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Experimental study on the stability of graphitic C3N4 under high pressure and high temperature
Diamond & Related Materials 20 (2011) 819–825
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Experimental study on the stability of graphitic C3N4 under high pressure and high temperature Leiming Fang ⁎, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577 Ehime, Japan
a r t i c l e
i n f o
Article history: Received 5 October 2010 Received in revised form 22 March 2011 Accepted 29 March 2011 Available online 5 April 2011 Keywords: g-C3N4 Carbon nitride High pressure High temperature Stability Decomposition
a b s t r a c t The stability and decomposition of graphitic C3N4 (g-C3N4) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C3N4 was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C3N4 has a positive slope (~0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic C\N framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C3N4 was found in the studied P–T range. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since β-C3N4, a carbon nitride phase, was theoretically predicted to have an extreme bulk modulus equivalent or greater than that of diamond [1–3], many efforts have been devoted to study this potential superhard material and related phases through theoretical and experimental approaches. To date, five types of dense C3N4 structures including β-C3N4 and cubic-C3N4, have been proposed [4–6]. Experimental studies have employed various techniques such as chemical (CVD) and physical (PVD) vapor depositions, solvothermal method, static and dynamic high pressure methods, and used various organic or inorganic chemicals as precursors, aiming to synthesize those predicted phases in the laboratory [5–8]. In spite of these extensive studies, there has been no conclusive evidence of successful synthesis of superhard bulk materials with stoichiometric C3N4 composition, although some studies reported that thin films of β- and cubic-C3N4 could be obtained by CVD. Exploring a reproducible method for obtaining low-compressible bulk C3N4 materials is therefore still a challenging task. At ambient conditions, graphitic carbon nitride (g-C3N4) is considered to be the most stable allotrope. It has a hexagonal structure composed of AB-stacked graphene sheets which are constructed ideally from tri-s-triazine units connected by planar amino groups. gC3N4 can be readily synthesized by various methods such as solvo-
⁎ Corresponding author. Tel./fax: + 81 927 8405. E-mail address: [email protected] (L. Fang). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.03.034
thermal synthesis [9–15], mechanochemical reaction [16–18], and thermal decomposition [11,19–22]. Among these, the solvothermal method yields relatively well-crystalline g-C3N4 in large quantity under relatively mild conditions. In recent years, many efforts were made to study the stability and potential phase transition of g-C3N4 under high pressure and high temperature conditions. Ming et al. [23] observed the phase transition to a new high pressure phase at pressures above 23 GPa and temperatures above 1600 K in laser-heated diamond anvil cell (LHDAC) experiments. The observed high pressure phase was not structurally comparable to any of the predicted C3N4 phases and could not be quenched to ambient conditions. Upon pressure release, it was transformed to a metastable phase with a cubic unit cell, which is, however, different from the predicted superhard cubic-C3N4. The reproducibility of these results was examined by large-volume press experiments [23] and also by a further LHDAC study [24]. It should be mentioned that a considerable amount of diamond is associated with their quenched products as clearly seen in the XRD profiles. It is unclear whether the diamond was derived from the phase transition of the organic (carbon-containing) residues that were possibly contaminated in the starting material [23,24] or simply formed as a result of the partial decomposition of g-C3N4 to carbon (diamond) and nitrogen-containing phase(s). Indeed, the decomposition of turbostratic carbon nitride (t-CN) was observed at below 20 GPa by high P–T in situ and quench experiments [25]. This study showed that at 4.7 GPa t-CN decomposed to form disordered graphite above 990 K, while at 17.8 GPa it decomposed directly to diamond above 1850 K and that no phase transitions to denser forms of carbon nitride could be obtained below 20 GPa and 2200 K.
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In this study, we performed a series of extensive high P–T experiments using graphitic carbon nitride with a C3N4 composition as a starting material and examined its stability and decomposition behavior in the P–T range up to 25 GPa and 2000 °C. 2. Experimental method We studied the stability and high pressure behavior of g-C3N4 up to 25 GPa and 2000 °C by using a Kawai-type 2000 t multianvil apparatus (Orange-2000). WC anvils of 4 mm truncation edge-length and MgO pressure medium (octahedron) of 10 mm edge-length were used. Fig. 1 shows a schematic illustration of the cell assembly used in this study. The starting sample was carefully enclosed in two platinum capsules and loaded in an MgO sleeve that is located in the middle of a cylindrical LaCrO3 heater. A W97Re3–W75Re25 thermocouple was also placed between the two sample capsules for temperature measurement during heating. The pressure calibration was performed at room temperature using conventional pressure standards (phase transition), ZnS (15.6 GPa), GaAs (18.7 GPa) and GaP (23 GPa). In each run, the starting sample was compressed to a desired pressure at room temperature, and then heated to a target temperature of 600–2000 °C. After a desired heating duration, the sample was quenched to room temperature by turning off the heating current and then decompressed to ambient pressure. The details of the experimental conditions are listed in Table 1. The samples recovered from each run were first analyzed by X-ray diffraction and Raman spectroscopy for phase identification. X-ray diffraction measurements were made on a micro-focus X-ray diffractometer (MAC Science M18XHF) using CuKα radiation (40 kV, 200 mA, and collimated to 50 μm). Raman spectroscopic measurements were performed using a confocal micro-Raman system (Renishaw RSSYS1000) equipped with an Ar-ion laser (λ=514.5 nm). The Raman shift was calibrated with an uncertainty of less than ±0.5 cm− 1 using a silicon standard. The samples were then examined by a field-emission SEM (JEOL JSM-7000F) equipped with an energy dispersive X-ray spectrometer (EDS) after gold coating (ca. 20 nm thickness). An accelerating voltage of 15 kV and a beam current of 0.53 nA were used for chemical quantitative analysis by EDS. SiC and h-BN were used as standard references, respectively, for C and N quantification. Some selected samples were further investigated by transmission electron microscopy (TEM, JEOL JEM-2010) operated at 200 kV for microtextural observation and selected area electron diffraction (SAED) analysis. The starting material used in this study was nanocrystalline g-C3N4 which was prepared via a benzene-thermal reaction between C3N3Cl3 and NaNH2 at 220 °C for 12 h by following the method described in Ref. [10]. The material was examined by X-ray powder diffraction, SEMEDS and TEM prior to the high pressure experiments. Fig. 2 shows the XRD pattern of the synthesized g-C3N4, in which the only 002 diffraction peak is observed at 2θ = 27.5° (d = 3.22 Å), being consistent with the result of the original reference [10]. The absence of other
Table 1 List of the experimental conditions and the phase(s) observed in the recovered products. Run no. P (GPa) T (°C) Heating time (min) Phase(s) identified by XRD and Raman OS1936 OS1930 OS1923 OS2002 OS1990 OS1980 OS1983 OS1984 OS1981 OS1948 OS1876 OS1976 OS1971 OS1871 OS1892 OS1958 OS2014 OS2012 OS2011 OD821 a
10 10 10 10 15 15 15 15 15 19 22 22 22 22 22 22 25 25 25 25
600 800 1000 1600 700 1000 1200 1400 1600a 25 800a 900 1000 1400 1600 1800a 600 800 1000 2000
30 30 30 30 30 30 30 30 30 Not heated 30 60 30 30 30 5 30 30 30 5
g-C3N4 Graphite Graphite Graphite g-C3N4 Graphite Graphite Diamond Diamond g-C3N4 g-C3N4 g-C3N4 Diamond (+amorphous carbon) Diamond Diamond Diamond g-C3N4 g-C3N4 + diamond Diamond Diamond
Temperature was estimated by the power–temperature relation.
