Modulation of Z-scheme photocatalysts for pharmaceuticals remediation and pathogen inactivation: Design devotion, concept examination, and developments

3.1. Theoretical proof

Theoretical investigations that have been used and accepted scientifically to provide enough evidence are simulation techniques that predict the behavioural interaction of different elemental materials input into a software. They can be employed towards structural, electronic, and interaction predictions. The parameters are important to understand different assumptions made towards establishment of the results. Density functional theory (DFT), frontier electron density calculation (FED), etc. are some of the theoretical methods that have been used for different applications [57], [58], [59]. Just recently, these theoretical investigations were employed towards validation of charge transfer in Z-scheme heterojunctions.

The DFT calculations is the most commonly used theory for different applications with different software. The detailed inspection and comparison of software performance towards a particular application would be an interesting study to actually determine the best software package to use for determining the charge transfer at the interface of two different semiconductors. With each different software, minor and major parameters are reported as they may affect the outcome of the whole process under investigation. The parameters include the module, cut-off energy, quality of k-points, and size of vacuum layer [59], [60], [61]. After the parameters are set, the most important part is the interpretation of the DFT data obtained towards understanding the interfacial charge transfer. Density of states, bandgap and work functions together with electron depletion and accumulation, and planar averaged electron density difference Δρ(z) along the z direction collaboratively determine electron transfer [62]. Wang et al. constructed a novel AgI/BiSbO₄ Z-scheme heterojunction via a hydrothermal-precipitation method towards degradation of acid red G (ARG) and tetracycline (TC) with in-situ generated surface plasmon resonance (SPR) from Ag⁰ during degradation [62]. They employed DFT calculations to study the interfacial charge transfer at the interface of AgI and BiSbO₄ (Fig. 2 a-e).

From Fig. 2a and b, the density of states of BiSbO₄ and AgI are conveyed and bandgaps of the respective semiconductors were determined as lower than the experimental values. This result is also similar to what was found by most studies that determines both experimental and theoretical bandgaps like the work of Du and co-workers [41]. Fig. 2c and d shows that AgI semiconductor donate free electrons to BiSbO₄ and BiSbO₄ shows positive value indicating it is an electron accumulation side while the negative value exhibited by AgI edicts an electron transfer route across the heterojunction. Generally, DFT calculations successfully portrayed formation of interlayer at the interface, and that the exhibited accumulation of electrons on BiSbO₄ after equilibrium demonstrated charge transfer from VB of AgI to BiSbO₄ CB. DFT calculations offer a clear insight into interfacial charge transfer that occurs between two semiconductors which can then be employed to affirm the nature of the heterojunction. More details of validation of Z-scheme charge transfer mechanism under different applications can be found in a review by Bao and co-workers on S-scheme photocatalytic systems [7].

3.2. Experimental validations

Despite theoretical investigations towards validation of Z-scheme heterojunctions formation which is normally done with DFT calculations, numerous scientific methods are employed to validate the formation of Z-scheme charge transfer mechanism in a formed heterojunction. These methods are grouped based on their mode of operation (Fig. 2f) into (1) methods used to explore chemical state of sample (metal nanoparticle photodeposition, in situ irradiated X-ray photoelectron spectroscopy (XPS)), (2) methods reflecting band energy structure (Mott-Schottky (M−S) test, UPS), and (3) methods demonstrating redox products (reactive oxygen species (ROS) generation test, photocatalytic CO₂ reduction, ESR), and just recently microscopic time resolved imaging technique [26], [39], [63]. The use of a single validation method may however not be sufficient and numerous methods may be required as complementary experimental or theoretical evidence for affirmation of Z-scheme charge transfer.

In metal nanoparticle photodeposition, the principle adopted is the photodeposition of noble metal co-catalysts that are described as (M^z+ + ze⁻ →M⁰), where M is a metal ion with charge z required for its reduction to zero valence state [63]. The photoreduction is normally done on the surface of the semiconductor with the intended metal precursor with a tendency for the photodeposition to occur where there is accumulation of electrons. Since Z-scheme mechanism has distinctive reduction and oxidation sides, photodeposition will occur on the reductive or sometimes on the oxidative semiconductor [39], [64]. It is possible to predict the charge transfer mechanism under light irradiation in the presence of electron donor or acceptor sacrificial agents, based on the metal photodeposition as the photo-deposited metals can be seen under TEM and mapping will show on which semiconductor the photodeposition occurred.

XPS is used for chemical composition and surface oxidation states analysis and it has also proved to be a vital technique to study the charge transfer pathway in a heterojunction by exploring the relative shift in binding energy of individual elements in the composite [65]. When a foreign material is added to a semiconductor, the binding energy shift occurs subsequent to existence of electron transfer between them, where a positive shift in binding energy validates decrease in electron density of the element while a negative shift affirms increase in electron density. Therefore, a shift in binding energy in XPS spectra can be used to confirm electron transfer mechanism at the interface of a Z-scheme heterostructure. Jo et al. reported that the decrease in the intensity and shift to higher binding energy of elements in high XPS spectra of composites compared to pristine semiconductors confirmed Z-scheme charge transfer between g-C₃N₄/TiO₂ [66].

