Crystal Growth and Characterization of Diamond from Carbonyl Iron

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Crystal Growth and Characterization of Diamond from Carbonyl Iron Catalyst under High Pressure and High Temperature Conditions Xiaobing Liu, Xiaopeng Jia, Zhuangfei Zhang, Yong Li, Meihua Hu, Zhenxiang Zhou, and Hong-an Ma* State Key Lab of Superhard Materials, Jilin University, Changchun 130012, P. R. China ABSTRACT: We have explored diamond synthesis from carbonyl iron powder catalyst to investigate the effects of minor elements on diamond crystallization in a series of experiments at temperatures of 1200 1800 °C and pressures of 5.0 7.0 GPa. Our results show that the diamond morphology is not only determined by pressure and temperature conditions but is also significantly influenced by the composition of crystallization medium. The stable growth forms of diamond are strip and lamellar shapes at relatively low temperature conditions. We have established the essential dependence of diamond morphology and nucleation peculiarities on the composition of crystallization medium and P/T conditions in the P T diagram. Furthermore, we find two different diamond growth modes existing in the studied systems. The current study suggests that the novel growth habits of the diamond could be attributed to the common effect of nitrogen and oxygen elements on the diamond crystallization.

1. INTRODUCTION To get a further understanding of the genesis of natural diamond, much research has been devoted to investigating the effects of the minor elements and impurity diffusion on diamond crystallization,1 6 especially nitrogen, oxygen, and hydrogen.7 13 It is well-known that nitrogen is the dominant impurity in most natural and synthesized diamonds. Most of the physical properties of diamond essentially depend on the form and concentration of nitrogen. Generally, the concentration of substitutional nitrogen is mainly determined by the catalyst. In addition, oxygen is also considered to play an important role in the genesis of natural diamond.14 16 Since the elements nitrogen and oxygen are readily available during the formation process of natural diamond, it is important to study the common effect of nitrogen and oxygen on diamond crystallization. Theoretically, both nitrogen and oxygen atoms could form donors to achieve some outstanding properties, such as mechanical, optical, and electronic applications. And the simultaneous incorporation of nitrogen and oxygen could tend to enhance the quality of chemical vapor deposition (CVD) diamond film.17 19 Furthermore, it is confirmed to be a reasonable alternative to investigate the genesis of natural diamond using catalyst under high pressure and high temperature (HPHT) conditions. To our knowledge, until recently there was no publication on the HPHT diamond synthesis with simultaneous incorporation of nitrogen and oxygen elements. In general, the nitrogen concentration is typically approximately 200 300 ppm in synthetic diamonds from conventional metal catalyst.10,20 Recently, iron nitride, as an effective catalyst, could help to synthesize diamonds with high nitrogen concentrations (∼3300 ppm).7 Such a high nitrogen concentration could be comparable with that of most natural diamonds. Since iron nitride is usually obtained from carbonyl iron, we here focus on the carbonyl compounds as a new catalyst rich in oxygen to investigate the common effect of nitrogen and oxygen on diamond crystallization. r 2011 American Chemical Society

In this work, we investigated in detail diamond crystallization using carbonyl iron powder as catalyst over a wide range of temperatures from 1200 to 1800 °C and pressures from 5.0 to 7.0 GPa. Both the diamond spontaneous nucleation and growth were obtained by film growth (FG) processes. The synthesized diamonds were characterized by optical, scanning electron microscope (SEM), powder X-ray diffraction (XRD), and infrared (IR) absorption. Our work attempts to clarify the common effect of nitrogen and oxygen on diamond crystallization under HPHT conditions. We believe that it will provide some important information and be helpful for the deep understanding of natural diamond genesis.

2. EXPERIMENTAL PROCEDURES Experiments on diamond crystallization were carried out using a chinatype large volume cubic high-pressure apparatus (CHPA) (SPD-6  1200) with sample chamber of 18 mm edge length. The starting materials were a high-purity graphite rod (99.9 wt.% purity) as carbon source. To study the common effect of nitrogen and oxygen on diamond crystallization, we choose carbonyl iron powders (the special iron with 99 wt.% in purity and 5 10 μm in size) as catalyst to synthesize diamond. These carbonyl iron powders were synthesized from carbonyl iron with a purity of 99.999 wt % deoxidizing by iron powders in a stream of ammonia (NH3) in a running quartz reactor at 300 400 °C. Then the carbonyl iron powders were deoxidized under hydrogen for 1 h. As a result, the major impurities in the carbonyl iron powders were nitrogen, oxygen, and a little hydrogen. Furthermore, some phosphating carbonyl iron powders were also used as catalyst to synthesize diamond. The graphite and catalyst powders (7:3, weight ratio) were mixed for 4 h and then were machined into samples for synthesizing diamond. The temperature was measured in each experiment using a Pt 30% RH/Pt 6% Rh Received: March 27, 2011 Revised: June 24, 2011 Published: July 20, 2011 3844

