Fig. 1. (Color online) In-situ characterization of semiconducting photocatalysts for material research and mechanism disclosure in solar energy conversion and the storage process.

Fig. 2. (Color online) Energy band diagram and electron distribution of three typical types of semiconductors, including (a) intrinsic type, (b) negative-type, and (c) positive-type. (d) Mechanism displays of semiconductor photocatalysis.

Fig. 3. (Color online) Schematic diagram of the interaction between particles (electrons and photons) and materials.

Fig. 4. (Color online) (a) In-situ STM observation of oxygen vacancies dynamic changes on TiO₂(110) surface. (b) In-situ XPS, in-situ EPR, in-situ XAS (XANES) and in-situ EXAFS spectra in R space of WO₃-600. (c) Series of in-situ TEM images of a CeO₂ nanoparticle. (d) In-situ TEM images of atom mobility on CeO₂ surface at {100} facet in high vacuum, oxygen, and carbon dioxide atmospheres. Modified with permission from (a) Ref. [32] Copyright 2018 American Chemical Society, (b) Ref. [33] Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, (c) Ref. [ 35] Copyright 2017 American Chemical Society, (d) Ref. [36] Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA.

Fig. 5. (Color online) (a) In-situ TEM images of TiO₂ under water vapor conditions, with (b) EELS spectra and (c) XPS spectra of TiO₂ particles. In-situ TEM study of TiO₂ photocatalytic water splitting under UV light: (d) formation and evolution of bubbles, (e) magnified views of the surface shell on TiO₂, (f) schematic diagrams of fluidic TEM holder with in-situ UV illumination, (g) surface shell thickness on TiO₂ under UV illumination and (h) EELS spectra of the TiO₂. (i) HRTEM images of Cu₂O samples irradiated for 1, 2, and 3 h, as well as schematic diagrams of Cu₂O structure change during irradiation. Modified with permission from (a), (b) and (c) Ref. [38] Copyright 2013 American Chemical Society, (d), (e), (f), (g) and (h) Ref. [39] Copyright 2018 Springer Nature, (i) Ref. [40] Copyright 2020 Elsevier Ltd.

Fig. 6. (Color online) (a) Schematic diagram of water reduction in Au-TiO₂ with UV- (right) and visible-light (left) driven. (b) In-situ EPR spectra of TiO₂ and Au-TiO₂. (c) In-situ XAS spectra of THMSs and Au THMSs. (d) Schematic illustration of the electron transfer mechanism between TiO₂/ZnIn₂S₄ S-scheme heterojunction. (e) In-situ XPS spectra of TiO₂, ZnIn₂S₄, and TiO₂/ZnIn₂S₄. (f) In-situ TEM observations about evolution and dynamic structural changes of the overlayer in a strong metal-support interaction. (g) Time-series in-situ ETEM image of Pt-loaded CeO₂. Modified with permission from (a) and (b) Ref. [43] Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, (c) Ref. [ 44] Copyright 2018 American Chemical Society, (d) and (e) Ref. [52] Copyright 2021 Wiley-VCH GmbH, (f) Ref. [53] Copyright 2021 Springer Nature, (g) Ref. [54] Copyright 2020 Springer Nature.

Fig. 7. (Color online) In-situ characterization of reaction intermediates in the photocatalytic HER and OER processes. (a) Mechanism scheme of HER and OER. (b) LSV curve (left) and (c) operando Raman spectra (right) of MoS_0.9Se_1.1. (d) Schematic illustration of the HER process in MoS_2xSe_2(1–x). (e) In-situ DRIFT spectra of H₂O on the ZnIn₂S₄@MoS₂ with 2 h H₂O-saturated flow under He and (f) with a 2 h irradiation time under Xe lamp, and (g) proposed reaction pathway for the HER process over the ZnIn₂S₄@MoS₂. In-situ FTIR spectra of the HER process on ReS₂/CdS with two different sacrificial agents, (h) Na₂S-Na₂SO₃ and (i) lactic acid. Modified with permission from (b), (c) and (d) Ref. [64] Copyright 2020 Wiley-VCH Verlag GmbH & Co. KGaA, (e), (f) and (g) Ref. [ 65] Copyright 2021 Elsevier Ltd., (h) and (i) Ref. [66] Copyright 2019 Elsevier Ltd.

Fig. 8. (Color online) In-situ characterization of reaction intermediates in the photocatalytic CO₂ conversion processes. (a) Mechanism scheme of CO₂ conversion. (b) In-situ FTIR spectra of CO₂ adsorbed on the BiOBr layers with oxygen vacancy in the presence of H₂O vapor. (c) Schematic illustration for CO₂ photoconversion into CO over the oxygen-vacancy BiOBr. (d) In-situ DRIFTS spectra for the conversion process of CO₂ in the presence of CH₃OH under Xe lamp for oxygen vacancy-rich-Bi₂O₃ nanosheets. (e) Schematic illustration for CO₂ photofixation to long-chain chemicals. (f) In-situ DRIFTS spectra of CO₂ reduction reaction and (g) possible photocatalytic pathways over the TiO₂@ZnIn₂S₄. In-situ FTIR spectra of atmospheric 0.03 vol % CO₂/Ar photothermal reduction over the oxygen vacancy-rich Zn₂GeO₄ nanobelts at (h) 293 K and (i) 348 K. (j) Function mechanism of photoinduced heat during CO₂ photoreduction to CH₃COOH. Modified with permission from (b) and (c) Ref. [80] Copyright 2021 Wiley-VCH GmbH, (d) and (e) Ref. [81] Copyright 2019 Springer Nature, (f) and (g) Ref. [52] Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, (h), (i) and (j) Ref. [ 82] Copyright 2021 American Chemical Society.

Fig. 9. (Color online) In-situ characterization of reaction intermediates in the N₂ photofixation process. (a) Mechanism scheme of N₂ fixation. In-situ IR spectra of Bi₅O₇Br-40 in the process of (b) N₂ adsorption from 0 to 40 min in the dark, (c) afterward receive visible light illumination, (d) then visible light illumination lasts 35 min, and (e) finally the visible light illumination is turned off. (f) In-situ DRFTIRS spectra were obtained and (g) a reaction pathway of N₂ photofixation over the Bi₅O₇Br nanotubes. Modified with permission from (b), (c), (d) and (e) Ref. [94] Copyright 2020 American Chemical Society, (f) and (g) Ref. [95] Copyright 2020 American Chemical Society.

Fig. 10. (Color online) In-situ characterization of reaction intermediates in other catalytic processes. In-situ FTIR spectra of (a) TiO₂-OV and (b) TiO₂. (c) Selective removal mechanism during the NO removal process over the TiO₂-OV and TiO₂. (d) In-situ FTIR spectra and (e) NO removal mechanism of Bi₀OVs-(BiO)₂CO₃. Modified with permission from (a), (b) and (c) Ref. [98] Copyright 2019 American Chemical Society, (d) and (e) Ref. [97] Copyright 2019 Elsevier Ltd.