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Review

Recent Developments in Heterogeneous Photocatalysts with Near-Infrared Response

by
Nan Cao
1,2,
Meilan Xi
1,
Xiaoli Li
3,
Jinfang Zheng
4,
Limei Qian
1,
Yitao Dai
1,2,
Xizhong Song
1,* and
Shengliang Hu
4,*
1
Jiangxi Fangzhu Pharmaceutical Co., Ltd., Xinyu 338000, China
2
Suzhou Institute for Advanced Research, University of Science and Technology of China, Suzhou 215123, China
3
Jiangxi Nafutang Pharmaceutical Co., Ltd., Yichung 336000, China
4
Xinyu Comprehensive Inspection and Testing Center, Xinyu 338000, China
*
Authors to whom correspondence should be addressed.
Symmetry 2022, 14(10), 2107; https://doi.org/10.3390/sym14102107
Submission received: 1 September 2022 / Revised: 3 October 2022 / Accepted: 9 October 2022 / Published: 11 October 2022
(This article belongs to the Special Issue Symmetry/Asymmetry in Heterogeneous Catalysis for the Activation of Small Molecules)
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Abstract

:
Photocatalytic technology has been considered as an efficient protocol to drive chemical reactions in a sustainable and green way. With the assistance of semiconductor-based materials, heterogeneous photocatalysis converts solar energy directly into chemical energy that can be readily stored. It has been employed in several fields including CO2 reduction, H2O splitting, and organic synthesis. Given that near-infrared (NIR) light occupies 47% of sunlight, photocatalytic systems with a NIR response are gaining more and more attention. To enhance the solar-to-chemical conversion efficiency, precise regulation of the symmetric/asymmetric nanostructures and band structures of NIR-response photocatalysts is indispensable. Under the irradiation of NIR light, the symmetric nano-morphologies (e.g., rod-like core-shell shape), asymmetric electronic structures (e.g., defect levels in band gap) and asymmetric heterojunctions (e.g., PN junctions, semiconductor-metal or semiconductor-dye composites) of designed photocatalytic systems play key roles in promoting the light absorption, the separation of electron/hole pairs, the transport of charge carriers to the surface, or the rate of surface photocatalytic reactions. This review will comprehensively analyze the four main synthesis protocols for the fabrication of NIR-response photocatalysts with improved reaction performance. The design methods involve bandgap engineering for the direct utilization of NIR photoenergy, the up-conversion of NIR light into ultraviolet/visible light, and the photothermal effect by converting NIR photons into local heat. Additionally, challenges and perspectives for the further development of heterogeneous photocatalysts with NIR response are also discussed based on their potential applications.

1. Introduction

Fossil fuels such as oil, coal, and natural gas are non-renewable energy sources that fail to afford long-term social development [1,2,3]. In addition to the continuous and irreversible energy loss, the employment of fossil fuels also releases toxic byproducts including wastewater, waste gas, and residues that seriously pollute the environment. Thus, the utilization of clean renewable energy has become crucial for the continuation of human life and sustainable development. Photocatalytic technology has been regarded as one promising way to help ease the issues of environmental pollution and energy shortage, because it can directly transform solar energy into valuable chemical energy through semiconductor materials [4]. Until now, semiconductor-based photocatalysis has been widely explored in several application fields, including H2O splitting, CO2 reduction, CH4 activation, organic synthesis, and the degradation of organic pollutants [5,6,7,8,9,10].
Typically, the working principles of semiconductor-based photocatalysis involve three main stages as follows: (1) the semiconductor-based heterogeneous photocatalysts absorb light with the photoenergy larger than bandgaps to produce electrons and holes; (2) electron-hole pairs are separated and transported to the surface of photocatalysts; (3) the photogenerated electrons and holes participate in the redox reactions whose reactants are adsorbed on the surface of photocatalyst [11]. However, due to the fast recombination of electrons and holes and limited active catalytic sites on surface, the solar energy conversion efficiency is suppressed to a quite low level (even <1%), which dramatically hinders the practical applications of photocatalysis [12]. Another reason comes from the common use of photocatalytic materials with wide bandgaps such as TiO2 and ZnO (~3.2 eV), which only respond to UV light consisting of a small part (5%) of solar energy [13,14,15,16]. Consequently, visible and near-infrared (NIR) light, which accounts for 46% and 49% of solar energy respectively, is hugely wasted [17]. Research efforts in recent decades have focused in improving quantum efficiency and broadening the responsive spectra of photocatalytic materials [18].
Through the design and preparation of narrow-bandgap photocatalysts [19], up-conversion materials [20], dye-sensitized materials [21], and plasmonic nanomaterials [22], the optical absorption spectra of photocatalysts can be effectively widened to even the NIR region. However, narrowed bandgaps of semiconductors are always accompanied by weaker redox ability, which may result in poorer photocatalytic activity [23]. This problem may be figured out by other synthesis protocols for the establishment of advanced photocatalytic systems, which involve up-conversion materials [24], dye sensitization [25], or photothermal effects [26]. These design concepts can lead to the asymmetric energy transformation (e.g., low-energy NIR light to high-energy UV or visible light) or the fabrication of asymmetric heterojunctions (e.g., dye-semiconductor or noble metal-semiconductor composites) with aligned energy level structures. Accordingly, the wide-bandgap semiconductor hosts (e.g., TiO2) with sufficient redox capability could couple with up-conversion materials, or dyes, or plasmonic metals to drive desired reactions with high activity under NIR light illumination [27,28,29]. At present, how to remarkably enhance photocatalytic efficiency with suppressed charge recombination and active catalytic sites on the surface under the irradiation of visible and NIR light is still challenging [30]. In addition, quite limited reviews have been published before for the comprehensive analysis of heterogeneous photocatalysis under NIR irradiation. For example, Wu et al. analyzed the recent progress of up-conversion photocatalytic systems with NIR response by demonstrating the synthesis and applications [31]. Tian et al. provided the review on the degradation of organic pollutants by the use of up-conversion materials under NIR illumination [24]. Currently, Yang et al. summarized the main photocatalysts with NIR response, including up-conversion, photothermal materials, and narrow-bandgap semiconductors [32]. Meanwhile, some similar reviews for heterogeneous photocatalysis under NIR light have been reported as well [25,33].
In contrast, this review mainly introduces the cutting-edge design approaches for the synthesis of efficient and robust heterogeneous photocatalysts with NIR response. Based on the reported cases, the photocatalytic performance and inherent mechanisms are thoroughly analyzed from the point of view of symmetry in material composition, electronic structure, and morphologies, which is not available in other reported reviews. In the conclusion part, the challenges and perspectives on developing novel efficient photocatalytic systems with NIR response are discussed, which may offer meaningful opinions and advice for future research directions of photocatalysis under NIR light.

