1 Introduction
When hydrogen gas (H2) is used as an energy source, only water will be produced after the combustion process. Therefore, H2 has been considered as a potential candidate of renewable energy to relieve the energy crisis caused by limited fossil fuel resources and the detrimental impact on the environment. Several techniques have been developed to produce hydrogen gas, such as electrolysis, plasma arc decomposition, water thermolysis, thermochemical water splitting, thermochemical conversion of biomass, gasification, biofuel reforming, photovoltaic (PV) electrolysis, photocatalysis, and the photoelectrochemical method, etc [1,2]. Intensive research efforts have been made in each of these approaches to develop an optimized process in both environmental and economic respects.
The photocatalytic H2 generation by water splitting is to use photocatalysts to induce the water splitting process as expressed in the Eq. (1) and generate H2 as a reaction product [3]. TiO2 is the first and the most widely investigated photocatalyst applied to photocatalytic water splitting [4]. Despite of its good stability and good efficiency, the main limitation of TiO2 is imposed by its wide band gap energy (~3.2 eV). This band gap energy allows TiO2 to only absorb light with the energy above the ultraviolet (UV) range, which only occupies 4% of solar energy spectrum [4]. Many different types of materials, including carbon nitrides, metal oxides, organic materials, etc. have been proposed and studied as candidates for promising high efficiency photocatalysts [5,6].
Ferrites (MFe2O4), such as NiFe2O4, CoFe2O4, etc., are iron oxide-based transition metal oxides with a spinel lattice structure. Ferrites have attracted much research attention as photocatalysts due to their suitable band gap energy around 2.0 eV for visible light absorption [7] and their room temperature magnetization for easy recollection [8,9]. Besides, different spinel ferrites could present various bandgaps and band positions [10]. More importantly, for water splitting generation of H2, ferrites possess a good oxidation ability to promote the oxygen evolution reaction (OER). During the OER process or the consumption of photoexcited holes, the valence state of metal M transforms to a high-valent species with a redox couple (the exchange of low and high valence state in the metal ions, for example, the redox couple of Ni2+/Ni3+ during photocatalytic reaction). These redox couples are active species for oxidative reaction [11–13]. Hence, the stronger oxidation activity could accelerate the consumption holes or the whole redox reactions in a water splitting process. This activity could be expected in the water splitting H2 production process to enhance the efficiency. Therefore, various ferrites and ferrite consisted composite materials have been reported for photocatalytic H2 production.
In this paper, the reported results in ferrite-based photocatalysts applied in H2 production by water splitting will be briefly reviewed. Moreover, the effect of synthesis methods, morphology, and microstructures on the H2 production performance from several ferrites will be discussed first. Furthermore, ferrites/TiO2 and ferrites/g-C3N4 composites will be presented to demonstrate the synergic effects from both materials in enhancing the light absorption and the separation of the photo-generated charge carriers.
2 Basic principle of photocatalytic hydrogen generation from water
Utilization of light energy on a semiconductor photocatalyst to split water and produce hydrogen is a multi-electron process as expressed in Eq. (1), which is also referred to as photoreduction.
The required energy for the reduction process is 0 eV versus normal hydrogen electrode (NHE), at pH= 0. The electrons are provided from the semiconductor photocatalyst by light absorption. To induce the reduction of water, the electrons should possess a higher energy than that required for Eq. (1), i.e., the conduction band energy level of the semiconductor should be negative. At the same time, the photo-generated holes in the valence band participate in an oxidation process, including the generation of O2 from water, or the oxidation of other chemicals used as scavenger [14,15]. The overall process of photoreduction of water to hydrogen is schematically demonstrated in Fig. 1 [16]. The process involves light irradiation (1), light absorption and excitation of electrons and holes to the conduction band (CB) and valence band (VB), respectively (2), migration of electrons and holes to reactive sites (3), reduction and oxidation reactions on the reactive sites (4). In addition, the recombination of photoinduced electrons and holes occurs through path (5)–inside the catalyst and (6)–on the surface of the catalyst, which are well known as the main factors limiting the photocatalytic efficiency.
