Xiaowei Mua,
Bihe Yuanb,
Xiaming Fenga,
Shuilai Qiua,
Lei Song*a and
Yuan Hu*a
aState Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China. E-mail: leisong@ustc.edu.cn; yuanhu@ustc.edu.cn; Fax: +86-551-63601664; Tel: +86-551-63601664
bSchool of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
First published on 28th October 2016
The doping of nitrogen into reduced graphene oxide (NRGO) is achieved by reduction of graphene oxide with hydrazine hydrate and ammonium hydroxide. Boron (BRGO) or phosphorus (PRGO) doped reduced graphene oxide (RGO) is obtained by annealing of RGO (prepared by reduction with sodium borohydride) with boric acid or phosphoric acid, respectively. The successful preparation of the doped RGO is confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. A remarkable enhancement in thermal oxidative stability of RGO is achieved by doping of these heteroatoms. The enthalpy values of BRGO and PRGO during the thermal oxidation decrease remarkably compared with that of RGO, indicating the reduced heat release by the doped heteroatoms. The mechanism for improvement in thermal oxidative resistance by doping of heteroatoms is demonstrated clearly. Doping of boron atom not only lowers the electrons density on the reactive carbon atoms and the Fermi level of carbon, but also contribute to graphitization of RGO, leading to inhibition of thermal oxidation of RGO. Phosphorus containing complexes, in the forms of metaphosphates, C–O–PO3 and C–PO3 groups, can poison the active sites on RGO and function as physical barrier for the access of oxygen. Retarding oxidation of RGO against air may be strengthened by forming more thermostable structure in NRGO, for example: pyrrolic-N (NC2), pyridinic-N (NC2) and graphitic-N (NC3). The doping of heteroatoms will provide an important strategy for broadening RGO application at the elevated temperature.
Graphene, one-atom-thick sp2 carbon network, is widely investigated around the world. Generally, the thermal oxidative resistance of graphite or perfect graphene is better than that of reduced graphene oxide (RGO) prepared by chemical methods, due to the defects, vacancies and functional groups in the RGO.11 Inspired by the fact that oxidation inhibition of carbon materials is enhanced significantly by doping of heteroatoms, it is logical to expect that thermal oxidative stability of RGO can be strengthened in the same way. There have been a large amount of articles concerning doped graphene published so far.3,12–17 However, there is few report about inhibiting thermal oxidation of graphene by doping heteroatoms. To verify the assumption, we have prepared boron/nitrogen and phosphorus/nitrogen co-doped RGO. In comparison with neat RGO, the temperature at maximum weight loss rate of boron/nitrogen and phosphorus/nitrogen co-doped RGO increase by as much as 83 °C and 161 °C, respectively.17 The RGO in the previous work was prepared by hydrazine reduction and nitrogen atoms were inevitably introduced. Thus, it is hard to evaluate the contribution of the single doped heteroatom to thermal oxidative stability of RGO.
In this study, RGO was synthesized by reduction of GO with sodium borohydride (NaBH4), thus, the nitrogen atoms are absent in the RGO obtained. Then, boron and phosphorus were introduced by thermal annealing. Their thermal oxidative stability was compared with that of RGO prepared by hydrazine. The influence of the single kind of doped heteroatom on the thermal oxidative stability of the doped RGO was investigated. Furthermore, the mechanism for enhanced thermal oxidative resistance of RGO by doping heteroatoms was demonstrated further.
Crystal structure of the samples is characterized with XRD and the results are shown in Fig. 2. The interlayer spacing of GO calculated from the characteristic (002) diffraction peak at 11.58° is 0.746 nm. The diffraction peak centered at approximately 26.1° is showed in the pattern of RGO, confirming that GO was reduced successfully.30,31 The XRD patterns of the doped RGO are similar to that of RGO, indicating that the doping has negligible influence on the crystal structure of RGO.
