Keywords

1 Introduction

The requirement to develop novel materials drives several worldwide research groups to focus their efforts on the synthesis and enhancement of advanced materials with application in the power generation field. Metals, ceramics, polymers, organics, and composites are involved in all the producing energy processes [1]. In the last decades, those novel materials related to renewable and sustainable methods to generate energy have attracted the attention of technological and power generation industries due to their feasibility to produce energy without causing damage as serious as that generated by the exploitation of non-renewable sources [2, 3]. Among the common renewable sources such as solar energy, wind energy, sea energy, bio-energy, geothermic, and hydrogen energy [4], those related with the exploitation of solar energy present a special interest, since the total amount of energy reaching our planet surface each year is estimated to be 3,400,000 EJ, this is between 7000 and 8000 times the annual global primary energy consumption [5]. Photovoltaic and photocatalysis are two important and arising technologies classified as branches of frontier sciences [6] since they involve sunlight harvesting and the constant development of advanced materials for energy generation purposes [7, 8]. Regarding the photocatalytic technology, this one is widely used in decontamination processes [9,10,11,12] and sustainable power generation, through the generation of H2 as fuel, [13,14,15] and photoreduction processes to convert the CO2 in solar-based fuels [16,17,18]. In this way, advanced photocatalytic materials such perovskite semiconductors with formula ABO3 containing metal transition ions in the structural sites A or B have generated great attention to develop numerous investigations, since such materials present a bandgap between 1.5 and 3.0 eV, which makes them suitable for use in the visible region of the electromagnetic spectrum for photocatalytic applications. Titanates such as ZnTiO3, CdTiO3, and PbTiO3 show favorable characteristics for the use of visible light with application potential for the production of hydrogen. Despite this, the study of such titanates perovskites has been reported in few publications [19,20,21,22]. Most research works focus their studies in the final catalytic activity of materials, and the synthesis process used to synthesize the ABO3 perovskites (solid-state, hydrothermal, microwave-assisted hydrothermal, and sol-gel); such reports do not establish well a relationship between the synthesis method, the crystalline structure, and catalytic activity caused by the substitution of the A or B perovskite cations and the photocatalytic activity. Lanthanum-based perovskites such as LaMnO3 and LaCoO3 also have been reported as suitable compounds to perform the degradation of polluting substances such as methyl orange [23], direct Green [24], and bisphenol A [9] using visible light as the excitation source. LaFeO3 is another perovskite which has shown excellent activity for the removal of organic pollutants from water [25] and NOx from the air [26]. However, there are few reports on the photocatalytic activity of LaFeO3, LaMnO3, and LaCoO3 as photocatalysts to carry out the photoelectrocatalytic hydrogen evolution reaction. In particular, it has been reported that LaCoO3 exhibits a deactivation process during the photocatalytic reaction. Such behavior was also reported for Cu/LaFeO3 and LaMnO3 powders, in all cases this deactivation process leads to low photocatalytic efficiency. In the same way that ATiO3 and LaBO3 present a variation in the photocatalytic activity as the cation A or B is substituted, the utilization of a secondary material to generate heterojunctions is another strategy to improve the photocatalytic activity. The addition of a second semiconductor (SB) to the bare main semiconductor structure (SA) modifies the levels of the valence and conduction bands of each material, overlapping in some cases the energetic levels and enhancing the separation of charge carrier’s and avoiding the recombination processes, thus increasing the photocatalytic activity [27]. NaTaO3 is a perovskite material widely used for photocatalytic degradation and energy generation [28,29,30,31]; its structure modification can lead to better photocatalytic performance. In addition to the bands overlapping, the doping or couple of NaTaO3 with other compounds can also produce the formation of intermediate energy levels; this feature can reduce the bandgap of the SA, inducing a shift to the red spectrum, and enhancing the visible light harvesting [17, 32,33,34]. Therefore, the design of highly efficient photoactive semiconductors requires an extensive study of the structural and superficial parameters. The aim of this chapter is focused on analyzing how the cation substitution in the tetrahedral and octahedral sites, the heterojunction formation, and the doping of the SA as structural and morphological methodology affects the psychochemical, optical, and photoelectrocatalytic properties of inorganic semiconductor perovskites ABO3 toward photocatalytic reduction processes. It is necessary a critical analysis of the physicochemical characteristics of bare perovskites and the existing relationship between the substitution of different cations, doping and heterojunction formation, in order to clearly identify the processes that enhance or limiting the photocatalytic activity of these advanced materials.

