Metal doped CdS nanoparticles were synthesized by a simple chemical precipitation route with different metals. The obtained nanoparticles were characterized by XRD and UV-vis reflectance spectroscopy. The results indicated that metal-doped CdS catalysts were successfully obtained with cubic structure and 4.0 - 4.5 nm crystallite size. The band gap energies of metal-doped CdS catalysts were estimated using UV-visible reflectance spectra to be about the range of 2.25 - 2.55 eV. Methylene blue was degraded by using metal doped CdS nanoparticles under a 400 W medium-pressure mercury lamp of visible light irradiation (
λ > 420 nm). Higher degradation efficiency was achieved by adding metals to the photocatalyst compared with the single CdS catalyst. In this case, the degradation efficiency of Co-CdS catalyst after 4h irradiation time was about 87%.
Chalcogenides Chemical Synthesis Optical Properties Photocatalytic Reaction1. Introduction
Environmental problems associated with organic pollutants provide the impetus for sustained fundamental and applied research in the area of environmental remediation. Semiconductor photocatalysis offers the potential for complete elimination of toxic chemicals through its efficiency and potentially broad applicability [1] . Dye-related industries such as textile, release various organic pollutants to the atmosphere. If these pollutants are discharged into the environment without any treatment, it will cause extensive toxicity to the various forms of lives. In order to eliminate the potential danger of these organic dyes, many technologies have been developed to treat these pollutants. Photocatalysis is an alternative route for water purification [2] . Photocatalysis is a process by which a semiconductor material absorbs light of energy greater than or equal to its band gap. It causes excitations of valence band electrons in the conduction band. II-VI semiconductors have attracted attention in recent years because of their easy synthesis in the size range and their potential applications in optoelectronic devices, solar cells, photo catalysis, and sensitized solar cells [3] .
CdS is an important group of II-VI compound semiconductors with excellent physical properties and wide band gap energy of 2.42 eV. Cadmium sulfide (CdS) has two abilities for photo catalytic reaction: its band gap responses to visible light and its conduction band is more negative than the reduction potential of H+/H2 [4] . It is obviously known that CdS has many properties such as crystalline phase, size, morphology, specific surface area which can affect its photo catalytic activity. It has been offered that morphology of photo catalyst could be effective for the constraint of recombination between photo generated electrons and holes and for the separation of H2 evolution sites from oxidation reaction sites [5] . A favorable shift of the optical response into the visible region occurs subsequently to the doping of transition metal, such as Ni2+ and Cu2+. Thus, CdS nanoparticles can also be used as promising catalysts for photocatalytic reactions under visible-light irradiation [6] [7] .
Metal doped semiconductors have appealed scientific attention due to their probable applications. Different techniques such as chemical precipitation, hydrothermal method, chemical vapor deposition, spray pyrolysis and other chemical routes have been used to synthesize metal doped CdS. [5] Metal (Ni, Co, Sb, Ce, etc.,) doped CdS has drawn remarkable attention as it presents a great opportunity to combine electrical and optical properties into a single material [5] [8] [9] .