2. Design Strategy of Near-Infrared Photocatalysts with Symmetric/Asymmetric Nanostructures and Properties

In the field of photocatalysis, NIR photocatalysts are attracting more and more attention because they present superior light-harvesting capability [25]. Currently, there exists four design routes to produce efficient photocatalytic materials with NIR response:
(1) Regulating the bandgap width of semiconductors [34]. Photocatalysts with NIR response can be prepared by altering their electronic structures to drive certain reactions with low redox potentials under NIR light irradiation [35,36]. Tuning the anisotropic facet of NIR-responsive photocatalysts can affect photocatalytic performance as well, which originates from the symmetry in crystal structures of photocatalysts [37].
(2) Converting long-wave NIR light into short-wave ultraviolet or visible light [31]. The absorbed NIR photons with low photon energy can be asymmetrically converted into high-energy photons by functional up-conversion materials. Simultaneously, the adjacent wide-bandgap semiconductor absorb the emitted UV/visible light to photo-catalyze the target redox reactions [38].
(3) Photosensitization. The establishment of asymmetric heterojunction systems consisting of sensitizers (such as organic dyes and metal complexes) and semiconductors is another appealing strategy [39]. These asymmetric hetero-structures can efficiently integrate multiple functions in one photocatalytic system, including the excitation of sensitizer by NIR light, the electron injection to semiconductors, and catalytic processes on the surface [40].
(4) Converting NIR photon energy into heat [41]. The heat produced via a photothermal effect can boost the surface redox reactions [42]. For instance, the surface plasmon resonance (SPR) not only generates the hot charges directly participating in the activation of reactants, but also offers a high local temperature on the surface of photocatalysts, which synergistically promote the photocatalytic performance [43]. Specifically, the asymmetric configuration in nanostructures, involving the loading of noble-metal nanoparticles on semiconductor-based supports, could result in the efficient two-step conversion of NIR photon energy into chemical energy with local heat as a transfer path [44].
According to the above design strategies, NIR-responsive photocatalysts can be divided into the following three categories: materials with direct NIR utilization, materials with indirect NIR utilization, and photothermal conversion materials.

2.1. Materials with Direct NIR Utilization

2.1.1. Narrow-Bandgap Materials

As shown in Figure 1, narrow-bandgap semiconductor-base materials can harvest NIR light (>700 nm) to initiate the photo-to-charge process. Tuning the valence/conduction band positions of semiconductors and developing novel narrow-bandgap materials are efficient protocols to achieve enhanced absorption of long-wavelength NIR light.
Due to the tunable narrow bandgaps, transition-metal sulfides have gained abundant attention in the field of photocatalysis driven by NIR light [45]. For example, by use of a simple ion-exchange method, Jiang et al. successfully prepared aggregated monoclinic Ag2S nanocrystals (0.2–0.7 μm), which were used as photocatalysts for the photocatalytic decomposition of methyl orange (MO) under visible or NIR light irradiation [46]. The crystal gains of as-prepared Ag2S material demonstrated a size range of 30~80 nm (Figure 2a) with the irregular shapes. The monoclinic phase of Ag2S sample indicates that the symmetry in space group belongs to P21/c (no. 14), designating the bond length of Ag2S as 2.692 Å with a bond angle of 105.894° (Figure 2b). This symmetry in crystal structure leads to the unique band structure of Ag2S, which presents an absorption edge in the NIR region (>800 nm) with a direct bandgap of 1.078 eV. Due to this narrow bandgap, the MO dye was completely photodegraded by Ag2S under visible light irradiation in 30 min, and under NIR light irradiation in 70 min (Figure 2c). Based on the proposed photocatalytic mechanism (Figure 2d), the asymmetric functions of photo-induced electrons and holes synergistically promote the oxidation of MO dye. Specifically, the holes could oxidize the organic molecules directly to generate decolored intermediates (because of losing conjugation), while the electrons may reduce O2 to reactive oxygen species such as superoxide anions (·O2−) [47]. The formed ·O2− could participate in the complete degradation of organic intermediates into CO2 and H2O. However, the proposed formation of ozonide radical (·O3−), as shown in Figure 2d, from O2 is quite doubtful, since the production of O3 always requires the ionization or high-energy UV excitation of O2 [48].
In addition, Sang et al. reported the synthesis of WS2 nanosheets with the hexagonal lattice structure from the calcination of (NH4)2WS4 at 1200 °C [49]. The WS2 nanosheet with a narrow bandgap of 1.39 eV decomposed 80% of MO dye within 300 min under the irradiation of a 5-W NIR LED light (with the emission range of 800–900 nm). This impressive NIR light activity mainly comes from the relatively long lifetime and suppressed recombination of photo-induced charge carriers. In addition, the WS2 hexagonal nanosheets demonstrated high stability with no obvious deactivation after five successive cycling tests. The promoted optoelectronic properties and strong stability of WS2 nanosheets under NIR light illumination probably originate from the symmetric and robust hexagonal microcrystals, presenting high-quality crystallinity with few lattice defects and high resistance of photo-corrosion.
Except for metal sulfides, black phosphorus (BP) as a two-dimensional material has gained plenty of attention due to the adjustable band structures for the NIR response [50,51]. The BP crystal presents a stacked layer structure, where layers interact with each other via van der Waals force with an interlayer distance of 3.21–3.73 Å. The light absorption data show that the bandgap of BP can be tuned from 0.3 to 2.0 eV when the bulk BP material is transformed into single-layer nanosheets. Thus, by precisely tailoring the number of BP layers, the absorption range can cover the whole ultraviolet-visible-NIR region with the controllable bandgaps, which may help to optimize the photocatalytic activity under NIR light [52].
In 2017, Zhu et al. firstly reported that BP nanoflakes coupled with carbon nitride (CN) as metal-free hybrid photocatalysts generated H2 gas in a water-methanol solution under NIR light (>780 nm) [53]. BP nanoflakes were obtained by exfoliating BP crystals in the presence of saturated basic NaOH/N-methyl-2-pyrrolidone (NMP) solvent. Without the use of any co-catalyst, the photocatalytic performance of BP/CN nanoflakes approached 101 µmol·h−1·g−1 under NIR light, while no H2 was detected with bare BP or CN as the photocatalyst. This incredible synergetic effect can be attributed to the asymmetric band structures of BP and CN, which lead to a type I heterojunction at their interface with BP as an electron acceptor. This electron trapping effect on BP originates in the formation of trap sites from P-N coordinate bonds, dramatically prohibiting the recombination of the photo-induced charges in BP/CN. Thus, more excited electrons under NIR light irradiation can be utilized for the efficient photocatalytic H2 generation via the promoted charge separation/transfer processes.

