Advances in nanomaterials for heterogeneous photocatalysis

Photocatalyst can be of different types based on a certain aspect according to the convenience and implementation like compositions, sizes, dimensions, electrical properties or others. Xiaogang et al [50] had grouped photocatalyst into six categories based on constituent materials—traditional semiconductor, molecular, plasmonic, 2D, quantum dots, and traditional semiconductor-based photovoltaic assisted photocatalyst. Serpone et al [56] and Emeline et al [57] summarized photocatalyst based on time and material, group into generations—first, second and third, and proposed concepts of photoactive materials with multi-photon excitation capability, selective photoexcitation and heterojunctions. In the review article by Fernando et al [58], the advancements of photocatalytic material from single-phase semiconductor to mix bi-functional material was discussed. By studying heterogeneous catalysis, G.M.Schwab and K.Gossner [59] classified photocatalyst based on the structure; metals, supported metal catalysts, oxide catalysts [33]. Continually, Ren et al [60] and Boyjoo et al [61] classified photocatalyst materials based on their composition; binary, complex metal oxides, metal sulfides, and metal-free materials. Due to the emergence of countless photocatalyst materials, it becomes difficult in categorising properly and found to be subtle to club most materials explicitly in a certain group. Yet this review article attempted to consolidate photocatalyst materials into eleven categories based on the type and composition of the material, covering almost every type of photocatalyst. The categories are: Oxides, [62, 63] Sulfides, [64, 65] Nitrides, [66–68] II-VI, [69] Halides, [19, 70] III-V, [71] Doped material, [72, 73] Carbon-based material, [74, 75] Elemental/Metal NPs, [76, 77] Molecular organic frameworks, [78] and other complex structures which are collectively known as Composites/Hybrids like core-shells [70, 79–81], solid solution oxynitrides which demonstrates enhanced performances as a photocatalyst [68, 82, 83]. The photocatalyst material in each category can be of unary (ex. one element oxide; ZnO [13]), binary (ex. two-element oxide; CoFe₂O₄ [84]), tertiary (ex. three-element oxide; GaZnON [85], GaN:ZnO [86], AgBiW₂O₈ [87]) and multi-elemental [88], GaZn:NO [83], GaZnON–rGO [89] (shown in table 1).

The new heterogeneous photocatalysts are emerged by manipulating certain facets like bandgap, carrier transport, crystallinity, surface area and stability based on the applications and requirements. These heterogeneous photocatalyst display divergent optical, magnetic, physical and chemical properties as the constituents vary from organic (ex. protein) to inorganic (ex. CdS), metals (ex. Pt) to non-metals (ex. N)/rare-earth elements (ex. La), intrinsic (undoped)-extrinsic (doped), and others. Tuning particle's shape, structure and sizes, microns to nano-size morphologies; 0 D (quantum dots), [74, 75] 1 D (rods, tubes, wires), [62, 90] 2 D (thin films), [74, 91] 3 D (nanowire bundles, bulk powders), [13, 92] and other complex structures [93, 94] mostly alter bandgap energies. This alteration in the bandgap results in inducing unique responses to the interaction of light as light-matter interaction phenomena is a very delicate process and are easily hindered by even a single change in composition, type of material, or change in reaction parameters. Therefore, the main focused area of photocatalyst properties is to have large visible spectral absorbance, high-energy conversion efficiency, and maximum charge generation-separation rates.

The heterogeneous photocatalysis includes a variety of reactions: oxidation, reduction, de-hydrogenation, radical formation etc. occurring in semiconductor surfaces where a wide range of weakly bound chemical adsorbates undergo light-initiated-induced redox reactions. The photocatalysis process is carried out in different media for different implementations like gas-phase (ex. CO₂ reduction or GHGs mitigation), solid-phase (ex. solar cells), liquid-phase or aqueous solution (ex. dye degradation/wastewater treatment, water splitting). For the water pollution mitigation, the photocatalytic reaction takes place in photocatalyst materials that are dispersed in a pollutant containing medium or on a photocatalyst that is anchored in a substrate. The execution of this kind of reaction takes place in a batch reaction system for analysing particular photocatalyst material under a known quantity of reaction conditions. The batch reaction system primarily uses powder catalyst dispersing in the medium through stirring or sonication (stirring throughout the process to prohibit agglomeration), acknowledging its simplicity and uncomplicated reaction step explorations. Furthermore, the catalyst material is also manoeuvred in different forms like electrodes, [74] powders, [95] fabric, [96] floating forms, [79] and many others [97]. The liquid phase photocatalysis process follows five predominant steps (illustrated in figure 2): (a) Catalyst introduction in the contaminated liquid medium, (b) Adsorption of contaminant molecules on the catalyst surface, (c) Oxidation/reduction reaction of the contaminants, (d) Desorption of product molecules from the catalyst surface, and (e) Removal of products from interface region [4].

