Polyaniline–Titanium Dioxide Heterostructures as Efﬁcient Photocatalysts: A Review

: This review paper focuses on present issues concerning the use of polyaniline–TiO 2 heterostructures as potentially efﬁcient photocatalysts. Conducting polymers such as polyaniline (PANI) are used in the preparation of heterojunction systems with metal oxides like titania to overcome their inherent limitations, e.g., their sole absorption of UV light and overly fast recombination of charge carriers. This review discusses preparation methods, the properties of resultant products and mechanistic aspects. An important part of this paper is its presentation of the major challenges and future perspectives of such photocatalytic materials.


Introduction
Titanium dioxide (TiO 2 ; titania) has been recognized as the most promising photocatalytic material, with high application potential in the fields of water or air treatment (mainly in the photooxidation of organic compounds) and in solar energy's conversion into chemical energy in the obtention of valuable products such as hydrogen and hydrocarbons [1][2][3][4][5].Its main strengths include the optimal positions of conduction (CB) and valence bands (VB) in a semiconductor band structure, corresponding to the redox potentials of various important chemical transformations; its low cost and perfect stability toward photocorrosion and relative nontoxicity are the reasons for researchers' widespread interest.
However, to better utilize light (solar or artificial cheap sources, e.g., light-emitting diodes (LEDs)) and thus increase photocatalytic efficiency, it is necessary to overcome the weak points of titania.The first is the relatively wide band gap of TiO 2 (3.0-3.2 eV), which only corresponds to the photoabsorption of UV light.As this has been calculated in Abe's review, if all UV light (in solar spectrum) up to 400 nm were utilized, the solar light conversion efficiency for photocatalytic water splitting would only be 2% [4].The possibility of utilizing visible light up to 600 nm or 800 nm will lead to efficiencies of 16% and 32%, respectively (Figure 1).The second limitation is the recombination of photogenerated charge carriers (electrons and holes), which reduces overall quantum efficiency [6].During the recombination process, the excited electron reverts to the valence band without reacting to adsorbed species.
Many strategies have been designed to improve the photocatalytic performance of titania and overcome the above-mentioned problems.These range from intrinsic modifications of TiO 2 in fields such as particle morphology, particle size, specific surface area, porosity and density of surface or bulk defects [3,[7][8][9][10][11] to chemical modifications.Therefore, the introduction of visible light activity into titania chemical modifications is required, although the preparation of defective titanium dioxide could also be a promising direction [3].Most of these chemical modifications concern the metal and non-metal modification/doping of TiO 2 , dye-sensitization and the design of titania-other semiconductor heterojunction systems [1,3,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].In comparison to other modification strategies, the usage of non-metal components is still very promising.The use of noble metals or metal complexes could be an unreasonable solution because of the cost of these components, which can limit their practical applications.The use of carbon-, nitrogen-, boron-or sulfur-doping/modification is much cheaper than the metals enumerated above.It is necessary to mention that a consequence of the application of some carbon (e.g., pentaerythritol) or nitrogen (e.g., urea) precursors in the modification process of TiO 2 can be the formation of specific organic sensitizers on the titania surface, which is responsible for visible-light-induced photocatalytic activity [14,16].Another possibility regarding the use of an organic sensitizer for titania is the application of conducting polymers such as polyaniline.The conductivity of polyaniline changes upon its addition to TiO 2 ; therefore, perspectives regarding the application of such materials not only focus on photocatalysis but also on other fields, e.g., metal oxide semiconductor devices and microactuators [27].In this review, a brief overview of titaniapolyaniline systems as photocatalysts is presented in relation to current knowledge and perspectives of application.This review also indicates the main problems in interpreting the research (including mechanistic aspects) and the reproducibility of the results.Many strategies have been designed to improve the photocatalytic performance of titania and overcome the above-mentioned problems.These range from intrinsic modifications of TiO2 in fields such as particle morphology, particle size, specific surface area, porosity and density of surface or bulk defects [3,[7][8][9][10][11] to chemical modifications.Therefore, the introduction of visible light activity into titania chemical modifications is required, although the preparation of defective titanium dioxide could also be a promising direction [3].Most of these chemical modifications concern the metal and nonmetal modification/doping of TiO2, dye-sensitization and the design of titania-other semiconductor heterojunction systems [1,3,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].In comparison to other modification strategies, the usage of non-metal components is still very promising.The use of noble metals or metal complexes could be an unreasonable solution because of the cost of these components, which can limit their practical applications.The use of carbon-, nitrogen-, boron-or sulfur-doping/modification is much cheaper than the metals enumerated above.It is necessary to mention that a consequence of the application of some carbon (e.g., pentaerythritol) or nitrogen (e.g., urea) precursors in the modification process of TiO2 can be the formation of specific organic sensitizers on the titania surface, which is responsible for visible-light-induced photocatalytic activity [14,16].Another possibility regarding the use of an organic sensitizer for titania is the application of conducting polymers such as polyaniline.The conductivity of polyaniline changes upon its addition to TiO2; therefore, perspectives regarding the application of such materials not only focus on photocatalysis but also on other fields, e.g., metal oxide semiconductor devices and microactuators [27].In this review, a brief overview of titania-polyaniline systems as photocatalysts is presented in relation to current knowledge and perspectives of application.This review also indicates the main problems in interpreting the research

Electronic and Optical Properties of Polyaniline
Polyaniline (PANI) is an example of a polymer belonging to the conducting polymer (CP) group.One can classify this group as containing electrically conducting organic polymeric compounds.Other examples of CPs are polypyrrole (PPy), polythiophene (PTh), poly(3,4-ethylenedioxythiophene) (PEDOT) and polyacetylene (PAc).CPs can act as a semiconductor or a conductor.These compounds are conjugated and show π electron delocalization along their polymer backbone.This main property determines their unique electrical and optical properties [26,[28][29][30][31].The conjugated structure of CPs is a necessary condition for the formation of delocalized electronic states.The degree of delocalization influences the value of band gap energy, which determines the conductivity of CPs.Single and double bonds contain a localized σ-bond (chemically strong).In the case of double bonds, it is also possible to distinguish a localized π-bond (less strong than the σ-bond).Therefore, π-bonds are responsible for an easier delocalization of electrons (Figure 2) [29,32].
PANI has many advantages, such as its low cost, its high environmental stability and its ability to enact electric switching between its conductive and resistive states via doping processes; it is important to emphasize that PANI is characterized by its diversity of structural forms [33].Polyaniline is a mixed oxidation state-conducting polymer, which consists of reduced benzenoid amine units (-NH-) and oxidized quinoid imine units (=N-) (Figure 3).PANI's average oxidation state is denoted as 1 − x.When 1 − x = 0 PANI exists as fully reduced leucoemeraldine (LE).If the value 1 − x is equal to 0.5, the half-oxidized Crystals 2023, 13, 1637 3 of 22 emeraldine base (EB) is present.The fully oxidized form of PANI is pernigraniline (PE) when 1 − x = 1 [28,34].
condition for the formation of delocalized electronic states.The degree of delocalization influences the value of band gap energy, which determines the conductivity of CPs.Single and double bonds contain a localized σ-bond (chemically strong).In the case of double bonds, it is also possible to distinguish a localized π-bond (less strong than the σ-bond).Therefore, π-bonds are responsible for an easier delocalization of electrons (Figure 2) [29,32].[32].
PANI has many advantages, such as its low cost, its high environmental stability and its ability to enact electric switching between its conductive and resistive states via doping processes; it is important to emphasize that PANI is characterized by its diversity of structural forms [33].Polyaniline is a mixed oxidation state-conducting polymer, which consists of reduced benzenoid amine units (-NH-) and oxidized quinoid imine units (=N-) (Figure 3).PANI's average oxidation state is denoted as 1 − x.When 1 − x = 0 PANI exists as fully reduced leucoemeraldine (LE).If the value 1 − x is equal to 0.5, the half-oxidized emeraldine base (EB) is present.The fully oxidized form of PANI is pernigraniline (PE) when 1 − x = 1 [28,34].If the EB form of PANI is doped with inorganic acids and organic acids such as phosphoric acid, hydrochloric acid, sulfosalicylic acid and sulfuric acid, emeraldine salt (ES) is then formed.In this process, imine nitrogen is protonated by acid, and the conductivity of ES significantly increases in comparison to other forms of PANI (Table 1) [35].Furthermore, as has been presented in Table 1, enumerated forms of PANI are characterized by different values of band gap energy.The colors of these materials are white, blue/violet, blue and green for LE, PE, EB and ES, respectively [34,35].[34][35][36][37][38].
PANI has many advantages, such as its low cost, its high environmental stability and its ability to enact electric switching between its conductive and resistive states via doping processes; it is important to emphasize that PANI is characterized by its diversity of structural forms [33].Polyaniline is a mixed oxidation state-conducting polymer, which consists of reduced benzenoid amine units (-NH-) and oxidized quinoid imine units (=N-) (Figure 3).PANI's average oxidation state is denoted as 1 − x.When 1 − x = 0 PANI exists as fully reduced leucoemeraldine (LE).If the value 1 − x is equal to 0.5, the half-oxidized emeraldine base (EB) is present.The fully oxidized form of PANI is pernigraniline (PE) when 1 − x = 1 [28,34].If the EB form of PANI is doped with inorganic acids and organic acids such as phosphoric acid, hydrochloric acid, sulfosalicylic acid and sulfuric acid, emeraldine salt (ES) is then formed.In this process, imine nitrogen is protonated by acid, and the conductivity of ES significantly increases in comparison to other forms of PANI (Table 1) [35].Furthermore, as has been presented in Table 1, enumerated forms of PANI are characterized by different values of band gap energy.The colors of these materials are white, blue/violet, blue and green for LE, PE, EB and ES, respectively [34,35].[34][35][36][37][38].

