3.1. Characterization of the As-synthesized Photocatalysts
3.1.1. XRD Analysis
X-ray diffraction is the most powerful and successful technique commonly used for determining the structure of crystals and the arrangement of atoms within a crystal [23]. The characteristic peaks on the XRD patterns of the as-synthesized photocatalyst: CeO2, CdS and CdS/CeO2 were shown in Fig. 1.
Accordingly, diffraction peaks observed at scattering angle observed at 2θ of 28.52, 33.19, 47.38, 56.54, 59.09, 70.01, 76.78, 78.99 and 88.24° corresponding to (111), (200), (202), (311), (222), (400), (313), (402) and (422) lattice plane respectively represents the cubic fluorite structure of CeO2 [24]. The broad peaks observed at 2θ of 25.02, 26.42, 28.19, 36.24, 43.55, 48.00, 52.02, 54.66, 67.03, 70.11, 71.34, 72.81, 76.03, 80.52 and 83.44° corresponding to (100), (002), (101), (012), (110), (013), (112), (044), (023), (210), (211), (114), (212), (300) and (213) crystal plane can be ascribed to the hexagonal greenockite structure of CdS nanoparticle [25, 26]. The broadening of the diffraction peak indicates the nanocrystalline nature of the samples and provides information about crystallite size. As the width increases, the particle size decreases and vice versa [27].
In the case of CdS/CeO2 binary system, most of the diffraction peaks observed could be ascribed to cubic fluorite structure of CeO2 nanoparticle. However, diffraction peaks at 2θ values of 26.82, 44.01 and 52.21° indicate the presence of hexagonal greenockite structure of CdS in the binary system [15, 19]. No other phases can be observed in the XRD patterns of CdS/CeO2 nanocomposite, suggesting that no impurity exists in the sample. It is found that the diffraction peaks for both CeO2 and CdS can be observed from the XRD pattern of the composite, indicating the formation of CdS/CeO2 composite. The absence of other peaks in the XRD pattern of core/shell composite implies that it only consists of CeO2 and CdS.
The average crystallite size of each of the as-synthesized photocatalyst was calculated using the Debye-Scherrer formula [28];
(3)
Where, D = crystallite size in nm, K = the shape factor constant taken as 0.9; β is the full width at half maximum (FWHM) in radians, λ is the wavelength of the X-ray (0.15406 nm) for Cu target Kα1 radiation and θ is the Bragg’s angle. Generally the calculated average crystalline size of the as-synthesized photocatalyst confirms the involvement of good crystalline nano range between 10 and 50 nm [29] as summarized in Table 1.
Table 1
Crystal size of the as-synthesized photocatalysts
Photocatalyst
|
2θ (degree)
|
β (radians)
|
D (nm)
|
CeO2
|
28.52
|
0.02804
|
18.406
|
CdS
|
43.55
|
0.02338
|
23.576
|
CdS/CeO2 (1:1)
|
28.61
|
0.01004
|
47.992
|
3.1.2. Determination of Surface Area (BET)
Surface area of the as-synthesized nanocomposite was determined by the Brunauer-Emmett-Teller (BET) method. Table 2 shows the specific surface area of each sample investigated by nitrogen adsorption-desorption isotherm analysis. The specific surface area is largest for CeO2 (77 m2g− 1) whereas lowest for Ag3PO4 (0.122 m2g− 1) showing the compacted nature of the later.
As indicated in Table 2, the specific surface area can be calculated and those of single and binary nanocomposite which is about 43.8627, 44.3277, 43.5162, 17.4935 and 37.6739 m2g− 1 for the obtained CdS/CeO2 (1:0.5), CdS/CeO2 (1:1), CdS/CeO2 (0.5:1), CdS and CeO2 samples, respectively. The CdS/CeO2 (1:1) composite has the largest specific surface area may be because the modification of CeO2 can increase the dispersion of the CdS nanoparticle, as well as because the CeO2 nanoparticles have relatively large surface area. It is expected that this can facilitate enhancing adsorption of reactant molecules and, thereby, the enhanced photocatalytic activity [30].
