Binary Semiconductors

Authored by: Rakshit Ameta , Suresh C. Ameta


Print publication date:  December  2016
Online publication date:  November  2016

Print ISBN: 9781482254938
eBook ISBN: 9781315372396
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Photocatalysis is a fantastic way to clean wastewater, industrial effluents, and our environment in general. One can reduce pollution in air and water by modifying and further developing this technology. It can also put a check on the spread of infections and diseases such as severe acute respiratory syndrome (SARS). This cleaner way of life would benefit everyone around the globe. A good photocatalyst should be photoactive, able to utilize visible and/or near UV light, biologically and chemically inert, photostable, inexpensive, and nontoxic in nature. The redox potential of the photogenerated valence band (VB) hole must be sufficiently positive for a semiconductor to be photochemically active so that it can generate OH radicals that can subsequently oxidize the organic pollutants. The redox potential of the photogenerated conductance band electron must be sufficiently negative to be able to reduce absorbed oxygen to a superoxide. TiO2, ZnO, WO3, CdS, ZnS, SnO2, WSe2, Fe2O3, and so on can be used as effective photocatalysts in combating against the problem of environmental pollution.

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Binary Semiconductors

3.1  Introduction

Photocatalysis is a fantastic way to clean wastewater, industrial effluents, and our environment in general. One can reduce pollution in air and water by modifying and further developing this technology. It can also put a check on the spread of infections and diseases such as severe acute respiratory syndrome (SARS). This cleaner way of life would benefit everyone around the globe. A good photocatalyst should be photoactive, able to utilize visible and/or near UV light, biologically and chemically inert, photostable, inexpensive, and nontoxic in nature. The redox potential of the photogenerated valence band (VB) hole must be sufficiently positive for a semiconductor to be photochemically active so that it can generate OH radicals that can subsequently oxidize the organic pollutants. The redox potential of the photogenerated conductance band electron must be sufficiently negative to be able to reduce absorbed oxygen to a superoxide. TiO2, ZnO, WO3, CdS, ZnS, SnO2, WSe2, Fe2O3, and so on can be used as effective photocatalysts in combating against the problem of environmental pollution.

The major advantage of photocatalysis is the fact that there is no further requirement for any secondary disposal methods as the organic contaminants are converted to carbon dioxide, water, inorganic ions, and so on. Other treatment methods such as adsorption by activated carbon and air stripping merely concentrate the chemicals present as pollutants and transfer them to the adsorbent or air. They do not convert them to nontoxic wastes as in the case of photocatalysis. Compared to other oxidation technologies, expensive oxidation methods are not required as ambient oxygen and water is used.

Photocatalysis is an important process of advanced oxidation processes (AOPs). These are widely used for the removal of recalcitrant organic constituents from industrial and municipal wastewater. The homogenous photocatalytic system is well studied and it has been reported to be a promising method for wastewater treatment, but the ions remain in the solution at the end of the process in homogeneous photocatalytic reactions. Therefore, the removal of sludge at the end of the wastewater treatment becomes necessary and increases the cost, as large amounts of chemicals and manpower is needed for this purpose. This disadvantage of homogeneous catalytic systems can be overcome by heterogeneous photocatalysis using an appropriate photocatalyst.

3.2  Oxides

3.2.1  Titanium Dioxide

Titanium dioxide (TiO2) is an excellent photocatalyst with wide applications in various fields. The main advantages of TiO2 are its high chemical stability on exposure to acidic and basic conditions, nontoxic behavior, relatively low cost, and environmentally safe and high oxidizing characteristics, which make it a prospective candidate for many photocatalytic applications. TiO2 exists in three different crystalline modifications. These are anatase, brookite, and rutile. Out of these forms, anatase exhibits the highest overall photocatalytic activity. The high oxidizing power of TiO2 in the presence of light makes it suitable for decomposition of organic and inorganic compounds even at very low concentrations. The photocatalytic effect of TiO2 can be used for self-cleaning surfaces, decomposing atmospheric pollutants (in water and air), self-sterilization, and so on.

The TiO2 is both intrinsic and n-type (nonintentionally doped), due to oxygen vacancies in the TiO2 lattice, and its properties are similar to ZnO. Of course, studies have been conducted to dope the TiO2 and introduced a p-type conductivity in it.

Irradiation of TiO2 with light of a suitable wavelength promotes an electron from its VB to the CB. The electron in the CB is now readily available for transfer, while the hole in VB is open for donation. Both possibilities are here: a reactant receiving the electron from TiO2 will be reduced, while a reactant donating electron to semiconductor to occupy the hole will be oxidized.

TiO2 possesses a large band gap value (3.2 eV), which is a disadvantage for its use in photocatalysis; hence, efforts have been made for increasing its efficiency by doping. Doping refers to the introduction of some impurities to the material for the purpose of modifying its physical properties or electrical characteristics. A dopant may increase the level of the VB edge or lower the CB level, and depending on the nature of dopant, whether it is a metal or a nonmetal; thus reduce the band gap. It improves or minimizes electron–hole recombination, so as to minimize any loss in quantum yield.

