Preparation, Growth, and Analysis of Bioactive Films on Corning Glass using TEOS and CaSiO3

Silicon oxycarbide (SiOC) is a widely investigated material due to its different applications and uses; it is currently being studied in medicine, due to its biocompatibility, antibacterial properties, and chemical and thermal stability. On the other hand, it is known that hydroxyapatite can function as a light-emitting element. This work presents the obtaining of thin films of silicon oxycarbide containing calcium atoms (SiCaOC) by the hot filament chemical vapor deposition (HWCVD) technique, using tetraorthosilicate (TEOS) and calcium silicate (CaSiO₃) as precursors, to obtain bioactive substrates for the growth of calcium hydroxyapatite (Ca₅(PO₄)₃(OH)) and thus have a film capable of emitting luminescence. In the XPS analysis, we found the elements silicon, carbon, oxygen, calcium, and phosphorus. On the other hand, in the photoluminescence spectrum (PL) three emissions were observed∼390 nm,∼513 nm,∼700 nm, corresponding to ultraviolet, green and red, respectively, the latter in the visible spectrum; in the SEM microscopies we can observe compact films composed of quasi-spherical grains, on the films clusters and filaments created by the growth of hydroxyapatite. 

Introduction

Silicon oxycarbide (SiOC) is currently being studied because of its ability to produce stable and intense photoluminescence [1]–[3]. In addition, it has been demonstrated that its light emission can be modulated over a wide range of wavelengths with high quantum efficiency, spanning from the infrared to the near ultraviolet.

SiOC has been reported to have advantages over other silicas, where partial substitution of oxygen by carbon species within an amorphous silicon oxide matrix improves thermal, chemical, and mechanical properties [4]–[6].

In addition, SiOC denotes a wide use within medicine due to its biocompatibility as it does not cause adverse reactions in the human body, its antibacterial property makes it effective against the proliferation of these; it is also used in the creation of biosensors [7], [8]; SiOC file; On the other hand, hydroxyapatite has great behavior within photoluminescent devices due to its crystalline structure, it has also been used in the manufacture of sensors and detectors since it can interact with light efficiently, not to mention the biocompatibility that this compound has [9].

Although SiOC does not present good optical and electrical properties, it has been reported that an increase in the number of defects present in the material, as well as the incorporation of impurities, could improve its conduction and emission properties. In addition, the quantum confinement phenomenon presents in the material by introducing Si nanocrystals inside the amorphous SiOC matrix would cause a large increase in these two properties [10]–[12].

There are several methods of ceramic coatings [13]–[15], the most prevalent being plasma atomization; however, it is limited by intrinsic disadvantages, as the process takes place at high temperatures, which affects the stability of phases, hindering the incorporation and stimulation of HAp growth in bone.

The literature mentions the application of hybrid coatings produced by sol-gel to a metallic implant, which allows it to be compatible and capable of being functionalized to stimulate the nucleation and growth of a hydroxyapatite layer when embedded in simulated physiological fluids (SBF) [16], [17].

In this work, we report on the use of TEOS and CaSiO₃ as precursors for the deposition of SiOC films containing calcium atoms and the use of these for calcium hydroxyapatite growth.

Experimental Procedure

SiOC and SiCaOC films were obtained by the HWCVD technique. TEOS (99.999% trace metal base, Sigma-Aldrich) was used as a solid source of oxygen and silicon atoms, and calcium silicate (CaSiO₃) was used as a source of calcium atoms. The tungsten filament was heated up to 2000°C by supplying 82 V, while maintaining a constant H₂ flow rate of 30sccm in the reactor. For all samples, the substrate temperature was maintained at ~300°C, with a deposition time of 5 min. The filament-substrate distance was 3.5 cm, maintaining a constant total system pressure of 1 atm.

The incorporation of calcium atoms was by making a solution with TEOS and CaSiO₃ in a molar ratio of 1:1; the solution was placed in a bubbler system to introduce it into the HWCVD equipment, passing through it a flow of H2 (Fig. 1).

Fig. 1. HWCVD system schematic.

SiOC and SiCaOC films were deposited on Corning glass substrates. The substrates were cleaned using ultrasonic baths with xylene, acetone, and methanol for 15 minutes in each process.