Table 2
Specific surface area of the as-synthesized photocatalysts
As-synthesized photocatalyst
|
BET surface area (m2g− 1)
|
CeO2
|
37.6739 ± 0.1923
|
CdS
|
17.4935 ± 0.0745
|
CdS/CeO2 (1:0.5)
|
43.8627 ± 0.7335
|
CdS/CeO2 (1:1)
|
44.3277 ± 0.0012
|
CdS/CeO2 (0.5:1)
|
43.5162 ± 0.1108
|
3.1.3. SEM-EDX Image Study
Morphological images of the as-synthesized photocatalysts; CeO2, CdS and CdS/CeO2 were investigated as shown in Fig. 2a-c respectively.
The EDX spectrum shows the presence of all the relevant components in each photocatalyst. According to the SEM images shown in Fig. 2a, the morphology of CeO2 nanoparticle is observed to be nearly spherical with slight agglomeration (single phase). From the SEM image shown in Fig. 2b it is noticed that the surface morphologies are in the form of assemblies of CdS nanoparticles and uniformly distributed over the entire surface. The EDX spectrum and elemental mapping shows that the CdS nanoparticle contains average stoichiometric composition with 80.45% of cadmium and 19.55% of sulphur. As can be seen from Fig. 2c, the CdS/CeO2 composite also exhibited no distinct morphology, though cubic like structures appeared rarely. On the other hand, the percent chemical distribution gradients across the surface of the sample (w/w %) were determined from results of energy dispersive X-ray (EDX) analysis. Elemental composition (weight %) of the CdS/CeO2 photocatalysts contains average stoichiometric composition with 24.85% of cerium, 62.5% of cadmium and 12% of sulphur. Interestingly, EDX spectrum result exhibits the presence of Ce, Cd and S peaks indicating the CeO2 and CdS nanoparticles [17].
3.1.4. UV/Vis Diffuse Absorption Spectra of the As-synthesized Photocatalysts
UV/Vis diffuse absorption edges of the as-synthesized photocatalysts are obtained from plot of absorbance against wavelength.
The intercept of the tangent line on descending part of the absorption peak at the wavelength axis gives the value of diffuse absorption edge (nm). In such case band gap energy (Eg) of the as-synthesized photocatalysts was obtained from Eq. 4 [31].
(4)
Where, Eg is bandgap energy in electron volts and λmax is wavelength (nm) corresponding to absorption edge.
But estimating the band gap using the above approach sometimes may not provide clear tangential line when the peak is not well resolved for the samples. To avoid the difficulties in obtaining band gap energy from UV/Vis absorption spectroscopy in dispersed samples, diffuse reflectance measurements of dry powders can be performed. The optical absorption properties of each of the as-synthesized photocatalyst were investigated by using a UV/Vis diffuse reflectance spectrometer in the range of 200–900 nm. The band gap values of the photocatalysts were determined by analyzing the optical data with the expression for the optical absorbance α and the photon energy hv using Tauc’s plot [32, 33, 34].
αhν = A (hν - Eg)n/2 (5)
where α is the absorption coefficient, which is proportional to the absorbance, h is the Planck’s constant (J.s), v is the light frequency (s− 1), A is the absorption constant, Eg the band gap energy and n is a constant related to the electronic interband transition. n = 2 for an indirect allowed transition (plotted as (αhv)1/2 versus Eg), n = 3 for an indirect forbidden transition (plotted as (ahv)1/3 versus Eg), n = 1/2 for a direct allowed transition (plotted as (ahv)2 versus Eg), n = 3/2 for a direct forbidden transition [plotted as (ahv)2/3 versus Eg] [35]. The band gaps was then determined by extrapolating the straight line portion of the (ahv)2 versus (hv) graphs to the (hv) axis until (ahv)1/n = 0 the linear section of this spectra as shown in the Fig. 3.