Nanosized titanium oxide powders were prepared by Bessekhouad et al. (2003) via sol–gel route. The preparation parameters were optimized using malachite green oxalate degradation. The catalyst was excellent as compared to TiO2–P25 using two pollutants, 4-hydroxy benzoic acid and benzamide (BZ); however, their performance was found strongly dependent on the type of pollutant. The photodegradation of malachite green was studied under different pH values and amounts of TiO2 (Chen et al. 2007). MG (99.9%) was degraded with addition of 0.5 g/L TiO2 to solutions containing 50 mg/L of dye in 4 hours. They indicated that the N-de-methylation degradation of MG dye took place in a stepwise manner to yield mono-, di-, tri-, and tetra-N-de-methylated MG species during degradation.

Aarthi et al. (2007) evaluated the dependence of photocatalytic rate on the molecular structure of azure (A and B) and sudan (III and IV) dyes. The photocatalytic activity of combustion-synthesized TiO2 (CS–TiO2) was compared with that of Degussa P25. It was observed that the photodegradation rate was higher in solvents with higher polarity. The effect of pH and the presence of metal salts on degradation of azure A was also investigated.

Photocatalytic degradation of acetamiprid, a widely used pyridine-based neonicotinoid insecticide, was observed by Guzsvány et al. (2009) in UV-irradiated aqueous suspensions of O2/TiO2. Acetaldehyde, formic and acetic acid, and pyridine-containing intermediates (e.g., 6-chloronicotinic acid) were formed during the process. The pH changed from 5 to 2 during the photocatalytic process.

Gas-sensitive materials (both n-type and p-type) were developed from NiOx- doped TiO2 thin films (Wisitsoraat et al. 2009). TiO2 gas-sensing layers were deposited over a wide range of NiOx content (0–10 wt.%). It was reported that NiOx content as high as 10 wt.% was needed to invert the n-type conductivity of TiO2 into p-type conductivity. Notable gas-sensing response differences were observed between n-type and p-type NiOx-doped TiO2 thin film. NiOx (p-type) doping results in enhanced response toward acetone and ethanol.

Chen et al. (2009) prepared TiO2 photocatalyst by a surface chemical modification process with toluene 2,4-diisocyanate (TDI). As-prepared TiO2–TDI had an absorption in the visible region because of the ligand-to-metal charge transfer (LMCT) excitation of the surface complex. This photocatalyst is stable and showed high photocatalytic performance for the degradation of 2,4-dichlorophenol. The turnover number (TON) of the photocatalyst for photodegradation of 2,4-dichlorophenol was 15.43 even after using it five times under visible light irradiation.

Rezaee et al. (2009) reported the photocatalytic degradation of reactive blue 19 (RB19) dye using TiO2 nanofiber as the photocatalyst in an aqueous solution under UV irradiation. Nanofiber was prepared using a templating method with tetraisopropylorthotitanate as a precursor. Chemical oxygen demand (COD) measurements were also carried out. A significant decrease in the COD values was observed indicating that the photocatalytic method is quite good for the removal of RB19. It was found that photocatalytic decomposition of RB19 was most efficient in acidic medium.

Nanoglued binary titania (TiO2)–silica (SiO2) aerogel has been synthesized on glass substrates by Luo et al. (2009). Anatase TiO2 aerogel was immobilized into a three-dimensional (3D) mesoporous network of the SiO2 with the help of an about-to-gel SiO2 sol as nanoglue. This binary aerogel exhibited high photocatalytic activity for the degradation of methylene blue (MB) under simulated solar light. It was also revealed that the hydroxyl radical was formed during the illumination of the binary TiO2–SiO2 aerogel, which acts as an oxidant in oxidative degradation of MB.

Mahmoodi and Arami (2009) studied the feasibility and performance of photocatalytic degradation and toxicity reduction of a textile dye (acid blue 25) at a pilot scale on immobilized titania. The effects of operational parameters such as H2O2, pH, and dye concentration were investigated on the photocatalytic degradation of Acid Blue 25. Daphnia magna bioassay was used to test the progress of toxicity during the treatment process and it was observed that the residual acute toxicity reduced during the photocatalytic degradation.

Saggioro et al. (2011) studied the photocatalytic degradation of two commercial textile azo dyes, namely reactive black 5 (RB5) and reactive red 239 (RR 239) using TiO2 P25 Degussa as a catalyst. Photodegradation was carried out in an aqueous solution with a 125 W mercury vapor lamp. The effects of the amount of TiO2 used, UV light irradiation time, pH initial concentration of the azo dye, and addition of different concentrations of hydrogen peroxide were investigated. It was observed that the degradation rates achieved in mono- and bi-component systems were almost identical. The rate of color lost was 77% of the initial rate even after five cycles.

A spray pyrolysis procedure was used by Stambolova et al. (2012) for preparation of nanostructured TiO2 films. Thin films of active nanocrystalline titania were obtained from titanium isopropoxide, stabilized with acetyl acetone. The activity of this TiO2 was tested for photocatalytic degradation of RB5 dye. The reduction of toxicity after photocatalytic treatment of RB5 was observed with TiO2 taking mortality of Artemia salina as a standard. It was observed that thickness of the film, conditions of postdeposition treatment, and the type of the substrate affected the photocatalytic reaction.

