AbstractSemiconductor photocatalysis is proven to be one of the potential approaches to solve energy crisis and environmental problems. Efficient solar energy utilization and superior charge carrier separation capacity are two crucial aspects in pho-tocatalysis. Herein, the photocatalytic performances of the pristine and modified tungsten-based materials with mixed valence state are summarized concisely. The narrow band gap energy, coexistence of W5+/W6+ and the oxygen vacancies all contribute to the pristine tungsten-based photocatalysts with unique ultraviolet (UV), visible (Vis), and near-infrared (NIR) light-induced photocatalytic activities. Furthermore, the enhanced localized surface plasmonic resonance (LSPR) effect, improved charge carrier separation efficiency and prolonged charge carrier lifetime all boost the performances of modified tungsten-based heterojunction photocatalysts. Moreover, multifunctional tungsten-based photocatalysts with mixed valence state are established to realize the full utilization of solar energy authentically. Concluding perspectives on the challenges and opportunities for the further exploration of tungsten-based photocatalysts are also presented.
In the period of sustainable development, photocatalytic technology continues to be a promising approach in con-trolling the energy crisis and environmental contaminations [1–3]. The exploration of TiO2, ZnO, etc. and the traditional high-efficiency ultraviolet (UV) light-triggered photocata-lysts never stopped. To overcome the inadequate solar light utilization of photocatalysts, several strategies such as ion doping [4, 5], dye sensitization [6, 7] and heterostructure construction [8, 9] were proposed, and various novel pho-tocatalysts with narrow band gap (Ag3PO4, BiVO4, CdS, etc.) were also investigated [10–14]. That is, UV- and visible (Vis)-light-active photocatalytic systems have been explored extensively. Nevertheless, the near-infrared (NIR) light that contains ~ 50% of sunlight energy still remains untapped because of its low photonic energy and intense thermal
effect, which extremely restrains the solar energy conver-sion [15, 16]. Thus, paving a direct pathway to fully harness NIR photons is a valuable challenge for the development of photocatalytic systems.
To date, several strategies have been devoted to investi-gate NIR-light-induced photocatalysts: (1) employing semi-conductors with a band gap of less than 1.6 eV to utilize NIR light directly, such as Ag2O [12, 17, 18], WS2 [19, 20] and Bi2WO6 with oxygen vacancies [21]; (2) introducing up-conversion materials into the traditional photocatalyst system, which converts NIR photons to Vis even UV light, and then excites the photocatalytic reactions mediately, such as NaYF4:Yb3+, Tm3+@TiO2 [22, 23], Pt/CdS/NaYF4:Yb3+, Er3+ [24], CQDs/Cu2O [25]; and (3) NIR-light-responsive dye-sensitized photocatalyst systems, such as MgPc/Pt/mpg-C3N4 [26] and CuPc/Bi2MoO6 [27]. Although these explorations reveal the possibility of further development of the NIR-light-triggered photocatalysis, some shortcom-ings hinder the sufficient utilization of solar light, including a narrow absorption range (808 nm or 980 nm), and low NIR to the UV/Vis conversion efficiency of up-conversion materials, as well as the high charge carrier recombination probability and the low charge carrier transfer process [12, 28, 29]. Hence, it is imperative to seek efficient and practical full-spectrum-response photocatalysts.
In recent years, a group of vacancy-rich tungsten-based materials with the general formula of MxWO3 (M = Na+, K+, Rb+, Cs+, NH4
+, and so on) or WO3–x have been fully inves-tigated as electroconductive devices [30, 31], electrochromic materials [32–34] and NIR-shielding films [35–40]. More importantly, these mixed valence state tungsten-based mate-rials appear to be well-suited candidates for photocatalysis with various unique optical properties [41–46]: (1) the nar-row bandgap energy promotes their strong UV and Vis-light absorption capacities; (2) the presence of the W5+ ion acceler-ates the absorption of NIR light owing to the metastable state induced photosensitive effect and localized surface plasmonic resonance (LSPR). Moreover, the conversion between W5+ and W6+ releases plenty of free electrons and then induces the photocatalytic performances; and (3) the existence of the oxy-gen vacancies constitutes localized states just below the con-duction band, which greatly boosts the charge carrier transfer. The exploration and development on the mixed valence state tungsten-based material points to a new path for the develop-ment of new full-spectrum photocatalysts.
