1. Introduction

Graphene acts as a single-atom thick 2D carbon crystal consist of carbon atoms forming by hexagonal arrangement, which has attracted much attentions from physicists and materials scientists [1–3], due to its excellent properties including high electrical conductivity [4–6], good flexibility [7], ultrahigh carrier mobility [8], high optical transmission [9] and other feasible characteristics [10–13]. Nevertheless, the production of high-quality and large-scale graphene single-crystal film for practical applications is still an urgent issue. As a precursor of graphene, the synthesis of graphene oxide (GO) has a relatively low cost, which has been widely researched since it was synthesized by Brodie in 1859 [14]. GO is an atomically thick carbon sheet functionalized with several oxygen-containing functional groups (OFGs) dominated by the epoxies and hydroxyls functional groups on the basal plane, with carboxyls and lactols on the sheet edges [15]. Therefore, GO and its derivatives not only contains local sp²-hybridized carbon atom domains, which are analogues of graphitic crystalline graphene, but also possesses sp³-hybridized carbon atom domains, as well as the boundaries between the two kinds of sp²- and sp³-hybridized (sp²-sp³ for short) carbon atom domains (i.e., small sp² domains isolated within a sp³ carbon matrix) [16]. Hence, it is tunable from an insulator to a semiconductor for GO and even potentially changes into a graphene-like semi-metal by regulating the proportion of sp²/sp³ through various reduction reactions [17, 18].

Originally, it was considered that large-scale production of graphene sheets could be readily fabricated by removing OFGs in GO via chemical [19] or thermal reduction [14]. Although chemical and thermal reduction processes are effective and easily realized merely in the condition of laboratory, industrial manufacturers would not be satisfied with the toxic reduction processes and high temperatures applied in chemical and thermal reductions. Therefore, photo-reduction as one of the most feasible, environment-friendly and high-efficient reduction methods has been investigated by a lot of ground-breaking experiments [16, 17, 20–25]. Sun et al have first observed intrinsic fluorescence from GO [20]. Their research has demonstrated that the inhomogeneous nature of the aromatic domains ranging from small to large in GO, were responsible for the fluorescence in the visible and NIR regions, respectively. In addition, it is well known that the reduction degree of GO has a close relationship with its fluorescence properties [26]. Namely, the greater degree of GO reduction, the weaker fluorescence intensity obtained, combined with a larger shift of fluorescence peak shown [15]. However, the photo-reduction mechanism of GO is still ambiguous. It is undisputed that a clearly explanation of photo-reduction mechanism of GO would essentially solve a variety of intractable issues and expand the applicability of GO.

Here, we illuminate the interpretation about photo-reduced GO mechanisms from the perspective of fluorescence evolution processes of visible/ultraviolet (UV) photo-reduced GO. Herein, the GO were prepared by modified Hummers method [14], and the reduction reaction wavelength from UV light to visible light was utilized to reduce the as-prepared GO. The wavelengths of the UV light and visible light used in the photo-reduction reactions are 365 nm and 532 nm, respectively. To avoid the thermal effect, the as-prepared GO is under continuous stirring condition during the photo-reduction treatments. After that, a variety of measurements including steady-state photoluminescence spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and femtosecond transient absorption spectroscopy had been applied to analyze the fluorescence evolution processes of photo-reduced GO. It has demonstrated that GO can be reduced by visible light and UV light to various degree. Furthermore, we consider that the OFGs in sp³ domains of GO were initially reduced by the visible light, however, the OFGs in the hybrid regions of sp²-sp³ domains of GO prefer to be reduced by the UV light. The detail evolution processes are discussed in the following sections.

