Research on controllable ozone oxidation on diamond surface

Abstract In recent years, there have been more and more researches on the surface modification of diamonds, however, the exact types and quantities of oxygen-related species on diamond surfaces and the method to control the condition parameters to obtain as many oxygen-containing groups as possible have been rarely studied so far. Therefore, in this work, we focused on these questions. And we find out that ozone oxidation would not affect the overall crystal structure and morphology of diamonds. Besides, changing oxidation time and ozone concentration would significantly influence the density of hydroxyl groups, which is manifested as a change of oxygen content. In order to make the hydroxyl density on diamond surface reach a high level (3.12 × 1014 units/cm2), so that diamonds can be better combined with the resin matrix, the ozone oxidation time should be 15 min, and the ozone concentration should be 115 g•m−3. And under these conditions, the thermal conductivity of diamond and polysiloxane composites can reach 8.02 W/mK.


Introduction
As a typical representative of diamond cubic crystals, because of its reaction inertia, extremely high electrical insulation and high thermal conductivity, diamonds are widely used in various fields, such as machinery, materials, electronics, lubrication, coating, filling, polishing and so on [1].
Since natural diamonds are extremely scarce, synthetic diamonds produced with high temperature and pressure are used as a substitute. Carbon atoms on the synthetic diamond surfaces frequently integrate with atoms of other elements to compose different surface terminations, due to the influence of diamond production processes (such as CVD, etc.). According to the different chemical states of diamond surface, diamond can be roughly divided into two types: C-H bond terminations (called hydrogen terminations) and C-O bond terminations (called oxygen terminations). Typically, diamond films end in C-H bonds, while diamond particles end in C-O bonds. The diamond surface ending with C-O bond usually contains hydroxyl (-OH), carboxyl (-COOH), bridge oxygen (C-O-C), carbonyl (C = O) and so on [2][3][4]. The physical and chemical properties of diamonds are greatly affected by them [5].
For diamond films terminated with hydrogen terminations, a hydroxyl-rich surface chemical state can be obtained on the film surface by oxidation. Oxidation methods include thermal methods, plasma methods, electrochemical methods, singlet oxygen treatment methods, ultraviolet irradiation (oxygen atmosphere) methods and so on. Girard [6] exposed diamonds terminated by C-H bonds to water vapor, supplemented by ultraviolet irradiation, so that the hydroxyl radicals generated in situ attacked the diamond surface to form a uniform surface covered by hydroxyl groups.
For diamonds terminated with oxygen terminations, hydroxyl terminations are more likely converted to other organic groups. Vigorous oxidation of diamonds with hydroxyl terminations can be carried out using a mixture of strong oxidants. Currently successful systems are H 2 SO 4 , HNO 3 and HClO 4 or HCl, HNO 3 and H 2 SO 4 mixed acid systems [7][8][9][10][11][12]. "Piranha water" systems (a mixture of H 2 SO 4 and H 2 O 2 ) [13], concentrated H 2 O 2 [10] or a 3:1 mixture of concentrated H 2 SO 4 and HNO 3 [14,15]. In addition, when hydroxyl groups are oxidized into carboxyl groups, these reagents can take the simultaneous removal of non-diamond carbon. Another method that effectively introduces oxygen-containing groups on diamond surface is air oxidation. Gogotsi [11] found out that there are plenty of oxidized surface groups on diamond surface with careful oxidation in air at high temperature of 425 °C. However, the above-mentioned oxidation methods have some problems more or less, such as the low reactivity of H 2 O 2 , the waste liquid of strong acid oxidation, and the high temperature of air oxidation. Ozone (O 3 ) is a highly oxidizing oxidant. Its oxidizing electrode potential is as high as 2.07 eV, and the hydrogen terminations on diamond surface can be oxidized into oxygen terminations by the method of ozone oxidation. Besides, the produced by-product oxygen (O 2 ) is completely harmless. So, it's an excellent reactive and environmentally friendly surface treatment oxidant.
Therefore, in this work, the surface of diamonds was modified by ozone, and the influence of factors such as ozone concentration and ozone reaction time on oxygen-containing groups was explored, so as to find a set of corresponding process conditions with the highest density of hydroxyl. Morphological and structural changes of diamonds and the effect of ozone oxidation reaction on thermal conductivity of composites were also explored.