Accordingly, the effective strategy to enhance the photocatalytic H2 production is to reduce the charge recombination by inducing more migration paths for separation. Especially, a higher possibility for charges transferring to the surface of the photocatalyst, i.e., in step (3) of Fig. 1, can enable more charges to participate the redox reactions. In addition, since the water only can be adsorbed on the surface of the photocatalyst, the photocatalysts with a hydrophilic surface and large specific surface area are considered favorable. The photocatalytic production of hydrogen is mainly conducted in a liquid environment or an aqueous suspension state. Therefore, the recovery and collection of photocatalyst should be considered. Based on these requirements, it is obvious that the narrow bandgap energy and room temperature magnetization make ferrites suitable for photocatalytic H2 generation.
3 Ferrite photocatalyst
3.1 Basic structure of spinel ferrite
The partial unit cell of spinel ferrite AB2O4 is depicted in Fig. 2. There are 8 A sites and 16 B sites in which the metal cations occupy the tetrahedral coordination (A) and octahedral coordination (B), respectively. Basically, when A sites are occupied by M2+ cations and B sites are occupied by Fe3+ ions, the ferrite is called a normal spinel. If the occupancies of A sites are completely coordinated with Fe3+ ions, it is called inverse spinel [17]. However, most of the ferrites display an intermediate state. Ferrites can exhibit several types of magnetism, and their magnetic behaviors are directly influenced and tuned by the distribution of the ions between A and B sites mentioned above [18]. Most of the spinel ferrites are soft magnetic materials. They can be used as the magnetic-recoverable photocatalyst because they can be easily magnetized and demagnetized. According to the description of soft ferrites from Mathew et al. [19], the magnetic properties of soft ferrites are mainly induced by the interactions between some particular metallic ions with the oxygen ions. These interactions could create magnetic domains, which can be aligned in a magnetic field, resulting in a net magnetic response. The band gap energy could also be tuned by selecting the A2+ cation. Normally, the electronic structures or energy bands are mainly affected by the cationic characters and the corresponding d-d hybridized orbitals. A partially inverted structure by swapping an octahedral ion with a tetrahedral ion site could be used to adjust the net magnetic moments and electronic structures [18,20,21]. Therefore, the spinel ferrites could be easily collected by a magnet, which present several band energy features with different A and B sites, as shown in Figs. 3(a) and 3(b). However, the investigation of such tunning effect has rarely been reported in the application of photocatalytic reaction.
3.2 Application in photocatalytic hydrogen production
Lopez et al. synthesized CoFe2O4 nanoparticles through coprecipitation and mechanical ball milling, and suggested that the high-density surface oxygen vacancies generated during the ball milling process improve the photocatalytic hydrogen production [22]. The surface oxygen vacancies presumably could enhance the water adsorption capacity of the material. The formation and effect of oxygen vacancies were also investigated in Refs. [30,31]. Peng et al. reported that the NiFe2O4 nanoparticles prepared through a hydrothermal and calcination process by using cetyltrimethylammonium bromide (CTAB) as a template directing agent presented much better performance of hydrogen evolution reaction (HER: 154.5 mol/(g·h) than NiFe2O4 aggregation (HER: 16.1 mol/(g·h) prepared without adding CTAB [32]. The authors attributed the improved photocatalytic efficiency to the relative larger surface area and small particle size in well dispersed NiFe2O4 nanoparticles. Hong et al. prepared high crystalline mesoporous NiFe2O4 by using an aerosol spray pyrolysis method (ASPM) with a structure directing agent of Pluronic F127 [33]. Their results further clarified the importance of a larger surface area and crystallinity through comparing amorphous and well-crystallized NiFe2O4 with a similar morphology and size. Rekhila et al. also demonstrated the stability of NiFe2O4 in an aqueous media and under illumination during the photocatalytic hydrogen production process [21]. Guzmán-Velderrain