PANI Form
Conductivity/S cm −1 Band Gap Energy/eV Leucoemeraldine 10 −5 -10  If the EB form of PANI is doped with inorganic acids and organic acids such as phosphoric acid, hydrochloric acid, sulfosalicylic acid and sulfuric acid, emeraldine salt (ES) is then formed.In this process, imine nitrogen is protonated by acid, and the conductivity of ES significantly increases in comparison to other forms of PANI (Table 1) [35].Furthermore, as has been presented in Table 1, enumerated forms of PANI are characterized by different values of band gap energy.The colors of these materials are white, blue/violet, blue and green for LE, PE, EB and ES, respectively [34,35].[34][35][36][37][38].

PANI Form
Conductivity/S cm −1 Band Gap Energy/eV The ES form of PANI is the main candidate for a good photocatalytic material, owing to its high electron transfer properties in conjunction with its low band gap energy.The reason for this can be the polaron band formation that occurs.Furthermore, the result of partial oxidation is the occurrence of crystal lattice distortion, which can be responsible for the formation of polaron and bipolaron bands.Polaron formation is due to an upward shift in the highest occupied molecular orbital (HOMO) and a downward shift in the lowest unoccupied molecular orbital (LUMO), which results in a reduction in band gap energy.In general, the π-π* conjugate electron and polaron band transition system improves electron mobility, positively influencing photocatalytic properties through a reduction in the charge carrier recombination effect [35,39].The low band gap is mainly due to the charge transfer exciton, such as the transition of the benzenoid rings from the HOMO (π b ) to the LUMO (π q ).Another transition, referred to as the UV region, is assigned to the π b → π* transition.Furthermore, ES-PANI behaves as a p-type semiconductor [37].
The optical properties of PANI are closely related to the form of PANI and preparation/modification conditions.For example, Figure 4 shows changes in the absorption spectrum of EB under different conditions of its protonation [34].EB has an absorption peak at 2.1 eV due to electronic excitation from the benzenoid (π b ) to the quinoid rings (π q ).When pH changes to 3, absorption at 2.1 eV shifts to 1.5 eV.The reason for this is the lattice distortion of polyaniline into a polaronic structure as a result of the protonation of imine nitrogen atoms.Its peak at 3.9 eV can be attributed to the following transitions: (1) π-π* transition, which is similar to that of LE; (2) the transition from low-lying orbitals to the π q orbital.However, an absorption study on the different concentrations of ES in 80% acetic acid did not show any interchain interactions [34].Generally, regardless of the form, PANI has a high absorbance coefficient over a wide spectral range, from the Vis to IR region [35].
preparation/modification conditions.For example, Figure 4 shows changes in the absorption spectrum of EB under different conditions of its protonation [34].EB has an absorption peak at 2.1 eV due to electronic excitation from the benzenoid (πb) to the quinoid rings (πq).When pH changes to 3, absorption at 2.1 eV shifts to 1.5 eV.The reason for this is the lattice distortion of polyaniline into a polaronic structure as a result of the protonation of imine nitrogen atoms.Its peak at 3.9 eV can be attributed to the following transitions: (1) π-π* transition, which is similar to that of LE; (2) the transition from lowlying orbitals to the πq orbital.However, an absorption study on the different concentrations of ES in 80% acetic acid did not show any interchain interactions [34].Generally, regardless of the form, PANI has a high absorbance coefficient over a wide spectral range, from the Vis to IR region [35].