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Regardless of innumerable photocatalysts, photocatalysis processes predominantly revolves around these five pivotal phases (illustrated in figure 3) and two reaction pathways. The five phases are: (i) Light absorption or harvesting, (ii) Excitation and exciton/charge generation, (iii) Charge separation-transfer-migration, (iv) Charge recombination, or Oxidation-reduction processes, and (v) Desorption of reaction products from the catalyst surface or reaction medium [4, 35, 98, 99]. The two reaction pathways for the photocatalytic reactions are: (a) Direct pathways, where photogenerated charge carriers are separated and trapped by pollutant molecule or surface defects consequently forming radicals, and (b) Indirect pathways, where photo-generated charge carriers are used to produced ions and radicals (reactive oxygen species) by reacting with water molecules [100, 101].

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For photo-degradation of contaminants, advanced oxidative process (AOP) provides complete oxidation of pollutants and mineralizing them into less harmful components by generating strong oxidants or oxidising species (like •OH radicals, superoxide radicals), called reactive oxygen species (ROS) triggered by an external force. Some of the pollutant molecules like Cl⁻, ${{{\rm{SO}}}_{4}}^{2-},$ Al³⁺, Cu²⁺, CN⁻, phosphates and others, present in the wastewater also act as charge carriers and attack the unsaturated bonds of the pollutants [102]. The elimination of pollutant molecule occurs through a complete/partial breakdown of conjugated unsaturated bonds attacked by the charged particles and radicals generated, collectively known as ROS [103]. In the case of complete mineralization, pollutants are degraded into less harmful products like CO₂, H₂O, or alcohol and gases with the formation of numerous reaction intermediates [103, 104].

Photocatalysis reaction is initiated by photo-absorption and photo-excitation by the photocatalyst when incident photon energy is equal to or greater than photocatalyst bandgap (E_g) (i.e. hυ ≥ E_g; where υ = frequency of incident light and h = Planck's constant). This photoexcitation creates an electronic vacancy, hole (h⁺) in the valence band edge by exciting an electron (e⁻) to the conduction band edge (Photocatalyst + hυ → Photocatalyst (e⁻ + h⁺ )). This photogenerated e⁻─h⁺ pairs are called an exciton pair that carries energy with their correlated motion of e⁻s and h⁺s not associating electronic current. Exciton pairs can be of loosely or strongly bound e⁻─h⁺ pairs with the Coulomb attractive interactions. The loosely bound excitons are known as Wannier-Mott exciton or free excitons, which are typically observed in semiconductor nanocrystals (quantum dots) and the exciton wavefunction is delocalized. The free movement of Wannier-Mott excitons are limited by the particle or crystal size (because of its large radius and easy for dissociation). The strongly bound excitons (small-radius exciton) are known as Frenkel excitons and are normally found in the aggregates of organic dyes [74, 105]. The h⁺ then initiate further interfacial e⁻ transfer and reaction rate is determined by the reaction of h⁺s with a substrate, as carrier trapping is a fast phenomenon. These transferred photogenerated e⁻─h⁺ pairs are noticeably powerful to induce various oxidation and reduction processes of the surface adsorbed chemical substances forming singly oxidized e⁻ donor $\left(D+{h}^{+}\to {D}^{+}\right)$ or singly reduced e⁻ acceptor $\left(A+{e}^{-}\,\to {A}^{-}\right)$ or witness surface recombination emitting radiative or non-radiative energy [98]. Table 2 provides a series of photocatalytic reactions decentralized in a simple semiconductor catalyst surface.