Mott-Scotty plots have also been implemented to understand band structure of individual composites that can then be used to predict the charge transfer dynamics in Z-scheme heterojunctions [67]. However, Mott-Schottky plots are more clearly used to further elucidate and complement the radical trapping experiments. In our previous work, we showed the use of M−S plots to portray formation of direct Z-scheme heterojunction between ceria and CN [68]. The M−S plots were used to give the band structure while the radical trapping experiments gave information on most active radicals which were used together to show existence of direct Z-scheme charge transfer between CN and ceria. Comparison of possible heterojunctions with generated radicals inevitably rules out formation of some heterojunctions and thus give insights into the charge transfer route. Also some reports exist for complementary use of M−S plots to further describe charge transfer in a Z-scheme [69]. Another common technique that is employed towards systematic analysis of charge transfer mechanism in Z-scheme is UPS spectra. In short, linear approximation of the UPS spectra gives the work function of the catalysts that can be used to determine their Fermi levels and valence and conduction bands. This can then be easily employed to determine the Z-scheme charge transfer at the interface and the details of this method is given by Aguirre et al. [70]. The surface potentials of semiconductors can be measured by spectroscopic techniques such as Kelvin probe force microscopy (KPFM), surface photovoltage spectroscopy (SPS) and transient absorption spectroscopy (TAS) to affirm Z-scheme charge transfer route [39], [71], [72], [73].

In radical trapping experiment, a chemical reagent is added to quench the effect of a particular set of expected radical species through capturing it [74]. Comparisons based on the activity with and without the radical trapping chemical reagent will give insights into existence and possible magnitude of the specific radical contribution [55], [75]. The understanding of the involved radicals aid towards prediction of charge transfer mechanism in a Z-scheme heterojunction [75]. Since the most common initiators of photocatalytic redox reactions stems from h⁺, hydroxyl radicals and superoxide anion radicals, radical scavengers for each one of them include TEOA, EDTA and ammonium oxalate (AO) for h⁺ [76], [77], tert-butyl alcohol (TBA) and isopropanol (IPA) for OH radicals [78], [79], and N₂ gas and p-benzoquinone (BQ) as superoxide anion radical quenchers [76], [80]. In our previous work, we proved existence of a Z-scheme charge transfer mechanism between CO₃O₄ and BiOI towards direct Z-scheme charge transfer using just radical trapping tests and some theoretical investigations that included electronic properties of the materials [74]. Similar reports exist from other groups for successful use of radical trapping experiments to confirm charge transfer pathway in Z-scheme [81].

In CO₂ photoreduction reaction, photoexcited electrons play a major role based on their conduction band edge potential energy after a heterojunction is formed. The product of a photoreduction reaction is used to predict the charge transfer mechanism when the band edge positions of the semiconductors are determined (experimentally or theoretically) such that if a traditional heterojunction is formed, the reduction in the redox capability of the semiconductor reaches a level that is undesirable for photoreduction of CO₂ to CO (-0.42 eV vs NHE), CH₃OH (-0.38 eV vs NHE) or CH₄ (eV vs NHE) [82], [83], [84], [85]. Therefore, CO₂ photoreduction gives different products based on the reduction potential [39]. Its application towards confirmation of Z-scheme transfer mechanism is portrayed in proposed Z-scheme BiOI/g-C₃N₄ towards CO₂ photoreduction under visible light by Wang and co-workers [83]. The use of CO₂ experiment towards confirmation of Z-scheme mechanism was employed after CB and VB potentials of BiOI were 2.42 and 0.67 eV, and that of g-C₃N₄ were determined to be 2.17 and − 0.52 eV, respectively. If the photocatalytic system obeys a traditional double charge transfer mechanism, the accumulated electrons on the conduction band of BiOI would be more positive than the CO₂ reduction potentials leaving the Z-scheme as the possible charge transfer mechanism. Studies towards confirmation of Z-scheme charge transfer mechanisms are influenced by (1) the reaction products obtained and (2) the band edge potentials of the individual semiconductors.

Electronic paramagnetic resonance (EPR) is a spectroscopic technique commonly employed towards systematic investigation of charge transfer mechanism at the interface of semiconductors. Its application in the determination of Z-scheme charge transfer mechanism has largely increased over the last decade and it is based on the determination of ^•OH and ^•O₂ ⁻ indirectly through use of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent to generate EPR measurable DMPO-^•OH and DMPO-^•O₂ ⁻ signals [84], [86], [87]. The increase of the signals intensity with increase in radiation time signals generation of OH radicals and superoxide anion radicals. From the present generated radicals and the band alignment of the individual semiconductors, the Z-scheme mechanism can be assured. In a study by Xu et al. [88], a Z-scheme mechanism was confirmed to occur during sulfadiazine (SDZ) oxidation and Ni(II) reduction by a Cu₂O/Bi/Bi₂MoO₆ composite ruling out the possibility of p-n heterojunction formation due to determination of ^•OH radicals signal in EPR that would not occur when holes concentrate on the VB of Cu₂O in traditional p-n heterojunction.

The ongoing attempts of studying charge transfer mechanism in Z-scheme resulted in novel methodology developments or modifications. Ebihara et al. applied pattern-illumination time-resolved phase microscopy towards in depth investigations that probe photogenerated charge carrier dynamics using a Mo-doped BiVO₄ and Rh- doped SrTiO₃ with indium tin oxide as the electron mediator [89]. They did not only manage to map reactive sites and recombination centres, but they also fruitfully observed position- and structure-dependency of photoexcited charge route. Therefore, characterization of Z-scheme photocatalytic systems promotes understanding that can be employed for optimisation of charge transfer and separation efficiency.

