Preparation of reduced graphene oxide/hydroxyapatite nanocomposite and evaluation of graphene sheets/hydroxyapatite interface

Highlights

• Various crystalline planes such as (002) and (300) played a role in the growth of hydroxyapatite crystals.

• The HA is coherently connected by its (300)-planes with the surface of the rGO sheets.

• Interface coherency is accomplished in the rGO cross-section with the (002) planes of HA.

• The hardness and the Young's modulus of the composite samples were 5.9–6.6 GPa and 132–146 GPa respectively.

Abstract

In this investigation, reduced graphene oxide/hydroxyapatite (rGO/HA) hybrid powders have been synthesized using hydrogen gas injection into hydrothermal autoclave. The powders were then consolidated with spark plasma sintering. The results showed that this method of synthesis caused the obtained powders to have high crystallinity. Microscopic analysis confirmed the presence of rGO sheets with folding and wrinkling in the nanocomposite and indicated that various crystalline planes such as (002) and (300) played a role in the growth of hydroxyapatite crystals. The results of interface analysis (HA||rGO) showed that the HA is coherently connected by its (300)-planes with the surface of the rGO sheets and this coherency is accomplished in the rGO cross-section with the (002) planes of HA. The hardness and the Young's modulus of the composite samples were 5.9–6.6 GPa and 132–146 GPa respectively.

Introduction

Synthesis of hydroxyapatite (HA) with nanotechnological approaches in nanosized dimensions and in various forms such as nanotubes and nanorods have improved the mechanical properties of this ceramic material [[1], [2], [3]]. The chemical composition (Ca₁₀(PO₄)₆(OH)₂), the ratio of calcium to phosphate (C/P ≈ 1.67), and the crystalline structure of HA are very similar to the mineral part of the human skeletal system. Also, having properties such as bioactivity and osteoconductivity make HA one of the most valuable materials for orthopedic applications [[4], [5], [6]]. The applications of this bioceramic are very broad include orthopedics, biosensors, catalysts and drug delivery [[7], [8], [9], [10], [11], [12]]. However, the poor mechanical properties of HA such as fracture toughness, intrinsic brittleness, and poor wear resistance still limit the applications of HA despite its excellent biomaterial properties [[13], [14], [15]].

To make HA usable as an implant, one of the strategies is to enhance its mechanical properties with a second, reinforcing, phase. Various materials have been investigated including titanium oxide, aluminum oxide, and carbon nanostructures [[16], [17], [18], [19], [20]]. But among these materials, graphene, with its unique mechanical properties (Young's modulus ≈ 1 TPa and fracture strength ≈ 130 GPa), has attracted a great deal of attention and research [21,22]. Graphene has a honeycomb-like structure of a single carbon atom with the SP² hybrid [[23], [24], [25]]. Its high specific surface area (2630 m² g⁻¹) has made its reinforcing properties more effective. Also, its good biological properties have led to much research for medical applications including orthopedic surgery, bioimaging, and drug delivery [[26], [27], [28], [29], [30]].

Several studies have been published on the use of graphene and its derivatives to improve the mechanical and biological properties of HA [[31], [32], [33], [34]]. The results of these investigations showed that the mechanical and biological properties of HA were improved by the addition of graphene. Graphene has been used as filler that blocks the crack growth with its two-dimensional structure [[35], [36], [37]]. One of the methods to synthesis of graphene is a hydrothermal method that the graphene oxide (GO) is used as precursor. Hydrogen under the high pressure and high temperature of the hydrothermal autoclave is the main factor in the reduction of GO, assembly as a gel, and forming three-dimensional rGOs. Using hydrothermal method is an in situ mechanism for the synthesis of HA-rGO hybrid powders because the presence of oxygen agents on GO sheets causes nucleation and growth of HA particles on the graphene surface [[38], [39], [40], [41]]. Due to the high mechanical properties of graphene recovered from GO, the degree of reduction is very important. One of the most attractive methods is utilizing of hydrogen gas as a reducing agent. In one published report, a mixture of argon and hydrogen gases was used alongside palladium and the process temperature was 200 °C. Palladium caused hydrogen gas to automatically reduce the oxygen present on the graphene oxide surface [42]. In another report, hydrogen-rich water was used to reduce GO [43]. Further researches have been published in which hydrogen gas is injected into a hydrothermal autoclave, and the results showed that the presence of hydrogen gas in hydrothermal conditions would also reduce GO [44,45].

The mechanical behavior of composites made of HA and rGO depends on interface type between the two phases. Therefore, studying the details and behavior of the interface is very important. In this study, the hybrid nanostructured powders were synthesized using a hydrothermal method utilizing hydrogen gas injection to increase the reduction rate of GO. However, to have a greater effect on reduction of GO, hydrogen gas needs to be decomposed by the catalysts to the hydrogen atoms, because the hydrogen gas has little solubility in solution. So, hydrothermal temperature and time will probably have a stronger role in the synthesis of powders [[42], [43], [44], [45], [46]]. In order to investigate the effect of this process on the final nanocomposite properties, the spark plasma sintering (SPS) method has been used to consolidate these powders and to fabricate bulk samples. High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy, and nanoindentation were used to investigate the type (coherent or incoherent), chemical bonds, and mechanical behavior of the interface.