Table 2. Major possible reactions in a photocatalysis reaction for the generation of charge carriers and reactive oxygen species for the degradation and mineralization of pollutants.

$Pollutant+{h}^{+}\to Oxidation;$	$Pollutant+{e}^{-}\to Reduction;$
${h}^{+}+\,{H}_{2}O\,\to {h}^{+}+\cdot OH;$	${h}^{+}+O{H}^{-}\to \cdot OH;$
${e}^{-}+{O}_{2}\to {O}_{2}^{{\cdot }^{-}}\,\,;$	${O}_{2}^{{\cdot }^{-}}+{h}^{+}\to H{O}_{2}^{\cdot };$
$2H{O}_{2}^{\cdot }\to {H}_{2}{O}_{2};$
${H}_{2}{O}_{2}+\,{O}_{2}^{{\cdot }^{-}}\to \cdot OH+O{H}^{-}+\,{O}_{2}\,;$	${H}_{2}{O}_{2}+{h}^{+}\to {O}_{2}+2{h}^{+};$
${H}_{2}{O}_{2}+h{\boldsymbol{\nu }}\to 2\cdot OH;$	${H}_{2}{O}_{2}+{e}^{-}\to \cdot OH+O{H}^{-};$
${O}_{3}+{e}^{-}\to {O}_{3}^{{\cdot }^{-}};$	${O}_{3}^{{\cdot }^{-}}+{H}^{+}\to H{O}_{3}^{{\cdot }^{-}};$
${O}_{3}+h\nu \to {O}_{2}+O\cdot ;$	$O\cdot +{H}_{2}O\to 2\cdot OH;$
$F{e}^{3+}+{H}_{2}O+h\nu \to F{e}^{2+}+\cdot OH+\,{H}^{+}$
${\boldsymbol{Pollutant}}+\left({{\boldsymbol{e}}}^{-},{{\boldsymbol{h}}}^{+},{\boldsymbol{ROS}}\right)\to {\boldsymbol{Degradation}}\,{\boldsymbol{products}}\left({\boldsymbol{C}}{{\boldsymbol{O}}}_{2}\,,\,{{\boldsymbol{H}}}_{2}{\boldsymbol{O}},\ldots \right)$
ROS: O2 ·− , ·OH, OH− , HO2 ·, H2 O2

The transfer of photogenerated charge carriers to surface adsorbed pollutant molecules occur when the Fermi level and redox potential of the absorbed molecules equilibrate or sufficiently positive to the valence band maxima or sufficiently negative to the conduction band minima. Because of the surface potential developed between semiconductor and electrolyte interface (known as Schottky barrier), an interfacial electric field is generated, driving e⁻s and h⁺s in opposite directions. This charge carrier separation inhibits recombination causing band flattening at the solid-liquid interface. This surface potential also induces charge carriers to drive towards the surface trapping sites, by either migration or diffusion. As every catalyst surface has unique electrical, physical and chemical properties, it possesses a distinct number of trapping active sites and responses uniquely to the charge carriers or pollutants. These sites are the bait or launching site for charge particles or carriers like anions, cations, contaminant molecules, which are either organic or inorganic that attached through chemisorption or physisorption processes by weak van der Waal's force of attraction. The efficiency and effectiveness of these reactions are indicated by the quantum yield, 'it is the number of molecules of a given pollutant degraded per photon of radiation absorbed' or 'it is the ratio of the number of molecules undergoing dissociation by several photons absorbed by the solution'. The efficiency is altered by tuning major factors like bandgap, catalyst compositions, catalyst loading concentration, surface defects, surface properties, shape and size, pH of the reaction medium, and light source. Here, table 3 shows the commonly used formulae for quantifying and determining photocatalysis reactions.

