Controlled Ag-TiO2 heterojunction obtained by combining physical vapor deposition and bifunctional surface modifiers
Graphical abstract
Introduction
The good photocatalytic properties of titanium dioxide (TiO2) make its use possible in many fields of science; for example, as an antibacterial agent [1], as a UV protecting agent [2], in solar energy conversion [3], in splitting of water to produce hydrogen fuel, in decomposition [[4], [5], [6], [7], [8]] and removal of organic or inorganic pollutants [[9], [10], [11]], and as a photocatalyst in 2D materials [12]. In this regard, organic pollutants in water sources are one of the biggest environmental problems as they are highly toxic and difficult to degrade [7,13]. The effectiveness of the photocatalytic process depends on several factors, such as the efficient generation of reactive oxygen species, which have high oxidizing power, being able to oxidize and mineralize almost any organic pollutant (especially •OH, E° = 2.8 V) [14]. Other factors include a wide range of light absorption, low recombination of electron-hole pairs, the availability of active sites for oxygen, electron-hole transfer capability, interphase connection between the pollutant and the photocatalyst, and the removal of interfacial photocatalyst from an aqueous medium [[15], [16], [17], [18]]. One of the most common persistent cationic organic pollutants is methylene blue (MB), which is widely used in the cotton, wood, and silk industries [19]. When it is discharged into water sources, it harms human health and ecosystems through bioaccumulation. This dye can be mineralized in an aqueous medium to CO2 and H2O or to less toxic molecules with use of TiO2 nanostructures, since these photocatalyst materials are chemically stable and easily recoverable in the solid phase, and therefore can be scalable for potential practical applications [20].
Kamat [21] pointed out that features such as shape- and size-controlled synthesis, new tools to understand the surface properties, and ease of chemical modification to tailor their surface properties have given many of these oxides prominence in recent years. However, TiO2 is a semiconductor with a wide energy gap, preventing further absorption of photons (it absorbs only UV light, which is less than 3% of incident solar light). It also exhibits a high rate of recombination of hole-electron pairs, and because of its small size it tends to agglomerate, preventing pollutant interaction on the photocatalyst surface. Therefore one of the most important aims in photocatalysis regarding environmental applications is the development of new modified TiO2 photocatalysts. Cong et al. [22] reported the importance of increasing the stability the photocatalyst and the transmission of photogenerated electrons between the electron acceptor molecule and the conduction band of TiO2 through chemical bond formation. They also reported that problems such as low quantum yield (associated with the rapid recombination of photogenerated electron-hole pairs) diminish adsorption and make recovery of the photocatalyst difficult, hampering the applications of these nanostructured semiconductors. In this regard, our group recently reported the impact of the shape, size, and distribution of semiconductor nanoparticles on photocatalytic degradation response since they are controlling factors that dictate the physical and chemical properties [23].
Several studies confirmed that the addition of noble metal nanoparticles such as gold nanoparticles and Ag nanoparticles (AgNPs) increases the photoresponse of TiO2 [[24], [25], [26], [27]]. Those nanoparticles act as an electron acceptor material, facilitating electron-hole separation because of the formation of a Schottky barrier at semiconductor-metal junctions. Additionally, there is considerable interest in the use of AgNPs to enhance the photocatalytic properties because of their high oxygen adsorption reactivity [28], high efficiency, and capacity to facilitate the excitation of electrons by creating a local electric field [29,30], and as a consequence, they improve the semiconductor photoresponse. Xu et al. [31] recently reported that some issues still occur in these kinds of semiconductor-metal photocatalysts, such as a weak junction, easy separation, and self-agglomeration, resulting in a lesser separation of photogenerated electron-hole pairs. In this respect, the development of Ag-TiO2 compounds with tight junctions is also a great challenge to efficiently separate the generated electron-hole pairs, thus increasing the photoresponse.
Vargas-Hernández et al. [32] recently reported that the concentration of photogenerated charge carriers and electron-hole recombination lifetimes are fundamentally important factors in the photocatalytic performance of TiO2 materials. They also addressed the impact of metal doping in TiO2 and how this affects the physical properties and therefore the photocatalytic process to degrade MB. They pointed out a clear redshift of the semiconductor band gap, improving visible light harvesting, and achieved greater photodegradation by Ag doping, which was attributed to greater intensity of light radiation at the semiconductor interface generated by surface plasmon resonance of AgNPs tightly attached to the semiconductor.
