Atomic layer deposition of ZnO on ultralow-density nanoporous silica aerogel monoliths

Kucheyev, S. O.; Biener, Juergen; Wang, Y. M.; Baumann, Theodore F.; Wu, Kuang Jen; Buuren, T. van; Hamza, A. V.; Satcher, Joe H.; Elam, Jeffrey W.; Pellin, Michael J.

doi:10.1063/1.1870122

Cited by 90 publications

(85 citation statements)

References 23 publications

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“…Figure 1 From the SEM images, the ZnO coatings appear conformal for each thickness of ZnO deposited, consistant with previous reports. [9,13] The aerogel films remain porous after coating with ZnO, and the porosity is observed to decrease as expected with additional growth of ZnO. In addition, the feature size of the coated aerogels closely matches twice the thickness of the deposited ZnO indicating that the original silica aerogel framework takes up a minimal amount of space as expected.…”

supporting

confidence: 58%

“…The aerogel templates are coated with ZnO via atomic layer deposition (ALD) to yield an electrically interconnected semiconductor core-shell nanoweb structure. [13,14] Because it is both a stepwise and conformal coating technique, ALD provides exceptional control over nanoscale device composition. The large number of materials accessible by ALD (including, but not limited to, TiO 2 , ZnO, SnO 2 , ZrO 2 , and NiO) makes the technique widely applicable for the development of new photoelectrodes.…”

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confidence: 99%

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Aerogel Templated ZnO Dye‐Sensitized Solar Cells

et al. 2008

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Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO 2 have exhibited solar energy-conversion efficiencies of over 10% and remain one of the most promising candidates for cost-effective solar energy conversion devices. [1,2] The most efficient DSSCs reported to date utilize a high surface area photoanode, which allows for good light harvesting with a moderate extinction dye, in contact with the I 3 À /I À couple which acts as a redox mediator.[3] The good performance of DSSCs is partially attributed to the slow dark reaction, i.e., electron transfer from TiO 2 to I 3 À via a potentially multielectron process. The slow dark reaction kinetics allows for excellent charge collection despite the relatively slow (millisecond) transport through the nanoparticle network.[4]The I 3 À /I À couple, however, has several disadvantages, including limitations on the open-circuit voltage related to the redox potential of the mediator species. In order to push device performance beyond its current limits, a faster redox shuttle, which requires a smaller overpotential to reduce the oxidized dye, is likely necessary, either to increase the photovoltage (provided that the dark current in not enhanced) or to allow alternate dyes with increased spectral coverage to be used. Replacing iodide/ triiodide with faster one-electron, outer-sphere redox reagents such as ferrocene, however, has resulted in significantly worse overall performance. [5,6] Although such fast redox species efficiently reduce the oxidized dye, rapid recombination between electrons in the TiO 2 and the redox shuttle results in poor electron collection. [5][6][7] Therefore, an alternate redox shuttle will concomitantly require faster charge transport through the metal oxide frameworks to allow for complete charge collection under cell operating conditions. Several interesting photoanode architectures have been fabricated with reduced dimensionality, a design feature that is expected to accelerate charge transport. Among these new architectures are hydrothermally grown ZnO nanorod arrays, ZnO nanotubes, and TiO 2 nanotubes. [8][9][10] The ZnO nanorod arrays have been shown to exhibit much faster transport than comparable ZnO nanoparticle networks. [11,12] While nanorod devices showed promising efficiencies of 1.5%, further improvement requires overcoming the technical challenge of increasing the relatively low surface area that currently limits light-harvesting.[8] Nominally one-dimensional ZnO nanotubes showed efficiencies of 1.6%, but with limited lightharvesting again a significant performance limiting factor.[9]Here we introduce a new core-shell material as a pseudo-one dimensional ZnO photoanode produced from coating templates of high aspect ratio substructures, exhibiting initial efficiencies up to 2.4% under AM 1.5 illumination when incorporated in a DSSC. Inert low density, high surface area silica aerogel films, featuring a large range of controllable thickness and porosity, are prepared as substructure templates. The aerogel templates are coated wi...

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confidence: 58%

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confidence: 99%

Aerogel Templated ZnO Dye‐Sensitized Solar Cells

et al. 2008

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“…Zn L-edge XAS was used to investigate the electronic transitions from the Zn 2p core electrons into the Zn 4s and Zn 4d electronic states in the conduction band [34] and the final states were recorded in total electron yield (TEY) and total fluorescence yield (TFY) modes, which are surface-and bulk-sensitive, respectively ( Figure 5). [35] The overall shape of the Zn L-edge XAS spectra is in good agreement with previously reported data.…”

Section: Resultsmentioning

confidence: 99%

Photocatalytic Activity and Selectivity of ZnO Materials in the Decomposition of Organic Compounds

