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Nanocomposites are multiphase materials that attracted considerable attention as very efficient photocatalytic materials. The nanocomposite photocatalysts contain semiconductors and metals as reinforced nanophase and photocatalytic activity is result of this heterojunction with matrix. The aim of this section is to explore some of the most representative nanocomposite materials with photocatalytic and electro-photocatalytic properties. These reactions are an alternative solution to use sunlight energy in degradation of contaminants from air and water, synthesis of new organic compounds, and as energy source. The reaction between photons and nanocomposite materials (powder, fiber, and film) is associated with generation of the reactive oxygen species that play a key role in these applications. The effects of heterojunctions between different semiconductors and metals and their considerable synergy that promote the photocatalytic properties of nanocomposites are evidenced. The mechanisms of various types of the photocatalytic reactions are thus presented highlighting the efficient strategy to suppress the recombination of e−/h+ pairs. The variation of the visible light absorption in the photocatalytic reaction and increasing of its efficiency, selectivity, and stability due the contribution of the surface plasmon resonance effect produced by precious metals nanoparticles is also considered.
Institute of Physical Chemistry - Ilie Murgulescu of the Romanian Academy, Bucharest, Romania
Gabriela Petcu
Institute of Physical Chemistry - Ilie Murgulescu of the Romanian Academy, Bucharest, Romania
*Address all correspondence to: vpirvulescu@icf.ro
1. Introduction
Nanocomposites are multifunctional materials with unexpected characteristics as a result of connection of each component. These materials were used as photocatalysts for many applications as synthesis of new organic compounds [1, 2, 3], as sustainable energy production [4, 5, 6, 7] and environmental remediation [8, 9, 10, 11, 12]. Whereas nanocomposites are considered to be the materials of twenty-first century [13], most of their applications represent solutions for the problems of this period. The synthesis of nanocomposite materials is an efficient way to improve the efficiency of the individual nanoparticles in the photocatalytic reactions. A single nanosemiconductor photocatalyst could not accomplish, most of the times, the required performances in the photocatalytic processes. Such nanocomposites contain a number of nano- and mezo-scale materials with distinctive chemical and physical properties compared to the individual component that are consequence of their synergistic effects. Nanocomposites used in photocatalytic reactions have different shapes (particles, fibers, and membranes). The matrix phase of the most nanocomposite photocatalysts is polymeric (PMNC) or ceramic (CMNC), and both influence their properties. Nanocomposites have more advantages than individual nanoparticles. They are a solution for adapting properties and improving stability and performance for certain applications due to their enriched synergistic effects. Photocatalytic reactions are advanced oxidation processes that represent an agreeable solution for transformation of various compounds by utilization of solar light under mild conditions. In this catalytic process the ultraviolet (UV) or visible light energy is absorbed by semiconductor or plasmonic phases of the photocatalysts. This energy excites the electrons of metals (plasmonic effect) and the valence band semiconductor and generates their transfer to the conduction band of semiconductor. The resulted electrons and holes react with oxygen and water molecules to produce reactive oxygen species (ROS) as hydroxyl (•OH) or superoxide anion (O2•−). These reactive oxygen species play a key role in reaction with various organic residues [9, 11, 13, 14, 15, 16, 17] and metals [18], and most of them present as pollutants in wastewater. The extensive development of nanocomposite-based photocatalysts has not yet offered sufficient solutions for the efficient degradation of some pollutants. The performances have been improved by using different integrated photocatalytic processes such as adsorptive photocatalysis, photo-Fenton, photo-sonocatalysis, photo-ozonation, photo-piezocatalysis [19, 20, 21], or by different types of tandem processes such as membrane, plasma, electro-, thermo-, and photocatalytic [9, 22, 23, 24]. All the reported results show the variety of semiconductor nanomaterials, their interaction with the matrix, and synergistic effect on the reaction mechanism and finally on the photocatalytic reactions.
