percentage obtained for CdS 1-x (0 B x B 0.1) applying Rietveld analysis are given in Table 1. The lattice parameters for both cubic and hexagonal phases are obviously decreased upon increasing (x). The decrease in lattice parameters may be attributed to the incorporation of O atoms to compensate for S atoms and/or the formation of some vacancies in the lattice. As in Table 1, the z-coordinate of the S/O atoms z(S/O) in the hexagonal phase is also decreased with the parameter (x), resulting in increasing the bond length distortion index.

Infrared absorption spectroscopy study
The FTIR spectra measured for the non-stoichiometric nano-CdS 1-x (x = 0.0, 0.03, 0.05 and 0.1) are depicted in Fig. 3. All the spectra exhibit peaks in the low wavelength region 445, 543, 580, 612, and 684 cm -1 representing the stretching mode of Cd-S bond [32]. The inset figure manifests the shift of these bands with the parameter non-stoichiometry (x) indicating the insertion of O 2 into the CdS lattice replacing the S atoms. The observed bands at 1400-1540 cm -1 can be ascribed to the carboxyl group [33], and the band at 1620 cm -1 represents C = O stretching modes and/or O -H bending vibration; both happened due to the absorbed atmospheric CO 2 and humidity on the surface of the nanocrystals [33]. The active vibrations related to the O -H are observed in the range from 3300 to 3500 cm -1 for the stretching mode [34]. The weak band around 2000-2250 cm -1 is attributed to C = C stretching vibration mode [35].    Fig. 4, using the following relation for the direct transition:

UV absorption analysis
where C is a constant, hm is the incident photon energy, and a is the absorption coefficient, respectively. Although the optical band gap of the CdS 1-x O x was expected to have a value intermediate to that of the CdS (* 2.4 eV) and CdO (* 3.1 eV) [36], the obtained band-gap energies are reduced below these values upon increasing the parameter (x) forming a band-gap ''bowing,'' Fig. 4b. Such a band-gap bowing was detected for ZnS 1-x O x [37] and was interpreted by applying first-principle calculations [38]. For small contents of oxygen (x), the energy value of the conduction-band (CBM) minimum is reduced whereas the valence band energy is nearly not changed, which caused a decrease in the optical band-gap energy. Furthermore, this shift in CBM is due to the hybridization between the oxygen 3 s state and ZnS conduction-band states [39]. As the concentration of oxygen (x) increased, the conduction band staying nearly unchanged and the valance band maximum (VBM) energy is reduced; therefore, the band-gap energy enlarged once more [38]. For the present system CdS 1-x O x , the same interpretation may be applied; hybridization between the oxygen 3 s state and the conduction-band states of CdS can occur leading to a narrowing in the band gap for low oxygen incorporation, then the band-gap energy increases again for high values of (x). Figure 5 shows the Gaussian fitting of the broad photoluminescence (PL) spectra of nano-CdS 1-x system under 350 nm excitation at room temperature. The PL intensity of the sample varied irregular depended on the x value, and it reached its maximum value in CdS 0.9 sample. Furthermore, samples with x = 0, 0.03, 0.05, and 0.1 can be deconvoluted using Gaussian function in ORIGIN PRO 9.0 software, into (violet, blue, green), (violet, blue, two greens, orange), (blue, two greens, orange), and (violet, two blues) sub-emissions colors, respectively, Table 2. The corresponding goodness-of-fitting parameter (R-square) values are written in each figure. On the other hand, as Cd 1-x system exited with 375 nm, Fig. 6, Gaussian fitting revealed (blue, two greens), (blue, green, orange), (violet, blue), and (two blues, green, orange) colors for x = 0, 0.03, 0.05, and 0.1, samples, respectively, Table 