4.1. Pharmaceuticals degradation

Pharmaceuticals are classified as organic pollutants of concern by environmentalists while pharmaceuticals production and consumption has steadily escalated due to increased number of patients in need of medical attention due to sicknesses and symptoms concomitant with Covid-19 pandemic, and emotional traumas triggered by effects of the Covid-19 on the economy and freedom. Inevitable use of antiviral and antibiotic medications and their prescription has ensued their secretion into drinking water systems which is very precarious in different perspectives: (1) the pollutants bio-accumulate and their concentrations increase in drinking waters and can become deadly under acute or chronic exposures, (2) they can form extremely lethal metabolites when interacting with the ecosystem, (3) and they may result in another pandemic of drug resistance as elongated ingestion of pharmaceutical polluted water may lead to emergence of drug resistant strains of pathogens. Their removal from water bodies is justified with the use of advanced oxidation processes based heterojunctions, especially Z-scheme heterostructures formation, considered a go-to technique with possible sustainability and low cost when sunlight is used as activation energy source.

The applications of Z-scheme systems have increased exponentially in the past decade for pharmaceuticals pollution remediation because of their ability to reserve the strong redox capability of oxidative and reductive semiconductors, improved charge separation efficiency, and provision of distinctive redox reaction sites for high photoactivity. It has numerous fabrication strategies that have to be observed for better performance in pharmaceutical degradation. The staggered band alignment, and intimate contact or a charge transfer channel are required during construction of the Z-scheme charge transfer heterostructures [10], [35], [90]. The charge transfer channel in traditional Z-scheme systems entails utilization of reversible redox ion pairs in liquid phase which has not been largely applied in water treatment as some pollutants are either electron acceptors or donors that would quench the effective charge transfer mechanism in this system. All-solid-state Z-scheme heterostructures normally employ the use of solid redox mediators (SRMs) and they are employed for degradation of pharmaceuticals and pathogens inactivation. Direct Z-scheme encompass the generation of a direct contact between the semiconductors which is aided and enhanced by internal electric field (IEF), interfacial defect states, and selective facet design [35]. In essence, the optimisation of Z-scheme systems can be achieved by enhancing the charge transfer at the interface of the semiconductors. In indirect Z-scheme, the optimisation of the catalyst ratios and the amount of solid redox mediator forms an integral aspect for enhancement of charge separation efficiency which in turn boosts the photocatalytic performance [91]. Except the universal optimisation conditions (catalyst amount, reactor design, amount and pH of pollutant, photons flux, etc.), direct Z-scheme heterostructures performance is boosted immensely through an intimate hetero-interface contact with low charge resistance that is established through Coulomb electrostatic interaction, van der Waals (vdW) forces, hydrogen bonding, and chemical bonding [91]. Coulomb electrostatic interaction are physical mechanisms induced by negative and positive charged particles electrostatic adsorption, hydrogen bonding results in low resistance interfaces induced by hydrogen bonds, and van der Waals (vdW) forces emerge from strong interaction and distinctively formed face-to-face contact of 2D layers which facilitate interfacial charge transportation with low charge resistance heterointerfaces [92], [93]. This ensures the popularity of direct and indirect Z-scheme systems towards pharmaceuticals degradation and pathogens destruction from water with comparative data of conditions and efficiency summarised by Table 1 . Dual-Z-scheme heterostructures promises to be very effective compared to single direct or indirect Z-scheme heterojunctions due to (1) the increase in the redox capability of the system by increasing either its oxidation power or reduction power, and (2) the systematic growth of catalytic reaction centres which upturns degradation performance despite tumbling the potential upscaling of the system due to cumulative cost. In this section, the degradation of pharmaceuticals with Z-scheme heterojunctions with exhaustive analysis focused on antibiotics and non-steroidal anti-inflammatory drugs (NSAIDs) is explored.

Except the formed heterojunction in the composite that is used for pharmaceutical degradation, important factors during the process needs to be taken into consideration and optimised as they can negatively affect the efficiency of the process. These factors can be grouped into (1) conditions of the photocatalytic reaction reactor and (2) properties of the materials. The degradation of pharmaceuticals requires the reactor volume to be appropriate for the intended application and since one of the most important requirements is adequate interaction of pollutant and catalyst, stirring is important to enhance the adsorption of the pollutant on the catalyst surface [107]. Adsorption should be allowed to occur for a certain period of time until it reaches equilibrium. Moreover, the source of the reactor light is an important aspect as UV and visible light is required to initiate electrons and holes pairs formation in the Z-scheme heterojunction for generation of radicals that pioneer photocatalytic reactions [90], [107]. The proximity and energy of the light source determines photons flux and energy absorption by the catalyst. The source of light to be employed is determined by the optical properties of the individual or composite semiconductors as UV or visible light active semiconductors require UV or visible light sources, respectively. The semiconductor and the pollutant should possess different charges (positive and negative) to allow Coulomb force induced attraction for enhanced pollutant adsorption on catalyst. The pharmaceutical pollutant charges in water solutions are normally determined by their PKa which in influenced by solution pH and is negative or neutral in pH values higher and lower than the PKa value, respectively [106]. Moreover, the solution volume and concentration ratio to catalyst concentration should be optimised while other factors such as temperature and pH of solution, and the composition of the pollutant solution are of paramount importance. The presence of cations and anions may compete for adsorption sites with positive or negatively charged pharmaceutical pollutants and retard degradation efficiency. The presence of certain anions may also retard photocatalytic degradation efficiency by scavenging the robust hydroxyl radicals to form other radicals that are less reactive such as OH radicals quenching with chlorine containing anions [101].