The absorption edges of the binary (CdS/CeO2) photocatalysts are well extended to visible regions of spectrum as compared to the single nanoparticles.
This may be due to the effect of modification in the electronic levels of each single nanoparticle by making them binary composite.
Based on Tauc’s plot as Eq. (5) the band gaps for all the as-synthesized materials were displayed in Table 3. The calculated band gaps of the single systems: CeO2 and CdS are found to be 3.26 eV and 2.48 eV respectively. These findings are similar with previous reports made on these nanoparticles [17, 19, 36, 37]. The binary systems CdS/CeO2 (molar ratio: 1:0.5, 1:1 and 0.5:1) have band gaps of 2.27 eV, 2.23 eV and 2.29 eV respectively. This finding also similar to previous reports made on the same nanoparticles [15, 17, 19].
Table 3
Maximum wavelength and band gap energy (Eg) of the as-synthesized photocatalysts
As-synthesized photocatalyst
|
Max. Wavelength
|
Band gap (Eg) eV
|
CeO2
|
380
|
3.26
|
CdS
|
500
|
2.48
|
CdS/CeO2 (1:0.5)
|
546
|
2.27
|
CdS/CeO2 (0.5:1)
|
542
|
2.29
|
CdS/CeO2 (1:1)
|
555
|
2.23
|
3.1.5. FTIR Study of the As-synthesized Photocatalysts
The FTIR spectrum in the mid-infrared region is the feature of a particular compound that gives the information about the functional groups, molecular geometry and intra/intermolecular interactions. Both inorganic and organic materials can be analyzed in the spectrum.
From the as-synthesized photocatalyst, the functional groups of CeO2 were analyzed by FTIR in the range from 400 to 4000 cm− 1 and displayed in Fig. 4. In case of CeO2, the band at 3435 and 1620 cm− 1 corresponds to the O-H stretching vibration and -OH scissor bending mode respectively, which is originated from physical absorbed (H-bonded) water molecules or surface -OH groups [38, 39]. The band located around 1047 cm− 1 has been attributed to the C-O stretching vibration may be from the additional CO2 that was absorbed at CeO2 surface [40]. The wide band at 1315 cm− 1 consists of the symmetrical stretching mode of N = O and the inside bending mode of N-H. The peak at 850 cm− 1 attributed to outside bending mode of N-H [41, 42].
The peaks observed at 1387 cm− 1 could be ascribed to the stretching vibration of N-O nitrate groups (NO3−) which resulted from a precursor solution of Ce(NO3)3.6H2O that was used to synthesis the nanoparticle (CeO2) [43]. The intense band at 521 cm− 1 corresponds to the Ce-O stretching vibration [17, 44].
3.1.6. Photoluminescence (PL) Study of the As-synthesized Photocatalysts
PL is mainly used as a diagnostic and development tool in semiconductor research, since it can provide information about the electronic structure and the emission mechanism of the material [45]. The PL emission spectra of different photocatalysts (CeO2, CdS and CdS/CeO2 (in 0.5:1, 1:1 and 1:0.5 molar ratios) were determined. The order of intensity is CeO2 > CdS > CdS/CeO2 (0.5:1) > CdS/CeO2 (1:0.5) > CdS/CeO2 (1:1) as indicated in the Fig. 5.
In binary nanocomposite the photoinduced electrons and holes can be effectively separated and hence excitation PL intensity goes down. This is because, lower the excitation PL intensity, stronger the capacity of coupled materials to capture photoinduced electrons, higher the separation rate of photoinduced electrons and holes, and higher the photocatalytic activity [46]. So, in this study it was clearly observed that the lower PL emission spectra were recorded for CdS/CeO2 (1:1) nanocomposite.
This might be also due to the photogenerated electron hole pair in CeO2 separated well due to the synergy effect of coupling of A-type heterojunction (CdS/CeO2). In general the PL intensities of single systems were found to be higher than the binary system photocatalyst.