Photocatalytic decolorization of brilliant green yellow (BGY), an anionic dye, has been observed in TiO2 and ZnO aqueous dispersions under UV light irradiation (Habib et al. 2012). Adsorption was found to be prerequisite for the metal oxide-mediated photodegradation/photodecolorization of the dye. Complete decolorization of water was achieved on the UV irradiation but only 75% degradation of BGY was found. ZnO-mediated decolorization was better and faster as compared to TiO2.

Zhang et al. (2014) successfully synthesized bicrystalline TiO2 supported acid activated sepiolite (TiO2/AAS) fibers by a simple method at low temperature. It was indicated that the binary mixtures of anatase and rutile exist in TiO2/AAS composites. Photocatalytic activity of the TiO2/AAS composites was evaluated by the degradation of gaseous formaldehyde (a main indoor air pollutant) under UV light irradiation, which was found to be excellent and superior to that of TiO2/sepiolite (raw sepiolite) as well as pure TiO2. This activity was attributed to the anatase–rutile mixed phase and bimodal pore structure in the TiO2/AAS composites.

A facile synthesis of hydrogenated TiO2 (H-TiO2) nanobelts was reported by Tian et al. (2015). This exhibited excellent UV and visible photocatalytic activity for decomposition of methyl orange (MO) and water splitting for hydrogen production. This improved photocatalytic property was attributed to the Ti3+ ions and oxygen vacancies in TiO2 nanobelts created by hydrogenation, which can enhance visible light absorption, promote charge carrier trapping, and hinder the photogenerated electron–hole recombination.

Black TiO2 may be an excellent solution to clean polluted air and water and to produce hydrogen. Black TiO2 had crystalline core-amorphous shell structure and was easily reduced by CaH2 at 400°C (Zhu et al. 2015). It harvests over 80% solar absorption, while white TiO2 harvests only 7%. It was 2.4 times faster in water decontamination and 1.7 times higher in H2 production than pristine TiO2. This method could provide a promising and cost-effective approach to improve the visible light absorption and performance of TiO2.

3.2.2  Zinc Oxide

ZnO is a wide band gap semiconductor and belongs to the II–VI semiconductor group. This semiconductor has several favorable properties, including good transparency, high electron mobility, and a wide band gap. Zinc oxide crystallizes in two main forms: hexagonal wurtzite and cubic zinc blende, out of which the wurtzite structure is most common.

The photocatalytic degradation of crystal violet (CV) dye over zinc oxide suspended in an aqueous solution has been carried out by Rao et al. (1997). The photocatalytic reaction was monitored spectrophotometrically by observing absorbance at different time intervals. The effect of various operating parameters was observed, including concentration of CV dye, pH, amount and nature of semiconductor, and light intensity. Observed similar to concentration of CV dye, pH, amount, and nature of semiconductor and light intensity.

Sunlight-mediated photocatalytic degradation of rhodamine B (RhB) dye was studied using hydrothermally prepared ZnO (Byrappa et al. 2006). Zinc chloride was used as the starting material in the hydrothermal synthesis of ZnO. The disappearance of RhB followed the first-order kinetics. The thermodynamic parameters of the photodegradation were determined similar to parameters of energy of activation, enthalpy of activation, entropy of activation, and free energy of activation. An actual textile effluent was also tried, which contained RhB as a major constituent along with other dyes and dyeing auxiliaries. The reduction in the COD of the treated effluent indicated a complete destruction of the organic molecules along with color removal.

ZnO nanostructures were synthesized by Mohajerani et al. (2009) in different shapes such as particle, rods, flower-like, and microsphere via a facile hydrothermal method. The effect of morphology on the decolorization of acid red 27 (AR 27) solution was observed under direct irradiation of sunlight. It was found that these have Wurtzite-type hexagonal structure with high crystallinity and crystallite size in the range of 67–100 nm. The decolorization of AR 27 followed the first-order kinetics.

ZnO particles with different morphologies such as rod-like, rice-like, and disc-like were successfully synthesized via the sol–gel approach by Pung et al. (2012) through adjustment of the reaction parameters such as amount of ammonia and reaction time as well as complexing agent aluminum sulfate Al2(SO4)3. Rod-like ZnO particles were found to be the most effective in degrading the RhB solution under the illumination of the UV light. The first-order rate constants were found to be in the following order:

Rod-like ZnO particles > Rice-like ZnO particles > Disc-like ZnO particles

Ameen et al. (2013) used a simple solution method for the synthesis of ZnO flowers using zinc acetate precursor in a basic medium. It was employed for the degradation of CV dye. The average length of ZnO flower with well-defined petals was ~300 ± 50 nm. As-synthesized ZnO flowers showed efficient degradation of CV dye with a degradation rate of ~96% within 80 minutes.