4.2. Activation of persulfate

Application of sulfate radical (SO₄ ^•−, E = 2.5–3.1 eV, half-life: 30–40 μs) based advanced oxidation processes (SR-AOPs) has been regarded as a hotspot of innovative research towards environmental pollution remediation [127]. The production of sulfate radicals is achieved through activation of peroxymonosulfate (HSO₅ ⁻, PMS) or peroxydisulfate (PDS, S₂O₈ ²⁻) by either homogeneous or heterogeneous photocatalysts [128], [129]. Most recently, Z-scheme photocatalysts are used for heterogeneous activation of persulfate in photocatalytic removal of various water pollutants [127], [130], [131]. In heterogeneous activation of PMS/PDS, the other component if not both should be a metal or metals (Co, Mn, Fe, Cu, etc.) that can effectively break the O—O peroxo bond of PMS/PDS via redox mechanism to generate sulfate radicals. Since this bond is highly stable at room temperature, breaking it is only realized when high energy and chemical activators are used [128], [132], [133]. Some researchers employed heat, ultraviolet irradiation, and ultrasound system as source of energy, whereas chemical activators include sulfides, transitional metal ions and their metal oxides together with their complexes. However, the choice of chemical activators should be investigated properly as they introduce secondary pollution from toxic metal ions. The common utilised precursors for sulfate radicals’ generation possess different properties and that lead to difference in activation and catalytic power. The PDS has higher potential (E^° (S₂O₈ ²⁻/SO₄ ^•−) = 2.01 V_NHE) than PMS (E^° (HSO₅ ⁻/SO₄ ^•−) = 1.75 V_NHE) and its cost is estimated at 0.74 USD/kg (0.18 USD/mol) while 2.2 USD/kg (1.36 USD/mol) is the cost for PMS [134], [135]. Furthermore, PDS has a longer O—O peroxo bond distance (1.497 Å) as opposed to PMS (1.460 Å) and this makes it easier to be activated. For the purpose of this review activation of both precursors by Z-scheme photocatalysts will be discussed for degradation of pharmaceutical water pollutants. It should be noted that the higher degradation efficiencies of PMS/PDS based photocatalytic systems emanates from the production of reactive radical (SO₄ ^•−, ^•OH, and ^•O₂ ^–∙) and non-radical (¹O₂) species due to synergistic effect of PDS/PMS and Z-scheme systems. The synergy can be better understood by the description of the processes that are involved during the activation of persulfate with Z-scheme systems. The persulfate can be activated directly by UV radiation to form both sulfate and hydroxyl radicals Eqn. (5) which can degrade pharmaceuticals [136]. The initial step of Z-scheme activation of persulfate is the generation of electrons and holes under light irradiation by the Z-scheme heterostructure. The adjacent persulfate anions accept an electron or hole generated by the Z-scheme system to directly or indirectly transform into sulfate radicals [137]. The general equations involved in the step by step direct or indirect sulfate radicals formation by PMS and PDS normally occur in different mechanisms, and the example using PMS is summarised by Eqn. (6), (7), (8) [138] while that of PDS is summarised by Eqn. (9), (10). The sulfate radicals can also generate OH radicals while the superoxide anion radicals can be transformed to sulfate radicals increasing the flux of strong oxidising sulfate and OH radicals. The formed persulfate radicals on the surface of the catalyst, and the catalyst generated radicals results in increased generated radical’s flux that synergistically attacks adsorbed pollutants for robust degradation of pollutants. The ability of Z-scheme systems to have continuous generation of electrons enhances the generation of sulfate radicals for better performance as illustrated in Fig. 6 a.

However, the co-existence of sulfate and hydroxyl radicals may decrease the degradation of pharmaceuticals through their reaction that generate a non-radical PMS (SO₄ ^•− + ^•OH → HSO₅ ⁻) [139].

Ji et al. [132] prepared a perylene diimide (PMI)/MiL-101(Cr) (PMS) Z-scheme heterojunction using one-pot method and used it for activation of PMS under visible light for degradation of iohexol (IOH) drug. The authors achieved degradation efficiency of ∼100 % in an illumination time and PMS concentration of 35 min and 3.0 mM, respectively. The IOH degradation mechanism portrayed the production of reactive species for initiation of both radical and non-radical based reactions. Electron spin resonance (ESR) tests (Fig. 6b-c) indicated that participation of Z-scheme was vital as the signals for SO₄ ^•−, ^•OH, ^•O₂ ^–, and singlet oxygen (¹O₂) was produced in the presence of visible light. The presence of oxygen containing radicals and holes was also confirmed by scavenger experiments which showed that the holes and ^•OH were dominant radicals that contributed to effective IOH degradation. The results proved the concept of high charge separation and utilization of visible light irradiation for effective activation of PMS. Likewise, a 2D/2D CoAl-LDH/BiOBr Z-scheme photocatalyst was prepared via hydrothermal method and its application revealed 96 % removal of ciprofloxacin (CIP) on 8 wt% LDH/BiOBr/PMS/Vis system within 30 min at optimum catalyst and PMS doses of 40 and 100 mg, respectively. The CIP degradation was initiated by reactive radicals as shown in Fig. 6e. The authors allied the high CIP elimination to several factors such as strong intimacy interaction between the materials, high specific surface area (91.38 m² g⁻¹) which was higher than that of CoAl-LHD (50.80 m² g⁻¹), and BiOBr (30.60 m² g⁻¹), and enhanced charge carrier separation and migration as demonstrated by EIS, transient photocurrent response and PL results (Fig. 6f-i). The 8 wt% LDH/BiOBr/PMS/Vis system was further extended to TC (98 %), ENR (88 %), NOR (88 %), and RhB (100 %), respectively. It was discussed that the high degradation efficiencies obtained confirmed the synergistic effect between photocatalysis and PMS activation in the LDH/BiOBr/PMS/Vis system. The scavenger and EPR experiments indicated the prevalence of radical (^•O₂ ^–) and non-radical (¹O₂) degradation of CIP [131]. The synergy between photocatalysis and PMS/PDS activation is the cause of observed short degradation times compared to when only Z-scheme heterojunctions are employed under similar conditions without PMS/PDS oxidants, and the attained efficiencies of this synergy are normally close to 100 % and accomplishes almost complete mineralisation of organic pollutants in water. Z-scheme mechanism activation of PMS/PDS have been explored by other researchers [127], [130], [140], [141], [142], [143], [144] for pharmaceuticals and other organic pollutant elimination.

