In situ doping and synthesis of two-dimensional nanomaterials using mechano-chemistry

Abstract

Doping foreign atoms into materials can modify their electronic configuration and tune their chemical and physical properties. Developing an efficient doping strategy is thus critically important for many new applications of two-dimensional (2D) nanomaterials. Here, we report an in situ process to produce and simultaneously dope carbon and nitrogen in a number of 2D materials including graphene, BN, MoS₂ and WS₂ using mechanochemistry. This new process produces large quantities of 2D materials with controlled doping contents and new properties.

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Conceptual insights

The new concept is a novel method that combines both doping and mechanical exfoliation processes to produce doped nanosheets in one step. The new chemistry behind this new in situ process is that the dopant gases (NH₃ or C₂H₄) also act as an exfoliation agent for the production of 2D structures. For the first time, a controlled mechano-chemical process is reported for the production of doped 2D materials, while most common doping methods are post-synthesis treatments. The advantages of the new method include large quantity production, controlled dopant contents and reliable process. Mechano-chemical reactions between materials and dopant gases were generated by ball milling at room temperature. The doped nanosheets have negative zeta potential and can be dispersed in water to form stable solutions.

Introduction

Two-dimensional (2D) nanomaterials have many new chemical, mechanical, electrical and thermal properties, and thus various exciting applications.^1–9 Doping of foreign atoms in them can modify the electronic configuration and further tune the chemical and physical properties of the materials.^10–12 For example, nitrogen (N) doped graphene is endowed with various enhanced properties; for example, the spin density and charge distribution of carbon atoms will be influenced, which is vital for catalytic reactions.¹³ Carbon (C) doped boron nitride (BN) has been used as a metal-free catalyst for enhanced oxygen reduction reactions (ORRs).¹⁴ The catalytic activity of molybdenum disulphide (MoS₂) nanosheets has been improved by C doping,¹⁵ while N doping enhanced the electrochemical performance and cycling stability.¹⁶ In addition, N doping of tungsten disulphide (WS₂) nanosheets can improve the intrinsic conductivity, resulting in enhanced catalytical performance in the hydrogen evolution reaction (HER).¹⁷ The common doping methods include arc discharge,¹⁰ chemical vapour deposition,¹⁸ thermal annealing,¹⁹ hydrothermal methods,²⁰ and sol–gel methods.¹⁷ However, these methods are post-synthesis, multistep processes that often require high-temperature calcination¹⁶ and plasma treatment,²¹ and can damage the material structure. In addition, it is difficult to control the doping content. Here, we demonstrate an in situ process that combines mechanical exfoliation and chemical doping processes together. Large-quantities of nanosheets with controlled doping contents can be produced via a mechano-chemical process at ambient temperature.

Experimental

Several grams of materials (graphite, MoS₂, WS₂, or BN) were loaded with several hardened steel balls of a diameter of 25 mm in a sealed milling container, which was evacuated and purged with argon several times, before filling with a selected gas (anhydrous ammonia (NH₃) or ethylene (C₂H₄)) at 300 kPa. Ball milling experiments were performed in a rotating high-energy ball mill as described in our previous report.²² The crystalline structure and phases of the samples were investigated by X-ray diffraction (XRD) using a PANalytical X’pert powder diffractometer (Cu K-alpha radiation, λ = 0.15418 nm) operated at 40 kV with 30 mA current. XRD data were recorded over a range of 10°–70° with a step time and size of 150 s and 0.02, respectively. The morphology of the milled samples was examined by scanning electron microscopy (SEM), using a Hitachi S4500 Zeiss Supra 55VP microscope operated at 3 to 10 kV. Elemental composition was measured by energy-dispersive X-ray spectroscopy (EDS) on cold-pressed pellets. Transmission electron microscopy (TEM) analysis was performed using a JEOL JEM 2100F instrument with an acceleration voltage of 200 kV and equipped with a Gatan Quantum ER 965 Imaging Filter. X-ray photoelectron spectra (XPS) were acquired on a Kratos AXIS Nova with an Al Kα X-ray source. Fourier transform infrared spectroscopy (FTIR) was carried out using a Bruker Vertex 70 infrared spectrometer in attenuated total reflection mode. The zeta potential of dispersed samples was measured by using a Malvern Zetasizer Nano ZS.

