Solar Photocatalysis

Few‐layered graphitic carbon nitride (g‐C3N4) has been considered a popular base for constructing composite materials in photocatalytic CO2 reduction and H2 generation applications, particularly for constructing g‐C3N4 based composites based on the idea of nanoarchitectonics. The photocatalytic activity of these constructed g‐C3N4 composites relies on the composition, morphology, microstructure of the material. This review aims to focus on the summary of latest progress in the development of few‐layered g‐C3N4 nanosheets based noble metal clusters/nanoparticles and other semiconductor nanomaterials modified composite materials constructed using the technology of nanoarchitectonics for photocatalytic H2 generation and CO2 reduction. This review covers a brief introduction of the enhanced photogenerated carrier separation and transfer as well as extended light absorption of superior thin g‐C3N4 nanosheets based photocatalysts; discussions on the mechanism of g‐C3N4 based photocatalyst involved photocatalytic H2O splitting and CO2 reduction system; the latest progress in the study of nanoarchitectonics of a variety of g‐C3N4 based heterostructures including type II, Z‐scheme, S‐scheme, and Schottky junction. Major challenges on nanoarchitectonics of superior thin g‐C3N4 based heterostructures, as well as the recommendations for future studies of these materials to attain higher efficiency in photocatalytic H2 generation and CO2 reduction, are also concluded.

Energy and environment are the key global challenges in the 21st century. Solar energy is considered the most promising clean and sustainable energy resource due to its university, inexhaustible and environmental friendliness. One of the most viable means of solar energy conversion and utilization is artificially converting solar energy into chemical energy as natural photosynthesis does. H2 produced from water splitting and CO2 reduction, are the major research topics of artificial photosynthesis. However, the effective conversion of solar energy into chemical energy by cost-effective artificial means on a large scale remains elusive. Metal oxide-based photocatalysts are the most studied materials for photocatalytic water splitting and CO2 reduction. Particularly, perovskite oxides with the chemical formula of ABO3 have been intensively studied as semiconductor photocatalysts. However, overwhelmingly of the perovskite oxides are only active under UV-light-irradiation, which limited their potential in solar energy application. Therefore, breakthrough technology and step-change materials, particularly, visible-light-responsive photocatalysts are highly desirable for the development of photocatalytic systems. In recent years, copious progress has been made in designing materials that function under visible-light-irradiation. So far, the most successful strategy is anion doping of oxide semiconductors, such as nitrogen or sulfur dope to form oxynitrides and oxysulfides, respectively. For example, nitrogen-doped oxynitrides such as (Ga1-xZnx)(N1-xOx) and ANbO2N (A=Sr, Ba, and La)1, and sulfur-doped oxysulfides such as Sm2Ti2S2O5 2 showed efficient photocatalytic overall water splitting activities under visible-light-irradiation. Particularly, oxynitrides (Ga1-xZnx)(N1-xOx) based photocatalyst sheet, showed remarkable photocatalytic overall water splitting activity with AQE of more than 30% at l»420 nm3. However, these oxynitrides and oxysulfides suffer stability problems due to photocorrosion, hindering their potential practical application in photocatalytic applications. Recently, it has been theorized that double perovskite oxides (DPOs) with the chemical formula of A2BʹB"O6 can function as efficient and stable visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction. However, DPOs have been rarely studied for photocatalytic water splitting and CO2 reduction due to the difficulty of obtaining pure phase materials and the paucity of exposed active sites. Thus, developing efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting and CO2 reduction becomes important. In this regard, recently, we have demonstrated a series of efficient and stable visible-light-responsive DPOs photocatalysts for photocatalytic water splitting. For instance, Sr2CoWO6 and Sr2CoTaO6 can serve as an efficient and stable bifunctional photocatalyst for both photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction(HER)4, 5, and Sr2NiWO6 can efficiently drive the photocatalytic OER6. Even though these DPOs have been demonstrated as visible-light-responsive photocatalysts with suitable CB and VB positions that straddle the theoretical potentials for overall water splitting, however, one-step overall water splitting has not been achieved so far, and their potential for photocatalytic CO2 reduction has not been studied yet. Overall water splitting under visible-light-irradiation based on