Analysis of the conversion of cadmium carbonate to cadmium oxide in thin films with photocatalytic application

Figure 4 shows the micrographs obtained by the SEM technique of polycrystalline thin solid films at (a) 30°C, (b) 150°C, (c) 300°C, (d) 400°C, and (e) 500°C. According to the morphological images of the thin solid films, plates assembled with a flat and smooth surface can be observed, supported on the surface of a small nanocrystalline thin layer. Granular stacks feature individual crystalline entities with pyramid-like geometries stacked in different directions in a disorderly fashion. The average length of the crystals was ∼2–5 μm. Another interesting surface characteristic of the thin solid films is the crystalline grain boundaries, which are very close to each other. Usually, the morphological and structural properties present a wide correlation with the optical properties, which are of wide interest in various scientific and technological applications [28]. It is possible to observe directly in SEM images that the effect of the TAT does not significantly modify the crystalline morphology of the thin solid films; this is best reflected in the histograms in Figure 3 for all samples (a), (b), and (c), where the average nanoparticle size does not vary significantly (2–5 μm). Generally, drastic morphological changes related to the effect of TAT have been reported in morphological studies of inorganic nanocrystals using SEM [29]. The experimental techniques applied for chemical synthesis in the preparation of inorganic compounds have been exhaustively investigated and associated with the phenomenon of crystalline growth, considering the geometric properties of the crystal and meticulously controlling the optimal physical and chemical parameters involved [30]. Such variations often correspond to a drastic change in the structural and physicochemical properties and offer an opportunity for nanocrystal fine-tuning [31]. Various theoretical-experimental models have been proposed to study the evolution of crystalline growth, considering the origin of morphology as a complex phenomenon that is difficult to fully understand [32]. Generally, in the phenomenon associated with crystalline growth, the following critical stages have been proposed: (a) creation of nanocolloids and the spontaneous or gradual transformation into long compact crystals and growth of the crystal geometry that acquires greater relative energetic thermodynamic stability, involving various possible precursor phases (amorphous or crystalline), which is governed by the parameters of the environment (chemical or physical); temperature, pressure, concentration of precursor reagents, pH, etc. [33]; and (b) nanonuclei generated by the gradual creation of an active center of minimum relative energy under experimental conditions evolve favorably until reaching the minimum relative energy stability of the crystal, as well as the optimum size.

Figure 4

Micrographs and histograms obtained by SEM of the polycrystalline thin solid films at (a) 30°C, (b) 150°C, (c) 300°C, (d) 400°C, and (e) 500°C.

Figure 5 shows FTIR spectra of 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films. The vibration bands in the FTIR spectra, located in the range ∼3,600–400 cm⁻¹, correspond to the characteristic vibrations of the carbonate ( CO 3 2 − {\text{CO}}_{3}^{2-} ) ion, which are visualized in this spectrum at ∼1,400, 851, and 716 cm⁻¹ [34]. The absorption band situated at ∼1,441cm⁻¹ corresponds to the asymmetric stretching vibrations, and a strong, sharp absorption band at ∼85 cm⁻¹ is assigned to the bending out-of-plane vibrations. We found FTIR absorption bands at ∼3,400 cm⁻¹ caused by the −O–H stretching of hydroxyl groups [35]. Two FTIR bands were observed at 1,461 and 1,586 cm⁻¹, which were attributed to the symmetric and asymmetric stretching vibrations of the carboxylate groups, respectively [36].

Figure 5

FTIR spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films.

The powder of CdCO₃ annealed at 500°C decomposes and, at this temperature, CO 3 2 − {\text{CO}}_{3}^{2-} breaks down, releasing CO₂ as a gas, resulting in the formation of CdO as a solid compound.