Noble metal co-catalysts such as Pt, Pd, and Rh on CdS, can trap light-in- duced electrons and act as the active sites for H2 production and degradation of dyes from water. [10] [11] . However, the high cost of noble metals seriously limits their practical application in hydrogen evolution through water splitting, which can well explain why metals are of particular significance as co-catalysts for production of hydrogen and oxygen through water splitting or for degradation of organic pollutants in water and air [12] [13] [14] [15] [16] . With the assistance of cocatalysts, researchers have successfully developed various CdS based photocatalysts possessing visible light catalytic activity, such as NiS/CdS, Ni (OH)2/CdS, and Ni@C/CdS [12] [13] [14] [15] [16] . These modified photocatalysts are valuable, since they can efficiently promote water splitting under visible light irradiation and provide high hydrogen production rates. [17]
A doping effect on metallic chalcogenides is available in the literature [18] [19] [20] , proving that the photoluminescent and electrical features of sulfide nanocrystals. Continuous attempts are being made to synthesize sulfide nanomaterials with controlled sizes, shapes, and phase purity by various chemical routes [21] [22] [23] . Doping cadmium sulfide has attracted considerable attention as it is a convenient way to tailor its physical properties. Recently, spectroscopic studies of transition metal and rare-earth doped nanocrystalline CdS have motivated to develop new and more efficient multicolor phosphor materials [24] . Similarly, the mixed Cd1−xZnxS [25] , Cd1−xMnxS, and CdSxSe1−x nanoparticles system [26] [27] and [28] [29] is studied because of its suitability for optical devices from the near-ultraviolet to the near-infrared [30] and [31] . Investigation of CdS nanostructure has received considerable attention due to its size dependent properties and quantized charging effects on metal nanoparticles and provides the basis for developing new and effective systems. These nanostructures provide innovative strategies for designing next generation energy conversion devices. Incorporate dopants into CdS nanocrystalline have been prepared by various techniques such as, ion exchange reaction, dry process, spray pyrolysis, chemical bath deposition, pulsed laser deposition and sol-gel spin coating method [32] and [33] . In literature, TiO2-graphene nanocomposite was also used for photocatalytic reactions under visible light and it was showed 93% degradation efficiency with a progress at photocatalytic activity studies [34] .
In this study, we investigate optical properties of metal-doped (Ni, Co, Ce, Sb) CdS nanoparticles prepared by chemical co-precipitation method. The prepared samples were characterized using X-ray diffraction and UV/Vis reflectance spectra for studying optical properties. Photocatalytic degradation of methylene blue was performed by using metal doped CdS nanoparticles under a 400 W medium-pressure mercury lamp of visible light irradiation (λ > 420 nm).
2. Experimental2.1. Synthesis
In the present study, metal-doped (2% mol) CdS catalysts have been synthesized through chemical precipitation technique. 5 mol aqueous solution of cadmium chloride (CdCl2) (50 ml) and 0.1 mol nickel nitrate (Ni (NO3)26H2O), cobalt nitrate (Co (NO3)26H2O), antimony chloride (SbCl3) and cerium chloride (CeCl3) (10 ml) were stirred for 30 min at room temperature, separately. 5 mol aqueous solution of sodium sulfide (Na2S) (50 ml) was added drop wise to mixed solution and was stirred for 2 h. A yellowish-orange color precipitate was obtained. The nanoparticles were purified by excess distilled water and then the sample was dried at 65˚C for 4 h [35] [36] .
2.2. Characterization
The phase as cubic, hexagonal, etc. and the crystallite size of the samples were carried out through X-ray Diffraction Analysis. Structural characterization for the phase identification and crystallite size analysis was carried out by Rigaku Ultima + powder X-ray diffractometer operating at 40 kV and 30 mA with Cu/Kα (λ = 1.54 Å) radiation. The crystallite size of CdS samples was calculated using Debye-Scherrer equation. UV-V is reflectance and absorbance spectrums were recorded on an Ocean Optics USB-4000 in the range 220 - 1000 nm.
2.3. Photocatalytic Degradation
The photocatalytic reaction was performed in a reactor (250 ml). 50 mg of the catalyst was dispersed in 200 ml of 8 ppm (aqueous) MB dye solution. The reaction mixture was stirred with a magnetic stirrer. It was photo irradiated at room temperature by using a 400-W medium pressure mercury lamp with UV filter (λ > 420 nm). The decomposition of MB dye was measured by UV-V is absorption spectrophotometer at regular intervals of time using (λmax = 665 nm) [37] . The procedure was repeated for all samples.
3. Results and Discussion3.1. X-Ray Diffraction
X-ray diffraction pattern gives information about crystalline structure and crystallite size. Figure 1 shows the X-ray diffraction pattern of the CdS catalysts with different metals(Ni, Co, Sb, Ce). XRD peaks are found at 2θ values of 26˚, 43˚ and 52˚ approximately, referring to diffraction from (111), (220) and (311) planes, respectively, which has only a single very broad neighbor peak near the (110) line [38] . There were not seen any different peaks corresponding to the metals because of their small quantity (2% mol). Figure 2 shows the XRD pattern of single CdS catalyst. It is clear that, XRD patterns showed for all catalysts
X-ray Diffraction pattern of metal-doped CdS catalysts
X-ray Diffraction pattern of as-prepared CdS catalyst
were composed of single cubic phase. Figure 3 shows standard X-ray Diffraction pattern of cubic CdS (JCPDS-89-0440) and hexagonal CdS (JCPDS-80-0006). The XRD patterns obtained for both single CdS and metal doped CdS corresponded to pure cubic CdS when compared with the standard reference (JCPDS-89-0440). All the peaks in the diffraction pattern is found to be characteristic of CdS, suggesting that incorporation of metals in the sample does not introduce appreciable change in the crystal structure of CdS. But also X-ray diffraction data indicates that the diffraction peak of metal doped CdS plane slightly shifts to larger angle. There is a small shift in broad peaks ofmetal-doped CdS with respect to the pure CdS nanocrystals;this small shift in intensity may be assigned to the presenceof dopant metal in doped nanocrystals. The broadening of diffraction peak provides information about crystallite size. As the width increases, the crystallite size decreases.
The crystallite size was calculated using Debye Scherrer’s formula as seen Equation (1) [39]
where D is the crystallite size, λ is the wavelength of X-rays, β is full-width at half-maxima (FWHM) in radians and θ is the diffraction angle. Corresponding to the maximum intensity peak crystallite sizes were in the range of 4.0 - 4.5 nm as seen in Table 1.
Crystallite size of metal doped CdS catalysts were decreased by adding metals as Ni, Co, Sb, and Ce. The diffraction pattern exhibits broad peaks revealing that
Standard X-ray Diffraction pattern of CdS [<xref ref-type="bibr" rid="scirp.72672-ref40">40</xref>]
Crystallite size of metal-doped CdS catalysts from X-ray diffraction analysis
Catalysts
Crystallite size of metal-doped and single CdS catalysts
D (nm)
Phase
Ni-CdS
4.4
Cubic
Co-CdS
4.0
Cubic
Ce-CdS
4.5
Cubic
Sb-CdS
4.5
Cubic
CdS
5.1
Cubic
the prepared particles are of very small crystallite size.
3.2. Diffuse Reflectance
The UV―visible reflectance spectroscopy is an efficient technique to identify the optical properties of CdS particles. Figure 4 shows the Kubelka Munk function spectrums of the metal doped CdS catalysts. These spectrums exhibit defined reflection edges at 550, 487, 513 and 524 nm, corresponding to band gaps of 2.26, 2.55, 2.42 and 2.37 eV, for metal-doped CdS catalysts, as seen in Table 2. For the study of the optical properties of the metal doped CdS nanoparticles, the band gaps were determined, which were calculated by means of the optic reflectance spectrum [39] . Because this is the most accurate way to find the correct band gap value for as-prepared catalysts. Electromagnetic radiation in the range of 440 - 490 nm is known as blue region of the visible light spectrum.
Generally, the wavelength of the maximum exciton reflection decreases when the particle size decreases due to the quantum confinement of the photo-gener- ated electron-hole pairs. But in this study, crystallite sizes were closed to each
Kubelka-Munk curve of metal-doped CdS catalysts
Band gap values of metal-doped CdS catalysts from UV-vis reflectance spectroscopy
Catalysts
Band gap values of metal-doped and single CdS catalysts
UV-Vis(Eg)-eV
UV-Vis(λ)-nm
Ni-CdS
2.26
550
Co-CdS
2.55
487
Ce-CdS
2.42
513
Sb-CdS
2.37
524
CdS
2.25
552
other as seen in Table 1. So it cannot be said that band gap values were changed because of only the crystallite sizes of the catalysts. It is clearly seen that the band gap of metal doped CdS catalysts were shifted in the blue region [5] .