The Concept of Improving Photocatalytic Activity via Binary System: Metal Oxide-Polyaniline Heterostructure Nanocomposites
Referring to the previous chapter, the chemical structure and form of PANI determine its electronic and optical properties.It is known that PANI usually behaves as a p-type semiconductor.This means that PANI has an E f (Fermi level) close to the HOMO-it can receive electrons and demonstrate downward band bending [38].PANI has the LUMO level in the range of −1 to −2 V vs. NHE and has the HOMO level in the range of 0.4-0.6V [35,37,40].Therefore, the formation of heterojunctions between PANI and metal oxides such as TiO 2 is possible.
First, if two semiconductors with different redox energy levels are coupled, it can lead to an increase in photocatalytic performance by improving charge carrier separation.Second, it is important that the conditions for an efficient interparticle transfer between the semiconductor and TiO 2 are met; the CB of titania is more anodic than the corresponding band of the sensitizer.Then, if one only considers the visible light activation of a semiconductor-sensitizer, this component is excited, and electrons photoexcited to its CB are injected into the conduction band of the titania [22,24,[41][42][43].
The coupling of an n-type semiconductor (e.g., TiO 2 ) with a p-type semiconductor (e.g., PANI) is advantageous from a photocatalytic point of view.This combination of semiconductors can ensure an efficient electron transfer between them [43].Given the band positions of PANI and n-type semiconductors, such as metal oxides, it is possible to design a heterojunction system with a high photocatalytic performance (Figure 5, left).The band positions of PANI and TiO 2 form a type II heterojunction (staggered band pattern) (Figure 5, right).Considering the fact that PANI can be activated by visible light, the following main advantages of TiO 2 properties can be mentioned: (1) the visible light sensitization of a titania-containing system, (2) the improvement of charge carrier separation.Therefore, Crystals 2023, 13, 1637 5 of 22 a synergistic effect from these two semiconductor components can be expected.As for the PANI/TiO 2 ratio, the influence of this issue on photocatalytic activity is often studied; in most research reports, the amount of titania is higher than PANI.
semiconductors can ensure an efficient electron transfer between them [43].Given the band positions of PANI and n-type semiconductors, such as metal oxides, it is possible to design a heterojunction system with a high photocatalytic performance (Figure 5, left).The band positions of PANI and TiO2 form a type II heterojunction (staggered band pattern) (Figure 5, right).Considering the fact that PANI can be activated by visible light, the following main advantages of TiO2 properties can be mentioned: (1) the visible light sensitization of a titania-containing system, (2) the improvement of charge carrier separation.Therefore, a synergistic effect from these two semiconductor components can be expected.As for the PANI/TiO2 ratio, the influence of this issue on photocatalytic activity is often studied; in most research reports, the amount of titania is higher than PANI.

PANI-TiO2 Binary Heterojunction Systems-The State of the Art
A prospective photocatalytic material with high application potential should meet three main criteria: (1) high photocatalytic performance over a broad light spectrum in major reactions, such as the degradation of organic pollutants, hydrogen production and CO2 reduction in high-value-added products; (2) reasonable cost for reproducible and

PANI-TiO 2 Binary Heterojunction Systems-The State of the Art
A prospective photocatalytic material with high application potential should meet three main criteria: (1) high photocatalytic performance over a broad light spectrum in major reactions, such as the degradation of organic pollutants, hydrogen production and CO 2 reduction in high-value-added products; (2) reasonable cost for reproducible and large-scale production; (3) high stability (the retention of photocatalytic properties after many cycles).A major problem concerning these standards is that, as more components are introduced into the structure of the resultant material, its stability tends to decrease.Therefore, this review focuses on binary PANI and TiO 2 heterojunctions to better understand the great potential of this type of material.However, many examples of ternary or multicomponent composites containing both PANI and TiO 2 can be found in the literature [45][46][47][48][49][50][51][52][53][54][55][56].Table 2 summarizes the binary PANI-TiO 2 heterostructures acting as photocatalysts.Three main areas were considered: preparation conditions, photocatalytic efficiency and aspects of the photocatalytic mechanism.

