Edited by: Maria Olea, University of Cambridge, United Kingdom
Reviewed by: Chuncai Kong, Xi'an Jiaotong University, China; Nageswara Rao Peela, Indian Institute of Technology Guwahati, India
This article was submitted to Heterogeneous Catalysis, a section of the journal Frontiers in Catalysis
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In this work, we provide new insights into the design of Ti@TiO2 photocatalyst with enhanced photothermal activity in the process of glycerol reforming. Ti@TiO2 nanoparticles have been obtained by sonohydrothermal treatment of titanium metal nanoparticles in pure water. Variation of sonohydrothermal temperature allows controlling nanocrystalline TiO2 shell on Ti0 surface. At 100 < T < 150°C formation of TiO2 NPs occurs mostly by crystallization of Ti(IV) amorphous species and oxidation of titanium suboxide Ti3O presented at the surface of Ti0 nanoparticles. At T > 150°C, TiO2 is also formed by oxidation of Ti0 with overheated water. Kinetic study highlights the importance of TiO2 nanocrystalline shell for H2 generation. Electrochemical impedance spectroscopy points out more efficient electron transfer for Ti@TiO2 nanoparticles in correlation with photocatalytic data. The apparent activation energy, Ea = (25–31) ± 5 kJ·mol−1, assumes that photothermal effect arises from diffusion of glycerol oxidation intermediates or from water dynamics at the surface of catalyst. Under the heating, photocatalytic H2 emission is observed even in pure water.
Hydrogen is a clean fuel that, when consumed in a fuel cell, yields only water. Today, 95% of hydrogen is produced from fossil fuels, such as natural gas and oil (Baykara,
Design of catalyst morphology is another important strategy to reach maximal photocatalytic activity. Core-shell nanoparticles have attracted a great deal of attention as promising photocatalysts for hydrogen production due to the synergism between the cores and shells and/or new properties providing by the interactions between the cores and shells (Gawande et al.,
The commercially available titanium nanopowder (Nanostructured & Amorphous Materials, Inc. Ti, 99%) is an air-sensitive material and it was stored in the argon-filled glove box prior use. Stable Ti@TiO2 NPs were prepared by SHT treatment of Ti nanopowder in pure water (Milli-Q 18.2 MΩ·cm at 25°C). The SHT reactor is shown in
Powder X-Ray Diffraction (XRD) diagrams were recorded with the use of a Bruker D8 Advance X-ray diffractometer equipped with a linear Lynx-eye detector (Cu Kα1,2 radiation, λ = 1.54184 Å). XRD patterns were collected between 10 and 90° (θ − 2θ mode) at room temperature, with a step size of Δ(2θ) = 0.02° and a counting time of 1.8 s·step−1. Quantitative phase analysis was performed by Rietveld refinement with the phase detection limit about 5% (León-Reina et al.,
Electrochemical Impedance Spectroscopy (EIS) was studied at dark conditions in the frequency range from 0.1 to 100 kHz with an AC amplitude 10 mV. The electrolytic cell was filled with 1 M KOH and bubbled with Ar for 20 min prior the measurements. Silver/silver chloride (Ag/AgCl) and platinum (Pt) were used as the reference electrode and counter electrode, respectively. The working electrode was prepared by ultrasonic dispersion of the synthesized photocatalyst (5 mg) in the mixture of 1 mL isopropanol (VWR, ≥99.7%) and 40 μL Nafion (Aldrich, 5 wt% lower aliphatic alcohols, 15–20% water). Then 5 μL of the photocatalyst suspension was deposited onto glassy carbon electrode.
The photocatalytic study was performed in aqueous glycerol (99% Sigma-Aldrich) solutions using a thermostated gas-flow cell made from a borosilicate glass and adapted to mass spectrometric analysis of the outlet gases. The image of the photocatalytic cell is shown in
TEM images depicted in
Typical TEM images of initial Ti particles
STEM/EDX mapping of air passivated Ti0 NPs
XRD diagram of air passivated Ti NPs in
XRD patterns of the initial Ti NPs and Ti@TiO2 NPs treated at different temperatures under SHT conditions
High-resolution Ti 2p XPS spectra shown in
Fitted Ti 2p high-resolution XPS spectra of air-passivated Ti0
At higher temperature, metallic titanium is also oxidized yielding TiO2:
According to the XPS data anatase nanocrystals at the surface of Ti NPs also can be formed by crystallization of TiO2·xH2O species. It is worth noting that SHT oxidation of Ti NPs is more effective than hydrothermal heating without ultrasound (
Variation of Ti NPs composition with the temperature of SHT treatment obtained by Rietveld refinement of XRD patterns.
