Journal of Nanotechnology and Smart Materials Research Open Access Influence of Temperature on Calcium Carbonate Polymorph formed from Ammonium Carbonate and Calcium Acetate Charles A Weiss, Jr.*, Kevin Torres-Cancel, Robert D Moser, PG Allison, E Rae Gore, Mei Q Chandler and Philip G Malone US Army Engineer Research and Development Center, Geotechnical & Structures Laboratory, Vicksburg, MS, USA *Corresponding author: Charles A. Weiss, Jr., US Army Engineer Research and Development Center, Geotechnical & Structures Laboratory, Vicksburg, MS, USA, Phone: 601-634-3928, Fax: 601-634-3242, E-mail: Charles.A.Weiss@ usace.army.mil Received Date: November 02, 2013 Accepted Date: January 15, 2014 Published Date: February 11, 2014 Citation: Charles A. Weiss, Jr., et al. (2014) Influence of Temperature on Calcium Carbonate Polymorph formed from Am- monium Carbonate and Calcium Acetate. J Nanotech Smart Mater 1: 1-6 Abstract This research used ammonium carbonate and calcium acetate in the preparation of various calcium carbonate polymorphs for biomimetic composite applications. Biominerals were synthesized at temperatures ranging from 25 to 80°C to investigate the effect of synthesis temperature on the abundance of vaterite, aragonite, and calcite, delineating regions that are favorable for the formation of these different calcium carbonate polymorphs. In particular, it was observed that lower temperatures of 25-30°C were favorable for vaterite formation with increasing crystallite size at the higher temperature. Calcite was the preferred polymorph formed at temperatures of approximately 50°C. Although a trace of aragonite was observed at 70°C, it was not observed at the highest temperature investigated (80°C). The absence of aragonite formation at elevated temperatures may be a function of the lower pH (~8.4) in the ammonium carbonate/calcium acetate system compared to ~9.9 for the so- dium carbonate/calcium acetate system that promotes the formation of vaterite over aragonite. These results are important when developing biomimetic materials incorporating calcium carbonate with different polymorphs and crystal sizes. Introduction texture in determining properties of materials. The precipi- tation of the polymorphs of calcium carbonate is critical to The syntheses of composites for protection or support that in- the formation of biomimetic materials[2,3]. Careful control clude both inorganic and organic components such as bones, of the critical variables in the process determines the shape shells, and teeth are an inspiration for bio-inspired materials’ and crystal structure of the calcium carbonate[3]. Research design[1,2]. Structurally, these materials typically are com- on calcium carbonate crystal precipitation from solution has posites made from organic molecules as the matrix/binding demonstrated that the final product that forms during this phase and very often are present in various polymorphs of reaction includes the polymorphs aragonite, vaterite, and cal- calcium carbonate. Nature has evolved mechanisms to con- cite[4-15]. Although calcite is the most thermodynamically trol crystal growth and morphology, thereby producing min- stable phase under ambient conditions, many authors have eralized structures that have engineering properties that far shown that it may take hours or longer for the transformation surpass the biomaterial calcium carbonate. These unique from the other polymorphs to calcite[5]. Previous research properties derive from the natural polymorphs and morphol- pointed out that many factors affect the yield of a specific ogy of CaCO . As such, the development of new strategies to 3 type of polymorph such as magnesium to calcium ion ratios, produce these materials with an eye toward controlling the phosphate and sulfate ions, organic compounds, and acids polymorph formed, and the size of the crystallites becomes (change in pH), but among the most important are the car- extremely important. These CaCO polymorphs with unu- 3 bonate ion controlled kinetics[11], metal ions[9], and tem- sual morphologies can potentially be promising candidates perature[3]. One author used a constant composition method for advanced materials due to the importance of shape and to produced calcite, aragonite, and vaterite[4] and found that, ©2013 The Authors. Published by the JScholar under the terms of the Crea- from the controlled variables that they tested, which included tive Commons Attribution License http://creativecommons.org/licenses/ solution pH, temperature, supersaturation[10], concentra- by/3.0/, which permits unrestricted use, provided the original author and tion ratio of the components, and concentration of additives, source are credited. JScholar Publishers J Nanotech Smart Mater 2014 | Vol 1: 105 2 solution