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Table 1. Experimental Results and the Characteristics of Obtained Diamond obtained cubic phase run

P (GPa)

T (°C)

time (min)

morphology

N-1

5.2

1250 1400

20

cubic, cub-octahedron, octahedron

yellow

0.2 0.3

N-2

5.5

1350 1500

10

cub-octahedron, octahedron

light yellow

0.4 0.5

N-3

6.5

1600 1650

15

irregular shape

green

0.2 0.3

N-4

6.5

1650 1700

15

strip and slice

green

0.5 0.9

N-5

6.5

1700 1750

15

octahedron

dark green

0.3 0.5

N-6

6.8

1650 1750

15

twinned crystal

green

0.7 0.8

N-7

6.8

1750 1800

15

twinned crystal

dark green

0.7 0.8

N-8 N-9

6.3 6.3

1600 1700 1700 1800

15 15

strip and slice octahedron

green green

0.5 0.8 0.3 0.4

color

size (mm)

thermocouple, whose junction was placed near the crystallization sample. Pressure was calibrated at room temperature by the change in resistance of standard substances and at high temperatures by the graphite diamond equilibrium. After experiments, crystallization sample columns were first cracked and examined with an optical microscope. Then, the products were dissolved in a hot mixture of H2SO4 and HNO3 to remove the remaining graphite and metal catalysts. Morphology and structural properties of the synthesized samples were characterized by SEM analysis and XRD. The IR spectra were obtained on a Perkin-Elmer 2000 Fourier transform infrared (FTIR) spectrometer in the spectral range between 400 and 4000 cm 1 with a spectral resolution of 2 cm 1 in the transmittance mode.

3. RESULTS AND DISCUSSION We perform the diamond crystallization under HPHT conditions and the experimental results are presented in Table 1. The diamonds are established in the Fe Ni C and Fe C systems at 5.2 GPa (N-1) and 5.5 GPa (N-2), respectively. At 6.5 GPa and temperature of 1550 °C, neither spontaneous nucleation nor diamond growth on seed crystals is found with carbonyl iron powders as catalyst. In run N-3, the diamond nucleation and growth are established with temperature increasing up to 1600 °C. Figure 1 shows the typical XRD pattern of the obtained samples in this system, which is after treatment with hot nitric acid to remove the remaining graphitic components and metal catalysts. It is found that the diffraction pattern of obtained samples is mainly composed by the characteristic {111}, {220}, and {311} lines, which correspond to cubic diamonds. However, we notice from Figure 2a that the obtained diamond exhibit irregular shape without typical {100} or {111} faces at temperatures of 1600 1650 °C. As the temperature increases up to 1650 °C (N-4), both the nucleation and growth rate of diamond increase and some crystals are up to 0.5 0.9 mm in a 15 min run. The diamond obtained under these conditions mainly exhibits lamellar and strip shapes (Figure 2b,c). At higher temperatures, from 1700 to 1750 °C (N-5), the roles of the {111} faces in the crystal habit increase and the crystallized diamonds show dominant {111} and minor {100} faces (Figure 2d). Hence, we conclude that the stable growth form under relatively higher temperature conditions is octahedron. Diamond crystals produced in all the experiments have green color with different intensities. In experiments performed at 6.8 GPa and 1600 1800 °C, the twinned crystals are usually prominent (Figure 3). At relatively low temperatures, 1650 and 1750 °C (N-6), the twinned crystals

Figure 1. Powder X-ray diffraction pattern of the obtained diamond samples from run N-3 (6.5 GPa, 1600 °C).

are composed of several strips or slices (Figure 3a,b). More interestingly, although most of the crystallized diamonds exhibit octahedron at higher temperatures, some strips or slices are found on the {111} faces (N-7). In general, the diamond strips grow vertically on the {111} faces (Figure 3c), while the slices always locate in parallel with the boundary of two neighboring {111} faces (Figure 3d). It is clear that the obtained diamond strips and slices always exhibit high purity and are nearly transparent. Furthermore, an additional series of experiments was performed with phosphating carbonyl iron powders as catalyst to grow diamond. Nevertheless, almost all the initial graphite has transformed to diamond at 6.3 GPa and 1600 °C (runs N-8 and N-9). Consequently, we could attribute it to the addition of phosphorus leading to a decrease in P T conditions for diamond synthesis. The experimental results on crystallization of carbon phases, including our previous experimental data,21,22 are summarized in a P T sketch diagram (Figure 4). First of all, it is clear that the P T conditions for diamond synthesis from the carbonyl iron powder catalyst (L3 and L4) are significantly higher than those established in the conventional catalyst (L1 and L2). Additionally, it is important to find that at a certain pressure condition the diamond nucleation and growth rate increase upon increasing the reaction temperature. Therefore, our results can indicate that the specific feature of the studied systems for 3845