2 Scientific and Technologic Interest in Perovskites ABO3

The constant improvement of new energy conversion systems has allowed the development of new technologies to produce energy by the use of non-expensive and abundant materials, thus reducing the materials manufacturing and energy production costs [35]. An emergent technology in the branch of novel processes for the power generation field is photocatalysis, nowadays classified as a branch of frontier science [6], which means it is a promising technology still in development. The photocatalytic technology began its technological breakthrough in the 1970s when Fujishima and Honda reported for the first time the water molecule splitting by a photocatalytic process using titanium dioxide (TiO2) as a photoactive material in 1972 [36]. From this first approach, many research groups have focused their efforts on the development of new and more efficient semiconductor materials to perform photocatalytic processes. Among an extensive variety of semiconductors, perovskite materials have played a remarkable role in different conversion energy applications. Perovskite materials with general formula ABO3 induce a high charge carrier’s production (electron and holes), capable to carry out electrochemical redox processes, decreasing the charge carrier’s recombination to improve photocatalytic reactions to produce new renewable energy sources. It is worth highlighting that in the last decade, perovskite materials have played a fundamental role in the generation of energy utilizing solar radiation sources with applications in photovoltaic panels and photocatalytic processes. The development of silicon-based solar panels (Crystalline Si cells) and thin films (thin-film technologies) started their production in the 1970s. In the first years, such materials presented photovoltaic efficiencies less than 15%. In general, the silicon-based panels have presented an efficiency average increase of 12% in 45 years, showing energy efficiencies around 28% in 2020. Despite this efficiency increase represents a significative technological development, the current energy demand requires high impact and well-developed strategies to supply the energetical requirements. The photovoltaic hybrid-perovskite materials have become the materials with the highest energy efficiency performance, increasing from 14% to 26% in the last 5 years. Similarly, the cell efficiency of perovskite-silicon-based tandems has shown an increase from 25% to 28% in the last 3 years. In this way, there is enough evidence that has demonstrated that perovskites are emerging as an exemplary candidate for the manufacture of photovoltaic devices. In this context, it is very important to continue with the exhaustive study and development of perovskite materials, to modify their physicochemical and optical properties to increase their efficiency in photovoltaic processes. In this context, perovskite semiconductors have also shown exceptional improvement in the development of photocatalytic systems with application in the branch of solar-based fuels production, such as the generation of hydrogen via photocatalytic water splitting process, or the production of low carbon-containing compounds and fuels from the photocatalytic carbon dioxide reduction reaction (CO2RR). Regarding the photocatalytic process and its applications, complex oxides with general formula ABO3, such as the SrTiO3 [37, 38], NaTaO3 [32, 34] and KTaO3 [39], have shown outstanding efficiency for the hydrogen generation; Huerta et al. reported a hydrogen production of 5672 μmol g−1 h−1 onto a NaTaO3 laser assisted chemical vapor deposition (LCVD) film [33]. Tedsuda Kida et al. [40] reported a photocatalytic hydrogen production of 29 μmol h−1 for the heterostructured material LaMnO3/CdS, employing Na2SO3/Na2S as a sacrificial agent. Meizini et al. [41] obtained a H2 production of 60 μmol h−1 with the compound LaCoO3/SnO2, employing also Na2SO3 as a sacrificial agent. Another research publication reported the use of Cu/LaFeO3 to produce 187 μmol g−1 h−1 of H2 using a solution containing water + 10 wt.% of triethanolamine [42]. Some studies report that LaFeO3 coupled with g-C3N4 formed a heterostructure capable to perform properly the photocatalytic hydrogen evolution reaction in presence of sacrificial agents [43, 44]. While these works highlight the use of different sacrificial agents to improve the photocatalytic reactions, they do not emphasize the use of rare earth elements to substitute cations, which promotes better characteristics for the sunlight harvesting compared to transition metals, due to a greater number of 4f energy levels which maximize the photon absorption during photocatalytic processes [45]. Similarly, Jin Luo et al. recently reported the coupling of g-C3N4/LaCoO3, and this time was applied to carry out the degradation of methyl orange under visible light irradiation [46]. Even though most of this research works attribute the enhanced photocatalytic activity to the heterostructure formation, it is not fully understood how the main physicochemical characteristics of such heterostructures affect the photocatalytic mechanisms in order decrease the charge carrier’s recombination, which improves the catalytic properties of the based g-C3N4 heterostructures.

Focusing on the structural aspects of perovskite-based materials, and its relationship with the photocatalytic activity, these materials present a propitious electronic band structure to perform photoelectrocatalytic reduction processes; besides, the substitution of the A and B cations, which conform the crystalline structure in the ABO3 formula, provides a broad panorama for designing and modifying the crystal and its electronic structure, which affects directly the physicochemical properties [47]. Many of these perovskite oxides contain metal transition ions d0 o d10 (Ti4+, Zr4+, V5+, Nb5+, Ta5+, W6+) in their electronic structure [48], which induced a higher charge carrier’s generation, responsible for carrying out the water splitting process. Most of these transition metals such as titanium, iron, and manganese, among others are earth abundant and non-toxic elements. Also, because they can easily lose electrons in their d orbitals, they can be found in nature with different oxidation (mostly a valence state +2), which bring them the characteristic to be easily substituted, and more importantly, the capability to be photocatalytic active in the region of the visible electromagnetic spectrum [49]. Among different perovskites reported as good photocatalytic materials to perform the hydrogen evolution reaction, tantalates (NaTaO3 [32, 34], KTaO3 [39], K3Ta3Si2O13 [50], BaTa2O6 [51], SrTa2O6 [52]), and titanates (SrTiO3 [37, 38], BaTiO3 [53], CaTiO3 [54]) have attracted great attention due to their high photocatalytic performance, mainly those composed by alkali and alkaline earth metals.

3 Strategies for the Development of New Functional and Efficient Photocatalytic Materials

Despite the great effort that research groups worldwide have dedicated to the development of efficient materials for obtaining hydrogen by photocatalysis, the current efficiencies are still below the desired level. For this reason, different strategies have been implemented to increase the efficiency of the photocatalysts used in hydrogen production and other photoreduction processes. In this way, the photocatalytic activity depends on the cumulative effect of the following processes: light absorption, charge separation, and charge carrier’s migration and transport [55, 56]. The current bottleneck in the photocatalysis technology lies in its low quantum yield, which is affected by the rapid recombination of the hole-electron pairs on the surface, but also in the bulk. The stability of the photocatalysts is also a very important parameter to improve the photocatalytic activity. The overall efficiency of photocatalytic hydrogen production depends directly on the thermodynamic and kinetic equilibrium, which depend on the electronic and crystalline structure [56]. The most important factors and characteristics that influence the efficiency of the photocatalytic processes, as well as their corresponding improvement strategies, are an effective separation and transfer of the electron-hole charge carriers, the light harvesting, an adequate bandgap, low cost and minimum toxicity of the photocatalyst, as well as an optimized kinetic reaction on the surface to carry out the redox processes. There are important strategies reported in the literature to improve the photocatalytic processes, such as the band structure modification, the design of micro/nanometric structures, the modification of the surface, and the modification of the semiconductors interfaces [57]. However, it is also important to consider the physicochemical properties, particularly the crystalline and electronic structure, and its relationship with the photocatalytic activity. These two characteristics are notably influenced in perovskite-type structure ABO3 , due to the effects of the substitution of cations A and B in the cationic sites [47, 48]. These properties are attributed to the position of the semiconductor conduction bands, a band with higher energy (more negative potential), thermodynamically favors the evolution of hydrogen [58]; in the same way, distortions in the crystal structure due to bond angles close to 180° (for example Ta-O-Ta in the KTaO3) promote better mobility and transference of the photogenerated charges [59]. Considering all the aforementioned factors, a deep understanding of these materials is necessary for the adequate development of photocatalytic systems capable to perform efficiently the hydrogen production and other photoreduction processes.