diffraction peaks such as 100 and 101 suggests that the synthesized gC3N4 has a turbostratic structure where individual graphene layers randomly translate to each other and rotate along the stacking direction (i.e. c-axis direction) [26–28]. SEM and TEM observations showed that the obtained g-C3N4 powder is so fine and poorly crystalline that individual crystals cannot be clearly seen even at high magnification (Fig. 3). The SAED pattern collected from an area of 1.2 μm diameter shows a faint but full diffraction ring which can be indexed as the 002 reflection of g-C3N4, indicating that the material consists of extremely small crystallites which are randomly oriented. The C/N atomic ratio of our starting material was determined to be 0.73, which is in good agreement with the C3N4 stoichiometry, as shown in Table 2. EDS analysis also revealed the presence of a nonnegligible amount of O in the starting sample, as is the case of the earlier studies using the same or similar starting materials [6,15,16,19]. Taking into account the poorly-crystallized, nano-structural feature of the synthetic g-C3N4, a considerable amount of O and/or OH might be trapped on the surface of the individual nano-particles, where dangling bonds are assumed to be dominant. Although the starting material was kept in a vacuum oven and heated at 110 °C for more than 2 weeks before use, it was not effective to prevent the contamination of these elements. This implies that these light elements are not simply surface-adsorbed, but are bound directly to the dangling bonds of the g-C3N4 particles. Another possibility to explain the oxygen contamination is the presence of C3N3 voids in the starting g-C3N4, which could be fully occupied by water molecules, as recently noted by Foy et al.
Fig. 1. A schematic illustration of the cell assembly used for the present multi-anvil experiments.
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Table 2 List of the chemical compositions of the recovered products. The compositions were obtained by averaging 20 measurements at randomly selected points on the polished surface of each sample. The standard deviation is given in bracket.
Fig. 2. XRD pattern (Cu Kα = 1.5418 Å) of the synthetic g-C3N4 used as the starting material.
[29]. The slight excess of nitrogen (C/N = 0.73) might be explained by the presence of the C3N3 voids. Whatever the case, the contamination of these elements may be inevitable, as long as using such poorly crystalline, nano-sized g-C3N4 particles. The trace amounts of Na and Cl are derived from the residues of the side product (NaCl) of the g-C3N4 synthesis [10].
3. Results and discussion Fig. 4(a) shows the variation of the X-ray diffraction patterns collected from the recovered samples, which were heated to different temperatures at 10 GPa. Compared with the starting sample (Fig. 2), the sample heated to 600 °C shows no change in the diffraction pattern, the broad peak of 002 reflection of g-C3N4 was only detected, apart from several smaller peaks derived from the surrounding Pt capsule. The samples heated above 800 °C also show a broad intense peak at 2θ = 26.5°, whose position is, however, slightly shifted toward the low-angle side. The d-value of the peak is calculated to be 3.33– 3.38 Å, which is comparable to the d (002) spacing of graphite rather than of g-C3N4. These observations suggest that the thermal decomposition of g-C3N4 to graphite (C) and nitrogen-containing phase(s) starts above 600 °C at this pressure condition. It is interesting to note that the observed graphite 002 peak is considerably broad (even broader than those of the original g-C3N4) when the sample was heated at 800 °C, but becomes sharper and more intense with increase in the heating temperature. This means that graphite first formed through the decomposition of g-C3N4 is poorly developed with dis-
Sample conditions
Chemical compositions (at.%)
Starting material 10 GPa, 600 °C 10 GPa, 800 °C 10 GPa, 1000 °C 15 GPa, 700 °C 15 GPa, 1000 °C 15 GPa, 1400 °C 15 GPa, 1600 °C 22 GPa, 800 °C 22 GPa, 900 °C 22 GPa, 1000 °C 22 GPa, 1400 °C 22 GPa, 1600 °C
34.8 37.6 93.0 93.7 39.6 95.0 95.8 96.1 40.0 47.3 96.9 99.1 99.9
C (9) (18) (39) (17) (13) (5) (10) (6) (20) (36) (20) (15) (1)
N
O
Na
Cl
47.8 (9) 44.6 (15) – – 41.6 (13) – – – 46.4 (22) 38.4 (48) – – –
16.8 (5) 16.9 (18) 3.7 (17) 4.4 (11) 17.1 (5) 3.7 (6) 2.6 (10) 3.7 (6) 13.1 (8) – 1.6 (4) – –
0.2 (0) 0.5 (1) 3.1 (14) 1.2 (4) 0.2 (1) 0.4 (0) 0.5 (4) – 0.2 (0) 14.0 (5) 1.2 (2) 0.4 (0) –
0.1 (0) 0.3 (1) 0.3 (1) 0.1 (1) 0.7 (3) 0.4 (1) 0.6 (5) – 0.3 (1) 0.3 (1) 0.4 (1) 0.5 (1) –
ordered nano-particles, which are, however, readily crystallized upon annealing at higher temperature. The XRD patterns of the samples heated to 700–1600 °C at 15 GPa are presented in Fig. 4(b). g-C3N4 remained stable after heating at 700 °C, but decomposed to form graphite after heating at 1000 °C, as can be seen from the low-angle shift of the 002 peak. Upon heating above 1400 °C, sharp intense peaks of cubic diamond appeared, and no graphite peaks were observed, suggesting that g-C3N4 has decomposed to directly produce diamond at this P–T condition. In the experiments conducted at 22 GPa, the 002 peak of g-C3N4 maintains its intensity and sharpness up to 900 °C and shows no signs of peak shifting (graphitization does not occur), as shown in Fig. 4(c). Above 1000 °C, the 002 peak of g-C3N4 completely disappeared and was replaced by diamond peaks. The observed diamond peaks are considerably broad when they first appeared at 1000 °C, but become sharper and more intense with increasing heating temperature probably due to the grain growth. At this pressure condition, the formation of pure carbon graphite was not observed in the studied temperature range. Fig. 4(d) shows the XRD patterns of the samples heated to 600– 2000 °C at 25 GPa. g-C3N4 remained stable up to 600 °C, as indicated by the diffraction peak from the 002 basal plane, and showed no signs of graphitization. With increasing temperature, the g-C3N4 peak was gradually replaced by broad peaks of diamond and eventually disappeared completely at 1000 °C. This indicates that the formation of diamond occurs directly through g-C3N4 decomposition without its graphitization as is the case at 22 GPa. Fig. 5(a) and (b) shows Raman spectra collected from the recovered samples which were heated to different temperatures at 10 GPa and 22 GPa, respectively. The starting material (g-C3N4) produces no visible band in the spectra (Fig. 5(a)) due to its strong fluorescence and
Fig. 3. SEM (a) and TEM (b) images of the synthetic g-C3N4. The inset of (b) is an SAED pattern showing a broad Debye–Scherrer ring from the 002 reflection.