Materials
In this work, diamonds (range from 41 μm to 68 μm) that had been shaped by air flow were provided by Henan huanghe whirlwind, China. Toluene, sodium hydroxide, 3-aminopropyltriethoxysilane, Sulfo-Cyanine5.5 NHS ester (molecular formula: C 44 H 42 N 3 K 3 O 16 S 4 ) and ethanol without treatment were obtained from Sinopharm, China.

Reaction system
In order to achieve a uniform oxidation effect, the method of exposing the diamond directly to the ozone stream (hereinafter referred to as "ozone gas phase oxidation" -OGO) is chosen. Besides, the reaction system ( Figure 1) can be divided into two parts provided by Changsha Xiangpu environmental protection technology, China. The first part is ozone generation part, the other part is ozone reaction part. The diamonds of different particle sizes are mixed according to the mass ratio of 53:30:10:7 from coarse to fine with reference to related studies [16,17], and the particle sizes of the four types of diamonds are 68, 57, 49 and 41 μm. Put the mixed diamond into a three-necked flask, and then slowly add 0.066 g/mL sodium hydroxide solution into the three-necked flask, and keep stirring. The reaction temperature was set as 90 °C, and the reaction was carried out for 4 h. After the reaction was completed, it was washed and dried. The purpose of this step is to remove organic deposits such as grease on the diamond surface.

The effect of ozone concentration
The method of OGO treatment was adopted to oxidize the diamond powder treated by NaOH solution with the device as shown in Figure 1. There are four groups of diamond powder, and each group reacts for 10 min, with the ozone concentrations of 95(±5) mg/L, 105(±5) mg/L, 115(±5) mg/L and 125(±5) mg/L.

The effect of OGO treating time
There are four groups of diamond powder, and each group reacts for 15 min, 10 min, 15 min and 25 min, with the ozone concentrations of 95(±5) mg/L.

XRD analysis
The basic structure of the sample was characterized by X-ray diffraction spectrometer (XRD, Emma), and CuKα radiation (λ = 1.541 84 Å) was carried out in the 2θ range of 5°∼80°.

SEM analysis
The surface topography of the diamond was observed by a scanning electron microscope (SEM, Thermo Scientific Apreo 2C).

FT-IR analysis
The surface functional groups of the samples to be tested were characterized by Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700) with a scanning range of 4000-400 nm. The samples were prepared by KBr tableting method, and the tableting pressure was 10 MPa.

XPS analysis
The oxygen content on the surface of the sample was detected by an X-ray photoelectron spectrometer (XPS, K-Alpha) with a detection depth of about 10 nm and a detection limit of about 5‰.

Quantitative analysis using fluorescent reagents
For the purpose of calculating the content of hydroxyl on the diamond surfaces before and after OGO treatment, a new method called "quantitative analysis using fluorescent reagents" was token. The main mechanism of "quantitative analysis using fluorescent reagents" can be illustrated in Figure 2. First, 3-aminopropyltriethoxysilane can combine with the hydroxyl groups on the diamond surfaces to expose amino groups. Then, sulfo-cyanine5.5 NHS ester can react with amino groups to graft fluorescent molecules onto the diamond surface. Finally, the fluorescence intensity was investigated by a Mindray MR-96A microplate reader. Then a standard curve was drawn to calculate the number of fluorescent molecules on the diamond surface, to reflect the number of 3-aminopropyltriethoxysilane grafted on the surfaces, and to infer the number of hydroxyl groups on the diamond surfaces. The calculation formulas are shown in formula (1) and formula (2).
n f is the number of fluorescent molecules, N A is Avogadro constant, c 0 is the initial concentration of the fluorescent reagent, x is the concentration of the fluorescent reagent remaining in the liquid, V is the volume of the solution, M f is the molar mass of the fluorescent reagent, σ h is the surface density of hydroxyl groups, σ a is the surface density of the grafted silane coupling agent and S is the mean specific surface area of diamonds.

