DOI : 10.2240/azojomo0303

Surface Modification of Diamond Powder Through Dissolution of CO2 in Water

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Dec 27 2010

Heidy Visbal, Chanel Ishizaki and Kozo Ishizaki

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AZojomo (ISSN 1833-122X) Volume 6 December 2010

Topics Covered

Abstract
Keywords
Introduction
Experimental
Results and Discussion
     DRIFT Spectral Characteristics
     Modified Surface
     Reaction Mechanism and Activation Energy
Conclusions
Acknowledgment
References
Contact Details

Abstract

Surface modification of diamond powder (5-12 µm) was achieved using a CO₂ disssolution. The effects of the reaction temperature were studied. The diamond surface was characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Four chemical structures; epoxy (), aliphatic ether (-C-O-C-) as well as methyl (-CH₃) and methyne (-CH) bands were identified in the diamond surface after the treatment. The total intensity of the bands increases proportionally by augmenting the treatment temperature. An activation energy of 51.2 kJ mol^-1 was obtained, that is in the range of a chemisorption reaction energy. From the results it is concluded that methyl formate (HCOOCH₃) was chemisorbed on the diamond surface to a carbon with two unsaturated valences.

Keywords

Diamond Powder, Surface Modification, Chemisorption, CO₂ Dissolution, Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy

Introduction

Several studies have been focused on diamond powder surface modification. They can be classified into two main groups: modifications in vapor phase [1-7] and in liquid phase [8-14]. Modifications in the liquid phase of diamond surface are more suitable for industrial applications, because the common technology of chemical engineering can be used. Another advantage is that organic substances with organic functional groups can be chemisorbed on a diamond surface and new organic-inorganic functional materials can be developed. Silane coupling reagents [10], carboxylic acids [9] have been reported as precursors for modifying diamond surface. In the previous paper it was shown that formaldehyde solution [14] could modify diamond surface. By this treatment, methyl formate is chemisorbed to diamond surface. However, even these chemical methods require solvents or expensive chemicals to activate the reaction. By far, the most common acid maker is simple carbon dioxide (CO₂). When this dissolves, it makes carbonic acid (H₂CO₃). It is well known that carbon dioxide is soluble in pure water and its value of solubility is less than 0.039 mol l^-1 at 40°C. Once it has dissolved, a small proportion of the CO₂ reacts with water to form carbonic acid. “Disolved carbon dioxide” consists mostly of the hydrated oxide CO₂ (aq) together with a small amount of carbonic acid. Carbon dioxide is an extremely abundant and inexpensive resource because it is a waste product of many industries, such as in the production of limestone and cement, fermentation of carbohydrates, and on a vast scale in fossil-burning power plants. It may be also produced by concentrated solar energy, by calcinations of limestone at 800-900°C. Therefore if carbon dioxide alone can be use to modify surfaces instead of expensive reactives, it will be a promising source for industrial applications. According to our knowledge nobody has reported about modifying the diamond surface using a carbonate water solution and its mechanism. In this paper, the feasibility of modifying the surface of diamond powder by dissolution of CO₂ in water is reported. The diamond surface after treatment was characterized by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The reaction between the species formed in the dissolution of CO₂ in water and the diamond powder is also discussed. The treatment was studied at several reaction temperatures.

