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Phase engineering is an emerging strategy for tuning the electronic states and catalytic functions of nanomaterials. Great interest has recently been captured by phase-engineered photocatalysts, including the unconventional phase, amorphous phase, and heterophase. Phase engineering of photocatalytic materials (including semiconductors and cocatalysts) can effectively affect the light absorption range, charge separation efficiency, or surface redox reactivity, resulting in different catalytic behavior. The applications for phase-engineered photocatalysts are widely reported, for example, hydrogen evolution, oxygen evolution, CO2 reduction, and organic pollutant removal. This review will firstly provide a critical insight into the classification of phase engineering for photocatalysis. Then, the state-of-the-art development of phase engineering toward photocatalytic reactions will be presented, focusing on the synthesis and characterization methodologies for unique phase structure and the correlation between phase structure and photocatalytic performance. Finally, personal understanding of the current opportunities and challenges of phase engineering for photocatalysis will also be provided. [ABSTRACT FROM AUTHOR]

This work presents a method for phase-selective synthesis of ln 2 O 3 and demonstrate the phase-dependent photocatalytic CO 2 reduction performance. [Display omitted] • Introduced a synthetic method for phase-engineered In 2 O 3 using MOFs with specific functional group. • Achieved superior CO 2 reduction with c-In 2 O 3 , reaching high CO production rate and selectivity. • Demonstrated that c-In 2 O 3 enhances charge separation and lower reaction barriers, essential catalysis. Crystal phase engineering emerges as a powerful strategy to enhance photocatalytic activity, yet controlled phase-selective synthesis and phase-dependent performance understanding remain challenging. In this study, we introduce a method for synthesizing phase-engineered In 2 O 3 via the pyrolysis of metal–organic frameworks (MOFs). We demonstrate that the functional group and pyrolysis temperatures of MOFs are critical for phase-selective synthesis. Specifically, pyrolysis of MIL-68(ln)–NH 2 at optimal temperatures yields In 2 O 3 with rhombohedral (rh-In 2 O 3), cubic (c-In 2 O 3), and rhombohedral/cubic heterophase (rh/c-In 2 O 3) structures. The photocatalytic CO 2 reduction tests reveal that c-In 2 O 3 outperforms rh-In 2 O 3 and rh/c-In 2 O 3 , achieving a CO production rate of 29.19 μmol g-1h−1 with 94.47 % selectivity. Spectroscopic and theoretical analyses show that c-In 2 O 3 has superior charge transfer efficiency and lower reaction energy barriers, particularly for the rate-determining *CO intermediate, which exhibits a lower Gibbs free energy on its surface. This work provides a significant advancement in optimizing photocatalytic CO 2 reduction efficiency through precise phase engineering, underscoring the vital role of phase control in enhancing catalytic performance. [ABSTRACT FROM AUTHOR]