The graphitization degree and defect density of the samples are investigated by Raman spectra. The peaks at 1355 cm−1 and 1398 cm−1 for GO are corresponding to D and G band, respectively. The intensity ratio of D to G band (ID/IG) is usually employed to measure of specific value of sp3-hybridized carbon atoms in the disordered graphite layer compared to sp2-hybridized carbon atoms in graphite layer.32 As shown in Fig. 3, the ID/IG value of BRGO is lower than those of NRGO and PRGO, which may be explained by the fact that boron can catalyze the graphitization of RGO.33 The ID/IG values of NRGO and PRGO are higher than that of RGO. This may be explained that the defect degree of RGO is changed along with doping heteroatoms.34 Besides, red-shift or blue-shift of D and G bands are observed in the Raman spectra of the doped RGO compared with that of RGO, because of redistribution electron density of RGO by the heteroatoms.30,35
Chemical composition and bond state of the samples are investigated by XPS spectra, as shown in Fig. 4. The peaks of nitrogen, boron and phosphorus are observed, confirming the successful preparation of doped RGO. The detailed XPS data are presented in Table 1. The atomic percentage of the doped heteroatoms in NRGO, BRGO and PRGO is 2.29 at%, 1.98 at% and 8.27 at%, respectively. The high-resolution XPS spectra of GO, RGO, NRGO, BRGO and PRGO are depicted in Fig. 5–9. The characteristic peaks of C1s in Fig. 5 are assigned to C–C (284.8 eV), C–OH (285.5), C–O–C (287.4 eV), CO (288.5 eV) and π–π* shake-up satellite peak (290.0 eV).36,37 The high-resolution XPS spectra of RGO can be divided into four peaks in Fig. 6, for example, C–C (284.6 eV), C–OH (286.0), C–O–C (287.3 eV) and π–π* shake-up satellite peak (290.8 eV).36,38,39 In Fig. 7a, the C–N peak at 285.2 eV is observed in C1s XPS spectrum of NRGO.40 There are pyridinic-N (398.6 eV), pyrrolic-N (400.2 eV) and graphitic-N (401.9 eV) in N1s XPS spectrum of NRGO in Fig. 6b.41–43 As shown in Fig. 8a, a new peak at 288.2 eV in C1s XPS spectrum of BRGO is corresponding to C–O–B bond.44 In Fig. 8b, three characteristic peaks are shown in B1s XPS spectrum, including BC3 (189.8 eV), BC2O (191.4 eV) and BCO2 (192.9 eV).43 It can be confirmed that C–P (285.2 eV) and C–O–P (286.7 eV) bonds are in C1s XPS spectrum of PRGO in Fig. 9a.35,45,46 The characteristic peaks of P2p are shown in Fig. 9b. The peaks at 132.7, 134.3 and 135.1 eV belong to PO3–C, PO3–O–C and metaphosphates, respectively.47 These results confirmed that the heteroatoms were doped successfully into graphene.
Sample | C (at%) | O (at%) | N (at%) | B (at%) | P (at%) |
---|---|---|---|---|---|
GO | 67.44 | 32.56 | |||
RGO | 92.24 | 7.76 | — | — | — |
NRGO | 92.45 | 5.26 | 2.29 | — | — |
BRGO | 88.49 | 9.53 | — | 1.98 | — |
PRGO | 50.56 | 41.17 | — | — | 8.27 |
TEM images of samples are shown in Fig. 10. There is slight distinction between the doped RGO and RGO. The doped RGO nanosheets are more crumpled than that of RGO and GO, which is ascribed to structure distortion caused by the doped heteroatoms. The bond length and bond angle of C–C are changed when the heteroatoms are incorporated into RGO nanosheets by substituting carbon atoms.35,48
Thermal stability of the samples under air atmosphere is shown by TGA/DSC in Fig. 11. The detailed data are listed in Table 2. The temperature at 5 wt% weight loss (T5 wt%) is named as initial degradation temperature. The main weight loss stage for RGO and doped RGO is assigned to the thermal oxidation. The T5 wt% of NRGO, BRGO and PRGO are increased by 100 °C, 139 °C and 220 °C, respectively, compared with that of RGO. The temperature at maximum weight loss rate (Tmax) of NRGO, BRGO and PRGO is 142 °C, 182 °C and 318 °C higher than that of RGO, respectively. The degradation stage below 200 °C in the TGA curve of PRGO is corresponding to the loss of water in the sample. In addition, the values of maximum mass loss rate of the doped RGO decrease significantly compared with that of RGO. Enthalpy values of samples during thermal oxidation under air atmosphere are characterized by TGA/DSC in Fig. 12 and detailed enthalpy values of the samples are listed in Table 3. The enthalpy values of BRGO and PRGO decrease significantly compared with that of RGO. It is shown that the thermal oxidation of BRGO and PRGO is inhibited. In conclusion, the thermal oxidative stability of graphene is enhanced impressively by the doping of heteroatoms (nitrogen, boron and phosphorus). PRGO shows the best thermal oxidative resistance. Thermal oxidative stability of heteroatom doped RGO is in the order of PRGO > BRGO > NRGO.
