DOI: 10.1021/cg9015146
Size-Dependent Crystalline to Amorphous Uphill Phase Transformation of Hydroxyapatite Nanoparticles
2011, Vol. 11 45–52
Christina Mossaad,† Mei-Chee Tan,† Matthew Starr,† E. Andrew Payzant,‡ Jane Y. Howe,‡ and Richard E. Riman*,† †
Rutgers, The State University of New Jersey, Department of Materials Science and Engineering, 607 Taylor Road, Piscataway, New Jersey 08854, United States, and ‡Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States Received December 3, 2009; Revised Manuscript Received September 28, 2010
ABSTRACT: The room temperature Ca(C2H3O2)2-K3PO4-H2O equilibrium system was examined for the preparation of hydroxyapatite nanopowder with sizes less than 10 nm. The reaction products were characterized with X-ray diffraction, transmission electron microscopy (TEM), nitrogen-adsorption surface area, helium pycnometry, thermogravimetric analysis, and Karl Fisher titration methods. TEM revealed that ∼5 nm nanopowders could be successfully prepared with this synthesis approach. However, the vast instability of these powders brought upon by the method of sample separation or the characterization method itself made it impossible to use other conventional methods of characterization to validate TEM data. This study has identified key processing steps that control the order and disorder of these nanomaterials, as well as the conditions that lead to surface area reduction. The most unique phenomenon from this work is the observed crystalline to amorphous phase transformation when washed or unwashed nanopowders are aged for 5 months in 30% relative humidity. This transformation, the first of its kind to be reported in the literature, is accompanied by a surface area loss by a factor of 3 or greater. The uphill phase transformation from the nanocrystalline to amorphous state appears to be driven by the reduction of the large positive surface energy inherent in the as-crystallized ∼5 nm nanopowder.
1. Introduction Hydroxyapatite is a well-known biomedical material that has a myriad of uses in powder, coating, and monolithic forms.1 For example, it is often used as a coating on orthopedic devices for its osteoconductive properties and, recently, on vascular implants for its drug delivery capabilities.2 Hydroxyapatite, Ca10(PO4)6(OH)2, is a hexagonal calcium orthophosphate with a significant chemical similarity to the inorganic mineral found in bone.3 It has been widely accepted as osteoconductive,4,5 but there has also been literature speculating osteoinductive properties;6 however, this has been continually under debate.7 Native bone apatite is found as platelets or rods with the largest dimensions in the range of 20-40 nm depending on local mechanical demands.8,9 However, synthesis of nanoscale hydroxyapatite in and below this size range has been a continuing focus.10 Hydroxyapatite synthesis methods have rapidly moved into the under-20 nm size range because of the increased cellular conductivity that this size range offers.10 For example, it has been observed that micro- and especially nanotopography on surfaces has a significant effect on cell adhesion, induction, viability and prompting of maturation10,11 and strengthening the interfacial bond between host tissue and implant. Biomaterial intramuscular osteoinduction was also enhanced with a higher specific surface area due to higher surface reactivity, which improves the cellular response in an implant site.6 Synthetic hydroxyapatite sizes in the literature range from 8 to 100 nm, through the use of various techniques that often employ harsh reaction conditions.12-15 Hydroxyapatite with a surface area as high as 180 m2/g was hydrothermally synthesized for 8 h at 120 °C, with a calculated particle size *To whom correspondence should be addressed. r 2010 American Chemical Society
ranging from 10 to 60 nm, although there was no microscopy evidence supporting these computations.16 Through the use of single and mixed surfactants, 8-20 nm lath-shaped hydroxyapatite particles were synthesized using a lengthy reaction time of 48 h, with surface areas as high as 364 m2/g.17 Continuous reactors can also be used to crystallize agglomerated 15 65 nm nanorods of hydroxyapatite at temperatures of 200 °C.18 One of the goals of the present research is to push the lower size limit for HA by synthesizing powders with particle sizes less than 10 nm. Another goal is to use a simple room temperature process that can run to completion in much less than 48 h using conventional batch reactor technology. The literature suggests that making nanopowders smaller than 10 nm presents some unique challenges. For significantly large particle sizes, the surface to volume ratio is low and properties such as surface free energy are neglected.19,20 In contrast, particles with sizes less than 10 nm have a significantly greater surface to volume ratio, where >50% of the atoms are found on the surface. Thus, surface-related properties such as surface energy can play a more dominant role. Dingreville et al. and Fischer et al. both studied the influence of surface energy in their theoretical treatments using published literature on nanoparticles, nanobelts, nanowires, and nanofilms.19,20 These papers are examples of treatments that explore how surface energetics can drive a variety of different phase transformation phenomena not observed in bulk materials, such as the lowering of melting points for gold nanoparticles as a result of reducing particle sizes. Vollath et al. found that ceramic oxides such as zirconia form nanocrystalline high temperature phases instead of the lower temperature polymorphs.21 Gilbert et al. studied the influence of water vapor adsorption on metastable ZnS (wurtzite).22 For ZnS, the sphalerite-wurtzite phase relation is inverted with a small Published on Web 11/23/2010
pubs.acs.org/crystal
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Crystal Growth & Design, Vol. 11, No. 1, 2011
particle size such that the high temperature wurtzite polymorph is the thermodynamically stable phase at the nanoscale (