et al. synthesized MgFe2O4 nanoparticles through a coprecipitation combined with hydrothermal treatment to keep the particles in a nanometric size [23]. A higher activity in photocatalytic hydrogen production of MgFe2O4 than TiO2 was demonstrated, which was attributed to the more efficient visible light absorption in MgFe2O4. Zazoua et al. reported the synthesis of MgFe2O4 nanoparticles with a diameter of approximately 1.8 nm and a specific surface area above 60 m2/g through layered double hydroxides [24]. Accordingly, the enlarged surface area induced a desired photocatalytic hydrogen production. Saadi et al. performed photoelectrochemical (PEC) studies on the hydrogen production property of CuM2O4 (M: Co, Fe, Al, Mn, and Cr) [25], the p type CuFe2O4 and CuCo2O4 presented a great promise as H2-photocathodes. In addition, the CuFe2O4 synthesized via a citric acid assisted sol-gel method had more uniform nanoparticles with a diameter of 80 nm and larger specific surface areas, as compared to those obtained from solid-state reaction and co-precipitation [26]. It is proposed that the larger surface area enables the photocatalysts to adsorb more photons, and the smaller crystalline size can provide a shorter traveled path and consequently a longer lifetime for the photocarriers. The oxalic acid is shown to be a promising sacrificial reagent for holes because its strong reductive ability could promote the consumption of photoinduced holes and enhance the separation of photoexcited electron/hole pairs. Lv et al. produced floriated ZnFe2O4 nanostructures constructed by porous nanorods with an average length of 122 nm and a diameter of 29 nm via a mild hydrogen thermal and thermal decomposition process, in the presence of CTAB as a template-directing reagent [27]. In comparison with flaky ZnFe2O4, floriated ZnFe2O4 presented an enhanced activity. It was attributed to the small crystalline size and special rod structure that are beneficial for the transfer and separation of the photo-induced carriers. Besides, the nanorods and mesoporous structure were favorable for releasing the CO2 produced from the oxidation of sacrificial reagent of methanol, which further promote the efficiency of hydrogen production. Dom et al. prepared highly crystalline ZnFe2O4 nanoparticles with a crystallite size of 35 nm using a rapid microwave irradiation method [28], and confirmed that a good crystallinity and a reduced particle size are beneficial for photocatalytic hydrogen generation. Recently, Rodriguez-Rodriguez et al. synthesized CoFe2O4, NiFe2O4, and ZnFe2O4 by utilizing a novel oil-in-water microemulsion method and compared their performance in photocatalytic hydrogen production [29]. ZnFe2O4 had the best performance, mostly due to more favorable electronic band positions as illustrated in Fig. 1(a). The more negative conduction band energy (–1.65 eV versus NHE) than other ferrites endorses ZnFe2O4 a stronger reduction force to promote the water splitting process.
The above literature review indicates that spinel ferrites can be used for hydrogen production through the photocatalytic water splitting reaction. The main factors that affect the hydrogen production efficiency, such as crystallinity, particle size, specific surface area, morphology, and bandgap structure, are collected in Table 1. Although it seems not meaningful to compare the efficiency between the study of different groups due to the different setups, the comparison in individual study could provide a clear guidance for the design of the spinel ferrite to be used in photocatalytic hydrogen generation. In addition, the stability and reusability are important for photocatalytic applications. In ferrite-based photocatalysts, the reusability is mainly attributed to the retrievability of the photocatalysts by using a magnet [34], and the stability of lattice structure is due to the existence of the Fe ions [13]. Jia et al. compared the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and FTIR spectrometer (FTIR) results before and after the photocatalytic reaction to demonstrate the good stability of CoFe2O4 [34]. Rekhila et al. demonstrated the remarkable corrosion stability of NiFe2O4 in an aqueous media [21]. The corrosion rates are 752 μm/a and 1427 μm/a under dark and light irradiation, respectively, indicating that NiFe2O4 has a very stable semiconductor property.