Experimental

Commercial available synthesized diamond powder of 5-12 µm grain size from Matsumoto Yushi-seiyaku Co., Ltd. was used in this study. First, 27 ml of distilled water was saturated with carbon dioxide (99.9% purity). Then, an amount of 0.06 grams of diamond powder was put inside the flask containing the saturated dissolution of CO₂. To study the effects of the temperature, the solution was heated in an oil bath with reflux at 60, 80 and 100°C for 3 h with continuous stirring. The temperature of the liquid inside the flask was monitored each 20 min and was kept constant. After the treatment, the solution was decanted and washed few times with distilled water and finally, the powder was dried in an oven at 120°C for 20 h. The surface of the diamond was characterized by DRIFT spectroscopy using a Shimadzu FT-IR 8300 spectrometer equipped with a Spectra-Tech diffuse reflectance accessory, a triglycine sulfate (TGS) detector and data processing software. Three samples were prepared for each condition. DRIFT spectra of three different portions of the samples in the range of 4000-400 cm^-1 were obtained in dry air atmosphere with no dilution. For each spectrum, 256 scans were accumulated at a resolution of 4 cm^-1. Spectral processing such as baseline adjustment and smoothing were performed using an OMNIC software package. The second derivative spectra derived from the absorbance spectra were deconvoluted and the peaks quantified using a Jandel peak separation and analysis software PeakFit™ 4.0 (AISN Software Inc., USA) in log (1/R) units. Peak fitting was carried out until squared correlation coefficients with r² greater than 0.999 and best F standard values were obtained.

Results and Discussion

DRIFT Spectral Characteristics

Diffuse reflectance DRIFT spectra of diamond modified by CO₂ as a function of reaction temperature is presented in fig. 1. This figure shows the average spectra of raw diamond powder (RD), and the diamond after treatment. The presented spectra are averaged spectra. In pure natural diamond, only two-phonon (or second order) absorption process (the broad bands in the region 2300-2000 cm^-1) are infrared-active, but the presence of impurities or defects in the structure causes the forbidden single phonon modes to become infrared active [15-16]. Therefore two bands in the region 2300-2000 cm^-1 can be observed in the spectrum of the diamond powder. Other small bands can be observed in the region 1400 -1100 cm^-1 due probably to impurities or defects in the diamond [16]. After the CO₂ chemical treatment the main differences observed in the spectra are the new bands that appear in the ranges of 3000-2800 cm^-1 and 1600-400 cm^-1 (these are superimposed to the defect bands observed in the raw diamond). The absorbance intensity of these bands normalized by the intensity of the diamond band from 2250-2100 cm^-1 for different reaction temperatures is shown in fig. 2. It can be observed that the total intensity of the bands increases by augmenting the reaction temperature. Since the results show that the samples prepared at 100°C shows the highest intensity, to study the effects of treatment time, the reaction temperature was set at this temperature. Deconvolution of all the spectra was performed and the identified peaks in both regions for a sample treated at 100°C for 3 h are shown in fig.3. In fig. 3 (a), correspondent to the C-H region, 4 peaks clearly defined around 2962, 2924, 2893 and 2855 cm^-1 are observed. In the ?CH frequency region, the spectra are very complex due to Fermi resonances between the ?CH fundamental and combinations or overtones, and therefore these bands can be assigned to -CH stretching vibration modes of methyl and or methyne [17-19]. Additionally, these bands have been reported on adsorption of methyl formate on powders [20]. In the region 1600-400cm^-1 four main peaks can be observed as seen in fig. 3 (b). The broad band from 1200 to 950 cm^{- 1} can be assigned to ether stretching vibrations (? C-O-C) according to the literatures [3, 7, 10, 16-19]. Two other peaks located at 1260 and 810 cm^-1 are observed. These two bands can be assigned to asymmetric and symmetric vibration of epoxide ( ) [17-19, 21-25] respectively. Normally the strong band is due to asymmetrical -C-O-C- stretching and the symmetrical stretching band is usually weak. However the -C-O-C- group in a ring like epoxide and as the ring becomes smaller, the asymmetrical -C-O-C- stretching vibration moves progressively to lower wave numbers, whereas the symmetrical -C-O-C- stretching vibration moves to higher wave numbers [21-22, 24]. Diamond has a very small lattice constant, so oxygen bonded to it would also be highly strained and could exhibit this kind of behavior [22]. This can explain why the peak at 810 cm^-1 has a higher intensity than the band located at 1260 cm^-1. An additional small peak appears on the spectrum around 1400 cm^-1. This peak can be assigned to carbonate [26- 27].