In this review, the emphasis on the single-component tungsten-based photocatalyst is introduced at first to uncover the fundamental principle of the unique photocatalytic per-formance. Then, we outline different tungsten-based com-posites to discuss the role of mixed valence state tungsten in the enhanced photocatalytic effects. Finally, multifunctional applications of the tungsten-based photocatalysts are enu-merated with the aim of the directing research into these areas.
2 Single‑component tungsten‑based photocatalyst
2.1 Single‑component tungsten bronze
In typical tungsten bronze MxWO3, the WO6 octahedra con-stitute a type of layered frame structures by edge-sharing
or corner-sharing connection [47, 48]. The layer is grown along the c-axis, and the solute ion M occupy in the holes or tunnels; that is to say, M is embedded in the channel of the WO6 framework. These tunnel alkali cations in the tungsten bronzes bring various interesting proprieties, such as super-conducting transition, NIR-absorbing property, and electri-cal conductivity [49].
CsxWO3, which is formed by doping Cs+ into the WO3 lat-tice, has been widely investigated as the NIR-shielding mate-rials these years, owing to its particular optical and electri-cal properties. Besides that, the strong Vis-light absorption capacity caused by the narrow band gap of 2.5 eV and the unique NIR-light absorption capacity generated by the LSPR both promote the optical absorption property of CsxWO3 in the wide range of 200–2500 nm (Fig. 1a) [50]. To evaluate the intrinsic photocatalytic performance of CsxWO3, methyl-ene blue (MB) degradation was carried out under respective irradiation of UV, Vis, and NIR light. As shown in Fig. 1b, it exhibited prominent photocatalytic decoloration activi-ties no matter which kind of light irradiated. In the W6+ and W5+ co-existed CsxWO3 compound, chromophore W5+ could be excited by the NIR light to release the electrons and converted to W6+ (Eq. 1). Subsequently, various reac-tive oxygen species such as ·OH, ·O2
−, and ·OOH are gener-ated by trapping photogenerated electrons, and then degrade kinds of pollutions (Eqs. 2–4). Meanwhile, W6+ ions could react with ·OH and convert back to W5+, realizing a full photocatalytic circle (Eq. 5) (Fig. 1c). Furthermore, as a semiconductor with a narrow band gap, CsxWO3 could also be sensitized by UV or Vis-light and accelerate the photo-catalytic decomposition of organic pollutants into non-toxic substances. Benefited from the above two factors, CsxWO3 possesses full-spectrum photocatalytic performance:
(1)W5+ + hv(NIR) → W6+ + e−
(2)e− + O2 → ⋅O−
2+ H+
→ ⋅OOH → ⋅OH
Fig. 1 a Powder absorbance spectra of CsxWO3 nanorods and con-trol of CsxWO3+x/2. b Photocatalytic activity in degradation of MB under conditions of (i) only CsxWO3 without light irradiation, (ii) CsxWO3 + NIR light irradiation (> 800 nm), (iii) CsxWO3 + visible light (400–800 nm), (iv) CsxWO3 + UV light irradiation (< 400 nm),
(v) NIR irradiation only, and (vi) CsxWO3+x/2 + NIR light irradiation (> 800 nm), respectively; c Schematic diagram of polaron hopping between the W6+ and W5+ states. Reproduced with permission from Ref. [50] Copyright 2016 Elsevier
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2.2 Single‑component tungsten oxide (WO3–x)
WO3–x is a class of suboxides including W3O8, W18O49, W17O47, W25O73, which has a suitable energy band gap (Eg) ranged from 2.4 to 2.8 eV [51, 52]. Among them, monoclinic W18O49 attracts plenty of attention as the high-efficient photo-catalyst owing to its mixed W5+/W6+ ions and plenty of oxygen vacancies on the surface (Fig. 2a, b), which endow W18O49 with a wide absorption tail in the solar spectrum and incompa-rable photocatalytic response [44, 53, 54]. To demonstrate the significant role of oxygen vacancies during the photocatalytic process, Xi et al. [55] employed H2O2 as the oxygen-vacancy sacrificial agent to prepare a series of WO3–x samples with different oxygen-vacancy concentrations. As shown in Fig. 2c, the as-synthesized W18O49 nanowires exhibited unexpected ability to photochemically reduce carbon dioxide to methane, while the photocatalytic activity decreased obviously with the prolonging of H2O2 oxidation time. This phenomenon proves the crucial roles of the oxygen vacancies and mixed valence state of tungsten in the charge carrier separation process, and further demonstrate the outstanding photocatalytic capacity of W18O49 under solar light irradiation.
