1. Introduction

Graphene oxide (GO) is an atomically thin sheet of graphite that is disordered and defective with both sp²- and sp³-hybridized carbon sites because of the introduction of oxygen-containing functional groups [1, 2]. With suitable chemical and structural modifications, GO can fluoresce over a broad wavelength range from ultraviolet, visible to near-infrared [1–5]. Although the exact fluorescence mechanisms are still highly debated, the feasibility of tailoring the optical properties of GO has enabled promising applications in photocatalysis [6], solar cells [7], light-emitting diodes [8], biological imaging [4], and biosensing [9]. Moreover, due to its chemical processability, GO can be easily doped [10], bonded with fluorescent ions [11], or integrated into other optical materials [1, 2] for the charge or energy transfer interactions, thus greatly broadening its already rich optoelectronic properties.

Semiconductor titania (TiO₂) is a wide bandgap material that has been one of the most promising photocatalysts for pollutant degradation and energy conversion applications [12, 13]. Recently, much research effort has been devoted to the construction of TiO₂ with nanostructured carbonaceous materials [14]. In particular, the light responsive activity of GO sheets coated with TiO₂ nanoparticles can be greatly enhanced along with the improved photocatalytic activity [15, 16]. In this report, we studied the optical properties of hybrid spheres consisting of alternating TiO₂ and GO single-layer nanosheets. Compared to reference samples with only TiO₂ spheres or GO nanosheets, a blue-to-red light emission band emerges and persists in this novel material system even after it was further reduced through microwave irradiation. From detailed time-resolved measurements and energy-level structure modeling, this unexpected fluorescent feature was attributed to the indirect optical transitions between TiO₂ and the localized sp² domains of GO. This finding not only furthers our current understanding of the optoelectronic properties of GO, but also provides a useful guidance for the synthesis of GO-based charge-transfer complexes for various device applications.

2. Results and discussion

The detailed procedures for the synthesis of single-layer TiO₂ and GO nanosheets, as well as their hybrid spherical structures have been reported elsewhere [17]. In brief, based on the layer-by-layer self-assembly technique, poly(methly methacrylate) (PMMA) solid spheres were successively immersed into a protonic polyethylenimine (PEI) aqueous solution, a colloidal suspension of negatively charged TiO₂ nanosheets, the protonic PEI aqueous solution again, and a suspension of negatively charged GO nanosheets. The above steps were repeated until core/shell composite spheres with five layers of PEI/TiO₂/PEI/GO were formed (here below denoted by GO/TiO₂). The coated PMMA spheres were then placed in a crucible surrounded with carbon powder to introduce strong microwave irradiation. The observed color change and measured weight loss indicate that both the PEI moiety and the PMMA template spheres were successfully removed to yield hybrid hollow spheres with alternating nanosheets of reduced GO (rGO) and TiO₂ (here below denoted by rGO/TiO₂). For the reference purpose, hollow spheres with five layers of TiO₂ nanosheets (here below denoted by TiO₂) were also synthesized using a similar procedure to that described above.

The transmission electron microscopy images of the rGO/TiO₂ and GO/TiO₂ hybrid spheres are presented in Figs. 1(a) and 1(b), respectively. The solid powders of these two samples, as well as the TiO₂ hollow spheres and the GO nanosheets, were each dispersed with appropriate amounts in deionized water by ultrasonic treatment to have comparable optical densities at the ultraviolet wavelength range. One drop from each of the solution samples was put and let dry on silica substrates to form solid films for the optical measurements. The 800 nm output of a 76 MHz, picosecond Ti:Sapphire laser was used to get the wavelengths of 400 nm and 266 nm from the second and third harmonic generation processes, respectively. For the photoluminescence (PL) spectral measurements, the 266 nm or 400 nm laser beam with a power density of ~100 W/cm² was focused onto the sample surface at an incident angle of ~45° relative to the normal direction. The sample PL was collected vertically from the surface by a microscope objective and sent through a 0.5 m spectrometer to a charge-coupled-device camera. For the time-resolved PL decay measurements, the 400 nm laser beam was focused on the sample surface by a microscope objective also at a power density of ~100 W/cm². The sample PL was collected by the same objective and sent to an avalanche photodiode in the time-correlated single-photon counting system with a time resolution of ~250 ps. All the measurements were performed at room temperature.