2.1.2. Defect-Engineered Materials

Defect doping is an effective method to compensate the intrinsic shortcomings of various semiconductors, including the poor optical absorption of wide-bandgap semiconductors and the weak redox capability of narrow-gap semiconductors [54]. In addition to the doping of metals or metal-free elements, vacancy-type defects can introduce impurity states between the valence band (VB) and conduction band (CB) to narrow the optical bandgap [55,56,57]. For example, Kong et al. successfully developed an ultraviolet-visible-NIR responsive Bi2WO6-OV photocatalyst via a facile one-step glycol-assisted solvothermal approach [58]. In comparison with pristine Bi2WO6 only permitting the excitation from VB to CB under UV-Vis light, the existence of oxygen vacancy (OV) defects on the surface of Bi2WO6 can introduce asymmetric band structures with sub-bands located below CB to capture low-energy NIR light (Figure 3). Thus, the presence of OV defects offers efficient electron excitation from sub-bands to CB for the photoreduction of CO2 to CH4 on the surface of Bi2WO6-OV photocatalyst under NIR light illumination. Furthermore, the fabrication of well-defined Bi2WO6-OV/TiO2 heterojunctions can facilitate the charge transmission between these two phases with diminished recombination of electron-hole pairs, which was evidenced by the decreased photoluminescence in comparison with that of Bi2WO6-OV and TiO2 alone [59]. In addition, this asymmetric heterostructure of Bi2WO6-OV/TiO2, where the bottom of teeth-like Bi2WO6-OV nanosheets closely attach to the surface of TiO2 nanobelts, provides a higher specific surface area (about 45.371 m2 g−1) to expose more active sites on the surface than Bi2WO6-OV (35.57 m2 g−1) and TiO2 (32.24 m2 g−1). Therefore, the promoted charge separation and larger surface area of Bi2WO6-OV/TiO2 hybrid photocatalyst leads to higher photocatalytic activity (degradation degree of 74.3%) than the single phase (58% from Bi2WO6-OV and 0% from TiO2) under NIR light irradiation for MO degradation.
In addition to Bi2WO6-based photocatalysts, BiO2−x monolayers with rich OV defects on the surface demonstrated the full-spectrum response with excellent photocatalytic activity for RhB decomposition under NIR light (reaching a degradation efficiency of 95%) [60]. In contrast with a bulk sample with a thickness of 5–10 nm, the BiO2−x monolayer material presented an average thickness decreasing to 1 nm, which belongs to the symmetric cubic phase with a space group of Fm 3 ¯ m. This ultrathin structure offered the (110) crystal plane as the main exposure facet, which possessed high surface energy (0.71 J m−2) based on density functional theory (DFT) calculations. The preferential exposure of high-energy facets may boost photocatalytic activity due to facile formation of rich OV defects as catalytically active sites [61,62,63]. Moreover, the asymmetric band structures of BiO2−x monolayers due to sub-states from OV defects not only narrow the optical bandgap for NIR absorption, but also promote the capture of photogenerated electrons under NIR light illumination with improved charge separation.
To further utilize OV defects in bismuth-based photocatalysts, Li et al. in situ fabricated a cost-effective Bi2O3−x photocatalyst with OV sites by facile calcination of commercial bismuth powders, as demonstrated by XRD data in Figure 4a [64]. The formation of the Bi2O3−x surface species can result in the strong absorption of NIR light from 600 to 1400 nm (Figure 4b), which mainly comes from the SPR effect induced by OV sites [57,65,66]. For this well-defined Bi@Bi2O3−x asymmetric heterostructure, the unique band structures and chemical environment on the surface caused the apparent quantum yield (AQY) of CO2 photoreduction at 940 nm to reach 0.113%, which was about 4.0 times that at 450 nm (Figure 4c). As expected, the pristine Bi sample without NIR response presented no activity at 650–940 nm (Figure 4d). Moreover, after three consecutive cycling tests, no obvious deactivation was observed for the Bi@Bi2O3−x photocatalyst under NIR light illumination (Figure 4e). This impressive photocatalytic performance can be ascribed to the rich OV defects on the surface, which not only bring about the NIR response due to th e SPR effect (Figure 4f), but also enhance the chemisorption of CO2 as trapping sites based on CO2 temperature-programmed desorption. Clearly, this Bi@Bi2O3−x photocatalyst with the NIR response demonstrates several advantages including low-cost synthesis, wide light absorption, and high stability for the photoreduction of CO2. Unfortunately, the low-level AYQ (<1%) still dramatically limits the practical application of photocatalytic technologies in the field of CO2 reduction. Determining how to make full use of NIR photons for target reactions would help break through the present bottleneck.