2. The proposed schematics of various possible transitions for all samples are revealed in Figs. 5 and 6. It is known that vacancies and interstitial of sulfur or cadmium (V S , V Cd , I S , I Cd ) atoms formed during the preparation of nano-CdS are the most adsorbed types of defects existed on its surface [40]. When sulfur ions are less than cadmium ions, sulfur vacancies and/or interstitial are formed [40], and therefore, the PL emission may be due to the surface electron traps by sulfur vacancy or cadmium atoms adsorbed on the surface [41]. The recombination of this trapped electron by sulfur vacancy with a hole in the valence band caused a broadband PL emission with a red shift [42]. On the other hand, when sulfur ions concentration is larger than cadmium ions concentration, interstitial sulfur or cadmium vacancies are formed [43]. These defects can work as hole traps located above the valence band, caused also a broadband PL emission and red shift [43]. CdS nanoparticles obtained by the co-precipitation method revealed a green emission [44,45]. Mishra et al. found that CdS quantum dot emitted green, yellow, orange, and red colors [46]. CdS quantum dots prepared by a post-synthesis modification with aqueous solutions of NaOH emitted violet-blue colors [47]. CdS nanoparticles obtained by the chemical precipitation method utilizing polyvinylpyrrolidone as a capping agent revealed blue and green colors [48]. CdS prepared by hydrothermal technique exhibited blue and green   emissions [49]. The emitted colors of CdS 1-x can be described as follows: (i) violet-blue band is ascribed to the band-edge emission [47], (ii) the green color can be accredited to transitions from the donor sulfur vacancy levels to the valance band, and (iii) the orange emission color could be due to the recombination of defect complexes (I cd -V cd ) [50]. The bandgap energies of bulk CdS and CdO are very near to each other (2.4 and 2.3 eV) [51]. Thus, the emission characteristics of both are comparable, and peak maxima roughly have a similar position. CdO-CdS synthesized in ethylene glycol water mixture emitted green and yellow colors [51,52]. It also observed that the PL intensity of CdO-CdS samples is larger than CdO or CdS nanoparticles [51]. This improvement in intensity may due to the fine mixing of both CdS and CdO phases in the sample. This also indicated the enhancement in the light absorption features and emphasized the high surface area of the samples due to their quantum dots sizes as compared with CdS or CdO alone [51].

Electronic analysis
CdS with cubic structure (space group F-43 m) was used in this calculation as an initial model. The lattice parameter (a) and atoms positions were obtained from X-ray refinement part. The lattice parameter was optimized after geometry relaxation for CdS, CdS 0.9 O 0.1 , and CdS 0.9 samples. The obtained lattice parameter after relaxation was 5.9133, 5.8412, and 5.7628 Å for CdS, CdS 0.9 O 0.1 , and CdS 0.9 samples, respectively. As noticed from the XRD part, the lattice parameter (Table 1) Fig. 4. The values of band-gap energies obtained from DFT calculations are underestimated, which always happen in DFT analysis due to the recognized limitation of calculating accurate energy band features [53]. In the case of sulfur vacancies sample, CdS 0.9 , the direct band gap enhanced to 1.823 eV as compared with the energy gap of CdS but it is smaller than the value extracted from UV analysis (2.09 eV). In the case of CdS 0.9 O 0.1 , the energy gap decreased as compared with CdS. In the case under the study, the energy gap got from UV analysis for CdS 0.9 sample is less than the energy gap of CdS sample, which indicated that as the content of sulfur reduced in the CdS 1-x samples, most of the empty places are occupied by oxygen as indicated from DFT analysis.
