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Categories of Quantum Photoinitiators
The use of novel photoinitiators (PIs) for free-radical polymerization has attracted significant attention from the scientific community. Quantum PIs, quantum-confined nanoscale crystals with semiconductor properties, have received interest for use in photopolymerization, due to their precisely tunable properties as a function of structural and surface engineering.
Current advancements in synthetic procedures have led to the development of a variety of quantum PIs with characteristic photoinitiating properties . The surface moieties introduced in the course of nanocrystal synthesis facilitate control over skeletal arrangements, dispersibility, and reactivity at the molecular level . Postsynthetic surface modifications enable the dispersal and stabilization of nanocrystals PIs in formulations for many applications .
Several classes of quantum PIs have been developed and evaluated for polymerization, with various absorption and excitation wavelength windows . The mode of action of most of the quantum PIs is generally similar to type I radical initiators, with highly conjugated aromatic moieties; others mimic the activity of type II initiators . Loir et al. indicated that two pathways exist for initiation in quantum PIs, which involve surface mediated hole transfer (Figure 1) . Typical quantum PIs can be classified as semiconductor nanoparticles (NPs), such as TiO2, ZnO, and CdS NPs ; hybrid photoinitiators, including composites of metal nanoparticles (MNPs)/silanized metal (organic PIs, MNPs), fluorescent dyes, oligomeric silsesquioxane, and parent PIs,  semiconductor NPs (metal/graphene oxide) , and organometallic nanoparticles; panchromatic photoinitiators, namely upconverting nanoparticles (UCNPs)  and plasmonic nanoparticle composites (e.g., Ag@SiO2@UC@BFO–Au core@triple-shell); near-infrared photoinitiators, such as luminescent lanthanides (e.g., Ln3+, Yb3+, Er3+, and Ho3+) and doped nanomaterials in a crystalline host lattice (NaYF4);  magnetic nanoparticles, such as Fe2O3; and metal core-shell nanoparticles (e.g., Ag@AgCl nanocubes) . Figure 2 describes the polymerization of acrylic acid sodium (AAS) using the Ag@SiO2@UC@BFO-Au photoinitiator.
2. Categories of Quantum Photoinitiators
2.1. Semiconductor QPIs
Quantum photoinitiators based on semiconductor nanocrystals are considered a viable alternative to traditional organic photoinitiators with low molecular weights. Semiconductor nanocrystals have attracted attention due to their capacity to function as photocatalysts for many types of chemical reactions; these materials offer unique advantages, such as efficient light-harvesting activity, tunable properties, and large surface area-to-volume ratios . These nanocrystals exhibit quantum confinement effects; the properties of these materials may be modified by synthetic control over nanocrystal size, shape, and composition .
Semiconductor quantum dots (QD) are solution-dispersible nanocrystals, which have found use as photocatalysts for light-induced polymerization. QDs exhibit strong absorption in the UV-visible range, with large extinction coefficients (ε > 105 M−1 cm−1) , large specific surface area values that allow for the interaction with multiple substrates, and higher photostability than organic-based photocatalysts and transition metal complexes , Semiconductor suspensions were first used for photopolymerization by Kuriacose et al.; ZnO powders were used for photopolymerization of methyl methacrylate (MMA) in water . The effect of the oxygen level on polymerization was investigated; higher amounts of oxygen were associated with a lower molecular weight and a larger number of chains. On the other hand, too much oxygen in the solution suppressed the propagation step; this phenomenon was reported to be a common limitation for polymerization initiated by oxidative anionic species-releasing photoinitiators.
2.2. Carbon-Based QPIs
2.3. Graphene-Based QPIs
2.4. UCNPs and Hybrid QPIs
2.5. Polymer–Hybrid QPIs
The entry is from 10.3390/polym13162694
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