Table 3. Commonly used formulae for photocatalysis reaction analysis:

Sl. No	Formula	Denotations
1.	Degradation rate, Pseudo-first-order kinetics, Langmuir-Hinshelwood mechanism	R = reactant degradation rate in mg/l*min; C = reactant concentration in mg/l, t = irradiation time in min; K = reactant adsorption coefficient in l/mg; k = reaction rate constant in mg/l*min
	$R=-dC/dt,$ $R=\left(kKC\right)/\left(1+KC\right)$
2.	Degradation rate for very small reactant concentration C,	C0  = initial concentration; kapp  = apparent first-order rate constant = 0.0343 min/t.
	$\mathrm{ln}\left({C}_{0}/C\right)=kKt=\,{k}_{aap}t;$
	Or $C={C}_{0}{e}^{-kt}$
3.	Removal efficiency,	A0  = absorbance at initial time (t = 0), At  = absorbance at time t
	$RE\,\left( \% \right)=\displaystyle \frac{{A}_{0}-{A}_{t}}{{A}_{0}}\times 100$
4.	Total organic content,	TOC0  = TOC value at initial time (t = 0),
	$TOC\,\left( \% \right)=\,\displaystyle \frac{TO{C}_{0}-TO{C}_{t}}{TO{C}_{0}}\times 100$	TOCt  = TOC value at time t
5.	Chemical oxygen demand,	COD0  = COD value at initial time (t = 0),
	$COD\left( \% \right)=\displaystyle \frac{CO{D}_{0}-CO{D}_{t}}{CO{D}_{0}}\times 100$	CODt  = COD value at time t
6.	Photonic efficiency,	R = initial rate, V = volume of suspension (ml), A = irradiation area (cm2), I = light intensity (W/cm2), NA  = Avogadro's number, h = Planck's constant, c = speed of light
	$\zeta =\displaystyle \frac{r}{{I}_{0}}\times 100$
	${I}_{0}=\displaystyle \frac{IA}{{N}_{A}hcV}$
7.	Quantum Yield,	PRR = Photochemical reaction rate,
	$QY\left( \% \right)=\displaystyle \frac{PRR}{PAR}\times 100$	PAR = Photon absorption rate
	Or,	R = amount of reactants consumed or product formed
	$QY\left(\lambda \right)=\displaystyle \frac{R}{P}\times 100$	P = amount of photons absorbed
		λ= absorbed photon wavelength
8.	Apparent quantum yield,	ne  = number of reacted electrons
	$AQY\left( \% \right)=\displaystyle \frac{{n}_{e}}{{n}_{p}}\times 100$	np  = number of incident photons
9.	Turnover number,	me  = number of reacted electrons in moles
	$TON=\displaystyle \frac{{m}_{e}}{{m}_{p}}\times 100$	mp  = number of moles of photocatalyst
10.	Turnover frequency,	TON = turnover number
	$TOF=\tfrac{TON}{t}$	t = duration of the reaction

One of the embryonic approach to enhance photocatalytic reaction is by doping process, introducing co-catalyst materials/sensitizers, and others. Doping can be grouped into four types: (i) Self-doping, (ii) Metal-doping, (iii) Non-metal doping, and (iv) Co-doping. In association with different types of doping, mid-gap energy states are established between the conduction band minima and valence band maxima (shown in figures 4(d), (e)). In metal ion-doped semiconductor oxides, reduction reaction primarily occurs through electron transfer reaction to the metal ions via single or multiple steps, additionally, interfacial charge transfer and separation. The presence of surface defects suppresses charge carrier recombination, acting as a reaction site [14, 35, 111, 112]. Also in a doped photocatalyst material, dopants act as trappers for the photo-generated e⁻s and h⁺s; trappings, and de-trapping of h⁺s and e⁻s (shown by a series of reactions in table 4) [113, 114].

Table 4. Role of defects and surface defects in photocatalysis reaction in charge transfer.

Trapping	${\left(Dopant\right)}^{n+}+{e}^{-}\to {\left(Dopant\right)}^{\left(n-1\right)+},$ ${\left(Dopant\right)}^{n+}+{h}^{+}\to {\left(Dopant\right)}^{\left(n+1\right)+}$
De-trapping	${\left(Dopant\right)}^{n+}+h\nu \to {\left(Dopant\right)}^{\left(n-1\right)+}+{h}^{+},$ ${\left(Dopant\right)}^{n+}+h\nu \to {\left(Dopant\right)}^{\left(n+1\right)+}+{e}^{-}$
Surface defects (SD)	$SD+{h}^{+}\to {\left(SD\right)}^{+},$ ${\left(SD\right)}^{+}+{e}^{-}\to SD,$ ${\left(SD\right)}^{+}+dye\to {\left(SD-dye\right)}^{+}$