Tab.1 Summary of morphology, particle or crystalline size, crystallinity, and specific surface areas with corresponding HER properties |
| Materials/keywords | Method | Morphology | Particle size (crystalline size) | Surface area/(m2·g–1) | HER/(mol·g–1·h–1) |
|---|---|---|---|---|---|
| CoFe2O4 [22]/oxygen defects | Coprecipitation | Nanoparticles | 25(20); | 20.0 | 310.0 |
| Mechanical ball milling | Agglomerates | 100–500(5) | 4.0 | 490.0 | |
| NiFe2O4 [32]/surface area and size | Hydrothermal and calcination (CTAB) | Nanoparticles | (18.1) | 76.0 | 154.5 |
| Hydrothermal and calcination | Agglomerates | (18.1) | smaller | 16.1 | |
| NiFe2O4 [33]/crystallinity | ASPM | Mesoporous sphere | 200 High crystallinity | 121.0 | 44.0 |
| ASPM | Mesoporous sphere | 200 Low crystallinity | 278.0 | 9.0 | |
| MgFe2O4 [23] | Hydrothermal | Cubic | 90 | 53.0 | 81.0 |
| CuFe2O4 [26]/size | Sol-gel | Nanoparticles | 80 | – | 1720.0 |
| Coprecipitation | Irregular particles | Irregular size | – | 1333.0 | |
| Solid state | Aggregate badly | 1000 | – | 1060.0 | |
| ZnFe2O4 [27]/morphology | Hydrothermal and calcination | Porous nanorod | Length: 122 Diameter: 29 | 52.0 | 47.0 |
| Hydrothermal and calcination | Flaky | – | 51.0 | 17.0 | |
| ZnFe2O4 [28]/size | Rapid microwave solid-state | Nanoparticles | (35) | 4.6 | 133.4 |
| Solid-state | Agglomerates | (53) | 2.2 | 31.7 | |
| ZnFe2O4 CoFe2O4 NiFe2O4 [29] /band structures | Oil-in-water microemulsion reaction | Nanoparticle | 12–20 | 49 | 44.3 |
| Same | Nanoparticle | 12–20 | 64 | 16 | |
| Same | Nanoparticle | 12–20 | 65 | 16.1 |
4 Ferrite based composite photocatalysts for hydrogen generation
Although spinel ferrites can be used alone as photocatalysts, the efficiency needs to be improved for practical applications. It is well known that the recombination of photo-induced electron/hole pairs is one of the dominant reasons for the low photocatalytic activity in single material. In this vein, spinel ferrites-based composite photocatalysts have been reported with different compositions for water splitting hydrogen production. As exhibited in Fig. 4, various types of heterojunctions (I and II) between two semiconductors can be designed to promote the electron/hole separation. In this section, the performance of selected composite photocatalysts, mainly composed of ferrites with g-C3N4 or TiO2, will be discussed. The preparation procedure, morphology, and hydrogen production rate are listed in Table 2.
4.1 TiO2/ferrites
TiO2 is the first and literally the mostly studied semiconductor photocatalyst, since its photocatalytic effect was reported for the photo-electrolysis of water in 1972 [4]. To overcome the limited solar light absorption due to the wide band gap energy of TiO2 and the relatively high rate of recombination of electrons and holes, various modification approaches have been reported, including doping, surface modification, facet and morphology control, and formation of heterojunctions [35].
Composites which consisted of TiO2 and ferrite materials can have the advantages of enhanced visible light absorption and easy magnetic retrievability, due to the relatively narrow band gap and room temperature magnetization in most ferrite nanoparticles. Most earlier studies applied TiO2/ferrite composites to the degradation of aqueous pollutants [36–41]. In the work of Haw et al., the 3D urchin-like TiO2 microparticles were hydrothermally synthesized and decorated with CoFe2O4 magnetic nanoparticles (NPs) via the co-precipitation method [36]. The designed composite presented much improved performance of photodegrading methylene blue due to the lower recombination rate of the photoexcited charge carriers (pseudo-first order rate constant k/h: 0.7432 (produced composite)>0.2605 (synthesized 3D urchin-like TiO2)>0.1073 (commercialized rutile TiO2)> 0.0321 (synthesized CoFe2O4)). Ghosh and Gupta synthesized CoFe2O4/TiO2/rGO (rGO: reduced graphene oxide) through the co-precipitation method, in which it is proposed that the photo-generated electrons transfer easily from CB of TiO2 to rGO via CoFe2O4, leading to an effective spatial separation of the electrons and holes. As a result, the suppressed charge recombination led to an improved performance in the photodegradation of chlorpyrifos, methyl orange, methylene blue, and rhodamine B [37,38]. Furthermore, Wei et al. synthesized a S-N doped CoFe2O4/rGO/TiO2 nanocomposite via the facial vapor-thermal method. The S and N dopant created new defect energy levels to decrease the bandgap of TiO2. The defects cannot only improve the light adsorption, but also improve the separation of photoexcited electrons and holes [39]. The presence of CoFe2O4 granted the above mentioned composite photocatalysts with the magnetic recoverable feature.
