Effect of TiO2 nanoshape on the photoproduction of hydrogen from water-ethanol mixtures over Au3Cu/TiO2 prepared with preformed Au-Cu alloy nanoparticles
Graphical abstract
Introduction
The photocatalytic production of hydrogen is one of the most sustainable processes to obtain renewable hydrogen, which is a powerful energy carrier that, combined with fuel cells, can help us to manage and store energy in a very efficient way. Upon light irradiation, electrons and holes accumulate on the surface of the photocatalyst and H2 can be obtained when these electrons reduce hydrogen ions [1]. Among the photocatalysts tested so far for hydrogen generation, TiO2-based materials are the most widely used due to a proper balance between photocatalytic activity, low cost, availability and stability [2]. Recently, the use of 1D, 2D and 3D nanostructured TiO2 has attracted interest to improve the photocatalytic performance and several works have been reported in the literature showing the beneficial effect of controlling the dimensionality and architecture of TiO2 for the photoproduction of hydrogen [[3], [4], [5], [6], [7], [8], [9], [10]]. To that end, different synthetic procedures, such as electrochemical anodization [[11], [12], [13]], atomic layer deposition [14], microwave-assisted synthesis [15,16], flame synthesis [17], solvothermal methods [[18], [19], [20], [21]] and hydrothermal methods [[22], [23], [24], [25], [26], [27], [28]] have been used to fabricate diverse types of micro- and nanoshaped TiO2. However, some of these methods are complex and include multiple steps or expensive equipment and reagents [[29], [30], [31]]. Among the above-mentioned methods, the hydrothermal route is the most used one since it has many advantages, such as simplicity, scalability and cost-effectiveness. Nevertheless, the hydrothermal methods used to produce 1D TiO2 nanostructures as nanotubes, nanowires and nanobelts present the inconvenience of requiring an acid treatment step and intensive washing, which are time-consuming and may generate a great variability in the characteristics and properties of the resulting materials [32,33]. In addition, hydrothermal methods usually require long reaction times (up to 72 h). Therefore, the development of synthetic methods of nanoshaped TiO2 with an improved efficiency and a short preparation time is highly desirable. In this work, we have used for the first time (to the best of our knowledge) TiO2 microrods for the photogeneration of hydrogen, which can be obtained at short reaction times by simple methods. We compare the photocatalytic efficiency of this material containing preformed Au3Cu nanoparticles to generate hydrogen from water-ethanol mixtures under dynamic gas-phase conditions against well-known nanoshaped TiO2 nanobelts, nanotubes, nanowires and urchins as well as standard P25.
It is important to highlight that neither the shape and dimensions of the metal nanoparticles nor the architecture of the metal-TiO2 interaction are comparable when the metal nanoparticles are synthesized by conventional impregnation or photoreduction methods [1,[34], [35], [36]]. The use of preformed nanoparticles guarantees the same metal particle size and a similar architecture of the Au-Cu/TiO2 interphase for all the samples tested. Here we have deposited the metal nanoparticles on the different TiO2 shaped materials in a way that the characteristics of the metal nanoparticles are common over all the photocatalysts. In this way, the effect of the shape of the support can be precisely evaluated. This approach has been used successfully, for instance, in deciphering the role of nanoshaped CeO2 (nanocubes, nanorods and nanopolyhedra) in the oxidation of CO over Au/CeO2 catalysts [37]. Here we have chosen bimetallic nanoparticles of Au-Cu alloy with a Au3Cu composition since they have shown an excellent performance in the photoproduction of hydrogen from water-ethanol mixtures in previous experiments using standard TiO2 decorated with bimetallic nanoparticles with different Au:Cu ratios [38]. Non-noble metal Cu not only facilitates the separation of carriers, but also reduces the overpotential of hydrogen evolution, thus promoting the photocatalytic activity for H2 production [39,40].
Section snippets
Materials
Commercial TiO2 (P25 ca. 80% anatase and 20% rutile, purity >99.55%) was purchased from Evonik; sodium hydroxide (NaOH), potassium hydroxide (KOH) and ethylene glycol ((CH2OH)2) were purchased from Fisher Scientific; hydrochloric acid (HCl) and titanium(IV) n-butoxide (Ti(OBun)4, 97%) were purchased from Sigma-Aldrich; absolute ethanol was purchased from Scharlau. All reagents were used without further purification.
Preparation of TiO2 nanotubes, TiO2 nanobelts and TiO2 nanowires
For the synthesis of TiO2 nanotubes, nanobelts and nanowires, sodium titanate
Characterization
The nanoshaped titania supports were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and UV–vis spectroscopy and their surface area was determined by BET measurements. Additionally, the AuCu/TiO2 photocatalysts were also characterized by high-resolution transmission electron microscopy, Raman spectroscopy, UV–vis spectroscopy and X-ray photoelectron spectroscopy. Fig. 1 shows the XRD patterns of the different nanoshaped titania supports prepared in this
Conclusions
A series of well-defined titania 1D architectures (tubes, wires, rods, belts and urchins) were synthesized, characterized and tested for the photogeneration of hydrogen under dynamic conditions and in gas phase from water-ethanol mixtures and their photocatalytic performances were compared with a standard TiO2 P25 sample. The hydrogen photoproduction rates normalized on a weight basis followed the trend:nanotubes>>microrods∼P25 > nanowires > nanobelts > microurchins
The good results obtained
Acknowledgments
This work has been funded by projects MINECO/FEDER ENE2015-63969-R and GC 2017 SGR 128. JL is a Serra Húnter Fellow and is grateful to ICREA Academia program. LM is grateful to CONACYT México for the PhD grant no. 409809.
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