Using Dyes for Evaluating Photocatalytic Properties: A Critical Review

Abstract

This brief review aims at analyzing the use of dyestuffs for evaluating the photocatalytic properties of novel photocatalysts. It is shown that the use of dyes as predictors for photocatalytic activity has its roots in the pre visible-light activity era, when the aim was to treat effluents streams containing hazardous dyes. The main conclusion of this review is that, in general, dyes are inappropriate as model compounds for the evaluation of photocatalytic activity of novel photocatalysts claimed to operate under visible light. Their main advantage, the ability to use UV-Vis spectroscopy, is severely limited by a variety of factors, most of which are related to the presence of other species. The presence of a second mechanism, sensitization, diminishes the generality required from a model contaminant used for testing a novel photocatalyst. While it is recommended not to use dyes for general testing of novel photocatalysts, it is still understandable that a model system consisting of a dye and a semiconductor can be of large importance if the degradation of a specific dye is the main aim of the research, or, alternatively, if the abilities of a specific dye to induce the degradation of a different type of contaminant are under study.

g-C3N4-Based Nanomaterials for Visible Light-Driven Photocatalysis

“…Note that the direct activation of reactants and intermediates through light absorption is the realm of photochemistry; in establishing whether a transformation is truly photocatalytic it is therefore crucial to establish that photons are absorbed by the catalyst rather than adsorbates [17,18]. In the photocatalytic production of so-called 'solar fuels', photoexcited charge carriers drive the conversion of water and CO2 into H2, CO, CH4, CH3OH and related oxygenates and hydrocarbons [19][20][21].…”

Probing the activities of UV and visible-light absorbing photocatalyst powders using a resazurin-based photocatalyst activity indicator ink (Rz Paii )

“…If the excitation light does comprise a significant level of photons of wavelengths which will be strongly absorbed by the dye, then a non-photocatalytic, dye-bleaching processes, such as dye photolysis [9,10], or dye-photosensitisation [8,11,12], can be very effective in bleaching the dye.…”

“…The dye photosensitisation process usually involves the electronic excitation of the dye, D, to D*, by light of sufficient energy, hν', which allows it to inject an electron into the conduction band of the semiconductor, SC, which in turn then reacts with an oxidizing agent, Ox, present in the system [8,11]. The process can be summarized as follows: …”

“…Usually, Ox is O2, although other oxidizing agents, such as H2O2, Fe 3+ and Cu 2+ , have been used for the same purpose. A classic example is provided the work of Wu et al, in their study of the photosensitized bleaching of Rhodamine B under visible light in aqueous dispersion of TiO2 [12], and the reviews of Rochin et al [11] and Rajeshwar et al [8] provide many more such examples. Note, however, that in most, if not all reported cases, an oxidizing agent, usually O2, is present and, in contrast, most of the work described here is carried out under anaerobic conditions, in the presence of a notable sacrificial electron donor, glycerol, rendering a dye-photosensitisation reaction unlikely.…”

PHOTOCATALYTIC DEGRADATION OF DYE OVER GRAPHENE-TiO2NANOCOMPOSITE

“…Como demonstrado por Rochkind et al, 20 o uso de corantes torna complexa a discussão de mecanismos de degradação e de efeitos sinérgicos associados a materiais híbridos, em razão da ocorrência de mecanismos secundários relacionados com processos de sensibilização. …”