Preparation Conditions
The main method for the preparation of PANI-TiO 2 composites is the in situ chemical oxidative polymerization of aniline, using ammonium persulfate (APS) as an oxidant.This process occurs in the presence of TiO 2 or a titania precursor.Key reaction parameters, such as the molar ratio of aniline/APS and aniline/TiO 2 , are different.Furthermore, polymerization conditions vary as well: polymerization time ranges from 1 h to 20 days, and process temperature ranges from ice bath conditions to room temperature.It is therefore difficult to find the optimal variant for this method.However, there are several reports that take into account the dominant amount of PANI.It is usually possible to find the best PANI/TiO 2 ratio for photocatalytic performance.Figure 6 shows this type of study with 2% PANI content as the best amount for photocatalytic activity [57].In general, these values vary in other reports according to experimental conditions.and process temperature ranges from ice bath conditions to room temperature.It is therefore difficult to find the optimal variant for this method.However, there are several reports that take into account the dominant amount of PANI.It is usually possible to find the best PANI/TiO2 ratio for photocatalytic performance.Figure 6 shows this type of study with 2% PANI content as the best amount for photocatalytic activity [57].In general, these values vary in other reports according to experimental conditions.A much smaller number of papers consider other preparation methods.Some reports include PANI and TiO 2 impregnation procedures [58][59][60][61].A novel, green and promising procedure for the preparation of PANI/TiO 2 was proposed by Cionti et al. [62].Figure 7 shows the main steps of this procedure: (1) UV light-mediated growth of PANI oligomers on a TiO 2 surface, (2) polymerization of the oligomers with H 2 O 2 .
The identification of the PANI form (oxidation state) present in a resultant product is not a common procedure.Only some authors include information about the PANI form.As for the form of TiO 2 that forms a part of the composites discussed, it is a commercial material in many cases, such as P25, which contains a mixture of anatase (85%) and rutile (15%).Some authors directly used a titania precursor in a polymerization procedure.Attempts have also been made to obtain special morphological forms of TiO 2 , such as a self-assembled monolayer [63], mesoporous material [64][65][66], flowerlike NPs [67], nanotubes [68,69], nanosheets [60], nanofiber membranes [70], fiber films [59] and nanorods [71].Some works have described the possibility of using defective forms of titania, e.g., grey or black titania [72,73].There are also reports of modified forms of TiO with noble metals such as Au, Pt and Ag [58,[74][75][76], core-shell structures with SiO 2 [77] and combinations with reduced graphene oxide [78].Furthermore, several applicable forms of PANI/TiO 2 have been considered, such as gypsum plasters [79], acrylic pseudo-paints [80], cotton fabric [81] and polyurethane foam [82].
The identification of the PANI form (oxidation state) present in a resultant product is not a common procedure.Only some authors include information about the PANI form.As explained earlier, the form of PANI directly affects its photocatalytic properties.Therefore, it is difficult to discuss the mechanism of photocatalytic action without this knowledge.There are also a very limited number of approaches for obtaining PANI with different morphological forms [83].

Photocatalytic Efficiency
Testing the photocatalytic efficiencies of PANI/TiO 2 is another crucial issue.As many as 65% of reports concern the use of organic dyes as model compounds, such as methylene blue, rhodamine B or methyl orange.This approach is highly controversial, especially for testing under visible light conditions [84].The photosensitizing effect of the dye itself, induced by visible light, can cause the misconception that a given photocatalyst enacts real activity under visible light.Fortunately, authors included colorless organic compounds, such as phenol or 4-chlorophenol, in some of their reports.Another issue is the need to study the effect of dye adsorption and include this information in the graphs.Therefore, the selection of dyes as model compounds for photocatalytic experiments requires careful planning.
When the types of photocatalytic reactions are considered, most of the publications are concerned with the photocatalytic oxidation of organic compounds, and the photocatalytic efficiencies reported in both UV and Vis were high in this field, but the materials from some of the papers, especially when methylene blue was used under visible light conditions, should probably be retested, using other model compounds.As shown in Figure 8, more than 80% of 4-chlorophenol was degraded after 7 h, confirming the synergistic effect of PANI and TiO 2 [64].Considering other reaction types, only 12% of the publications are concerned with testing hydrogen evolution process [72,75,76,78,85], and one report deals with photocatalytic reduction in carbon dioxide [86].These reaction fields still require further research in order to test the suitability of these materials.It should be noted that most PANI/TiO 2 photocatalysts that are active in hydrogen evolution contain modified titania (with noble metals or with defective structure-grey titania).Moreover, the work of El-Bery et al. (Figure 9) demonstrated that the PANI/TiO 2 sample is more active in photocatalytic hydrogen evolution than PEDOT/TiO 2 and PPy/TiO 2 , confirming the good prospects of PANI composites with titania compared to other conductive polymers [61].
When the types of photocatalytic reactions are considered, most of the publications are concerned with the photocatalytic oxidation of organic compounds, and the photocatalytic efficiencies reported in both UV and Vis were high in this field, but the materials from some of the papers, especially when methylene blue was used under visible light conditions, should probably be retested, using other model compounds.As shown in Figure 8, more than 80% of 4-chlorophenol was degraded after 7 h, confirming the synergistic effect of PANI and TiO2 [64].Considering other reaction types, only 12% of the publications are concerned with testing hydrogen evolution process [72,75,76,78,85], and one report deals with photocatalytic reduction in carbon dioxide [86].These reaction fields still require further research in order to test the suitability of these materials.It should be noted that most PANI/TiO2 photocatalysts that are active in hydrogen evolution contain modified titania (with noble metals or with defective structure-grey titania).Moreover, the work of El-Bery et al. (Figure 9) demonstrated that the PANI/TiO2 sample is more active in photocatalytic hydrogen evolution than PEDOT/TiO2 and PPy/TiO2, confirming the good prospects of PANI composites with titania compared to other conductive polymers [61].