The band gap of CdS is increased by adding metals as seen in Table 2. It means that wavelength which corresponds with band gap is decreased. This decrease is interpreted due to sp-d exchange interaction between the band electrons and the localized d electrons of the metal ions (Co2+, Ni2+, Sb3+, Ce3+) substituting for host ions.
3.3. Photocatalytic Decomposition of Methylene Blue (MB)
Visible light photocatalytic activity of the catalysts was subsequently evaluated by measuring the discoloration of MB in aqueous solution under visible light illumination. The peak maximum for the absorbance spectra of MB at 665 nm decreases gradually with increasing irradiation time [41] .
Equation (2) shows % Degradation calculation which is seen below.
The rate of MB photo degradation in the Co-CdS was higher than that in the other CdS catalysts; the rate of MB photo degradation in the CdS with metals were also higher than that of the single CdS catalyst as seen in Table 3. In this study, the adsorption time for CdS catalysts keep constant about 30 minutes. The maximum wavelength of MB in the absorption spectra was 665 nm and almost kept the same. After being irradiated for 240 min, the degradation rate of MB with the Co-CdS catalyst was up to 87%. This faster degradation rate of MB with using Co-CdS is referred to the increase in defect sides caused by Co doping, and leading to an enhanced optical absorption in the visible region. Kubelka-Munk curve is also indicated that Co doped CdS slightly placed at the right side of the spectrum, as seen in Figure 4.
Figure 5 shows also the medium pressure mercury (Hg) lamp spectrum with UV filter.
Figure 6 shows the photodegradation of MB in the presence of metal doped CdS catalysts under visible light. The photo catalytic degradation of MB which
% Degradation values of metal doped CdS binary catalysts and single CdS catalysts
Catalysts
% Degradation values of metal-doped and single CdS catalysts
% Degradation
Ni-CdS
73
Co-CdS
87
Ce-CdS
76
Sb-CdS
70
CdS
68
The medium pressure mercury lamp spectrum with UV filter
Photodegradation of MB in the presence of metal doped CdS catalysts under visible light
contains metal doped CdS catalysts under visible light obeys pseudo-first-order kinetics with regard to the concentration of MB, as seen in Equation (3): [40]
The integration of the equation (CA = CA0 at t = 0, CA0 is the initial concentration in the bulk solution after dark adsorption and t is the reaction time) will prevent with Equation (4):
where CA and CA0 are the reactant concentrations at time t = t and t = 0, respectively; kapp and t are the apparent reaction rate constant and time, respectively. According to the equation, a plot of ln (CA/CA0) versus t will yield a slope of kapp. Figure 7 shows the the plot of ln (CA/CA0) versus time for as-prepared catalysts in order to find the kapp. The linearity of plot suggests that the photo degradation reaction approximately follows the pseudo-first-order kinetics with a kapp of 0.0051 min−1, 0.0086 min−1, 0.0030 min−1 and 0.0019 min−1 for the degradation under visible light for Ni, Co, Ce and Sb doped CdS catalysts, respectively as seen in Table 4.
It is certain that metal doping on CdS increase the photo catalytic degradation of the MB under visible light. It has been seen that the photo catalytic activity of metal doped CdS is intensely depending on the metal ion type. Doping of Co increases the life time of excited charge carriers, which improves photo catalytic
The plot of ln (C<sub>A</sub>/C<sub>A</sub><sub>0</sub>) versus time for as-prepared catalysts
Reaction Rate Constants of metal-doped CdS Catalysts
Catalysts
Reaction Rate Constants of metal-doped CdS Catalysts
CA0 (ppm)
CA (ppm)
k*10−3 (min−1)
Ni-CdS
5.30
1.44
5.1
Co-CdS
5.62
0.75
8.6
Ce-CdS
5.35
1.26
3.0
Sb-CdS
5.30
1.56
1.9
activity. When the doping ion changes, some CdS crystal defects could be caused, and this may act as recombination centers to decrease the photo activity. It is obvious that doping of CdS with metal ions improves photo catalytic activities of CdS, and therefore metal doped CdS is capable of degradation of MB under visible light.