2.1.3. Conjugated Polymeric Materials

Currently, conjugated polymers (CPs) used as photocatalytic materials have aroused the interest of researchers due to their decent charge separation/transport and improved photocatalytic activity [67]. In particular, tuning the energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is one effective protocol to obtain CPs with NIR response [68]. The symmetric molecular structures of monomers lead to the construction of robust regular CPs, which facilitate the precise adjustment of the energy gap, as well as tuning the backbone (e.g., from benzene to thiophene ring) [69]. Moreover, the asymmetric block structures from the copolymerization of diverse chemical components can efficiently regulate the optical absorption, surface properties (such as hydrophilicity and hydrophobicity), charge transport, and the nanostructures of CPs [70,71]. These chemical-physical properties always strongly affect the ultimate photocatalytic performance through the optimization of light absorption, charge separation, and catalytically active sites [72].
Wang et al. have condensed hexa-ketocyclohexane octahydrate and 1,2,4,5-benzenetetramine tetrahydrochloride in NMP by use of sulfuric acid to synthesize aziridine nanosheets (aza-CMP) with high structural symmetry in the unit cell (Figure 5a) [73]. By increasing the stacking layer number of polymer nanosheets from 1 to 4, the bandgap of aza-CMP gradually becomes narrow, from 1.65 eV to 1.26 eV (Figure 5b). The UV-Vis-NIR absorption spectra of aza-CMP nanosheets demonstrates a broad light absorption extended to the NIR region (Figure 5c). Under NIR light illumination (λ > 800 nm), aza-CMP provided an average photocatalytic oxygen production rate of 0.4 µmol·h−1 (Figure 5d), while the typical polymer photocatalyst (C3N4) without NIR response presented no activity. As depicted in Figure 5e, the photo-induced holes with the powerful oxidative ability led to successful oxygen evolution under NIR excitation. Regarding the reaction mechanism for photocatalytic oxygen evolution over the aza-CMP photocatalyst, the water could be decomposed to O2 via four one-electron oxidation steps. Specifically, the accumulated holes on aza-CMP participate in the formation of adsorbed *OH, *O, and *OOH intermediates, resulting in the release of O2 after losing protons. Even though this work has systematically investigated the influence from nanostructures of CPs (stacking layer numbers) on their band structures by calculations, how to precisely control the layer numbers of aza-CMP by experiments and correlate it with the photocatalytic performance is still unrealized. The reported literatures clearly show that the fine adjustment in the chemical structure of CPs can vitally alter their band structures and thereby photocatalytic performance, offering a promising protocol for constructing efficient CP-based photocatalysts with NIR response [74,75]. In addition, how the symmetry of CPs’ chemical structures affects the optoelectronic properties and photocatalytic behavior could be another meaningful research question.

2.2. Materials with Indirect NIR Utilization

Although a narrow bandgap is one basic requirement for the semiconductors with NIR response, most narrow-bandgap semiconductors present poor NIR photocatalytic activity [76]. The deep recombination of photogenerated carriers in a single narrow-bandgap semiconductor is one main reason [77]. Therefore, it is highly urgent to design photocatalytic materials that can make full use of NIR photons through improved charge transport/separation to accelerate the occurrence of target reactions on surface. So far, the asymmetric combination of wide-bandgap semiconductors with up-conversion materials, photosensitizers, or photothermal materials can indirectly utilize NIR light with promoted transport and separation of photo-generated charges, which consequently boost the ultimate photocatalytic performance [24,30,78,79].

2.2.1. Up-Conversion Luminescent Materials

In comparison with the ordinary luminescence typically involving only one ground state and one excited state [80], up-conversion luminescence originates from multiple intermediate states [81]. As a result of an anti-Stokes process, the excitation of up-conversion materials by low-energy NIR light leads to the emission of high-energy luminescence including UV or visible light in an asymmetric way [82]. The basic working principles of photo-energy upconversion can be categorized into three paths (Figure 6): (1) excited state absorption (ESA in Figure 6a), where an ion is excited from the ground state to a high-energy excited state by sequential two- or multi-photon absorption [83]; (2) the energy transfer up-conversion path (ETU), where one excited ion receives the energy transferred by proximate excited ions to reach the excited states with high levels (Figure 6b) [84]; (3) photon avalanche (PA), which can be regarded as the combination of the above two up-conversion luminescence paths to result in a population of the high-energy states (e.g., E3 in Figure 6c) rapidly in an avalanche-like mode. When the ion at the excited state on E3 falls down to the ground states, it emits high-level photons to realize up-conversion luminescence [24].
At present, up-conversion inorganic materials have been successfully employed for NIR light-driven photocatalysis via the asymmetric coupling with wide-bandgap semiconductors. For instance, in 2016, Chatti et al. prepared a NIR-response photocatalyst composite consisting of the semiconductor MoS2 and an up-conversion material (NaYF4:Yb3+/Er3+) [85]. This hybrid photocatalytic system had the capability to transform NIR light (at 980 nm) into visible light (408, 523, 545 and 655 nm), which was absorbed by MoS2 to decontaminate organic pollutants. The asymmetric nanocomposites of MoS2-NaYF4:Yb3+/Er3+ presented a cauliflower-like morphology with up-conversion nanocrystals grown on the surface of MoS2 nanoflowers. Under 980 nm irradiation, 61% of Rhodamine B dye was decomposed over the MoS2-NaYF4:Yb3+/Er3+ photocatalyst, while negligible activities were observed in the case of pristine MoS2 and NaYF4:Yb3+/Er3+ samples. The much higher photocatalytic performance of this nanocomposite photocatalyst can be attributed to the excellent up-conversion properties of the NaYF4:Yb3+/Er3+ material and the flexible nanostructures of MoS2 nanosheets with rich catalytically active sites. Moreover, the authors claimed that the reactive oxidative species (ROS) of hydroxyl radicals came from the oxidation of water molecules on surface. However, this conclusion is quite suspicious for MoS2-based photocatalysts, because the holes from the VB position (locating at ~2.0 eV) of MoS2 present relatively insufficient oxidative ability to oxidize water directly (requiring a redox potential of 2.38 eV vs. NHE) [86]. Meanwhile, no characterization of electron paramagnetic resonance was conducted in this work to verify the presence of possible ROS.
Furthermore, nanomaterials with core-shell structures have attracted considerable attention due to the diverse functions from different core and shell materials and the close interaction between them, resulting in the well integration of multi-functions in a single system [87]. Zhang et al. prepared a rare earth-doped NaYF4@TiO2 core-shell composite by a simple hydrothermal method [27]. As shown in Figure 7a,b, a highly crystalline titanium dioxide shell with controlled thickness was coated on the fluoride rod without high-temperature treatment. The shell thickness of the monodisperse composite can be easily modified by changing the ratio between fluoride rods and titanium precursors. In addition, the doped NaYF4@TiO2 composite demonstrated a uniform rod shape with high symmetry in morphology, which offers homogeneous elemental distribution and controllable activity. After the excitation of NIR light, the up-conversion property of NaYF4:Yb,Tm rod materials could emit UV luminescence, which well matched the light absorption range of TiO2, as shown in Figure 7c. Figure 7d demonstrates the photocatalytic degradation of methylene blue (MB) over as-prepared rod samples. For contrast, both bare NaYF4:Yb,Tm and undoped NaYF4@TiO2 composites (without up-conversion properties) were also tested. Obviously, the NaYF4:Yb,Tm@TiO2 sample showed excellent photocatalytic decontamination of MB (~90% conversion) after 12 h irradiation of NIR light, while the other photocatalysts offered poorer activity. This work showed that the asymmetric core-shell structure with wide-bandgap semiconductors as shells can effectively utilize the high-energy luminescence from the up-conversion core materials under NIR irradiation. It should be noted that the thickness of shell materials plays a key role in the ultimate photocatalytic performance. Too thin shell may lead to the inefficient absorption of emitted UV luminescence, while too thick shell would bring the hinderance of NIR irradiation on core materials and the severe charge recombination in the shell layer with deceased activities. In a whole, this material synthesis strategy can successfully fabricate the core-shell hybrid photocatalysts with NIR activity based on up-conversion luminescence, presenting great potential for developing novel efficient photocatalytic materials with NIR response.

