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Abstract
Transient features of graphene oxide (GO), graphene oxide nanoribbon (GOR) and graphene quantum dot (GQD) have been investigated by femtosecond transient absorption (TA) spectroscopy. It is found that for pristine GO and HNO3-/NaOH-treated GO, a hybrid sp2/sp3 state at about 430 nm (∼2.88 eV) always appears. However, this hybrid state becomes less apparent in GOR, and completely disappears in GQD. It indicates that there are absolutely different electronic structures in GO, GOR and GQD. Furthermore, compared with pristine GO and GQD, GOR presents a faster decay for carrier dynamics. This could be attributed to the relatively weak interaction between the sp2 domains and sp3 matrix in GOR, which facilitates the carrier recombination.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) graphene has drawn extensive attention due to its unique physical properties [1–3] and numerous applications [4–10]. However, graphene is a zero band gap material and insoluble, which hinders it from the other potential applications in nanoelectronics, optical and optoelectronics [9–19]. By contrast, graphene oxide (GO), graphene oxide nanoribbon (GOR) and graphene quantum dot (GQD) could overcome the above shortcomings of graphene. Firstly, GO has the tunable bandgap and exhibits broad fluorescence (FL) from visible to near-infrared (IR) after functional modifications [11,15,2021–26]. GO even could be used to fabricate soft robots [26,27] or applied in solar energy cells [28]. Secondly, GOR is a kind of quasi-one-dimensional (1D) material with a width of about 10 nm and a length of more than 100 nm, where the carries could be confined within the transverse dimension. In addition, there could be much less functional groups on the plane of GOR. GOR also could improve the stability and efficiency of polymer solar cells dramatically [13]. At last, GQD is a quasi-zero-dimensional (0D) carbon nanomaterial with a size of <10 nm. GQDs can be merely considered as sp2 hybrid carbon atoms encircled by functional groups, which could be easily functionalized by chemical methods, too [29]. Thus, GO, GOR and GQD are promising candidates in biological labeling, photovoltaic devices and drug delivery [29–36].
All these fascinating properties in GO, GOR and GQD should be attributed to the electronic structures, which are still needed to be further validated. Gao et al. have been fully reviewed the computational insights into the electronic structures of graphene oxide [37]. The electronic structure of GO has been reported by Wang et al. through experiments. It is proved that there are amorphous carbon atoms sp2/sp3 matrix that exist in the boundaries between the sp2 domains and sp3 matrix [38]. As previously reports, GO treated by HNO3 and NaOH exhibits distinctive optical properties [22,39,40], but the electronic structures of those chemical treated GO have never been directly observed. Meanwhile, the comparison between the transient spectra of quasi-1D GOR and quasi-0D GQD have never been reported, too. So, it is becoming more challenging and has great significance to clarify the electronic structures of GO, GO treated by HNO3 and NaOH, GOR and GQD.
Herein, we carried out an elaborate study on the transient features of GO, GO treated by HNO3 and NaOH, GOR and GQD via femtosecond transient absorption (TA) spectroscopy. Our research reveals that the common hybrid sp2/sp3 state at around 430 nm (∼2.88 eV) in GO, GO treated by HNO3 and NaOH. This hybrid state is not sensitive to the chemical treatments, which could be originated from the amorphous carbon atom matrix between the sp2 domains and sp3 matrix. However, the hybrid state becomes less evident in GOR and disappears in GQD. The typical dynamics of GO, GO with HNO3 and NaOH treatments, GOR and GQD are revealed and analyzed in the following experiment.
2. Experimental section
2.1 Materials
Graphite powder (Tianjin Dingshengxin Chemical Industry Co., Ltd), NaNO3, KMnO4, NaOH, HNO3 (69 wt %), H2O2, single-walled carbon nanotubes (SWCNTs), CX-72 carbon black (Cabot Corporation).