In this study, we assumed that metal ions with CdS generate electron-hole pair at conduction and valence band under visible light. The electrons transfer to the adsorbed MB molecule on the particle surface. The excited electron which comes from the conduction band enters in to the molecular structure of MB and complete degradation of MB. Hole at the valence band generates hydroxyl radical via reaction with water. This obviously indicates that the doped CdS catalysts can be as promising photocatalyst for the environmental detoxification from organic pollutants which can operate under visible light irradiation.
4. Conclusions
This study demonstrated an easy method for preparation of efficient visible light photocatalysts, namely metal doped CdS nanoparticles, by chemical precipitation method. The obtained metal doped CdS nanoparticles exhibit cubic structures with average crystallite sizes in the range of 4.0 - 4.5 nm. Crystallite size of metal doped CdS catalysts was decreased by adding metals as Ni, Co, Sb, Ce.
Optical diffuse reflectance spectra of metal-doped CdS nanoparticles exhibited a blue shift in the visible range compared with bulk CdS. The band gap is increased with adding metals. It means that wavelength which corresponds with band gap is decreased. This decrease is interpreted as mainly due to sp-d exchange interaction between the band electrons and the localized d electrons of the Co2+ ions substituting for host ions. The usages of different metals on CdS catalysts are promising work for photocatalytic degradation of methylene blue. Especially, CdS with Co was exhibited larger band gap value than the others and this result was a direct consequence associated with highest % degradation.
Methylene blue was degraded with using CdS and metal doped CdS nanoparticles under visible light irradiation. This study reveals that the photocatalytic activity of the catalysts depends on the metal doping (Ni, Co, Ce, Sb) on CdS. The highest photodegradation efficiency was observed for Co doped CdS with a percentage of 87%.
Acknowledgements
This work was supported by the Research Fund of Istanbul University by the Project number 12096.
Cite this paper
Ertis, I.F. and Boz, I. (2017) Synthesis and Characterization of Metal-Doped (Ni, Co, Ce, Sb) CdS Catalysts and Their Use in Methylene Blue Degradation under Visible Light Irradiation. Modern Research in Catalysis, 6, 1-14. http://dx.doi.org/10.4236/mrc.2017.61001
ReferencesZhu, L., Meng, Z., Cho, K. and Oh, W.C. (2012) Synthesis of CdS/CNT-TiO2 with a High Photocatalytic Activity in Photodegradation of Methylene Blue. New Carbon Materials, 27, 166-174. https://doi.org/10.1016/S1872-5805(12)60011-0Yogamalar, R.N., Sadhanandam, K., Boseb, A.C. and Jayavel, R. (2015) Quantum Confined CdS Inclusion in Graphene Oxide for Improved Electrical Conductivity and Facile Charge Transfer in Hetero-Junction Solar Cell. RSC Advances, 5, 16856-16869. https://doi.org/10.1039/C4RA13061HThuy, U.T.D., Liem, N.Q., Parlett, C.M.A., Lalev, G.M. and Wilson, K. (2014) Synthesis of CuS and CuS/ZnS Core/Shell Nanocrystals for Photocatalytic Degradation of Dyes under Visible Light. Catalysis Communications, 44, 62-67.
https://doi.org/10.1016/j.catcom.2013.07.030Saikia, D., Gogoi, P.K. and Saikia, P.K. (2010) Structural and Optical Properties of Nanostructured CdS Thin Films Deposited at Different Preparative Conditions. Chalcogenide Letters, 7, 317-324.Kotkata, M.F., Masoud, A.E., Mohamed, M.B. and Mahmoud, E.A. (2009) Synthesis and Structural Characterization of CdS Nanoparticles. Physica E: Low-dimensional Systems and Nanostructures, 41, 1457-1465.