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Fig. 4. The variation of X-ray diffraction patterns collected from the samples heated to different temperatures at 10 GPa (a), 15 GPa (b), 22 GPa (c) and 25 GPa (d), respectively. The peaks marked Pt are from the surrounding platinum capsule.
Fig. 5. Raman spectra of selected samples recovered from 10 GPa (a) and 22 GPa (b) and various high temperatures.
photoluminescence induced by visible (Ar 514.5 nm) laser excitation [30]. In the experiments at 10 GPa, the sample heated to 600 °C shows no peaks as is the case for the starting material, while those heated to 800 °C and 1000 °C show two intense peaks at 1350 and 1580 cm− 1, which can be assigned respectively to the D-band and G-band of (disordered) graphite (Fig. 5(a)). It is thus suggested that at 10 GPa the decomposition of g-C3N4 occurs at temperatures between 600 and 800 °C, showing good agreement with the XRD results. When the sample was heated to 1600 °C, the two Raman peaks of graphite became noticeably sharp and the D-band intensity (relative to G-band) dropped significantly, indicating considerable grain growth and an increase in structural order of the produced graphite crystals. In the experiments at 22 GPa, no peaks were visible below 900 °C, while a broad peak at 1331 cm− 1 attributed to nanocrystalline diamond was observed when heated above 1400 °C (Fig. 5(b)). The sample heated to 1000 °C shows continuous broad bands ranging at 1400–1600 cm− 1, so the Raman peak from the diamond was not clearly recognized due to signal overlap, despite that the formation of nanocrystalline diamond was certainly confirmed by XRD (Fig. 4(c)). The detection of the broad bands implies the presence of minor amounts of disordered (amorphous) carbon species such as diamond-like carbon [31] in the recovered sample. We further examined the microtexture and the chemical composition of the recovered products by FE-SEM/EDS and TEM. Fig. 6 shows representative SEM images of samples recovered from different temperatures
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Fig. 6. SEM (secondary electron) images of samples recovered from various P–T conditions. (a) g-C3N4 recovered from 15 GPa and 700 °C, (b) graphite from 15 GPa and 1000 °C, (c) euhedral diamond crystals synthesized respectively at 1400 °C at 15 GPa, and the diamond samples recovered from 22 GPa and 1000 °C (d), 1400 °C (e), and 1600 °C (f).
at 15 and 22 GPa and Table 2 summarizes the results of the elemental analysis. The sample recovered from 15 GPa and 700 °C, which was found to consist purely of g-C3N4 by XRD and Raman analyses, exhibits a massive texture where individual grains are not clearly recognized even at high magnification (Fig. 6(a)), as is the case of the starting material (Fig. 3(a)). EDS analysis revealed that the sample consists mainly of nitrogen and carbon, although the observed C/N ratio (0.95) is somewhat higher than that obtained for the starting material (0.73). It is also found to contain oxygen in an equivalent amount to that detected in the starting sample (Table 2). The samples heated to 1000 and 1400 °C at 15 GPa consist of an aggregate of fine particles of 50–100 nm (Fig. 6(b) and (c)). They are composed of mostly carbon and a small amount (b4 atom%) of oxygen and contain virtually no nitrogen (Table 2), which is consistent with the observed XRD and Raman data characterized by the decomposition to pure carbon phases (graphite and diamond, respectively) plus nitrogen. TEM observation of the latter sample revealed that the individual grains are euhedral crystals (50–100 nm) of diamond, which are loosely aggregated but are not tightly bound to each other (Fig. 7(a)). The samples recovered from the 22 GPa runs also exhibit similar textural features: both graphite and diamond as decomposition products of g-C3N4 consist of relatively loose aggregate of nanocrystalline particles (Fig. 6(d–f)). Upon temperature increase, significant grain growth was observed in diamond; grain (crystallite) size ranges from an almost amorphous-like state (~1 nm, Fig. 7(c)) at 1000 °C to half of a micron (Fig. 6(f)) at 1600 °C.
It should be noted that all the diamond samples obtained in this study are not sintered at all, but are in the form of white powder. SEM and TEM observations showed that the individual diamond grains are discrete euhedral crystals and are not tightly bound to each other (Fig. 6(c), (e), and (f); Fig. 7(a) and (d)). These morphological features imply that they have crystallized in a free space such as a fluid medium. It is naturally expected that the decomposition of gC3N4 produces nitrogen or nitrogen-bearing compound(s) as well as diamond (and/or graphite) as pure carbon phase(s). However, the EDS analysis of those decomposition products revealed the absence of nitrogen-bearing phases, although a trace amount of nitrogen may be included in diamond/graphite grains or present as an amorphous phase at their grain boundary. This discrepancy can be explained by the following scenario: the decomposition of g-C3N4 also involves the formation of pure nitrogen, which initially exists in the sample as a liquid phase under the studied P–T conditions [32,33], but is eventually released from the sample capsule in the form of gas when the capsule is opened after decompression. Therefore, the diamond formation through thermal decomposition of g-C3N4 is considered to have occurred in the liquid nitrogen medium. This is likely the reason why the obtained diamond samples are all in the form of a loose powder of euhedral crystals unlike the products from the direct conversion of pure carbon graphite and related materials which are characterized to have well-sintered nanocrystalline textures and high optical transparency [34].