3 Tungsten‑based heterojunction
Although the single tungsten-based mixed valence state pho-tocatalyst exhibits some full-spectrum photocatalytic capaci-ties, its photocatalytic efficiency is still poorly compared
(3)⋅O−
2+ ⋅OH →
1O2 + OH−
(4)W6+ + OH−→ W5+ + ⋅OH
(5)⋅O−
2, ⋅OH, 1O2 +MB → Degradation ofMB
with other photocatalysts, which could be attributed to the high charge carrier recombination probability, so it is impor-tant to optimize the carrier dynamics [56, 57]. One of the most effective methods is to construct the heterojunction with other traditional photocatalysts properly, which can not only further improve the solar light utilization efficiency, but also restrain the recombination rate of the photogen-erated charge carrier [4, 58, 59]. Basically, two different heterojunctions were proved to be efficient for enhancing the photocatalytic activity: conventional type-II heterojunc-tion and Z-scheme heterojunction, as illustrated in Fig. 3. Then recent state-of-the-art accomplishments of different tungsten-based heterojunction are concisely summarized and highlighted, which is categorized through diverse forms and compositions.
3.1 Tungsten‑based type‑II heterojunction
The fabrication of the g-C3N4/CsxWO3 composite was car-ried out by an electrostatic attraction method [60]. Herein, to reveal the superiority of type-II heterojunction in regu-lating carrier dynamics, the performances and mechanism of this composite are introduced in considerable detail. As displayed in Fig. 4a, b, the tight contact between two phases greatly expedite the operation of the synergistic effect in these composites, which is proved by the photocatalytic and photoelectrocatalytic H2 production, rhodamine B (RhB)/phenol degradation under UV, Vis or NIR-light irradiation. As can be seen in Fig. 4c, d, bare CsxWO3 can generate H2 or degrade RhB under full-spectrum light (365–940 nm) irradiation in a certain extent. As expected, after the intro-duction of the single layered C3N4 photocatalyst, the g-C3N4/CsxWO3 composite exhibits an evidently enhanced photo-catalytic H2 evolution activity and RhB degradation rate, which are nearly as 1.5 times as bare g-C3N4 and 23 times higher than bare CsxWO3, respectively. Furthermore, when NIR laser with a specific wavelength is used as the light
Fig. 2 XPS of W18O49 crystals. a W4f core-level spectrum with peaks corresponding to W6+ and W5+; b O 1 s core-level spectrum; c time courses of CH4 production over the tungsten oxide samples with
different oxygen-vacancy concentrations. a, b Reproduced with per-mission from Ref. [42] Copyright 2017 Elsevier, c reproduced with permission from Ref. [55] Copyright 2012 Wiley
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source, dramatic enhancements of the water splitting and RhB degradation capacities also could be detected after
the combination of g-C3N4 with CsxWO3 (Fig. 4e, f), while pure g-C3N4 and CsxWO3 show nearly no response under the same conduction. Furthermore, as shown in Fig. 4g, h, the photocurrents and the Nyquist impedance plots under different circumstances all demonstrate that the enhanced full-spectrum photocatalytic activities can be ascribed to the improvement of the charge carrier separation efficiency, indicating the synergistic effect inside the g-C3N4/CsxWO3 heterojunction.