The nanocomposite-based photocatalysts were extensively developed as a result of their applications and nanotechnology progress. Although the most nanocomposite photocatalysts can be polymeric (PMNC) or ceramic (CMNC), their wide variety is due both to the diversity of matrix (polimetric, ceramic, or hybrid) and reinforced nanophases. A high variety of nanomaterials as metals (Au, Ag, Pd, Fe, Sn, W, Ni, Pb, and Co Cu), semiconductors (TiO2, ZnO, Cu2O, SiO2 Nb2O5, Fe2O3, FeO, WO3), SiO2, zeolite, polymeric materials, and other semiconductors such as carbon-based nanomaterials (graphene oxide, reduced graphene oxide, and activated carbon) and 2D-hexagonal boron nitride have been used in synthesis of the nanocomposite photocatalysts [6, 7, 11, 14, 15, 18, 21, 25]. Recent reports have confirmed that hybrid nanocomposites with polymer matrix are efficient photocatalysts [11, 15, 25, 26, 27, 28]. The interpenetration of polymeric matrix with inorganic phases at nanoscale level improves the individual properties and influences the global synergetic effect. The conductive polymers, as polypyrrole, polythiophene, and polyaniline, are used as sensitizing agents in photocatalytic nanocomposites [29, 30, 31, 32]. These conducting organic polymers are efficient as matrix phase, due to their extended π-conjugated electrons, to obtain nanocomposite photocatalyst in association with semiconducting materials. The hybrid framework conducting polymer/carbon along with semiconducting materials is also nanocomposite photocatalysts for degradation of dangerous and toxic pollutants [15]. Low-density (LDPE) and high-density polyethylene (HDPE) with intrinsic hydrophobicity and good chemical and thermal stability were also selected to coating TiO2 nanoparticles [11]. The activity in visible light of the obtained photocatalysts was explained by higher ionic character of Tix+ species that promotes transfer of electrons in the TiO2 valence band and increasing the appearance of holes. XPS results suggested the formation of Ti‒O‒C bonds at the interface TiO2-polyethylene. Nanoparticles of another semiconductor oxide such as ZnO were coated with 3-glycidoxypropyltriethoxysilane (GPTES) [33].
Biopolymers are another type of polymeric material from composition of the nanocomposite photocatalysts. Chitosan is one of the often-used biopolymer, with excellent adsorption ability due its abundant hydroxyl and amino functional groups [34, 35]. This polymer has been associated with inorganic materials like zinc oxide, cadmium sulfide, zinc sulfide, and cuprous oxide for photocatalytic degradation of various pollutants. Geopolymers, as alkali-activated aluminosilicates, are another alternative for the polymeric matrix of nanocomposite photocatalysts that can readily incorporate photocatalytically active materials such as nanoxide semiconductors. Thus, nanocomposite photocatalysts for dye degradation were obtained by immobilization of TiO2, Cu2O and Fe2O3, or carbon nanotubes and graphene on alkali-activated aluminosilicates [36]. As well, carbon materials are also promising materials for embedded with semiconductor photocatalysts such as reduced graphene oxide (rGO) or graphene, carbon, and graphitic carbon nitride (g-C3N4). The graphene from nanocomposites with oxide semiconductors (WO3-Fe3O4, MoS2-Fe2O3) enhanced the photocatalytic efficiency [6]. Very active nanocomposite photocatalysts were obtained with CS-biochar, a carbonaceous material derived from pyrolysis of carbon-rich biomass. The optimal photocatalytic activity was obtained for ternary WO3@TiO2/CS-biochar (20 wt%) in degradation, under visible-light irradiation, of organic dyes and antibiotics from aqueous solution [37]. The graphitic carbon nitride (g-C3N4) is a new two-dimensional (2D) organic polymer