3.2. Photocatalytic Studies
3.2.1. Photocatalytic Activities of the As-synthesized Photocatalysts
Methyl orange (MeO), with a characteristic absorption at 464 nm is chosen as a typical organic pollutant for testing the photocatalytic activity of the as-prepared products. The photocatalytic activities of all the as-synthesized sample were evaluated by testing their ability in the degradation of MeO (initial dye concentration of 10 ppm and catalyst load of 0.2 g/L) under 160 min visible irradiation time. The degradation rate was analyzed by plotting Ct/Co versus irradiation time. The results show that the characteristic absorption peaks corresponding to MeO decrease rapidly as the exposure time increases, indicating the decomposition of MeO and the significant reduction in the MeO concentration.
The photocatalytic performance of CeO2, CdS and CdS/CeO2 (in 0.5:1, 1:1 and 1:0.5 molar ratios) photocatalyst was first evaluated by the degradation of model pollutant MeO under visible light irradiation. Before the photocatalytic reaction, an adsorption step in dark conditions was allowed to take place during 60 min. The result indicates that the adsorption capacity of binary and single photocatalyst become in the order of CeO2 < CdS < CdS/CeO2 (0.5:1) < CdS/CeO2 (1:0.5) < CdS/CeO2 (1:1). The photocatalytic activities of pure CeO2 and CdS were lower than that of the composite photocatalyst. They decolorized 18.58, 27.07, 32.06, 35.98 and 53.73 % of MeO in 160 min of irradiation time respectively. The photocatalytic activities of pure CeO2 are lower than CdS due to its higher band gap energy which makes it less sensitive to visible irradiation unlike other single counterparts [47].
The increment of photocatalytic degradation efficiency of CdS from that of CeO2 over MeO solution is due to the efficient charge separation, narrow band gap that makes it sensitive to visible portion of the spectrum, less electron-hole recombination, and a wide range of optical absorption of light by the composite could be possible reasons for the enhanced photoactivity [36]. In the A-type heterojunction system CdS/CeO2 (1:1), the increased photocatalytic activities of CeO2 could be initiated by conduction band electrons of CdS that involve in the photoreduction process [26].
There are many reports available on the photocatalytic degradation efficiency of binary nanocomposite (A-type) over different azo dyes (MeO). For A-type heterojunction, CdS/CeO2, CdS/ZnO and CdS/TiO2 also shows percent decolorization about 38.00, 49.30 and 55.00% over methyl orange dye respectively in different irradiation time [17, 48, 49]. In our case 53.73% for A-type heterojunction in 160 min irradiation time which is partially in line with the previous reports on both heterojunction.
The kinetics of the photocatalytic degradation was also exhibited using the pseudo-first-order reaction [50]. The rate constant of the binary CdS/CeO2 (1:1) nanocomposite was higher than single and binary in other ratio photocatalysts, indicating the presence of a synergistic effect as indicated in Table 4. Among them, the apparent rate constants of CdS/CeO2 (1:1) was the highest, calculated based on the equation ln(Ct/Co) per irradiation time, and it was almost 18, 12, 8.9 and 6.13 times higher than that of CeO2, CdS, CdS/CeO2 (0.5:1) and CdS/CeO2 (1:0.5) respectively.
Table 4
The apparent rate constants and % degradation of the as-synthesized photocatalysts after 160 min
As-synthesized photocatalyst
|
Degradation (%)
|
Rate constant, k (×10− 4 min− 1)
|
CeO2
|
18.58
|
9.006
|
CdS
|
27.07
|
55.210
|
CdS/CeO2 (1:1)
|
53.73
|
162.11
|
CdS/CeO2 (0.5:1)
CdS/CeO2 (1:0.5)
|
32.06
35.98
|
80.150
108.072
|
3.3. Photocatalytic Stability of the As-synthesized Photocatalyst
In addition to photocatalytic efficiency, the stability of photocatalyst is also very important for practical application. To evaluate the stability of photocatalytic performance of binary CdS/CeO2 (1:1), the circulating run in the photocatalytic degradation of MeO was carried out under visible light irradiation. The process was repeated up to four times. As exhibited in Fig. 7 the studies revealed that the composite demonstrated moderate stability after recovery.