particulate photocatalysts is a challenging reaction, which is regarded as one of the “Holy Grail” of sciences. And oxide semiconductors showing both photocatalytic OER and HER activities are rare. Nevertheless, bifunctional photocatalytic OER and HER were successfully demonstrated based on Sr2CoWO6 and Sr2CoTaO6. Further improvement of the material designs and developing appropriate cocatalysts may play a key role in achieving one-step overall water splitting under visible-light-irradiation based on DPOs photocatalysts. This work is mainly to explore the possibility of utilizing DPOs materials as visible-light-responsive photocatalysts for photocatalytic water splitting and CO2 reduction reaction, which is challenging work but has great potential value in advancing science and technology in photocatalysis. We anticipated that this work will provide a novel and rational strategy for improving the light absorption, charge separation and charge utilization in DPOs photocatalysts. References Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. The Journal of Physical Chemistry B 2006, 110 (26), 13107-13112.Ma, G.; Kuang, Y.; Murthy, D. H.; Hisatomi, T.; Seo, J.; Chen, S.; Matsuzaki, H.; Suzuki, Y.; Katayama, M.; Minegishi, T. The Journal of Physical Chemistry C 2018, 122 (25), 13492-13499.Kato, H.; Asakura, K.; Kudo, A. Journal of the American Chemical Society 2003, 125 (10), 3082-3089.Idris, A. M.; Liu, T.; Shah, J. H.; Zhang, X.; Ma, C.; Malik, A. S.; Jin, A. Solar RRL 2020, 4 (3), 1900456.Idris, A. M.; Liu, T.; Hussain Shah, J.; Han, H.; Li, C. ACS Sustainable Chemistry&Engineering 2020, 8 (37), 14190-14197.Idris, A. M.; Liu, T.; Hussain Shah, J.; Malik, A. S.; Zhao, D.; Han, H.; Li, C. ACS Applied Materials&Interfaces 2020, 12 (23), 25938-25948.

Open AccessCCS ChemistryRESEARCH ARTICLE11 Apr 2022Slow Photon-Enhanced Heterojunction Accelerates Photocatalytic Hydrogen Evolution Reaction to Unprecedented Rates Jing Liu†, Yin-Hao Guo†, Zhi-Yi Hu†, Heng Zhao, Ze-Chuan Yu, Lihua Chen, Yu Li, Gustaaf Van Tendeloo and Bao-Lian Su Jing Liu† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan , Yin-Hao Guo† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan , Zhi-Yi Hu† State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Nanostructure Research Centre (NRC), Wuhan University of Technology, 430070 Wuhan , Heng Zhao State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan , Ze-Chuan Yu School of Civil Engineering and Architecture, Wuhan University of Technology, 430070 Wuhan , Lihua Chen State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan , Yu Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan , Gustaaf Van Tendeloo Nanostructure Research Centre (NRC), Wuhan University of Technology, 430070 Wuhan EMAT, University of Antwerp, B-2020 Antwerp and Bao-Lian Su *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 430070 Wuhan Laboratory of Inorganic Materials Chemistry (CMI), University of Namur, B-5000 Namur https://doi.org/10.31635/ccschem.022.202101699 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail In photocatalysis, both the photogenerated charge separation and transport and the induced light utilization greatly influence performance. In this work, highly ordered [email protected] core-shell inverse opal ([email protected]) nanocomposites have been successfully designed as a model to couple the heterojunction system with the slow photon effect for photocatalytic H2 production. Theoretical calculations and experimentation provide direct evidence for the slow photon effect in the [email protected] nanocomposites. The type II heterojunction is responsible for promoting the migration and separation of photogenerated charges, and the slow photon effect is in charge of enhancing light harvesting in the [email protected] nanocomposites. This synergy of two functions gives rise to a significantly enhanced photocatalytic H2 production rate under simulated solar light for the [email protected] nanocomposites. The highest H2 production rate reaches 48.7 mmol g−1 h−1 under simulated solar light with the benchmark performance for all reported [email protected] composites. Our work provides proof-of-principle that coupling the heterojunction system with the slow photon effect can greatly enhance the photocatalytic activity of composite photocatalysts. Download figure Download PowerPoint Introduction Solar light utilization has been extended to limit global warming and decrease carbon emissions. Consequently, photocatalytic hydrogen generation enabled by semiconductors under light illumination has attracted increasing attention. In fact, this strategy has been widely explored as an environmentally friendly method for converting solar energy to stored chemical energy.1–3 Generally, the photocatalytic reaction involves the following steps: light harvesting, charge separation, transportation of photogenerated electrons and holes to the