semiconductor. It was extensively investigated as photocatalytic material due to its moderate band gap and piezoelectric effect. The integration of photocatalysis and piezocatalysis by coupling NaNbO3 and g-C3N4 into a heterojunction system [21] evidenced the effect of carbon quantum dots (CQDs) content on photocatalytic properties. The influence of the composition for nanocomposite based on CODs was also studied for CQDs/CoO/g-C3N4 [38] or CQDs/CeO2/SrFe12O19 [39]. Another type of nanocomposite was obtained by mixing of g-C3N4 nanosheets with MOF-derived porous CoFe2O4. The obtained nanocomposites were efficient photocatalysts for degradation of pollutants [19]. For similar applications were obtained nanocomposite catalysts with CoWO4 semiconductor and graphitic g-carbon nitride [40]. The preparation of g-C3N4 with chlorophyll (Chl) organic heterojunction structures in Chl@g-C3N4/Ti3C2Tx [7], AgNPs/PDA/g-C3N4 nanocomposite by decoration polydopamine-grafted g-C3N4 (PDA/g-C3N4) with silver nanoparticles (AgNPs) led to active photocatalysts in visible light [25]. Other active photocatalytic materials for wastewater treatment under solar light were obtained by dispersion of AuNPs on the ZnO nanospheres to obtain ZnO/Au/g-C3N4 nanocomposites [41]. 3D porous N-doped carbon/CuO nanocomposites were also obtained and used for efficient removal of antibiotics from aqueous solution by synergistically utilizing adsorption and photocatalytic oxidation processes [42]. A matrix used to improve the photocatalytic efficiency of various semiconductors is graphene oxide (GO). GO has a key role in the efficient separation of the photogenerated electron–hole pairs and significantly diminishes the recombination of charge carriers. The remarkable enhancement of the photocatalytic activity in visible light was obtained for Ag,F co-doped Bi2MoO6/rGO [25], Ag-AgX-ZnO-rGO (X = Cl&Br) [43], Ag-ZnO-rGO (5%Ag) [44], Cu2O/MoS2/rGO [45], NiFe3O4/rGO [46], and Ag@ZnO/rGO [14] nanocomposites.
A high variety of the nanocomposite photocatalysts contain semiconductor oxides and mixed oxides as ceramic matrix and supported nanoparticles. The addition of noble metals such as gold and silver nanoparticles on TiO2 and ZnO materials is a general method used to improve the photocatalytic properties [47]. Various active photocatalytic nanocomposites were thus obtained: Bi/BiOF/Bi2O2CO3 [48], Ag2S/2D Bi4TaO8Cl [49], MoS2/Co3O4 [50], Fe2O3/TiSBA-15 [51], Au/TiO2/zeolite Y [52], TiO2−x/AgBiO3 [53], Ag/Ag2O/C/P-TiO2 [54], Co3O4, NiO/TiO2/zeolite Y [55], Fe3O4, Co3O4,NiO/Ti-zeolite Y [56], binary Bi2O3/CuS (2, 3 and 5 wt%) and ternary Bi2O3/CuS (3%)/Ag (3, 10, 15 wt%) [57] nanocomposites, and Ag|AgNbO3/Ag/Er3+:YAlO3@Nb2O5 [58], Ag/Au-WO3−x [59], and Ag|AgBr/Ag/FeTiO3 [60] nanocomposite films.
In the view of solar energy utilization were obtained photocatalysts with response in visible light by modification of the semiconductor oxides (TiO2, WO3, Fe2O3, CuO, ZnO, BiVO4, and SnO2-Sb2O4) of the films electrodes with noble metal particles (Au, Ag, Pt, and Ir) or another oxide. Photosensitive materials with different band gaps have been obtained and used, and how to improve their photoresponse has been reported. Metal or non-metal doping is a significant and effective method for improving the photoresponse of nanomaterials. Thus, was obtained the increase of PEC activity in the degradation of pollutants by doping the BiVO4 film with noble metal particles as Ag [61]. Other examples of nanocomposite materials obtained for applications in electro-photocatalytic reactions are: Ti/SnO2-Sb2O4 [62], CdS/TiO2 [63], Ni-TiO2 nanotube [61] Si-Au-TiO2 [64] C and N codoped TiO2 nanotube [65], TiO2/ZnO nanorods [66], BaTiO3-CoFe2O4 [5], SnS2/ZnTe electrode [62], and AgNPs/SnO2/g-C3N4 [67].
3. Applications of the nanocomposite materials in photocatalytic reactions
The main photocatalytic processes in which there is interest for the applications of nanocomposite materials are: photocatalytic synthesis of oxidation compounds [1, 2, 3], photocatalytic degradation of pollutants [7, 8, 9, 10, 11, 12], photoelectric energy conversion [4, 5, 6], photocatalytic CO2 reduction [68, 69], and photocatalytic water splitting [6, 70, 71]. In the photocatalytic process, the energy of light which irradiates the semiconductor or plasmonic phases is absorbed and stored into chemical fuels [72]. The typical stages of the photocatalytic reaction are: the electrons from valence band (VB) are excited by photon with equal/higher energy to/than the bandgap of the photocatalyst and jump to conduction band (CB), leading to positive holes formed at VB; the resulted charge carriers will be separated and migrated to the surface of the photocatalyst to initiate redox reactions; the electrons from CB will eventually participate into reduction reactions while the holes are involved into oxidation processes [73]. Table 1 shows some examples of applications of photocatalytic reactions. It is observed that most applications are photocatalytic degradation reactions of pollutants from air or water. In water, most applications are in degradation reactions of organic pollutants. It is observed that the reactions take place either under UV light or in visible light irradiation, and in many cases, values of conversion or photocatalytic efficiency are very high. A special interest has been given in recent years to CO2 reduction reactions in order to obtain hydrocarbons or their oxidation products. In the case of reactions from gas phase, both reactions were carried out for CH4 or nitrogen oxide removal from the air.
Photocatalytic reactions with nanocomposite materials.
Another type of photocatalytic reaction is those applied for electricity generation. These are either reactions that produce hydrogen, which is then used as fuel or to generate electricity, or reactions that produce electricity. Also, the production of electricity can be accompanied either by the production of hydrogen or by the degradation of some organic compounds from the wastewater. The most important factor, which prescribes the efficiency of photocatalytic degradation, is separation of the photogenerated charge carriers, i.e., electrons and holes, which have the tendency to recombine, releasing energy as heat. Among other possibilities, an efficient way to separate photogenerated (e−, h+) pairs is the electrochemical method. This can be obtained in a photoelectrochemical cell (PEC). A PEC cell contains two electrodes, i.e., the photo anode with the photocatalytic material, the dark cathode, and an electrolyte. Photoelectrochemical decomposition of many biomass on nanocomposite film photoanode and an O2-reducing cathode was successfully achieved to generate photocurrents. Electricity production by photocatalytic degradation of some organic waste using a photo electrochemical (PEC) cell is an attractive method and presents two benefits on the environment: use of waste and conversion of solar radiation into energy (electricity). PEC cell that runs with a cathode in the absence of oxygen produces hydrogen. A diversity of biomass and bio-related materials such as poly/saccharides, proteins, cellulose, lignin, amino acids, alcohols, organic acids, urea, etc., is able to be photodecomposed by the anode nanocomposite in combination with an O2-reducing cathode. PEC can be associated with photoelectrochimic cell in a composite system. The composite photoelectrocatalytic system, with a photoelectrocatalytic reactor and a photocatalytic fuel cell (PFC), was made to effectively degrade refractory organic compounds. Various nanostructures, such as nanoparticles, nanorods, nanothin film, or nanotubes, were obtained for applications in photo fuel cell using various techniques. Typical photo-sensitive materials such as TiO2, ZnO, Fe2O3, CuO, and WO3 with different band gaps have been studied. Many researchers tried to enhance their photoresponse by different methods. Metal or non-metal doping is one of the most efficient methods used for improving the photoresponse of nanomaterials. Besides, organic doping, co-doping alloys, and muti-component materials also lead to increased performance of PECs. Organic conductors can be easily deposited on various electrodes leading to the formation of porous structures and large specific surface area. For example, poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) have recently been studied as electrocatalysts, with encouraging results reported [79]. Some applications of the nanocomposite materials in photoelectrocatalytic reactions are presented in Table 2.