The promising photocatalytic properties of TiO2/ferrite composites promoted their applications in photocatalytic hydrogen production. Hafeez et al. synthesized a magnetically separable reduced graphene oxide-supported CoFe2O4-TiO2 photocatalyst via the simple ultrasound-assisted wet impregnation method. It has been found that the integration of CoFe2O4 with TiO2 could induce the formation of Ti3+ sites or substitution of Ti4+ by Fe3+ and Co2+, which could induce the impurities or defects energy band levels and reduce the optical bandgap of TiO2 from 3.2 to 2.8 eV, as shown in Figs. 5(a) and 5(b) [42,43]. Additionally, the induced defect levels cause the photoinduced electron in the conduction band to move to these extra levels before returning to the valance band, leading to the suppression of electron-hole recombination and increased photocatalytic performance (HER in mol/(g·h)–1: 16673 (TiO2/CoFe2O4)>5336 (TiO2)) [43]. Similar observation was also reported for CuFe2O4-TiO2/rGO composite from the same group [44]. The hydrogen production rate was improved (see Table 2) due to a double charge separation, i.e., the photoinduced electrons transfer from CuFe2O4 to TiO2, then to rGO. Since the CB edge position of CuFe2O4 is more negative than that of TiO2, the electron transfer direction is different from that in the case of CoFe2O4/TiO2 composite. Interestingly, Kim et al. constructed a NiFe2O4/TiO2 core/shell structure and achieved a good hydrogen productivity [45]. Since the band energy levels of NiFe2O4 are included in the range of TiO2, the mechanism of charge transfer was described as follows: the photoexcited electrons from the conduction band of TiO2 shell moved on the surface of the shell and fall into the conduction band of the NiFe2O4 core, then the excess accumulated electrons fall into the valence band of NiFe2O4 and TiO2, and be excited to the conduction band again, which accordingly form a full flow cycle of the charge carriers. Consequently, NiFe2O4 can be considered as a co-catalyst to promote the separation of photoexcited electrons and holes, leading to an improved photocatalytic performance. It is obvious that due to the different band energy levels in each ferrite, the photoinduced carriers would flow in different pathways between ferrites and TiO2. The injection of the impurity level of metal ions to TiO2 is also a special feature coming from ferrites. On the other hand, it should be mentioned that despite the favorable effect of ferrite/TiO2 composite, more systematic experimental and theoretical investigation on the charge migration mechanism and photocatalytic mechanisms are desired.
4.2 g-C3N4/ferrites
g-C3N4 is a typical polymer semiconductor that has been recently discovered to have promising properties as photocatalysts [46]. Wang et al. first discovered the photocatalytic water splitting evolution over g-C3N4 in 2009 [47]. g-C3N4 has attracted intensive research attention due to its suitable bandgap energy (2.7 eV), easy preparation with low cost, good chemical stability, and non-toxicity [48]. Additionally, the C and N atoms hybridize with the sp2 orbital to form a highly delocalized π conjugated layer structure, which guarantees a high specific surface area [49]. However, when used alone, the insufficient visible light absorption, and especially the high recombination rate of the photogenerated electron-hole pairs, limit the photocatalytic activity of g-C3N4. The easy synthesis process, layered morphology, and the surface functional groups of g-C3N4 provide advantages in forming a well-contacted interface when used in heterojunction photocatalysts. In addition, the graphite like structure could support the growth of inorganic nanoparticles with a controlled size [50,51]. The combination of ferrites with g-C3N4 has been shown to effectively enhance the hydrogen production in water splitting.