2.2.2. Photosensitization Materials

Establishing asymmetric heterojunctions of photosensitization materials and semiconductors is another efficient method to indirectly utilize low-energy NIR light for photocatalysis. As shown in Figure 8, when the photosensitization material (such as a dye molecule) is irradiated by the incident NIR photons, the photo-induced electrons can jump from the HOMO to the LUMO level. Later, these electrons can transfer to the semiconductor’s CB level for suppressed charge recombination. Finally, the accumulated CB electrons with the suitable reducing capability can participate in the target photocatalytic reactions.
Dye-sensitized photocatalytic systems have been studied for many years, but there are still limited reports using photosensitive systems for photocatalytic reactions under NIR irradiation [28,88]. Generally, the utilization of photo-induced charges in dye-sensitized systems can be well controlled by dyes adsorbed on semiconductor photocatalysts [89]. Photosensitizers used for photocatalysis in the NIR region should exhibit several basic properties, including a wide light absorption range, high electron transfer efficiency, strong regeneration ability after light excitation, good recovery, and low cost. Moreover, each dye has its own unique light absorption profile covering only hundreds of nanometers, which is also deeply relevant to their asymmetric/symmetric molecular structures. Accordingly, the integrated dyes can significantly affect photocatalytic activity under NIR irradiation. Presently, phthalocyanine dyes and their derivatives are popular organic dyes used in NIR photocatalysis since they have good chemical stability with unique band structures, whose light absorption is typically centered at 600–700 nm [90].
Takanabe et al. first prepared magnesium phthalocyanine (molecular structure presented in Figure 9a) modified mesoporous magnesium carbonitride (MPG-C3N4) by an impregnation method in the presence of tetrahydrofuran, which extended the light absorption spectrum from the visible to the NIR region [91]. Although its photocatalytic efficiency in hydrogen evolution was not high, this work was a good pioneering example for the exploration of dye-sensitized photocatalytic systems with NIR activity. Later, Zhang et al. successfully used the asymmetric zinc phthalocyanine (Zn-tri-PcNc) (Figure 9b) and the symmetric zinc naphthalocyanine derivative (Zn-tetra-Nc) to decorate the wide-bandgap semiconductor (TiO2) for photocatalytic H2 evaluation (Figure 9c) [28]. The optical absorption edge of the dye-sensitized semiconductor was successfully shifted to 600–800 nm. Meanwhile, the AQY of Zn-tri-PcNc/TiO2 hybrid photocatalyst for photocatalytic H2 production reached 0.2% under the irradiation of NIR light at 700 nm, while the Zn-tetra-Nc counterpart presented only half activity. This big difference mainly came from the distinct symmetry in the molecular structures of the above two dyes. The asymmetric structure of dyes could lead to the orientated electron transfer with a longer lifetime and suppressed charge recombination, which consequently boosted the photocatalytic H2 evolution under NIR irradiation. In addition, the asymmetric functional groups may promote the high dispersion of dyes on the surface of semiconductors, offering better heterojunction contact. Furthermore, this highly asymmetric zinc phthalocyanine derivative (Zn-tri-PcNc) with strong NIR absorption (650–800 nm) as a sensitizer can help to extend the spectral response of g-C3N4 from 450 nm to ~800 nm. Its AQY under the irradiation at 700 nm was reported to be 1.85% (Figure 9d). According to the mechanism study, Zn-tri-PcNc adsorbed on the surface of g-C3N4 generates photogenerated carriers under the excitation of NIR light, which are then injected into g-C3N4 and trapped by the Pt nanoparticles for hydrogen evolution [92].
Obviously, organic dyes or metal complexes as the powerful photosensitizers can extend the optical absorption of wide-bandgap semiconductors to the NIR region. The asymmetric hybrid photocatalysts consisting of dye-based molecular photosensitizers and semiconductors can also demonstrate high redox capability under Vis-NIR irradiation through the rapid electron transfer in these heterojunctions. However, the dye-sensitized systems usually exhibit poor recycling performance and a limited light absorption range. Therefore, it may be a better choice to integrate multiple dyes with different absorption ranges and bridge functional groups for robust deposition, which demonstrates the advantages of asymmetric design in compositions and molecular/electronic structures well.