Sample | T5 wt% (°C) | Tmax (°C) |
---|---|---|
RGO | 410 | 470 |
NRGO | 510 | 612 |
BRGO | 549 | 652 |
PRGO | 625 | 788 |
Sample | ΔH (J g−1) |
---|---|
RGO | 2644.7 |
NRGO | 2834.5 |
BRGO | 1213.6 |
PRGO | 707.6 |
To study the thermal oxidative resistance of doped RGOs further, the RGO and doped RGOs were calcined at 700 °C in muffle furnace under air atmosphere for 5 min. The gained oxidized products were characterized by FTIR and XPS to figure out the variation in their chemical composition. FTIR spectra of oxidized samples are shown in Fig. 13. It is obvious that no new chemical bond of oxidized products occurs. Thus, the chemical composition of samples after calcination is similar to that of untreated specimens. High-resolution C1s XPS spectra of oxidized RGO and doped RGOs are displayed in Fig. 14. The detailed XPS data of samples are listed in Table 4. In Fig. 14, it can be seen that the content of C–OH and C–O–C band of oxidized samples increases and decreases, respectively. It is referred from Table 4 that heteroatom content of oxidized products decreases except for BRGO. It may be explained that there is a good compatibility between boron and carbon atom due to similar covalent radius between them.49 The ratio (Cox/Ca) between oxidized carbons and unoxidized carbons (aliphatic and aromatic carbons) can function as a standard of thermal oxidative resistance.50 As shown in Table 4, the values of (B − A)/A of treated doped RGOs are smaller than that of oxidized RGO, which means the present of heteroatom inhibits the thermal oxidation of samples. Besides, the Cox/Ca values of BRGO and PRGO after calcination are lower than that before annealing, which can be illuminated that the thermal oxidative resistance of BRGO and PRGO is enhanced after calcining. In conclusion, the thermal oxidative resistance of RGO is enhanced by heteroatom doping.
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Fig. 14 High-resolution C1s XPS spectra of (a) RGO, (b) NRGO, (c) BRGO and (d) PRGO after calcination at 700 °C. |
Sample | C (at%) | O (at%) | N (at%) | B (at%) | P (at%) | A | B | (B − A)/A |
---|---|---|---|---|---|---|---|---|
a A: Cox/Ca value of samples after calcination, B: Cox/Ca value of samples before calcination. | ||||||||
RGO | 88.63 | 11.37 | — | — | — | 0.208 | 0.263 | 26.4% |
NRGO | 90.51 | 7.58 | 1.91 | — | — | 0.274 | 0.327 | 19.3% |
BRGO | 79.13 | 14.18 | — | 6.69 | — | 0.379 | 0.278 | −26.6% |
PRGO | 73.83 | 21.42 | — | — | 4.75 | 0.326 | 0.307 | −5.8% |
The oxidation of RGO occurs at the active sites on nanosheets, like defects, vacancies and functional groups.17,51 In this study, thermal oxidative resistance of RGO is enhanced by two ways. The first is heat treatment. The elimination of functional groups and increased graphitization of RGO are achieved after heat treatment, which will contribute to the enhancement in thermal oxidative stability of doped RGO. Incorporating of heteroatoms into RGO is another more efficient way to enhancing thermal stability, according to TGA results. Boron atom is electron-poor and is inclined to attract the electrons in carbon atoms, resulting in the collapsing of C–C band and the rearrangement of the crystal lattice. Thus, the graphitization degree of RGO is increased.49 Besides, substitutional boron atoms in the RGO result in redistribution of π-electrons in the graphene layer and lowering the Fermi level of carbon, leading to poorer electrons donation ability of carbon.5,33 The ability of the atom to donate electrons in oxidation reaction is regarded as oxidation reactivity of the atom.52 Thus, boron doped RGO is less susceptible to oxidation. Meanwhile, the electrons density on the reactive carbon atoms is reduced by substitution of boron, leading to a suppression effect on oxygen chemisorption.53 As confirmed by Raman results, the order and crystallite dimensions of RGO increase due to the boron-catalyzed graphitization.54 Phosphorus possesses the best inhibition effect on the thermal oxidation of RGO. Excess phosphorus containing deposit, in the forms of metaphosphates, C–O–PO3 and C–PO3 groups, may be dispersed as discrete clusters on the basal plane of RGO.55 A part of deposit forms complexes with the carbon on the active sites of RGO, leading to poisoning of active sites.56 In another word, inhibition oxidation of RGO is achieved through forming of C–O–P bond and it can block active sites on graphene.4,57,58 Some phosphorus-containing products may work as a barrier for access of oxygen to RGO surface. In this study, poisoning of active sites by the formed C–O–P bond may be the main factor affecting the enhancement in thermal oxidative stability of RGO due to no obvious deposit found in the TEM image of PRGO. Pyrrolic-N (NC2), pyridinic-N (NC2) and graphitic-N (NC3) are formed after incorporating nitrogen atoms into RGO. It is confirmed that stability of these nitrogen bonds against combustion are higher than graphene (CC3) fragments.42 Therefore, thermal oxidative stability of nitrogen doped RGO is strengthened. The doped boron and phosphorus atoms have better effect on the thermal oxidative stability of RGO than that of nitrogen.
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