2.2 Preparation of samples
The GO sheets were acquired by modified Hummers’ method [33,41]. Natural graphite powder (2 g) and NaNO3 (2 g) were added to concentrated sulphuric acid (98 wt %, 96 mL) in an ice-bath. KMnO4 (12 g) was gradually added in several portions by stirring over 30 minutes at 0°C. The color of as-prepared solution changed from dark to yellow-brown. After that, the mixture was further stirred for about 90 min. Then, the mixture was allowed to be heated to 35°C and kept with constant stirring for 2 h. Next, the mixture was diluted by slowly dripping of deionized water for nearly 30 min. Deionized water (100 mL) was poured into the mixture. Crude GO suspension was then acquired by the addition of H2O2 (30%, 10 mL).
To gain the purified GO suspension, we must wash the crude GO dispersion with water repetitively. The resultant crude GO dispersion was collected. Then it was diluted by water and centrifuged (12000 rpm) for 15 min. We removed the supernatant and collected the precipitate. The collected solids were redispersed in water and then ultrasonicated in a water bath for 15 minutes. The resultant crude GO dispersion was collected, diluted by water and then centrifuged (12000 rpm, 15 min) again. After 15 times repetition, we almost removed the acid in the solution and got the suspension with pH ∼7. At last, the GO suspension was further centrifuged (3000 rpm, 15 min) to remove the residual graphite solid particles and a transparent light yellow GO suspension was formed finally.
We mixed the above GO suspension with the HNO3 (69 wt %) solution. The ratio of GO to HNO3 is 10:1. Meanwhile, the GO suspension was treated with NaOH solution (1 mol L-1). The ratio of GO to NaOH is also 10:1. The GO treated by HNO3 and GO treated by NaOH were then centrifuged and washed with deionized water until the pH reached ∼7.
The GOR was synthesized and purified according to the published literature [13]. Firstly, we mixed the purified single-walled carbon nanotubes (SWCNTs) (100 mg) with 98% H2SO4 (150 mL). The mixture was ultrasonicated and stirred at 25 °C overnight. Secondly, we added KMnO4 (500 mg) slowly in five portions and constantly stirred the mixture at 25 °C for about 2 hours. Then, we stirred the mixture at 70 °C for 2 hours. Soon afterwards, another KMnO4 (200 mg) was continually added again. After the mixture was cooled down to room temperature, 50 g water, 400 g ice, and 5 mL H2O2 under constant stirring were poured into the mixture. The reaction mixture was then centrifuged (20 min, 22000rpm) to obtain crude GOR. The obtained GOR was redispersed in 20 mL water and then one drop of hydrochloric acid was added in. We removed the inorganic impurities by filtration through 0.2 µm dialysis bag. Finally, the collected solids were then dispersed in water and centrifuged (6000 rpm, 20 min). We discarded the precipitate and the obtained transparent dispersion was the purified GOR.
The GQD was prepared according to the typical method in literature [29]. This procedure was as follows: CX-72 carbon black (0.2 g) and the HNO3 (50 mL 6 mol L-1) was mixed followed by refluxing for 24 h. When the suspension was cooled to the room temperature, the sample was centrifuged at 4000 rpm for 10 min. Then the obtained sediment was washed with 1 M HCl and centrifuged for three times again. Next, the sample dried under vacuum was dissolved in the deionized water (50 mL), where 10 mM ammonia water was added to make the pH reach to 8. Finally, the suspension was ultrafiltered by a centrifugal filter device (molecular weight cut off membrane: 100 kDa). The filtrate was dried, and then collected as the GQD.
So far, we obtained the pristine GO, GO treated by HNO3 and NaOH, GOR and GQD, and they were labeled as Sample 1, Sample 2, Sample 3, Sample 4 and Sample 5, respectively.