For hydroxyapatite growth on substrates, a methanol solution was made with hydroxyapatite (Ca₅OH₁₃P₃ from Sigma-Aldrich powder 5 µm), this solution was left in magnetic stirring for 24 hours at room temperature (32°C) in a molar ratio 1:0.5 (Fig. 2); the substrates were placed on a Teflon base and left inside the solution for 10, 15 and 20 days for different stages of hydroxyapatite growth (Fig. 3).

Fig. 2. Scheme to obtain methanol and hydroxyapatite solution.

Fig. 3. Teflon base with silicon substrates and Corning glass.

Discussion of Results

XRD

The spectra presented in Fig. 4 are of SiCaOC and SiCaOC-HAp films, the latter corresponding to samples immersed in hydroxyapatite solution and methanol.

Fig. 4. XRD spectra: (a) SiCaOC film; (b), (c), and (d) SiCaOC-HAp film in solution for 10, 15, and 20 days, respectively.

In the spectrum of Fig. 4a, we can observe the planes (311), (212), and (023). The first plane corresponds to calcium oxide (CaO₂); the other planes observed in the spectrum of the red box correspond to silicon oxide (SiO₂).

The crystallographic planes of hydroxyapatite (HAp), it can be observed, in Fig. 4b whose sample was kept in solution for 10 days, we find the planes (300), (241) and (600) corresponding to HAp; also in this spectrum we see a plane for calcium (Ca) (310) and carbon oxide (CO₂)(013), as well as silicon oxide (SiO₂). From the Fig. 4c, we found a new plane (422) corresponding to the HAp, this sample that was kept in solution for 15 days, in this spectrum we can also observe the previous spectra of Ca, CO₂ and SiO₂; finally in Fig. 4d the planes of the HAp of the previous spectra are maintained, however, we found the planes (026), (211) and (306) corresponding to calcium silicate (Ca₂SiO₄).

SEM

In the microscopies presented in Fig. 5a, the image of a film of silicon oxycarbide containing calcium (SiCaOC) is shown, in which it can be observed that the film is compact and is composed of quasi-spherical grains, which are agglomerated and uniformly distributed on the surface of the substrate; in Figs. 5b and 5c are shown magnified images, in which we can observe in better detail the quasi-spherical grains and the agglomeration that these grains have on the substrate.

Fig. 5. SEM images of the SiCaOC film using TEOS and CaSiO₃ at: (a) 10 µm, (b) 1 µm, and (c) 100 nm magnification.

Fig. 6 presents an SEM image of a silicon oxycarbide (SiOC) film, in which we can observe the compact film and the quasi-spherical grains as in the previous images.

Fig. 6. SEM image of SiOC film using TEOS only.

The SEM images obtained from SiCaOC films with HAp (hydroxyapatite) growth after passing in solution for 10, 15, and 20 days are presented below.

In the SEM image of Fig. 7a, several dispersed clusters can be observed on the substrate surface, in Fig. 7b, with the amplification we can observe more clearly these clusters, they look like a kind of joined filaments, these clusters show a small growth of hydroxyapatite on the surface of the film, a compact morphology similar to that observed in Fig. 6 was also observed.

Fig. 7. SEM images of Corning glass substrate at 10 days of HA growth at: (a) 10 µm, (b) 1 µm, and (c) 100 nm magnification.

In Fig. 8a, we can observe both the SiCaOC film and the HAp growth. In this figure, we can appreciate grains of different sizes and in a large part of the substrate film. In Fig. 8b, we observe large agglomerates and filaments on the surface, possibly due to the growth of HAp on the SiCaOC surface; in Fig. 8c, it is observed that the growth of HAp is given as small lamellae that come together to form a spherical cumulus on the substrate.

Fig. 8. SEM images of Corning glass substrate at 15 days of HA growth at: (a) 10 µm, (b) 1 µm and (c) 100 nm magnification.

In Fig. 9a, we can observe a more homogeneous growth on the surface of our film; on the other hand, in Fig. 9b, we can observe in greater detail the growth on the HAp, where the surface presents small agglomerates distributed randomly. In Fig. 9c, we observe how the HAp grew in the form of short overlapping and slightly dispersed tubes.

Fig. 9. SEM images of Corning glass substrate at 20 days of HA growth at: (a) 10 µm, (b) 1 µm, and (c) 100 nm magnification.