4. Heterojunction and plasmon resonance effects on photocatalytic reactions
The performances of nanocomposite materials in photocatalytic reactions are determined by the interfaces between two or more materials. The main attributes of heterojunction photocatalysis are: (a) improving and extending the range of light absorption because different materials absorb light of different wavelengths generating an electric field and a potential-energy difference at the heterojunction interface; (b) extended electron/hole lifetimes because the electrons transfer occurs due to Fermi level differences which inhibits electron/hole recombination; (c) higher numbers of reactive sites which favor the photocatalytic reaction; (d) improving of the photocatalysts stability in condition of low electron and hole recombination which enhances redox efficiency [81]. Therefore, heterojunctions play important roles in photocatalysis. At the interface of two combined semiconductors, based on the bands energy values and their alignment, can have three types (I, II, III) of heterojunctions [73]. In the case of Type I heterojunction, the photoexcited electrons and the resulted holes migrate from CB with higher energy and VB, respectively, with lower energy of one of the semiconductors to the other (Figure 1). Thus, the accumulation of photoexcited charge carriers in the first semiconductor and their recombination are unfavorable for the photocatalytic activity. In Figure 1, the interface was marked with FTI, which represents internal electric field. Type II is the most efficient and most often used. In this case, the photoinduced electrons and holes are spatially separated, and the probability of their recombination decreases and is also reduced their redox capacity, making it unfavorable for photocatalytic redox reactions [48].
Figure 1.
Mechanisms of different types of heterojunctions.
To overcome this, Z-scheme has been proposed [21, 31, 49]. In the traditional type II heterojunction, the photogenerated electrons in the CB2, with higher energy transfer, transfer to CB1, with lower energy while photogenerated holes in the VB1, with lower energy, transfer to VB2, with higher energy. For type III heterojunction, the photoexcited carriers migrate in the same direction with type II, but the variation of energy is the following: VB1 < CB1 < VB2 < CB2. The Z-scheme is formed of a reducing photocatalyst (RP) and an oxidizing photocatalyst (OP). When RP and OP contact each other, due to a higher Fermi level of RP, electrons are spontaneously transferred from RP to OP. It leads to an interfacial electric field between RP and OP, OP being negatively charged and RP positively charged. Based on the internal electric field, were explained more W5+ and oxygen vacancies in g-C3N4/Mo-doped WO3 than Mo-doped WO3 nanocomposites [82]. The electrons of g-C3N4 transfer, at the interface of g-C3N4, through Mo-doped, combine with W6+ and then turn W6+ into W5+.
Another strategy to increase catalytic activity and electron transfer rates because of the surface plasmon resonance (SPR) effect is by doping the semiconductor with a metal as Au, Ag, and Pt [15, 18, 47, 52, 53, 54, 57, 58, 59, 60, 61]. Plasmonic photocatalysis utilizes noble metal nanoparticles embedded with semiconductor nanocatalyst and utilizes Schottky junction and surface plasmonic resonance (SPR). Ag nanoparticles are able to produce the surface SPR effect and act as electric channel. The use of Ag nanoparticles contributes to construct the immobilized and forced Z-scheme photocatalytic system [58]. Ag-AgX-ZnO-rGO nanocomposites, particularly, showed a higher degradation efficiency compared to the two-component ZnO-rGO nanocomposite due to enhanced visible-light absorption capacity and separation of photogenerated electron–hole pairs [43]. Moreover, different from the common type II heterojunction, it seems that a Z-scheme heterojunction can be formed in Bi/BiOF/BOC [48]. This novel Z-scheme nano-heterojunction possessed a high redox efficiency, which apparently improves the performance of photocatalysis H2 evolution and degradation of organic dye [50, 60]. Z-scheme Ag|AgBr/Ag/FeTiO3 photocatalyst [63] realizes the effective separation of CO2 and H2 because the photocatalytic oxidation and reduction reactions can simultaneously occur. Ag nanoparticles induced the SPR effect and facilitated the e− transfer in the Ag|AgBr/Ag/FeTiO3 photocatalytic system. For another photocatalyst like ZnO/Au/g-C3N4, the photocatalytic activity was ascribed to the surface plasmon resonance effect given by Au NPs and the synergistic effects between ZnO and g-C3N4 [41].