3. Femtosecond transient absorption setup
A mode-locked Ti: sapphire laser/amplifier system (Solstice, Spectra-Physics, 800nm, ∼100 fs duration, and 1.5 mJ/pulse) was used in the transient absorption setup [42–49]. The 800 nm femtosecond laser pulse with 250 Hz repetition rate from the amplifier was divided into two beams. The stronger beam was used to produce the excitation light of a specific wavelength through the TOPAS system. The smaller beam was used to produce the broadband white-light probe pulses ranging from 400 to 850 nm through 2-mm-thick water. The TA signal was picked up by an Avantes spectrometer. The group velocity dispersion was calibrated by a chirp program. All the experiments were conducted at room temperature.
4. Results and discussion
4.1 Steady-state absorption experiments
The diagram of GO, GOR and GQD are presented in Fig. 1. For GO, there are many different sizes of sp2 clusters inserted in the sp3 matrix, which could consist of a huge number of oxygen-containing functional groups. Besides, there are also many amorphous hybrid carbon atoms in the boundaries between the sp2 domains and sp3 matrix. For GOR, there could be much less oxygen-containing functional groups on the plane. For GQD, it is composed of sp2 carbon atoms surrounded by oxygen-containing functional groups.
The steady-state absorption spectra of GO samples are shown in Fig. 2(a). Typically, there are two bands in the steady-state absorption spectrum of GO: one exists at around 230 nm, and the other exists at around 305 nm. The steady-state absorption peak at 230 nm is assigned to the π→π* transition, and the steady-state absorption peak at 305 nm is assigned to the n→π* transition [12,22]. After chemical treatment by HNO3, Sample 2 shows that the π→π* transition peak blue-shifts to 228 nm. When treated by NaOH, Sample 3 presents that the n→π* transition strength decreases and it is barely observed. Although the steady-state absorption of GO, GO treated by HNO3 and NaOH gradually decreases from ultraviolet to near IR, it shows a clearly decreased absorption from 400 nm to 800 nm for the two chemical treated GO. Meanwhile, we also find that the solution color of GO treated with NaOH varied from yellow brown to dark, suggesting a chemical reduction occurs. In Fig. 2(b), GOR merely shows an apparent absorption peak at about 230 nm and a long absorption tail extends to 800 nm. It implies much less oxygen-containing functional groups in GOR, in comparison with GO samples. With regard to GQD, we find that there is a new absorption band in the spectral range of 430 nm to 450 nm, which could be ascribed to the new molecule-like states, corresponding to its green FL.
Fig. 2. (a) Steady-state absorption spectra of GO, GO treated by HNO3 and NaOH. (b) Steady-state absorption spectra of GO, GOR and GQD.
4.2 X-ray photoelectron spectroscopy (XPS)
The XPS spectra for these samples are shown in Fig. 3. For GO shown in Fig. 3(a), the XPS bands at 284.6 eV, 286.8 eV and 288.2 eV are mainly attributed to the sp2 carbon atoms, sp3 carbon atoms (represented by C-O bond) and C = O bond, respectively [38]. Compared with that of GO, the XPS spectrum of GO with HNO3 treatment has barely any changes, except for the slightly increased intensity for the XPS band corresponding to the C = O bond, which implies a slightly oxidization, shown in Fig. 3(b). However, in Fig. 3(c), we can see that the C-O in GO treated by NaOH diminished dramatically. It indicates that the NaOH greatly reduced the C-O functional groups. Meanwhile, for GOR shown in Fig. 3(d), the distinctive difference is that XPS spectrum of GOR is like that of GO treated by NaOH, suggesting much less C-O bonds than that of pristine GO. That could be because of the limited transverse dimension, which is harder to attach C-O oxygen functional groups on the surface of GOR. For GQD in Fig. 3(e), there are only a little C = O groups, and no C-O groups. It implies that when using the CX-72 carbon black as the precursor of GQD, which treated by HNO3, the as-prepared GQD is mainly enveloped by C = O functional groups.