Photocatalytic Mechanism
Upon analyzing the mechanisms of photocatalytic reactions based on the PANI/TiO 2 materials presented in the publications surveyed, there is general agreement on the mechanistic routes.Figure 10 shows the mechanism of reactive oxygen species generation under UV and Vis conditions using PANI/TiO 2 photocatalysts.As shown in Figure 5, the HOMO of PANI is located between the VB and CB bands of titania, while the LUMO of PANI is located higher than the CB of TiO 2 .This has been experimentally verified and reported by Hidalgo et al. [85].Considering UV radiation (Figure 10a), both PANI and TiO 2 absorb photons.The charge is immediately transferred from the HOMO to LUMO levels in PANI.Photogenerated electrons are transferred from the LUMO of PANI to the CB of TiO 2 .An electron center is formed.Charge separation is enhanced, and superoxide radicals are generated in the presence of oxygen (reduction reaction).Subsequently, these radicals react with water to form hydroxyl radicals (OH • ).Simultaneously, photogenerated holes from the VB of TiO 2 migrate to the surface of the HOMO of PANI.A hole center is then formed, which is involved in the oxidation of water to OH • (oxidation reaction).The resulting reactive oxygen species (mainly hydroxyl radicals) participate in the oxidation of organic compounds.The synergistic effect of PANI and TiO 2 relates to an efficient charge carrier separation, which reduces the recombination effect and increases photocatalytic activity [87,88].
Under visible light irradiation, only the PANI component is excited (Figure 10b).The charge migrates from the HOMO to LUMO of PANI.The excited-state electrons from PANI can then be transferred from the LUMO of PANI to the CB of TiO 2 .An electron center is formed.Simultaneously, the transitions π → polaron and polaron → π* in PANI are induced.Photogenerated holes in the π orbital of PANI are transferred to the interface and react with water to form hydroxyl radicals.The synergistic effect of PANI and TiO 2 is associated not only with visible light sensitization of TiO 2 through PANI but also with improved charge separation [87,88].
There are also reports of modifications to the PANI-TiO 2 binary system.The modification of the mechanism is the result of such a concept.For example, Zhang et al. modified the PANI-TiO 2 composite with gold [58].In this mechanism, additional hot electrons from Au NPs are injected into the CB of titania, inducing a visible-light-mediated surface plasmon resonance (LSPR) mechanism (Figure 11).The presence of Au NPs in this system enhances the charge separation and visible light sensitization effect due to the additional synergy between Au@TiO 2 and PANI-TiO 2 .There are also reports of modifications to the PANI-TiO2 binary system.The modification of the mechanism is the result of such a concept.For example, Zhang et al. modified the PANI-TiO2 composite with gold [58].In this mechanism, additional hot electrons from Au NPs are injected into the CB of titania, inducing a visible-light-mediated surface plasmon resonance (LSPR) mechanism (Figure 11).The presence of Au NPs in this system enhances the charge separation and visible light sensitization effect due to the additional synergy between Au@TiO2 and PANI-TiO2.The role of the core-shell structure in visible light activity.
[64] PANI nanofibers/TiO 2 Aniline (0.91 mL) was used as a precursor; 24 h polymerization occurred at 0 • in the presence of HCl.Finally, the product was dispersed in absolute ethanol to form the nonaqueous suspension of PANI nanofibers.The PANI/TiO 2 mass ratio of 1/100 was optimal.
Photodegradation of methylene blue and rhodamine B was performed under UV and Vis (λ > 420 nm).UV: activity (both MB and RhB) was similar to pure TiO 2 ; Vis: activity was 8 and 2.5 times better than pure titania for MB and RhB, respectively.
UV: proposed passivation of photocatalytic activity by PANI layer; Vis: proposed sensitization mechanism of TiO 2 by PANI.
UV: proposed passivation of photocatalytic activity by PANI layer; Sunlight: proposed sensitization mechanism of TiO 2 by PANI.
[67] PANI/TiO 2 Aniline was used as a precursor; polymerization for 3 h occurred at 0 • C in the presence of 2.5 g TiO 2 and HCl; hydrothermal treatment was subsequently performed; different molar ratios of TiO 2 and aniline were used: 4:1, 2:1 and 1:1.