Many research groups show interest regarding the creation of magnetic separation photocatalyst for SrFe12O19-based heterojunction photocatalyst to enhance its photocatalytic activity [39]. Among them, semiconductor materials with n-n type heterojunction represent an important class of photocatalysts. The n-n type heterojunction photocatalyst has shown high photocatalytic activity in decomposition of water for hydrogen production and degradation of the organic pollutants. Nanocomposites with carbon quantum dots matrix (CQDs/CeO2/SrFe12O19) were used as magnetic separation photocatalyst and showed much higher adsorption capacity and photocatalytic degradation efficiency of MB dye under simulated sunlight irradiation. In this photocatalyst, CeO2/SrFe12O19 is n-n type composite semiconductor, and CQDs were introduced to enhance the transfer and separation of charge carriers between their CB and VB bands. To prevent the accumulation of photo-induced electrons on the conduction band of a single semiconductor, heterojunction structures with another semiconductor are made. In the case of the p-n heterojunction, a faster transfer of photogenerated electrons and holes occurs compared to p-p or n-n heterojunctions. For a p-n heterojunction, an internal electric field is created at the interface which increases the probability of separation of photogenerated electrons and holes.
Localized surface plasmon resonance (LSPR) was observed in nanocomposite media with semiconductor or dielectric matrices and metallic, especially noble metals, nanoparticles. The surface plasmon resonance induced in Au, Ag, and Cu nanoparticles generates energetic carriers (electrons or holes), called hot carriers, which can promote photocatalytic reactions. Since pure plasmonic metal nanoparticles can be photocatalysts for limited reaction types, their hybridization with semiconductors (TiO2, ZnO, WO3, g-C3N4, and SnO2) has been actively studied to obtain new photocatalysts with high efficiency under visible light. The resulted plasmonic nanocomposites are of significant interest due to their potential applications in various fields, among which photocatalytic processes are of significant interest [34, 47, 52, 53, 59, 68, 75]. A distinctive property of these nanocomposites is the effect of localized surface plasmon resonance which consists in the resonant absorption of incident light due to its strong interaction with the collective oscillations of electrons in the metal nanoparticles. Introduction of noble metal nanoparticles such as gold (Au) and silver (Ag) on semiconductor photocatalysts based on TiO2 and ZnO is a commonly used process. The improvement of the photocatalytic activity for these materials is the result of the LSPR effect and the formation of semiconductor–metal interfaces. The plasmonic effect can thus extend the optical absorption in the visible region allowing the absorption of more photons. At the same time, the metal–semiconductor interface can effectively suppress the recombination of charge carriers (electrons and holes). For the interface of ZnO-Ag-TiO2, nanocomposite photocatalyst was proposed as a plasmonic Z scheme [47]. The unfavorable effect of the excess visible light on the photocatalytic activity was thus explained. The charge transfer of electrons from ZnO to TiO2, via Ag interface, leads to higher electron desist in titania that suppresses the electron transfer from ZnO to Ag which reduces the charge separation. Another example of plasmon resonance effect evidenced that the photocatalytic reaction on single-site Co species in the g-C3N4 layer was facilitated by the SPR of the core AuNPs [68]. Another type of the surface plasmon resonance (SPR)-assisted photocatalyst was obtained by growing Ag nanoparticles on the surface of SnO2-coupled g-C3N4 nanocomposite. The photocatalytic activity was evidenced for hydrogen generation from water under visible-light irradiation [75]. Ag nanoparticles are excited under visible light, and the electrons exhibit surface plasmon resonance phenomena. The excited electrons are transferred to the CB of g-C3N4 and then to the CB of SnO2 for the reduction of water into hydrogen gas. The LSPR effect of Ag NPs and the formation of Z-scheme heterojunction were evidenced for nanocomposites containing photoluminescent material Sr2MgSi2O7:(Eu, Dy) (SMSO) and g-C3N4@Ag [83]. The plasmonic effect of AgNPs and heterojunction accelerated the transfer of electrons and also improved the separation efficiency of e−/h+ pairs. In the case of nanocomposites obtained by functionalizing, with TiO2 and Au, of the supports with different pore geometries and architectures (MCM-48, KIT-6, zeolite Y, hierarchical zeolite Y), the effect of plasmonic resonance was highlighted by comparing the photocatalytic results obtained in amoxicillin degradation under UV (254 nm) and visible (532 nm) irradiation [52]. Figure 2 shows a representation of the proposed mechanism for these reactions.
Figure 2.