Doping of metals or non-metals introduces either new band levels (acceptor or donor levels based on both the type of material and dopant) also known as impurity state levels in between conduction band and valence band or shifting of Fermi level. By co-doping with S and V into Sr₂Ta₂O₇ shifted the valence band maxima upward and conduction band minima downward [115]. Also, doping Cu ions into the ZnO resulted in narrowing the bandgap by shifting valence band downward due to the presence of Cu²⁺ ions and with the increase in Cu²⁺ ion concentration, the narrowing of band gap increases due to the difference in electron distribution [116]. However, not all alterations in a photocatalyst yield positive results as the introduction of dopants into a pristine semiconductor may slow down the reaction processes due to an increase in the recombination of e⁻─h⁺ pairs in the dopant sites [33, 100]. Yin et al [117] widely studied bandgap and band edge modification using generalized gradient approximation (also validated by hybrid density function method) by substituting/co-doping certain percentage of O and Ti from TiO₂. It was found that the concentration of dopants affects bandgap and band edge positions. The binding energy of co-doped material is calculated by [110]:

$$\begin{eqnarray*}&&{E}_{b}=E\left({D}_{Ti}\right)+E\left({A}_{O}\right)-E\left({D}_{Ti}+{A}_{O}\right)-{E}_{Ti{O}_{2}}\end{eqnarray*}$$

where: $E\left({D}_{Ti}\right){\rm{and}}\,E\left({A}_{O}\right)\,$are total energies with donor and acceptor on Ti and O sites, ${E}_{Ti{O}_{2}}$ is the total energy of TiO₂.

Shifting to multicomponent photocatalysts, designing of multicomponent photocatalysts give heterojunction interfaces between the constituting materials and therefore suitable band edge materials are chosen to build heterostructures. In the case of semiconductor-metal heterostructured photocatalyst, a space charge region called Schottky barrier is formed in the interface. The alignment of Fermi level to attain equilibrium takes place in the interface by flowing e⁻s from material with higher Fermi level or lower work function to lower Fermi level or higher work function forming an electric field in the contact region [76, 118, 119]. Here, metal with higher work function acts as e⁻ trappers and make itself excessively negatively charged, consequently making the corresponding semiconductor into positively charged in excess. Therefore, these e⁻ trapping phenomena assist in improving exciton separation and inhibit in charge carrier recombination. For example, Ag nanoparticles in AgBr or AgCl, produce surface plasmon resonance induced electric field developed in the metal and semiconductor contact region. This phenomenon enhanced photocatalytic activity in sunlight illumination by transferring e⁻s from Ag metal to semiconductor AgBr, equilibrating Fermi levels (shown in figure 5) [76].

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One of the hot emerging photocatalyst material is plasmonic material, where noble metal nanoparticles like Ag, Au, are incorporated with another photocatalyst to form complex hybrid photocatalyst. These plasmonic photocatalysts are known to absorb solar energy and form an internal electric field when illuminated by the light of sufficient energy where charge generation takes place followed by a similar photo-redox mechanism to that of conventional reaction mechanisms. The localized surface plasmon resonance originates when the incident light wavelength is greater than the size of the metal nanoparticle, and free e⁻s oscillate in the nanoparticle surfaces in phase with the incident electromagnetic field [50, 118, 120]. Here, charge carriers are separated by the developed surface plasmon resonance electric filed, increasing the quantum efficiency and with the direct e⁻ transfer from plasmonic metal to the counter material [77, 118]. In plasmonic photocatalysis cases, two main mechanism take place: plasmon- assisted photocatalysis (by the influence of irradiation: energy transfer and charge transfer) and plasmon-assisted catalysis (by the influence of heating: plasmonic heating). The plasmon-assisted catalysis are due to the photothermal effect caused by the strong local electromagnetic field by the noble metal nanoparticles to the adjacent semiconductor or environment. Thus, the process assisted in generating additional charge carriers, and hence affecting photocatalytic reactions [121, 122]. In the case of bimetallic photocatalyst like Au-Pd nanoparticles, the catalytic activity is attributed by the plasmonic photocatalysis. However, when bimetallic photocatalyst coupled with semiconductor photocatalyst, ex. Au-TiO_x-Pd catalyst, both the photothermal as well as plasmonic photocatalysis affect overall catalytic reactions thereby enhancing the process [123].