2. Experimental

2.1 Preparation of GO

GO were synthesized by modified Hummers method from natural graphite flakes, that is, graphite flakes were oxidized by concentrated (98%) H₂SO₄ and KMnO₄, same as previously reported [27]. GO solution were prepared by reaction with a mixture of graphite, KMnO₄ and concentrated (98%) H₂SO₄. 2 g graphite flakes were mixed with 96 mL concentrated H₂SO₄ in a 500 mL glass beaker in an ice-water bath atmosphere. Subsequently, 12 g KMnO₄ was added into the reaction system slowly. Then the reaction system was heated to 35 °C in a water bath kettle and stirred for 1 hour with a continuous magnetic stirrer. Afterwards, 80 mL deionized water was added in to the reaction system bit by bit, and then heated to 90 °C in a water bath and continuously stirred for 1 hour with a magnetic stirrer. At last, the reaction system was stopped by pouring into 12 mL 30% H₂O₂ slowly. Then the reaction system was diluted by adding into 200 mL deionized water and heating is stopped. As the sample cooling down to room temperature, stop stirring, sealed, and set at room temperature for 24 h. The resulting mixture was washed several times with deionized water by sonication and centrifugal process to remove aggregated GO sheets, until the solution changed from acidic to neutral.

2.2 Preparation of photo-reduced GO

The as-prepared GO aqueous solutions were divided into several shares in the sample bottles for reduction reactions. For visible light reduction reactions, we selected the continuous semiconductor laser with the wavelength of 532 nm (the power is 200 mW). The sample bottles with the same volume of GO were irradiated for 1 h, 5 h, 12 h, 24 h, 48 h, 7 day, 18 day and 23 day, respectively, with continuous stirring in the same condition, which were labeled as rGO-Vis-1h, rGO-Vis-5h, rGO-Vis-12h, rGO-Vis-24h, rGO-Vis-48h, rGO-Vis-7day, rGO-Vis-18day and rGO-Vis-23day, correspondingly. For UV light reduction reactions, the laboratorial UV lamp with the central wavelength of 365 nm was used (the power is 250 W). The samples were stored in the sample bottles to be reduced directly by the UV lamp in the same condition. The time for reduction reactions were 5 min, 10 min, 20 min and 30 min, and then the reduced samples by UV light were labeled as rGO-UV-5min, rGO-UV-10min, rGO-UV-20min and rGO-UV-30min, respectively. These samples were stored in the cuvettes with a thickness of 2 mm for further characterization and testing of optical properties at room temperature.

2.3 Femtosecond transient absorption spectroscopy setup

The output laser pulses from a mode-locked Ti:sapphire laser/amplifier system (Solstice, Spectra-Physics) centered at 800 nm (repetition rate: 250 Hz) with 100 fs pulse width and 1.5 mJ pulse energy was split into two parts: the stronger beam was frequency-doubled to 400 nm (or through TOPAS to generate 530 nm) pump pulses used as excited light; the other beam was penetrated through 2 mm thick water, generating broad-band (from 400 nm to 850 nm) white-light probe light. The transient absorption data were collected by a fiber-coupled spectrometer connected to a computer. The group velocity dispersion of the transient spectra was compensated by a chirp program. All the measurements were performed at room temperature.

3. Results and discussion

The steady-state absorption spectra of GO and rGOs measured by a Shimadzu UV-2550 UV-VIS scanning spectrophotometer are presented in Figs. 1(a) and 1(b). They show that the steady-state absorption spectra of GO and rGOs reduced by visible light of 532 nm and UV light of 365 nm both varying with different reduction reaction time. According to the steady-state absorption spectra, there is a main absorption peak at ~230 nm attributed to the π→π* transitions of C = C [14, 27–31]. In addition, a broad shoulder located at ~300 nm was observed during the entire course of photo-reduction, which ascribed to n→π* transitions of one or more functional groups such as O−C = O, C = O, and C−OH [31]. The intensity of steady-state absorption is gradually increasing with longer reduction reaction time, which is consistent with the stronger darkening of these rGOs samples in appearance [insets of Figs. 1(a) and 1(b)]. Comparing Fig. 1(a) with Fig. 1(b), we can clearly observe that the reduction degree of GO by UV light is much stronger than that by visible light which need a longer reduction time up to 23 day. This could be mainly attributed to the high power of UV lamp. Similarly, the color of rGOs reduced by visible light is shallower than that reduced by UV light, which can also be seen from insets of Figs. 1(a) and 1(b). Furthermore, the color of the rGOs suspension solution changed from light-brown to dark-brown after various irradiation time, which could be attributed to the restoration of sp² π-conjugated network and the elimination of OFGs in GO. On the other hand, there is an obvious difference related to absorption peaks between the two steady-state absorption spectra. For visible light reduction [Fig. 1(a)], the representative absorption peak in the steady-state absorption spectra of rGO-Vis at ~230 nm greatly increased. As for UV light reduction [Fig. 1(b)], however, the absorption peaks in the steady-state absorption spectra of rGO-UV at ~300 nm and longer wavelengths increased evidently, while the absorption peak at ~230 nm changed little. After that, the photoluminescence spectra of GO and rGOs were measured by a Hitachi F-4600 fluorescence spectrophotometer. The steady-state photoluminescence spectra of GO and rGOs excited by 400 nm are presented in Figs. 1(c) and 1(d). Comparing with pristine GO, the fluorescence peak of rGOs reduced by visible light shows a blue shift from 580 to 520 nm [Fig. 1(c)], while the fluorescence peak of rGOs reduced by UV light appears a red shift from 580 to 650 nm [Fig. 1(d)]. Meanwhile, the fluorescence intensity for rGO gradually decreased as the reduction time increasing. Finally, the fluorescence peaks of rGO-Vis-23day and rGO-UV-30min disappeared due to the excessive reduction time.

 

Fig. 1 Steady-state absorption spectra of GO and rGOs reduced by (a) visible light and (b) UV light with different reduction time. The insets of Figs. 1(a) and 1(b) show the corresponding photographs of GO and rGOs reduced by visible light and UV light, respectively. Steady-state photoluminescence spectra (excited by 400 nm) of GO and rGOs reduced by (c) visible light and (d) UV light with different reduction time. The left panel of Figs. 1 (c) and 1(d) is the schematic diagram of visible/UV photo-reduced GO.

Download Full Size | PDF

Actually, the origin of fluorescence [30, 32] from GO is very complicated, and our group has made a lot of investigation on it. We have found that the blue side of the emission from GO was attributed to the sp² domains, while we ascribed the red side of the emission to sp³ domains. In addition, Sun et al have first observed intrinsic fluorescence from GO [20]. Their research has demonstrated that the inhomogeneous nature of the aromatic domains ranging from small to large in GO, were responsible for the fluorescence in the visible and NIR regions, respectively. These aromatic domains usually refer to quantum-confined graphene-like states [29], in this paper is so called sp²-hybridized carbon atom domains, which have a size-dependent band gap according to theoretical calculation [32]. Since the size effect of sp² domains, fluorescence from GO or its derivate should be shift from blue to red as the size of sp² domains change from small to a large one. According to the above steady-state photoluminescence spectra, we supposed that the blue or red shifts of different rGOs fluorescence peaks could be mainly attributed to the selective reduction reactions. In other words, the OFGs in sp³ domains of GO reduced by visible light could be initially reduced, which are responsible for the red side of the emission, and the fluorescence from new generated small-sized sp² domains could probably be remained, which correspond to the blue side of the emission. Thus, the fluorescence peaks of rGO-Vis show a blue shift. As for GO reduced by UV light, the OFGs around hybrid region of sp²-sp³ domains could prefer to be easily reduced, and the fluorescence from OFGs in sp³ domains and the size expanded sp² domains could probably be remained, so the fluorescence peaks of rGO-UV show a red shift.