3.1 Crystalline Structure Modification: Cation A Substitution

To understand how the physicochemical, optical, and catalytic properties of some perovskites are related to the structural modifications depending on the substitution of the cation A in the ABO3 formula, Fig. 1a shows the XRD patterns for the perovskite ATiO3 , where A = Zn, Cd, Pb. The three perovskites were synthesized by a solvo-combustion methodology [32]. The ZnTiO3 presents a rhombohedral structure with the spatial group R-3 [60]. Besides, a TiO2 rutile phase (3 wt.%) is present in this compound. Similarly, the cadmium titanate (CdTiO3) presents a rhombohedral structure with the same spatial group R-3. However, for the PbTiO3 it presents a tetragonal structure with the spatial group p4/mmm (01-074-2495) (Fig. 1b). Since we are substituting the cation A, the difference in the crystalline structures is directly related to the difference between the atomic radii of the A = Zn, Cd, and Pb cations. According to the atomic radii reported in the literature: Zn2+ (0.74 Å) < Cd2+ (0.97 Å) < Pb2+ (1.19 Å) [61], the materials containing cations with shorter atomic radii present crystalline structures with higher distortion rates, meanwhile larger cationic radii promotes the formation of a more-symmetric structural phase. Research works have reported that the cubic structure of different perovskite materials, particularly metallic tantalates present an enhanced photocatalytic activity compared with distorted structures such as the orthorhombic. This difference in photocatalytic activity relies on the Ta-O-Ta angle of the crystalline structure, which is closer to 180° in the cubic structure which induces an easy transport of the photogenerated charge carriers through the bulk crystalline structure, thus improving the photocatalytic activity [59, 62].

Fig. 1
figure 1

(a) X-ray diffraction patterns for ZnTiO3, CdTiO3 and PbTiO3. (b) Crystalline structure for ZnTiO3 and CdTiO3 (rhombohedral) and PbTiO3 (tetragonal). (c) Diffuse reflectance spectroscopy measurements for ATiO3, A = Zn, Cd, Pb. (d) Electrochemical potentiometry measurements. ((a) Reprinted with permission from O. Carrasco-Jaim et al. J Photoch Photobio A, 371, 98–108. Copyright (2019) Elsevier)

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Figure 1c shows the diffuse reflectance UV-Vis spectra for the metal transition titanates perovskites. From this figure, it is important to notice that ZnTiO3 presented an absorbance signal at 410 nm, which can be associated with the rhombohedral phase [60]. Additionally, it is possible to observe a signal around 300 nm, which corresponds to the rutile phase TiO2 [63] previously identified. The CdTiO3 spectrum depicts a signal transition around 300 nm, which is attributed to the rhombohedral phase [64], and an additional band at 380 which corresponds to an energetic value around 3.26 eV, this link energy is associated to intrinsic defects caused by oxygen vacancies in the crystalline structure of CdTiO3 [65]. Regarding the PbTiO3, this perovskite shows an extended absorption signal in the visible range (200–450 nm) which indicates a better visible light harvesting. It is worth to mention that ZnTiO3 and CdTiO3 show energy transitions below 300 nm (≥ 4.13 eV). According to the DOS calculation reported for these materials, such transitions correspond to the d orbitals from the level Ti-3d from 3 to 9 eV. Such energetic levels are responsible for the major contribution in the conduction band for these perovskites [66]. On the other hand, it is possible to observe a similar absorption band for ZnTiO3 and CdTiO3, and this effect is attributed to the d-states in their respective conduction bands (Zn-3d and Cd-5d), in comparison to the Pb-6p state for PbTiO3 [66, 67].

A piece of clear evidence that how the substitution of the cation A in the ATiO3 structure modifies the photocatalytic activity is described as follows; when a material presents a lower conduction band energy level, as in the case of PbTiO3, it is possible to observe an absorption rate extended to the visible light range [68]. In this way, the substitution of the cation of the site A from an electronic configuration with lower energy values induces the obtention of photocatalytic materials active to harvest light in the visible region. Another significant effect observed when the cation of the site A is substituted in materials ATiO3 (A = Zn, Cd, Pb) is related to the nature of the generation and transference of the photogenerated charges. This effect can be explained by the variation of the electrochemical potential under dark and light conditions (UV light, 254 nm). Since the electrochemical potential of ZnTiO3 presents a shift toward negative potentials (Fig. 1d), which means an electron accumulation in the conduction band [69], we can classify this material as an n-type semiconductor. On the other hand, CdTiO3 and PbTiO3 present a contrary behavior under light irradiation, the displacement to positive potentials indicates a p-type semiconductor. In particular, PbTiO3 presents a slight negative shift when it is irradiated, this effect is associated with the accumulation of negative charges on the material surface (charge accumulation layer) which compensates the excess of positive charges present in a p-type semiconductor. Once the negative charges are consumed, the plot presents a potential change caused by the movement of the majority charge carriers (holes) [70].

