Aerosol assisted chemical vapour deposition of hydroxyapatite-embedded titanium dioxide composite thin films
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.
References (50)
- et al.
Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution
Chem. Eng. J.
(2009) - et al.
Pilot plant scale continuous hydrothermal synthesis of nano-titania; effect of size on photocatalytic activity
Mater. Sci. Semic. Proc.
(2016) - et al.
Photocatalytic degradation of ethylene on mesoporous TiO2/SiO2 nanocomposites: effects on the ripening of mature green tomatoes
Biosys. Eng.
(2015) - et al.
Silver-modified titania with enhanced photocatalytic and antimicrobial properties under UV and visible light irradiation
Catal. Today
(2015) - et al.
Photocatalytic decomposition of perfluoroocatanoic acid by noble metallic nanoparticles modified TiO2
Chem. Eng. J.
(2016) - et al.
Synthetic hydroxyapatite in pharmaceutical applications
Ceram. Int.
(2016) - et al.
Composite hydroxyapatite/TiO2 materials for photocatalytic oxidation of NOx
Mater. Sci. Eng. B
(2012) - et al.
Preparation of nanocrystals hydroxyapatite/TiO2 compound by hydrothermal treatment
Appl. Catal. B
(2006) - et al.
Hydroxyapatite/titanium dioxide nanocomposites for controlled photocatalytic NO oxidation
Appl. Catal. B
(2011) - et al.
Synthesis methods for nanosized hydroxyapatite with diverse structures
Acta Biomater.
(2013)
Critical process parameters and their interactions on the continuous hydrothermal synthesis of iron oxide nanoparticles
Chem. Eng. J.
Solar photocatalytic TiO2 sol-gel thin films: optical and morphological characterization
Sol. Energy
Spectr. Chim. Acta A
Enhancement of stability and photoactivity of TiO2 coatings on annular glass reactors to remove emerging pollutants from waters
Chem. Eng. J.
Hydroxyapatite coating on titanium by low energy plasma spraying mini-gun
Surf. Coat. Tech.
Hybrid chemical vapour and nanoceramic aerosol assisted deposition for multifunctional nanocomposite thin films
Thin Sol. Films
Synthesis and properties of hydroxyapatite-containing porous titania coating on ultrafine-grained titanium by micro-arc oxidation
Acta Biomater.
An XPS study of the mixing effects indiced by ion bombardment in composite oxides
Appl. Surf. Sci.
Vanadyl pyrophosphate catalysts: surface analysis by XPS and LEIS
Appl. Catal. A
Structure, corrosion resistance and in vitro bioactivity of Ca and P containing TiO2 coating fabricated on NiTi alloy by plasma electrolytic oxidation
Appl. Surf. Sci.
Photocatalytic degradation of bilirubin on hydroxyapatite-modified nanocrystalline titania coatings
Catal. Comm.
A new measurement for nitrogen oxides in the air using an annular diffusion scrubber coated with titanium dioxide
Atmos. Environ.
Selective oxidation of benzyl alcohol over TiO2 nanosheets with exposed {0 0 1} facets: catalyst deactivation and degeneration
Appl. Catal. A
Self-cleaning coatings
J. Mater. Chem.
Multiphasic bi-component TiO2-ZnO nanocomposite: synthesis, characterization and investigation of photocatalytic activity under different wavelengths of light irradiation
J. Mater. Sci. Mater. Electr.
Cited by (38)
Augmentation of photocatalytic activity of nano-crystallite hydroxyapatite by fluoride doping
2024, Journal of Photochemistry and Photobiology A: ChemistryProduction of smart packaging from sustainable materials
2023, Green Sustainable Process for Chemical and Environmental Engineering and Science: Methods for Producing Smart PackagingProbing the photocatalytic competency of hydroxyapatite synthesized by solid state and wet chemical precipitation method
2022, Journal of Molecular StructureNanostructured titanium dioxide coatings prepared by Aerosol Assisted Chemical Vapour Deposition (AACVD)
2020, Journal of Photochemistry and Photobiology A: ChemistryCitation Excerpt :For higher acac content, on the other hand, the TiO peak is the dominant one; for AA_4, in particular, Ti-O-H peak is just a shoulder. This latter situation is the most similar to that previously observed in AACVD TiO2 coatings prepared with only TIPP as the precursor [16,25]. These different oxygen environments indicate different interactions of the surface oxygen with the possible atmospheric moisture, which can result in higher or lower OH surface content.