To investigate the structural and microstructural properties of the polycrystalline thin films, the XRD technique is applied. We performed angular scanning (2θ) in the range of 2θ ∼20°–80° on all samples. The XRD diffractograms are arranged in descending order, starting with the 30°C thin solid film, followed by the treated samples (TAT). Figure 6 shows the XRD diffractograms of the thin films, labeled 30°C, 150°C, 300°C, 400°C, and 500°C, respectively. The 30°C, 150°C, and 300°C thin solid films present six angular reflections of different relative intensities when compared with each other. The crystalline reflections recorded at the angular positions located at 2θ ∼23.4°, 30.16°, 32.96°, 36.26°, 48.0°, and 49.66°, associated with the corresponding (012), (014), (116), ( 110), (024), and (116) crystalline planes, were identified with CdCO₃ rhombohedral phase, space group Rc-187, according to reported standards (00-042-13-42). In the angular reflections, the presence of materials other than CdCO₃ was not recorded, showing that with the CBD technique, it is possible to prepare inorganic materials with acceptable purity. In the 400°C sample, six additional angular reflections emerge at 2θ ∼33.06°, 38.3°, 55.3°, 65.98°, and 69.36°, corresponding to the (111), (200), (220), (311), and (222) crystal planes, respectively, as well as ae decrease in the relative intensity identified in the CdCO₃ thin solid films. In the 500°C sample, the crystalline planes associated with CdCO₃ completely disappeared, making it possible to identify the crystalline planes of CdO, according to the standards (card number 96-9006674). No other material other than CdO was identified, which implies that the effect of TAT produces the gradual and total transformation of CdCO₃ to CdO, obtaining it in pure form. The transition from CdCO₃ → CdO is observed in this analysis.

Figure 6

XRD diffractograms of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films.

We propose gradual structural evolution according to the following reaction: (5) CdCO 3 ( s ) 500 ° C ⇒ CdO ( s ) + CO 2 ( g ) ↑ . {\text{CdCO}}_{3(\text{s})}\text{ }500^\circ C\Rightarrow {\text{CdO}}_{(\text{s})}+{\text{CO}}_{2(\text{g})}\uparrow .

The crystalline phase transition of the inorganic nanocrystals examined here implies a complex microstructural phenomenon in the crystalline lattice; these have not yet been specifically presented and must be methodically analyzed. This phenomenon involves spatial disruption around the nucleus of a new phase owing to the inherent volume change. Atomic, molecular, and/or crystalline relaxation during chemical and physical changes produces lattice strain, which in turn affects the nucleation kinetics and dynamics of the transition, as in the case of nanocrystals [37].

The nanocrystals undergo a rhombohedral → cubic phase transition, which is discernible through the loss of rhombohedral features and shifting of high-symmetry reflections. Gradual creation of microstructural changes associated with ion and cation slippage, among other parameters in the crystal lattice; interplanar distance, lattice parameter related to the translation in the corresponding geometric position (lengthening and/or decrease of the lattice parameters) that have the minimum relative crystalline energy, reaching relative thermodynamic stability of the crystal lattice [38]. In this context, it is suggested that the external energy applied serves to rearrange the crystal lattice, thereby selectively creating a new position of minimum crystal energy for each TAT.

The crystal size for each sample was obtained by using the Scherrer equation as follows: D = K λ / β Cos θ , D=K\lambda /\beta \hspace{.5em}\mathrm{Cos}\hspace{.25em}\theta , where D is the average grain size, K is the constant of the dimensional form factor, λ is the wavelength of X-rays, β is the FWHM of the crystalline plane, and θ is the angle of the Bragg diffraction; therefore, the grain size was calculated using the FWHM for each phase present in the process.

Figure 7 shows the grain size (GS), interplanar distance (ID), and temperature (°C) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C nanocrystals. The GS decreases from ∼50 to 25 nm in the 30–400°C temperature interval, then presents a drastic increase at ∼43 nm on the 500°C film. The experimental results obtained here provide information on some of the microstructural properties of the nanocrystalline system, which are associated with the gradual transition of the crystalline phase; in this case, it corresponds to CdCO₃ to CdO, such as tunable GS, morphology, crystalline phase, tailored porosity, surface area, and stacking faults [39]. On the other hand, the changes at ID present oscillatory behavior, in which it reaches a relative minimum of crystalline energy for a 500°C layer. The microstructural phenomenon is justified in principle by the evolution corresponding to the CdCO₃ to CdO transition, which is imperative to consider the disordered crystalline energy change of the nanocrystals in the films subjected to TAT. In turn, the previous arguments focus on the relationship involved in this physical phenomenon associated with spatial distortions of the crystal lattice created by various rearrangements of ions and cations, associated with the different chemical bonds present in the Cd < CO 3 2 − {\text{CO}}_{3}^{2-} , a Cd–O, −Cd–C═O, etc. Thus, the microstructural parameters associated with the evolution change significantly at each stage examined. The characteristics of the microstructural evolution of inorganic materials in the solid state are related to phase change and solidification, among other physicochemical parameters [40]. Our results are consistent with a damaged microstructure dominated by vacancies, highlighting the importance of free surfaces in determining the defects retained.