Since we have elucidated how the substitution of the cation A modifies the structural arrangement of the ATiO3 perovskites, these crystallographic modifications affect the photocatalytic activity to perform redox processes. Figure 2 depicts the micrographs and photocatalytic production of molecular hydrogen related to the A cation, A = Zn, Cd, Pb. From this plot, it is clear that ZnTiO3 generates the highest amount of hydrogen, 470 μmol g−1 in 3 h of reaction; this value corresponds to a solar-to-hydrogen conversion efficiency (STH) equal to 0.48%. The highest hydrogen production presented by ZnTiO3 is attributed to two main factors: (i) a higher crystallite size which presents a lower number of crystalline defects, thus reducing the charge carrier’s recombination process, and (ii) the presence of a secondary phase TiO2, which acts as an efficient co-catalyst to form the heterostructure ZnTiO3/TiO2 , thus enhancing the separation of the photogenerated charges and minimizing their recombination rate. To compare the hydrogen production rates, as well as the crystallographic characteristics, Table 1 summarizes the most relevant data. Since PbTiO3 generated a higher hydrogen amount than CdTiO3 (140 μmol g−1 and 80 μmol g−1, respectively). It is suggested this variation can be attributed to the different crystalline structures presented by each compound. As aforementioned, ZnTiO3 and CdTiO3 present a rhombohedral structure, meanwhile PbTiO3 crystalized with a tetragonal structure. A more ordered structure induces the formation of Ti-O-Ti angles close to 180°, which enhances the light harvesting; this characteristic enhances the photocatalytic activity of PbTiO3 over the CdTiO3 to produce hydrogen.

Fig. 2
figure 2

Photocatalytic hydrogen production and scanning electron micrographs for ATiO3 (A = Zn, Cd, Pb)

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Table 1 Crystalline properties and photocatalytic H2 production for ATiO3 (A = Zn, Cd, Pb)
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3.2 Crystalline Structure Modification: Cation B Substitution

Section 1.3.1 was dedicated to understanding how the substitution of the cation A in the perovskite ATiO3, A = Zn, Cd, Pb can tailor the photocatalytic activity to perform the hydrogen production since the different atomic radio of each cation promotes the formation of different crystallographic arrangements. This modifies the lattice distortion promoting a redshifting for PbTiO3 and the formation of heterostructures ZnTiO3/TiO2 , which improves in both cases the generation and separation of photogenerated charge carriers and decrease the recombination rate. In this context, it is important to study how the modification of the cation B in the perovskite structure ABO3 affects the photocatalytic parameters. Figure 3a depicts the X-ray diffraction patterns of three different perovskites LaBO3 (B = Fe, Co, Mn) synthesized by the sol-gel method [34]. The perovskite LaFeO3 presents a cubic phase (JCPDS-01-075-0541); however, it is possible to notice the presence of an orthorhombic phase (JCPDS-01-070-777). Meanwhile, the LaCoO3 semiconductor powders show a double reflection 2θ = 32.8° – 33.3°, feature characteristic of the rhombohedral phase (JCPDS 01-084-0848). Since the perovskite LaMnO3 is easily affected by the oxygen content, this fact can modify the unitary cell structure (Fig. 3b). It is common to find the nomenclature LaMnO3+δ, where δ represents the oxygen excess. Commonly, if δ increases the crystalline structure changes form the orthorhombic arrangement to the rhombohedral structure [71]. The material LaMnO3 described in this section was indexed with the orthorhombic phase JCPDS 01-089-2471, an additional peak is observed at 2θ = 30°, such signal corresponds to La2O3 since it is very difficult to obtain the pure phase LaMnO3. Regarding Co+3, Mn+3, and Fe+3, the ionic radii are 2.18 Å < 2.10 Å < 2.01 Å, respectively [71]. Their internal angles B-O-B are 163°< 157°< 155°. As occurs for the family of materials ATiO3, a larger ionic radio promotes the formation of angles B-O-B closer to 180° which makes easier the charge carrier’s transference to the material surface. The optical characteristics are also an important parameter to observe the effect of the B ion substitution, and the UV-Vis spectra for the LaBO3 (B: Co, Mn, Fe) perovskites (Fig. 3c) show light harvesting in the visible range. LaMnO3 and LaCoO3 present a wide absorption range in the visible electromagnetic spectrum; meanwhile, LaFeO3 shows different transitions, the most notable at 550 nm. It is well known that the valence band of these materials is assigned the O2-2p orbital, and the conduction band is assigned to the B+3 3d orbitals. However, the crystalline field of the orbitals B+3: d (B: Co, Mn) is divided into two sets t2g, which contributes to the highest levels of the valence band and the levels eg added in the conduction band [72]. Hence, it exists a spatial overlapping of the metal d orbitals and the oxygen 2p orbital, thus leading the presence of d→d transitions, which require less energy than p→d transitions. Particularly, the LaBO3 (B = Fe, Co, Mn) perovskites present localized states in the levels eg, which makes them more susceptible to d→d transitions, thus inducing a higher visible light absorption [41, 72, 73]. In comparison with the ATiO3 perovskites (A = Zn, Cd, Pb), the substitution of the B cation changes significatively the optical properties, because as above-mentioned, the energy levels in this position conform the semiconductor conduction band.

Fig. 3
figure 3

(a) X-ray diffraction patterns for LaFeO3, LaCoO3, and LaMnO3. (b) Crystalline structures for LaBO3, B = Fe, Co, Mn. (c) Diffuse reflectance spectroscopy measurements for LaFeO3, LaCoO3, and LaMnO3. (d) Cyclic voltammetry characterization. ((a) Reprinted with permission from L. Ibarra-Rodriguez et al. J Phys Chem Solids, 136, 109189. Copyright (2020) Elsevier)

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The active species of cations B+3 located onto the material surface allow the performance of the redox reactions due to their multiple oxidation states. Since the electrochemical techniques are very sensible procedures to characterize and observe the oxidation transition states of heterostructured materials, cyclic voltammetry in Fig. 3d allows the cation oxidation transition from B+2 to B+3, and also a partial transition in LaMnO3 from carbonated species to MnO. From these results, it is possible to point out that LaMnO3 y LaCoO3 present a higher surface area and a higher concentration of active metal ions B (B = Co, Mn) [74] onto the perovskite surface, these species perform a key role in the capability of such semiconductor to photogenerated molecular hydrogen [75].