In detail, the synergistic effect can be divided into vari-ous categories under different conditions. First, under UV light irradiation, both of g-C3N4 and CsxWO3 can be excited to produce the electron–hole pairs. Driven by the conduc-tion band (CB) and valance band (VB) potential differ-ences, photogenerated electrons and holes transfer direc-tionally between g-C3N4 and CsxWO3. In this situation, the charge separation efficiency and reactive oxygen species (·OH and ·O2
−) concentration are improved dramatically, which greatly promote the UV light-induced photocatalytic responses. Second, under short-wavelength Vis-light irra-diation, π–π* transition occur inside g-C3N4, while CsxWO3 still maintains the ground state owing to its wide band gap
Fig. 3 Schematic illustrations of the a type-II heterojunction and b direct Z-scheme system. Reproduced with permission from Ref. [61] Copyright 2016 Wiley
Fig. 4 a, b TEM images of the g-C3N4/CsxWO3 heterojunction; Pho-tocatalytic c, e H2 production and d, f RhB degradation over pure g-C3N4, pure CsxWO3 and different g-C3N4/CsxWO3 heterojunctions under various light irradiation; g photocurrent measurements under visible light irradiation and h Nyquist impedance plots in dark for
g-C3N4, CsxWO3 and g-C3N4/CsxWO3 heterojunction (the y axis of Fig. 4h should be “−Z’/ohm”); i schematic photocatalytic mechanism for the g-C3N4/CsxWO3 heterojunction under UV, Vis, and NIR-light illumination. Reproduced with permission from Ref. [60] Copyright 2018 Elsevier
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(Eg = 3.21 eV). Similar to the UV light irradiation situation, driven by the CB potential difference, the charge separation is accelerated and then realizes the efficient photocatalytic processes directly. Furthermore, when the incident light wavelength is broadened to the red region and even NIR region, the LSPR is induced inside CsxWO3. In this situa-tion, the hot electrons injected from CsxWO3 to g-C3N4 sig-nificantly increase the electron concentration and accelerate ·O2
− generation, while more holes are accumulated on the VB of CsxWO3 to degrade RhB directly, so as to enhance the Vis and NIR-light-triggered photocatalytic performance dramatically.
Similar tungsten bronze-based hybrid photocatalysts with accelerated photocatalytic activities could be detected in various type-II heterojunctions. Owing to the synergistic effect between two components, P25/(NH4)xWO3 hetero-junction exhibits dramatical improvement in the photogen-erated charge carrier separation efficiency, which results in the enhanced photocatalytic RhB degradation response under UV or Vis-light illumination [41]. Besides, the stepped band structure inside the heterojunction promotes the transfer of the hot electrons as soon as the NIR-light irradiates, which promotes the full-spectrum photocatalytic capacities ultimately. Ag3VO4/RbxWO3 nanocomposites also perform enhanced photocatalytic activities in decom-position of MB under various light illumination [62], which are originated from the extended optical absorption in the entire UV–Vis–NIR region, intimate contact between the
two semiconductors and efficient separation of photogen-erated electron–hole pairs.
3.2 Tungsten‑based Z‑scheme heterojunction
Although the photogenerated charge carriers are separated effi-ciently under the driving of type-II heterojunction, the redox ability of electrons and holes is sacrificed due to the spontane-ous transfer to low reduction and oxidation potentials for elec-trons and holes, respectively [61, 63]. In contrast, Z-scheme heterojunction endows the tungsten-based composite with more positive VB and more negative CB potentials, which pos-sess stronger oxidative holes and reductive electrons to carry out the enhanced photocatalytic performances [59, 64, 65].
Zhang et al. [66] reported a fascinating nonmetal plas-monic Z-scheme W18O49/g-C3N4 heterostructure, which exhibits strong photon energy harvesting capacity from the UV to NIR-light region and possesses intense photocata-lytic H2 evolution activities compared with pure W18O49 and pure g-C3N4 (Fig. 5a–d). It is worth noting that pure W18O49 always remains photoinertness in H2 generation under irra-diation of various light, which is restrained by its low CB potential (0.2 eV vs. normal hydrogen electrode). Upon the introduction of W18O49 onto g-C3N4 layers, band bending takes place between these two phases, and the electrons on the CB of W18O49 flow to the VB of g-C3N4, which quench the holes to realize the “Z-scheme” charge transfer pathway. In this situation, the recombination of excitons is hindered,
Fig. 5 a UV–Vis–NIR absorption spectra of the as-synthesized sam-ples; Time-dependent photocatalytic H2 evolution curves of the as-synthesized samples under b simulated sunlight irradiation, c visible-light irradiation, and d IR-light irradiation: (i) g-C3N4 nanosheets, (ii) W18O49/g-C3N4 heterostructure, and (iii) W18O49; e energy band
configuration and photoinduced charge carriers generation/transfer process in the nonmetal plasmonic Z-scheme photocatalyst with the W18O49/g-C3N4 heterostructure under UV–Vis–NIR light excitation. Reproduced with permission from Ref. [66] Copyright 2017 Wiley
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and the lifetime of the electrons maintained in the CB of g-C3N4 is prolonged to produce much more H2 under UV or Vis-light illumination. When NIR light irradiates, elec-trons stored in W18O49 are excited to form the plasmonic hot electrons with high energy, which are injected to the CB of g-C3N4 to reduce the protons into H2 (Fig. 5e). That is to say, the Z-scheme heterojunction and the LSPR-induced hot electron injection based on the Z-scheme band structure both promote the enhancement of the photocatalytic H2 evolution, whether under irradiation of UV, Vis or NIR light.
