Control of the morphology and chemical properties of carbon spheres prepared from glucose by a hydrothermal method

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Carbon spheres (CSs) with regular shapes (Ø 300–1200 nm) and numerous oxygen groups (–OH, C6H5–C5O, and C5O) were prepared by a simple glucose hydrothermal process. CSs were studied by x-ray powder diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, x-ray photoelectron spectra, and elemental analysis. Their size was directly proportional to temperature and glucose concentration. Their shape could be controlled by temperature and reaction time. To prepare CSs (Ø 300–800 nm) with regular shapes and smooth surfaces, temperature and reaction time in the range of 180–230 °C and 3–4 h, respectively, were optimal. The chemical properties of the CSs were affected by temperature. A phase transformation from amorphous to turbostratic structure took place at T . 230 °C. The number of oxygen groups decreased as the temperature increased, and T # 230 °C were optimal to prepare oxygen-rich CSs. Comparison of oxygen contents and O/C ratios indicated a further carbonization, and the degree was directly related to temperature. A possible formation mechanism for the CSs is proposed.

I. INTRODUCTION

Perfectly spherical functional carbonaceous materials have attracted interests because of their unique morphology and electromagnetic, thermodynamic, and mechanical properties.1–3 These materials have potential applications in the areas of purification, drug delivery, adsorbents, catalysts, and catalyst supports.4–7 Because of problems related to the depletion of fossil fuels and environmental pollution, development of preparation methods for carbonaceous materials has focused on the use of environmental friendly resources and methods. Recently, many attempts have been made to produce spherical carbon from biomass by hydrothermal synthesis.8 Biomass is a promising starting material because it is inexpensive, abundant, readily available, and environmental friendly and rapidly regenerates.9 Compared with traditional approaches,10–12 hydrothermal synthesis is an environmental friendly route. It proceeds under mild conditions (e.g., T , 300 °C and t , 14 h) in a sealed container without any organic solvent. In hydrothermal synthesis, it is easy to control the shape and chemical structure of the products for targeted applications13,14 that depend on specific properties (e.g., size, diameter distribution, shape, and chemical functionalities). Oxygen-rich carbon spheres (CSs) (Ø 0.25–5 lm) were first reported by Wang et al.15 in 2001. Sucrose was used as the starting material, and hydrothermal carbonization was carried out at 190 °C for 5 h. Since then, many relevant

studies16–34 using saccharides as raw materials have been reported. CSs with diameters ranging from several hundred nanometers to several micrometers have been prepared by hydrothermal treatment at 160–250 °C from glucose,15 sucrose,16,17 fructose,18 starch,19 and cellulose.21,22 The CSs formed in these studies have many surface oxygen groups (mainly –OH and –C5O).23–25 Benefited by these groups, CSs can be used as supports, loaded with metal particles (