Regarding the photocatalytic performance toward the hydrogen evolution reaction, the results show a correlation between the H2 generation and the B (B = Fe, Co, Mn) cation employed in the LaBO3 structure. LaMnO3 exhibits the highest production rate compared to the perovskites LaFeO3 and LaCoO3 (Fig. 4). A general summary of the photocatalytic activity and structural parameters are displayed in Table 2. Such behavior is attributed to the following suggestions: (i) LaMnO3 presents a higher superficial area; in this way, there exist more actives sites which increase the surface contact and the photocatalytic activity; (i) a higher concentration of metal ions onto the material surface; (iii) the presence of the La2O3 phase which improves the charge carrier’s transport since its lanthanide 4f level plays an important role in the interfacial charge transference and the decrease in the charge carrier’s recombination rate [12, 76]. It is worth remembering that titanates family ATiO3 (A = Zn, Cd, Pb), PbTiO3 present the most ordered structure, and a Ti-O-Ti angle closer to 180°, hence the highest photocatalytic activity. In this context, the LaBO3 (B = Fe, Co, Mn) perovskites show a similar result. The LaMnO3 presents an angle B-O-B value of 163°, being this material the one that presents the highest photocatalytic activity. Such results show the influence of the crystalline structure over the photocatalytic performance. A free defect and highly symmetric structure enhance the charge carrier’s transport. However, once the photogenerated charges are transported to the surface, the superficial characteristics like the number of metal active ions play a key role to improve the photocatalytic hydrogen evolution reaction.

Fig. 4
figure 4

Photocatalytic hydrogen production and scanning electron micrographs for LaBO3 (A = Fe, Co, Mn). (Reprinted with permission from L. Ibarra-Rodriguez et al. J Phys Chem Solids, 136, 109189. Copyright (2020) Elsevier)

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Table 2 Crystalline properties and photocatalytic H2 production for LaBO3 (B = Fe, Mn, Co)
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3.3 Superficial Modification: Key Role in the Photocatalytic Performance

To further understand the influence rate of the cation B substituted in the LaBO3 (B = Fe, Co, Mn) perovskites and its relation with the photocatalytic activity, further analyses have to be discussed. Superficial measurements by XPS allow the determination of the relation O-ads/O-lattice (O-ads = adsorbed oxygenated species), this relation is used as a reference to appreciate the material surface changes. The O-ads/O-lattice for LaMnO3 presents higher values after the material was subjected to the photocatalytic evaluation (1.35 and 1.67 before (Fig. 5a) and after photocatalytic tests (Fig. 5b), respectively), this feature is related with a higher number of oxygenated species on the LaMnO3 surface mainly CO3−2 and OH−. To corroborate the presence of carbonate (CO3−2) or hydroxyl (OH−) ions absorbed on the LaMnO3 surface, the infrared FTIR measurements (Fig. 5c) confirm the presence of these species. It is possible to observe two vibrational bands (symmetric curves) at 723 and 857 cm−1, as well as two asymmetric signals at 1427 and 1485 cm−1 corresponding to the carbonated anions [77, 78]. These signals become more intense after the photocatalytic reaction, thus indicates the formation of carbonate links. Such behavior can be attributed to the carbon dioxide adsorption over the perovskite surface which is later converted into carbonated species during the photocatalytic reaction. In this way, it is suggested that carbonate ions (CO3−2) compete for the available active sites over the semiconductor surface, causing in this way the deactivation of catalytic sites to perform the hydrogen generation. Considering the aforementioned explanation, Fig. 6 shows a mechanism to elucidate the role of the oxygenated species in the photocatalytic reaction. Once the light source promotes the photogeneration of the charge carriers (e− and h+), different phenomena take place. The photoexcited electrons are displaced to the conduction band, which is commonly composed of the d transition levels. From this point, the electrons can take two different transport ways: d → d (t2g) or O−2: 2p → M: d, this suggests that the exposed transition ions over the surface promote the formation or partial charges, increasing the available active sites to perform the redox reactions. However, partial charges are not attracting just water, they also attract another oxygenated species [79], such carbonated and oxygenated species, CO3−2 and OH- block the active sites decreasing the photoinduced hydrogen generation.

Fig. 5
figure 5

XPS spectra, O 1s signal for LaMnO3 (a) Before the photocatalytic tests and (b) After the photocatalytic test. (c) FTIR spectra for LaMnO3 before and after the photocatalytic tests. ((a), (b) and (c) Reprinted with permission from L. Ibarra-Rodriguez et al. J Phys Chem Solids, 136, 109189. Copyright (2020) Elsevier)

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Fig. 6
figure 6

Photocatalytic hydrogen evolution mechanism for LaBO3 (A = Fe, Co, Mn) semiconductors. (Reprinted with permission from L. Ibarra-Rodriguez et al. J Phys Chem Solids, 136, 109189. Copyright (2020) Elsevier)

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It has been demonstrated that the substitution of cations A or B in the perovskite ABO3 affects the structural, morphological, and optical characteristics, thus modifies the photocatalytic activity of such materials. However, it is necessary the combination of multiple factors to obtain an adequate synergy. A very important factor is that the compounds ZnTiO3 y LaMnO3, which showed the highest photocatalytic activities, present a secondary phase. It is suggested that these secondary compounds can generate intermediate energetic levels which induce the charge carrier’s generation. The theoretical diagrams for the materials ZnTiO3, CdTiO3 y PbTiO3 show that the conduction band for CdTiO3 is more negative than the conduction band for ZnTiO3 and PbTiO3 (Fig. 7a). This condition has been related to a higher photocatalytic activity to perform reduction processes [58, 68]. Although, regarding the ATiO3 (A = Zn, Cd, Pb) materials, PbTiO3 showed the best photocatalytic efficiency producing more hydrogen than CdTiO3. This enhancement is related to the ferroelectric properties of PbTiO3 [80] which induce spontaneous polarizations capable to produce dislocation of electric charges. This charge carrier’s separation is favorable to reduce the holes – electrons recombination, enhancing in this way the photocatalytic activity [81]. On the other hand, the band diagram for LaBO3 (B: Co, Mn, Fe) shows that La2O3 presents a wide bandgap ~4 eV (Fig. 7b); however, it is suggested a section these energy levels generates transition levels to act as a heterostructure [82].