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Fig. 7. TEM images and SAED patterns of different diamond crystals recovered from various P–T conditions. (a) 1400 °C at 15 GPa, (b) 1000 °C at 25 GPa, (c) 1000 °C at 22 GPa, and (d) 1600 °C at 22 GPa, respectively.
Considering all the data obtained through XRD, Raman and SEMTEM analyses, we propose a decomposition diagram of g-C3N4 in the pressure and temperature range up to 25 GPa and 2000 °C, respectively (Fig. 8). It is demonstrated by the present investigation that g-C3N4 decomposes to graphite (and probably nitrogen) at temperatures above 600–700 °C and at a relatively low pressure range (10– 15 GPa), while it decomposes to diamond directly (plus nitrogen) above 800–900 °C at a higher pressure range (22–25 GPa). The formation of diamond from g-C3N4 also occurs at 15 GPa, when the heating temperature exceeds 1400 °C. The estimated decomposition curve of g-C3N4 has a positive slope (~ 0.05 GPa/K) at pressures up to ~ 22 GPa, but becomes inverted above this pressure. It seems that gC3N4 remains stable after heating at 600–900 °C for 30–60 min in the pressure range studied (~ 25 GPa). However, the observed increase in C/N ratio (i.e. decrease in nitrogen content) of those recovered pro-
Fig. 8. P–T diagram showing the stability of g-C3N4. The solid black line indicates the decomposition boundary for g-C3N4, determined from the results of our quench experiments (not a thermodynamic boundary). The broken line indicates the reaction (kinetic) boundary between graphite and diamond observed in the present study.
ducts compared with that of the starting sample suggests that the decomposition reaction is kinetically slow and might proceed slowly even at lower temperatures (below the threshold line). It is also interesting to note that the decomposition boundary of g-C3N4 estimated from our study is located at lower temperature than that reported for turbostratic CN [25], particularly in the higher pressure range. There seem to be no signs of a phase transition of g-C3N4 in the studied P–T range, even at the highest pressure of 25 GPa, at which pressure a cubic C3N4 phase was claimed to be obtained as a quench product [23,24]. In our study, g-C3N4 was observed to be beginning to decompose at relatively mild temperatures (N800 °C), despite using a starting sample of g-C3N4 prepared by the same procedure and employed the same kind of capsule (Pt) as those used in the earlier studies. However, it seems unlikely that g-C3N4, which starts to decompose at 800 °C, transforms to another phase even if taking it to higher temperature conditions. The high pressure phase and its quenched form (cubic phase) of C3N4 reported in earlier studies [23,24] may be metastable products, as they are also not observed in recent LHDAC experiments [35]. In the present study, we observed the formation of cubic diamond as a decomposition product of g-C3N4 at temperatures more than 1400, 1000 and 800 °C and at pressures of 15, 22 and 25 GPa, respectively (Table 1 and Fig. 8). Interestingly, these temperature conditions are significantly lower than those required for diamond formation by the direct conversion of pure carbon graphite and related carbon materials under similar pressure conditions. For example, the minimum temperature necessary for cubic diamond formation from graphite and even from non-graphitic carbons (which are more dominated by sp3 bonding) was reported to be 1500–1600 °C in the pressure range of 15–21 GPa [36]. Moreover, to obtain pure, single-phase cubic diamond without unreacted residues high temperatures of more than 2200 and 1600 °C are necessary, respectively, for graphite and non-graphitic starting materials at those pressures [35,36]. The lower threshold temperatures for diamond formation from g-C3N4 may be attributed to the physicochemical nature of the carbon nitride itself. Klyui et al. [37] studied the diamond formation from both nitrogen-containing (amorphous CNx) and nitrogen-free (amorphous carbon) precursors and found that at the condition of 7.7 GPa and 2000 °C crystalline
L. Fang et al. / Diamond & Related Materials 20 (2011) 819–825
diamond was obtained only from the former precursor. They suggested the following scenario to explain why the diamond formation from carbon nitride can occur under milder conditions. The decomposition of carbon nitride involves the breakdown of C\N bonds and the formation of N2 molecules, which consequently induces the generation of highly active dangling bonds of carbon. These dangling bonds can be preferentially reconstructed under high pressure into sp3-coordinated C\C bonds and provide preferential sites for diamond nucleation. This, in turn, means that the overall activation energy required for diamond nucleation (formation) is significantly lowered through the decomposition of carbon nitride. This model also seems applicable to the present case. In addition, the presence of liquid nitrogen as a decomposition product of g-C3N4 may also play an important role in enhancing the grain growth rate of diamond. In this study, the diamond crystals formed at 1400–1600 °C at 15 and 22 GPa were observed to be ~ 500 nm or even more in size, which is more than two times larger than those synthesized from pure carbon precursors at even higher temperatures of ~2500 °C in a similar pressure range [36]. This may be because the diffusion rate of carbon atoms is faster in the liquid nitrogen regime than in solid, although further study is needed for more detailed and quantitative discussion. Alternatively, the lower threshold for diamond formation and the fast grain growth rate might be due to the presence of water (dissociated or not) or hydrogen molecules, which are likely adsorbed on the surface or trapped in C3N3 voids [29] in the initial g-C3N4, as previously suggested by several studies [38]. In this case, hydrogen and/or hydroxyl may play a catalytic role in promoting the diamond crystallization. 4. Conclusions
proofreading. This work was supported by Global COE program “Center for Advanced Experimental and Theoretical Deep Earth Mineralogy” and also partly by a Grant-in-Aid for Young Scientists (B) (#20740255) to HO from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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The stability of graphitic C3N4 under high pressure and high temperature was studied up to 25 GPa and 2000 °C on the basis of quench experiments using a multi-anvil apparatus. The result demonstrates that g-C3N4 remains stable at relatively mild temperatures within the experimental time scale (N30 min), but decomposes to C and N upon heating above 600 and 900 °C, respectively, at 10 and 25 GPa. The decomposition products consist only of pure carbon phases, either graphite or diamond depending on the P–T conditions, and do not contain any nitrogen phases. It is likely that nitrogen had existed as liquid during the high P–T treatments, but was released from the sample chamber in the form of gas upon pressure release. No phase transition of g-C3N4 was observed at P–T conditions up to 25 GPa and 2000 °C, and the theoretically predicted, low-compressible superhard C3N4 phases cannot be obtained. Further studies, for example at higher pressures and/or using well-crystalline g-C3N4 with a stoichiometric composition as a starting material, are needed to investigate whether such superhard C3N4 can really be synthesized in laboratory. Acknowledgments The authors are grateful to L. Lei, N. Nishiyama and T. Inoue for useful advice on the experimental works and also to Y. Kojima for assistance in preparing the starting material. We also thank S. Whitaker for English
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Experimental study on the stability of graphitic C3N4 under high pressure and high temperature Leiming Fang ⁎, Hiroaki Ohfuji, Toru Shinmei, Tetsuo Irifune Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577 Ehime, Japan
a r t i c l e
i n f o