https://doi.org/10.1016/j.physe.2009.04.020Thambidurai, M., Muthukumarasamy, N., Agilan, S., Sabari, A.N., Murugan, N. and Balasundaraprabhu, R. (2011) Structural and Optical Characterization of Ni-Doped CdS Quantum Dots. Journal of Materials Science, 46, 3200-3206.
https://doi.org/10.1007/s10853-010-5204-yAwati, P.S., Awate, S.V., Shah, P.P. and Ramaswamy, V. (2003) Photocatalytic Decomposition of Methylene Blue Using Nanocrystalline Anatase Titania Prepared by Ultrasonic Technique. Catalysis Communications, 4, 393-400.
https://doi.org/10.1016/S1566-7367(03)00092-XRahimi, R., Zargari, S., Ghaffarinejad, A. and Morsali, A. (2016) Investigation of the Synergistic Effect of Porphyrin Photosensitizer on Graphene-TiO2 Nanocomposite for Visible Light Photo Activity Improvement. Environmental Progress & Sustainable Energy, 35 642-652. https://doi.org/10.1002/ep.12267Patel, N.H., Deshpande, M.P. and Chaki, S.H. (2015) Study on Structural, Magnetic Properties of Undoped and Ni Doped CdS Nanoparticles. Materials Science in Semiconductor Processing, 31, 272-280. https://doi.org/10.1016/j.mssp.2014.11.039Bacaksiz, E., Tomakin, M., Altunbas, M., Parlak, M. and Çolakoglu, T. (2008) Structural, Optical and Magnetic Properties of Cd1–xCoxS Thin Films Prepared by Spray Pyrolysis. Physica B: Condensed Matter, 403, 3740-3745.
https://doi.org/10.1016/j.physb.2008.07.006Thambidurai, M., Muthukumarasamy, N., Agilan, S., Murugan, N., Sabari, A.N., Vasantha, S. and Balasundaraprabhu, R. (2010) Studies on Optical Absorption and Structural Properties of Fe Doped CdS Quantum Dots. Solid State Sciences, 12, 1554-1559. https://doi.org/10.1016/j.solidstatesciences.2010.06.020Reyes, P. and Velumani, S. (2012) Structural and Optical Characterization of Mechanochemically Synthesized Copper Doped CdS Nanopowders. Materials Science and Engineering: B, 177, 1452-1459. https://doi.org/10.1016/j.mseb.2012.03.002Bhargava, R.N. (1996) Doped Nanocrystalline Materials—Physics and Applications. Journal of Luminescence, 70, 85-94. https://doi.org/10.1016/0022-2313(96)00046-4Wang, Y., Heron, N., Moller, K. and Bein, T. (1991) Three-Dimensionally Confined Diluted Magnetic Semiconductor Clusters: Zn1–xMnxS. Solid State Communications, 77, 33-38. https://doi.org/10.1016/0038-1098(91)90421-QCounio, G., Gacoin, T. and Boilot, J.P. (1998) Synthesis and Photoluminescence of Cd1-xMnxS (x ≤ 5) Nanocrystals. Journal of Physical Chemistry B, 102, 5257-5260.
https://doi.org/10.1021/jp980511wFuyu, Y. and Parker, J.M. (1988) Quantum Size Effects in Heat Treated, Cd(S, Se) Doped Glasses. Materials Letters, 6, 233-237.
https://doi.org/10.1016/0167-577X(88)90028-6Bhargava, R.N., Gallenger, D., Hong, X. and Nurmikko, A. (1994) Optical Properties of Manganese-Doped Nanocrystals of ZnS. Physical Review Letters, 72, 416-419. https://doi.org/10.1103/PhysRevLett.72.416Pileni, M.P. (2000) II-VI Semiconductors Made by Soft Chemistry: Syntheses and Optical Properties. Catalysis Today, 58, 151-166.