Fig. 7
figure 7

(a) Theoretical band diagram for ATiO3 (A = Zn, Cd, Pb). (b) Theoretical band diagram for LaBO3 (A = Fe, Co, Mn) semiconductors. ((a) Reprinted with permission from O. Carrasco-Jaim et al. J Photoch Photobio A, 371, 98–108. Copyright (2019) Elsevier)

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3.4 Tailoring the Photocatalytic Activity Through the Heterojunction Formation

Several strategies have been carried out to increase the photocatalytic activity of semiconductor materials to perform redox processes. The doping with metal and non-metal nanoparticles [54, 83], structural changes to reduce the bandgap values [84], sensitization with visible light materials [85], and the formation of heterojunctions [11, 27, 86] are some of the most used strategies to improve the visible light harvesting and avoid the charge carrier’s recombination rate. Regarding these strategies, the heterojunction formation is one of the most efficient methodologies to improve the photocatalytic activity of semiconductors by controlling the electron-hole recombination rate [27]. Semiconductor heterojunction involves the stacking of two photocatalytic materials with different bandgaps. A direct bandgap transition is better than an indirect bandgap. The direct bandgap refers to the highest level of the valence band aligned with the lowest level on the conduction band; in this way, the direct recombination takes place releasing an energy amount equal to the energy difference between the recombining particles [87].

The sodium tantalate NaTaO3 is an extensively studied perovskite reported as a suitable material to perform the photoelectrocatalytic hydrogen production from the water splitting under UV-Vis irradiation [32, 33, 88]. Taking into account the NaTaO3 presents a bandgap ~4 eV [88], this perovskite is capable to generate charge carriers only under UV irradiation, and this fact decreases its effectiveness in technological scaling applications in devices irradiated with sunlight. On the other hand, although barium bismuth (BaBiO3) does not show a high production of H2 when it is used as a photoanode [89], this semiconductor presents a bandgap ~2 eV [90], which makes it a photocatalytic active material under visible irradiation. In this way, and continuing with the task to obtain better and more efficient photocatalytic materials, the following paragraphs describe the formation and photocatalytic performance of a heterojunction BaBiO3/NaTaO3 to elucidate the advantages of the heterostructures formation. Fig. 8a–c shows the particle sizes for the heterostructure, and this one decreases its particle size in comparison with bare NaTaO3 and BaBiO3. In turn, the heterojunctions containing 20 and 25% by weight of BaBiO3 present the largest crystal size, which from the photocatalytic point of view reduces the recombination effects of charge carriers, improving the catalytic activity (Fig. 8d).

Fig. 8
figure 8

(a) Scanning electron micrographs for (a) NaTaO3, (b) BaBiO3, and (c) 20%-BaBiO3/NaTaO3. Theoretical band diagram for ATiO3 (A = Zn, Cd, Pb). (d) Crystallite sizes as wt.% BaBiO3 is added. ((a) and (b) Reprinted with permission from J.M. Mora-Hernandez et al. J Photoch Photobio A, 391, 112363. Copyright (2018) Elsevier)

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While it is true a larger crystal size affects and improves the photoelectrocatalytic activity of the heterojunction, this structural modification affects the optical and electrocatalytic parameters. The heterostructured material shows a decrease in the charge carrier’s recombination rate as the amount of BaBiO3 increases until 20%-BaBiO3/NaTaO3, this ratio presents the lowest recombination rate (Fig. 9a, b); therefore, the highest photocatalytic hydrogen production (Fig. 9c) [91]. The photoelectrochemical evaluation reveals the high effectivity in the formation of the heterojunction 20%-BaBiO3/NaTaO3 to perform the water splitting by the generation of ∙OH radicals to form oxygen onto the electrode surface and hence the hydrogen generation on the counter electrode (Fig. 9d). These results allow considering this material as an effective compound to perform the photoelectrocatalytic water splitting as an energy vector. It is suggested the photocatalytic enhancement is caused by: (i) an increase in the crystallite size compared to the bare perovskites, (ii) an optimal addition of BaBiO3 (20 wt.%) which enhances the heterostructure synergy and decreases the charge transfer resistance, and iii) La continuous charge carrier’s transference induced by energy bands overlapping, thus avoiding the recombination issues and improving the charge transference processes.

Fig. 9
figure 9

(a) Charge carrier’s recombination rate by photoluminescence measurements. (b) Photocatalytic hydrogen generation. (c) Charge transfer resistance under dark and light conditions. (d) Photoelectrochemical activity under oxidation scan potential. ((a), (b), (c) and (d) Reprinted with permission from J.M. Mora-Hernandez et al. J Photoch Photobio A, 391, 112363. Copyright (2018) Elsevier)

