Shape-Selective Mesoscale Nanoarchitectures - Preparation and Photocatalytic Performance

At a Glance

Metadata	Details
Publication Date	2020-05-12
Journal	Catalysts
Authors	Simona E. Hunyadi Murph, Katie Heruox
Institutions	University of Georgia, Savannah River National Laboratory
Citations	13
Analysis	Full AI Review Included

Executive Summary

This research details the successful fabrication and performance testing of novel, shape-selective mesoscale hybrid nanoarchitectures designed for enhanced photocatalysis under both UV and Visible (Vis) light.

Multiscale Control: A hybrid fabrication strategy merging top-down lithography (E-beam patterning, RIE) and bottom-up solution chemistry (ALD, AuNR synthesis) achieved unprecedented control over material size (mesoscale 100 µm to 5 mm), shape, and spatial arrangement.
Shape Dependence: Photocatalytic activity toward Methyl Orange (MO) degradation under UV light is strongly dependent on the nanoparticle geometry, decreasing in the order: diamonds > squares > triangles > spheres. Nanostructures with more edges and corners exhibited superior reactivity.
Compositional Enhancement (UV): Core-shell SiO₂-TiO₂ pillar arrays demonstrated significantly enhanced UV photocatalytic response, achieving degradation rates up to 4 x 10^-3 min^-1, which is up to four times higher than pure TiO₂ arrays of the same shape.
Visible Light Activation: Au Nanorods (AuNRs) were successfully anchored to the TiO₂ and SiO₂-TiO₂ surfaces using an L-glutathione bifunctional linker, sensitizing the arrays to Vis light, where bare TiO₂ arrays are inactive.
Kinetic Improvement: Repetitive UV exposure of the SiO₂-TiO₂ arrays resulted in a doubling of photocatalytic efficiency after the first three exposures (rate increased from 20 x 10^-4 to 83 x 10^-4 min^-1), suggesting surface cleaning and maximization of active sites.
Material Phase: Post-deposition annealing at 500 °C successfully converted the amorphous TiO₂ layer into the highly active crystalline anatase phase.

Technical Specifications

Parameter	Value	Unit	Context
Mesoscale Array Dimensions	3 x 3	mm	Size of individual patterned areas
Nanopillar Length	~1	µm	Height of etched Si posts
Nanopillar Pitch	2	µm	Center-to-center spacing
Nanopillar Width Range	200 to 330	nm	Width/Diameter after ALD coating
TiO₂ Crystal Phase	Anatase	N/A	Achieved after annealing at 500 °C
Annealing Temperature	500	°C	Post-deposition treatment (2 h)
Highest UV Degradation Rate (k)	4 x 10^-3	min^-1	SiO₂-TiO₂ diamond pillar array (initial exposure)
UV Degradation Rate (Film Control)	1 x 10^-4	min^-1	TiO₂ or SiO₂-TiO₂ thin film (no nanoarray structure)
AuNR Length / Width	48 (± 6) / 20 (± 2)	nm	Aspect ratio 2.4
AuNR Longitudinal LSPR	660	nm	Peak absorption wavelength for Vis activation
AuNR Transverse LSPR	520	nm	Peak absorption wavelength
MO Degradation (Vis Light)	3-6	%	Degradation over 3 hours for AuNR-modified arrays
Si Etch ICP Power	1200	W	Reactive Ion Etching (RIE) parameter
Si Etch RF Power	30	W	Reactive Ion Etching (RIE) parameter

Key Methodologies

The fabrication of the hybrid nanoarchitectures relied on a precise sequence combining lithography, deposition, etching, and wet chemical functionalization.

