Introduction
The renewed interest toward silver-based semiconductors is not surprising. The applicability of Ag nanoparticles is well-known even from ancient times due to their antibacterial character; however, their practical applications were only popular in the 1900s.1 Moreover, due to their low stability (formation of silver nanoparticles on their surface), the applicability of silver-containing semiconductors is still low. Nevertheless, they are excitable under visible light irradiation (having a relatively narrow band gap energy, e.g., Ag2O: 1.2 eV;2 Ag2S: 0.9-1.0 eV;3 and Ag3PO4: 2.43 eV (ref. 4)) and can be synthesized easily. There is still a dispute regarding whether their instability is an advantage or a disadvantage; by noble metal deposition, although the structure and properties change, they are usually beneficial.5
One of the most interesting silver-based materials is Ag2O, a p-type semiconductor with relatively low stability. Due to its low stability, it disproportionates under visible light irradiation and gives Ag and AgO.2 Another interesting material is Ag2S, an n-type semiconductor with a large visible light absorption coefficient,6 showing luminescent properties.7 Because of the low stability of the semiconductors mentioned above, other Ag-based photocatalytic materials have been investigated, such as Ag3PO4,8 Ag2SO4,9 Ag2CO3,10 and delafossite-type Ag-based semiconductors (e.g., AgGaO2 (ref. 11) or AgAlO2 (ref. 12)). Moreover, the affinity of Ag-based materials for photocorrosion could be decreased using the composites of two Ag-based semiconductors such as Ag2O/Ag2CO3,13 Ag2S/Ag2WO4,14 Ag2S@Ag2CO3,15 AgCl/Ag2CO3,16,17 AgBr/AgIO3,18 and [email protected]
Bạn đang xem: Shape tailoring of AgBr microstructures: effect of the cations of different bromide sources and applied surfactants†
Silver halides also appeared in different applications, including photographic techniques.20 Moreover, silver halides are more prevalent in photocatalytic processes (e.g., AgCl,21 AgBr,22 and AgI23). Silver halides usually have narrow band gap energy (about 3.2 eV for chlorides,24 2.6 eV for bromides,25 and 2.8 eV for iodides23), can be synthesized rather simply (e.g., by ion exchange,26 precipitation,27 or hydrothermal crystallization processes28), and possess relatively increased photosensitivity. Among these types of halides, silver bromide is one of the most widely used as a photocatalyst.26 Also, in the case of AgBr, silver nanoparticles/nanoclusters can be formed during photocatalytic processes.29 Interestingly, the as-formed Ag nanoclusters can be selectively adsorbed on the (110) crystallographic plane of AgBr, according to a theoretical calculation.29 Therefore, researchers working in this field have been focusing on manipulating the (111)/(110) ratio to control the amount of the as-formed and deposited Ag.
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Moreover, the amount of the deposited/formed Ag nanoparticles as essential as the obtained shape of the photocatalyst since AgBr octahedra with exposed (111) facets showed higher activity than cubes and spheres.30
One approach to control the shape of the catalyst could be the usage of surfactants/shape-tailoring agents during the synthesis since, depending on their structure, the morphology and size of the semiconductor crystal can be controlled.31 The most-studied shape-tailoring/capping agent is polyvinylpyrrolidone (PVP), which is a polymer (monomer, N-vinylpyrrolidone) and a non-ionic surfactant at the same time. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) are among the most widely-applied surfactants. SDS is an anionic,32 while CTAB is a cationic surfactant, both with broad applicability spectra, which have already been used simultaneously.33
Differently shaped AgBr microcrystals have already been synthesized using different surfactants/shape-tailoring agents, such as PVP34,35 and CTAB (which can act as a shape-tailoring agent and can be used as bromide source as well28,36). In many cases, PVP is used as a capping agent to increase the formation of the (111) crystallographic plane,34 thereby increasing the number of edges and corners with specific morphologies, such as polyhedral,28 nanorods,37 and hollow cubic.38 Moreover, it can be used to influence the primary crystallite size.39
Until now, to the best of the authors’ knowledge, there is no available data/research concerning the application of SDS as a shape-tailoring agent in the case of AgBr. However, there have been reports about the synthesis of Ag2S where SDS has been applied successfully.40 In several other cases,41,42 SDS has been used as an anionic surfactant in the synthesis of semiconductors with high monodispersity. Furthermore, even if CTAB is mainly considered as a surfactant, AgBr microcrystals can be obtained using CTAB as a bromide source,28 using precipitation,36 ion exchange,43 and hydrothermal44 methods.
Besides CTAB, different alkali metals, such as sodium45 and potassium38 ions are used as alkali metal-based Br sources to synthesize AgBr microcrystals. Moreover, the alkali metal cations could be incorporated in the structure of AgBr, creating interstitial defects in the surface.46
Accordingly, the current work’s main aim was to systematically investigate the effect of different surfactants/capping agents and alkali metal-based Br sources on the morpho-structural, optical, and stability parameters of AgBr-based materials. To the best of our knowledge, no such investigation has been conducted so far in the literature. CTAB, SDS, and PVP were used as surfactants/capping agents, while H+, Li+, Na+, K+, Rb+, and Cs+ were used as the Br sources’ cations.
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