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Hierarchical Porous TiO Embedded Unsymmetrical Zn-Phthalocyanine Sensitizer for Visible-Light-Induced Photocatalytic H Production 2
Amritanjali Tiwari, Narra Vamsi Krishna, Lingamallu Giribabu, and Ujjwal Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09759 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017
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The Journal of Physical Chemistry
Hierarchical Porous TiO2 Embedded Unsymmetrical Zn-Phthalocyanine Sensitizer for Visible-Light-Induced Photocatalytic H2 Production Authors: Amritanjali Tiwari, †‡ Narra Vamsi Krishna,†‡ L. Giribabu,* †‡ Ujjwal Pal*†‡ †
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India
‡
Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India
ABSTRACT In this study, a novel visible-light-driven photocatalyst was designed based on unsymmetrical zinc-phthalocyanine photosensitizer on hierarchical porous TiO2 (HPT) semiconductor. The HPT material has been prepared by a simple self-formation route. The present work successfully uses zinc phthalocyanine with spectral response extended to 700 nm triggers light harvesting centre and HPT semiconductors for high photocatalytic H2 production. This novel unsymmetrical zinc-phthalocyanine (PCH001) containing three tert-butyl and two carboxylic acid groups that act as “push” and “pull” electron transfer properties from the excited dye to the TiO2 conduction band. The carboxylic group in the sensitizer serves as an anchoring group on to the surface of TiO2 and to provide intimate electronic coupling between its excited-state wave function and the conduction-band manifold of the semiconductor. The excellent photophysical properties was governed further by choosing three tert-butyl groups which tuned the LUMO level of the sensitizer that provides directionality in the excited state in addition to low aggregation and high solubility. The Zn-PCH@TiO2 composites exhibited promising activity and enhanced stability a photocatalytic system for visible-light-induced hydrogen production from water. The photocatalyst (HPT-0.25) shows H2 production yield 2260 µmol and high turnover number (TON 18080) under visible/near IR light irradiation. Moreover, HPT-0.25 photocatalyst shows a broad visible/NIR light responsive range (400−800 nm) with high apparent quantum yields (AQY) of 7.15, 2.70, 11.57, 3.90 and 0.50% under λ = 420, 550, 690, 730 and 800 nm monochromatic light irradiation, respectively. The present work gives a new advance toward efficient solar energy conversion with promising visible/near IR light-driven photocatalytic activity.
INTRODUCTION In recent years, hierarchically porous structured materials plays an important role to achieve the advanced materials applications targets like optoelectronic, biomedical, energy storage, etc.1-2 In the field of energy harvesting and new energy sources, production of clean energy H2 from water under visible light source is the most attractive and challenging in this 20th century. With the aim of visible light harvesting efficiency, several attempts have been made over the past decades after the pioneering invention made by Fujishima and Honda in 1972.3 Taking into account a large number of patents and scientific literature published on the development of active photocatalysts. Recently, semiconductor metal oxide photocatalysts with variety of controlled morphologies and exposed facets have been fancied to acquire high photocatalytic activity.4-10 Amongst them, hierarchical porous nanostructured materials with tunable porous framework are found to be efficient semiconductor photocatalytic system due to their
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better mass transport of reactants and radiation, large specific surface area, abundant active sites for the reactions, thus enhancing light harvesting from multi-reflections in the pores.2,11 The synthesis of fine control hierarchically porous nanostructures is still a tremendous challenge, mostly involves complicated and tedious templated routes. However, the metal alkoxide precursors spontaneously form unique hierarchical porous metal oxides in the presence of porogenic solvent through self-formation phenomenon. This process is simple and gained significant progress in recent years.12 The photoactivity of TiO2 powders with hierarchically porous structures is found to be higher as it has better transport of radiation light absorption and enhanced charge transfer. We