Recently, studies have shown that direct Z-scheme heterojunctions are preferred over all solid state/mediator-assisted Z-scheme heterojunctions for activation of PMS/PDS [130], [132], [141]. The mediator-assisted Z-scheme has been plagued with notable drawbacks such as the cost of noble metals normally used, absorption of photons and electrons by mediators which tend to limit the performance of the process while direct Z-scheme is intimately designed to operate as a sieve for the used photo-electron-hole pairs possessing low redox potential, thus minimising the light shielding effect and uplifting the redox capabilities [135]. The intimate contact in a direct Z-scheme heterojunction also induces formation of electric field which is vital for redox ability needed for redistribution of carrier charges. Afterwards, the Z-scheme photocatalytic system results in more positive valence band and negative conduction band which are vital for both PMS/PDS activation and absorption of more solar light throughout the entire solar spectrum.

4.3. Pathogens inactivation

Numerous micro-organisms or disease-causing pathogens have been determined in surface water making their removal very important. This review focusses on bacteria and virus inactivation only. Viruses have been evaluated directly for their photocatalytic removal with foodborne, airborne and waterborne viruses of supreme prominence [145]. The photocatalytic virus inactivation was first reported on TiO₂ photocatalyst by Sierka and Sjogren in 1994 leading to widespread application of photocatalysis for viral decomposition [146]. The mechanism (like in other pathogen degradation) involves generation of ROS like ^•OH and H₂O₂ that directly attacks and destroys the virus capsid or cell wall and the cytoplasm releasing contents inside the virus like genetic makeup and proteins [147]. The mechanisms of photocatalytic pathogens inactivation can be subdivided into metal ion toxicity, physical damage and chemical oxidation as suggested by Zhang et al. (Fig. 7 a) [1]. In brief, metal ion toxicity hastens inactivation kinetics through synergistic metal ions release and Z-scheme photocatalysis. The metals used in fabrication of Z-scheme systems can be leached into the surface of the pathogens and its synergistic involvement with Z-scheme is the one responsible for heightened virucidal activity. In physical damage, the Z-scheme photocatalyst ROS attack is induced and restricted to the capsid virus protein shells destruction and the contents (Virus RNA, etc.) in the cell leaks out and eventually the pathogen is inactivated after sometime. A chemical oxidation process involves the use of semiconductors under illumination by light of appropriate wavelength for initial generation of electrons and holes which are transformed through redox reactions towards formation of ROSs (^•OH, h⁺, ¹O₂, H₂O₂, and ^•O₂ ^–). The generated ROSs attacks the virus membrane and destruct it and continue to completely destruct the contents inside of the membranes to completely inactivate it. It is noteworthy that interaction of catalyst and virus surface, like in photocatalytic pollutant degradation, will enhance efficiency of the processes.

The most commonly studied inactivation of micro-organisms involves bacterial investigations. The antimicrobial evaluation of photocatalytic mechanism involves studies on different bacterial species. Bacterial contamination is a serious concern with numerous bacterial species determined to cause diseases that may result in death from severe diarrhoea making their inactivation a priority in surface and drinking water systems. Liu et al. studied Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) inactivation with direct Z-scheme BiO_2-x/BiOBr heterojunction synthesised in-situ with oxygen vacancies with bactericidal efficiency of 100 % in 20 min [45]. The increase in irradiation time supervenes decrease in the number of viable bacteria on the culture dish (Fig. 7b). In a recent review by Wang and co-workers [1], different g-C₃N₄ modifications like doping and heterojunction formation were summarised towards photocatalytic inactivation of different classes of bacteria with specific conditions and efficiency highlighted. Xia et al. hydrothermally synthesised a Z-scheme g-C₃N₄/m-Bi₂O₄ towards visible light driven inactivation of 6 log10 cfu/mL of E. coli K-12 in 1.5 h [105]. Fluorescence microscopy images after staining with fluorescent dye portrayed an increase of green cells and a decrease in red cells attributed to membrane rapture. SEM images affirmed destruction and distortion of the bacterial structure due to leaking out of important contents resulting in instantaneous inactivation of bacteria by metabolic arrest. Fig. 7c shows a schematic presentation of the step-by-step process that occurs during the inactivation of viruses or bacteria with Z-scheme heterojunctions. Firstly, there should be an interaction between the catalyst and the pathogen, under visible light irradiation, the catalyst attached on the cell wall of bacteria or virus generate ROSs that rapture the cell membrane. The attack leads to destruction of the membrane and the continued release of the ROSs would ensure that contents inside the cell membrane are also destroyed. There are numerous traditional, all solid state and direct Z-scheme heterojunctions associated studies that principally emphasise only bacterial inactivation or bacterial inactivation plus an additional application under visible light irradiation [98], [105], [148]. The illustration of the charge transfer at the interface during pathogens (virus) direct, indirect, and dual Z-scheme heterojunctions which are similar to pharmaceuticals (antibiotics) degradation to water and carbon-dioxide as shown (Fig. 8 a). Indirect Z-scheme comprises the use of an electron mediator as charge transfer route at the interface of the semiconductors to aid recombination of oxidation catalyst CB electrons with VB holes in a reductive semiconductor for enhanced generation of ROSs to initiate bacterial deactivation. In dual Z-scheme, the use of electron mediators may be avoided or collectively used with direct charge transfer with 2 oxidative semiconductors and 1 reductive semiconductor or 2 reductive semiconductors and 1 oxidative semiconductor for enhanced oxidative or reductive performance of the entire Z-scheme system towards antimicrobial performance, respectively.