surface of the photocatalyst, and the redox reaction.4 Because the balance of the thermodynamics and kinetics of the photocatalytic reaction processes play the key role in the photocatalytic hydrogen generation efficiency, the single-component photocatalyst, owing to its low absorption of solar light and photocatalytic inefficiency, has hindered its commercial application. To overcome this obstacle, heterojunction photocatalytic water-splitting systems (including type I, type II, and type III heterojunctions, Z-scheme heterojunction, and S-scheme heterojunction systems) have been adopted because of their ability to accelerate the migration of photogenerated electrons and holes to various counterparts for reduction and oxidation reactions to prevent their recombination rate, and to prolong their lifetimes.4–16 Such heterojunction photocatalysts exhibit broadband light response and high hydrogen evolution rates.4,17–19 This internal charge-transfer process leads to a spatial separation of photogenerated electrons and holes, resulting in efficient electron transfer for H2 reduction when the conduction bands (CBs) of the composite photocatalyst couple favorably.2,13 Furthermore, the heterojunction system yields photogenerated carriers that benefit photocatalysis in terms of thermodynamic requirements.4,20,21 For the heterojunction system, the structural design is very important to make full use of the induced light. Hierarchically structural porous materials with high surface area, large pore volume, and interconnected hierarchical porosity of different length scales display significant structural advantages in electron and ion transport, and mass diffusion has roused widespread attention and applications.22–26 As a special hierarchically porous structure, three-dimensional ordered marcroporous (3DOM) structures as good light harvesters have given rise to considerable research activity. They allow deep penetration of light and lead to longer light pathways. They significantly improve induced light utilization efficiency. Previous work on single (TiO2, ZnO, BiVO4, etc.), binary (TiO2/CdS, TiO2/Au, TiO2/ BiVO4, etc.), and ternary (TiO2/BiVO4/ZnO, TiO2/Au/CdS, etc.) semiconductor-based composites with 3DOM structure show that the 3DOM structure can greatly improve the absorption and utilization of light and significantly enhance photocatalytic performance.17,22,27–32 Particularly, the inverse opal photonic crystal structures benefitting the slow photon effect have further exhibited highly enhanced photocatalytic activity.18,29,33–35 The slow photon effect generally occurs when the wavelength of blue and/or red edges of the photonic band gap (PBG) match the electronic band gap (EBG) of a semiconductor. When this happens, light propagates with extremely low group velocity, leading to a longer lifetime of the photons and a largely increased optical path length of the light wave in the material to promote the interaction between light and material.36–38 This greatly improves the photon absorption and conversion efficiency, as broadly evidenced and applied in various kinds of single semiconductors for enhanced photocatalytic activity.17,39 Since the heterojunction system plays a significant role in separating photogenerated electron-hole pairs and slow photon effect contributes a lot to enhancing light harvesting, binary/ternary inverse opal composites seldom provide evidence or theoretical calculations that reveal the slow photon effect in the inverse opal composites. Thus, it is worth systematically investigating and revealing the existence of the slow photon effect in inverse opal composites and demonstrating the advantages of coupling the slow photon effect with the heterojunction system in such an efficient solar-powered photocatalysis. Herein, we choose the very common semiconductors (ZnO, CdS) and report the construction of three-dimensional (3D) [email protected] core-shell inverse opal ([email protected]) nanocomposites as a photocatalyst model by coating CdS nanoparticles onto a ZnO inverse opal (ZnO-IO) open framework using the successive ion layer absorption and reaction (SILAR) method.19,29 To tune and monitor the slow photon effect, we tailor the macroporous sizes of the inverse opal nanocomposite to modulate the relative positions of PBG and EBG in the [email protected] composites, aiming to compare photocatalytic enhancement originating from the slow photons between the red edge and the blue edge.33,36 These results show that by coupling a heterojunction system with the slow photon effect, [email protected] films can enhance light absorption and facilitate the separation of photogenerated electrons and holes, resulting in significantly improved photocatalytic H2 production. This is the first evidence that the heterojunction system can synergistically be coupled with the slow photon effect in such a [email protected] photonic crystal