Chen et al. reported the improved photocatalytic efficiency through coupling MgFe2O4 to g-C3N4/Pt [52]. Because the bandgap energy of MgFe2O4 is 2.0 eV (<2.7 eV), and the conduction band edge position is more positive than that of g-C3N4, the MgFe2O4/g-C3N4 heterojunction could be regarded as the type I band alignment (Fig. 4), and the recombination of electrons and holes may not be quite effective. However, the authors found that the loading of Pt on g-C3N4 surface can prevent the migration of electrons from g-C3N4 to MgFe2O4 because the electrons can be easily accepted by Pt nanoparticles. Accordingly, the photocatalytic hydrogen production rate was improved by 100 times (up to 300.9 mol/(g·h)) due to Pt induced charge separation. To investigate the advanced catalytic oxidation abilities of MgFe2O4, the linear sweep voltammetry (LSV) measurement was applied to study their electrocatalytic oxidation activities in oxygen evolution reaction. The results presented a lower onset potential of MgFe2O4/g-C3N4 than g-C3N4. It was concluded that MgFe2O4 cannot only extract the photoinduced hole from g-C3N4 to accelerate the charge transfer between MgFe2O4 and g-C3N4, but also act as an oxidative catalyst accelerating the oxidation reaction kinetics at g-C3N4 surface, as shown in Fig. 5(c) [52]. Similar characterizations have also been performed for CoFe2O4 and NiFe2O4 coupled with g-C3N4/Pt, and the results demonstrated that ferrites promote the separation of photoexcited electrons as well as possessing a superior surface oxidative catalytic activity. However, although both heterojunctions form a type II band alignment, the more negative VB edge position of CoFe2O4 relative to g-C3N4 supports a stronger driving force for the hole transfer from g-C3N4 to CoFe2O4 than in the case of g-C3N4/NiFe2O4. Therefore, the CoFe2O4/g-C3N4/Pt composite had a better performance than NiFe2O4/g-C3N4/Pt [53]. Recently, Aksoy et al. made a comparison of the photocatalytic hydrogen production activity among several types of ferrites (MFe2O4, M: Mn, Fe, Co, and Ni) coupled with g-C3N4 and found that NiFe2O4/mesoporous carbon nitride (mpg-CN) exhibited the best performance (Table 2) [54]. It is suggested that the better efficiency in NiFe2O4 may be explained by considering Ni as a good candidate for hydrogen evolution reaction catalyst due to its compact orbitals and low binding energy between hydrogen (1s orbital) and nickel (d orbital). The different morphology of NiFe2O4 nanoparticles (concave shape) as compared with other ferrites (spherical) should also be considered. It should be mentioned that although the oxidation ability of ferrites can be of advantage in enhancing the hydrogen generation, hole scavengers are normally necessary in promoting the production of hydrogen for most ferrite-based composites [54].
Tab.2 Summary of performances in composites of TiO2/ferrites and CN/ferrites |
| Materials | Synthesis methods | Morphology | HER/(mol·g–1·h–1) | Lamp | Ref. |
|---|---|---|---|---|---|
| TiO2 TiO2/rGO TiO2/CoFe2O4 TiO2/CoFe2O4/rGO | Ultrasound-assisted wet impregnation method (composite) | Non-special | 5336 9421 16673 76559 | UV-Vis | [43] |
| TiO2 TiO2/rGO TiO2/CuFe2O4 TiO2/CuFeO4/rGO | Ultrasound-assisted wet impregnation method (composite) | Non-special | 4640 9397 14719 35981 | UV-Vis | [44] |
| TiO2 TiO2/NiFe2O4 | Sol-gel/precipitation | Core/shell | 0 mL 18.5 mL | UV | [44] |
Note: In addition to enhanced light absorption and separation of h+/e− pairs, ferrites could also induce the metal and oxygen defect levels in TiO2 to further improve the photocatalytic activity. |
| Materials | Synthesis methods | Morphology | HER/(mol·g–1·h–1) | Lamp | Ref. |
|---|---|---|---|---|---|
| g-C3N4 (CN) CN/Pt CN/MgFe2O4 CN/MgFe2O4/Pt | Sol-gel /calcination | Non-special | 12.5 100.0 3.0 300.9 | Vis >420 nm | [52] |
| CN/Pt CN/NiFe2O4/Pt CN/CoFe2O4/Pt | Sol-gel /calcination | Non-special | 53.7 161.1 187.9 | Vis >420 nm | [53] |
| CN CN/MnFe2O4 CN/CoFe2O4 CN/NiFe2O4 | Liquid self-assembly | Uniformly nano-ferrites | 0.27 1.07 1.51 1.82 | Vis >420 nm | [54] |
Note: Different type of ferrites would induce different multi-step electron transfers in a composite with g-C3N4 due to different band energy alignments. Especially, the strong oxidation ability of ferrites would accelerate the hydrogen production in the composite of CN/ferrites. |
4.3 Other ferrite composites
