Aerosol assisted chemical vapour deposition of hydroxyapatite-embedded titanium dioxide composite thin films

https://doi.org/10.1016/j.jphotochem.2016.08.010Get rights and content

Highlights

Abstract

This work describes the first Aerosol Assisted Chemical Vapour Deposition (AACVD) synthesis of photocatalytic titanium dioxide thin films embedded with synthetic hydroxyapatite, [Ca10(PO4)(OH)2], nanoparticles. The hydroxyapatite nanoparticles were prepared using a low temperature continuous hydrothermal flow synthesis method; analysis of the hydroxyapatite powder showed that it was phase pure and that the as-prepared material was made up of nano-needles. The nanoparticles were then embedded into TiO2 coatings using the AACVD technique by suspending them in a solution of the titania precursor (titanium tetra-isopropoxide).

Results showed that the hydroxyapatite, although present in very low concentrations in the coatings (not detectable by XRD or Raman spectroscopy), heavily affected their morphology, depending on their concentration in the precursor solution. Tests of the photocatalytic activity of the composite films showed that the inclusion of the hydroxyapatite led to an increase in methylene blue photodegradation (up to 50% higher) and that the materials were photostable.

This study shows that TiO2 coatings embedded with hydroxyapatite nanoparticles have potential as highly efficient photocatalysts.

Introduction

Photocatalytic materials are of interest due to their potential application for environmental remediation and for self-cleaning structures [1]. Under appropriate light irradiation, such materials can generate active species (electrons (e), holes (h+), reactive oxygen species (ROS)), which can degrade organic molecules, including pollutants [2]. Titanium dioxide (TiO2) is the most common photocatalytic material and can exist in three different forms—anatase, rutile and brookite. The anatase form is particularly efficient as a photocatalyst, with a band gap of 3.2 eV [3]. TiO2 can also be mixed/combined with different compounds, in multiphase systems as a route to achieve simultaneous multifunctional properties. For example, there are literature reports of TiO2 combined with ZnO, PbO, SnO2 or SiO2, where the presence of the additional phase led to higher photocatalytic activity and/or photoactivity using a visible light source [4], [5], [6], [7]. Improved and/or additional functional properties were also achieved with the incorporation of nanoparticles (NPs) into a titanium dioxide matrix; in the majority of cases, such composite systems were prepared using preformed metallic nanoparticles (NPs), such as Au, Ag and/or other noble metals [8], [9], [10].

Hydroxyapatite (HAp), [Ca10(PO4)6(OH)2], is a calcium phosphate mainly known for its applications in bone replacement [11]. Literature data, however, suggests that some forms of HAp also have photocatalytic activity [12], [13]. Moreover, its combination with TiO2 looks particularly promising; TiO2-HAp biphasic composites have been shown to possess superior photocatalytic activities compared to the corresponding individual phases [14], [15], [16]. Despite the large volume of literature on the photoactivity of titania-based multiphase films, to the best of our knowledge, no study has ever looked at photocatalytic properties of HAp-embedded TiO2 films.

Hydroxyapatite can be made via a number of ways such as batch co-precipitation or flow methods [17]. In flow methods such as Continuous Hydrothermal Flow Synthesis (CHFS), supercritical water can be used as a reagent to drive the rapid synthesis of HAp [18], as well as a wide range of metal oxides [19], [20], [21], [22]. CHFS-prepared HAp nanoparticles and doped variants were also prepared [18], [23], [24]. More recently, lower temperature flow methods for HAp nanoparticle synthesis have also been developed, which do not require high pressures [25].

TiO2 thin films for photocatalysis or other applications can be prepared using several methods [26], [27], [28], [29], including Chemical Vapour Deposition (CVD). The Aerosol Assisted Chemical Vapour Deposition (AACVD) technique, in particular, is very versatile for the synthesis of TiO2, as by using appropriate deposition solvents and process temperatures, it has been possible to tailor phase composition [30]; moreover, different deposition precursors can affect the morphology of films [31]. NP­-containing multiphasic TiO2­-based coatings can also be deposited using the AACVD process [5], [32].

Herein, we report the synthesis of a composite coating of HAp embedded in a TiO2 matrix (HAp@TiO2) using the AACVD technique. HAp nanoparticles were first prepared using a plastic flow reactor and then the freeze dried HAp powder was mixed at different loadings with a Ti-precursor (in toluene solution) that formed the feed for the AACVD process; this resulted in needle like HAp being embedded in TiO2 films. The deposited thin films were characterised using several analytical techniques such as powder X-Ray Diffraction (XRD) and electron microscopy, to assess their composition and morphology. The photocatlytic activity of the coatings was also evaluated, to see whether the inclusion of HAp nanoparticles had any effect on such properties.

Section snippets

Preparation of HAp NPs

HAp NPs were prepared using a continuous plastic flow synthesis (CPFS) reactor using an approach similar to that described elsewhere (Fig. 1) [25]. This simple, single step synthesis method was used for HAp synthesis under near ambient conditions, with affordable and readily available reagents. The CPFS system consists of two HPLC Gilson pumps (Gilson Model 307 Pumps with 25 SC Pump heads). The first and the second pumps (P1 and P2) supplied the calcium and phosphate precursors respectively.

Characterisation of HAp NPs

Fig. 2(a) shows the XRD patterns of the NPs, both as prepared and after a 1000 °C heat-treatment. Both patterns are similar to the HAp reference pattern (JCPDF 01-072­-1243, see bottom of the figure); moreover, in both cases, no other phases apart from HAp can be detected. The data confirm that single-phase HAp was produced with the plastic flow system and that the 1000 °C heat-treatment did not lead to the formation of other phases, which suggests the material was stoichiometric (i.e. the Ca:P

Discussion

The use of HAp NPs during TiO2 deposition by AACVD showed to have a significant effect on the characteristics of the coatings; indeed, although the phases obtained did not change, features such as crystallinity and morphology of the titania were heavily affected. Regarding the crystal structure, overall the use of HAp NPs during the deposition led to a decrease in crystallinity of the titania coatings. It is interesting to highlight, however, that the effect was very different, depending on the

Conclusions

Composite thin films of anatase titanium dioxide with needle-like hydroxyapatite nanoparticles incorporated in the structure were prepared using Aerosol Assisted Chemical Vapour Deposition. Characterisation of the films showed that the quantity of NPs in the precursor solution affected the morphology and crystallinity of the films. Choosing appropriate deposition conditions, allowed coatings with superior photocatalytic activity to be prepared; selected materials also showed photostability,

Acknowledgments

This work was supported by National Funds from FCT—Fundação para a Ciência e a Tecnologia through the project UID/Multi/50016/2013 and developed in the scope of the project CICECO−Aveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. CP and RP thank FCT for the grants SFRH-BPD-86483-2012 and SFRH/BPD/97115/2013.

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