heteronanostructure. The [email protected] nanocomposites demonstrate an unprecedented photocatalytic H2 production rate, proven to originate from the type II heterojunction system with ∼2.7× enhancement under simulated solar light. The slow photon effect enhances photocatalytic H2 production with ∼1.4× and 1.7× at the red and blue edges respectively, providing direct experimental evidence that the slow photon effect occurring at the blue edge can bring a higher photocatalytic enhancement than that in red edge.17,39,40 The highest H2 production rate for [email protected] reached 48.7 mmol g−1 h−1 under simulated solar light with the benchmark performance of ∼fivefold enhancement of [email protected] (10.5 mmol g−1 h−1). Experimental Methods Fabrication of ZnO inverse opal film Highly ordered and continuous ZnO-IO films coated on fluorine-doped tin oxide (FTO) substrate with different air sphere sizes were successfully prepared by a metal salt-based sol–gel infiltration according to our previous work.29 Three different-sized colloidal templates ranging from 230, 290, and 440 nm have been used in this work. After removing the templates by calcination, various ZnO-IO films with different pore sizes were obtained. The samples were then designated as ZnO-IO-230, ZnO-IO-290, and ZnO-IO-440, respectively. The synthesis process of ZnO-IO-3M was the same with ZnO-IO film except using the mixture of three colloidal spheres (230, 290, and 440 nm with the volume ratio of 1∶1∶1) as template. Fabrication of 3D [email protected] nanocomposites To obtain 3D [email protected], the CdS shell layer was coated on the ZnO-IO film by the SILAR technique, as in previous reports.41 Typically, the ZnO-IO film is successively immersed in solutions containing 0.5 M Cd(NO3)2 in ethanol for 5 min, to allow Cd2+ to adsorb onto the ZnO-IO film and is then rinsed with ethanol for 5 min to remove the excess Cd2+. The film was rinsed in a solution containing 0.5 M Na2S in methanol for 5 min, to allow S2− to adsorb onto the ZnO-IO film and to react with preadsorbed Cd2+ to form the desired CdS. The above procedure is termed a SILAR cycle, and the loading of CdS can be increased by repeating these assembly cycles. This immersion cycle was repeated eight times in this study. The final samples were designated as [email protected], [email protected], and [email protected], respectively. The [email protected] was synthesized via the same method to coat the same amount of CdS shell on the ZnO-IO-3M film. Characterizations The crystalline phase of the samples was examined by X-ray diffraction (XRD) with a Bruker D8 ADVANCE diffractometer (Karlsruhe, Germany) using Cu Kα radiation (λ = 1.54 Å) in the 2θ range of 20–70°. Field-emission scanning electron microscopy (FESEM) was performed on a Hitachi S-4800 electron microscope (Tokyo, Japan) equipped with an energy-dispersive spectroscopy facility to characterize the sample morphology and composition. The absorption spectra were collected with a UV2550 (SHIMADZU, Chukyo, Japan) UV–vis spectrometer. The reflection spectra were recorded using an Avaspec 2048/2 fiber-optic spectrometer (Beijing, China). Transmission electron microscopy (TEM) was carried out on an FEI Tecnai Osiris microscope (Massachusetts, United States), operated at 200 kV. The photocatalytic H2 production was performed with an online photocatalytic analysis system (Labsolar-6A, Beijing Perfectlight Technology Co., Ltd., Beijing, China). The gas products were analyzed periodically by an Agilent 7890A (California, United States) gas chromatograph (GC) with a thermal conductivity detector (TCD). Photocatalytic hydrogen reduction Typically, the photocatalytic H2 production was performed in a Pyrex reactor with an entry window of optical quartz glass (Labsolar-6A, Beijing Perfectlight Technology Co., Ltd., Beijing, China). One piece of the obtained [email protected] film coated on FTO substrate (geometrical area of each piece was 2 cm × 2 cm) was placed in the reaction cell with 100 mL of an aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as the sacrificial agents. The light source was irradiated under simulated solar light irradiation (250–780 nm, PLS-SXE-300DUV (Beijing, China) with an UV light intensity of 34 mW cm−2 and a visible light intensity of 158 mW cm−2, Beijing Perfectlight Technology Co., Ltd., Beijing, China). When under visible light irradiation, the light passed through a cutoff filter (λ > 420 nm). The amount of H2 produced was analyzed using on-line GC (Agilent 7890A, California, United States) with TCD, using N2 as the carrier gas. The mass of photocatalyst on one piece of FTO was ∼1 mg. Photoelectrochemical measurements The photocurrent measurements in our experiments were performed on an electrochemical workstation (CHI 660D, Chenhua Instrument Co., Shanghai, China) in a standard three-electrode electrochemical cell with a Pt plate counter electrode and