Proposed mechanism for photocatalytic degradation, under UV and visible light irradiation, for silica (alumosilica)/TiO2/AuNPs nanocomposites. Reprinted from Petcu et al. [52] with permission from MDPI.
It was thus considered that AuNPs may have different roles. Thus, under UV light irradiation, AuNPs act as electron traps, while under visible light irradiation acts as electron generator. UV irradiation activates TiO2, and the electron transfer occurs from the valence band to the conduction band, leading to the formation of photogenerated charge carriers. In this case, Au NPs improve the separation of e−/h+ pairs, acting as an electron trap. In contrast, under visible light irradiation, the activated species are Au NPs through the SPR effect. The photogenerated hot electrons from gold nanoparticles are inserted into the conduction band of TiO2. After oxygen molecules adsorption these electrons are used to obtain highly reactive oxygen species as •O2− and HO•.
5. Synergic effects and mechanisms of the photocatalytic reactions
The photocatalytic performances of nanocomposite materials are the result of the synergistic effect of the components in most reactions. Therefore, the synergic photocatalytic degradation of pollutants was evidenced for many nanocomposites. An example can be the photocatalysts obtained by ZnO modification with 3-glycidoxypropyltriethoxysilane (GPTES) at different concentrations [33]. These nanocomposites have been successfully used in removal of dyes (methylene blue/methyl orange) in wastewater and benzene in air. Another example of a synergistic mechanism is the one responsible for the photocatalytic performances of the ZnO nanorods/Au samples [84] for which was observed: the improvement of sunlight absorption through the surface plasmon resonance of Au nanoparticles; improving the separation of charge carriers due to the appropriate energy band positions of the semiconductor and the gold nanoparticle that act as electron trap; hydroxylation of the ZnOR surface after interaction with the acidic HAuCl4 solution. For WO3@TiO2/CS-biochar photocatalysts [37] that exhibited enhanced synergic adsorption/photocatalytic removal was evidenced higher performance for methylene blue and antibiotic tetracycline. The enhancement of photocatalytic activity was linked to the S-scheme heterojunction formed between TiO2 and WO3 under the action of biochar which acts as electron transport channels and photosensitizer. The junction formed at the interface between biochar and semiconductors helps the separation and transfer of charge carriers. As an electron collector, biochar benefits the use of sunlight, and its photocatalytic activity increases in the presence of metal oxides or sulfides [85]. The better photocatalytic activity of nanocomposites containing microporous or hierarchical zeolite Y matrix and TiO2-NiO or TiO2-Co3O4oxides, than those only with one supported oxide, was also explained by a synergistic effect [55]. Efficient separation of electron–hole pairs resulted under irradiation, as an effect of internal electric field obtained in a typical p-n heterojunction, and also the beneficial alignment of the bands of NiO and TiO2 semiconductors (Figure 3) is presented in contrast with the photocatalytic systems containing TiO2-Co3O4, where the electrons transfer across the electric field is thermodynamically hindered because of the lower conduction band of Co3O4 than that of TiO2.
Figure 3.
Schematic representation of charge transfer in the p-n heterojunction for zeolite Y/TiO2/NiO and zeolite Y/TiO2/Co3O4. Reprinted from Petcu et al. [55] with permission from MDPI.
In addition to the synergistic action of the components from nanocomposites, associated processes with photocatalytic reactions were also used. Thus, the synergistic effect of photocatalytic reactions and adsorption, membrane filtration, and electrochemical or ultrasonic processes were studied [5, 74, 85, 86]. The membrane separation process integrated in photocatalytic reactors increases the efficiency of the reaction process. It has demonstrated the high qualities of the treatment process and the synergies of the photocatalytic reactor with membrane in practical applications. The fabricated RbxWO3@Fe3O4/rTAC/PET Janus membrane with porous recycled triacetate cellulose (rTAC) fiber membrane, as the host matrix, modified polyethylene terephthalate (PET), as substrate for nanosemiconductors, presented excellent results in photothermal conversion, water evaporation, desalination sewage, and photocatalytic applications under solar light [87]. This new developed Janus membrane was obtained to be applied in simultaneous evaporation of wastewater and photocatalytic reduction of carcinogenic pollutants in the remaining bulk water. Also, the Janus membrane has a significant capacity to adsorb the pollutant, and then, the adsorbed Cr(VI) is reduced to Cr(III) upon solar light irradiation.
