Utility of Squaraine Dyes for Dye-Sensitized Photocatalysis on Water or Carbon Dioxide Reduction

Abstract

Red light-sensitized squaraine (SQ) dyes were developed and incorporated into dye-sensitized catalysts (DSCs) with the formula of SQ/TiO2/Cat, and their efficacies were evaluated in terms of performance on either water or carbon dioxide reduction. Pt nanoparticles or fac-[Re(4,4′-bis-(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)3Cl] were used as each catalytic center within the DSC frame of SQ/TiO2/Pt (Type I) or SQ/TiO2/Re(I) (Type II). In order to convey the potential utility of SQ in low energy sensitization, the following catalytic reductions were carried out under selective lower energy irradiation (>500 nm). Type I and II showed different catalytic performances, primarily due to the choice of solvent for each catalytic condition: hydrogenation was carried out in H2O, but CO2 reduction in dimethylformamide (DMF), and SQ was more stable in aqueous acid conditions for hydrogen generation than CO2 reduction in DMF. A suspension of Type I in 3 mL water containing 0.1 M ascorbic acid (pH = 2.66) resulted in efficient photocatalytic hydrogen evolution, producing 37 μmol of H2 for 4 h. However, in photocatalysis of Type II (SQ/TiO2/Re(I)) in 3 mL DMF containing 0.1 M 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole, the TiO2-bound SQ dyes were not capable of working as a low energy sensitizer because SQ was susceptible to dye decomposition in nucleophilic DMF conditions, resulting in DSC deactivation for the CO2 reduction. Even with the limitation of solvent, the DSC conditions for the utility of SQ have been established: the anchoring group effect of SQ with either phosphonic acid or carboxylic acid onto the TiO2 surface; energy alignment of SQ with the flat band potentials (Efb) of TiO2 semiconductors and the reduction power of electron donors; and the wavelength range of the light source used, particularly when >500 nm.

Inorganometallic Photocatalyst for CO2 Reduction

“…29−32 In practical terms, this spectroelectrolysis technique is the most reliable for determining the E fb of polycrystalline TiO 2 materials, which are commonly used in Although the photosensitizing unit (PS) is known to be the most vulnerable component where unwanted side reactions between reactive radical species (photogenerated oxidized (PS •+ ) or reduced (PS •− ) forms) can occur to initiate degradation pathways during extended photolysis, such weakness can be suppressed by (1) a strong electronic coupling between the dye and n-type TiO 2 semiconductor (electron collector) that effectively drains the excited electrons of the photosensitizer and (2) lower-energy irradiation modulated by controlling the spectral range and intensity. 33,34 As a successful example of this approach, we have demonstrated that TiO 2 immobilization of Zn(II) porphyrin dyes significantly enhances the conversion activity and durability of porphyrin-sensitized MIOM systems (porphyrin/TiO 2 / ReC), in contrast to the mixed homogeneous system, which has a short photocatalysis lifetime because of photodegradation of the dye (Figure 4). 33 In this hybrid, the neighboring n-type TiO 2 semiconductor efficiently ameliorates the mass-transfer limitation between components and prevents the accumulation of reactive radical species, ensuring steady photosensitization of the porphyrin dye during long-term photolysis (>95 h).…”

“…Although the photosensitizing unit (PS) is known to be the most vulnerable component where unwanted side reactions between reactive radical species (photogenerated oxidized (PS •+ ) or reduced (PS •– ) forms) can occur to initiate degradation pathways during extended photolysis, such weakness can be suppressed by (1) a strong electronic coupling between the dye and n -type TiO 2 semiconductor (electron collector) that effectively drains the excited electrons of the photosensitizer and (2) lower-energy irradiation modulated by controlling the spectral range and intensity. , …”

Collisional Electron Transfer Route between Homogeneous Porphyrin Dye and Catalytic TiO2/Re(I) Particles for CO2 Reduction

“…2,5−12 In particular, given that the molecular photosensitizing unit has been known to be the most vulnerable under the long-term photolysis, ensuring the dye stability by chemisorption on a semiconductor is essential for the photocatalytic system to attain a smooth, steady redox process. 11,13 This strategy also contributes to strong interfacial electronic coupling between the unoccupied orbital of the photosensitizer and the conduction band (CB) of the semiconductor, mainly TiO 2 , which facilitates the efficient electron injection from the excited dye to the semiconductor. 8 However, controversy remains regarding the electron injection route (dye → TiO 2 CB), which is caused by the general detachment of the chemically anchored dye (i.e., typically a weak chemical interaction of carboxylic acid).…”

Photosensitive Hybrid Nanostructured Materials: The Big Challenges for Sunlight Capture

“…Squaraine dyes, based on squaric acid (diketocyclobutenediol), are gaining interest in several applications, including red light and NIR harvesting in DSCs and organic solar cells, owing to their high absorptivity in the region around 600-800 nm and to their easy structure tunability [68]; nonetheless, their use in photocatalytic systems has been limited. Two examples are: a zwitterionic squaraine dye, anchored through π-π interaction to graphene in TiO 2 /rGO nanocomposites, active in H 2 production [69], and squaraines featuring dodecyl spacers and carboxylic or phosphonate anchoring groups, coupled with Pt/TiO 2 or Re(I)/TiO 2 to obtain photocatalysts tested in H 2 evolution and CO 2 reduction [68].…”