an Ag/AgCl reference electrode. And the electrolyte solution was performed using 0.1 M Na2S and Na2SO3, which was the same as with the sacrificial agents. The working electrodes with the exposed area of 1 × 1 cm2 were illuminated from the front side with solar light sources, the same as with the photocatalytic H2 production. All the [email protected] photoanodes were measured at 0.5 V external potential versus hydrogen electrode (RHE). Apparent quantum yields (AQY) are defined by the following equation because of a 2-electron process for H2 production in the [email protected] with the slow photon effect and type II heterojunction system. The energy density of irradiated solar light with 365 and 500 nm are 19 and 16 mW cm−2. AQY ( % ) = [ Number of the reacted electrons ] / [ Number of incident photons ] × 100 = [ Number of evolved hydrogen molecules × 2 ] / [ Number of incident photons ] × 100 Theoretical simulation of reflectance spectra The theoretical simulation of the reflectance spectra of [email protected] and [email protected] inverse opal films was calculated using the finite-difference time-domain (FDTD) method in the MEEP package.42 Computational details The density functional theory (DFT) calculation was performed by the Cambridge Sequential Total Energy Package. Generalized gradient approximation of the Perdew–Burke–Ernzerhof functional was used as the commutative correlation functional. The cutoff energy was set to 450 eV, and a 30 Å vacuum layer was constructed to eliminate the interaction between the periodic structures of the surface model. The Brillouin area was sampled at 2 × 2 × 1 K points. Results and Discussion Figure 1a illustrates the synthesis process for [email protected] nanocomposites. Figures 1b–1d present the FESEM images of the as-synthesized [email protected] films with different macroporous diameters. The hexagonal close-packed structure is well copied from the hexagonally packed colloidal crystal templates ( Supporting Information Figure S1). The obtained [email protected] films are highly ordered with continuous macroporous structure on a large scale ( Supporting Information Figure S2), similar to those of the ZnO-IO films ( Supporting Information Figure S3). The air-spheres of the ZnO-IO films are distributed uniformly, and the average pore diameter is ∼220, 260, and 380 nm, respectively ( Supporting Information Figure S3). After coating CdS nanoparticles on ZnO-IO structures, the average pore diameter for [email protected], [email protected], and [email protected] was ∼200, 240, and 360 nm. The thickness of the CdS shell was estimated at ∼10 nm via the SILAR method (Figures 1b–1d). SEM-electron dispersive X-ray spectroscopy (EDXS) results further show that the molar ratio of ZnO and CdS in all nanocomposites was close to 1∶1. Supporting Information Figure S4 presents the mapping images of [email protected] ( Supporting Information). These results demonstrate that we have successfully prepared well-ordered, large-area, high-structural quality [email protected] nanocomposites. These impressive core-shell inverse opal films would have an interesting influence on the photocatalytic H2 production based on our previous work for ZnO-IO film.29 Figure 1 | Schematic illustration and structural characterizations of [email protected] films. (a) Schematic illustration of the preparation of [email protected] films. The FESEM images of the as-prepared (b) [email protected] csIO-230, (c) [email protected] csIO-290, and (d) [email protected] csIO-440 heteronanostructures. Download figure Download PowerPoint A hexagonal wurtzite structure (JCPDS No. 079-0206) of the [email protected] films with different macroporous diameters and three strong diagnostic diffraction peaks at (10-10), (0002) and (10-11), respectively were observed based on the XRD patterns ( Supporting Information Figure S5). The diffraction peaks located at 37.76° and 51.75° belong to the FTO substrate. Three characteristic diffraction peaks at 26.46°, 43.89°, and 54.47° were ascribed to the (111), (220), and (222) crystal planes of cubic CdS (JCPDS No. 089-440), respectively. The broader diffraction peaks of cubic CdS implied that very small CdS nanoparticles were decorating the surface of ZnO-IO. Furthermore, the XRD results suggest that the CdS coating did not influence the crystalline phase of ZnO-IO film, indicating the successful synthesis of a heterojunction inverse opal structure. Since the only difference for these [email protected] films was the macroporous diameter, [email protected] was selected to demonstrate the morphological quality of the [email protected] structures. Figure 2a presents a typical TEM image of [email protected] structure, showing the stable core-shell inverse opal structure. Figure 2b presents the high-resolution transmission electron microscopy (HRTEM) image from the enlarged squared area in Figure 2a. The fast Fourier transform (FFT) from the whole area, clearly displayed the main zone axis of ZnO along [2-1-10] and verified the quasisingle crystal domains in the ZnO-IO framework, suggesting that the ZnO nanoparticles formed inverse opal structure with the same orientation (Figure 2b). These quasisingle crystal domains in this ZnO photonic crystal skeleton could improve the separation and the transport of photogenerated electrons and holes43 although details of the formation of such quasisingle crystal domains in inverse opal structure are not clear. However, the CdS (111) ring revealed that the CdS crystals coated on the surface of the framework had different orientations. Figure 2 | Compositional and structural characterizations of [email protected] film. (a) TEM images of low magnification. (b) HRTEM image of the area indicated in (a) and FFT image (inset). (c) Enlarged image of the area indicated in (b) and FFT image (inset). (d) HAADF-STEM image of one typical cell of inverse-opal structure and (e–h) the corresponding STEM-EDXS color maps in red box (d). Download figure Download PowerPoint Figure 2b also displays the ∼10 nm thickness of CdS located on the surface of ZnO, consistent with the SEM observations. The enlarged image of a zone on the surface presents the CdS crystal along the [10-1] zone axis (Figure 2c), further revealing the CdS polycrystalline coat on the ZnO-IO framework. Figures 2d–2h present the CdS distribution on the ZnO-IO framework via high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and STEM- STEM-EDXS images on the randomly selected regions of [email protected] The uniform distribution of Zn, Cd, and S throughout the [email protected] was clearly observed, showing the homogeneous distribution of CdS nanocrystallites on the ZnO-IO structure and intimate contact interface between the CdS and ZnO phases. These results confirm that the CdS nanocrystals were successfully coated onto the ZnO-IO framework to form a core-shell inverse opal structure. And the intimate contact interface is also expected to bring efficient electron transfer and high separation effect. It is well documented that reflectance spectra reflect the morphological quality of the inverse opal photonic crystal structure and the judgment criteria for the occurrence of slow photon effects.17,18,36 The theoretical simulation of the reflectance spectra of [email protected] inverse opal films with 230 and 290 nm templates has been performed to feature the experimental spectral observations (Figures 3a–3d). The FDTD method was implemented by using the MEEP package to calculate the optical response of the ZnO-CdS inverse opal system.42,44 Reflectance spectra with wavelengths ranging from 250 nm to 1000 nm were obtained. In the FDTD simulation, and the system was periodic and of along the The two with thickness to the wavelength of the at the and the two vacuum to and thickness the and one material layer which the thickness was times the diameter, and in the The material layer the inverse opal system used in the which was constructed following the cubic with its surface to the Materials were by the that ZnO was by a with to and CdS of wave source was placed in the vacuum and two were placed at the between the vacuum layer and the on both Reflectance was calculated by the the of the two For [email protected] and [email protected], the calculated reflectance peaks were and nm. In both the reflectance to longer wavelengths as the was as expected from the photonic crystal structure of the films. the were located the electronic band edge of CdS blue in Figure which evidence of the existence of the slow photon effect. It is then expected that the occurrence of the slow photon effect on the red edge of [email protected] and on the blue edge of [email protected] bring high enhancement in light absorption and photocatalytic H2 Figure | Theoretical simulation of reflectance of the material layer in the FDTD simulation in three side regions with higher than 1 and regions are the is to (d) Theoretical simulation of reflectance spectra of [email protected] and [email protected] inverse opal films using FDTD and the blue is the of CdS. and UV–vis absorption spectra and reflectance spectra of [email protected] with different macroporous diameters. The the electronic absorption band of CdS. Download figure Download PowerPoint UV–vis and the reflectance spectra of these [email protected] films were performed in to the relative between the reflection and the absorption of the obtained [email protected] composites from an experimental of (Figures and UV–vis was performed to the electronic structures and the optical absorption of these [email protected] films (Figure The strong absorption at wavelength in the nm range the absorption of hexagonal ZnO, showing its UV absorption After coating the CdS crystal shell on the surface of the ZnO macroporous framework, the absorption edge was extended