Article history: Received 5 October 2010 Received in revised form 22 March 2011 Accepted 29 March 2011 Available online 5 April 2011 Keywords: g-C3N4 Carbon nitride High pressure High temperature Stability Decomposition
a b s t r a c t The stability and decomposition of graphitic C3N4 (g-C3N4) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C3N4 was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C3N4 has a positive slope (~0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic C\N framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C3N4 was found in the studied P–T range. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Since β-C3N4, a carbon nitride phase, was theoretically predicted to have an extreme bulk modulus equivalent or greater than that of diamond [1–3], many efforts have been devoted to study this potential superhard material and related phases through theoretical and experimental approaches. To date, five types of dense C3N4 structures including β-C3N4 and cubic-C3N4, have been proposed [4–6]. Experimental studies have employed various techniques such as chemical (CVD) and physical (PVD) vapor depositions, solvothermal method, static and dynamic high pressure methods, and used various organic or inorganic chemicals as precursors, aiming to synthesize those predicted phases in the laboratory [5–8]. In spite of these extensive studies, there has been no conclusive evidence of successful synthesis of superhard bulk materials with stoichiometric C3N4 composition, although some studies reported that thin films of β- and cubic-C3N4 could be obtained by CVD. Exploring a reproducible method for obtaining low-compressible bulk C3N4 materials is therefore still a challenging task. At ambient conditions, graphitic carbon nitride (g-C3N4) is considered to be the most stable allotrope. It has a hexagonal structure composed of AB-stacked graphene sheets which are constructed ideally from tri-s-triazine units connected by planar amino groups. gC3N4 can be readily synthesized by various methods such as solvo-
⁎ Corresponding author. Tel./fax: + 81 927 8405. E-mail address: [email protected] (L. Fang). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.03.034
thermal synthesis [9–15], mechanochemical reaction [16–18], and thermal decomposition [11,19–22]. Among these, the solvothermal method yields relatively well-crystalline g-C3N4 in large quantity under relatively mild conditions. In recent years, many efforts were made to study the stability and potential phase transition of g-C3N4 under high pressure and high temperature conditions. Ming et al. [23] observed the phase transition to a new high pressure phase at pressures above 23 GPa and temperatures above 1600 K in laser-heated diamond anvil cell (LHDAC) experiments. The observed high pressure phase was not structurally comparable to any of the predicted C3N4 phases and could not be quenched to ambient conditions. Upon pressure release, it was transformed to a metastable phase with a cubic unit cell, which is, however, different from the predicted superhard cubic-C3N4. The reproducibility of these results was examined by large-volume press experiments [23] and also by a further LHDAC study [24]. It should be mentioned that a considerable amount of diamond is associated with their quenched products as clearly seen in the XRD profiles. It is unclear whether the diamond was derived from the phase transition of the organic (carbon-containing) residues that were possibly contaminated in the starting material [23,24] or simply formed as a result of the partial decomposition of g-C3N4 to carbon (diamond) and nitrogen-containing phase(s). Indeed, the decomposition of turbostratic carbon nitride (t-CN) was observed at below 20 GPa by high P–T in situ and quench experiments [25]. This study showed that at 4.7 GPa t-CN decomposed to form disordered graphite above 990 K, while at 17.8 GPa it decomposed directly to diamond above 1850 K and that no phase transitions to denser forms of carbon nitride could be obtained below 20 GPa and 2200 K.
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In this study, we performed a series of extensive high P–T experiments using graphitic carbon nitride with a C3N4 composition as a starting material and examined its stability and decomposition behavior in the P–T range up to 25 GPa and 2000 °C. 2. Experimental method We studied the stability and high pressure behavior of g-C3N4 up to 25 GPa and 2000 °C by using a Kawai-type 2000 t multianvil apparatus (Orange-2000). WC anvils of 4 mm truncation edge-length and MgO pressure medium (octahedron) of 10 mm edge-length were used. Fig. 1 shows a schematic illustration of the cell assembly used in this study. The starting sample was carefully enclosed in two platinum capsules and loaded in an MgO sleeve that is located in the middle of a cylindrical LaCrO3 heater. A W97Re3–W75Re25 thermocouple was also placed between the two sample capsules for temperature measurement during heating. The pressure calibration was performed at room temperature using conventional pressure standards (phase transition), ZnS (15.6 GPa), GaAs (18.7 GPa) and GaP (23 GPa). In each run, the starting sample was compressed to a desired pressure at room temperature, and then heated to a target temperature of 600–2000 °C. After a desired heating duration, the sample was quenched to room temperature by turning off the heating current and then decompressed to ambient pressure. The details of the experimental conditions are listed in Table 1. The samples recovered from each run were first analyzed by X-ray diffraction and Raman spectroscopy for phase identification. X-ray diffraction measurements were made on a micro-focus X-ray diffractometer (MAC Science M18XHF) using CuKα radiation (40 kV, 200 mA, and collimated to 50 μm). Raman spectroscopic measurements were performed using a confocal micro-Raman system (Renishaw RSSYS1000) equipped with an Ar-ion laser (λ=514.5 nm). The Raman shift was calibrated with an uncertainty of less than ±0.5 cm− 1 using a silicon standard. The samples were then examined by a field-emission SEM (JEOL JSM-7000F) equipped with an energy dispersive X-ray spectrometer (EDS) after gold coating (ca. 20 nm thickness). An accelerating voltage of 15 kV and a beam current of 0.53 nA were used for chemical quantitative analysis by EDS. SiC and h-BN were used as standard references, respectively, for C and N quantification. Some selected samples were further investigated by transmission electron microscopy (TEM, JEOL JEM-2010) operated at 200 kV for microtextural observation and selected area electron diffraction (SAED) analysis. The starting material used in this study was nanocrystalline g-C3N4 which was prepared via a benzene-thermal reaction between C3N3Cl3 and NaNH2 at 220 °C for 12 h by following the method described in Ref. [10]. The material was examined by X-ray powder diffraction, SEMEDS and TEM prior to the high pressure experiments. Fig. 2 shows the XRD pattern of the synthesized g-C3N4, in which the only 002 diffraction peak is observed at 2θ = 27.5° (d = 3.22 Å), being consistent with the result of the original reference [10]. The absence of other
Table 1 List of the experimental conditions and the phase(s) observed in the recovered products. Run no. P (GPa) T (°C) Heating time (min) Phase(s) identified by XRD and Raman OS1936 OS1930 OS1923 OS2002 OS1990 OS1980 OS1983 OS1984 OS1981 OS1948 OS1876 OS1976 OS1971 OS1871 OS1892 OS1958 OS2014 OS2012 OS2011 OD821 a
10 10 10 10 15 15 15 15 15 19 22 22 22 22 22 22 25 25 25 25
600 800 1000 1600 700 1000 1200 1400 1600a 25 800a 900 1000 1400 1600 1800a 600 800 1000 2000
30 30 30 30 30 30 30 30 30 Not heated 30 60 30 30 30 5 30 30 30 5
g-C3N4 Graphite Graphite Graphite g-C3N4 Graphite Graphite Diamond Diamond g-C3N4 g-C3N4 g-C3N4 Diamond (+amorphous carbon) Diamond Diamond Diamond g-C3N4 g-C3N4 + diamond Diamond Diamond
Temperature was estimated by the power–temperature relation.