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As mentioned above, heterojunctions have been developed for several applications, pollutant degradation [92], microbial disinfection [93], energy generation by H2 production by water splitting [94], and CO2 reduction [18]. Depending on the application needs, the heterojunction components are selected to generate specific reactive species. Graphitic carbon nitride (g-C3N4) is a novel semiconductor material that has been widely used conjointly with several perovskite materials such as MnTiO3 [95], CoTiO3 [96], SrTiO3 [10], and NaTaO3 [97], with application in degradation processes. On the other hand, the family of perovskites LaMO3 (M: Co, Fe, Mn) has been employed in catalysts, sensors, and membranes in syngas applications [98]. There are several reports regarding the use of g-C3N4/LaFeO3 in the hydrogen production in presence of sacrificial agents [43], as well as the employment of g-C3N4/LaCoO3 to carry out the methyl orange degradation under visible light [46]. A comparative study of photocatalytic hydrogen evolution employing g-C3N4/(2, 5, 10%)LaMO3 (M: Co, Mn, Fe) composites is analyzed to elucidate the mechanism involved in the enhanced charge transfer processes over the heterostructured semiconductor. Table 3 shows a summary of the physicochemical properties and the photocatalytic activity for g-C3N4/LaBO3 (B: Co, Mn, Fe) heterostructures. In almost all cases, a LaBO3 percentage above 2 wt.% affects negatively the compound performance. It is worth to mention that samples containing 2 wt.% of LaBO3 (B: Co, Mn, Fe) exhibit the highest hydrogen generation. This feature can be related to a surface saturation where the perovskite particles block the g-C3N4 active sites. The addition of the second material enhances the photocatalytic activity, increasing the separation and transference of charge carriers. An energy band diagram (Fig. 10) allows us to observe when the materials are coupled, the electrons are transferred from the g-C3N4 conduction band to the perovskite conduction band. The electron transference continues until the Fermi energy level in both compounds is in equilibrium, and this improves the charge carrier’s separation and increases the photocatalytic activity. As the perovskite co-catalyst percentage increases, the spatial charge region becomes narrower, and the depth of light penetration into carbon nitride exceeds the space charge layer, this promotes the hole – electron recombination (Fig. 11) [99].

Table 3 Physicochemical properties and photocatalytic activity of g-C3N4 /LaBO3, B = Co, Mn, Fe
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Fig. 10
figure 10

Band diagrams, charge transference mechanism, and heterojunctions obtained by the coupling of LaBO3 (B = Fe, Co, Mn) and g-C3N4

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Fig. 11
figure 11

Surface saturation diagram for the heterojunctions obtained by the coupling of LaBO3 (B = Fe, Co, Mn) and g-C3N4 . (Reprinted with permission from L. Ibarra-Rodriguez et al. Mater Sci Semicond Process, 103, 104643. Copyright (2019) Elsevier)

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At this point, it is worth mentioning the definition and physicochemical and optical characteristics of the most common heterojunctions. Based on the different band and electronic structures, heterojunctions can be typically classified into three groups: type-I (straddling gap), type-II (staggered gap), and type-III (broken gap) (Fig. 12) [27]. For the type-I, the conduction band and the valence band levels for the semiconductor A (SA) are higher and lower than the CB and VB of semiconductor B (SB), the charge carriers migrate from the SA to the CB and VB of the SB, the charge carriers cannot separate effectively due to their accumulation on the SB (Fig. 12a). In type-II heterojunctions, the level of both, conduction and valence band of SB, is higher than both band levels in SA (Fig. 12b). In this way, electrons migrate from the to the CB of SA, and holes migrate to the VB of SB which results in effective spatial charge separation and transference. Fig. 12c depicts the band positions of type-III heterojunctions, in this case, the staggering gap becomes so wide that the bandgaps do not overlap. Thus, the synergic charge carrier’s migration and separation between SA and SB do not occur. Among these three conventional heterojunctions, the type-II heterojunction is the most effective due to its spatial migration and charge carrier’s separation. From the band diagram in Fig. 10, it is possible to observe that only the compounds g-C3N4 and LaFeO3 present suitable position bands to form a type-ll heterostructure, which makes it the most efficient material to carry out the photocatalytic hydrogen evolution. On the other hand, the herejunctions generated by g-C3N4 and LaCoO3 or LaMnO3 form a type-l heterostructure. These two heterostructures act as light sensitizers; however, this better light harvesting does not improve the photocatalytic activity since all the charge carriers are accumulated on the LaCoO3 or LaMnO3.

Fig. 12
figure 12

Semiconductor heterojunctions and charge carriers transfer processes. (a) Type-l, (b) Type-ll, (c) Type-lll. (Reprinted with permission from A. Kumar et al. Water Res, 170, 115356. Copyright (2020) Elsevier)

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The heterojunctions formation is one of the best strategies to enhances the photoelectrocatalytic activity in perovskite semiconductors; however, the coupling of two catalytic materials not always results in a heterojunction formation, sometimes the addition of a second material promotes the formation of intermedium states or defect levels capable to reduce the semiconductor bandgap [100, 101]. To elucidate this feature, this section presents the tailoring of a NaTaO3 perovskite by the incorporation of amorphous carbon to produce the generation of intermediate states to decrease the bandgap broadening and enhance the photocatalytic activity to perform reduction processes. The carbon-doped NaTaO3 (C-NaTaO3) was obtained by a solvo-combustion method, which involves the use of organic and volatile substances that are responsible for the incorporation of amorphous carbon into the NaTaO3 lattice (Table 4) [17]. The as-prepared C-NaTaO3 contains a 47 at. % of carbon, and this sample was subjected to an annealing process to remove different amounts of carbon. The sample annealed at 650 °C (30 at. % carbon) presented the optimal physicochemical and optical properties to perform photocatalytic reduction processes. Figure 13a shows the DTA/TGA measurements, the mass loss before 200 °C corresponds to the water evaporation, meanwhile the decrease in mass from 250 to 520 °C evidence the carbon removal as the temperature increases. The peak appreciated at 550 °C is attributes to the NaTaO3 crystallization. From Fig. 13b, it is evident that the heat treatment process modifies the specific surface area SBET. Considering that most of the carbon allotropes possess a high specific surface area [102, 103], the temperature increase promotes a significant SBET loss above 600 °C. Additionally, the NaTaO3 recrystallization causes the increase of the grain sizes promoting also the decrease of the SBET [104]. Although, a high crystallinity enhances the photocatalytic activity of semiconductors due to a low number of crystalline defects responsible for the charge carrier’s recombination [105]. In this way, an efficient photocatalytic semiconductor must exhibit an optimal relationship between a high crystallinity and a high surface area, and this optimal relationship is found in the sample annealed at 650 °C.