Results and discussion

Fig. 1a shows the XRD patterns of the graphite milled in ammonia (NH₃) for different time intervals; as the milling time increases, the (002) peak becomes wider and weak, indicating the exfoliation of graphite particles. The SEM image, Fig. 1b, shows that the graphite milled for 20 h in NH₃ transforms from large particles to thin layers/sheets. We found 2.6 wt% of nitrogen in the graphene sheets after milling for 70 h in our previous work.²² Pressure reduction of the NH₃ gas during the milling process was observed, which is due to the formation of N–C bonds as previously revealed by using near-edge X-ray absorption fine structures (NEXAFS) spectroscopy,²² leading to a gradual increase in N content in the milled samples and allowing the control of the doping content. The high-resolution TEM images (Fig. 1c and d) show a good crystallinity of individual nanosheets. The inserted fast Fourier transform (FFT) spectrum contains multiple sets of dots with a sixfold symmetry revealing an undamaged in-plane structure of the nanosheets. The XRD, SEM, and TEM results show that bulk graphite has been exfoliated to thin nanosheets during the milling process in a NH₃ atmosphere.

Fig. 1 XRD patterns of graphite milled in NH₃ (a) for different time intervals. SEM images of the graphite sample milled for 20 h in NH₃ (b); TEM images of graphite samples after 20 h milling in NH₃ (c and d).

Hexagonal boron nitride particles have been milled in ethylene (C₂H₄) over different time intervals to dope C into BN nanosheets. From the XRD patterns (Fig. 2a), it is clear that the structure remains the same after 30 h of milling. The SEM image (Fig. 2b) shows the layered structure of thin sheets. XPS spectra (Fig. 2c) of BN milled for 30 h in C₂H₄ show 32.4 at% of carbon present, and show C–N bonds at 285.5 eV and N=C–O at 288.8 eV;²³ the initial carbon content in BN is shown in Fig. S1 (ESI†). Energy-filtered transmission electron microscopy (EFTEM) images with overlay mapping (Fig. 2d and e) show a homogeneous distribution of boron and carbon, which is confirmed by the electron energy loss spectroscopy (EELS) spectra displayed in Fig. 2f. The B-K edge at 188 eV, C-K edge at 283.8 eV, and N-K edge at 401.6 eV can be seen clearly.

Fig. 2 XRD patterns of BN milled in C₂H₄ (a) for different time intervals, (b) SEM image of the BN sample milled for 30 h, (c) XPS spectra of C 1s for BN after 30 h of milling, (d) EFTEM image, (e) elemental mapping, and (f) EELS spectra of BN after 30 h milling.