diffraction peaks such as 100 and 101 suggests that the synthesized gC3N4 has a turbostratic structure where individual graphene layers randomly translate to each other and rotate along the stacking direction (i.e. c-axis direction) [26–28]. SEM and TEM observations showed that the obtained g-C3N4 powder is so fine and poorly crystalline that individual crystals cannot be clearly seen even at high magnification (Fig. 3). The SAED pattern collected from an area of 1.2 μm diameter shows a faint but full diffraction ring which can be indexed as the 002 reflection of g-C3N4, indicating that the material consists of extremely small crystallites which are randomly oriented. The C/N atomic ratio of our starting material was determined to be 0.73, which is in good agreement with the C3N4 stoichiometry, as shown in Table 2. EDS analysis also revealed the presence of a nonnegligible amount of O in the starting sample, as is the case of the earlier studies using the same or similar starting materials [6,15,16,19]. Taking into account the poorly-crystallized, nano-structural feature of the synthetic g-C3N4, a considerable amount of O and/or OH might be trapped on the surface of the individual nano-particles, where dangling bonds are assumed to be dominant. Although the starting material was kept in a vacuum oven and heated at 110 °C for more than 2 weeks before use, it was not effective to prevent the contamination of these elements. This implies that these light elements are not simply surface-adsorbed, but are bound directly to the dangling bonds of the g-C3N4 particles. Another possibility to explain the oxygen contamination is the presence of C3N3 voids in the starting g-C3N4, which could be fully occupied by water molecules, as recently noted by Foy et al.
Fig. 1. A schematic illustration of the cell assembly used for the present multi-anvil experiments.
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Table 2 List of the chemical compositions of the recovered products. The compositions were obtained by averaging 20 measurements at randomly selected points on the polished surface of each sample. The standard deviation is given in bracket.
Fig. 2. XRD pattern (Cu Kα = 1.5418 Å) of the synthetic g-C3N4 used as the starting material.
[29]. The slight excess of nitrogen (C/N = 0.73) might be explained by the presence of the C3N3 voids. Whatever the case, the contamination of these elements may be inevitable, as long as using such poorly crystalline, nano-sized g-C3N4 particles. The trace amounts of Na and Cl are derived from the residues of the side product (NaCl) of the g-C3N4 synthesis [10].
3. Results and discussion Fig. 4(a) shows the variation of the X-ray diffraction patterns collected from the recovered samples, which were heated to different temperatures at 10 GPa. Compared with the starting sample (Fig. 2), the sample heated to 600 °C shows no change in the diffraction pattern, the broad peak of 002 reflection of g-C3N4 was only detected, apart from several smaller peaks derived from the surrounding Pt capsule. The samples heated above 800 °C also show a broad intense peak at 2θ = 26.5°, whose position is, however, slightly shifted toward the low-angle side. The d-value of the peak is calculated to be 3.33– 3.38 Å, which is comparable to the d (002) spacing of graphite rather than of g-C3N4. These observations suggest that the thermal decomposition of g-C3N4 to graphite (C) and nitrogen-containing phase(s) starts above 600 °C at this pressure condition. It is interesting to note that the observed graphite 002 peak is considerably broad (even broader than those of the original g-C3N4) when the sample was heated at 800 °C, but becomes sharper and more intense with increase in the heating temperature. This means that graphite first formed through the decomposition of g-C3N4 is poorly developed with dis-
Sample conditions
Chemical compositions (at.%)
Starting material 10 GPa, 600 °C 10 GPa, 800 °C 10 GPa, 1000 °C 15 GPa, 700 °C 15 GPa, 1000 °C 15 GPa, 1400 °C 15 GPa, 1600 °C 22 GPa, 800 °C 22 GPa, 900 °C 22 GPa, 1000 °C 22 GPa, 1400 °C 22 GPa, 1600 °C
34.8 37.6 93.0 93.7 39.6 95.0 95.8 96.1 40.0 47.3 96.9 99.1 99.9
C (9) (18) (39) (17) (13) (5) (10) (6) (20) (36) (20) (15) (1)
N
O
Na
Cl
47.8 (9) 44.6 (15) – – 41.6 (13) – – – 46.4 (22) 38.4 (48) – – –
16.8 (5) 16.9 (18) 3.7 (17) 4.4 (11) 17.1 (5) 3.7 (6) 2.6 (10) 3.7 (6) 13.1 (8) – 1.6 (4) – –
0.2 (0) 0.5 (1) 3.1 (14) 1.2 (4) 0.2 (1) 0.4 (0) 0.5 (4) – 0.2 (0) 14.0 (5) 1.2 (2) 0.4 (0) –
0.1 (0) 0.3 (1) 0.3 (1) 0.1 (1) 0.7 (3) 0.4 (1) 0.6 (5) – 0.3 (1) 0.3 (1) 0.4 (1) 0.5 (1) –
ordered nano-particles, which are, however, readily crystallized upon annealing at higher temperature. The XRD patterns of the samples heated to 700–1600 °C at 15 GPa are presented in Fig. 4(b). g-C3N4 remained stable after heating at 700 °C, but decomposed to form graphite after heating at 1000 °C, as can be seen from the low-angle shift of the 002 peak. Upon heating above 1400 °C, sharp intense peaks of cubic diamond appeared, and no graphite peaks were observed, suggesting that g-C3N4 has decomposed to directly produce diamond at this P–T condition. In the experiments conducted at 22 GPa, the 002 peak of g-C3N4 maintains its intensity and sharpness up to 900 °C and shows no signs of peak shifting (graphitization does not occur), as shown in Fig. 4(c). Above 1000 °C, the 002 peak of g-C3N4 completely disappeared and was replaced by diamond peaks. The observed diamond peaks are considerably broad when they first appeared at 1000 °C, but become sharper and more intense with increasing heating temperature probably due to the grain growth. At this pressure condition, the formation of pure carbon graphite was not observed in the studied temperature range. Fig. 4(d) shows the XRD patterns of the samples heated to 600– 2000 °C at 25 GPa. g-C3N4 remained stable up to 600 °C, as indicated by the diffraction peak from the 002 basal plane, and showed no signs of graphitization. With increasing temperature, the g-C3N4 peak was gradually replaced by broad peaks of diamond and eventually disappeared completely at 1000 °C. This indicates that the formation of diamond occurs directly through g-C3N4 decomposition without its graphitization as is the case at 22 GPa. Fig. 5(a) and (b) shows Raman spectra collected from the recovered samples which were heated to different temperatures at 10 GPa and 22 GPa, respectively. The starting material (g-C3N4) produces no visible band in the spectra (Fig. 5(a)) due to its strong fluorescence and
Fig. 3. SEM (a) and TEM (b) images of the synthetic g-C3N4. The inset of (b) is an SAED pattern showing a broad Debye–Scherrer ring from the 002 reflection.