Similar mechanisms of the enhanced photocatalytic performances are also demonstrated in other Z-scheme heterojunctions. g-C3N4@CsxWO3 nanocomposites show markedly enhanced photocatalytic decomposing of volatile organic compounds (VOCs, i.e., HCHO or/and toluene) under the full-spectrum of solar light irradiation [67]. Fur-ther researches prove that the spontaneous Z-scheme hetero-junction greatly promotes the spatial separation of charge carriers, and the small polaron injected from localized states to CB of CsxWO3 results in the NIR-catalytic H+ reduc-tion. Besides, WO2–NaxWO3 (x > 0.25) hybrid photocata-lyst was synthesized via the high-temperature reduction of the NaxWO3 semiconductor (x < 0.25) [68]. The coupling of these two narrow energy band semiconductors (0.6 eV for WO2 and 1.3 eV for NaxWO3, respectively) urges the formation of Z-scheme heterojunction, which greatly boosts the generation of oxidizing holes and reducing electrons to carry out infrared (IR)-driven photocatalytic water splitting.
3.3 Other kinds of tungsten‑based heterojunctions
Complexing tungsten-based photocatalysts with other materi-als to form novel heterojunctions is also an efficient pathway to adjust the charge carrier concentration and improve the full-spectrum photocatalytic activities. NaYF4: Yb3+–Er3+ sensitized W18O49 nanowires exhibit distinct improved catalytic H2 evolu-tion from ammonia borane (BH3NH3) upon irradiation at the
wavelength of 980 nm [69]. As shown in Fig. 6a, in this novel hierarchical heterostructure, a unique NIR-plasmonic energy up-conversion process is detected: Upon 980 nm laser excita-tion, the LSPR effect is triggered inside W18O49 nanowires first. And then, the concentrated hot electrons transfer to adjoining NaYF4: Yb3+–Er3+ resonantly, resulting in the enhanced up-conversion luminescence. Finally, the emission with a suitable wavelength region further reacts to the LSPR effect of W18O49 and enhances the catalytic H2 evolution from BH3NH3. This synergistic effect detected between NaYF4: Yb3+–Er3+ and W18O49 offers a unique pathway to improve the photocatalytic H2 evolution response based on the NIR‐plasmonic energy tran-sition process. Furthermore, W18O49/carbon fiber heterojunction was constructed, which employs carbon fibers as the electron mediator [70]. In datil, under the potential difference between the CB of W18O49 and the fermi level of carbon fiber, plenty of NIR-light excited plasmonic hot electrons transfer from W18O49 to the surface of carbon fibers (Fig. 6b), and this ultrafast trans-port effectively hinders the relaxation process of the photogen-erated charge carrier inside W18O49, which enhances the cata-lytic hydrolysis of NH3BH3 for H2 production remarkably.
In summary, different types of heterojunctions have their own advantages and disadvantages. The rational construc-tion of the heterojunction is essential to improve the sepa-ration efficiency of the photogenerated electron–hole pairs and prolonging the carrier lifetime of the tungsten-based photocatalysts, which greatly promotes their full-spectrum photocatalytic activities in practical.