Ag–N-codoped zinc oxide nanoparticles (NPs) were synthesized by Welderfael et al. (2013) through impregnation of Ag–N-doped ZnO NPs. Photocatalytic degradation of methyl red (MR) dye using Ag–N-codoped ZnO was studied under solar as well as UV irradiation. Codoped zinc oxide photocatalysts showed higher photocatalytic activity as compared to pure zinc oxide. Calcined zinc oxide showed better photocatalytic activity than commercial zinc oxide. The high catalytic activity of Ag–N-codoped zinc oxide was attributed to the lower rate of recombination of the photogenerated electrons and holes and its lower band gap energy.

Pure and nitrogen-doped ZnO nanospheres were successfully prepared by Lavand and Malghe (2015) using a microemulsion method. It was indicated that nanosized N-doped ZnO was spherical and had wurtzite phase. The incorporation of nitrogen into the oxygen site of ZnO causes lattice compression. It was observed that N doping significantly enhanced the light absorption capacity of ZnO in the visible region and exhibited higher photocatalytic activity as compared to that of commercial and pure ZnO NPs. As-prepared nanosized N-doped ZnO was found to be highly stable and reusable.

Graphdiyne–ZnO nanohybrids as an advanced photocatalytic material. (Adapted from Thangavel, S. et al.,

Figure 3.1   Graphdiyne–ZnO nanohybrids as an advanced photocatalytic material. (Adapted from Thangavel, S. et al., J. Phys. Chem. C, 119, 22057–22065, 2015. With permission.)

A novel graphdiyne−ZnO nanohybrid was prepared by Thangavel et al. (2015) using a hydrothermal method. Photocatalytic properties of as-prepared sample were evaluated on the degradation of two azo dyes (MB and RhB). The graphdiyne−ZnO nanohybrids showed superior photocatalytic properties as compared to bare ZnO NPs. The rate of degradation with graphdiyne−ZnO nanohybrids was nearly two times higher than ZnO NPs (Figure 3.1).

3.2.3  Others

Bi2O3 NPs were prepared by means of ammonia precipitation, polyol-mediated, and microemulsion chemical methods (Peng et al. 2004). The photocatalytic oxidation reactions of benzene, toluene, and xylene were used as the model reaction to observe the photocatalytic activity of Bi2O3 NPs. The photocatalytic activity of Bi2O3 NPs prepared with the microemulsion chemical method was higher than that of the particles prepared with the polyol-mediated method. The degradation rates of the three pollutants decreased in the following sequence:

Xylene > Toluene > Benzene

A new crystal structure for nanostructured VO2 with body-centered cubic (bcc) structure and a large optical band gap of 2.7 eV was reported by Wang et al. (2008). It showed excellent photocatalytic activity in hydrogen production. The bcc phase of VO2 had a high quantum efficiency of 38.7% in the form of nanorods. The hydrogen production rate can be tuned by varying the incident angle of the UV light in the presence of films of the aligned VO2 nanorods. A high rate of 800 mmol/m2/h was obtained for hydrogen production from a mixture of water and ethanol under the UV light.

Efficient removal of phenol was carried out by Gondal et al. (2009) using a UV laser induced photocatalysis process in the presence of Fe2O3 semiconductor catalysts. The effect of operational parameters in the removal process was also investigated with variation of laser irradiation time, laser energy, and concentration of the catalysts. Maximum phenol removal (more than 90%) was achieved in this process during 1-hour irradiation.

CeO2 nanocrystals were synthesized by a simple precipitation method and calcination at 600°C by Pouretedal and Kadkhodaie (2010) using (NH4)2Ce(NO3)6 and ammonia as precursors. They studied photodegradation of MB in the presence of CeO2 NPs under UV and sunlight irradiation. Maximum degradation was obtained with 1.0 g/L CeO2 at pH 11 within 125 minutes. Results indicated the effect of photogenerated holes in the degradation mechanism of the dye.

Yuan and Xu (2010) prepared nanometer tin oxide (SnO2) by constant temperature hydrolysis, microwave hydrolysis, chemical precipitation, and solid-state reaction. The photocatalytic activities of nanometer in oxides were studied using MO as a model organic pollutant. The effects of amount of photocatalysts, photocatalysts doped with different metal ions, and pH were also observed. Well-crystallized SnO2 of 30–40 nm was obtained by constant temperature hydrolysis sintered at 800°C. The 97% decoloration of MO was achieved in 120 minutes, which shows higher photocatalytic activity of SnO2 prepared by this method than that obtained from other methods.

Nanoporous structured tin dioxide was used by Kim et al. (2011) for photocatalytic destruction of endocrine, bisphenol A. The photocatalytic destruction of bisphenol A was enhanced by combining the nanoporous structured SnO2 with TiO2. It was observed from the photoluminescence curve that the recombination between electron and hole largely decreased in the TiO2/nanoporous SnO2 composite. Seventy-five percent decomposition of 10.0 ppm of bisphenol A was achieved after 24 hours in the presence of light and composite.

Chu et al. (2011) synthesized ultrafine SnO2 nanocrystals via a surfactant-assisted solvothermal route in water–ethanol mixed media. Photocatalytic performance of the as-synthesized catalyst for decomposing acetaldehyde was observed at ppb level in a continuous glass-plate reactor. It was shown that SnO2 photocatalyst of about 4 nm with a large BET surface area of 130 m2 /g exhibited the best photocatalytic oxidation properties, comparable to the properties of Degussa TiO2 P25.