https://doi.org/10.1016/S0920-5861(00)00250-9Murase, N., Jagannathan, R., Kanematsu, Y., Watanabe, M., Kurita, A., Hirata, K., Yazawa, T. and Kushida, T. (1999) Fluorescence and EPR Characteristics of Mn2+-Doped ZnS Nanocrystals Prepared by Aqueous Colloidal Method. Journal of Physical Chemistry B, 103, 754-760. https://doi.org/10.1021/jp9828179Peng, H., Bai, L., Yu, L., Li, J., Yuan, F. and Chen, Y. (2009) Shape-Controlled Synthesis of ZnS Nanostructures: A Simple and Rapid Method for One-Dimensional Materials by Plasma. Nanoscale Research Letters, 4, 1047-1053.
https://doi.org/10.1007/s11671-009-9358-yYan, C., Liu, J., Liu, F., Wu, J., Gao, K. and Xue, D. (2008) Tube Formation in Nanoscale Materials. Nanoscale Research Letters, 3, 473-480.
https://doi.org/10.1007/s11671-008-9193-6Zuo, T., Sun, Z., Zhao, Y., Jiang, X. and Gao, X. (2010) The Big Red Shift of Photoluminescence of Mn Dopants in Strained CdS: A Case Study of Mn-Doped MnSCdS Heteronanostructures. Journal of the American Chemistry Society, 132, 6618-6619. https://doi.org/10.1021/ja100136aArora, S. and Manoharan, S.S. (2007) Large Shift in the Photoluminescent Properties of Mn2+-Doped Nanosized CdS-ZnS Solid Solutions. Solid State Communications, 144, 319-323. https://doi.org/10.1016/j.ssc.2007.08.038Sathyamoorthy, R., Sudhagar, P., Balerna, A., Balasubramanian, C., Bellucci, S., Popov, A.I., et al. (2010) Surfactant-Assisted Synthesis of Cd1–xCoxS Nanocluster Alloys and Their Structural, Optical and Magnetic Properties. Journal of Alloys and Compounds, 493, 240-245. https://doi.org/10.1016/j.jallcom.2009.12.063Romcevic, M., Romcevic, N., Kostic, R., Klopotowski, L., Dobrowolski, W.D., Kossut, J. and Comor, M.I. (2010) Photoluminescence of Highly Doped Cd1-xMnxS Nanocrystals. Journal of Alloys and Compounds, 497, 46-51.
https://doi.org/10.1016/j.jallcom.2010.03.072Wang, H., Chen, W., Zhang, J., Huang, C. and Mao, L. (2015) Nickel Nanoparticles Modified CdS—A Potential Photocatalyst for Hydrogen Production through Water Splitting under Visible Light Irradiation. International Journal of Hydrogen Energy, 40, 340-345. https://doi.org/10.1016/j.ijhydene.2014.11.005Zhang, W., Wang, Y., Wang, Z., Zhong, Z. and Xu, R. (2010) Highly Efficient and Noble Metal-Free NiS/CdS Photocatalysts for H2 Evolution from Lactic Acid Sacrificial Solution under Visible Light. Chemical Communications, 46, 7631-7633. https://doi.org/10.1039/c0cc01562hZhang, L., Tian, B., Chen, F. and Zhang, J. (2012) Nickel Sulfide as Co-Catalyst on Nanostructured TiO2 for Photocatalytic Hydrogen Evolution. International Journal of Hydrogen Energy, 37, 17060-17067.