Lin¹,

Cojocaru²,

Chou³

et al. 2013

ChemCatChem

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ZnO and Li‐doped ZnO photocatalysts were prepared by using a solvothermal method, aided by a supercritical drying technique. The structure and morphology of the photocatalysts were investigated by using SEM, X‐ray diffraction (XRD), UV/Vis and Raman spectroscopy. The photocatalytic activity and selectivity were investigated in the aqueous‐phase photodegradation of methylene blue and phenol as model reactions. Herein, it is reported for the first time that Li doping can lead to significant deactivation of the photocatalytic activity (i.e., decreased oxidization capability) of ZnO materials. The distribution of intermediate products (i.e., selectivity) was also significantly modified in the decomposition of phenol catalyzed by Li‐doped ZnO compared to that catalyzed by ZnO. Photoluminescence (PL) and soft X‐ray absorption spectroscopy (XAS) studies suggested that dopant‐induced surface‐defect states acted as electron–hole combination centers and changed the adsorbate/surface binding, thus causing the deactivation of photocatalytic activity and altering the photocatalytic selectivity in Li‐doped ZnO materials.

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“…Thus the Knudsen diffusion coefficient (D K ) of the gas in the capillary is given by (14) Because the porous alumina is not well represented by a collection of straight cylindrical capillaries, the d C of the capillary is generally replaced by an equivalent diameter (d e ) of pores obtained from experimental values of the specific surface area (S), porosity (ε) and apparent density (ρ M )of the porous monolith. 28 The equivalent diameter (d e ) is simply the diameter of the pore when the alumina has the same surface-to-volume ratio with the capillary, i.e., area of a capillary/volume of a capillary = 4/d C , and, (15) Then the Knudsen diffusion coefficient is given by (16) By using Eq. (16), the Knudsen diffusion coefficients of DEZ and TiCl 4 are evaluated to be ~1.02 × 10 −2 and ~8.27 × 10 −3 cm 2 /s, respectively.…”

Section: Methodsmentioning

confidence: 99%

“…[10][11][12] However, for the conformal deposition on porous materials, the exposure time of precursor molecules should be much longer comparing with the exposure time on a flat substrate, because the chemisorption of the precursor is limited by the Knudsen diffusion on the internal surface of the porous materials. [13][14][15][16][17][18][19][20][21] In smaller pores with a longer depth, it is more difficult for the precursor to reach vacant sites which locate at the core of the porous materials. In addition, the number of adsorption sites in the porous materials is much higher than that in the flat substrate, due to their high specific surface area.…”

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confidence: 99%

Modified Shrinking Core Model for Atomic Layer Deposition of TiO₂on Porous Alumina with Ultrahigh Aspect Ratio

Park¹,

Leem²,

Lee³

et al. 2013

Bulletin of the Korean Chemical Society

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When atomic layer deposition (ALD) is performed on a porous material by using an organometallic precursor, minimum exposure time of the precursor for complete coverage becomes much longer since the ALD is limited by Knudsen diffusion in the pores. In the previous report by Min et al. (Ref. 23), shrinking core model (SCM) was proposed to predict the minimum exposure time of diethylzinc for ZnO ALD on a porous cylindrical alumina monolith. According to the SCM, the minimum exposure time of the precursor is influenced by volumetric density of adsorption sites, effective diffusion coefficient, precursor concentration in gas phase and size of the porous monolith. Here we modify the SCM in order to consider undesirable adsorption of byproduct molecules. TiO 2 ALD was performed on the cylindrical alumina monolith by using titanium tetrachloride (TiCl 4 ) and water. We observed that the byproduct (i.e., HCl) of TiO 2 ALD can chemically adsorb on adsorption sites, unlike the behavior of the byproduct (i.e., ethane) of ZnO ALD. Consequently, the minimum exposure time of TiCl 4 (~16 min) was significantly much shorter than that (~ 71 min) of DEZ. The predicted minimum exposure time by the modified SCM well agrees with the observed time. In addition, the modified SCM gives an effective diffusion coefficient of TiCl 4 of ~1.78 × 10 −2 cm 2 /s in the porous alumina monolith.

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Atomic layer deposition of ZnO on ultralow-density nanoporous silica aerogel monoliths

Cited by 90 publications

References 23 publications

Aerogel Templated ZnO Dye‐Sensitized Solar Cells

Aerogel Templated ZnO Dye‐Sensitized Solar Cells

Photocatalytic Activity and Selectivity of ZnO Materials in the Decomposition of Organic Compounds

Modified Shrinking Core Model for Atomic Layer Deposition of TiO₂on Porous Alumina with Ultrahigh Aspect Ratio

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Atomic layer deposition of ZnO on ultralow-density nanoporous silica aerogel monoliths

Cited by 90 publications

References 23 publications

Aerogel Templated ZnO Dye‐Sensitized Solar Cells

Aerogel Templated ZnO Dye‐Sensitized Solar Cells

Photocatalytic Activity and Selectivity of ZnO Materials in the Decomposition of Organic Compounds

Modified Shrinking Core Model for Atomic Layer Deposition of TiO2on Porous Alumina with Ultrahigh Aspect Ratio

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Modified Shrinking Core Model for Atomic Layer Deposition of TiO₂on Porous Alumina with Ultrahigh Aspect Ratio