to nm, the of CdS. This indicated that both UV light and visible light were by the [email protected] films. This is very for the utilization of solar light for hydrogen production. It also that the the of the the higher the light absorption for these This is because the enhances light and the optical path length of the light the inverse opal structure, its optical absorption For a high quality photonic crystal structure, the PBG can be by the and/or the average This is revealed by the reflectance spectra of the [email protected] and ZnO-IO films. the CdS the PBG of located at nm ( Supporting Information Figure After the CdS shell the of reflection peaks of [email protected] located at nm (Figure showing a of nm to the high average of the composites of ZnO = and CdS = This to the PBG red edge of [email protected] the electronic band of CdS nm). When the was increased to 290 nm, the corresponding reflection from nm ( Supporting Information Figure to nm. The PBG blue edge side of [email protected] then with the electronic band of CdS (Figure the red and blue edges of the PBG the occurrence of the slow photon The PBG of [email protected] was further from nm ( Supporting Information Figure to nm its PBG was at nm, the electronic wavelength of CdS (Figure the band reflection the light absorption to decrease the photocatalytic activity of [email protected] because of the of the slow photon the measured reflectance of [email protected] and [email protected] were in good with calculated spectra measured reflectance peaks were and nm, and the calculated reflectance peaks were and nm, (Figure the that were observed on the red edge of [email protected] and the blue edge of [email protected] reflectance were also in the calculated experimentation and theoretical calculations provide direct evidence of the existence of the slow photon on the red edge of [email protected] and the blue edge of [email protected], which can bring high enhancement for photocatalytic H2 Photocatalytic H2 generation was performed to show the slow photon effect in [email protected] using 0.1 M Na2S and Na2SO3 as sacrificial (Figure [email protected] and [email protected] exhibited significant enhancement of photocatalytic H2 production with the hydrogen amount of and mmol g−1 in 5 respectively under visible light irradiation (λ > 420 However, the [email protected] low photocatalytic H2 production with the hydrogen amount of only mmol g−1 in 5 This difference in H2 production for these samples originate from the slow photon effect that at the red

MXene-based photocatalysts for photocatalytic CO2 reduction have attracted great attention in recent years. This chapter discusses the mechanism of photocatalytic CO2 reduction reaction and summarizes the strategies for improving the performance of photocatalytic CO2 reduction on MXene-based photocatalysts. In addition, different kinds of MXene-based photocatalysts for photocatalytic CO2 reduction were reviewed, such as the MXene/nitride composite photocatalysts, MXene/metal oxide composite photocatalysts, MXene/perovskite composite photocatalysts, MXene/metal–organic framework composite photocatalysts. Moreover, the effect of surface-functional groups of MXene on the photocatalytic CO2 reduction and the theoretical calculation of MXene-based photocatalysts was summarized. Finally, the challenges and the application prospect of the MXene-based photocatalysts for photocatalytic CO2 reduction were proposed.

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Recently, transition metal carbide or nitride (MXene)-based nanomaterials have been broadly investigated as new photocatalysts and electrocatalysts for the reduction of CO2 into valuable energy-rich fuels due to their unique properties such as rich surface chemistries, flexible morphologies, bandgap structures, considerable electrical conductivities, thermal stabilities, and significant specific surface areas. Nevertheless, only a few reviews have been reported on the application of MXenes or MXene-based nanomaterials as advanced photocatalysts and/or electrocatalysts for CO2 reduction, which do not cover new findings and the current development in the application of MXene-based nanomaterials for CO2 reduction. Accordingly, herein, we present a comprehensive review of current findings on the photocatalytic and electrocatalytic reduction of CO2 by various MXene-based nanomaterials. Particularly, this review focuses on the (i) photocatalytic reduction of CO2 by functionalized Ti3C2, TiO2/Ti3C2, g-C3N4/Ti3C2, and other/Ti3C2 catalysts, (ii) electrocatalytic CO2 reduction; (iii) CO2 reduction associated with photothermal catalysis and hydrogenation, and (iv) stability of MXene-based photoelectrocatalysts. Additionally, we have briefly explored the challenges in the large-scale fabrication of MXene-based nanomaterials and proposed the future research prospects of MXene-based nanomaterials.