Figure 7

Grain size (GS), interplanar distance (ID), and temperature (°C) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C nanocrystals.

To calculate the lattice parameters a and c, we apply the following expression:

1 d 2 = h 2 + k 2 a 2 + l 2 c 2 . \frac{1}{{d}^{2}}=\frac{{h}^{2}+{k}^{2}}{{a}^{2}}+\frac{{l}^{2}}{{c}^{2}}.

In Table 1, the lattice parameters a and c, microstrain, and dislocation density (δ) of the 30°C, 150°C, 300°C, 400°C, and 500°C thin films are compiled. The variations of the experimental microstructural parameters registered here have oscillatory behavior, confirming the registered and analyzed results of the GS and ID. The disordered variations are associated in principle with the creation of different chemical bonds in the transition from Cd < CO 3 2 − {\text{CO}}_{3}^{2-} to Cd–O is imperative, as a consequence, crystalline distortion is created until the minimum energy is reached gradually in the lattice crystalline, considering possible distorted elongations of the lattice parameters, which implies promising structural stability [41]. The microstructural phenomenon is associated with the order–disorder effect typical of some reversible systems or vice versa. In particular, the crystalline system cation distributions are proven to be random and homogeneous, and new phases of crystalline matter and untapped opportunities to reach a relative thermodynamic maximum of disorder at the 300°C layer are discovered.

The strain of the powders was obtained using the following formula: ε = β h k l 4 tan tan θ . \varepsilon =\frac{{\beta }_{hkl}}{4\hspace{.25em}\tan \hspace{.25em}\tan \hspace{.25em}\theta \hspace{.25em}}.

The strain values are shown in Figure 8. The observed values are positive, indicating that the strain is tensile. The increment for powder annealed at 300°C and 400°C, and the increment with temperature is due to thermal expansion, and higher strain rates improve the strength. The higher temperature during strain-controlled fatigue tends to reduce the fatigue life for a given strain, and a considerable decrement can be observed at 500°C because of the phase transition from CdCO₃ to CdO. In particular, under moderate tensile strains, the materials are suitable for photocatalytic conversion.

Figure 8

(a) Williamson–Hall plot and (b) strain vs temperature.