Different morphologies of Cu2O have been synthesized by Sharma and Sharma (2012) via solution grown reduction route. Anhydrous dextrose and ascorbic acid were used as reducing agents and poly(vinyl pyrrolidone) (PVP) and sodium dodecyl sulfate (SDS) as surfactants. It was reported that Cu2O polyhedrons possess higher (99.56%) photocatalytic activity and lower adsorption capacity (32.81%) as compared to Cu2O NPs.

Liu et al. (2012) prepared CuO nanowires (NWs) using a Cu foil as substrate via a solution route. It was confirmed that the synthesized products were wire shaped. As-obtained NWs were well-crystalline pure CuO possessing good optical properties. About 90% of the MO was degraded in the presence of CuO NWs after 180 minutes under light.

Shao and Ma (2012) synthesized mesoporous CeO2 NWs through a facile hydrothermal process by using a triblock copolymer F127 as the template. The surface area of the as-prepared sample was to be 273 m2/g with pore width distribution of 6.9–13.8 nm. These mesoporous CeO2 NWs could be used as efficient photocatalysts for organic dye degradation under UV light irradiation and were found superior as compared to the commercial photocatalyst P25 and CeO2 powders. The NW structure facilitates the separation of CeO2 by sedimentation so that these can be reused.

Nb2O5 NPs have been successfully prepared and modified by Hashemzadeh et al. (2013) via a hydrothermal process. Commercial Nb2O5 powder, H2O2, and NH3 aqueous solutions have been used as precursors. The photocatalytic activity of modified NPs on degradation of MB and RhB dyes was examined and a higher photocatalytic efficiency for MB dye was obtained as compared to RhB.

SnO2 NPs were synthesized by Singh and Nakate (2013) using a microwave method. SnO2 NPs had spherical morphology with crystallite size of 35.42 nm. Synthesized NPs were used for photodegradation of methylene blue dye under UV light. These NPs were found to show 55.97% photodegradation efficiency.

Hierarchically assembled SnO2 microflowers were prepared by Liu et al. (2013) using a facile hydrothermal process. It was found that these hierarchical nanostructures were made from two-dimensional (2D) nanosheets (50 nm thick). The as-prepared sample exhibited excellent photocatalytic performance.

ZrO2 nanopowder was prepared by Mehrdad et al. (2013) via a sol–gel autocombustion method. The average crystalline size of ZrO2 was determined to be 62 nm. The photocatalytic removal of nitrophenol from aqueous solution was observed in the presence of zirconia under UV light irradiation. Nitrophenol was degraded by 84%, 78%, and 66% in the presence of 0.04 g of ZrO2 within 70 minutes using initial concentrations of 3, 5, and 10 ppm, respectively.

Cerium oxide (CeO2) NPs have been synthesized hydrothermally by Khan et al. (2013) in the presence of urea. As-prepared CeO2 had a well-crystalline cubic phase and NPs were optically active. CeO2 was explored as a redox mediator for the development of a chemi-sensor for ethanol. Acridine orange (AO) was degraded to 50% in the presence of ceria in a short time. They concluded that reduction in the particle size enhanced the active surface area of the CeO2 resulting in increase of chemical sensing of ethanol and its photocatalytic properties.

Semiconductor-based gas sensors using n-type WO3 or p-type Co3O4 powder were fabricated by Akamatsu et al. (2013). They studied their gas-sensing properties toward NO2 or NO (0.5–5 ppm in air) at 100°C or 200°C. The resistance of the WO3-based sensor was found to increase on exposure to NO2 and NO, while the resistance of the Co3O4-based sensor varied depending on the temperature and the gas species. The chemical states of the surface of WO3 or Co3O4 powder on exposure to 1-ppm NO2 and NO were also investigated.

Nanocrystalline α-Fe2O3 was synthesized by Jahagirdar et al. (2014) using a solution combustion method. As-formed α-Fe2O3 nanopowder was used as the photocatalyst for the degradation of the dye RhB under UV light. The effects of pH, amount of the photocatalyst, amount of H2O2, and irradiation time were observed. It was found that it acted as an efficient photocatalyst in the presence of H2O2. Maximum degradation of RhB was achieved only in 40 minutes with 0.8 g of the catalyst, pH 10, H2O2, and UV light.

Sharma et al. (2014) reported synthesis at temperature ~90°C and characteristics of well-crystalline iron oxide NPs. It was observed that the pure Fe2O3 NPs had a well-crystalline, rhombohedral crystal structure. They also showed highly super paramagnetic behavior. As-synthesized Fe2O3 NPs were used as an efficient photocatalyst for the photocatalytic degradation of MO dye. About 80% MO was degraded in the presence of Fe2O3 NPs within 210 minutes under UV light irradiation.