Figure 1. Common scale DRIFT spectra of raw and chemically modified diamond powder with CO₂ dissolution for 3 h at different temperatures.

Figure 2. Normalized absorbance intensity of bands as a function of reaction temperature by region, a) 3000-2800 cm^-1, b) 1300-750 cm^-1. The total intensity of the bands increases by augmenting the reaction temperature.

Figure 3. Deconvolution of the spectrum obtained for a diamond sample treated at 100°C for 3 h by region, a) 3000-2800 cm^-1 b) 1300-750 cm^-1

Modified Surface

Based on the four chemical units obtained from the spectral results; epoxy (), aliphatic ether (-C-O-C-), methyl (-CH₃) and methyne (-CH), we proposed that methyl formate: is attached to one surface carbon with two unsaturated valences as follows: where C_s represents diamond surface carbons.

Reaction Mechanism and Activation Energy

When carbon dioxide gas (CO₂) dissolves in water (H₂O), its molecules often cling to water molecules in such a way that they form carbonic acid molecules (H₂CO₃). Carbonic acid is a weak acid, an acid in which most molecules are completely intact at any given moment. But some of those molecules are dissociated and exist as two dissolved fragments: a negatively charged HCO₃^- ion and a positively charged H⁺ ion. The H⁺ ions are responsible for acidity—the higher their concentration in a solution, the more acidic that solution is. For the water dissociation reaction, it had been shown that the value of the free-energy change, hence the electrochemical potential, decreases as the temperatures is raised. The following reaction has been reported in photo catalysis of CO₂

CO₂(g)+2 H₂O → HCOOH + ½ O₂

CO₂(g)+ H₂O → HCHO + O₂

CO₂(g)+ 2H₂O → CH₃OH + 3/2O₂

CO₂(g)+ 2H₂O → CH₄ + 2 O₂

The electrochemical potentials of these reactions of CO₂ are affected by changes in temperature. The formation of methanol (CH₃OH) and formaldehyde (HCHO) is enhanced by temperature. If formaldehyde and methanol are formed in the system, formaldehyde can be produced to methyl formate as explain in previous paper [13]. Therefore methyl formate could chemisorb to the diamond surface as follows

This mechanism is in agreement with the spectral data. Figure 4 shows the ln of the absorbance intensity of the 810 cm^-1 epoxy band normalized by the diamond peak area versus 1/T. The extent of the reaction is temperature dependent. From Arrhenius equation we can calculate the activation energy, as shown in Equation 1.

ln C = ln A - E/RT (1)

where A is the pre exponential factor, E the activation energy, T the absolute temperature, C the concentration of the reaction product and R the gas constant. The activation energy of this process was calculated from the slope of the graph shown in the fig. 4 in accordance with equation (1). The calculated activation energy is 51.2 kJ mol^-1. This value is in the range of activation energy reported in the literature for chemisorption mechanisms [28-29].

Figure 4. Ln normalized intensity of the 810 cm^-1 epoxy band vs. 1/T. The activation energy is calculated from the slope of this graph.

Conclusions

The experimental results show that it is possible to modify the surface of diamond powder using a CO₂ dissolution. Four chemical structures that increase proportionally with reaction temperature and time could be identified as follows; epoxy (), aliphatic ether (-C-O-C-), methyne (-CH) and methyl (-CH₃). From the spectral results and the obtained value of activation energy for the reaction (51.2 kJ mol^-1), it is concluded that methyl formate (HCOOCH₃) chemisorbed to a diamond surface carbon with two unsaturated valences.

Acknowledgment

The authors are indebt to the Japanese government for financial support to Heidy Visbal through the Monbukagakusho scholarship as well as the partial support to the research through the 21^st century Centers of Excellence (COE) program of the Ministry of Education, Culture, Sports, Science and Technology.

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