Air passivated Ti |
86 | 14 | |
SHT 101°C | 88 | 12 | |
HT 150°C |
88 | 5 | 7 |
SHT 164°C | ≥80.5 | ≤1.5 | 18 |
SHT 214°C | ≥62 | ≤3 | 35 |
Thermal stability of Ti and Ti@TiO2 NPs obtained from TGA analysis.
Tox±10°C | 230 | 240 | 263 | 304 |
All Ti-based materials studied in this work have intense black color indicating extented photoresponse with nearly full solar spectrum. The solid state reflectance spectra of both Ti and Ti@TiO2 NPs (
Solid-state reflectance spectra of Ti0 NPs before and after sonohydrothermal treatment at different temperatures. The spectrum of Ti particles SHT treated at 101°C is very similar to the spectrum of air-passivated Ti0 particles reported recently (Nikitenko et al.,
The electrochemical impedance spectroscopy (EIS) provides a valuable information about the charge transfer and charge recombination processes at the interface of the catalysts and electrolytes (Barsoukov and Macdonald,
Nyquist plots for Ti0 NPs and Ti@TiO2 NPs and the equivalent circuit for Ti0
EIS parameters for Ti and Ti@TiO2 NPs.
Ti@TiO2 SHT 214°C | 59.95 | 7.96 | 5.44 |
Ti@TiO2 SHT 164°C | 48.63 | 22.85 | 7.67 |
Ti@TiO2 SHT 101°C | 42.06 | 34.03 | 9.12 |
Air passivated Ti NPs | 49.84 | 9.05 (kΩ) | not been observed |
Typical hydrogen emission profiles and calculated H2 yields for studied photocatalysts shown in
Typical hydrogen emission profiles with catalysts obtained at 101, 164, and 214°C SHT temperatures in solutions of 0.5 M glycerol under Xe-lamp white light and Ar flow
Furthermore, photocatalytic process with Ti@TiO2 NPs exhibits strong photothermal effect in an agreement with our previous results (Nikitenko et al.,
Plot of the H2 formation yields against glycerol concentrations over SHT 214°C Ti@TiO2 photocatalyst at 53 and 87°C.
Mass spectrometric measurements indicate the absence of CO2 emissions (
On the basis of the above data, it can be concluded that the mechanism of H2 formation in studied system involves two reaction pathways illustrated in
where RCOOH in the Equation (11) could represent a mixture of mentioned above carboxylic acids. It is worth mentioning that the direct water splitting is practically observable at the sufficiently high temperature only. In general, higher yield of hydrogen for glycerol compared to water was attributed to more effective hole scavenging by glycerol than by water (Jiang et al.,
Graphical sketch of suggested mechanism of H2 photocatalytic formation in the presence of Ti@TiO2 core-shell nanoparticles.
Finally, the stability of Ti@TiO2 photocatalysts at studied conditions was tested using HRTEM and ICP-OES techniques. Modification of the particle's morphology was not detected by HRTEM after photothermal experiments as shown in
In summary, this work provides some new insights into the structure and thermally-assisted photocatalytic properties of Ti0 and Ti@TiO2 nanoparticles. Rietveld refinement of XRD data revealed the presence of scarce titanium suboxide Ti3O with a
The original contributions presented in the study are included in the article/
SN and SR conceived the study. SN, SR, TC, and SE prepared the manuscript. TC and SE prepared the samples and performed their characterization. SE and SN performed photocatalytic experiments. AN and SE performed electrochemical study. All authors contributed to this work and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors kindly acknowledge Dr. Adel Mesbach for Rietveld refinement of XRD data, Dr. Xavier Le Goff for HRTEM measurements, Dr. Cyrielle Rey for TGA measurements, and Dr. Valerie Flaud for XPS analysis.
The Supplementary Material for this article can be found online at: