Control of the nanoscale crystallinity in mesoporous TiO<sub>2</sub>shells for enhanced photocatalytic activity

Joo, Ji Bong; Zhang, Qiao; Dahl, Michael; Lee, Ilkeun; Goebl, James; Zaera, Francisco; Yin, Yadong

doi:10.1039/c1ee02533c

Cited by 284 publications

(231 citation statements)

References 42 publications

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“…Surprisingly, however, similar fluorescence decay kinetics were seen here with Au@Void@TiO 2 versus Void@TiO 2 samples [ Fig. S1A; synthetic details of the latter catalyst are provided elsewhere (20,21)], indicating that the photophysics of the titania shells is not affected by the presence of the metal. This behavior proved to be quite general: it was also observed with catalysts made by dispersing either gold or platinum nanoparticles…”

Section: Resultsmentioning

confidence: 72%

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Joo

Dillon

Lee

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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116

113

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The production of hydrogen from water with semiconductor photocatalysts can be promoted by adding small amounts of metals to their surfaces. The resulting enhancement in photocatalytic activity is commonly attributed to a fast transfer of the excited electrons generated by photon absorption from the semiconductor to the metal, a step that prevents deexcitation back to the ground electronic state. Here we provide experimental evidence that suggests an alternative pathway that does not involve electron transfer to the metal but requires it to act as a catalyst for the recombination of the hydrogen atoms made via the reduction of protons on the surface of the semiconductor instead.photocatalysis | hydrogen production | gold-titania | time-resolved fluorescence | core-shell nanostructures P hotocatalysis is a promising technology to address problems in chemical synthesis (1), environmental remediation (2, 3), and energy utilization (4). The potential for harvesting solar energy and storing it as a chemical fuel using photocatalysts is particularly appealing. Specifically, H 2 may be produced via water splitting this way (5-7). In fact, many photocatalysts have been reported already capable of producing hydrogen out of water, even if none of those are yet commercially viable. Such photocatalysis rely on capturing photon energy via excitation of electrons from the valence band to the conduction band of appropriate semiconductors such as titania, which is often cited as a prototypical example (8-10), creating an excited electron-hole pair that is then used to promote redox reactions (11,12). However, for the photocatalysts to be useful, the lifetime of the electron-hole pair needs to be long enough to be accessible for chemical conversions. The search for photocatalytic systems that fulfill this and other key requirements continues, and would be greatly facilitated by a clearer understanding of the underlying chemical process.It is well known that the addition of metals such as platinum or gold to semiconductors enhances their activity as photocatalysts, in particular for water splitting to produce H 2 (13-15). This effect is currently explained by a mechanism where the excited electrons produced by absorption of light are transferred from the semiconductor to the metal before they have the opportunity to recombine with their corresponding holes and return to the ground electronic state (Fig. 1A) (16-18). In this scheme, the protons from water are reduced on the surface of the metal, in sites physically separated from those on the semiconductor, where oxygen production takes place. Here we present evidence that challenges this conventional explanation, and offer an alternative mechanism for the promotion of photocatalysis by metals. Specifically, using the Au/TiO 2 metal-semiconductor pair as a model system, we show that electron transfer to the metal may not play a significant role in photocatalysis. Instead, the data support a model where the excited electron promotes the reduction of protons on the surface of the se...

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Section: Resultsmentioning

confidence: 72%

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Joo

Dillon

Lee

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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116

113

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“…In addition to the intrinsic materials characteristics including low cost, low toxicity, and high chemical and optical stability [15,16], hollow nanostructures of TiO2 are believed to be able to provide a high active surface area, reduced diffusion resistance, and improved accessibility, which are beneficial features for photocatalysis. We have previously synthesized mesoporous hollow TiO2 shells with high surface areas through a surface-protected calcination process, revealed how the surface coating of another oxide or polymer could affect the crystallinity and the catalytic activity of the shells, and demonstrated that the phase composition, degree of crystallinity, surface area, and dispersity of TiO2 were important features required for photocatalysis [17][18][19][20].…”

Section: Introductionmentioning

confidence: 99%

Contribution of multiple reflections to light utilization efficiency of submicron hollow TiO2 photocatalyst

Liu

Joo

et al. 2016

Sci. China Mater.

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In this work, we carried out both theoretical calculation and experimental studies to reveal the contribution of hollow geometry to the light utilization efficiency of the TiO2 photocatalysts in diluted aqueous solution. It is found that the single or multi-shelled hollow structures do not induce significant multiple reflections within the shells as widely believed in previous reports, and therefore the geometric factor has minimal contribution to the improvement of the light utilization efficiency of the photocatalyst. To design TiO2 photocatalysts with higher activity, it is more appropriate to focus on the improvement of the crystallinity, diffusion, surface area, and dispersity of the catalysts, rather than their geometric shapes.