The optical properties of transmittance (%T), reflectance (%R), and absorbance (α) of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films are examined at the wavelength (λ) located in the UV-Vis region corresponding to λ ∼350–850 nm (∼3.54–1.45 eV) at normal incidence for all the samples. The optical phenomenon commonly recorded in thin films is observed when the incident radiation penetrates the analyzed sample and undergoes a combination of scattering and absorption inside the sample. Therefore, some of the radiation was reflected toward the surface. In the spectral study of %R and %T of various thin films, the standard relationship T + R = 1 is generally applied. Figure 9 displays (a) transmittance (% T), (b) reflectance (%R), and (c) absorbance (α) vs λ (nm) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C films. The optical phenomenon related to the scattering of incident external radiation in different regions of the electromagnetic spectrum (UV-VI-IR) created by the different changes in morphology, crystalline phase, stoichiometry, grain boundaries, stacking faults, and other properties is correlated with the intrinsic optical properties of each material [42]. Considering the above, related to the different properties of the materials examined in the region used, we have the 30°C and 150°C layers show a similar increase at %T ∼55%. The rough crystalline surface of the 30°C and 150°C thin films presents morphology with various defects, which generate scattering of the incident radiation, which is scattered in different random directions [43]. For the 30°C and 150°C films, the surface roughness is much smoother than that of TAT polycrystalline films. In the Vis-region, %T spectrum, the fundamental electronic valence band (VB) and conduction band (CB), (VB) VB → CB can be seen with a shift located at λ ∼ 450–550 (2.92–2.25 eV). This allows us to visualize in a semi-empirical way the band gap (E _g) energy located at the λ (nm) interval examined in our nanocrystals. On the other hand, the 300°C, 400°C, and 500°C films show a drastic increase of %R ∼40–95% at the Vis region. The behavior of α at 30°C and 150°C layers decreases similarly in the region located at ∼350–500 nm (∼3.54–2.48 eV) in both samples, with an insignificant increase for λ > 500 nm. This optical phenomenon is correlated with the different morphological and structural properties and is related to the gradual transformation into a different inorganic material. Similar to surface phenomena, shape-related properties are also apparent, particularly at the nanometer scale. An interesting property, observed in the Vis-region, on the 300°C, 400°C, and 500°C layers, is the existence of an inflection point, which presents a shift in the range of ∼506 nm (∼2.45 eV), ∼536 nm (∼2.31 eV), and ∼556 nm (∼2.23 eV), respectively. From the geometric point of view, the origin of the inflection point in the α spectrum of an inorganic material is considered an optical phenomenon associated with the dispersion of the light beam in single crystals, according to Raleigh dispersion is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the λ radiation. We propose that this structural behavior is correlated with the scattering of external electromagnetic radiation when λ is greater than the GS [44].

Figure 9

(a) Transmittance % T, (b) reflectance %R, and (c) absorbance (α) vs λ (nm) of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films.

Figure 10 shows the normalized absorbance vs wavelength (λ) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films. The 30°C and 150°C samples show an AB of relatively low intensity located at ∼463 nm (∼2.67 eV). However, in the 300°C, 400°C, and 500°C samples, a prominent AB emerges at ∼442 nm (∼2.80 eV), ∼495 nm (∼2.50 eV), and ∼516 nm (∼2.24 eV), respectively. The origin of AB at ∼442 nm (∼2.80 eV) and at ∼516 nm (∼2.24 eV) is associated with the electronic transition, fundamental of CdCO₃ and CdO [45]. The oscillatory shift of the relative maximum at AB toward lower energy is correlated with the creation of a CdCO₃/CdO solid solution [46] in proportions not quantified here, as well as the variation of the GS of the nanocrystals. The observed AB shift toward higher λ (nm) was ∼74 nm (∼0.16 eV), leading to promoted Vis-region light absorption.

Figure 10

Normalized absorbance vs wavelength (λ) spectrum of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films.

For a better understanding, deconvolution was performed and displayed in Figure 11 of the (a) 30°C, (b) 150°C, (c) 300°C, (d) 400°C, and (e) 500°C films. The (a) and (b) films show a weak and broad AB situated in the UV-Vis region at ∼400–500 nm (∼3.10–2.48 eV), recorded at ∼463 nm (∼2.67 eV) and ∼467 nm (∼2.65 eV) for the 30°C and 150°C samples. In the (a) 30°C and (b) 150°C samples, a weak and broad AB is recorded in the UV-Vis region (∼400–500 nm), recorded at ∼463 nm (∼2.67 eV) and ∼467 nm (∼2.65 eV), respectively. The AB was identified in these optical results, and the fundamental transition assigned to the VB → VC of CdCO₃ was identified. The small shift toward a higher λ (nm) and lower photon energy (hν) is associated with CdO and, in turn, with the gradual experimental formation of a CdCO₃/CdO solid solution promoted by the TAT effect. The deconvolution in (c) and (d) carried out at AB for the 300°C and 400°C films records the AB signals located at ∼392 nm (∼3.16 eV), ∼481 nm (∼2.57), and 661 nm (1.87). Finally, the 500°C film records two AB of different relative intensities located at ∼412 nm (3.00 eV) and ∼516 nm (∼2.40 eV).

Figure 11

Normalized absorbance deconvolution vs wavelength (λ) spectra of the (a) 30°C, (b) 150°C, (c) 300°C, (d) 400°C, and (e) 500°C films.