4 Multifunctional tungsten‑based photocatalysts
Benefitted by the unique structure and the mixed valence state of tungsten, the tungsten-based materials exhibit not only great photocatalytic performances, but also various novel physical and chemical properties such as NIR-shielding
Fig. 6 a Schematic diagram of the interaction between the LSPR effect of W18O49 nanowires and the energy-transfer up-conversion process of NaYF4: Yb3+‐Er3+ nanoparticles; b scheme of the kinet-
ics process of plasmon-induced hot electrons in the W18O49/C hetero-structures for enhancing the catalytic H2 production from NH3BH3. Reproduced with permission from Ref. [70] Copyright 2018 Elsevier
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character, gas sensitive capacity, electronic conductivity, and bio-thermal therapy. Compared with other thermal block-ing coatings such as indium tin oxide (ITO) [71, 72] and LaB6 [73, 74], tungsten-based materials show remarkably enhanced NIR-shielding ability and Vis-light transmittance, which are widely used as the smart window coating. Nev-ertheless, the normal tungsten-based NIR-shielding coating only transfers the absorbed NIR light into wasted heat energy, instead of making full use of solar energy [67, 75].
Recently, some efforts are focused on the multifunc-tional tungsten-based photocatalysts, which couple the energy saving and environmental cleanup properties to fully exploit the solar energy. As mentioned in Sect. 3.2, the Z-scheme g-C3N4@CsxWO3 heterojunction is a typi-cal multifunctional composite, which not only displays the blocking effect of UV or NIR light as well as high trans-mittance of Vis light (Fig. 7a), but also demonstrates the great photocatalytic depollution ability under excitation of
Fig. 7 a Optical properties of g-C3N4, CsxWO3 and g-C3N4@CsxWO3 heterojunctions; b photocatalytic HCHO photodegradation of the opti-mal g-C3N4@CsxWO3 heterojunction under different light irradiation; Schematic diagram for the c multifunctional properties of Z-scheme g-C3N4@CsxWO3 heterostructure and d F–TiO2–KxWO3 composite film applied to different conditions; e typical response curves of the Ag/
AgCl/W18O49 sensor to various detected vapors or gases of 100 ppm at the operating temperature of 300 °C; f photocatalytic degradation of methyl orange (MO) with Ag/AgCl/W18O49 and other reference sam-ples. a–c Reproduced with permission from Ref. [67] Copyright 2018 Elsevier, d reproduced with permission from Ref. [75] Copyright 2016 e, f reproduced with permission from Ref. [76] Copyright 2011 Elsevier
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the blocked UV and NIR light (Fig. 7b). In this case, the synergistic effect between g-C3N4 and CsxWO3 greatly pro-motes the generation of the multifunctional properties, and the shielded NIR light is further utilized instead of wasting as heat, which fully utilizes the full-spectrum solar energy authentically (Fig. 7c). Another composite manufactured by KxWO3 and surface-fluorinated TiO2 (F–TiO2) was also highlighted owing to its multifunctional properties [75]. As summarized in Fig. 7d, driven by the synergistic effect, the F–TiO2–KxWO3 composite film exhibits an outstanding NIR/UV light-shielding performance, high photocatalytic degradation activity, and excellent hydrophilic capacity. Moreover, many kinds of composites were also reported, such as CsxWO3/ZnO [77], RbxWO3/ZnO [78], and CsxWO3/Nb-doped TiO2 [79]. Beyond that, tungsten-based resistive-type gas sensors are widely used in a number of applications from health and safety to energy efficiency and emission control. Similarly, incorporating two or more gas-sensing materials to form a heterojunction interface could make drastic effects on gas sensor performances, especially the selectivity [80, 81]. As shown in Fig. 7e, f, after the decora-tion of Ag/AgCl nanoparticles onto W18O49 nanorods, the Ag/AgCl/W18O49 multifunctional heterojunction exhibits superior reducing gas-sensing properties to bare W18O49 nanorods as well as possesses a better photocatalytic per-formance than bare W18O49 nanorods for degradation of MO [76].
All these multifunctional tungsten-based photocatalysts provide a novel and convenient approach to make full use of solar energy, which solve the energy crisis and dete-riorating environmental issues nowadays. Meanwhile, the synergetic effect inside the tungsten-based heterojunction not only enhances the intrinsic photocatalytic properties of tungsten-based materials, but also extends their utilizing scope in human life.