Raman spectroscopy is an important tool for characterizing GO information on crystalline size, the degree of hybridization, crystal disorder. So, Raman spectra of GO, rGO-Vis-23day and rGO-UV-30min have been carried out. In Fig. 2(a), there are well-defined peak D (1352cm⁻¹) and peak G (1590cm⁻¹). Additionally, peak D/G intensity ratio (I_D/I_G) of GO, rGO-Vis-23day and rGO-UV-30min are ~0.898, 0.948 and 0.845, respectively. The value of I_D/I_G from small to large indicates a decrease of sp² domain size [31, 33]. It demonstrated the above conjecture that GO reduced by visible light generated new small-sized sp² domains, while GO reduced by UV light expanded the size of sp² domains. In order to evaluate the reduction degree of visible/UV photo-reduced GO, XPS was performed on GO, rGO-Vis-23day and rGO-UV-30min [Figs. 2(b)-2(d)], corresponding O1s PP AT% are 57.7%, 45.17% and 35.74%, respectively. It has suggested that UV light have a stronger effect for photo-reduced GO compared with visible light. Notably, the peaks at 284.4, 285.1, 286.8, 287.1 and 288.8 eV are attributed to Csp², Csp³, C−O−C, C-O and COOC, respectively [31,33,34]. For GO, rGO-Vis-23day and rGO-UV-30min, the percentage of Csp2 is 19.35%, 20.30% and 23.13%, the percentage of Csp3 is 21.67%, 19.64% and 20.69%, respectively. It is obvious that rGO-UV-30min shows distinct enhancement in the peak intensities of Csp² compared to the as-synthesized GO, rGO-Vis-23day, indicating a stronger reductive degree. It was consistent with the Raman data that showed evidence of graphitic regions. FTIR spectra have been employed to measure the deoxygenation of GO [Fig. 2(e)] [35]. Observably, pristine GO shows a rich collection of transmission bands corresponding to OFGs (e.g., O−H of COOH at around 3445 cm⁻¹, C = O of COOH at around 1734 cm⁻¹, C−OH at around 1240 cm⁻¹ and C−O−C at around 1133 cm⁻¹). And then, the C−O−C and C−OH on the basal plane of GO reduced by visible light were eliminated, while the C−OH, O−H and C = O of COOH at the sheet edges of GO reduced by UV light were removed, respectively. It demonstrates that the C−OH and C−O−C on the basal plane in sp³ domains of GO are initially reduced by visible light, while the C−OH, O−H and C = O of COOH at the sheet edges around hybrid region of sp²-sp³ domains of GO prefer to be reduced by UV light.

 

Fig. 2 (a) Raman spectra of GO, rGO-Vis-23day and rGO-UV-30min. XPS C1s spectra of (b) GO, (c) rGO-Vis-23day and (d) rGO-UV-30min. (e) FTIR spectra of GO, rGO-Vis-23day and rGO-UV-30min.

Download Full Size | PDF

On the basis of our wide probe range from entirely visible to near-infrared light by femtosecond transient absorption spectroscopy [29, 36–40], we can explore the inside characteristics on electronic structure of our experimental materials. Determined by the excitation of femtosecond laser, the transient spectral evolution of photo-generated carrier populations in corresponding excited-state energy levels can be revealed in the form of ground state bleaching (GSB), excited-state absorption (ESA) and stimulated emission (SE). To further support the assumption we proposed above, the fluorescence evolution processes of visible/UV photo-reduced GO is further confirmed by femtosecond transient absorption spectroscopy experiments with pump wavelength at 400 nm and 530 nm. The femtosecond transient absorption spectra of GO, rGO-Vis-23day and rGO-UV-30min with the 400 nm pump light are listed in Figs. 3(a)-3(c), respectively. The disturbances near 800 nm come from the limitation in probe light. As for GO, a broad ESA signal around 470 to 850 nm, as well as a GSB signal around 400 to 470 nm are observed. Similarly, the GSB signals and broad ESA signals of rGO-Vis-23day and rGO-UV-30min can also be observed distinctly. Compared with GO, the GSB and ESA signals intensity of rGO-Vis-23day show slightly increased, and the width of GSB signal maintains almost unchanged. In addition, the GSB signal of rGO-UV-30min shows a red shift and its intensity increases obviously. On the basis of our previous research about selective excitation of GO with various wavelength by femtosecond transient absorption spectroscopy [29], the 400 nm pump light can only excite the hybrid region of sp²-sp³ domains and small-sized sp² domains while 530 nm pump light is able to excite the larger size of sp² domains. Accordingly, we can sufficiently explain the variation of GSB and ESA signals of GO and rGOs. For rGO-Vis-23day, the OFGs in sp³ domains are initially reduced by visible light, producing a lot of new small-sized sp² domains. Thus, there is a slightly increased GSB and ESA signal intensity. As for rGO-UV-30min, OFGs around hybrid region of sp²-sp³ domains prefer to be reduced by UV light. As a result, more and more sp² domains are expanded, resulting in the GSB signal red shift with increasing intensity. The normalized carrier dynamics probed at 450 nm of GO, rGO-Vis-23day and rGO-UV-30min in 400 nm excitation is shown in Fig. 3(d). The carrier dynamics in rGOs are slightly faster than that in as-prepared GO.