The effect of ozone concentration
The IR spectras of diamond oxidized by different ozone concentration and untreated diamond in each group are shown in Figure 3. In Figure 3, there should be an O-H stretching vibration peak at 3446 cm −1 , but this peak didn't appear due to the interference of moisture in the test environment. However, the stretch vibration peak of C-O at 1086 cm −1 would not be disturbed by the moisture in the environment, so this peak was used as the comparison basis for the change of hydroxyl content in the following. As shown in Figure 3, with the continuous increase of ozone concentration, the stretching vibration peak of C-O continues to increase, and after the ozone concentration reaches 115(±5) mg/L, the stretching vibration peak of C-O was almost absently enhanced. Figure 2. mechanism of "Quantitative analysis using fluorescent reagents. " In addition, according to relevant research [18], the existence form of C-O may be bridge oxygen (-C-O-C-), hydroxyl (-OH), etc., which are marked as C +I below; the existence form of C = O may be carbonyl, aldehyde group, etc., which will be marked as C +II below; and the existing form of O = C-O-may be carboxyl group, ester group, etc., which will be marked as C +III below. The results in Figure 3 shows that when the ozone concentration reaches 115(±5) mg/L, the reaction sites on the diamond surface and ozone were basically occupied, it means that the ozone is already excessive under this condition. Therefore, continue to increase the ozone concentration did not change the reaction conversion, and the C +I content on the diamond surface is difficult to increase significantly. Figure 4 is the XPS spectrum of diamond treated with different ozone concentrations, and Figure 5 is the fine spectrum of diamond O element treated with different ozone concentrations. The peak areas of C1s and O1s in the XPS spectra were calculated, respectively, and analyzed by the atomic sensitivity factor method in the semi-empirical relative quantitative method. The theoretical formula is [19] K K In Formula 1, η 1 /η 2 is the relative content of atom 1 and atom 2 in the sample; I 1 /I 2 is the relative spectral peak intensity measured by the instrument; S 1 /S 2 is the relative atomic sensitivity factor. The calculation results are shown in Table 1. Table 1 shows that, compared with the untreated diamonds, the oxygen content on the surface of the diamond particles after ozone oxidation has been significantly improved. Among them, the diamond powder with an ozone concentration of 125(±5) mg/L has the highest oxygen content, reaching 16.08%, with an increase of 46.85%. And when the ozone concentration reaches 115(±5) mg/L, the increase of oxygen content is not obvious. The results show that when the ozone concentration reaches 115(±5) mg/L, the amount of ozone adsorbed on the diamond surface has reached the maximum, as the reaction sites are basically occupied. Therefore, if the ozone concentration continues to increase, the oxygen content on the diamond surface will not increase significantly, and the oxygen content is basically saturated. However, the exact form of these increased oxygen elements still needs to be explored.
In order to take a further exploration of the added oxygen species on diamond surface, it is necessary to study the chemical shift of C1s to distinguish the types and quantities of related organic groups. The C1s peak in the spectrum is formed by the superposition of several different components, and the peaks of all components are Gauss-Lorentzian product functions with the same shape. The width and shape of the C1s peak are mainly determined by the sp 3 C-C bond, and the peak position corresponding to the sp 3 carbon atom is 284.8 eV [20]. Due to high electronegativity of oxygen atoms, the binding energy of oxidized carbon atoms will become higher. The displacement of carbon monooxide atoms C +I (such as -C-OH and -COC-) is 1.3 ± 0.1 eV, and the   displacement of carbon dioxide atom C +II (such as -C = O and -OCO-) is 2.5 ± 0.1 eV, the displacement of carbon trioxide atom C +III (such as -COO-) is 3.5-4.5 eV [18]. For O1s peaks, the binding energies of oxygen-containing functional groups (such as -C-OH, -C = O, -OCO-, -COO-and -COC-) are not very different from each other, making accurate analysis difficult. Therefore, this paper focused on the C1s peak. Table 2 shows the XPS peaks ratio of each component after the diamond treated with different concentrations of ozone.
The adsorption mechanism of ozone on the diamond surface is shown in Figure 6. Ozone firstly decomposes into oxygen radicals O* and oxygen, which are then adsorbed by hydrogen terminations on diamond surface through hydrogen bonding [21]. It can be seen from Table 2 that with the increase of ozone concentration, the contents of C +I and C +II also increased gradually, and both of them reached their maximum when the ozone concentration became 125(±5) mg/L. Besides, after the ozone concentration increased to 115(±5) mg/L, the increase of C +I content was not obvious. The results show that when the ozone concentration reaches 115(±5) mg/L, the amount of ozone adsorbed by the hydrogen terminations on the diamond surface reaches saturation, and then the C +I content on the diamond surface also basically reaches saturation.