 

Fig. 1 Transmission electron microscopy images of the (a) rGO/TiO₂ and (b) GO/TiO₂ hybrid spheres.

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In Figs. 2(a) and 2(b), we plot the PL spectra (blue) of TiO₂ excited at 266 nm and 400 nm, respectively. With the 266 nm excitation, the emission peak centered around ~600 nm, and it shifted to ~650 nm when the laser wavelength was changed to 400 nm. Since the bandgap energy of bulk TiO₂ is at ~3.2 eV (~387 nm), the red-shifted fluorescence observed here should have its origins inside the band gap, such as defect levels generated by the lattice site vacancies [12, 13, 15]. This kind of defect-mediated transitions is normally associated with an extremely low quantum yield (<1%) [13], as confirmed by the PL decay curve (blue) shown in Fig. 3 with a lifetime component that is only a little longer than the ~250 ps resolution of our time-resolved system. For simplicity, we introduce a hole energy level within the TiO₂ band gap in Fig. 4 so that photoexcited holes in the TiO₂ valence band can nonradiatively relax to this defect level and recombine with the photoexcited electrons in the TiO₂ conduction band, leading to the defect-mediated optical emission.

 

Fig. 2 PL spectra of the TiO₂ (blue), rGO/TiO₂ (red), and GO/TiO₂ (black) samples excited at (a) 266 nm and (b) 400 nm, respectively. The inset of (b) shows the PL spectrum of as-synthesized GO nanosheets measured with the 400 nm excitation.

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Fig. 3 PL decay curves of the TiO₂ (blue), rGO/TiO₂ (red), and GO/TiO₂ (black) samples measured with the 400 nm excitation and plotted at the 0-3 ns range on a linear scale. Inset: The same PL decay curves plotted from 0 to 12 ns on a logarithmic scale. The fluorescence signal was collected over the entire spectrum of each sample for the PL decay measurement. IRF: instrument response function.

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Fig. 4 A schematic diagram of the energy levels for TiO₂ and the localized sp² domains of GO with respect to the water reduction and oxidation potentials. The water oxidation potential was set at 0 eV for convenience. The dotted arrow line (red) marks the IOT discussed in the text.

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As can be seen from Figs. 2(a) and 2(b), the PL spectra (black) of GO/TiO₂ are dramatically different from those of TiO₂, with a blue-shifted emission band centering at ~500 nm (550 nm) with the 266 nm (400 nm) laser excitation. This new PL feature persisted and became significantly weak when GO/TiO₂ was further reduced to rGO/TiO₂, whose PL spectra (red) excited at 266 nm and 400 nm are also shown in Figs. 2(a) and 2(b), respectively. One possible origin for this blue-shifted emission is the fluorescence quenching effect of GO or rGO on optical emitters [1], which has been reported to work more efficiently on the long-wavelength side of the TiO₂ PL spectrum [15]. If the whole PL spectrum of GO/TiO₂ or rGO/TiO₂ still originates from TiO₂, a faster PL decay process should be accompanied due to the nonradiative decay channels additionally introduced by the fluorescence quenching effect. However, as seen in Fig. 3, much slower lifetime components are resolved in the PL decay curves of both GO/TiO₂ (black) and rGO/TiO₂ (red), which implies that the fluorescence quench effect only plays a minor role in the appearance of the blue-shifted emission in Fig. 2. In Fig. 3, the PL decay curves of the GO/TiO₂ and rGO/TiO₂ samples were measured over the entire spectral range since the blue-shifted emission is dominant in their PL spectra and no obvious difference was detected from the wavelength-dependent lifetime measurement.