Al2O3 was synthesized by Tzompantzi et al. (2014) using the sol–gel method, dried at 100°C and annealed at 400°C, 500°C, 600°C, and 700°C. Al2O3 is a well-known insulator, but its feasibility as a catalyst was shown for the photomineralization of hazardous organic molecules. The photocatalytic activity of the sample of Al2O3 calcined at 400°C was higher than Degussa P25 TiO2. It was proposed that the UV irradiation of the hydroxylated Al2O3 induced an effective separation of the electron–hole pairs resulting in enhancement of the photodegradation of phenolic compounds.

Liu et al. (2015a) obtained porous Fe2O3 nanorods using a facile chemical solution method followed by calcination. The BET surface area of the porous Fe2O3 nanorods was determined as 18.8 m2/g. The porous Fe2O3 nanorods were used as a catalyst to photodegrade RhB, MB, MO, p-nitrophenol, and eosin B (EB). As-prepared porous Fe2O3 nanorods exhibited higher catalytic activities as compared to the commercial Fe2O3 powder, due to their large surface areas and porous nanostructures. This catalyst had better stability and reusability.

A novel catalytic performance of simple-synthesized porous NiO NWs as catalyst/cocatalyst for the hydrogen evolution reaction (HER) has been reported by Shen et al. (2015). High-resolution transmission electron microscopy (HRTEM) exhibited a strong dependence of NiO NW photocatalytic and electrocatalytic HER performance on the density of exposed high-index facet (HIF) atoms. It was observed that the optimized porous NiO NWs had a long-term electrocatalytic stability of over 1 day, and 45 times higher photocatalytic hydrogen production as compared to the commercial NiO NPs.

Flower-type V2O5 hollow microspheres having diameters of about 700–800 nm were obtained by Liu et al. (2015b) with the assistance of carbon-sphere templates. These were used in the photodegradation of 1,2-dichlorobenzene (o-DCB) under visible light. The V2O5 hollow structure showed high photocatalytic activity in the degradation of gaseous o-DCB under visible light due to its strong adsorption capacity and large specific surface area. Intermediates, such as o-benzoquinone-type and organic acid species, and final degradation products (CO2 and H2O) were also confirmed.

Nanosized ZrO2 powders having different structures with near-pure monoclinic, tetragonal, and cubic structures were prepared by Basahel et al. (2015) and used as catalysts for photocatalytic degradation of MO. The performance of as-synthesized ZrO2 NPs in the photocatalytic degradation of MO was evaluated under UV light irradiation. It was found that the photocatalytic activity of the pure monoclinic ZrO2 sample was higher than the tetragonal and cubic ZrO2 samples. It was also revealed that monoclinic ZrO2 NPs possessed high crystallinity and mesopores with a diameter of 100 Å. The higher activity of the monoclinic ZrO2 sample for the photocatalytic degradation of MO was attributed to the combined effects of the presence of a small amount of oxygen-deficient zirconium oxide phase, high crystallinity, large pores, and the high density of surface hydroxyl groups.

CeO2 NPs have strong redox ability, nontoxicity, long-term stability, and low cost. Ravishankar et al. (2015) synthesized CeO2 NPs via a solution combustion method using ceric ammonium nitrate as an oxidizer and ethylenediaminetetraacetic acid (EDTA) as fuel at 450°C. It was observed that the particles were almost spherical, and the average size of the NPs was 42 nm. Ceria NPs exhibited photocatalytic activity against trypan blue (TB) at pH 10 under UV light irradiation. These NPs also reduced Cr(VI) to Cr(III) and showed antibacterial activity against Pseudomonas aeruginosa.

Sood et al. (2015) reported the synthesis of α-Bi2O3 nanorods and used it as an efficient sunlight-active photocatalyst for the photocatalytic degradation of RhB and 2,4,6-trichlorophenol. A simple surfactant-free sonochemical route was used for the synthesis at ambient conditions. They observed that the prepared nanorods exhibited a high purity, well-crystalline monoclinic α-Bi2O3 structure, and excellent optical properties. The degradation of a cationic dye (RhB), its simulated dye bath effluent, and 2,4,6-trichlorophenol under solar light irradiation were used as model systems. As-synthesized α-Bi2O3 nanorod catalyst exhibited excellent solar light driven photocatalysis toward RhB (97% degradation in 45 minutes) and 2,4,6-trichlorophenol (88% degradation in 180 minutes).

Metastable β-Bi2O3 microcrystals were prepared from Bi(NO3)3 by an aqueous crystallization strategy without calcination or other complex treatment (Lu et al. 2015). The introduction of cetyltrimethylammonium bromide (actually Br ions) facilitated the formation of β-Bi2O3 crystals. Photocatalytic activities of metastable β-Bi2O3 samples were evaluated in the degradation of RhB under visible light irradiation. As-prepared β-Bi2O3 showed excellent photocatalytic efficiency of up to 77.9% (total removal 97.2%) in 2 hours of irradiation and the sample can be used for four cycles of degradation without any loss of activity. It was concluded that a minor amount of BiOCl crystallites appearing at the surface of β-Bi2O3 crystals facilitated their photocatalytic performance.

3.3  Sulfides

3.3.1  Cadmium Sulfide

Cadmium sulfide is yellow in color and acts as a semiconductor. It exists in nature in two different minerals, hexagonal greenockite and cubic hawleyite. Cadmium sulfide is a direct narrow band gap (2.42 eV) semiconductor.