AgNPs are typically synthesized by chemical reduction of Ag precursors in the solution phase [33] because greater reproducibility, less dispersion, and smaller particle size are achieved in comparison with solid-state methods. However, solid methods are often required for practical applications such as photocatalysts, sensors, supported materials, and solid matrices [23]. In this sense, with respect to wastewater remediation using photocatalytic nanomaterials, first, the catalyst obtained is in the solution phase and therefore it is difficult to remove it from treated water after photodegradation of the pollutant, which therefore limits their practical application and scalability [14]. The second factor is that the prepared nanomaterial needs an appropriate stabilizer in the solution phase to prevent agglomeration. Additionally, some reports have shown that the size, morphology, stability, and properties of metal nanoparticles are strongly influenced by the experimental conditions, the different kinetics with reducing agents, and the adsorption processes [34,35]. Therefore the design of synthetic methods in which the size, morphology, stability, and properties can be controlled has become a major topic of interest to obtain efficient photocatalyst materials.
One interesting physical method is physical vapor deposition (PVD), which has several advantages; for instance: there is no contamination from solvent or precursor molecules on the surface, the process is environmentally friendly, and there is no liquid waste. However, the greater dispersion and diameter of particles with respect to solution-phase methods suggests new tools are needed [[33], [34], [35]]. In this sense, our group improved this method through use of macromolecular systems, allowing the stabilization of ordered arrangements of metal nanoparticles and thus, reducing the size dispersion and improving the particle diameter [36].
Here we report an alternative and more versatile method to grow AgNPs by PVD tightly attached to nanocrystalline TiO2 by use of a bifunctional surface modifier such as mercaptoacetic acid (HOOCCH2SH). The efficient photocatalytic activity of TiO2 combined with the excellent electron acceptor abilities and surface plasmon resonance of AgNPs resulted in very efficient organic pollutant photodegradation. The stability, particle size, and particle size distribution of AgNPs were controlled and improved via chemical bonds between TiO2, HOOCCH2SH, and AgNPs and with use of an appropriate sputter time.
Section snippets
Chemicals and starting materials
HOOCCH2SH, MB, toluene, and ethanol were obtained from Sigma-Aldrich and were used without further purification. Degussa P25 TiO2 (nanopowder composed of 70% anatase and 30% rutile) was used to generate nanocrystalline anatase TiO2 (TiO2 nanoparticles). The AgNPs were obtained by PVD (PELCO SC-6 magnetron sputter coater). The cathode used was of electrolytic grade (99.99% purity).
Synthesis of photocatalytic material
TiO2 nanoparticles were obtained by sintering at 500 °C. This heat treatment allows a crystal phase transition from
Characterization
When metal particles are deposited on TiO2, the color changes from white to yellow-pink. This change is indicative of small AgNPs (around 10–50 nm). The presence of anatase crystalline particles and their interaction with bifunctional surface modifiers and AgNPs were analyzed by two different techniques, XRD and Raman spectroscopy, as described below. The XRD pattern of TiO2-OOCCH2S–Ag/Ag2O is shown in Fig. 2a. The XRD reflections for TiO2 correspond mainly to the anatase crystalline structure,
Conclusions
Through the use of a surface modifier, it was possible to achieve a chemical bond between Ag and TiO2, which results in an improved heterojunction and increased transmission of the photogenerated electrons, preventing desorption of the metal from the surface of the semiconductor. By means of a chemical bond, it was possible to dope the semiconductor with small spherical AgNPs, which have a superficial plasmon resonance in the visible zone, allowing an improvement of the separation of the
Acknowledgments
The authors acknowledge Vicerrectoría Académica UC grant VRA-39131781 and Faculty of Chemistry financing UC-3913-529-81 for financial support. This research received funding from Consejo Superior de Investigaciones Científicas, Spain, under grant I-COOP LIGHT 2015CD0013. The use of Servicio General de Apoyo a la Investigación (University of Zaragoza) is also acknowledged.
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