Carbon dots (CDs), a class of carbon-based sub-ten-nanometer nanoparticles, have attracted great attention since their discovery fifteen years ago. Because of the outstanding photoluminescence properties, photostability, low toxicity, and low cost, CDs have potential to replace traditional semiconductor quantum dots which have serious drawbacks of toxicity and high cost. This review covers the common top-down and bottom-up methods for the synthesis of CDs, different categories of CD precursors (small molecules, natural polymers, and synthetic polymers), one-pot and multi-step methods to produce CDs/photocatalyst composites, and recent advances of CDs on photocatalysis applications mostly in pollutant degradation and energy areas. A broad range of precursors forming fluorescent CDs are discussed, including small molecule sole or dual precursors, natural polymers such as pure polysaccharides and proteins and crude bio-resources from plants or animals, and various synthetic polymer precursors with positive, negative, neutral and hydrophilic, hydrophobic, or zwitterionic feature. Because of the wide light absorbance, excellent photoluminescence properties and electron transfer ability, CDs have emerged as a new type of photocatalyst. Recent work of CDs as sole photocatalyst or in combination with other materials (e.g., metal, metal sulfide, metal oxide, bismuth-based semiconductor, or other traditional photocatalysts) to form composite catalyst for various photocatalytic applications are reviewed. Possible future directions are proposed at the end of the article on mechanistic studies, production of CDs with better controlled properties, expansion of polymer precursor pool, and systematic studies of CDs for photocatalysis applications.

Conductive polymers have been widely investigated in various applications. Several conductive polymers, such as polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT)), and polythiophene (PTh) have been loaded with various semiconductor nanomaterials to prepare the composite photocatalysts. However, a critical review of conductive polymer-based composite photocatalysts has not been available yet. Therefore, in this review, we summarized the applications of conductive polymers in the preparation of composite photocatalysts for photocatalytic degradation of hazardous chemicals, antibacterial, and photocatalytic hydrogen production. Various materials were systematically surveyed to illustrate their preparation methods, morphologies, and photocatalytic performances. The synergic effect between conductive polymers and semiconductor nanomaterials were observed for a lot of composite photocatalysts. The band structures of the composite photocatalysts can be analyzed to explain the mechanism of their enhanced photocatalytic activity. The incorporation of conductive polymers can result in significantly improved visible-light driven photocatalytic activity by enhancing the separation of photoexcited charge carriers, extending the light absorption range, increasing the adsorption of reactants, inhibiting photo-corrosion, and reducing the formation of large aggregates. This review provides a systematic concept about how conductive polymers can improve the performance of composite photocatalysts.

Metal sulfide-based photocatalysts have gained much attention due to their outstanding photocatalytic properties. This review paper discusses recent developments on metal sulfide-based nanomaterials for H2 production, acting as either photocatalysts or cocatalysts, especially in the last decade. Recent progress on key experimental parameters, in-situ characterization methods, and the performance of the metal sulfide photocatalysts are systematically discussed, including the forms of heterogeneous composite photocatalysts, immobilized photocatalysts, and magnetically separable photocatalysts. Some methods have been studied to solve the problem of rapid recombination of photoinduced carriers. The electronic density of photocatalysts can be investigated by in-situ C K-edge near edge X-ray absorption fine structure (NEXAFS) spectra to study the mechanism of the photocatalytic process. The effects of crystal properties, nanostructure, cocatalyst, sacrificial agent, electrically conductive materials, doping, calcination, crystal size, and pH on the performance of composite photocatalysts are presented. Moreover, the facet effect and light trapping (or light harvesting) effect, which can improve the photocatalytic activity, are also discussed.

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