A review of material aspects in developing direct Z-scheme photocatalysts

A review of metal oxide-based Z-scheme heterojunction photocatalysts: actualities and developments

Summary Harnessing solar power to convert carbon dioxide (CO2) into fuels is a crucial channel to alleviate the global energy shortage and environment pollution. Photothermal catalytic conversion of CO2 into value-added fuels or chemicals, e.g., carbon monoxide (CO), methane (CH4), and methanol (CH3OH), offers an effective, economical, and eco-friendly solution to obtain “sunshine” feedstocks by coupling renewable solar energy with heat energy. In this review, we begin by briefly describing the fundamentals for photothermal catalytic CO2 reduction. Subsequently, we summarize the types of nanocatalysts for photothermal catalysis and their design strategies, where we propose two major photothermal catalytic mechanisms based on semiconductors and plasmonic metals, followed by three key design strategies of nanomaterials for photothermal catalytic CO2 reduction. Then, we expatiate the recent typical applications of photothermal catalytic CO2 reduction. Finally, we present our vision of the future developments and challenges in this exciting research field.

Photothermal catalytic CO2 reduction over nanomaterials

The continuous growth of fossil fuel consumption and large amounts of CO2 emissions have caused global energy crisis and climate change. The employment of semiconductor photocatalysts to convert CO2 into value-added products has attracted extensive attention and research worldwide in recent years. However, it is difficult for a single-component semiconductor photocatalyst to achieve this goal efficiently due to its drawbacks, such as low quantum efficiency, limited surface area, limited number of active sites, the short lifetime of photogenerated carriers, poor long-term stability, and the weak redox ability of carriers. Fortunately, inspired by photosynthesis, the construction of an artificial Z-scheme heterojunction has brought a new dawn for the realization of this goal. The Z-scheme heterojunction has a high separation efficiency of electron-hole pairs with strong redox ability and a wide light response range. The abovementioned advantages make the Z-scheme heterojunction provide a great opportunity for the conversion of CO2 to value-added chemicals. This review concisely reports the progress of the Z-scheme heterojunction in the field of photocatalytic CO2 reduction in recent years, photocatalytic mechanism, choice of oxidation and reduction systems, strategies for improving efficiency, confirmation of the Z-scheme charge transport mechanism, problems and challenges, and the prospects for the future.

Construction of a Z-scheme heterojunction for high-efficiency visible-light-driven photocatalytic CO2 reduction

Colored wide-bandgap semiconductor oxides with abundant mid-gap states have long been regarded as promising visible light responsive photocatalysts. However, their catalytic activities are hampered by charge recombination at deep level defects, which constitutes the critical challenge to practical applications of these oxide photocatalysts. To address the challenge, a strategy is proposed here that includes creating shallow-level defects above the deep-level defects and thermal activating the migration of trapped electrons out of the deep-level defects via these shallow defects. A simple and scalable solution plasma processing (SPP) technique is developed to process the presynthesized yellow TiO2 with numerous oxygen vacancies (Ov), which incorporates hydrogen dopants into the TiO2 lattice and creates shallow-level defects above deep level of Ov, meanwhile retaining the original visible absorption of the colored TiO2. At elevated temperature, the SPP-treated TiO2 exhibits a 300 times higher conversion rate for CO2 reduction under solar light irradiation and a 7.5 times higher removal rate of acetaldehyde under UV light irradiation, suggesting the effectiveness of the proposed strategy to enhance the photoactivity of colored wide-bandgap oxides for energy and environmental applications.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.202000204

Enhanced Solar Photothermal Catalysis over Solution Plasma Activated TiO2

All-solid-state Z-scheme systems are attracting increasing interest in artificial photosynthesis of solar fuel. Recent research has revealed that solid electron mediators facilitate shuttle of electrons and thereby enhance photocatalytic activity. However, for a mediator-free direct Z-scheme system, it still remains a challenge to promote the Z-scheme-type charge transfer at the interface. Herein, we report that the photocatalytic reduction of CO2 over the direct Bi4TaO8Cl/W18O49 (BiW) Z-scheme system, which is assembled from Bi4TaO8Cl nanosheet and nanosized W18O49, is markedly enhanced by simple integration of the photocatalytic process with external heating. The CO yield over a typical BiW Z-scheme system is increased by 87 times when the system temperature is increased from 298 K to 393 K. This yield via BiW photocatalysis at 393 K is higher than Bi4TaO8Cl photocatalysis at 298 K by a factor of 167. The superior performance of BiW Z-scheme system at elevated temperature is believed to result from the thermo-enhanced shuttle of electron from W18O49 to Bi4TaO8Cl. Moreover, this BiW Z-scheme enables long-lasting catalytic reduction of CO2 in the dark after light irradiation, benefiting from the thermal release of stored electrons in W18O49 to the conduction band of Bi4TaO8Cl. This study suggests that photothermal synergy can enhance the Z-scheme behavior, providing a promising means for designing photothermal catalysts for the solar-catalytic reduction of CO2 to fuel.

Photothermal synergic enhancement of direct Z-scheme behavior of Bi4TaO8Cl/W18O49 heterostructure for CO2 reduction

Revisiting cocatalyst/TiO2 photocatalyst in blue light photothermalcatalysis

Coupled adsorption and photocatalytic oxidation brings high expectations regarding the fast and efficient removal of gaseous pollutants in air However, to fabricate an adsorbent-photocatalyst composite, coating of a photocatalyst on adsorbent support inevitably results in loss of adsorption and light blocking on interior surfaces In this work, we attempt to develop an adsorbent-photocatalyst monolith composite, which not only perfectly retains original high adsorption capacity, but also allows complete penetration of UV light through the whole monolith We employ a SiO2 aerogel monolith with a diameter of 25 cm and thickness of 07 cm as adsorbent and support After atomic layer deposition (ALD) followed by calcination, 032-125 wt% TiO2 is dispersed on the skeleton of the SiO2 aerogel In spite of such a low level of loading, the monolith composites exhibit fast and efficient removal of gaseous acetaldehyde and NO Therein, the best performance is achieved at a loading of 06 wt% TiO2 By dark adsorption, the acetaldehyde pollutant with initial concentration of 200 ppm can be adsorbed by 54% within 10 min Moreover, the light transmittance at 387 nm can be retained as high as 6% after penetrating through the whole monolith, confirming that all loaded TiO2 nanoparticles can participate in the photocatalytic oxidation of acetaldehyde Under UV irradiation with intensity close to natural sunlight, the preadsorbed acetaldehyde can be completely mineralized into CO2 by photocatalytic oxidation in another 60 min, benefiting from the ultradispersion of TiO2 nanoparticles inside the SiO2 aerogel The study provides a novel three-dimensional model of an adsorbent-photocatalyst composite for the fast and efficient removal of gaseous pollutants

SiO2 aerogel monolith allows ultralow amounts of TiO2 for the fast and efficient removal of gaseous pollutants.

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Photothermal synergic enhancement of direct Z-scheme behavior of Bi4TaO8Cl/W18O49 heterostructure for CO2 reduction

Revisiting cocatalyst/TiO2 photocatalyst in blue light photothermalcatalysis

SiO2 aerogel monolith allows ultralow amounts of TiO2 for the fast and efficient removal of gaseous pollutants.