PANI-TiO 2 Material Preparation Conditions Photocatalytic Efficiency Type of Photocatalytic Mechanism
Ref.
PANI/TiO 2 film PANI was first synthesized from aniline dimer with the addition of poly (sodium 4-styrenesulfonate), which was used as an emulsioning and doping agent to stabilize a structure; next, green powder of PANI was deposited in the TiO 2 films through a dip-coating method.
Photoelectrochemical behavior was evaluated under UV-Vis, using prepared photoanodes for the water photoelectrolysis reaction in a NaOH solution; an increase in photocatalytic activity was observed compared to unmodified titania; ES form of PANI was more active than the PE form.
Enhanced electron transfer and efficient charge separation.
[85] PANI/TiO 2 Aniline (2.5 mmol) was used as a precursor; polymerization occurred for 12 h in the presence of various amounts of TiO 2 with respect to aniline: 0, 5, 10, 15 and 20 wt%; dark green powder was obtained.
Photodegradations of rhodamine B, methylene blue and phenol were performed under UV light; photocatalyst with 20 wt% of titania displayed the highest activity (>80% for RhB after 3 h of irradiation).All modified photocatalysts were more active than pristine TiO 2 .
Photodegradations of rhodamine B and methylene blue were performed under visible light (λ > 400 nm).The photodegradation efficiencies for RhB and MB were 99% and 97%, respectively, for the sample with the molar ratio (TiO 2 /ANI) 40:1.
Proposed sensitization mechanism of TiO 2 by PANI, supported by the presence of Au NPs (surface plasmon resonance effect).
[58] PANI/TiO 2 Aniline was used as a precursor; polymerization occurred for 24 h at room temperature in the presence of 0.8 g TiO 2 ; different weight ratios of PANI to titania were considered: Aniline was used as a precursor; polymerization occurred for 3 days in the presence of TiO 2 ; in the next step, carbonization in argon atmosphere was performed; different mole ratios of titania/aniline were considered: 20, 50 and 80.
Photodegradations of rhodamine B and methylene blue were performed under simulated solar irradiation; photocatalytic performance was optimum for molar ratio 80:1: almost 100% of MB degradation (60 min) and 97% of RhB degradation (90 min) occurred.
The role of the carbonization process-changes in TiO 2 structures-for the improvement of charge separation.
[99] PANI/TiO 2 Aniline was used as a precursor; polymerization occurred for 24 h at room temperature in the presence of 0.512 g of TiO 2 ; different molar ratios of aniline to titania were considered: 1/6, 1/5, Photodegradation methylene blue was performed under UV/Vis irradiation; the optimum photocatalytic performance was for the sample with molar ratio 1/5: 60% of MB degradation occurred during 180 min, and for pristine titania, it was 30%.
Proposed sensitization mechanism of TiO 2 by PANI.
[100] PANI/grey-TiO 2 Aniline was used as a precursor; polymerization occurred for 3 h at 0 • C in the presence of 2 g of grey-TiO 2 ; different weight percentages for aniline were considered: 2.5, 5 and 12 wt%.
Photodegradations of rhodamine B and hydrogen evolution were performed under visible light (λ > 420 nm).The most active sample for aniline weight content of 5 wt%; for RhB resulted in almost 100% degradation (180 min); for H 2 evolution: 9 mmol/g after 5 h of irradiation; activities were better than for single grey-TiO 2 .
PANI serves as a co-catalyst in promoting separation of photoinduced charges, and photoelectrons and holes can efficiently transfer at the interface of PANI and grey-TiO 2 .
[72] PANI/black anatase TiO 2 (BAT) Aniline (0.2 M) was used as a precursor; polymerization occurred for 16 h at 0-4 Aniline was used as a precursor; polymerization occurred for 6 h at 5 • C in the presence of 1 g of TiO 2 ; dark green materials were obtained; for the preparation of pseudo-paint films, the aqueous dispersion of dodecyl benzene sulfonic acid and PANI/TiO 2 were prepared and added to the acrylic latex.Pt-loaded TiO 2 NPs were prepared via photoreduction method; next, aniline was used as a precursor; polymerization occurred for 6 h at 0 • C in the presence of 0.5 g of Pt-TiO 2 ; aniline/titania ratio was 4/100.
Proposed sensitization mechanism of Pt-TiO 2 by PANI. [76] PANI/M-TiO 2 Aniline was used as a precursor; polymerization occurred for 12 h at 0 Photocatalytic hydrogen production was performed under UV light (LED).The highest efficiency was for samples with 5 wt% content of PANI; 15 mmol g −1 of hydrogen evolved during 5 h; this result was better than for other titania composites containing different conducting polymers such as PEDOT (10 mmol g −1 ) and PPy (9.5 mmol g −1 ).