2.3. Noble Metal-Based Photothermal Conversion Materials

The well-known photothermal effect of NIR absorption has been widely integrated into photocatalytic systems for redox reactions (such as CO2 reduction, H2O splitting, organic synthesis, and dye degradation) [93,94,95]. In particular, the plasmonic photothermal effect is considered as one promising approach to utilize NIR light indirectly for redox reactions, which is mainly because of the complementary combination with semiconductors and the high activity of metal nanoparticles for chemical reactions in traditional heterogenous catalysis [96]. As shown in Figure 10, the basic mechanism can be explained as follows: SPR occurs on the surface of metal nanoparticles after absorbing light with the subsequent decay of plasmons. The high-energy electrons (hot electrons) generated in this process escape from the metal nanoparticles and transfer to adjacent semiconductor’s CB. Meanwhile, the accumulated energy from SPR can decay through non-radiative pathways, which can release heat to the surrounding environment. This conversion process from photo energy to heat by SPR is called the plasmonic photothermal effect [93].
So far, the available photothermal materials mainly involve noble-metal nanoparticles, nanocarbons, metal-organic frameworks, and transition-metal composites. Due to their unique photothermal properties, these nanomaterials have been widely employed to establish efficient hybrid photocatalytic systems in the field of photocatalysis [95]. Among them, metal particles with the plasmonic effect have received huge attention [78]. Moreover, the SPR effect is strongly influenced by the symmetry in micro-shapes of metals (e.g., nano-cubes, rods, wires, sheets, or spheres) [97]. The variation of symmetry in the geometry of metal shapes can lead to the shift in the electric field density on the metal surface, which consequently alters the oscillation frequency of electrons for SPR and thus the optical absorption [98]. One typical synthesis strategy for the preparation of plasmonic photothermal materials is introducing asymmetric hybrid nanostructures, which involves the uniform loading of noble-metal nanoparticles on semiconductor supports. Metal nanoparticles can improve the photocatalytic properties of semiconductors through SPR energy transfer, plasmon sensitization, and photothermal effect. For instance, Zhang et al. investigated the SPR-induced photothermal effect from Ag on the optical absorption and photocatalytic properties of Ag/Bi2WO6 [99]. The loading of Ag nanoparticles obviously improved the light-harvesting ability of bare Bi2WO6, with an absorption edge extending from 450 to 800 nm. Then, the Ag-loaded Bi2WO6 exhibited higher photoactivity for phenol degradation than bare Bi2WO6 at the same reaction temperature. Furthermore, the difference in degradation rate between these two samples became greater if the solution temperature was not strictly controlled. Based on the study of photocatalytic mechanisms, both electronic and photothermal effects of Ag nanoparticles contributed to the enhancement of the photocatalytic activity of the Ag/Bi2WO6 supported photocatalyst, which could lead to the promoted charge excitation of semiconductors and the increase of solution temperature (from 48 to 68 °C), respectively. However, no monochromatic light sources were employed to specifically study the influence from the light absorption induced by Ag nanoparticles with the SPR effect on the ultimate photocatalytic performance. Additionally, the promotion from electronic and photothermal effects on reaction activity was mixed without unambiguous distinction by well-designed control experiments.
Similarly, Hu et al. prepared Au/Bi2WO6 heterojunctions by combining gold nanorods (Au NRs) with Bi2WO6 nanosheets using a thiol-assisted hydrothermal approach [100]. After Au NRs were attached onto Bi2WO6 nanosheets (Figure 11a,b), the visible absorption band became stronger and broader (Figure 11c). Impressively, the absorption band at 900 nm appeared for Au/Bi2WO6 heterojunctions due to the introduction of Au-NRs. Under visible light irradiation, the photocatalytic degradation rates of MO over Bi2WO6 nanosheets and Au/Bi2WO6 heterojunctions reached 58% and 71%, respectively, which matched well with the optical absorption data. In addition, the better photodegradation performance under NIR illumination for Au/Bi2WO6 heterojunctions mainly originated in the SPR effect of Au NRs with improved NIR light absorption. Regarding the electronic structure of this asymmetric heterojunction system, a Schottky barrier with the height of 0.7 eV was also formed at the interface, which could promote the charge separation with suppressed recombination of electron-hole pairs by trapping more charges. However, the photothermal effect from Au NRs was not discussed and investigated, which may lead to inadequate insights of the real photocatalytic mechanisms. While substantial improvements have been made in developing noble metal-semiconductor photocatalytic systems, there are still a few challenges and unclear questions to figure out. For example, due to the complexity of synthesis methods, the obtained noble metal particles usually have wide particle size distribution and uncontrolled morphology. Another interesting aspect involves the symmetry in the micro-shapes of metal nanoparticles, which should be thoroughly investigated to understand how it affects the photocatalytic performance. Therefore, developing more precise, controlled, and facile preparation strategies for noble-metal particles with well-defined nanostructures is quite important.