Table 4 Quantitative composition obtained by EDXS. Carbon contained in the C-NaTaO3 samples
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Fig. 13
figure 13

(a) DTA/TGA plots for the as-prepared C-NaTaO3 photocatalyst . (b) Specific surface area (SBET) and sample pore size (BHJ) for the as-prepared C-NaTaO3 and samples annealed from 500 to 700 °C. ((a) Reprinted with permission from J.M. Mora-Hernandez et al. J CO2 Util, 27, 179–187. Copyright (2018) Elsevier)

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Since the sample C-NaTaO3 annealed at 650 °C show the optimal relationship between surface area and crystallinity. This sample presents the highest photocatalytic activity to carry out the photoconversion of CO2 to produce formaldehyde (Fig. 14a), overpassing the photocatalytic efficiency of samples annealed at different temperatures, and bare NaTaO3. The high conversion rate achieved by C-NaTaO3 at 650 °C is attributed to three main factors: (i) the reduction of the bandgap caused by the generation of intermediate states in the energy bands, (ii) an enhancement of the electrical conductivity by the addition of carbon in the NaTaO3 structure, and (iii) an optimal ratio between the crystallization degree and the active surface area. Regarding the generation of intermediate states, Fig. 14b depicts the mechanism to carry out the photocatalytic generation and transportation of charge carriers. Typically, NaTaO3 presents a UV light absorption attributed to the band transition for O2p (valence band) to the Ta5d (conduction band); however, the carbon incorporation induces the apparition of mid-gaps which reduce the original bandgap value [106], and this better light harvesting and a major number of photogenerated charges is indicative of the presence of oxygen vacancies and impurities in the NaTaO3 lattice [107]. Nevertheless, the defects induced by the carbon-doping play a detrimental role, acting also as recombination centers and decreasing the number of active charge carriers to perform the redox processes. This is the reason why an appropriate combination of surface area, light absorption, and crystalline structure are determinant to improve the photocatalytic activity.

Fig. 14
figure 14

(a) Photocatalytic CO2 reduction over C-NaTaO3 samples . (b) Mechanism diagram for induced visible light activity in C-NaTaO3. (Reprinted with permission from J.M. Mora-Hernandez et al. J CO2 Util, 27, 179–187. Copyright (2018) Elsevier)

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4 Conclusions

In the last decades, inorganic perovskite semiconductors have attracted the attention of the scientific community since these materials have presented remarkable results in the photocatalytic energy generation field. The modification of structural parameters induced by the substitution of cations A or B in the ABO3 general formula, the heterojunction formation, and doping strategies have been employed as photocatalytic tailoring strategies. Such improvement strategies affect the psychochemical, optical, and photoelectrocatalytic properties of such semiconductors toward photocatalytic reduction processes. This chapter highlights the results obtained for three different perovskite families following the above-mentioned tailoring strategies. Regarding the ATiO3 family, A = Zn, Cd, Pb, the ZnTiO3 showed the best photocatalytic performance toward the hydrogen production (470 μmol g−1), since this compound presented the highest crystallite size, thus a lower number of crystalline defects. The substitution of cation A in the ATiO3 perovskites showed that cations with lower ionic radio generate a significant structure distortion, the compounds with a Ti-O-Ti angle closer to 180 °C presented better light harvesting. Additionally, the cations presenting energy levels lower than Ti3+ promote a photocatalysts redshifting. On the other hand, in the perovskites family LaBO3, B = Co, Mn, Fe, the compound LaMnO3 presented the highest photocatalytic hydrogen production (103 μmol g−1), such enhancement is attributed to the presence of the secondary phase La2O3 which promotes a 4f modification levels improving the charge carrier’s transference. Also, this material presented a higher superficial area, thus exists a higher number of photocatalytic active sites, as well as a higher number of metallic ions responsible to generate partial charges where water molecules can be adsorbed and later reduced. Similarly, to the ATiO3 family, the cation B substitution induced promoted a structural distortion. The use of lower cation radii generates less ordered structures, the perovskite structure presents enough internal space to generate higher structural distortions. The transition metals Co, Fe, and Mn present energetic transitions d→d, which require less energy and allow the activation of the LaBO3, B = Co, Mn, Fe in the visible light region. On the other hand, the heterojunction formation and doping strategies also promote better light harvesting and enhancement in the photocatalytic activity of some perovskites. For the BaBiO3/NaTaO3 heterojunction, the addition of 20 wt.% BaBiO3 enhanced the visible light absorption and increase the heterostructures crystallite size in comparison with bare semiconductors, this structural modification decreases the grain limit defects number, thus reducing the charge carrier’s recombination rate. Such photocatalytic improvement was also presented by the heterojunction ZnTiO3/TiO2, where the heterostructure formation induced better charge carrier’s transference by the modification of the energy levels to generate a type-ll heterojunction. Regarding the carbon-doped NaTaO3 compound (C-NaTaO3), the presence of 30 at. % of amorphous carbon improved the photocatalytic activity of the C-NaTaO3 composite. The carbon amount was a key parameter to improve and reduce the bandgap value and diminish the charge transference resistance by an improvement in the electrical conductivity, but more important, by the generation of intermediate energy levels in the NaTaO3 structure capable to reduce the bandgap. The optimal ratio between the surface area and crystallinity of the sample was achieved by the C-NaTaO3 annealed at 650 °C. These parameters play an important role to explain an improved photocatalytic activity caused by a higher reactive surface and lower recombination of the charges photogenerated onto larger crystals. The photoelectrochemical tests revealed that the material annealed at 650 °C is the only sample with a conduction band value more negative than the potential required to perform the generation of formaldehyde from the photocatalytic reduction of carbon dioxide in aqueous solution. Is it suggested that the appropriate combination and synergy of crystalline structure, light absorption, and surface area properties of the samples are determinant factors in the photocatalytic activity in carbon-doped materials.