In order to more accurately detect the hydroxyl density on the diamond surface, the diamond samples were tested by the method described in 2.3.5. For the purpose, the degree of the reaction between KH-550 and diamonds which contain the largest amounts of hydroxyl in this work was first investigated. Figure 7 is an XPS spectra of diamonds treated with KH-550 for different durations, and Figure 8 is a fine spectra of N elements of those diamonds.
It can be seen from Table 3 that with the prolongation of the treatment time, the ratio of C:N decreases continuously. This shows that with the progress of the reaction, the KH-550 is continuously adsorbed on the diamond surface, and reacts with the hydroxyl groups, thereby introducing amino groups, resulting in a     significant increase in the N element content. And after the reaction time reached 4 h, although the ratio of C:N was also reduced, but the gap between them was not significant. That's because the hydroxyl sites on diamond surface which can react with KH-550 had been basically occupied, so it is difficult for KH-550 to graft to diamond surface, and the reaction basically reached equilibrium. In addition, the KH-550 was in large excess in this reaction system, and the reaction would continue to move in the direction of grafting more KH-550 until all the hydroxyl sites were occupied. Therefore, it can be approximately considered that when the reaction time of the silane coupling agent is 4 h, all the hydroxyl sites on diamond surface had all participated in the reaction. Figure 9 is the fluorescence spectra of diamonds treated with different ozone concentrations obtained by the method "Quantitative analysis using fluorescent reagents. " The peaks of the fluorescence spectrum were respectively integrated to calculate final hydroxyl density by formula (1) and formula (2), and the results are shown in Table 4.
It can be seen from Table 4 that with the increase of ozone concentration, the density of hydroxyl groups increased gradually, and then reached a maximum value of 3.1238 × 10 14 units/cm 2 when the ozone concentration was 125(±5) mg/L. Besides, when the ozone concentration came to 115(±5) mg/L, the increase of the ozone concentration was not obvious. That's because with continuous increase of ozone concentration, more and more oxygen radicals and oxygen were adsorbed on the hydrogen end groups on diamond surface. When the ozone concentration reaches 115(±5) mg/L, the hydrogen terminations on diamond surface had been completely occupied by oxygen radicals and oxygen. Therefore, when the ozone concentration was increased, the hydroxyl density hardly increased. Figure 10 shows the IR spectra of diamonds oxidized for different durations with the same ozone concentration. There are the stretching vibration peaks of C-O at 1086 cm −1 . When the OGO treating time was prolonged, the intensity of the peaks at 1086 cm −1 also increased continuously, and after OGO treating time came to 15 min, the intensity of these peaks was almost unchanged. With the continuous prolonging of OGO treatment, a large amount of oxygen radicals and oxygen were adsorbed on diamond surface. The reactivity between oxygen and diamonds is relatively poor, so it is difficult for oxygen to oxidize the hydrogen terminations to oxygen terminations. However, oxygen radicals can easily be inserted between the C atom and the O atom of hydrogen terminations to convert hydrogen terminations into hydroxyl groups due to their high reactivity. And these hydroxyl groups were all attached to C atoms, so the intensity of the C-O peaks increased continuously. When OGO treating time reached 15 min, the hydrogen terminations which could adsorb ozone on diamond surface were basically occupied. Therefore, if OGO treating time was extended, ozone cannot continue to react with the hydrogen terminations. That shown in the spectrum was that the intensity of the C-O peaks didn't change. Figure 11 is the XPS spectrum of diamond with different OGO treating time, and Figure 12 is the fine spectrum of O element with different OGO treating time. Each peak was integrated to obtain peak area, and to calculate the oxygen content of each group by Formula (3), as shown in Table 5.   It can be seen from Table 5 that compared with the diamonds untreated, the oxygen content of diamond particles after OGO treatment had been significantly improved. Among them, the oxygen content of diamond particles with an OGO treating time of 25 min was the highest, reaching 14.61%, showing an increase of 33.42%. And after OGO treating time came to 15 min, the change of oxygen content was not obvious. With the prolongation of OGO treatment, ozone molecules were continuously decomposed into oxygen radicals and oxygen, and both of them were adsorbed through the action of hydrogen bonds. So, the oxygen radicals could be inserted between C-H to generate hydroxyl groups, that's why the oxygen content was constantly rise. When OGO treating time reaches 15 min, the reaction sites for oxygen radicals were basically occupied and then became hydroxyl groups, so the oxygen content wouldn't increase. Table 6 shows the ratios of each component after with different OGO treating time.