https://doi.org/10.1016/j.ijhydene.2012.08.120Zhang, J., Qiao, S., Qi, L. and Yu, J. (2013) Fabrication of NiS Modified CdS Nanorod p-n Junction Photocatalysts with Enhanced Visible-Light Photocatalytic H2-Production Activity. Physical Chemistry, 15, 12088-12094.Sreethawong, T., Suzuki, Y. and Yoshikawa, S. (2005) Photocatalytic Evolution of Hydrogen over Mesoporous TiO2 Supported NiO Photocatalyst Prepared by Single-Step Sol-Gel Process with Surfactant Template. International Journal of Hydrogen Energy, 30, 1053-1062. https://doi.org/10.1016/j.ijhydene.2004.09.007Guo, L., Zhao, L., Jing, D., Lu, Y., Yang, H., Bai, B., et al. (2010) Reprint of: Solar Hydrogen Production and Its Development in China. Energy, 35, 4421-4438.
https://doi.org/10.1016/j.energy.2010.08.010Yang, J., Yan, H., Wang, X., Wen, F., Wang, Z., Fan, D., et al. (2012) Roles of Cocatalysts in Pt-PdS/CdS with Exceptionally High Quantum Efficiency for Photocatalytic Hydrogen Production. Journal of Catalysis, 290, 151-157.
https://doi.org/10.1016/j.jcat.2012.03.008Rao, B.S., Kumar, B.R., Reddy, V.R., Rao, T.S. and Chalapathi, G.V. (2011) Preparation and Characterization of CdS Nanoparticles by Chemical Co-Precipitation Technique. Chalcogenide Letters, 8, 39-44.Thambidurai, M., Muthukumarasamy, N., Velauthapillai, D., Agilan, S. and Balasundaraprabhu, R. (2012) Structural, Optical, and Electrical Properties of Cobalt-Doped CdS Quantum Dots. Journal of Electronic Materials, 41, 665-672.
https://doi.org/10.1007/s11664-012-1900-5Chauhan, R., Kumar, A. and Chaudhary, R.P. (2013) Visible-Light Photocatalytic Degradation of Methylene Blue with Fe Doped CdS Nanoparticles. Applied Surface Science, 270, 655-660. https://doi.org/10.1016/j.apsusc.2013.01.110Shi, J.W., Yan, X., Cui, H., Zong, X., Fu, M.L., Chen, S. and Wang, L. (2012) Low-Temperature Synthesis of CdS/TiO2 Composite Photocatalysts: Influence of Synthetic Procedure on Photocatalytic Activity under Visible Light. Journal of Molecular Catalysis A: Chemical, 356, 53-60.
https://doi.org/10.1016/j.molcata.2012.01.001Tsuji, I. and Kudo, A. (2003) H2 Evolution from Aqueous Sulfite Solutions under Visible Light Irradiation over Pb and Halogen-Codoped ZnS Photocatalysts. Journal of Photochemistry and Photobiology A: Chemistry, 156, 249-252.
https://doi.org/10.1016/S1010-6030(02)00433-1Li, Y., Du, J., Peng, S., Xie, D., Lu, G. and Li, S. (2008) Enhancement of Photocatalytic Activity of Cadmium Sulfide for Hydrogen Evolution by Photoetching. International Journal of Hydrogen Energy, 33, 2007-2013.
https://doi.org/10.1016/j.ijhydene.2008.02.023Fan, Y., Chen, G., Li, D., Li, F., Luo, Y. and Meng, Q. (2011) Enhancement of Photocatalytic H2 Evolution on Hexagonal CdS by a Simple Calcination Method under Visible Light Irradiation. Materials Research Bulletin, 46, 2338-2341.
https://doi.org/10.1016/j.materresbull.2011.08.040Soltani, N., Saion, E., Hussein, M.Z., Erfani, M., Abedini, A., Bahmanrokh, G., Navasery, M. and Vaziri, P. (2012) Visible Light-Induced Degradation of Methylene Blue in the Presence of Photocatalytic ZnS and CdS Nanoparticles. International Journal of Molecular Sciences, 13, 12242-12258.
https://doi.org/10.3390/ijms131012242Hoffmann, M.R., Martin, S.T., Choi, W. and Bahnemann, D.W. (1995) Environmental Applications of Semiconductor Photocatalysis. Chemical Reviews, 95, 69-96.
https://doi.org/10.1021/cr00033a004