5 Summary and outlook
Photocatalytic water splitting and pollutant degradation attract an ever-growing number of scientists to solve the environment and energy issues. Benefited from the wide absorption region from UV to NIR light and suitable band structure, MxWO3, and WO3–x boost full-spectrum pho-tocatalytic performances and greatly enhance the solar energy conversion efficiency compared with the traditional UV/Vis-sensitized photocatalysts. For all this, the limited photocatalytic activity still restrains the practical applica-tion of single-component tungsten-based photocatalysts with mixed valence state. Thus far, the fabrication of mixed valence state tungsten-based heterojunctions with diverse mechanism models, including the type-II heterojunction, Z-scheme system, and other hierarchical heterostructure,
further expands the utilization of solar light especially NIR light, accelerates the separation/transportation of the charge carriers, and promotes the full-spectrum photocatalytic activities substantially.
Great effort and significant progress were achieved in the investigation of the efficient tungsten-based photocatalyst with mixed valence state. However, considering the prac-tical application prospects, there are still some challenges and opportunities of tungsten-based materials which need further investigation.
1. Clarify the photocatalytic mechanisms. A great num-ber of WO3–x and MxWO3 are proven to be potential full-spectrum photocatalysts, which provides a new strategy for making full use of solar energy. However, exploring new photocatalysts with high and unique per-formances is the core of photocatalysis, and clarifying the inner mechanisms of the photocatalytic process is indeed the prerequisite. Inside the single-component tungsten-based materials, there is no visualized under-standing of the charge carrier generation and the hot electron transfer process, which greatly limits the full play of their photocatalytic performances under solar light irradiation. Meanwhile, the in-depth explorations of the interfacial charge kinetics and the synergistically photocatalytic mechanisms inside heterostructures also should be put forward to further promote the modifica-tion of the mixed valence state tungsten-based materials.
2. Improve the photocatalytic performances. Charge ther-modynamics and kinetics are highly critical in deter-mining the solar–chemical conversion efficiency in photocatalysis, and various strategies are proposed to adjust the generation and transfer of the photogenerated electron–hole pairs in mixed valence state tungsten-based photocatalysts. Although the electrical conduc-tivity and optical properties are well documented via ion doping, heterostructure construction, and so on, the energy band structure and LSPR of the tungsten-based materials still need to be further optimized, regulat-ing the separation of photoexcited electron–hole pairs and the rational utilization of plasmonic hot electrons. Many improvements are proposed like the controllable synthesis of tungsten-based materials with the unique morphology or specific crystal facet, surface modifica-tion by noble metal or cocatalyst, oxygen vacancy, and W5+/W6+ proportion regulation in the tungsten-based materials.
3. Develop the practical multifunctional applications. To date, a wide variety of physical–chemical properties of tungsten-based materials have been reported, and some multifunctional tungsten bronze/WO3–x-based photocat-alysts are constructed successfully. Even though, it is still essential to explore novel multifunctional materials
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via combining with other traditional functional materi-als, which paves the way to achieve the better synergistic effect and broaden their application scope. More impor-tantly, the present researches on the mixed valence state tungsten-based materials still remain in the laboratory stage. Several aspects including large-scale and low-cost processability, long-term stability and corrosion resist-ance in harsh environments should be taken into account in practical applications. All in all, there is a long and wide way to fully explore the unique multifunctional properties of tungsten-based photocatalysts.
Acknowledgements This work was financially supported by the Gansu Province Development and Reform Commission (NDRC, Grant No. 2013-1336) and the Light Function and Fight Conversion Material Discipline Innovation Base Cultivation Project (Grant No. G20190028011).
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Prof. Yuhua Wang received his Ph.D. degree in materials chem-istry from Tohoku University in 2001, and has been a full-time professor in Lanzhou University since 2004. He received the National Outstanding Youth Sci-ence Foundation in 2009. He is a fellow of the Royal Society of Chemistry (RSC) and has authored more than 300 original research papers and 20 patents. The research interests of Prof. Wang are engaged to the discov-ery, development and application of novel functional materials
based on rare-earth ions doped oxides, nitrides and oxynitrides. Recent research topics of Prof. Wang’s group include luminescent materials for lamps and display applications, such as long-afterglow luminescent materials, LED phosphor, quantum dot, up-conversion luminescent nano-materials, photocatalysis materials and f lameresistant materials.
