Oxygen vacancies, the optical band gap (Eg) and photocatalysis of hydroxyapatite: Comparing modelling with measured data
Graphical abstract
Introduction
Pollution of the environment is one the key challenges for our modern industrial society; the increasing level of toxic compounds, both in air and waters, requires effective methods and techniques for their degradation and/or conversion into harmless species [1]. Photocatalysis is an important methodology for environmental remediation; through which it is possible to degrade hazardous chemicals both in liquid and in the gas phase [1], [2].
Photocatalysts are semiconductors, i.e. materials with an energy band gap (Eg). When irradiated with energy ≥ Eg, a negatively charged electron (e−) can be promoted to the conduction band from the valence band; this leads to the formation of a positively charged hole (h+) in the valence band. These two charged species can react with molecules adsorbed on the surface of the material, and such reactions can eventually lead to the degradation of these molecules [3]. Examples of the degraded molecules include dyes and pharmaceuticals in the liquid phase, while species such as nitrogen oxides (NOx), volatile organic compounds (VOCs) and alcohols/alkanes can be decomposed in the gaseous phase [4].
Beyond being used for pollution degradation, photocatalysts can also be employed for bacteria inactivation. Microorganisms can be killed by interactions with the e−/h+ couple or with other Reactive Oxygen Species (ROS), such as radicals, which are generated with light irradiation [5].
Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is a calcium phosphate employed both in biomedicine and for environmental remediation. Its use in biomedicine as a material for bone implants is due to its high biocompatibility and bioactivity [6]. In environmental remediation, HAp can be used for heavy metals absorption and removal, especially for bivalent cations such as Pb (II), Cd (II) and Zn (II) [7].
In recent years, the use of HAp as a photocatalyst has also been investigated, and several studies reported some forms of HAp as being photocatalytically active. Biphasic HAp-TiO2 materials, for instance, showed greater photocatalytic activity compared to either single phase HAp or TiO2 [8], [9]; titanium-doped HAp has also exhibited photocatalytic behaviour. In the latter case, the insertion of the Ti4+ ion into the HAp lattice led to light absorption in the UV region, with the absorption being stronger for higher Ti4+ concentrations [10], [11]. Different band gap values were measured for Ti-doped HAp; generally they were between 3.0 and 3.2 eV (around the values for anatase and rutile TiO2) [12], and higher values up to 3.65 eV were also measured [13].
Some forms of single-phase HAp, with no dopant, also showed photocatalytic activity; indeed, Nishikawa reported the degradation of pollutants such as methyl mercaptane and dimethyl sulphate by HAp under UV irradiation [14], [15]. Some HAp of natural origin has also exhibited photocatalytic behaviour − HAp derived from both cod fish bones and mussel shells has been used to degrade methylene blue [16], [17].
Despite these interesting results, however, it is not yet clear which are the structural features affecting and/or imparting the photocatalytic behaviour to some forms of HAp. According to Nishikawa, UV irradiation can lead to an oxygen vacancy in the HAp lattice. This causes an electron transfer from the HAp vacancy to the atmospheric oxygen which, in turn, leads to the formation of the charged O2− species, and indeed, O2− was observed with EPR spectroscopy [14], [18]. O2− can then react with liquid/gaseous molecules and degrade them.
Although the detection of O2− can be linked to the photocatalytic activity of HAp, it does not explain why some HAp forms are photocatalytic while others are not, i.e. why some forms can generate O2− when irradiated while others do not. Piccirillo et al., for instance, made two single-phase HAp materials from cod fish bones; of these two, only one showed photocatalytic activity, while the other did not [16]. According to the authors, a possible explanation for such a difference was the existence of an oxygen vacancy present in the photoactive HAp material, whose formation was due to the more reducing environment the material was prepared in.
In this work we present a density functional theory (DFT) modelling study of HAp, which correlates various possible oxygen vacancies in the HAp lattice (in PO4, OH, and PO4 + OH sites) with the variations in the value of the optical band gap Eg. To achieve this, first principles calculations of the HAp lattice and Density of States (DOS) modelling of defects in this system, specifically oxygen vacancies, were used [19]. The modelling results are compared with the band gap (Eg) values measured by the authors in actual HAp samples, of both marine origin and commercial products, using the Tauc plot method [20].
Section snippets
Sample preparation
The preparation of HAp samples of marine origin was as previously described [16]. Briefly, the clean and dried cod fish bones were treated in solution containing an excess of either calcium chloride or calcium acetate (samples indicated with the symbols B_Ac and B_Cl respectively). After the solution treatment, the bones were dried and calcined at temperatures of 1000 °C (B_Ac_1000 and B_Cl_1000) or 1200 °C (B_Cl_1200), using a heating rate of 5 °C/min and an annealing time of 1 h. Commercial HAp
Experimental measurements
Fig. 2(a) and (b) show the measured UV spectra for the HAp samples of marine origin, and the commercial ones, respectively, while Table 2 reports the band gap values calculated from the spectra. As already reported [16], the sample obtained from the bones treated with calcium acetate and annealed at 1000 °C (B_Ac_1000) shows a band gap of 3.4 eV. Moreover, some absorption in the visible region was also observed, between 500 and 650 nm. In fact, this sample had a sky blue colour. This phenomenon is
Conclusions
Density functional theory calculations and Density of States (DOS) modelling, was used to calculate the change of the optical band gap energy (Eg) upon introduction of various possible oxygen vacancies in the HAp lattice: O from PO4, O from OH, simultaneous O from PO4 + OH sites, and the loss of an entire OH group. The results were compared to measured data on both samples of marine origin and commercial HAp powders, and the modelled values were found to match the measurements very closely.
For
Acknowledgements
VSB acknowledges financial support via his RFBR (Russia) grant no. 15-01-04924. RCP acknowledges financial support from the Fundaçao para a Ciência e a Tecnologia (FCT, Portugal) via grant no. SFRH/BPD/97115/2013. JC thanks the FCT for support via grant no. UID/CTM/50025/2013. 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
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