In the first cycle 53.16% of the dye was degraded after 160 min of irradiation time. Subsequently the second, third and fourth cycle degraded 38.71, 23.17 and 17.41% of the dye corresponding to the rate constant (k) from 0.00493 to 0.00310, 0.00201 and 0.00120 min− 1 respectively. Agglomeration and sedimentation of the dye around the composite particles after each cycle of photocatalytic degradation is a possible cause of the observed decrease on the degradation rate, because each time the photocatalyst is reused new parts of the photocatalyst surface become unavailable for dye adsorption and thus photon absorption, reducing the efficiency of the catalytic reaction [51, 52]. Besides, one might expect a loss in the photocatalyst during recycling which eventually affect catalytic activity after each cycle.
This decreases the degradation rate of the as-synthesized photocatalyst may be due to the weakening of the absorbance ability of the catalyst or the loss of some catalyst during the cycling reaction. The result demonstrates that CdS/CeO2 (1:1) nanocomposite shows a good photocatalytic performance as well as moderate stability after 4 cycles in 160 min.
3.4. General Proposed Mechanism of degradation of MeO Using CdS/CeO2
According to the above experimental research, a probable mechanism of charge transfer and photocatalytic degradation of organic pollutant methyl orange (MeO) over the CdS/CeO2 nanocomposite under visible light irradiation is put forward and illustrated in Fig. 8. The light illumination on CdS/CeO2 nanocomposite causes the generation of electron (e−) in conduction band (CB) and holes (h+) in the valence band (VB).
Under visible light irradiation, the CdS can be activated, and the photoinduced electrons at the conduction band of CdS are transferred to the conduction band of CeO2. The electrons will then react with water to produce hydrogen. On the other hand, the photoexcited holes at the valence band of CdS nanoparticles are captured by the MeO dye.
At CB site, molecular oxygen (O2) forms superoxide radical •O2− in the presence of the photoexcited CB e− and subsequently reacts with H+ to form HO2• radical species. During the e− transfer from CdS to CB of CeO2, the generated photoinduced h+ in VB might react with water (H2O) and the adsorbed MeO dye molecule to yield hydroxyl radical (•OH) and MeO•− anions radical respectively [53]. On the other hand, electrons in the CeO2 CB can migrate to its VB, while the holes generated in the CeO2 VB move to CdS VB surface [54]. It is known that these oxygenous radicals (•O2−, •OH and HO2•) act as potential oxidizing and reducing species for the degradation of organic molecules (MeO) [55]. The proposed photoreaction mechanism of CdS/CeO2 composite over MeO degradation under visible light follows:
CdS/CeO2 + hv → h+ VB + e−CB (6)
Oxidative reaction
h+ VB + H2O → H+ + •OH (7)
•OH + MeO → (Intermediates) → CO2 + H2O
Reductive reaction
e− CB + O2 → •O2− (8)
•O2− +MeO → (Intermediates) → CO2 + H2O
Generally:
•O2− (•OH, h+) + MeO + O2/H2O → Degradation products (9)
In our case, on comparison with single and binary photocatalysts, the superior photocatalytic activity of binary nanocomposite under visible light might result from the enhanced charge separation and the formation of more active radicals (•O2− and •OH) which are induced by the synergetic effect between CdS and CeO2. As a result, CdS/CeO2 nanocomposite delivers high photogenerated e−-h+ pair charge separation and produces sufficiently high amount of radicals for the high degradation of MeO dye under visible light irradiation. The photocatalytic performance of the photocatalyst mainly depends on: (i) its light absorption properties; (ii) the rates of reduction and oxidation on the surface of the catalyst by the electrons and holes; and (iii) the electron-hole recombination rate [56].