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“…The relatively small band gap structure of rutile extends the photo-generating region of visible light and charge separation through the electron transfer from rutile to anatase delays the electron-hole recombination that enhances the duration of the existing electron-holes, followed by considerable defects at the anatase/rutile interface. 31,32 As a result, the nanoparticle surface interacts with organic compounds more frequently, yielding a noticeable increase in photocatalytic performance.…”

Section: Journal Of the Electrochemical Society 165 (2) E64-e69 (2018)mentioning

confidence: 99%

Bulk-Direct Synthesis of TiO₂Nanoparticles by Plasma-Assisted Electrolysis with Enhanced Photocatalytic Performance

Kim

Jeong

Lim

et al. 2018

J. Electrochem. Soc.

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A new plasma-assisted electrolysis method has been developed to synthesize amorphous TiO 2 nanoparticles and exploited for the enhanced photocatalytic performance. The method is simple, environmentally friendly, produces nanoparticles directly from bulk metal, and is suitable for mass production. The process was conducted in low-concentration nitric acid electrolyte under a voltage of 450 V, the minimum necessary to produce plasma on the anode surface. The average nanoparticle size was tuned between 16 and 28 nm by controlling electrolyte concentration within the range of 5 to 15 mM. The production rate increased with time, with the maximum of 11.27 g/h. The amorphous TiO 2 nanoparticles were calcined at various temperatures to determine the crystalline structures and to compare their photocatalytic effects. The structure ranged from pure anatase to rutile under various calcination temperatures; the anatase-rutile mixed phase produced at 600 • C showed the highest catalytic performance, with 94% degradation of methylene blue within 30 min owing to a synergetic effect between the phases. This liquid-phase plasma-assisted electrolysis method can pave the way for large-scale synthesis of highly pure metal-based ceramic nanoparticles with narrow size distributions. Titanium dioxide is a semiconducting material that shows chemical stability and noble characteristics. For several decades, it has been extensively studied due to its physical and chemical properties such as wide band gap and efficient photon electron transfer, which allow its application in photocatalysis, 1-3 antireflection coating, 4-6 water splitting, 7-9 and dye-sensitized solar cells. 10-13 Among these, its photocatalytic effect has attracted the most attention; TiO 2 possesses powerful organic decomposition properties under ultraviolet light.Formation of the three main crystal phases of TiO 2 , namely anatase, rutile, and brookite, depends on the synthesis method and calcination conditions. Generally, mixed phases of anatase and rutile show the best photocatalyst performance due to a synergistic effect. Among the pure phases, anatase yields more efficient catalytic performance than rutile because it has a larger band gap (anatase 3.2 eV, rutile 3.0 eV). The wider band gap reduces light absorption and raises the maximum of the valence band to a higher level that is closer to adsorbed molecules' redox potentials. Then, it facilitates electron transfer from the TiO 2 surface to molecules due to the increased oxidation potential of the electrons. This enhances the decomposition of organic molecules absorbed on the surface of the TiO 2 .14 The photocatalytic performance originating from absorbed ultraviolet light is also affected by photogenerated charge carriers such as electrons and holes. 15 Highly excited holes in the valence band diffuse into the outer layer of a particle and react with adsorbed water molecules on the surface, creating reactive •OH radicals. In addition, electrons remaining in the conduction band produce oxide radicals on the parti...

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Control of the nanoscale crystallinity in mesoporous TiO₂shells for enhanced photocatalytic activity

Cited by 284 publications

References 42 publications

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Contribution of multiple reflections to light utilization efficiency of submicron hollow TiO2 photocatalyst

Bulk-Direct Synthesis of TiO₂Nanoparticles by Plasma-Assisted Electrolysis with Enhanced Photocatalytic Performance

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Control of the nanoscale crystallinity in mesoporous TiO2shells for enhanced photocatalytic activity

Cited by 284 publications

References 42 publications

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H 2

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H 2

Contribution of multiple reflections to light utilization efficiency of submicron hollow TiO2 photocatalyst

Bulk-Direct Synthesis of TiO2Nanoparticles by Plasma-Assisted Electrolysis with Enhanced Photocatalytic Performance

Contact Info

Product

Resources

About

Control of the nanoscale crystallinity in mesoporous TiO₂shells for enhanced photocatalytic activity

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Promotion of atomic hydrogen recombination as an alternative to electron trapping for the role of metals in the photocatalytic production of H ₂

Bulk-Direct Synthesis of TiO₂Nanoparticles by Plasma-Assisted Electrolysis with Enhanced Photocatalytic Performance