3. Conclusions

In the design and preparation of photocatalytic materials with NIR response, a number of effective synthesis protocols have been put forward by introducing asymmetric/symmetric nano-architectures, hybrid composites, and defective states. These approaches include narrowing bandgaps of semiconductors, indirectly converting NIR light into ultraviolet or visible light and producing heat based on the photothermal effect. Several diverse photocatalytic systems with asymmetric/symmetric micro-morphologies, compositions, and electronic structure have been created to harvest long-wavelength NIR light. For example, the narrow-bandgap semiconductors (e.g., metal sulfides and black phosphorus), the conjugated polymers with symmetric or asymmetric unit chemical structures, and the defected wide-bandgap semiconductors with asymmetric band structures via rich vacancy sites (e.g., oxygen vacancy) can offer the direct utilization of NIR light for desired redox reactions. However, the redox capability of photo-induced charges is severely limited by the low-level NIR photons, consequently shrinking their application scope. More importantly, the fast charge recombination in these semiconductors leads to the low quantum efficiency.
On the other hand, several efficient hybrid photocatalytic systems have been established with indirect NIR utilization. For instance, the asymmetric combination of up-conversion materials with semiconductors and the heterojunction of photosensitizers (e.g., dyes) or noble metal-based photothermal materials with wide-bandgap semiconductors can achieve decent photocatalytic activities under NIR light by maintaining sufficient redox capabilities. The indirect utilization of NIR photons demonstrates a few advantages, including asymmetric energy conversion by transforming low-energy NIR light into high-energy UV or visible photons in up-conversion systems. Meanwhile, the presence of photothermal materials can asymmetrically convert NIR photons to local heat, which could increase the reaction temperature with promoted photocatalytic activities. In dye-sensitized photocatalytic systems, the excited electrons from NIR irradiation on dyes could transfer to the conduction bands of adjacent wide-bandgap semiconductors, where accumulated electrons with strong reducing powder can boost the target reactions. Clearly, the nanostructures and element compositions of photocatalytic systems should be well designed in symmetry to precisely regulate their morphologies, electronic structure, and optical features, which thus significantly affect the final photocatalytic properties.
Unfortunately, the present applications of photocatalysts with NIR response in photocatalysis are quite limited, and mainly lie in the environmental decontamination and water splitting reactions. Thus, in the future, more potential applications in various fields, including water splitting, CO2 photoreduction, pollutant photodegradation, photocatalytic disinfection, as well as selective organic synthesis, should gain abundant research attention. Furthermore, from the perspective of practical applications, the existing photocatalytic materials still present poor utilization efficiency of solar energy, which leads to a bottleneck for their scalable application. This is mainly due to several factors, which involve the limited absorbed photon energy, unmatched redox capabilities, fast charge recombination, and less active catalytic sites from the photocatalytic materials. Therefore, more research work should be carried out to design robust and efficient photocatalytic systems by focusing on the above problems to realize higher solar-to-chemical conversion efficiency, excellent stability, and low cost catalyst preparation. At present, how symmetry in geometric, crystal and electronic structures of photocatalysts influence the photocatalytic performance remains an open question, which therefore deserves further systematic study.