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Fig. 4. The variation of X-ray diffraction patterns collected from the samples heated to different temperatures at 10 GPa (a), 15 GPa (b), 22 GPa (c) and 25 GPa (d), respectively. The peaks marked Pt are from the surrounding platinum capsule.
Fig. 5. Raman spectra of selected samples recovered from 10 GPa (a) and 22 GPa (b) and various high temperatures.
photoluminescence induced by visible (Ar 514.5 nm) laser excitation [30]. In the experiments at 10 GPa, the sample heated to 600 °C shows no peaks as is the case for the starting material, while those heated to 800 °C and 1000 °C show two intense peaks at 1350 and 1580 cm− 1, which can be assigned respectively to the D-band and G-band of (disordered) graphite (Fig. 5(a)). It is thus suggested that at 10 GPa the decomposition of g-C3N4 occurs at temperatures between 600 and 800 °C, showing good agreement with the XRD results. When the sample was heated to 1600 °C, the two Raman peaks of graphite became noticeably sharp and the D-band intensity (relative to G-band) dropped significantly, indicating considerable grain growth and an increase in structural order of the produced graphite crystals. In the experiments at 22 GPa, no peaks were visible below 900 °C, while a broad peak at 1331 cm− 1 attributed to nanocrystalline diamond was observed when heated above 1400 °C (Fig. 5(b)). The sample heated to 1000 °C shows continuous broad bands ranging at 1400–1600 cm− 1, so the Raman peak from the diamond was not clearly recognized due to signal overlap, despite that the formation of nanocrystalline diamond was certainly confirmed by XRD (Fig. 4(c)). The detection of the broad bands implies the presence of minor amounts of disordered (amorphous) carbon species such as diamond-like carbon [31] in the recovered sample. We further examined the microtexture and the chemical composition of the recovered products by FE-SEM/EDS and TEM. Fig. 6 shows representative SEM images of samples recovered from different temperatures
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Fig. 6. SEM (secondary electron) images of samples recovered from various P–T conditions. (a) g-C3N4 recovered from 15 GPa and 700 °C, (b) graphite from 15 GPa and 1000 °C, (c) euhedral diamond crystals synthesized respectively at 1400 °C at 15 GPa, and the diamond samples recovered from 22 GPa and 1000 °C (d), 1400 °C (e), and 1600 °C (f).
at 15 and 22 GPa and Table 2 summarizes the results of the elemental analysis. The sample recovered from 15 GPa and 700 °C, which was found to consist purely of g-C3N4 by XRD and Raman analyses, exhibits a massive texture where individual grains are not clearly recognized even at high magnification (Fig. 6(a)), as is the case of the starting material (Fig. 3(a)). EDS analysis revealed that the sample consists mainly of nitrogen and carbon, although the observed C/N ratio (0.95) is somewhat higher than that obtained for the starting material (0.73). It is also found to contain oxygen in an equivalent amount to that detected in the starting sample (Table 2). The samples heated to 1000 and 1400 °C at 15 GPa consist of an aggregate of fine particles of 50–100 nm (Fig. 6(b) and (c)). They are composed of mostly carbon and a small amount (b4 atom%) of oxygen and contain virtually no nitrogen (Table 2), which is consistent with the observed XRD and Raman data characterized by the decomposition to pure carbon phases (graphite and diamond, respectively) plus nitrogen. TEM observation of the latter sample revealed that the individual grains are euhedral crystals (50–100 nm) of diamond, which are loosely aggregated but are not tightly bound to each other (Fig. 7(a)). The samples recovered from the 22 GPa runs also exhibit similar textural features: both graphite and diamond as decomposition products of g-C3N4 consist of relatively loose aggregate of nanocrystalline particles (Fig. 6(d–f)). Upon temperature increase, significant grain growth was observed in diamond; grain (crystallite) size ranges from an almost amorphous-like state (~1 nm, Fig. 7(c)) at 1000 °C to half of a micron (Fig. 6(f)) at 1600 °C.
It should be noted that all the diamond samples obtained in this study are not sintered at all, but are in the form of white powder. SEM and TEM observations showed that the individual diamond grains are discrete euhedral crystals and are not tightly bound to each other (Fig. 6(c), (e), and (f); Fig. 7(a) and (d)). These morphological features imply that they have crystallized in a free space such as a fluid medium. It is naturally expected that the decomposition of gC3N4 produces nitrogen or nitrogen-bearing compound(s) as well as diamond (and/or graphite) as pure carbon phase(s). However, the EDS analysis of those decomposition products revealed the absence of nitrogen-bearing phases, although a trace amount of nitrogen may be included in diamond/graphite grains or present as an amorphous phase at their grain boundary. This discrepancy can be explained by the following scenario: the decomposition of g-C3N4 also involves the formation of pure nitrogen, which initially exists in the sample as a liquid phase under the studied P–T conditions [32,33], but is eventually released from the sample capsule in the form of gas when the capsule is opened after decompression. Therefore, the diamond formation through thermal decomposition of g-C3N4 is considered to have occurred in the liquid nitrogen medium. This is likely the reason why the obtained diamond samples are all in the form of a loose powder of euhedral crystals unlike the products from the direct conversion of pure carbon graphite and related materials which are characterized to have well-sintered nanocrystalline textures and high optical transparency [34].