The popularity of bacterial inactivation with different heterojunctions resulted in fabrication of handy prototypes towards real environmental applications of bacterial inactivation, giving hope for large scale applications of these systems. In medical applications, thermodynamic therapy involves the production of radicals to block electron transport to the tumor cells for energy and nutrients deficiency induced cell death [149]. In wound healing, the ROSs induced by light illumination inhibit bacterial growth resulting in healing of wounds. For example, an indirect Z-scheme ZnO/C-dots/g-C₃N₄ heterojunction fabricated through ultrasonication method was evaluated for wound healing by Xiang et al with antibacterial efficacy of 99.97 % and 99.99 % for S. aureus and E. coli after 15 min of visible light irradiation, indexed to synergistic contributions of ROSs generation and hyperthermia [150]. In their study, both gram-negative and gram-positive bacteria was selected for visible light driven inactivation on a spread plate method. The results of E. coli in 15 min showed no reduction of bacterial count with the control, little reduction with CN and complete removal with a Z-scheme ZnO/C-dots/g-C₃N₄ ternary heterojunction (Fig. 8b). The SEM analysis of the bacteria structure shows more distortion with holes when the Z-scheme ZnO/C-dots/g-C₃N₄ ternary heterojunction was employed compared to no changes and little change for control and CN, respectively (Fig. 8c). In-vivo wound healing was organized with rats wounds (20 mm diameter) infested with 20 µL of S. aureus (1 × 108 CFU/mL) to assess the wound therapeutic ability of the catalyst. The images of the wounds confirmed faster healing in 12 days when the Z-scheme catalyst is used compared to the control experiment after initial S. aureus infection after 2 days in both cases (Fig. 8d). Further inspection of the hematoxylin and eosin (H&E) was performed to stain wound tissues. After 2 days, the composite had less inflammatory cells (GRAN, neutrophil granulocyte, red arrows) than the control group, yet after 12 days, necrosis of epidermal cells and epidermal crevices were witnessed in the control group while restoration curative outcome, and unharmed hypodermal tissue arrangement was observed with Z-scheme composite group (Fig. 8e).

The direct degradation of viruses has been studied and it is an emergent area of research with numerous articles focusing on the topic of their possible removal with advanced oxidation processes in water, air and food [147], [151], [152], [153]. Z-scheme heterojunction inactivation of viruses is no exception, but only a handful of research outputs exist to this regard similarly to Z-scheme remediation of antiviral drugs in water systems. Zhang et al. used a facile solvothermal-hydrothermal method to fabricate a direct Z-scheme heterostructure of oxygen-doped graphitic carbon nitride microspheres (O-g-C₃N₄) and carbonation carbon (HTCC) for visible light inactivation of human adenovirus type 2 (HAdV-2) with virucidal efficiency of −5log[C/C₀] in 120 min [106]. The increase of temperature to 37 °C further ensued complete removal efficiency at pH 5. The viral particle damage was studied with TEM where after 120 min of irradiation, HAdV-2 structure was ruptured and only virus debris was observed indicating the severity and lethality of the all-organic O-g-C₃N₄/HTCC-2 Z-scheme heterostructure under visible light irradiation. The fabricated nanoparticles cytotoxicity was evaluated through the XTT assay with human A549 cells portraying that within photocatalytic inactivation dosages, the nanoparticles toxicity is negligible with 85 % remaining even at higher concentrations of 150 μg/mL. This validates the biocompatibility and the negligible cytotoxicity indexed to stability and chemical composition of the composites. Cheng and co-workers investigated Ag₃PO₄/g-C₃N₄ heterojunction with Z-scheme charge transfer at the interface synthesised by hydrothermal method towards complete inactivation of bacteriophage f2 with concentration of 3 × 10⁶ PFU/mL under visible light irradiation in 80 min [154]. Scavenger tests were performed to validate charge transfer mechanism of Z-scheme and humic acid retarded the virucidal activity of the composites. As with bacterial inactivation, increase with irradiation time decreased the cultured viral count with none observed after 80 min of irradiation. Numerous investigations into Z-scheme heterojunctions have propelled knowledge broadening proved by recent increase of publications in this regard with numerous novel discoveries like in-situ generation of electron shuttles in direct Z-scheme composites.