believe that the wormhole mesoporosity further increase the diffusion of hole-electron from the surface by decreasing the diffusion length. Although HPT materials have shown satisfactory photocatalytic activity, many challenges still persist towards the efficient and easy synthesis pathways and limitation of harvesting wide spectral region of solar energy. In this regard, many research groups are working on designing of hybrid materials for targeting high solar energy conversion efficiency and photostability. Presently, several authors have used numerous dyes in developing visible light active semiconductor photocatalyst for photocatalytic water splitting to produce hydrogen.13-15 Since the great success achieved by O’Regan and Grätzel in 1991, Ru-bipyridyl complexes are the most commonly used sensitizers amongst the developed dye-sensitized system for photocatalytic hydrogen production, their spectral response regions usually located at λHPT-500> DCT > HPT-1.0 > HPT-0.5 > HPT.0.25 after dye sensitization onto HPT-500 and cTiO2 surface at optimal concentration. Additionally, it has been signified that the peak intensity of PCH-001 solution dye around 350-500 nm dye was disappeared and Q absorption band (655-690 nm) slightly red shifted (640-770 nm), after being sensitization onto the HPT-500 and cTiO2 surface. That could be ascribed to the suitable adsorption of the sensitizers on the HPT-500 and cTiO2 surface and is desirable for light harvesting of solar energy towards visible to near IR region. 100 80
%R
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HPT-500 DCT HPT-0.25 HPT-0.5 HPT-1.0 ......... PCH-001
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Wavelength (nm)
Figure 5. Diffuse reflectance spectroscopy (DRS) curves of the used samples.
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The Journal of Physical Chemistry
Theoretical Study To know the electronic distribution over PCH-001 sensitizer, we performed DFT geometry optimizations in ground state at the B3LYP/6-31g(d) level of theory. As can be seen from the Figure S5, both HOMO and HOMO+1 are delocalized π-electron system over the phthalocyanine macrocycle. On the other hand, both LUMO and LUMO-1 are π* orbitals delocalized across the phthalocyanine ring and phenyl ring and carboxyl group facilitating electron injection from the excited state of PCH-001 sensitizer to the conduction band of TiO2. These results are good agreement with other phthalocyanines reported in the literature.53-55
Water Splitting Reaction The performance of Pt-HPT-500/cTiO2 photocatalysts sensitized by PCH-001 dye for photocatalytic hydrogen production under simulated solar light were examined using 10 vol% of triethanolamine (TEOA) aqueous solution as sacrificial agents. The Pt-HPT-500/cTiO2 nanocomposite photocatalysts were prepared by irradiation of TiO2 suspended in a methanol solution containing H2PtCl6 (details in experimental section). The photo-irradiation with a UV-vis lamp results in the reduction of Pt2+ ions and deposition of Pt0 nanoparticles onto the surface of the TiO2 surface. The Pt deposition on TiO2 surface is confirmed by TEM analysis (Figure S3). The resultant data tabulated in the Table 1 and 6a, shown the photocatalytic H2 evolution activity of the HPT catalysts which followed the sequence: cTiO2< HPT-RT< DCT< HPT-400< HPT-500. While HPT-500 exhibits the best H2 evolution under similar reaction conditions. We have further optimized the dye concentration, a series of experiments were screened between 0.1 and 1.0 µmol/15 mg Pt-HPT-500/cTiO2 and results are incorporated in Figure 6b. The photoactivity increases with increasing the amount of the dye and reached at maximum at 0.25 µmol/15 mg Pt-HPT-500 nanocomposite (HPT-0.25) while above concentration the activity of H2 generation is continuously decrease. Typically, the fraction of incident light absorbed by the dye increases with increasing loading of dye and saturates at a certain level, because in general, a higher sensitization effect is expected at higher surface concentrations. However, excessive surface concentration of dyes is found poor sensitization effect in this study. The decrease of photocatalytic activity is due to the agglomerating nature of the dyes and the reduction of the penetration depth of incident light.
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As shown in the Figure 6a, H2 production activity of the as-prepared samples follows the
sequence: cTiO2< HPT-RT< DCT< HPT-400< HPT-500< HPT-N719