E-beam Lithography (Patterning): P-type Si wafers were coated with ZEP-520A resist. Patterns for four distinct shapes (cylinder/sphere, rhombic prism/diamond, square prism/square, triangular prism/triangle) were written in 3 x 3 mm arrays at a 2 µm pitch, followed by development and cleaning.
Cr Mask Deposition and Lift-off: A 15 nm Chromium (Cr) mask was deposited via E-beam thermal evaporation, followed by lift-off in acetone and IPA to define the Si nanofeatures.
Deep Reactive Ion Etching (DRIE): Masked wafers were etched using RIE (1200 W ICP, 30 W RF) with a gas mixture of Ar, SF₆, and C₄F₈ for 6-7 minutes to create ~1 µm tall Si posts.
Atomic Layer Deposition (ALD): The Si post arrays were coated with 25-50 nm of either pure TiO₂ or core-shell SiO₂-TiO₂ films.
Crystallization Annealing: Coated arrays were annealed at 500 °C for 2 hours to convert the amorphous TiO₂ into the photocatalytically active anatase crystalline phase.
Au Nanorod (AuNR) Synthesis: AuNRs (48 nm x 20 nm, aspect ratio 2.4) were synthesized using a silver-mediated surfactant (CTAB) wet chemical approach.
Surface Functionalization (Self-Assembly): The annealed TiO₂-based arrays were submerged in 10 mM L-glutathione (a bifunctional linker) in ethanol overnight. The arrays were then immersed in concentrated AuNR solution for >4 hours, allowing the carboxylate groups to bind to TiO₂ and the thiol group to bind to the AuNRs.
Photocatalytic Testing: Arrays were tested in 10 µM Methyl Orange (MO) solution under UV (365 nm LED) and Visible (fiber optic) light, monitoring degradation via UV-Vis spectroscopy and fitting results to pseudo-first-order kinetics.

Commercial Applications

The development of highly structured, shape-selective, and Vis-light-activated hybrid photocatalysts has direct relevance across several engineering and environmental sectors:

Wastewater Treatment and Remediation:
- High-efficiency degradation of persistent organic pollutants (dyes, pharmaceuticals, industrial chemicals) using reusable, structured catalyst beds.
- Deployment of catalysts that utilize the broader solar spectrum (Vis light) for continuous, energy-efficient water purification in large-scale reactors.
Environmental Safety and Stewardship:
- Creation of sequestered, chemically bound nanomaterials, eliminating the risk of environmental release associated with traditional unbounded nanoparticle slurries.
Solar Energy Conversion:
- Enhanced photocatalytic hydrogen production (water splitting) by leveraging the plasmonic effects of AuNRs to maximize solar energy harvesting efficiency.
Self-Cleaning and Anti-Microbial Surfaces:
- Integration of these structured, Vis-activated TiO₂-Au nanoarrays onto architectural glass, medical equipment, or protective coatings for continuous self-decontamination under ambient light conditions.
Advanced Chemical Sensing:
- Utilization of shape-selective facets (e.g., diamond tips) to control adsorption and reaction selectivity, enabling highly specific chemical sensors or catalytic converters.

View Original Abstract

We create ordered arrays of shape-selective gold-titania composite nanomaterials at the mesoscale (100 µm to 5 mm) by a combination of both bottom-up and top-down approaches for exquisite control of the size, shape, and arrangement of nanomaterials. Lithographic techniques along with wet chemical synthetic methods were combined to create these composite nanomaterials. The photocatalytic activity of these TiO2, TiO2-Au and SiO2-TiO2-Au nano-composite mesoscale materials was monitored by the photodegradation of a model analyte, methyl orange, under UV and visible (Vis) illumination. Bare TiO2- and SiO2-TiO2-coated pillar arrays showed significant activity toward methyl orange in UV light with degradation rates on the order of 10−4-10−3 min−1. The photocatalytic activity of these arrays was also found to depend on the nanoparticle shape, in which particles with more edges and corners were found to be more reactive than spherical particles (i.e., the photocatalytic activity decreased as follows: diamonds > squares > triangles > spheres). SiO2-TiO2-Au nano-composite pillar arrays were tested in both UV and Vis light and showed increased activity in Vis light but decreased activity in UV light as compared to the bare semiconductor arrays. Additionally, the Au nanorod-functionalized nanoarrays exhibit a strong shape-dependence in their photocatalytic activity toward methyl orange degradation in Vis light.

Tech Support

Original Source

DOI: https://doi.org/10.3390/catal10050532

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