4.3 Fluorescence emission experiments
To gain insight to the difference of optical properties for these carbon nanomaterials, we also conducted the steady-state emission experiments. We choose several different excitation wavelengths from 400 nm to 560 nm. As is shown in Fig. 4(a), there are broad photoluminescence (PL) spectra from 450 nm to 800 nm in GO. It indicates that there are many radiative recombination centers, where the PL central wavelength varies from 575 nm to 625 nm as the excitation wavelength changes from 400 nm to 560 nm. In GO treated by HNO3 shown in Fig. 4(b), the PL spectra are similar to that of pristine GO, which are also excitation wavelength dependent. This suggests that the emissions in pristine GO and HNO3 treated GO are mainly resulted from the superposition of localized sp2 clusters with different sizes, which are inserted in the matrix with large number of oxygen-containing functional groups. However, in Fig. 4(c), after the treatment with NaOH, the Sample 3 has nearly no emissions under the excitations of visible light wavelengths. It could be due to that NaOH reduces the amount of oxygen-containing functional groups on the GO plane; thus, it destroys the surrounding environment of the sp2 clusters, which results in the quenching of PL, and facilitates the nonradiative recombination processes. In Fig. 4(d), there is also very weak emission in GOR, like GO treated by NaOH, where the sp2 clusters are lack of the physical environment composed of huge amount of oxygen-containing functional groups, too. In Fig. 4(e), GQD has evident excitation-wavelength-dependent emissions, and its PL line width is narrower than that of pristine GO and HNO3 treated GO. From the PL results of all these samples, we suggest that the PL emissions in the visible range cannot appear unless there are many sp2 clusters surrounded by a huge amount of oxygen-containing functional groups.
Fig. 4. Steady-state emission spectra of (a) GO, GO treated by (b) HNO3 and (c) NaOH, (d) GOR and (e) GQD.
4.4 Femtosecond transient absorption spectroscopy
To further reveal the underlying electronic structures of the GO, GO treated by HNO3 and NaOH, GOR and GQD, broadband (400–850 nm) TA experiments are performed. The TA spectra of these samples are presented in Fig. 5. As is shown in Fig. 5(a-c) for three kinds of GO samples, large ground state bleaching peaks appear at around the excitation wavelengths. This implies that the localized sp2 states are excited and these bleaching signals represent sp2 clusters with the specific size. In Fig. 5(d), GOR has the similar ground state bleaching signal at around the excitation wavelength. This bleaching signal related to sp2 domains is much broader than that of GO samples. This could be due to the smaller superposition between ground state bleaching signals and the excited state absorption signals in GOR, where the excited state absorption signals could be related to the oxygen-containing functional groups. For GQD shown Fig. 5(e), the ground state bleaching signal corresponding to the sp2 domains has the smallest width, and its amplitude is much smaller than that of excited state absorption signal. Because the C = O groups are dominant in GQD as demonstrated in the XPS experiments, the observed large excited state absorption signal in GQD could be responsible for the C = O groups. Those excited state absorption signals in GO, GOR and GQD are different, too. The pristine GO and HNO3 treated GO have the larger excited-state absorption signals than that of NaOH treated GO, in comparison with their ground state bleaching signals. For GOR, the ratio of the excited state absorption signal to the ground bleaching signal is the smallest among those samples. For GQD, beside the strong excited state absorption signal at the red side of ground bleaching signal, there is a small excited state absorption signal at the blue side of ground bleaching signal, which could be related to the disappearing of the hybrid state in GQD.
Fig. 5. Transient absorption spectra of (a) GO, GO treated by (b) HNO3 and (c) NaOH, (d) GOR under 530 nm excitation, and (e) GQD under 500 nm excitation.
In GO, GO treated by HNO3 and GO treated by NaOH, there are extra bleaching signals appearing at around 430 nm (∼2.88 eV). It is assigned to the new hybrid state, which is related to the amorphous carbon-like matrix with high sp3/sp2 carbon ratio in the boundaries between the sp2 domains and sp3 matrix [38]. It implies that GO treated by HNO3 and NaOH still retain the amorphous carbon atom boundaries, though the distribution of function group is different. This hybrid state signal also exist in GOR, where the ration of hybrid state signal to its ground state signal around the excitation wavelength is rather smaller than that of GO. It further implies that there are much less amorphous hybrid sp2/sp3 states on the plane of quasi-1D GOR. For GQD, there is no such a hybrid state signal, instead of a small excited state absorption in the range of 400 nm to 440 nm. It suggests that for the electronic structure of GQD, it is merely sp2 hybrid carbon atoms which are surrounded by oxygen-containing functional groups.