Tristao et al. (2006) used nanometric particles of CdS to impregnate TiO2 in order to optimize its photocatalytic properties. They used CdS/TiO2 semiconductor composites in 1, 3, 5, and 20 mol% proportion. It was observed the CdS had hexagonal geometry and TiO2 was in anatase form. The photocatalytic activity of the CdS/TiO2 was investigated using UV light for the degradation of textile azo-dye, drimarene red. Better efficiency was observed for the CdS/TiO2 5% composite as compared to other CdS/TiO2 proportions and bare TiO2.

CdS semiconductor photocatalyst was synthesized by Li et al. (2008) via a solvent thermal process and then platinum was added to it by photodeposition. The activities of Pt/CdS photocatalysts under visible light were evaluated in terms of hydrogen production using formic acid as the electron donor. The photoetching of CdS was monitored by measuring dissolved Cd2+. The photocatalytic activity of Pt/CdS was enhanced by 126% with 1-hour photoetching treatment. However, excess photoetching was found to decrease the photocatalytic activity. Oxygen and platinum both play an important role in the photoetching process. The photocatalytic activity was enhanced through decreasing agglomeration of the CdS particles, which led to increased specific surface area, modified particle morphology, and selective removal of grain boundary defects, which acted as recombination centers of photoinduced electron–hole pairs.

Cadmium sulfide quantum dots (QDs) sensitized mesoporous TiO2 photocatalysts were prepared by Li et al. (2009) via preplanting cadmium oxide as crystal seeds into the framework of ordered mesoporous TiO2. Then this CdO was converted to CdS QDs through ion exchange. The presence of CdS QDs in the TiO2 framework was responsible for its photoresponse to visible light by enhancing the photogenerated electron transfer from the inorganic sensitizer to TiO2. This photocatalyst showed excellent photocatalytic efficiency for both the oxidation of NO gas in air and degradation of organic compounds in aqueous solution under visible light irradiation.

High-efficiency photocatalytic H2 production was achieved by Li et al. (2011) using graphene nanosheets decorated with CdS clusters as the visible light driven photocatalysts. They used the solvothermal method, where graphene oxide (GO) served as the support and cadmium acetate as the CdS precursor. These nanosized composites achieved a high H2 production rate of 1.12 mmol/h, which was about 4.87 times higher than pure CdS NPs, at graphene content of 1.0 wt.% and Pt 0.5 wt.% under visible light irradiation. It was attributed to the presence of graphene, which served as an electron collector and transporter to lengthen the lifetime of the photogenerated charge carriers from CdS NPs.

Nanosized cadmium sulfide containing different phase structures was synthesized by Yu et al. (2011) via complex compound thermolysis using different molar ratios of thiourea to cadmium acetate (S/Cd). Photocatalytic degradation of RhB was carried out, which showed that CdS with the cubic phase had the best photocatalytic activity due to its larger adsorption and absorbance abilities and smaller particle size of about 10–13 nm.

Cadmium sulfide is a representative material as the visible light responsive photocatalysts for hydrogen production. There has been significant progress in water splitting on CdS-based photocatalysts using solar light, especially in the presence of cocatalysts. The field of photocatalytic water splitting on CdS-based photocatalysts has been reviewed by Chen and Shangguan (2013), and includes controllable synthesis of CdS, as well as modifications with different kinds of cocatalysts, intercalated with layered nanocomposites and metal oxides, hybrids with graphenes, and so on.

Wang et al. (2015b) synthesized a novel 3D flower-like CdS via a facile template-free hydrothermal process. Cd(NO3)2·4H2O and thiourea were used as precursors and l-histidine as a chelating agent. Higher photocatalytic activity was observed on the flower-like CdS photocatalyst under visible light irradiation; it was almost 13 times that of pure CdS. The imidazole ring of l-histidine captured the Cd ions from the solution, and as a result, the growth of the CdS was prevented. As-synthesized flower-like CdS with l-histidine was found to be more stable than CdS without l-histidine in hydrogen generation.

Ding et al. (2015) developed a novel heterojunction structured composite photocatalyst CdS/Au/g–C3N4 by depositing CdS/Au with a core (Au)-shell (CdS) structure on the surface of g–C3N4. Photocatalytic hydrogen production activity of this photocatalyst was evaluated under visible light irradiation using methanol as a sacrificial reagent. The activity was found to be about 125.8 times higher than g–C3N4 and was even much higher than Pt/g–C3N4. The enhancement was attributed to efficient separation of the photoexcited charges due to the anisotropic junction in the CdS/Au/g–C3N4 system.

3.3.2  Zinc Sulfide

Zinc sulfide is a white- to yellow-colored powder. It is normally available in the more stable cubic form, also known as zinc blende or sphalerite. Its hexagonal form is known as the mineral wurtzite. Both these forms, sphalerite and wurtzite, are intrinsic and wide band gap semiconductors. The band gap of the cubic form is 3.54 eV, whereas the hexagonal form has a higher band gap of 3.91 eV. It can be either used directly as a photocatalyst or it may be modified to improve its efficiency.