Surface plasmon resonance effect enhances spin-polarized electrons to promote photocatalytic CO2 reduction.

Emission reduction of CO2 is an urgent global environmental problem. The photocatalytic reduction of CO2 has attracted lots of interest as a novel method for producing organic fuel, and this technology does not require the consumption of additional energy. So in this paper, the photocatalytic reduction processes of gas phase CO2 in the different optofluidic microreactors were simulated to analyze the reaction mechanism and the reactants transfer process. Firstly, the applicability of the classical Langmuir–Hinshelwood equilibrium adsorption model and the self-defined dynamic mass transfer model for the numerical simulation of optofluidic planar reactor were initially verified. The results of the simulation based on the Langmuir–Hinshelwood model were not consistent with the experimental results when the flow rate were high, while those of the simulation based on the self-defined dynamic mass transfer model were consistent with the experimental results. The result indicates that the mass transfer process of the reactants is the dominant factor which affects the reaction efficiency of the photocatalytic process in the microreactor. Then, the corresponding Sherwood number was calculated according to the mass transfer coefficient under different inlet flow rates, and the mass transfer characteristic correlation was established. Finally, the planar microreactor was optimized to be an inverted convex cylinders microreactor, and the numerical simulation of photocatalytic reduction process of gas phase CO2 was carried out by using self-defined dynamic mass transfer model. It was found that the inverted convex cylinders in the microreactor could significantly enhance the transfer effect of reactants during the reaction process and improve the conversion efficiency of CO2.

Construction of a bismuth-based perovskite direct Z-scheme heterojunction Au-Cs3Bi2Br9/V2O5 for efficient photocatalytic CO2 reduction

Boosting photocatalytic CO2 reduction through self-nitrogen-functionalization on bifunctional porous carbon macro-spheres metal-free catalyst.

Advances in molecular photocatalytic and electrocatalytic CO2 reduction

The development of efficient artificial photocatalysts and photoelectrocatalysts for the reduction of CO2 with H2O to fuels and chemicals has attracted much attention in recent years. Although the state-of-the-art for the production of fuels or chemicals from CO2 using solar energy is still far from practical consideration, rich knowledge has been accumulated to understand the key factors that determine the catalytic performances. This Feature article highlights recent advances in the photocatalytic and photoelectrocatalytic reduction of CO2 with H2O using heterogeneous semiconductor-based catalysts. The effects of structural aspects of semiconductors, such as crystalline phases, particle sizes, morphologies, exposed facets and heterojunctions, on their catalytic behaviours are discussed. The roles of different types of cocatalysts and the impact of their nanostructures on surface CO2 chemisorption and reduction are also analysed. The present article aims to provide insights into the rational design of efficient heterogeneous catalysts with controlled nanostructures for the photocatalytic and photoelectrocatalytic reduction of CO2 with H2O.

In this study, to overcome the low charge transportation efficiency and poor visible light absorption ability, and achieve highly efficient photocatalytic applications, carbon nitride nanosheets with oxygen and carbon co-doping were successfully designed and fabricated. The resultant carbon nitride nanosheets exhibited efficient photocatalytic H2 evolution and CO2 reduction performance, highlighting the efficacy of such a strategy. The highest H2 evolution rate could reach 698.43 μmol g-1 h-1, higher than that for graphitic carbon nitride (GCN). For CO2 reduction, the photocatalytic system shows a high CO selectivity, and MG3.0 achieves the largest CO generation amount of 55.2 μmol g-1. This enhanced photocatalytic reduction performance could be attributed to oxygen and carbon co-doping, which achieves fast electron extraction and transfer, and improved visible light absorption ability. It should be noted that the excessive addition of glucose in the synthesis process could enhance conductivity and promote visible light absorption of carbon nitride, but suppress the H2 evolution and CO2 reduction ability. Simultaneously, the photocatalytic reduction mechanism is discussed. This work confirms that a carbon nitride semiconductor with oxygen and carbon co-doping could be easily prepared by this strategy, achieving efficient photocatalytic applications.