The effect of ozone treating time
It can be seen from Table 6 that when OGO treating time came to 15 min, the content of C +I reached the maximum value of 14.41%. Then when OGO treatment was prolonged, parts of C +I component were further oxidized to C +II component. Therefore, the content of C +I component decreased. The results show that with continuous OGO treatment, the hydrogen terminations were continuously oxidized to C +I by oxygen radicals. And at the same time, C +I component was also continuously converted to C +II by ozone. When the OGO treating time is short, the conversion efficiency of hydrogen terminations to C +I is higher than that of C +I to C +II , so the content of C +I can be continuously increased. When the reaction time is longer, the hydrogen terminations on diamond surface were basically transformed into C +I , while C +I is still being transformed into C +II , so the content of C +I decreased. Figure 13 is the fluorescence spectra of diamonds with different OGO treating time obtained by the method "Quantitative analysis using fluorescent reagents. " The peaks of the fluorescence spectrum were respectively integrated to calculate final hydroxyl density by formula (1) and formula (2), and the results are shown in Table 7.
It can be seen from Figure 13 and Table 7 that when the OGO treating time was 15 min, the hydroxyl density was the highest, which was 3.1229 × 10 14 units/cm 2 . And then the hydroxyl density decreases with the prolongation of the reaction time. The results above show that the hydroxyl density on the diamond surface was the highest when OGO treating time came 15 min. With the continuous extension of the reaction time, the hydroxyl groups on diamond surface gradually transforms into C +II , and it is consistent with the results of XPS test.