Shifting to multicomponent photocatalyst, suitable band edge materials are integrated forming hybrid heterostructures. The hybrid multicomponent photocatalyst improves light absorption efficiency and optimizes charge separation-recombination process where one material act as a photosensitizer for the other material. This heterojunction is conceivably in between semiconductor-semiconductor, semiconductor-metal, metal-metal, and metal-insulator. Here, semiconductors can be organic or inorganic and depending on their majority charge carriers, it can be n-type or p-type. Hence, heterojunctions are the combination of n and p-type semiconductor, forming n-n, p-p, n-p/p-n heterojunctions [5, 124, 125].

In few cases of heterostructures, semiconductors are assumed to be an insulator when partnered with less bandgap material since absorption of light is limited to UV region of the electromagnetic spectrum and less electric property [5, 56, 126]. In n-p/p-n type heterojunction, one n-type semiconductor is in contact with other p-type semiconductor forming heterojunction interface with space charge region developed by e⁻s and h⁺s diffusion in opposite directions, thereby creating a built-in electric potential. This built-in potential uplift charge transfer rate, eliminating recombination and increasing carrier lifetime. The potential drive e⁻s from higher conduction energy level to lower conduction energy level and h⁺s will transfer from the lower valence level to higher valence band level. Succinctly, the mechanisms of e⁻─h⁺ transfer from one material to another strongly depend upon work function or conduction band and valence band levels, with the type of triggering reaction- oxidation or reduction. In heterostructures, charge transfer encounters less barrier and therefore due to the built-in potential and band matching, more number of e⁻s and h⁺s migrate in the opposite directions, thereby amplifying photo-redox reactions. However, to construct an efficient heterojunction, constituting materials should acquire matching bandgap or potential [76, 108, 127].

Xie et al [125] illustrated band alignment and charge separation between n-type SrFe₁₂O₁₉ and p-type Bi₂O₃ heterojunction; e⁻s were transferred from n-type to p-type and h⁺s from p-type to n-type (shown in figure 6). The magnetic field generated inside the magnetic component further cooperated in propelling photogenerated e⁻s and h⁺s, thereby complementing each other [125]. The optical band gap of n-n type hetero-structured ZnFe₂O₄/BiVO₄ fall to be in between the bandgap of an individual component and photons are absorbed by less bandgap material (ZnFe₂O₄), where e⁻s are transferred to less negative conduction band material (BiVO₄) to the counter component [124].

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Based on the charge excitation-separation and conduction band minima-valence band maxima alignment of the constituting semiconductor materials where the discontinuity or band-offset lies, semiconductor-semiconductor heterostructures are of four types. They are shown in figure 7 with semiconductor A and semiconductor B: (i) Type I (straddling gap), (ii) Type II (staggered gap), (iii) Type II (direct Z-scheme), and (iv) Type III (broken gap). Thus, due to the presence of band-offset, charge carriers experience a potential barrier in the junction and therefore can control the motion and flow of charge carriers. In Type I straddling gap semiconductor heterojunction, the CB and VB of one semiconductor shows more positive and more negative respectively as compared to the counter semiconductor material. Therefore, photoexcited e⁻s are transferred from the more negative CB to less negative CB and h⁺s are transferred from more positive VB to less positive VB between the two comprising semiconductors (from semiconductor A to semiconductor B) [128].