As-synthesized GO rarely emits light [18], however, there are still limited reports on its broad fluorescence spanning the red to near-infrared range [1, 2, 4, 5], which is proposed to come directly from the localized sp² domains owing to the quantum confinement effect [1]. In contrast, blue PL was observed from GO only after it received specific treatments, such as appropriate control of the sp² cluster concentration [3], surface passivation of the reactive sites [19], chemical bonding with fluorescent ions [11], or hydrothermal cutting into nanometer sized quantum dots [18]. In our experiment, we excited the as-synthesized GO nanosheets at 266 nm and 400 nm also with a power density of 100 W/cm². In both cases, the PL signals are extremely weak and are hardly discernible from the background fluorescence of the substrate, as can be seen from a representative PL spectrum excited at 400 nm in the inset of Fig. 2(b). Under the above excitation conditions, similarly weak fluorescence were obtained from the PMMA solid materials, which exist in GO/TiO₂ and should be almost completely removed from TiO₂ and rGO/TiO₂ [17].

After excluding the sole contribution of either TiO₂ or GO to the blue-shifted emission, we tentatively attribute it to an indirect optical transition (IOT) between these two materials due to the formation of a hybrid interface. Because of the introduction of oxygen functional groups, the carbon sheet is greatly modified to contain localized molecular sp² clusters dispersed within the sp³ matrix. The conduction band of the localized GO sp² domains can be still assigned to the π* orbitals, while the valence band changes from the π to the O 2p orbitals [20]. The energy of this indirect bandgap between the π* and O 2p orbitals will increase with the increasing degree of oxidation, e.g. from ~2.7-3.2 eV for the GO samples studied in Ref. 6. Based on the available data in the literature [6, 21], we plot in Fig. 4 a schematic diagram of the energy levels for TiO₂ and the localized sp² domains of GO with respect to the water reduction and oxidation potentials, the latter of which is set at 0 eV for convenience. Between the GO π* and O 2p levels of this simplified energy diagram, we ignored the possible presence of any defect-related states, which can give rise to visible fluorescence [22] that is not observed in our as-synthesized GO nanosheets.

For the localized sp² domains of GO, the excitation energy of 266 nm is high enough to create electrons and holes in the π* and π orbitals [2, 18], respectively, which are either dissipated as heat or injected to the adjacent metallic phase of carbon sheet since almost no PL was detected from pure GO nanosheets in our experiment. Meanwhile, electrons cannot be excited from the O 2p to the π* orbitals by the 400 nm laser since this corresponds to an indirect bandgap of the GO sp² domains, which might be one of the reasons for the extremely weak absorption normally reported for GO in the visible range [6, 11]. One the other hand, when TiO₂ is excited at 266 nm or 400 nm, the electrons will stay at the TiO₂ conduction band, and the holes can either relax to the defect level or be injected to the O 2p level. Due to the reduced symmetry at the hybrid interface [23], optical recombination of electrons in the conduction band of TiO₂ and holes in the O 2p levels of GO is allowed, thus giving rise to the blue-shifted emission in Fig. 2 for both GO/TiO₂ and rGO/TiO₂. PL resulting from this kind of IOT, also called type-II fluorescence, has been widely observed in semiconductor nanocrystals with heterogeneous interface structures [24].

We believe that the intimate contact between TiO₂ and GO (rGO) nanosheets, with a separation distance of only ~1.608 nm (~1.156 nm) [17], can facilitate efficient charge transfer across the interfaces. Due to the reduced overlap of electron and hole wave functions, the carrier recombination lifetime would be increased [24], which explains the longer PL decay lifetime component appearing in Fig. 3 for the GO/TiO₂ sample. In the rGO/TiO₂ case, the chemical reduction will elevate the inter-connectivity of localized sp² sites as well as increase the percentage of zero gap regions on the carbon sheets [3]. Thus the migration of carriers out of the TiO₂ and GO energy levels is expedited, leading to an enhanced PL quenching effect and the shortening of the PL decay lifetime [3]. The residual fluorescence observed in rGO/TiO₂ may reflect the fact that GO cannot be fully reduced to graphene [25]. However, the O 2p level in Fig. 4 will be lifted up with the decreasing oxygen content so as to cause a red shift in the PL peak of rGO/TiO₂ relative to that of GO/TiO₂, which is observed more obviously in Fig. 2(b) with the 400 nm excitation.