PANI/ TiO 2 -Ag
The first step: the preparation of Ag-TiO 2 NPs; The second step: the use of aniline as a precursor; polymerization occurred for 4 h in ice bath conditions in the presence of Ag-TiO 2 ; different amounts of Ag-TiO 2 were used: 5 wt%, 9 wt%, 11 wt% and 13 wt%.
Photocatalytic disinfection activity was tested using E. coli under visible light; the best sample for disinfection efficiency was a composite with 13 wt% content of Ag-TiO 2 ; photodegradation of lipopolysaccharides were performed under visible light; ca.40% of endotoxin was degraded in 180 min.Not studied.
[74] The first step was the preparation of core-shell SiO 2 @TiO 2 particles; subsequently, PANI was deposited on SiO 2 @TiO 2 ; aniline was used as a precursor; polymerization occurred in the presence of SiO 2 @TiO 2 ; molar ratios between aniline and TiO 2 were as follows: 1: Photodegradation of RfOM (refractory organic matter) was performed under simulated sunlight; the synergistic effect between PANI and TiO 2 was more pronounced in lower PANI ratios, whereas higher PANI ratios reflected a retardation effect up to a specific mass ratio of polyaniline to TiO 2 .
Not studied.[106] modification of the mechanism is the result of such a concept.For example, Zhang et al. modified the PANI-TiO2 composite with gold [58].In this mechanism, additional hot electrons from Au NPs are injected into the CB of titania, inducing a visible-light-mediated surface plasmon resonance (LSPR) mechanism (Figure 11).The presence of Au NPs in this system enhances the charge separation and visible light sensitization effect due to the additional synergy between Au@TiO2 and PANI-TiO2.

Conclusions
Binary PANI-TiO 2 heterostructures are very promising photocatalytic materials.They can be used in many types of reactions, such as the photooxidation of organic compounds, photocatalytic hydrogen generation and carbon dioxide reduction.These materials can be efficient in both UV and Vis due to the synergistic effect of both components.This effect manifests itself in two aspects: (1) the presence of PANI as an effective visible light sensitizer of titania and (2) the formation of a heterojunction (type II) between PANI and TiO 2 , which is a decisive factor in reducing the charge carrier recombination effect and increasing the efficiency of UV-induced reactions.Based on the literature survey, the following conclusions and remarks can be drawn: (I) there are reports on the use of different morphological forms of titania, but there are still too few reports on the influence of different forms of TiO 2 used in heterostructures with PANI on the efficiency of these binary systems; (II) there are almost no reports on the evaluation of the influence of PANI with different properties, e.g., particle size on photocatalytic activity, including the use of different preparation methods of PANI or PANI-TiO 2 heterostructures; (III) the model organic compounds in the photooxidation reactions were mainly dyes, making it advisable to use other, preferably colorless, organics, e.g., phenols, especially for visible light-induced reactions; (IV) mechanistic studies of photocatalytic reactions based on PANI-TiO 2 materials should be further investigated via experiments to confirm existing concepts or to propose other mechanism variants; (V) research into alternative, simple and low-cost preparation methods of PANI-TiO 2 structures with high application potential is highly desirable; (VI) studies on the stability of these heterostructures should be more frequently included in research reports.

Figure 1 .
Figure 1.Solar spectrum and maximum solar light conversion efficiencies for a photocatalytic water-splitting reaction.Reprinted with permission from [4].Copyright 2011, Elsevier.

Figure 5 .
Figure 5. Band positions and band gap energies of PANI and exemplary n-type metal oxide semiconductors.Data collected from [43,44] (left).Symbolic presentation of a type II heterojunction system of coupled semiconductors; A-TiO2, B-PANI (right).

Figure 5 .
Figure 5. Band positions and band gap energies of PANI and exemplary n-type metal oxide semiconductors.Data collected from [43,44] (left).Symbolic presentation of a type II heterojunction system of coupled semiconductors; A-TiO 2 , B-PANI (right).

Figure 10 .
Figure 10.Mechanism of the generation of reactive oxygen species under (a) UV and (b) visible light by a PANI-TiO2 nanocomposite photocatalyst.Reprinted with permission from [88].Copyright 2021, Elsevier.

Figure 10 .
Figure 10.Mechanism of the generation of reactive oxygen species under (a) UV and (b) visible light by a PANI-TiO 2 nanocomposite photocatalyst.Reprinted with permission from [88].Copyright 2021, Elsevier.

Table 2 .
Photodegradation of methyleneorange was performed under sunlight.Almost all the amounts of MO degraded during 150 min.

Figure 11 .
Figure 11.Mechanism of the photocatalytic degradation of RhB by the Au-PANI@TiO2 NPs under visible light irradiation.Reprinted with permission from[58].Copyright 2016, Royal Chemical Society.

Figure 11 .
Figure 11.Mechanism of the photocatalytic degradation of RhB by the Au-PANI@TiO 2 NPs under visible light irradiation.Reprinted with permission from [58].Copyright 2016, Royal Chemical Society.

Table 1 .
The main properties of different forms of PANI

Table 1 .
The main properties of different forms of PANI

Table 1 .
The main properties of different forms of PANI 2

Table 2 .
• C in the presence of 0.2 g of BAT.Cont.