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In contrast to Type I heterojunction where photoexcited h⁺s and e⁻s are transferred from one semiconductor to the corresponding semiconductor (example: TiO₂/BiVO₄ [129]), Type II heterostructures transfer photoexcited h⁺s and e⁻s in opposite directions. The charge transfer in Type II heterojunctions may follow two different pathways: conventional and direct Z-scheme, as shown in figures 7(b) and (c). The oxidation and reduction reaction takes place in different semiconductor surfaces for Type II heterostructure while the redox reaction for Type I heterostructure takes place in a single semiconductor surface, therefore charge separation and involvement of charge carriers in a redox reaction are more in Type-II heterostructures. The above-mentioned p-p, n-p/p-n, n-n heterojunctions are introduced and designed under Type II heterojunction category [130]. The photoexcited e⁻s in the more negative CB transfer either to the less negative CB (conventional charge transfer, example CuWO₄/Mn₃O₄ [131]) or to the VB of the counter material (direct Z-scheme, example g-C₃N₄/TiO₂ heterostructure. And, these e⁻s recombined with the h⁺s, thereby, increasing photoexcited e⁻- h⁺ separation, generating more e⁻s and h⁺s for reductive and oxidative reactions, resulting in the enhancement of photocatalytic efficiency [132].

The type III heterostructures are quite different from the above types as the chances of charge transfer in between the dissimilar semiconductors are rare, therefore losing the significance of engineering heterostructures [130]. The h⁺s of semiconductor A recombines with the photoexcited e⁻s of semiconductor B and redox reaction takes place in different semiconductor surfaces [128]. With the development in different catalyst types, the major concern of retrieving and reusability of catalyst was solved by using magnetic-based catalyst materials like ferrites based catalyst for its easy magnetic separation, e.g. n-n heterogeneous structure of ZnFe₂O₄ and BiVO₄ [124]. Apart from multi-component photocatalyst and doping, size variation is further one of the major influencing factors of bandgap manipulation. It is noted that band gap increases and is quantised with the decrease in particle sizes by localization of energy bands and edges [13, 133].

Another development in the field of hybrid photocatalyst has been a carbon-based heterogeneous catalyst as an approach towards environmental friendliness and availability, despite the well-accepted contribution by pure carbon-based materials. These catalysts are essentially composites of two or more semiconductor compounds co-partnered with carbon nanotubes (multi-walled or single-walled), [134] carbon quantum dots, [135] two-dimensional graphene with quantum dots [74, 75] or graphite composites, [74] and reduced graphene oxides [136].

The carbon-based heterogeneous photocatalysts are organic and known for its potential green catalyst; surface porosity properties of carbon-based materials are an asset in providing trapping active sites and modulates electrical properties of the composite, as carbon-based materials are conductive. Apart from enhancing photocatalytic properties, it acts as an anchoring surface or template for other partnered elements or compounds. [74, 134, 135] B.A deli and F.Taghipour had studied the effect of graphene oxide hybridization on Rh-loaded GaZnON–rGO photocatalyst and found an enhancement in the H₂ evolution after graphene oxide hybridisation due to the effective electrochemical interaction between the graphene oxide sheets and composite photocatalyst [89]. Choi et al showed that ZnO coupled with single walled carbon nanotubes (SWCNTs) exhibit higher performance as compared to the ZnO coupled with multi walled carbon nanotubes (MWCNTs) by providing efficient transport of charge carriers and stabilization of free electrons in contrast with the MWCNT due to their metallic property [137]. The presence of heterojunction interface between CNTs and ZnO reduces charge carrier recombination and increases hole extraction by the semiconducting SWCNTs and free electron transport by the metallic SWCNTs thereby improving the performance [137, 138]. The charge carrier generation and transfer in the heterojunction interface of the metal oxide and graphene quantum dot were explored by Sudhagar et al and showed the enhancement of photocurrent in the composite by 70% due to the compatible type II bandgap alignment. Here, the photogenerated charge carriers were separated where photogenerated h⁺s in graphene quantum dots were transferred to the solution for water oxidation and photogenerated e¯s were collected by the semiconductor oxide (TiO₂), thereby enhancing the photocurrents [6]. To understand the photophysical processes, Godin et al had studied the charge trapping-detrapping in graphitic carbon nitride photocatalyst through photoluminescense and time-resolved absorption as deep trapping of charge carriers reduced photocatalytic efficiency. In this context, the presence of deep traps slowed the charged transfer kinetics from the carbon nitride to the neighbouring material, also, due to the deep trappings, large energy loss were took place resulting in decreasing the driving force of charge transfer [139]. Therefore, certain material has to be tuned, designed and optimized to enhance the photogenerated charge carriers generation and transfer, and tune the charge carriers trapping-detrapping for a productive involvement in the photocatalytic reactions.