3. Conclusion

It should be noted that, besides the IOT model, there might exist other explanations for the blue-shifted emission of the GO/TiO₂ or rGO/TiO₂ sample studied here. As reported by Chien et al. in Ref. 22, two PL peaks were observed from the as-synthesized GO samples, centering around 600 nm and 470 nm, respectively. Upon continuous photo-thermal reduction, the two PL peaks both shifted to the blue side, with the relative PL intensity of the shorter-wavelength peak becoming gradually stronger. The above optical properties were attributed to the reduction-induced creation of more fluorescent sp² domains, which we believe only plays a minor role in the blue-shifted emission shown in Fig. 2. First, the as-synthesized GO nanosheets used in our experiment have almost no fluorescence, implying that there are significant variations in the sample structures and fluorescent mechanisms between them and those used in Ref. 22. Second, we have employed X-ray photoelectron spectroscopy (XPS) measurement to confirm that there is a significant amount of oxygenated carbon in the GO/TiO₂ sample [17, 26] so that the chance of its unintentional reduction is really small. Moreover, there was no detectable difference in the PL spectral shape and intensity of the GO/TiO₂ sample before and after UV illumination, thus excluding the light-induced reduction of GO by the TiO₂ material. Third, as shown in Fig. 2(b), the whole PL peak shifted to the red side when GO/TiO₂ was further reduced to rGO/TiO₂ through microwave irradiation, which is in contrast to the blue shift observed in Ref. 22 but can be well explained by the IOT model as already discussed in the text.

It was shown that the nonradiative electron-hole recombination centers of epoxy and carboxylic groups in GO could be removed after reacting with alkylamines, leading to the appearance of a blue emission band [19]. However, this kind of covalent interactions is not expected between the TiO2 and GO materials in our case since their nanosheets are stacked together with a layer-by-layer electrostatic deposition technique. This is evidenced by the XPS measurement of the TiO2/GO sample showing a significant amount of epoxy and carboxylic groups [17, 25], which is not too much less than those measured in our GO nanosheets and reported in other GO samples [6, 22]. An opposite scenario, whereby the nonradiative defect centers of TiO₂ are passivated by GO, can only increase the overall fluorescent quantum yield of the former material instead of causing the change of its PL spectral shape shown in Fig. 2. Based on the above considerations, we tentatively exclude the mutual passivation of the nonradiative defect centers between TiO₂ and GO as the major source of the blue-shifted emission observed in our GO/TiO₂ and rGO/TiO₂ samples.

To summarize, we have detected IOT in a hybrid spherical structure consisting of alternating nanosheets of TiO₂ and GO (or rGO) from the PL spectral and time-resolved measurements. For GO-based composite materials, such as GO/TiO₂ studied here, much research effort has been devoted to the reduction-induced restoration of the graphene phase and the subsequent electron transfer from TiO₂ to this metallic region, which favors a large enhancement of the photocatalytic activity [17, 26]. In contrast, the IOT observed in this report is a direct consequence of the hole transfer from TiO₂ to the localized sp² domains of GO. This finding has provided a new perspective on the interactions between the localized sp² domains of GO and other semiconductor materials, which has long been neglected in the literature. Technically, an appropriate control over the band alignment of each component in the GO-based composites will not only lead to tunable fluorescent properties, but also promote efficient carrier injection and collection in optical devices of solar cells and light-emitting diodes [2].