The above discussed methods fabricated with the indicated conditions, mechanisms and material configurations can be improved through structural modifications aimed at improving factors that affect pharmaceuticals degradation and pathogens inactivation like the photocatalysts light utilization, enhanced pollutant adsorption and interfacial charge transfer dynamics. Z-scheme heterostructure modifications and fabrication strategies are governed by geometry, morphology variations, or addition of another material for synergy [6], [155], [156]. The universal method that is employed as an improvement mechanism to Z-scheme systems performance is the directed modulation of its morphology. The alterations of morphology can significantly boost photodegradation efficiency of catalysts for pharmaceuticals degradation by reduction of the diffusion length of charge carriers. The variations of photocatalytic composites formation conditions (use of different methods) results in changes in morphology that can be targeted based on semiconductor properties. The modification of semiconductors band structure with doping, generation of defects or molecular grafting is another important materials modulation strategy that assists with increasing the efficient charge transportation [156], [157]. With vacancies as an example, charge transfer efficiency between PS I and PS II interface will be enhanced by generated defects such as oxygen vacancies, sulfur vacancies or dopant-induced interfacial defects to augment the interfacial charge transfer (Fig. 9 a). The induced defects (especially oxygen defects) increases the light harvesting ability of the photocatalyst composite, enhance interfacial charge separation, and increase available reaction sites for photocatalytic reactions.

The effect of coupling surface plasmon resonance materials (SPR) applied mainly to Z-scheme systems is a promising strategy (Fig. 9b). The semiconductor and SPR material synergy is reflected by superior light harvesting ability of the Z-scheme system in visible or NIR region of the solar spectrum and SPR-induced electromagnetic fields that can enhance charge separation at the semiconductor and SPR interfaces [35]. The SPR that are commonly used are noble metals [56]. The development of full solar spectrum (UV–vis-NIR) active photocatalytic systems is another interesting strategy that employs 100 % light source from the solar spectrum when non-linear up-conversion photoluminescence (UCPL) materials are integrated into Z-scheme systems [35]. Co-catalysts can be introduced to Z-scheme heterojunctions for effective suppression of photogenerated electron and hole pairs (Fig. 9c). This increases photocatalytic activity through reduction of some reactions activation energy and improves the charge transfer for recombination of useless electrons in the conduction band of the oxidative semiconductor and useless holes in the valence band of the reductive semiconductor with specific co-catalysts classified as electron co-catalysts and holes co-catalysts for electron or hole capturing semiconductors, respectively. Co-catalysts examples include carbon materials (rGO), noble metals (Pd, Pt), and some semiconductors (CeO₂, ZnO, etc.) [28]. This could further be described as cascade dual Z-scheme heterostructure when one semiconductor is used as both electrons and holes co-catalyst [158]. Another fabrication strategy that has been determined to increase photocatalytic performance efficiency of Z-scheme heterostructures include facet engineering. The different surface energies possessed by different crystalline facets of a single semiconductor can be tailored to enhance certain properties of Z-scheme heterojunctions. This property has been employed to channel charge transportation route and direction between Z-scheme semiconductor interfaces [35]. The most common example of this fabrication strategy with anatase TiO₂ co-exposed {1 0 1} and {0 0 1} facets as an oxidative semiconductor and reductive (PS I) semiconductor generates a Z-scheme charge transfer at the interface of {1 0 1} anatase TiO₂ exposed facets and PS I (Fig. 9d). Inter-semiconductor heterojunction will form when electrons transfer from {0 0 1) to {1 0 1} and holes transfer from {1 0 1} to {0 0 1} exposed facets of TiO₂. The incorporation of a PS I to {1 0 1} anatase TiO₂ exposed facets induces charge transfer from electron donor {1 0 1} facet to PS I valence hand to react with its holes resulting in Z-scheme mechanism. The geometry modulation has been employed for the degradation of pollutants with Z-scheme catalysts with Janus-type, core–shell, and surface decorated architectures commonly employed. Core-shell involves the complete coverage of a semiconductor to be a core of another catalyst with the size of the outer layer important to allow passage of light to the inner semiconductor to induce excited electrons and holes pairs (Fig. 9e). Janus type heterostructures can be varied into spherical or any similar shaped semiconductors attached parallel to one another (Fig. 9f). Surface functionalization may be applied for strong interaction of the semiconductors at the interface for improved charge transfer efficiency. Surface decorated systems strategic fabrication is governed by decorating the surface of one semiconductor with another with similar or different semiconductor shapes uniformly or non-homogeneously distributed (Fig. 9g) [159]. The fabrication strategies have been largely employed for direct Z-scheme heterojunctions due to their high comparative degradation activity for pathogen inactivation and pharmaceutical pollutants remediation compared to indirect Z-scheme heterojunctions.