 

Fig. 3 Femtosecond transient absorption spectra of (a) GO, (b) rGO-Vis-23day and (c) rGO-UV-30min at different probe delay times which are excited by 400 nm pump light. The disturbances near 800 nm come from the limitation in probe light. (d) Normalized carrier dynamics probe at 450 nm of GO, rGO-Vis-23day and rGO-UV-30min in 400 nm excitation.

Download Full Size | PDF

To scrutinize the spectral species evolution in the sp² domains of GO and rGOs, femtosecond selective excitation transient absorption experiments on GO, rGO-Vis-5h, rGO-Vis-48h, rGO-UV-5min and rGO-UV-10min at 530 nm excitation are also performed. Related transient absorption spectra are presented in Figs. 4(a)-4(e). Similarly, the vertical lines at 530 nm are the scatters of pump light, while the disturbances near 800 nm come from the limitation in probe light. As the reduction time increased, we can find that GSB signals of rGO-Vis gradually expand and increase [Figs. 4(b) and 4(c)] due to the new generated small-sized sp² domains, which link together to form larger sp² domains. On the other hand, the greatly broadening and enhanced intensity of GSB signals of rGO-UV [Figs. 4(d) and 4(e)] can be attributed to the expanding dimensions of sp² domains by the elimination of OFGs around hybrid region of sp²-sp³ domains via UV light reduction. The normalized carrier dynamics probed at 415 nm of GO, rGO-Vis-5h, rGO-Vis-48h, rGO-UV-5min and rGO-UV-10min in 530 nm excitation is shown in Fig. 4(f). The normalized carrier dynamics in rGOs are gradually accelerating with the increasing of reduction time, indicating that photo-reduced GO changes from an insulator to a semiconductor progressively.

 

Fig. 4 Femtosecond transient absorption spectra of (a) GO, (b) rGO-Vis-5h, (c) rGO-Vis-48h, (d) rGO-UV-5min and (e) rGO-UV-10min at different probe delay times which are excited by 530 nm pump light. The vertical lines at 530 nm are the scatters of pump light while the disturbances near 800 nm come from the limitation in probe light. (f) Normalized carrier dynamics probed at 415 nm of GO, rGO-Vis-5h, rGO-Vis-48h, rGO-UV-5min and rGO-UV-10min in 530 nm excitation.

Download Full Size | PDF

4. Conclusions

In summary, the partial elimination of OFGs and reconstruction of the C = C bonds could be proved by Raman, XPS and FTIR spectroscopy experiments, despite of GO reduced by visible or UV light. In addition, we found that the C−O−C and C−OH on the basal plane of GO are eliminated after reduced by visible light, while the C−OH, O−H and C = O of COOH at the sheet edges of GO reduced by UV light were removed. Furthermore, the fluorescence evolution processes of visible/UV photo-reduced GO have also been investigated in detail by steady-state photoluminescence spectroscopy and femtosecond transient absorption spectroscopy. These experimental results demonstrate that the OFGs in sp³ domains of GO are initially reduced by visible light, then generated new small-sized sp² domains and some of these small-sized sp² domains connected together to form larger sp² domains. Moreover, UV light prefers to reduce the OFGs around hybrid region of sp²-sp³ domains of GO, therefore, the sp² domains are expanded. The schematic diagram of visible/UV photo-reduced GO is shown in Fig. 5. In addition, the normalized carrier dynamics of photo-reduced GO demonstrate that GO reduced by visible or UV light gradually transforms from an insulator to a semiconductor. We consider that our work gives insights into the fluorescence evolution mechanisms of visible/UV photo-reduced GO, which might expand the applicability of rGOs in many fields, such as optoelectronics and photocatalysis.

 

Fig. 5 Schematic diagram of visible (532nm) /UV (365nm) photo-reduced GO.

Download Full Size | PDF