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Fig. 7. TEM images and SAED patterns of different diamond crystals recovered from various P–T conditions. (a) 1400 °C at 15 GPa, (b) 1000 °C at 25 GPa, (c) 1000 °C at 22 GPa, and (d) 1600 °C at 22 GPa, respectively.
Considering all the data obtained through XRD, Raman and SEMTEM analyses, we propose a decomposition diagram of g-C3N4 in the pressure and temperature range up to 25 GPa and 2000 °C, respectively (Fig. 8). It is demonstrated by the present investigation that g-C3N4 decomposes to graphite (and probably nitrogen) at temperatures above 600–700 °C and at a relatively low pressure range (10– 15 GPa), while it decomposes to diamond directly (plus nitrogen) above 800–900 °C at a higher pressure range (22–25 GPa). The formation of diamond from g-C3N4 also occurs at 15 GPa, when the heating temperature exceeds 1400 °C. The estimated decomposition curve of g-C3N4 has a positive slope (~ 0.05 GPa/K) at pressures up to ~ 22 GPa, but becomes inverted above this pressure. It seems that gC3N4 remains stable after heating at 600–900 °C for 30–60 min in the pressure range studied (~ 25 GPa). However, the observed increase in C/N ratio (i.e. decrease in nitrogen content) of those recovered pro-
Fig. 8. P–T diagram showing the stability of g-C3N4. The solid black line indicates the decomposition boundary for g-C3N4, determined from the results of our quench experiments (not a thermodynamic boundary). The broken line indicates the reaction (kinetic) boundary between graphite and diamond observed in the present study.
ducts compared with that of the starting sample suggests that the decomposition reaction is kinetically slow and might proceed slowly even at lower temperatures (below the threshold line). It is also interesting to note that the decomposition boundary of g-C3N4 estimated from our study is located at lower temperature than that reported for turbostratic CN [25], particularly in the higher pressure range. There seem to be no signs of a phase transition of g-C3N4 in the studied P–T range, even at the highest pressure of 25 GPa, at which pressure a cubic C3N4 phase was claimed to be obtained as a quench product [23,24]. In our study, g-C3N4 was observed to be beginning to decompose at relatively mild temperatures (N800 °C), despite using a starting sample of g-C3N4 prepared by the same procedure and employed the same kind of capsule (Pt) as those used in the earlier studies. However, it seems unlikely that g-C3N4, which starts to decompose at 800 °C, transforms to another phase even if taking it to higher temperature conditions. The high pressure phase and its quenched form (cubic phase) of C3N4 reported in earlier studies [23,24] may be metastable products, as they are also not observed in recent LHDAC experiments [35]. In the present study, we observed the formation of cubic diamond as a decomposition product of g-C3N4 at temperatures more than 1400, 1000 and 800 °C and at pressures of 15, 22 and 25 GPa, respectively (Table 1 and Fig. 8). Interestingly, these temperature conditions are significantly lower than those required for diamond formation by the direct conversion of pure carbon graphite and related carbon materials under similar pressure conditions. For example, the minimum temperature necessary for cubic diamond formation from graphite and even from non-graphitic carbons (which are more dominated by sp3 bonding) was reported to be 1500–1600 °C in the pressure range of 15–21 GPa [36]. Moreover, to obtain pure, single-phase cubic diamond without unreacted residues high temperatures of more than 2200 and 1600 °C are necessary, respectively, for graphite and non-graphitic starting materials at those pressures [35,36]. The lower threshold temperatures for diamond formation from g-C3N4 may be attributed to the physicochemical nature of the carbon nitride itself. Klyui et al. [37] studied the diamond formation from both nitrogen-containing (amorphous CNx) and nitrogen-free (amorphous carbon) precursors and found that at the condition of 7.7 GPa and 2000 °C crystalline
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diamond was obtained only from the former precursor. They suggested the following scenario to explain why the diamond formation from carbon nitride can occur under milder conditions. The decomposition of carbon nitride involves the breakdown of C\N bonds and the formation of N2 molecules, which consequently induces the generation of highly active dangling bonds of carbon. These dangling bonds can be preferentially reconstructed under high pressure into sp3-coordinated C\C bonds and provide preferential sites for diamond nucleation. This, in turn, means that the overall activation energy required for diamond nucleation (formation) is significantly lowered through the decomposition of carbon nitride. This model also seems applicable to the present case. In addition, the presence of liquid nitrogen as a decomposition product of g-C3N4 may also play an important role in enhancing the grain growth rate of diamond. In this study, the diamond crystals formed at 1400–1600 °C at 15 and 22 GPa were observed to be ~ 500 nm or even more in size, which is more than two times larger than those synthesized from pure carbon precursors at even higher temperatures of ~2500 °C in a similar pressure range [36]. This may be because the diffusion rate of carbon atoms is faster in the liquid nitrogen regime than in solid, although further study is needed for more detailed and quantitative discussion. Alternatively, the lower threshold for diamond formation and the fast grain growth rate might be due to the presence of water (dissociated or not) or hydrogen molecules, which are likely adsorbed on the surface or trapped in C3N3 voids [29] in the initial g-C3N4, as previously suggested by several studies [38]. In this case, hydrogen and/or hydroxyl may play a catalytic role in promoting the diamond crystallization. 4. Conclusions
proofreading. This work was supported by Global COE program “Center for Advanced Experimental and Theoretical Deep Earth Mineralogy” and also partly by a Grant-in-Aid for Young Scientists (B) (#20740255) to HO from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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The stability of graphitic C3N4 under high pressure and high temperature was studied up to 25 GPa and 2000 °C on the basis of quench experiments using a multi-anvil apparatus. The result demonstrates that g-C3N4 remains stable at relatively mild temperatures within the experimental time scale (N30 min), but decomposes to C and N upon heating above 600 and 900 °C, respectively, at 10 and 25 GPa. The decomposition products consist only of pure carbon phases, either graphite or diamond depending on the P–T conditions, and do not contain any nitrogen phases. It is likely that nitrogen had existed as liquid during the high P–T treatments, but was released from the sample chamber in the form of gas upon pressure release. No phase transition of g-C3N4 was observed at P–T conditions up to 25 GPa and 2000 °C, and the theoretically predicted, low-compressible superhard C3N4 phases cannot be obtained. Further studies, for example at higher pressures and/or using well-crystalline g-C3N4 with a stoichiometric composition as a starting material, are needed to investigate whether such superhard C3N4 can really be synthesized in laboratory. Acknowledgments The authors are grateful to L. Lei, N. Nishiyama and T. Inoue for useful advice on the experimental works and also to Y. Kojima for assistance in preparing the starting material. We also thank S. Whitaker for English
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