Author Contributions

N.C., M.X., X.L., J.Z., L.Q., Y.D., X.S. and S.H. wrote the paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are openly available.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The scheme of photocatalytic materials with direct photo-to-charge conversion. (CB means conduction band, while VB indicates valence band; e− is electron and h+ means hole).
Figure 1. The scheme of photocatalytic materials with direct photo-to-charge conversion. (CB means conduction band, while VB indicates valence band; e− is electron and h+ means hole).
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Figure 2. (a) The scanning electron microscopic (SEM) image of Ag2S. (b) The unit−cell crystal structure of Ag2S. Yellow spheres indicate S atoms, blue ones indicate Ag atoms. (c) Photodecomposition of MO in the presence of Ag2S crystals under visible and NIR light irradiation. (d) The proposed photocatalytic mechanism for dye degradation over Ag2S photocatalysts. Adapted with permission from [46].
Figure 2. (a) The scanning electron microscopic (SEM) image of Ag2S. (b) The unit−cell crystal structure of Ag2S. Yellow spheres indicate S atoms, blue ones indicate Ag atoms. (c) Photodecomposition of MO in the presence of Ag2S crystals under visible and NIR light irradiation. (d) The proposed photocatalytic mechanism for dye degradation over Ag2S photocatalysts. Adapted with permission from [46].
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Figure 3. The proposed photocatalytic mechanisms of CO2 reduction to methane over (a) pristine Bi2WO6 (P) and (b) Bi2WO6 with oxygen vacancy defects (OV). Adapted with permission from [58].
Figure 3. The proposed photocatalytic mechanisms of CO2 reduction to methane over (a) pristine Bi2WO6 (P) and (b) Bi2WO6 with oxygen vacancy defects (OV). Adapted with permission from [58].
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Figure 4. (a) XRD profiles of pristine Bi and Bi2O3−x−based samples. (b) UV−Vis−NIR absorption data (inset: photos of Bi (left) and Bi2O3−x (right)). (c) Wavelength dependence of the AQYs for photocatalytic CO2 reduction on the Bi2O3−x-based photocatalyst. (d) Wavelength dependence of the AQYs for photocatalytic CO2 reduction on the Bi powder sample. (e) The stability test for photocatalytic CO production. (f) The localized SPR excitation on Bi2O3−x-based samples. Adapted with permission from [64].
Figure 4. (a) XRD profiles of pristine Bi and Bi2O3−x−based samples. (b) UV−Vis−NIR absorption data (inset: photos of Bi (left) and Bi2O3−x (right)). (c) Wavelength dependence of the AQYs for photocatalytic CO2 reduction on the Bi2O3−x-based photocatalyst. (d) Wavelength dependence of the AQYs for photocatalytic CO2 reduction on the Bi powder sample. (e) The stability test for photocatalytic CO production. (f) The localized SPR excitation on Bi2O3−x-based samples. Adapted with permission from [64].
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Figure 5. (a) The basic reaction for the synthesis of 2D layer−structured aza−CMP. (b) The energy diagram demonstrating the calculated positions of VBM and CBM of aza−CMP with different amounts of stacking layers. (c) UV−Vis−NIR absorption data of as−prepared aza−CMP nanosheets. (d) The photocatalytic O2 evolution over aza−CMP nanomaterial under NIR illumination (λ > 800 nm) including g−C3N4 for comparison. (e) The band structure of aza−CMP nanomaterial. Adapted with permission from [73].
Figure 5. (a) The basic reaction for the synthesis of 2D layer−structured aza−CMP. (b) The energy diagram demonstrating the calculated positions of VBM and CBM of aza−CMP with different amounts of stacking layers. (c) UV−Vis−NIR absorption data of as−prepared aza−CMP nanosheets. (d) The photocatalytic O2 evolution over aza−CMP nanomaterial under NIR illumination (λ > 800 nm) including g−C3N4 for comparison. (e) The band structure of aza−CMP nanomaterial. Adapted with permission from [73].
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Figure 6. The basic principles of up-conversion processes. (a) Excited state absorption path. (b) Energy-transfer up-conversion path. (c) Photon avalanche path. Adapted with permission from [32].
Figure 6. The basic principles of up-conversion processes. (a) Excited state absorption path. (b) Energy-transfer up-conversion path. (c) Photon avalanche path. Adapted with permission from [32].
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Figure 7. (a) The up-conversion based photocatalytic mechanisms over the NaYF4:Yb,Tm@TiO2 core-shell hybrid photocatalysts with different shell thicknesses (left: thick shell, right: moderate shell). (b) The synthetic procedure for the construction of NaYF4:Yb,Tm@TiO2 core-shell hybrid photocatalysts. (c) Optical absorption profiles of NaYF4:Yb,Tm@TiO2 core-shell nanocomposite and NaYF4:Yb,Tm rods. (d) The photocatalytic degradation of MB under NIR irradiation over NaYF4:Yb,Tm@TiO2 photocatalyst, including NaYF4@TiO2 and NaYF4:Yb,Tm microrods as references (inset: the optical images of the MB solutions after photodegradation over (left) NaYF4@TiO2 and (right) NaYF4:Yb,Tm@TiO2). Adapted with permission from [27].
Figure 7. (a) The up-conversion based photocatalytic mechanisms over the NaYF4:Yb,Tm@TiO2 core-shell hybrid photocatalysts with different shell thicknesses (left: thick shell, right: moderate shell). (b) The synthetic procedure for the construction of NaYF4:Yb,Tm@TiO2 core-shell hybrid photocatalysts. (c) Optical absorption profiles of NaYF4:Yb,Tm@TiO2 core-shell nanocomposite and NaYF4:Yb,Tm rods. (d) The photocatalytic degradation of MB under NIR irradiation over NaYF4:Yb,Tm@TiO2 photocatalyst, including NaYF4@TiO2 and NaYF4:Yb,Tm microrods as references (inset: the optical images of the MB solutions after photodegradation over (left) NaYF4@TiO2 and (right) NaYF4:Yb,Tm@TiO2). Adapted with permission from [27].
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Figure 8. The scheme of basic working principles over a hybrid photocatalytic system, involving the asymmetric heterojunction between a photosensitization material and wide-bandgap semiconductor. Reprinted with permission from [32].
Figure 8. The scheme of basic working principles over a hybrid photocatalytic system, involving the asymmetric heterojunction between a photosensitization material and wide-bandgap semiconductor. Reprinted with permission from [32].
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Figure 9. (a) The molecule structure of MgPc [91]. (b) The molecular structure of Zn−tri−PCNC [28]. (c) The UV−Vis−NIR absorption and the AQY data of Zn−tri−PcNc/TiO2 [28]. (d) The possible photocatalytic mechanism for H2 production over the Zn−tri−PcNc/g−C3N4 photocatalyst [92]. Reprinted with permission from references [28,91,92]. Copyright © 2014, American Chemical Society.
Figure 9. (a) The molecule structure of MgPc [91]. (b) The molecular structure of Zn−tri−PCNC [28]. (c) The UV−Vis−NIR absorption and the AQY data of Zn−tri−PcNc/TiO2 [28]. (d) The possible photocatalytic mechanism for H2 production over the Zn−tri−PcNc/g−C3N4 photocatalyst [92]. Reprinted with permission from references [28,91,92]. Copyright © 2014, American Chemical Society.
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Figure 10. The scheme of a hybrid photocatalytic system constructed by a photothermal material and semiconductor for desired redox reactions. Reprinted with permission from [32].
Figure 10. The scheme of a hybrid photocatalytic system constructed by a photothermal material and semiconductor for desired redox reactions. Reprinted with permission from [32].
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Figure 11. SEM data from Bi2WO6 nanosheets (a) and Au/Bi2WO6 heterojunctions (b). (c) Optical absorption profiles of Au NRs, Bi2WO6 nanosheets, and Au/Bi2WO6 heterojunctions. (d) The photocatalytic decontamination of MO over Bi2WO6 nanosheets and Au NR/Bi2WO6 heterojunctions under the irradiation of UV−, visible−, and NIR light. Adapted with permission from [100].
Figure 11. SEM data from Bi2WO6 nanosheets (a) and Au/Bi2WO6 heterojunctions (b). (c) Optical absorption profiles of Au NRs, Bi2WO6 nanosheets, and Au/Bi2WO6 heterojunctions. (d) The photocatalytic decontamination of MO over Bi2WO6 nanosheets and Au NR/Bi2WO6 heterojunctions under the irradiation of UV−, visible−, and NIR light. Adapted with permission from [100].
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Cao, N.; Xi, M.; Li, X.; Zheng, J.; Qian, L.; Dai, Y.; Song, X.; Hu, S. Recent Developments in Heterogeneous Photocatalysts with Near-Infrared Response. Symmetry 2022, 14, 2107. https://doi.org/10.3390/sym14102107

AMA Style

Cao N, Xi M, Li X, Zheng J, Qian L, Dai Y, Song X, Hu S. Recent Developments in Heterogeneous Photocatalysts with Near-Infrared Response. Symmetry. 2022; 14(10):2107. https://doi.org/10.3390/sym14102107

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Cao, Nan, Meilan Xi, Xiaoli Li, Jinfang Zheng, Limei Qian, Yitao Dai, Xizhong Song, and Shengliang Hu. 2022. "Recent Developments in Heterogeneous Photocatalysts with Near-Infrared Response" Symmetry 14, no. 10: 2107. https://doi.org/10.3390/sym14102107

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