pH and temperature were the most important factors tration system operated under vacuum. Immediately after the in determining the polymorphs of CaCO3 formed. Further- precipitate was recovered, it was rinsed in deionized water and more, they found that at room temperature, solution pH is the dried in a furnace at 105°C. The resulting precipitated powder most significant factor, and high-yield polymorphs are present was stored in a desiccator for further characterization using at different pH values. At a high temperature, aragonite be- SEM and XRD analysis. comes the major product at a pH below[12]. The temperature and pH values affect the kinetics of crystal formation, which Methods favors certain polymorphs of calcium carbonate over the oth- ers. It was similarly found that aragonite was the predomi- X-ray Diffraction patterns, to be used for semi-quantitative nant phase at temperatures over 50°C and was virtually the phase identification and phase fraction analysis, were obtained only phase present at temperatures above 70°C[5]. Another using an X’Pert Pro Materials Research Diffractometer (Pana- author used a novel method for production of calcium car- lytical, Inc.) equipped with a Co-Kα X-ray source operated at bonate through the use of ammonium carbamate and calcium 45 KV and 40 mA. Diffraction patterns were obtained from acetate that showed vaterite formed at low temperatures and 2.5 to 70°2Ѳ with a step size of 0.016°2Ѳ with a total scan time calcite was formed at 50°C, although their research did not of 2 hr. Phase identification was performed using MDI Jade attempt crystallization at higher temperatures than 50°C[6]. 2010 analysis software for qualitative phase identification with This study examined the effect of temperature on calcium car- ICSD powder diffraction file (PDF) reference databases and bonate polymorphism with synthesis being performed using a semi-quantitative phase fraction analysis using whole pattern different system of ammonium carbonate and calcium acetate. fitting techniques. The study also extends the range of temperatures studied up to SEM imaging was performed using a Nova NanoSEM 630 80°C. Morphology and structure were investigated using Scan- (FEI, Inc.) field-emission SEM, capable of high-resolution im- ning Electron Microscopy (SEM) and x-ray diffraction (XRD). aging on non-conductive materials. Imaging was performed The following sections present details on the methods utilized in low-vacuum mode at pressures of 0.1 mbar and an accel- and the results of the investigation. erating voltage of 15 kV. All images were acquired using a Materials and Methods backscattered electron detector to improve phase contrast. Samples for SEM imaging were prepared by depositing ap- Materials proximately 5 mg of material onto double-sided carbon tape followed by light dusting with clean compressed air to remove Calcium acetate with a molecular weight of 158.17 g/mol free particles. (≥99%, Reagent Plus) was obtained from Sigma-Aldrich. Am- monium carbonate at 90.07 g/mol (Certified ACS, > 99%) was Results and Discussion purchased from Fisher Scientific. A digital stirring hot plate X-ray Diffraction Analysis of CaCO Polymor- and thermometer were utilized to control and monitor the 3 temperature of the solutions. phism In-solution synthesis of calcium carbonate XRD analysis was used to determine the phase compositions produced at the various temperatures. The predominant phase present for crystals precipitated at 25°C and 80°C was vaterite, The crystallization experiments to produce CaCO3 were per- formed by preparing 0.5 M solutions of ammonium carbon- while at 50°C the pattern showed clearly that the phase present was calcite (Figure 1). The presence of less than ~5 % arago- ate (NH4)2CO3 and calcium acetate Ca(C2H3O2)2 followed by mixing at temperatures of 25, 30, 40, 50, 60, 70, and 80 °C. The nite was observed in one sample made at 70°C. initial solutions were preheated at the desired temperature and stabilized for 30 min. Ammonium carbonate solutions were The temperature control was crucial during precipitation re- tightly sealed with parafilm paper (American National Can) actions as demonstrated by the results presented herein and to minimize the loss of ammonia and the introduction of car- is one of the most critical factors during calcium carbonate bon dioxide from the atmosphere. All solutions were stirred crystal growth4. Based on the results from experiments per- moderately until the reactants were completely dissolved. The formed by preheating the solutions at 25°C through 80°C, a pH of the solutions was measured before and subsequent to distribution of the fraction of calcium carbonate polymorphs mixing. The measured pH values were 8.39 for the ammonium is presented in Figure