In addition to the popular composites of ferrites/TiO2 and ferrites/g-C3N4, here two representative reports of ferrites composites are also listed. A sufficient charge transfer ability could promote the photocatalytic performance in a composite system. Kim et al. fabricated a bulk heterojunction of CaFe2O4/MgFe2O4 by using the simple polymer complex method [55]. The CaFe2O4 and MgFe2O4 phases formed an interpenetrating network on a nanometer scale in a particle, which contained many randomly mixed and interfacing 20–30 nm particles. The photogenerated carriers in each phase could easily diffuse to the interface and be separated, due to the similar diffusion length of carriers with the domain size in either phase and sufficient contact at the interface. The composite supported with co-catalysts of RuO2 and Pt had a quite high quantum yield for hydrogen evolution of 10.1% and 82.7 mol/(g·h) under visible light irradiation (450 W W-Arc lamp with UV cut-off filter, wavelength>420 nm). This work provides a guidance for fabricating a highly efficient photocatalyst configuration with bulk heterojunctions. A proper energy level distribution in a composite can also be used to protect a certain component from being photo-corroded during the photocatalytic reaction. For example, the accumulated holes on the CdS will oxidize S2– to S, resulting in an increased risk of photo-corrosion in CdS. In this vein, Yu et al. synthesized a composite of ZnFe2O4 decorated CdS nanorods through the solvothermal process [56]. The type II junction between ZnFe2O4 and CdS with a more negative conduction band level of ZnFe2O4 promotes the photoinduced electrons transfer from ZnFe2O4 to CdS, and the migration of holes occurs in the opposite direction, resulting in the CdS protection, electron/hole separation, and magnetic recyclability. Finally, the specific hydrogen evolution rate was achieved as 2.44 mmol/(g·h) with a much longer-term stability than CdS alone.
5 Conclusions
This paper briefly reviewed some of the ferrites and ferrite heterojunction composites (ferrite/TiO2 and ferrite/g-C3N4) for photocatalytic hydrogen production from water splitting. In the case of ferrites, the special feature of magnetic property promotes the easier collection of catalyst from aqueous suspension by using the magnetic field. Various approaches, including control of crystallinity, particles size, specific surface area, and morphology, have been used to efficiently improve the separation of photoinduced electrons and holes. ZnFe2O4 seems to have the best photocatalytic activity among common ferrites due to its higher conduction band level which can provide a stronger driving force for the reduction of water. Ferrites-based composites, especially with TiO2 and g-C3N4 are selected to review the photocatalytic hydrogen generation activity. Besides the convenient collection process, the addition of ferrites to TiO2 could induce some impurity or defects energy levels to increase light absorption, and suppress the recombination of electron/hole pairs. In the case of ferrite/g-C3N4 composite, normally, the photocatalysts shows an enhanced light absorption and separation of the charge carriers, and the highly efficient surface oxidation ability of ferrites also lead to enhanced kinetic reactions of electrons and holes to H+ ions and scavengers, respectively. However, even though the junction of ferrites-based composites could promote the efficient charges separation and magnetic recyclability etc., the low surface activity still limits the hydrogen evolution rate. Co-catalysts of noble metals or conductive carbonaceous materials are necessary to accelerate the kinetic reaction rate with water. Currently, to improve the surface reactivity and suppress the recombination of electrons and holes, most of the spinel-ferrites are loaded with noble metal nanoparticles of Pt and Au or graphene-based materials. Researchers can, in the future, investigate the synthesis and performance of loading single atoms (Pt, Au, etc.) or nonnoble cocatalyst on ferrites with a low cost and a high surface activity. The built-in electric field induced by facets controlling is also worth exploring to improve the separation of charge carriers and study the reactivities to hydrogen evolution reaction. Besides, various composite systems can be investigated depending on the different features of virous ferrites. Moreover, according to the requirements of HER, a comprehensively theoretical study on the structures of different types of ferrites could be conducted, as there are few reports in this regard.