EDS

In the EDS analysis of the samples, we found silicon, oxygen, carbon, calcium, and phosphorus.

Fig. 10 is the spectrum obtained from the SiOC film, where we can observe the intensities of silicon, carbon, and oxygen; Table I shows the percentages of each element in the film studied. And we corroborate the own elements of SiOC.

Fig. 10. EDS SiOC film.

Table I. Elemental Content of SiOC Film

Element	Atomic no.	% wt.	% at.
O	8	45	69
C	6	7.45	5.2
Si	14	28	25

In Fig. 11, the SiOC film containing calcium, a small amount of calcium was found, as well as amounts of oxygen, carbon, and silicon. Table II shows the weight percentage as atomic percentage, in which we corroborate that the amount of calcium in the film is very small.

Fig. 11. EDS spectra of the SiCaOC film.

Table II. Elemental SiCaOC Film Content

Element	Atomic no.	% wt.	% at.
O	8	51.74	51.11
C	6	28.86	37.98
Si	14	19.39	10.91
Ca	20	0.01	0.01

XPS

XPS characterization was performed to verify the composition of our films, both the film grown with calcium atoms and the film with hydroxyapatite growth.

Fig. 12 shows the high-resolution XPS spectrum. In this image, it is possible to observe the presence of calcium in the SiCaOC films. From this figure, we can also corroborate the presence of oxygen (1s) at ~530 eV, carbon (1s) at ~280 eV, calcium (2p) at ~350 eV, and silicon (2p) at ~100 eV, which tells us that we are indeed obtaining SiCaOC films.

Fig. 12. XPS spectrum of SiCaOC film.

In Fig. 13a corresponding to the SiCaOC film that was in solution for 10 days, we can observe the presence of oxygen (1s), calcium (2p), carbon (1s), silicon (2p) and phosphorus (2p), in this spectrum was found in small intensity the presence of phosphorus which is part of the hydroxyapatite. In addition, as shown in Fig. 13b, high-resolution XPS spectra are presented with a deconvolution of the peaks to better understand the elemental contribution that is being generated in the growth of this film.

Fig. 13. XPS spectra of the SiCaOC-HAp film at 10 days of growth (a) and individual elemental contribution (b).

In the spectrum of Fig. 14a, an increase in the intensity of the elements found in Fig. 13a can be observed; the intensities of these elements are well defined. Fig. 14b shows high-resolution XPS spectra with deconvolution in each element to obtain the contribution in each spectrum.

Fig. 14. XPS spectra of the SiCaOC-HAp film at 15 days of growth (a) and individual elemental contribution (b).

Fig. 15a shows the increase of oxygen, and the decrease of calcium and carbon, phosphorus, and silicon remained in intensity compared to Fig. 14a; Fig. 15b shows high-resolution XPS spectra that have been deconvoluted to determine the contribution of each element.

Fig. 15. XPS spectra of the SiCaOC-HAp film at 20 days of growth (a) and individual elemental contribution (b).

Photoluminescence

Fig. 16 shows the photoluminescence spectrum in which we can observe three prominent peaks: the first at approximately ~390 nm indicates emission in the ultraviolet region; at approximately ~513 nm emission in the green region of the visible spectrum; and at approximately ~700 nm emission in the red region of the visible spectrum.

Fig. 16. Photoluminescence spectra of SiCaOC and SiCaOC-HA samples.

We can also observe that the intensity of the peaks varies between samples; the SiCaOC-20D HA sample shows a higher intensity at the approximately ~700 nm peak compared to the other samples.

These results suggest that hydroxyapatite (HAp) growth significantly influences the photoluminescent properties of the material, especially in the red region of the visible spectrum.

Conclusions

With the SEM images, we could observe how hydroxyapatite is growing on the film and how it is formed after several days in the prepared solution.

With the XPS spectra, we corroborated the presence of calcium, oxygen, carbon, and silicon in the deposited films. We can also observe the presence of phosphorus due to the growth of hydroxyapatite on films.

In the photoluminescence results the emission in ultraviolet (~390 nm) could be associated with defects in the structure of the material, the green emission (~515 nm) may be caused by electronic transitions or by the influence of calcium, the high intensity of the SiCaOC-20D HA sample (~700 nm) at this wavelength could indicate a higher emission efficiency in the red region of the visible spectrum, potentially due to a higher concentration of calcium.