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Figure 2. Optical images of diamond obtained from the carbonyl iron powders catalyst at 6.5 GPa and different reaction temperatures: (a) 1600 °C, (b, c) 1700 °C, and (d) 1800 °C.

Figure 3. Optical images of the twinned crystal synthesized at 6.8 GPa and temperatures: (a, b) 1700 °C and (c, d) 1800 °C.

diamond crystallization is the high temperature. A similar tendency was previously found in the sulfur carbon system and Fe3N carbon system.7,23 The other typical feature is the special growth habits of morphology depending on the reaction temperatures. We find that there are no characteristic {100} and

{111} faces for the obtained diamond at relatively low temperature conditions, and the diamonds show strip and lamellar shapes as the temperature increases. Most of the diamond with strip shape elongate or extend in the {111} direction, and no obvious {100} faces are observed. Additionally, although the stable 3846

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Crystal Growth & Design growth form under higher temperature conditions is octahedron, some diamond strips or slices are always found on the diamond {111} faces. However, there is no definite boundary for the diamond synthesis of the strips and slices in the P T sketch diagram. On the basis of the above results, we can conclude that

Figure 4. Schematic P T diagrams of diamond and graphite crystallization: L1 and L2 show the growth regions for diamond crystallization in the Fe Ni C and Fe C systems, respectively; L3 and L4 show the regions for diamond crystallization from the carbonyl iron powder catalyst and phosphating carbonyl iron powder catalyst, respectively. The regions A, B, and C demarcate different fields of diamond crystallization from the carbonyl iron powder catalyst. The number of green solid circles in the top right corner indicates the relative amount of the diamond nucleation in the system.

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the formation of the diamond morphology is not only determined by the P and T conditions but also obviously influenced by the composition of crystallization medium. The SEM analysis reveals the surface character of the obtained diamond (Figure 5), and two typical growth defects are found on the diamond surfaces. As illustrated in Figure 5a,b, some growth steps arrange disorderly just like waves and extend from one side to the other side on the diamond surface. The successive arrays of layers evidently indicate that the diamond growth belongs to a two-dimensional (2D) nucleation growth. We describe the 2D growth mechanism in the schematic diagram in Figure 6a. As illustrated, the diamonds usually grow up by deposition and extension of the two-dimensional nucleus on the surface. On the other hand, spiral steps are also found on the diamond surfaces from Figure 5c,d. It may indicate a dominant spiral growth during the diamond growth process. Figure 6b shows the spiral growth process of the diamond layers. The well-known spirals are

Figure 6. Schematic drawings showing the two growth mechanisms of diamond synthesized from the carbonyl iron catalyst: (a) the twodimensional nucleation growth and (b) the spiral growth.

Figure 5. The SEM images of the representative growth defects on the diamond surface: the wavy steps (a, b) and spiral steps (c, d). The inset images lying at top right corners are the corresponding optical images of the diamond samples. 3847

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Table 2. Concentration of Nitrogen Impurity in Diamond Synthesized in the Fe C System (S-1), Fe Ni C System (S-2), and Carbonyl Iron Powders C System (S-3 and S-4) sample

Figure 7. Typical FTIR spectra recorded for diamond crystals synthesized from the carbonyl iron catalyst at 1700 °C (a) and 1800 °C (b) and in Fe Ni C (c) and Fe C (d) systems. Spectra have been displaced vertically for clarity.