Other layered materials from the transition metal dichalcogenide family such as molybdenum disulphide (MoS₂) have been milled in hydrocarbon gas ethylene (C₂H₄) or ammonia (NH₃) for C and N doping. The XRD patterns in Fig. 3a and e show that MoS₂ retains its structure after 100 h of milling in both gasses. SEM images show clearly a layered structure of the MoS₂ after milling in NH₃ for 100 h (Fig. 3b) and in C₂H₄ gas for 15 h (Fig. 3f). HR TEM images (Fig. 3c and g) show nice crystallinity of the MoS₂ milled in NH₃ for 15 h and in C₂H₄, respectively. As the milling time increases the pressure of the gas decreases, and the carbon and nitrogen content increases in the milled samples, Fig. 3d and h. The carbon content is around 12.7 at% in the sample milled in C₂H₄ for 70 h. Similar behaviour was also observed in the samples milled in NH₃, whereas the nitrogen content is 9.7 at% for the sample milled in ammonia for 70 h. The EELS spectrum shown in Fig. 4a clearly shows the doping of nitrogen in MoS₂, with the peaks for the Mo-M edge at 227 eV, N-K edge 401 eV, and S-L edge at 164.8 eV confirming the presence of nitrogen. The C-K edge at 283.8 eV shows the presence of carbon, which is most likely due to environmental contamination. XPS spectral (Fig. 4b) results also prove the nitrogen doping in MoS₂. The sample milled in NH₃ for 20 h has been selected for XPS measurement, and curve fitting and deconvolution shows that the peaks at 397.6 eV and 399.1 eV corresponds to Mo–N,¹⁶ and the peak at 401.4 eV corresponds to –NO^16,24 adsorbed to the surface of MoS₂, in the Mo 3p_3/2 and N 1s spectra. Thus, the XPS and EELS results confirm the doping of nitrogen in MoS₂. The XPS spectra of MoS₂ milled in C₂H₄ for 50 h are displayed in Fig. 4c and d. Mo–C bonds are found from the C 1s spectrum at 283.8 eV and also in the Mo 3d–S 2s spectrum at 228.2 eV and 230.5 eV.²⁵

Fig. 3 XRD patterns of MoS₂ milled in NH₃ (a), and C₂H₄ (e). SEM images of the MoS₂ sample milled for 100 h in NH₃ (b), and in C₂H₄ for 15 h (f); TEM images of MoS₂ samples after 15 h milling in NH₃ and in C₂H₄ for 15 h (c and g); variation of gas pressure, nitrogen and carbon content in the milled samples as function of milling time (d and h).

Fig. 4 EELS spectra of MoS₂ (a); XPS spectra of the N 1s and Mo 3p region of MoS₂ milled in NH₃ for 20 h (b); XPS spectra of MoS₂ milled in C₂H₄ for 50 h (c) Mo 3d + S 2s spectra, and (d) C 1s spectra.

To check if the same approach would work for other transitional metal chalcogenides, tungsten disulphide (WS₂) was milled in ethylene (C₂H₄) and ammonia (NH₃) for different intervals. The XRD patterns (Fig. 5a and d) show the same XRD patterns for all milled samples but the diffraction peaks become broader and weaker, which suggests that the layered structure remains the same at the end of 100 h of milling. The SEM images (Fig. 5b and e) show the remaining layered structure of WS₂ nanosheets milled in C₂H₄ (Fig. 5e) and NH₃ (Fig. 5b) for 15 hours. The TEM image in Fig. 5f confirms the layered structure in the WS₂ after milling for 30 h in C₂H₄. The SAED patterns in Fig. 5c show a good crystalline structure and the (105), (110), and (103) rings confirm a well-retained crystalline structure even after milling for 100 h in NH₃. As the milling time increases, the pressure decreases, and the carbon and nitrogen content increases (Fig. S2a and b, ESI†) in the samples milled in ethylene and ammonia gas respectively. The carbon content is around 21.1 at% in the sample milled in C₂H₄ for 100 h, whereas the nitrogen content is about 7.8 at% for the sample milled in NH₃ for 100 h. The FTIR spectra in Fig. 5g reveal the C–H bonds at 2847 cm⁻¹, 2915 cm⁻¹ and 2960 cm⁻¹, and N–H bonds at 1409 cm⁻¹ and 3183 cm⁻¹ in the WS₂ nanosheets milled in C₂H₄ and NH₃ respectively.²⁶ Carbon and sulphur overlay mapping (Fig. S3, ESI†) obtained by using EFTEM, confirms the uniform distribution of carbon in WS₂ milled in C₂H₄ for 20 h.