Moreover, in Fig. 6(a) and Fig. 6(b), we observe that the dynamics of ground state bleaching signal (probed at 480 nm) for GOR and NaOH treated GO exhibit a dramatically decay than the other samples. This is very similar to that of photothermally reduced GO [38]. The possible explanation for NaOH treated GO is that the treatment of NaOH restores the surrounding environment of sp2 clusters by removing oxygen-containing functional groups, which facilitates the carrier recombination in sp2 clusters. For GOR, the limited transverse dimension and much less oxygen-containing functional groups around the sp2 clusters are responsible for the fast decay of ground state bleaching signal. The dynamics for the excited state absorption signals of all samples (Fig. 6(c-f), probed at 650 nm and 750 nm, which could be corresponding to different kinds of oxygen-containing functional groups) are also in agreement with this observation. Since the excitation pulses used in our TA experiments cannot directly excite the oxygen-containing functional groups, those dynamics could reflect that the interaction between the sp2 domains and the oxygen-containing functional groups dominate the observed dynamics. Especially, the interaction between sp2 clusters and the oxygen-containing functional groups in GOR could be much weaker than that in pristine GO, HNO3 treated GO and GQD.
Fig. 6. Normalized kinetics of GO, GO treated by HNO3 and NaOH probed at (a) 480 nm, (c) 650 nm and (e) 750 nm. Normalized kinetics of GO, GOR and GQD probed at (b) 480 nm, (d) 650 nm and (f) 750 nm.
On the other hand, we find that the treatment of HNO3 has no distinctive influence on the ground state bleaching dynamics and the excited state absorption dynamics, compared with that of GO. The ground state bleaching dynamics of GQD has a long lifetime component since it also possesses the PL emissions, but it is almost the same as that of pristine GO and HNO3 treated GO within the first 10 ps. It implies GQD has a good sp2 core, and its emission centers are also different from that in pristine GO and HNO3 treated GO.
5. Conclusion
In summary, the different electronic structures for pristine GO, GO treated by HNO3 and NaOH, GOR and GQD have been investigated by TA experiments. We have discovered the common hybrid sp2/sp3 state at around 430 nm in pristine GO, GO treated by HNO3 and NaOH, which is originated from the amorphous nature structure of GO itself. Besides, we have compared their transient spectral features of pristine GO, GO treated by HNO3 and NaOH, GOR and GQD. In comparison with that of GO samples, the excited state absorption signals of GOR from 600 nm to 850 nm are very small, and the hybrid state at about 430 nm is less apparent, which indicates that there is less sp2/sp3 amorphous carbon atom matrix in GOR. In GQD, there is no sp2/sp3 state at around 430 nm completely, implying that there are merely sp2 carbon atoms enveloped by oxygen-containing functional groups. Furthermore, we have found that the treatment of NaOH reduces the pristine GO, and facilitates the carries recombination. The carrier recombination in GOR seems to be close to that in NaOH treated GO, but it is due to the weaker interaction between sp2 clusters and the oxygen-containing functional groups. Our findings suggest that in all those carbon nanomaterials mentioned above, the carrier dynamics are greatly dependent on the interaction between sp2 domains and the oxygen-containing functional groups.
Funding
National Natural Science Foundation of China (22073037, 61590930, 21603083, 61805159, 61927814, 21773087); National Key Research and Development Program of China (2017YFB1104300).
Acknowledgement
This work was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China (NSFC) under Grants 2017YFB1104300, 21773087, 61927814, 61805159, 61590930, 21603083, 22073037.
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time, but may be obtained from the authors upon reasonable request.
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