Ni-doped ZnS photocatalyst (Zn0.999Ni0.001S) had 2.4 eV energy gap and showed activities for the reduction of nitrate and nitrite ions to ammonia, and dinitrogen under visible light irradiation using methanol as a reducing reagent (Hamanoi and Kudo 2002). The reduction of nitrate ions was compatible with that of water to form hydrogen. The concentration of nitrate ions and loading of a platinum cocatalyst affected the selectivity for the reduction products of nitrate ions.

Stengl et al. (2008) prepared photocatalytically active TiO2/ZnS composites by homogeneous hydrolysis of mixture of titanium oxo-sulfate and zinc sulfate in aqueous solutions with thioacetamide. Photoactivity of the prepared TiO2/ZnS nanocomposites was evaluated by the photocatalytic decomposition of Orange II dye in an aqueous slurry under different irradiations (255, 365, and 400 nm). The highest catalytic activity was obtained with the composite sample prepared by hydrolysis of mixture solutions of 0.63 M TiOSO4 and 0.08 M ZnSO4·7H2O.

Pouretedal et al. (2010) synthesized NPs of zinc sulfide doped with iron by controlled coprecipitation. They used these NPs in the photodegradation of Congo red (CR) as water pollutant. The effect of mole fraction of Fe3+ to zinc ion, pH, dosage of photocatalyst, and concentration of dye on photoreactivity of doped zinc sulfide was observed. The size of NPs was determined to be 10–40 nm. The degradation efficiency was 92% under UV and 98% under sunlight irradiation in 120 minutes and 12 hours, respectively, using 0.8 g/L of ZnS:Fe (0.5%) nanophotocatalyst, and 12 mg/L of CR at pH 6. The photoreactivity of photocatalyst was reproducible up to 90%–92% degradation in four cycles of photodegradation.

Ma et al. (2013) synthesized N-doped ZnO/ZnS photocatalysts via a simple heat treatment approach using l-cysteine as the source of N and S. Anthraquinone dye (Reactive Brilliant Blue KNR) was used as the model contaminant to evaluate the photocatalytic activity of as-synthesized samples under sunlight illumination. N-doped ZnO/ZnS synthesized by this method showed better photocatalytic activity as compared to pure ZnO. This enhanced photocatalytic activity was attributed to N doping, ZnS/ZnO heterostructure, and covered abundant carbon species on the photocatalyst surface causing high absorption efficiency of light, efficient separation of electron–hole pairs, and quick surface reaction in doped ZnO.

ZnS–Ag nanoballs were synthesized using a chemical precipitation method at room temperature aided with the capping agent cetyl trimethyl ammonium bromide (Sivakumar et al. 2014). Photocatalytic activity of ZnS and ZnS–Ag nanoballs was evaluated by irradiating the solution of organic dye MB under visible light irradiation. The different operating parameters affecting degradation such as dye concentration, catalyst loading, and pH were studied. It was observed that the catalyst can be reused up to three cycles. It was found that ZnS–Ag nanoballs showed higher degradation efficiency as compared to ZnS.

ZnS NPs (ZnS QDs) were prepared by Al-Rasoul et al. (2014) using a simple microwave irradiation method under mild conditions. Zinc acetate and thioacetamide were used as a source of zinc and sulfur, respectively, and ethylene glycol as a solvent. As-prepared ZnS sample displayed a blue shift as compared to bulk ZnS from 310 to 345 nm. Photocatalytic degradation of MB dye catalyzed by ZnS NPs was studied under solar radiation and it was observed that photocatalytic degradation increased with increasing time exposure to solar light.

Controlling the amount of intrinsic S vacancies was achieved in ZnS spheres by Wang et al. (2015a) using a hydrothermal method. Zn and S powders were used in concentrated NaOH solution with NaBH4 added as a reducing agent. Absorption spectra of ZnS was extended to the visible region because of these S vacancies. Photocatalytic activities were evaluated for H2 production under visible light. They reported that the concentration of S vacancies in the ZnS samples can be controlled by variation in the amount of the reducing agent NaBH4. As-prepared ZnS samples exhibited photocatalytic activity for H2 production under visible light irradiation without loading any noble metal. The activity of ZnS increases steadily with increase in the concentration of S vacancies reaching an optimum value.

Apart from cadmium and zinc sulfide, various other sulfides have also been used for photocatalytic degradation of various organic pollutants particularly dyes. Some sulfides were used by Ameta et al. (2005), Sharma et al. (2011, 2013a, 2013b), Andronic et al. (2011), Yadav et al. (2012), and so on.

Some metal chalcogenides (oxide, sulfide, etc.) have been extensively used as photocatalytic materials because they have their band gaps in the desired range. They are insoluble in a wide range of pH and absorb solar radiation in the visible as well as UV range. Majority of these are low cost and eco-friendly as well, but some of them are inefficient because they cannot utilize visible radiation to the desired extent. This limitation can be overcome with slight modification of such systems like doping, sensitization, composite formation, metallization, and so on. Binary semiconductors are likely to maintain their status as effective photocatalysts in the future also, of course with certain modifications.


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