Fig. 5 Structural changes of WS₂ during ball milling in NH₃ and C₂H₄. XRD patterns of WS₂ milled in NH₃ (a), and C₂H₄ (d). SEM images of the WS₂ sample milled for 15 h in NH₃ (b), and C₂H₄ (e); TEM images of WS₂ samples after 100 h milling in NH₃ (c), and in C₂H₄ for 30 h (f). (g) FTIR spectra of bulk WS₂ and WS₂ milled in C₂H₄ and NH₃ for 30 h. The enlarged picture shows clearly the peaks of the N–H and C–H bonds.

The influence of carbon or nitrogen doping on the nanosheets was investigated using the zeta potential method. 6 mg of nanosheets and starting materials (before milling) were dispersed in 20 mL of DI water and bath-sonicated for 1 h and then the zeta potential was measured. The colloidal dispersion of doped nanosheets and bulk MoS₂ and WS₂ is shown in Fig. 6a inset.

Fig. 6 Zeta potential of samples milled in C₂H₄ and NH₃: MoS₂ (a) and WS₂ (b).

It is clearly visible that, bulk MoS₂ and WS₂ settled at the bottom of the container, but at the same time, the doped nanosheets were well dispersed in water. As shown in Fig. 6, the stability of the dispersions increases with the doping amount of foreign element. The zeta potential is −34.3 mV for MoS₂ milled in NH₃ for 100 h (N doping 9.7 at%), whereas it is −34.1 mV for the sample milled in C₂H₄ (C doping 12.7 at%), in comparison with that of the starting MoS₂ of 0.11 mV. Similar results were observed for WS₂, where the starting WS₂ has the zeta potential of 0.31 mV, which is decreased to −34.3 mV for the sample milled in C₂H₄ (C doping 21 at%) and −37 mV for the sample milled in NH₃(C doping 7.8 at%). Generally, a solution with a zeta potential more positive than +30 mV or more negative than −30 mV is considered as a stable dispersion due to interparticle electrostatic repulsion.²⁷ These negative potentials suggest that carbon and nitrogen doping develops negative charges on the nanosheet surfaces and edges, which acts as anionic surfactants, and develop electrostatic repulsion between individual nanosheets and help to disperse them in water and maintain the stability of the dispersion. Thus, nitrogen and carbon doped nanosheets are well dispersed and stable in water, without the help of any surfactant.

Under high-energy impacts, C₂H₄ and NH₃ dissociated and chemisorbed onto the defects and edges of nanosheets, which avoids cross-linking of the nanosheets. Significant pressure drops of NH₃ and C₂H₄ gasses were observed inside the sealed milling chamber during the entire milling process. The pressure reduction could be explained by gas absorption onto the newly created surfaces of the milled materials by high-energy impacts²⁸ which is confirmed by the nitrogen and carbon presence in the milled samples, with increasing contents. Other gases such as CH₄, C₃H₈ and N₂ can also be used as possible doping gases. High milling intensity creates more fresh surface area, active edges and defects in the milled samples, and provides more sites for gas adsorption, decomposition and chemical bond formation. The doped content can be increased by using higher milling intensity/energy or a high-speed milling system with a high impact frequency. This new in situ doping process ensures homogeneous distribution of the dopant in the materials, and a simple and economical method to produce doped nanosheets in large quantities.

Conclusions

We have developed a new in situ method for the production of nanosheets with controlled doping contents, including nitrogen doping in graphene, carbon doping in BN nanosheets, and carbon and nitrogen in MoS₂ and WS₂ nanosheets via a mechanochemical process with the usage of either C₂H₄ or NH₃ gas. This method produces large quantities of doped nanosheets for various applications. The doped nanosheets have negative zeta potential and can be dispersed in water to form stable solutions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Y. C., and L. H. L., acknowledge financial support from the Australian Research Council under the Discovery Programs (DP150102346). Q. M. and Y. C. thank the Australia Research Council for a Linkage Project (LP140100608). The authors also acknowledge Deakin University's Advanced Characterisation Facility for the use of electron microscopy facilities. Experimental assistance from Mr Yuchen Liu is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nh00369f

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