Changes in the structure and thermal conductivity of diamonds after OGO treatment
In order to increase density of hydroxyl groups on diamond surface as much as possible, the ozone concentration in the system of OGO treatment should be 115(±5) mg•L −1 , and the OGO treating time should be 15 min.   note: Since the ratio of c +iii is close to 0, it is not listed here. Figure 14 is the XRD patterns of diamonds before and after the reaction with above conditions. It can be seen from Figure 14 that the diffraction peaks before and after OGO treatment were extremely sharp. The crystal plane corresponding to the peak with 2θ of 43.84° is the (111) face, and the peak with 2θ of 75.20° corresponds to the crystal plane is the (220) face. The positions of the highest and second highest peaks were almost unchanged, which indicated that the crystal structure of diamond did not change before and after OGO treatment. And the ratio of peak intensity of the highest peak to second highest peak decreased significantly, that's because the diamond (111) face group has high reactivity. Then some of them were oxidized by ozone, which makes them no longer obey the Law of Braggs, so the peak intensity decreases. Figure 15 is SEM photos of diamonds before and after OGO treatment. It can be seen from the Figure 15 that the surface of the diamond remained flat after OGO treatment. Besides, almost all of them maintain the complete hexoctahedron morphology. Therefore, the surface morphology of diamond before and after OGO treatment did not change significantly.
With above conditions, diamonds before and after OGO treatment were compounded with polysiloxane with different filling ratios, and the thermal conductivities of composite material were shown in Table 8. In polymer-based thermally conductive composites, if the content of thermally conductive fillers is few, the fillers will form a "sea-island" phase structure, which are wrapped in the "ocean" of resin, making it difficult to contact each other to form a thermally conductive chain. At this time, the thermal conductivity of the composite material mainly depends on the thermal conductivity of the matrix resin. When the amount of thermally conductive fillers is relatively high, fillers in resin can form a heat-conducting network chain connected to each other, so the heat could be conducted along the heat-conducting chain. When the filling amount of thermally conductive fillers is below the percolation threshold, the higher the content of thermally conductive fillers, the higher the probability of the formation of thermally conductive chains and the higher the thermal conductivity of composites.
The hydroxyl density of diamonds after OGO treatment was significantly higher than that of diamonds before OGO treatment, so they are easier to combine with the polysiloxane matrix to form a heat-conducting network chain. At the same time, due to the powerful mechanical kneading of the vacuum kneader and the action of vacuum negative pressure, the turbulent diffusion of polysiloxane was extremely violent. It not only made polysiloxane coat diamonds evenly, but also made polysiloxane penetrate into the gap between diamond particles through action of turbulence. As a result, almost all the air in system was exhausted, so the thermal Figure 14. Diamond XRD patterns before and after ozone oxidation reaction; (a) before oxidation reaction, (b) after oxidation reaction.  conductivity was also significantly improved. It can be seen from Table 8 that the thermal conductivity of composites after OGO treatment was significantly higher than that of composites before OGO treatment. But at the same time, the fluidity of system will also be significantly reduced due to the formation of a cross-linked network structure between diamonds and polysiloxane after ozone oxidation.

Summary
In this work, ozone was used to modify the surface of diamonds, and the type and number of oxygen-containing groups on diamond surface were studied under conditions of different ozone concentrations and different OGO treating time. Finally, changes in the structure and thermal conductivity of diamonds before and after OGO treatment were also studied under conditions with the highest surface hydroxyl density. From the previous analysis, the following conclusions can be drawn: 1. Ozone oxidation does not change the surface morphology and overall structure of diamond; 2. The hydroxyl density on diamond surface would increase with the increase of ozone concentration, and it would basically reach saturation when the ozone concentration came to 115(±5) mg/L; 3. The hydroxyl density on diamond surface would increase with the prolongation of OGO treating time, but when the reaction time came to 15 min, the hydroxyl density was the highest, and then some hydroxyl groups would be converted into carbonyl groups with the extension of reaction time, which causes a decrease of hydroxyl density; 4. The thermal conductivity of the composites composed of polysiloxane and diamonds after OGO treatment was significantly higher than that without OGO treatment, and the thermal conductivity of the system can be 8.02 W/mK.  Figure 15. SEm photos of diamond before and after ozone oxidation; (a) before oxidation reaction, (b) after oxidation reaction.