Bismuth-based nanomaterials represent a rapidly advancing class of sustainable semiconductors for photocatalytic and solar-to-chemical energy conversion. Their unique Bi 6s–O 2p hybridization and strong spin–orbit coupling produce narrow bandgaps and efficient charge separation, enabling superior visible-light activity and chemical stability. This chapter provides a concise overview of synthesis strategies—from sol–gel and hydrothermal to green and mechanochemical routes—linking structural design, electronic configuration, and photocatalytic performance. Key advances in doping, heterojunction engineering, and defect control are analyzed in relation to charge-transfer pathways and reactive radical generation. Environmental and energy applications, including pollutant degradation, CO2 reduction, and hydrogen evolution, are discussed alongside prospects for integrating computational modeling and sustainable synthesis. Collectively, these developments position bismuth-based nanomaterials as scalable, eco-efficient photocatalysts for next-generation clean-energy and environmental technologies.