Figure 12 shows the spectrum of the derivative of the optical density (OD) to the photon energy d(DO)/dE of the 30°C, 150°C, 300°C, 400°C, and 500°C films. In the d(DO)/dE experimental spectrum, all the films are placed in ascending order, with the 30°C layer in the lower part, followed by 150°C, and so on, to appreciate the differences correlated with electronic transitions related to the VB and the CB at the gradual evolution of CdCO₃ to CdO. From this spectroscopic analysis, it is possible to record some AB created by intraelectronic photonic replicas caused by microstructural changes, differences in stoichiometric balance, grain boundaries, and crystalline stacking faults, which are generally recorded by applying this spectroscopic technique. Two broad AB with different relative intensities and higher magnification were recorded at ∼2.58 and ∼2.78 eV. The AB corresponds to the CdCO₃ rhombohedral phase assigned to the 30°C and 150°C films, respectively. In the 300°C film, a very broad band with greater relative intensity is located at 2.10 eV. A broad AB situated at ∼1.60 eV and another at ∼1.73 eV are related to the creation of CdO associated with the fundamental electronic transition from CdCO₃. The transition to CdO is recorded at the 400°C and 500°C thin solid films. The TAT effect provides the relative and quantitative critical experimental values in the gradual transition, experimentally recorded in the transformation of CdCO₃ to CdO.

Figure 12

Derivative of the OD concerning the energy of the photon d(DO)/dE spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C samples.

Directly applying the Tauc equation and the previously obtained results of the absorbance (α), we constructed an experimental graph of (αhν)² vs E (eV). Optical analysis was performed at a photon energy in the range of ∼1.5–3.0 eV, considering the fundamental transitions of CdCO₃ and CdO, respectively. The band gap (Eg) energy of the experimental plot was directly obtained by the intersection of the vertical axis (αhν)² with the horizontal axis E (eV) [46]. Figure 13 displays the (αhν)² vs E (hν) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C layers. As a first approximation, we considered that the nanocrystals presented a direct band transition according to the format of the experimentally obtained spectral curve [47]. The results associated with the optical behavior in the experimental curve of CdCO₃ present the characteristic direct band spectral behavior in which the line that intercepts the numerical E _g = 2.37 eV value is drawn. However, in the 15°C, 300°C, and 400°C samples, the optical behavior changes gradually, and it is difficult to formally record E _g located in the region ∼2.31–2.06 eV, which can be assigned to overlap of the optical signals generated by the mixture or solid solution of CdCO₃/CdO created gradually in quantities not experimentally quantified in this report. In the 500°C sample, a drastic change in the curve is observed, in which an optical phenomenon is observed in some nanocrystals. Table 2 compiles the experimental values for the bandgap energy (E _g) of the 30°C, 150°C, 300°C, 400°C, and 500°C samples. According to the values registered here, a gradual decrease in the gap associated in a specific way with the fundamental electronic transition CB → VB in these nanocrystals can be seen. The structural effect observed here is correlated with the shift of the wavelength and, of course, toward greater E _g associated in a specific way with the fundamental electronic transition CB → VB in these nanocrystals. The structural effect observed here is correlated with the shift of the wavelength and, of course, toward greater E _g due to the increase in GS, which is commonly reported in the literature [48].

Figure 13

(αhν)² vs E (hν) spectra of the 30°C, 150°C, 300°C, 400°C, and 500°C thin solid films.

Figure 14(a) shows the logarithm of the absorption coefficient vs photon energy. The band tails are associated with the edge broadening exciton-phonon that can be attributed to the indirect electronic transitions in the band gap, and also the broadening reflects the disorder degree in the lattice.

Figure 14

(a) Ln of the coefficient absorption, and (b) Urbach energy.

Under tensile strain, the band gap decreases, and this reduction in the band gap significantly affects the photocatalytic performance of the material. The photocatalytic activity depends on the E _g and the carrier mobility.

The Urbach energy values are shown in Figure 13(b), and they depend on the structural disorder. The sample annealed at 500°C has a lower Urbach energy value, although it has been reported that high Urbach energy enhances the photocatalytic degradation [49].