The total and partial density of state (PDOS) for Cd and S atoms are revealed in Fig. 8   constructed from the p-states of both sulfur and cadmium atoms near the Fermi level in all samples, as revealed in the partial density of state (PDOS). The S-p and Cd-d states created the middle region of the valence band (VB). The S-s and Cd-s,p,d states built the lower part of VB. In the case of CdS 0.9 O 0.1 and CdS 0.9 samples, the intensity of VBM is lower than of CdS sample. In addition, the conduction band (CB) is mostly formed from Cd-s,p atom and a small part shared from the S-p atom. Furthermore, the density of the state of CB exhibited more split and shifted to low energy part in CdS 0.9 O 0.1 sample but it merged and shifted to high energy part in CdS 0.9 sample. In order to obtain further features of these three samples, several optical parameters such as optical absorption, dielectric function e(x) (real part, e r (x), and imaginary part, e i (x)), refractive (n), extinction (k) indices, and optical conductivity have been calculated using Kramers-Kronig equations [54]. Figure 9 shows the variation of absorption spectra of CdS, CdS 0.9 O 0.1 , and CdS 0.9 samples with photon energy. Absorption spectrum may come out as a consequence of electron excitations from the valence to conduction band [55]. Furthermore, the exciton peak (Absorption maxima) of CdS 0.9 O 0.1 and CdS 0.9 samples are shifted slightly to higher energy (blue shift) as compared with CdS sample, which could be owing to quantum confinement effects [55]. The absorption spectra revealed also that the samples exhibited a high absorption in UV range and low absorption in visible and near IR regions. Figure 10 reveals the variation of refractive index, extinction coefficient, dielectric constant, and optical conductivity in the energy range of 0-7.7 eV for CdS, CdS 0.9 O 0.1 , and CdS 0.9 samples. Figure 10a   higher than CdS 0.9 sample. In general, the behavior of an increase or decrease in dielectric constant is related to the enhancement or reduction in absorption coefficient and states density [56]. Figure 10c demonstrates the increase in the extinction coefficient (k) value with raise the photon energy. The extinction coefficient represented the absorption of energy at the surface of material [57], as revealed from the figure that the absorption of energy at the surface of CdS 0.9 sample is higher than other two samples in energy range (2-5 eV), which may be due to the defect exists in this sample due to the decrease of the sulfur amount. The large value of the extinction coefficient of CdS 0.9 sample allows it to be used in several light harvesting technologies such as photovoltaics and thermo-spectronic-based biomedical applications [57]. Figure 10d shows that the optical samples are located at 6, 6.2, and 5.2 eV, respectively. After those maximum, optical conductivity decreased with energy. The optical conductivity peaks emerged in UV range, and they are caused by interband transitions [55]. Furthermore, the optical conductivity for CdS 0.9 sample is lower than other samples due to the presence of defects result from the decrease of the amount of sulfur in the matrix.

Conclusion
X'pert HighScore Plus program manifested that all CdS 1-x (0 B x B 0.1) samples have biphasic CdS structures (cubic F À 4 3m and hexagonal P 6 3 m c). The cubic phase is a little bit dominant over the hexagonal phase. The crystallite sizes of the two phases are around 5 nm. Rietveld refinement confirmed the presence of O atoms in CdS 1-x lattice to recompense the sulfur deficiency producing CdS 1-x O x alloy. The lattice parameters for both cubic and hexagonal phases are decreased upon increasing (x) due to the insertion of oxygen atoms to compensate the S atoms and/or the formation of some vacancies in the lattice. FTIR measurements confirmed the presence of Cd-S stretching mode with a shift in IR bands with the non-stoichiometry parameter (x) that indicated the inclusion of O 2 into the CdS lattice. The non-stoichiometric CdS 1-x samples have direct band-gap energy less than of CdS and CdO; it changed with (x) forming a band-gap bowing due to the change of relative position of the conduction and valance bands. The PL intensity of the samples varied irregular depended on the x value, and it reached its maximum value in CdS 0.9 sample. CdS 1-x samples with x = 0, 0.03, 0.05, and 0.1 revealed (violet, blue, green), (violet, blue, green, orange), (blue, green, orange), and (violet, blue) sub-emissions colors under 325 nm excitation wavelength, respectively. Under 375 nm excitation wavelength, they revealed (blue, green), (blue, green, orange), (violet, blue), and (blue, green, orange) colors, respectively. DFT confirmed that CdS 0.9 and CdS 0.9 O 0.1 have a direct band gap such as CdS sample. The energy gap of CdS 0.9 is larger than CdS while CdS 0.9 O 0.1 is larger than CdS. DFT calculation confirmed the occupied that empty places (sulfur deficiency) by oxygen. The absorption spectra revealed also that the samples exhibited a high absorption in UV range and low absorption in visible and near IR regions. The refractive index, extinction coefficient, dielectric constant, and optical conductivity of CdS 0.9 and CdS 0.9 O 0.1 are different from CdS due to the defects in the matrix. The obtained properties nominated CdS 1-x to be used in different optical and sensor applications.