5.1. Evolution of S-scheme

After numerous investigations of the direct Z-scheme, another mechanism referred to as the S-scheme was introduced in 2019 by Fu et al [160]. There is no difference in the reported S-scheme mechanism (Fig. 10 a) and the direct Z-scheme (Fig. 10b) based on materials and thermodynamic perspectives, and the pre-requisite conditions are the same with the same properties at the interfaces that results in their formation as evident from already reported S-scheme heterojunctions [61], [110], [123] in comparison with some direct Z-scheme heterostructures [113], [161], [162]. For example, as correctly pointed out by Liao and co-workers in their review based on Z-scheme catalytic systems, both S-scheme and direct Z-scheme mechanisms require a unique principle of high work function of oxidative semiconductor (PS II) compared to that of the reductive semiconductor (PS I) [163]. The build-in electric field is reported as the only means of interface charge recombination driving force in Z-scheme while coulomb attraction and bend bending also act as driving forces for interface recombination of charges in S-scheme system. Equivocally, some reports on Z-scheme mechanisms exist with Xu et al. reporting band bending induced by In-O-Cd bond as a driving force for interface charge recombination enhancement in ZnIn₂S₄/CdS material fabricated via cation exchange reaction towards efficient photoelectrochemical water splitting [164]. Therefore, the coulomb force, band bending and internal-build in electric field contribution for recombination of electrons and holes at the interface of semiconductors can be considered a good discovery for enhancement of knowledge and understanding in direct Z-scheme heterostructures. A strong recommendation is made that the word S-scheme can be regarded as another name for direct Z-scheme heterojunction by scientific community. It is reported that the S-scheme comprises mainly of two n-type photocatalysts in which one is oxidation photocatalyst and another reduction photocatalyst. The migration and movement of electrons follows a “step” (macroscopic viewpoint) or “N” (microscopic viewpoint) type for effective separation of carrier charges [34]. However, this kind of heterojunction is at its infancy stage and need to be explored more in the future and all-encompassing assessment with analogous scientific procedures maybe directed to determine a reasonable alteration with direct Z-scheme systems to evade confusion and misconception of forthcoming works.

5.2. Laboratory Up-scaling

The possible applicability of Z-scheme photocatalytic heterojunctions for real wastewater applications needs laboratory validations mimicking real wastewater compositions for proper analysis of actual expected behavioural activities and the minimum required time towards achieving meaningful efficiencies. In this regard, Wang et al. used a hydrothermal method to synthesise a 0D/2D/2D ZnFe₂O₄/Bi₂O₂CO₃/BiOBr double Z-scheme heterojunctions for visible light degradation of three tetracycline antibiotics like tetracycline (TC), oxytetracycline (OTC), and doxycycline (DOX) with 93 %, 90.1 %, and 89.4 % degradation efficiencies under sunlight and PMS system for 20 min [115]. Direct double Z-scheme mechanism was established with quenching experiments, DFT calculations, EPR measurements, and XPS valence band spectra with two oxidative semiconductors (Bi₂O₂CO₃ and BiOBr) and one reductive semiconductor (ZnFe₂O₄) forming an arrow-up heterojunction. From this study, despite the analysis of the effects of different anions, the analysis of different secondary effluents from a sewage treatment plant were used to prepare 20 ppm solutions of TC OTC, and DOX with 85.6 %, 82.8 %, and 82.5 % degradation efficiencies in 20 min, respectively. The small decrease of less than 10 % gave extraordinary confidence towards real wastewater applications with oxidants (PMS) enhanced dual Z-scheme photocatalytic systems in real wastewater matrixes for remediation of pharmaceuticals in polluted water.

Different reactor designs have been investigated to address the challenge of expensive filters required to dodge nanomaterials discharges into the environment. This dispersion which results in high quantum efficiency due to good pollutant and catalyst interaction is common despite causing secondary pollution [165]. The widely employed fluidized bed reactor contains a system for easy recovery of catalysts after application. However, if utilized for industrial, commercial, and domestic wastewater treatment plants, it should be integrated into the conventional plants as a tertiary treatment process following other tertiary processes like filtration and followed by discovery methods to reduce nanomaterials route into clean water which may leak into the solution from the fixed bed reactor (Fig. 10c). This reactor-type can allow batch and continuous modes of operation but necessary retention time will be required for pollutant degradation process in both systems with more time required in the former than the latter. The fluidized-bed photoreactors are designed such that the light source can either be tubular UV lams or LED lights placed at the centre, externally or wrapping the interior of the reactor walls with a barrier between them and the solution (Fig. 10d). The reviews on Z-scheme photocatalysis for degradation of pollutants do not have a discussion on possible application of Z-scheme photocatalytic reactors at pilot scale [91], [166]. Therefore, it is important to analyse this aspect with a view to encourage more studies to be tailored towards pilot scale testing of Z-scheme heterojunctions towards water pollution remediation for drinking water security and safety. Most reported Z-scheme photoreactors have been reported towards water splitting for HER and OER. Chandran et al. [167] assessed a micro-scale photocatalyst suspension in aqueous solution with a traditional Z-scheme charge transfer mechanism on a tandem particle-suspension reactor design for solar water splitting. Expedition of redox species conveyance between hydrogen and oxygen evolution reaction compartments to forfeit gas crossover is achieved through a porous separator while its superior performance is attributed to proton-coupled electron mediators (para-benzoquinone/ hydroquinone and iodide/iodate) with zero visible light absorption capacity. A 1 cm high reaction stall containing 2.2 mg of BiVO₄ and SrTiO₃: Rh particles was predicted to sustain reactor operational efficiency at 1 % with fast diffusive species transportation. Su et al. designed a mediator free Z-scheme twin reactor for isolated HER and OER during photocatalytic water splitting reaction [168]. A single Nafion membrane divided reactor was segmented into two distinctive chambers of CuFeO₂ and Bi₂₀TiO₃₂ as HER and OER powder catalysts, respectively. Reduced graphene oxides were used as electron mediators with branched copper wires employed to further enhance the electron collection in a Z-scheme interfacial charge transfer mechanism for water splitting. HER and OER efficiency was 2.23 and 1.14 in this reactor showing a stoichiometric ration of 2:1 with advantages such as hydrogen purification cost avoidance, and avoidance of potential explosion. It was suggested that the system can potentially be employed for wider range of applications like water pollution remediation. However, alarming scarcity of industrial systems in water purification sums up the lack of interest and trust from investors and thus limiting research aimed at large scale application of Z-scheme heterostructures and advanced oxidation processes in general.