A possible photogenerated charge transfer mechanism between a semiconductor material and conducting CNT is depicted in figure 8(a). The figure 8(b) shows the illustration of the charge transfer mechanism in a semiconductor photocatalyst molecules that are grown or anchored in a substrate by chemical or physical deposition or lithography techniques for the mineralization of wastewater pollutants via the production of charged particles.

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One of the simplest and fundamental way to engineer photocatalyst properties like optical, electrical, size, morphology is to adopt variable synthesizing routes and preparation parameters. In conjunction with material properties, factors like pH, concentration and type of pollutants, amount of catalyst loading, and inclusion of charge ion scavengers are also a major factor for revising demineralization or degradation of pollutants with high efficacies (shown in figure 9).

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It is prominent that photocatalysis reaction efficiency is the synergy of all these properties and considering single properties confines solely in that singular material and reaction conditions by keeping other characteristics as constants. However, one of the main contributing factors is the pH of the reaction medium as the production of ROS and interaction of catalyst-pollutant are majorly influenced by pH. On that account, by keeping other characteristics constant, the pH of pollutant containing solution affects surface charge properties of the catalyst by its alkaline and acidic nature, either negatively or positively charged and therefore, hinders the interaction between catalyst and pollutant molecules thereby affecting van-der-Waals attraction or electrostatic repulsion. Due to variability in functional groups/types present in catalyst like oxides/sulphides, their responses to a modest change in pH of untreated water (either acidic or basic) fluctuates degradation/demineralization reaction rate. The increase in pH implies the presence of extra OH⁻ ions and less pH means more H⁺ ions. However, controversy arises as the main charge carriers for photodegradation reaction includes OH‾ ions/radicals and, e⁻s/h⁺s, because of the pH variable effect and its charge carrier contribution in different pH range [140]. Thus, for the catalyst that tends to attract OH‾ ions like oxides (TiO₂, ZnO etc.), formed aggregates due to excessive OH‾ ions in the solution, and obstruct light penetration to each catalyst molecules. Thus, the formation of aggregates reduces the interaction of light in addition to reducing the amount of pollutant molecules absorption. In some reactions H₂SO₄, H₂O₂, NaOH and other pH changing solutions are used to adjust the desired pH level. Semiconductors like ZnO, SnO₂, TiO₂ prefer for near-neutral pH (pH = 6 or 7) in their suspensions as compared to its acidic (catalyst surface gets protonated) and basic (catalyst surface gets deprotonated) suspensions owing to its surface charges and nanoparticles distribution in the suspension [140].

Analysing apart from a material point of view, the effect of pH can also be seen from molecular interaction, which is the electrostatic interaction between pollutant molecules and reactive oxygen species (attractive/repulsive). Therefore, depending upon the molecular interaction, a certain reaction is either accelerated or decelerated. The pollutants possess different charge when suspended in a medium (cationic/anionic/amphoteric). To conclude all theories, the relation between solution pH and surface charge of catalyst is defined by point zero charges, pH_ZPC of the catalyst as a deciding factor of the optimum pH of a catalyst for a certain reaction [141–143]. The reaction pH higher than pH_ZPC delivers negatively charged catalyst surface and lower than pH_ZPC provide positively charged catalyst surface. At pH_ZPC, the catalyst is uncharged and electrostatic repulsion is outweighed by van-der Waals attraction and consequently, the size of catalyst increases by agglomeration [144]. Agglomeration of catalyst material also led to decreasing degradation rate due to the aggregate formation inhibiting light penetration by scattering and decline in pollutant-catalyst interaction. Summing up, the photocatalytic activity is a synergistic effect of several inter-dependent parameters and a small modification in any of these parameters showed merits or demerits, yet some controlling factors for photocatalytic activity are pH, [145] catalyst concentration & loading, [146] surface defects, [90] particle size and porosity, [62] surface hydroxyl groups present, [13] photocatalyst calcination [86, 103], irradiation time & intensity [142]. From all the study it is found that the formation and release of H₂O₂ is the main event that takes place during photochemical degradation via photochemical oxidation of the presence of oxygen/moist atmosphere/aqueous reaction medium.