Another nanocomposite with biochar (WO3@TiO2/CS-biochar) possessed optimal photocatalytic activity in complete degradation of organic compounds from aqueous solution [37]. The enhancement of photocatalytic activity was mainly ascribed to the S-scheme heterojunction formed between TiO2 and WO3 under the action of CS-biochar with electron transporter and photosensitizing properties. The proposed mechanism showed that electrons from the TiO2 surface spontaneously trend to transfer to WO3 across their interface. Thus, at interface TiO2 surface becomes positively charged, while WO3 is negatively charged. Finally, the electrons from CB of WO3 and holes from CV of TiO2 are eliminated but are retained the electrons from CB of TiO2 and holes from VB of WO3 with the highest redox activity.
Generally, the photocatalytic reaction consists of the next steps: the first is light absorption by the photocatalyst; the second step is photoexcitation of charge carriers; the third consists of separation, transfer, and recombination of photogenerated charge carriers; the fourth is adsorption of reactants and finally are the oxidation and reduction reactions. Figure 4 shows a proposed mechanism of cefuroxime photocatalytic degradation over Ti-zeolite Y modified with Fe, Co or Ni oxides [56]. These nanocomposites were obtained by immobilization of the various p-type semiconductors such as Fe3O4, Co3O4, and NiO on Ti-containing zeolite Y (n-type semiconductor) with titanium incorporated in the framework. The mechanisms proposed the formation of a p-n type heterojunction for all the photocatalysts.
Figure 4.
Mechanisms of cefuroxime photocatalytic degradation over Ti-zeolite Y modified with Fe (a), Co (b), or Ni (c) oxides. Reprinted from Petcu et al. [56] with permission from MDPI.
The internal electric field provides the spatial separation of the photogenerated charge carriers and guides the transfer of holes from an n-type to a p-type semiconductor and the transfer of electrons in reverse. According to the band edge position, a difference appears between the three types of synthesized materials. The holes generated in the valence band (VB) of TiO2 are transferred to the VB of all the used p-type semiconductors. Only for the photocatalyst with NiO, the electrons transfer from the conduction band (CB) of p-type semiconductors to the CB of TiO2. Thus, the recombination of e−/h+ pairs is reduced. The high dispersion of iron oxide ensures an efficient p-n heterojunction and hence a better separation of photogenerated charge, improving the photocatalytic performances.
The nanocomposite photocatalysts contain one, two, or more photoactive semiconductors (n- or p-type). The choice of these components is depending on their valence/conduction band potentials and band gap energy. Depending on the semiconducting type and number of components, different types (n-n, p-p, p-n, n-p-n, p-n-p) of nanocomposite photocatalysts can be obtained [72]. This results in different heterojunctions at the nanocomposite phase interface and different reaction mechanisms.
In this chapter, we have systematically summarized the main application of nanocomposite materials in the photocatalytic reactions and the processes that influence their activity and reaction mechanism. The applications of nanocomposite materials in the photocatalytic reactions offer significant advantages compared to each single component. The synergistic action of the components led to a significant increase in the activity and selectivity of photocatalytic processes in the synthesis of the oxygenated organic compounds, in the degradation of pollutants from wastewater or air, and in the generation of electricity. Since the photocatalytic reactions are non-polluting and the energy that activates them is renewable, they have a high potential to be used and developed. The implementation of photocatalytic reactions on a larger scale and in multiple fields represents both a solution for the present and a future perspective. As one of the most attractive technologies, photocatalysis represents nanocomposite materials, an application for present and future for producing green fuels and a wide range of environmental solutions. In order to increase the performance of nanocomposite materials in photocatalytic reactions, new systems and synthesis methods determined by the effects of heterojunction, synergism, and plasmonic resonance will be developed in the future.
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Written By
Viorica Parvulescu and Gabriela Petcu
Submitted: 03 April 2024Reviewed: 03 June 2024Published: 19 July 2024
Open access peer-reviewed chapter