generally believed to be a step that terminates at a screw dislocation.24 26 In the early growth stage, the screw dislocation appears as line defect on the crystal surface. If the screw dislocation intersects the crystal surface, the discontinuous change yields a step on the crystal surface. This step then provides a preferred site for atoms to bond. Since the step is immobile at the screw dislocation, it moves around the screw dislocation in a spiral form. Because of the smooth interface between the diamond and metal solution, the spiral growth mode rarely appears during the diamond synthesis from the conventional metal catalysts. Thus, we suggest that the formation of the spiral steps is probably due to the impurity incorporation into the crystal structure during the diamond growth process. Taking account of the larger size of iron atoms than carbon atoms, the most likely possibility is the incorporation of the nitrogen and oxygen atoms into the diamonds in this study. The IR absorption is used for quantitative measurements of the nitrogen concentration. For IR absorption measurements, a number of diamond crystals with suitable sizes and high quality are selected. The IR beam size is limited to a 150 μm square by apertures so as to pass only the diamond particles. Typical FTIR spectra of the synthesized diamond are shown in Figure 7. We can notice from curves a and b that nitrogen impurities in the samples are mainly existing in the A-form at 1282 cm 1 (pairs of nearest neighboring substitutional nitrogen atoms) and C-form at 1130 and 1344 cm 1 (single substitutional nitrogen atoms).27 29 These spectra show that the absorption intensities in one phonon region are obviously higher than that of the diamond synthesized in the Fe Ni C and Fe C system (c and d). The nitrogen concentration can be determined by the FTIR spectra.30,31 Specifically, the concentration of nitrogen in the C-form is obtained from the strength of absorption of the peak at 1130 cm 1, namely, 25.0 ( 2 atomic ppm per cm 1 of absorption at 1130 cm 1. The concentration of nitrogen in the A-form could be determined by the strength of absorption of the peak at 1282 cm 1, namely, 16.1 ( 2 atomic ppm per cm 1 of absorption at 1282 cm 1. We summarize the estimated nitrogen concentrations in Table 2. It can be seen that the nitrogen concentrations are much higher than that of the diamond synthesized in the Fe-based catalyst. The highest nitrogen concentration

diamond shape

concentration of nitrogen (ppm)

S-1

octahedron

30 50

S-2

cub-octahedron

200 300

S-3

strip and slice

900 1200

S-4

octahedron

1100 1500

of both A-form and C-form defects is approximate 1500 ppm in the studied systems, while the nitrogen impurity in the diamonds obtained from the Fe C system is less than 50 ppm. Probably the most interesting aspect of this work is the special growth habits for the crystal morphology at different temperature conditions. In our previous work,10,20,32 strip-shaped diamond crystals were found under nitrogen-rich conditions. These diamonds, having elongated {100} and {111} crystal faces, seemed to be formed from the stretched cub-octahedron. Furthermore, Borzdov et al. have also found that diamonds always exhibited specific morphology with high nitrogen concentrations.7,33 Thus, we suggest that the nitrogen concentration and substitutional form play very important roles in the formation of the diamond strip morphology. Additionally, it is interesting to note that most of the obtained diamond strips elongate or extend only with the {111} direction and no obvious {100} faces are observed in the present work. Considering the results of preceding studies, we suppose that this can be partly accounted for by the fact that the oxygen ions in the catalyst have obvious effects on the suppression of the growth of the {100} faces. A similar tendency has been previously revealed, and Pal’yanov et al. reported only octahedral diamond crystals formed in the presence of the C O H fluid.34,35 However, as far as we know, there is still no publication on the synthesis of diamond slices. Herein, we suggest that the special morphology could be attributed to the simultaneous incorporation of the nitrogen and oxygen atoms into the diamond lattice structures. When the diamonds are synthesized at relatively low temperatures, the nitrogen and oxygen elements in catalyst easily influence the diamond crystallization. As a result, the obtained diamonds always exhibit some special morphology (Figures 2a c). However, the oxygen and nitrogen atoms are apt to escape from the catalyst at relatively higher temperatures. Therefore, the diamond nucleation and growth rate increase due to the strong catalytic capability of iron metal in the catalyst, and the stable form of the diamond is usually octahedron (Figure 2d).

4. CONCLUSIONS We have explored the diamond crystallization from the carbonyl iron powder catalyst under HPHT conditions. We find that the specific feature for diamond crystallization is the high temperature, which can enhance the catalytic capability for the nucleation and growth of diamond, with the minimal temperature being approximately 1600 °C. The stable growth form is strip and lamellar shape at relatively low temperature conditions. FTIR reveals that synthesized diamond crystals contain A- and C-formed nitrogen impurities with a total nitrogen concentration up to 1500 ppm. We suggest that the novel growth habits for the diamond can be attributed to the common effect of nitrogen and oxygen elements during the diamond crystallization. We believe that our work is greatly helpful for the deep understanding of the effects of the minor elements and impurity 3848

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Crystal Growth & Design diffusion on diamond crystallization and provides a lot of valuable information on the search for the genesis of natural diamond.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial supported by the National Natural Science Foundation of China under Grant Nos. 50572032, 50731006, and 50801030 and Project 20111022 supported by Graduate Innovation Fund of Jilin University. ’ REFERENCES

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