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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 27 — Dec. 17, 2012
  • pp: 28801–28807
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Indirect optical transitions in hybrid spheres with alternating layers of titania and graphene oxide nanosheets

Shanshan Bao, Zheng Hua, Xiaoyong Wang, Yong Zhou, Chunfeng Zhang, Wenguang Tu, Zhigang Zou, and Min Xiao  »View Author Affiliations


Optics Express, Vol. 20, Issue 27, pp. 28801-28807 (2012)
http://dx.doi.org/10.1364/OE.20.028801


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Abstract

In this report, we studied the optical properties of hybrid spherical structures consisting of alternating nanosheets of titania (TiO2) and graphene oxide (GO) prepared by a layer-by-layer self-assembly technique. Compared to samples with only TiO2 spheres or GO nanosheets, a blue-to-red light emission band emerges and persists in this novel composite material 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 in a charge-separated configuration.

© 2012 OSA

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

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

, 2

2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

]. With suitable chemical and structural modifications, GO can fluoresce over a broad wavelength range from ultraviolet, visible to near-infrared [1

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

5

5. Z. Luo, P. M. Vora, E. J. Mele, A. T. C. Johnson, and J. M. Kikkawa, “Photoluminescence and band gap modulation in graphene oxide,” Appl. Phys. Lett. 94(11), 111909 (2009). [CrossRef]

]. 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

6. T. Yeh, F. Chan, C. Hsieh, and H. Teng, “Graphite oxide with different oxygenated levels for hydrogen and oxygen production from water under illumination: The band positions of graphite oxide,” J. Phys. Chem. C 115(45), 22587–22597 (2011). [CrossRef]

], solar cells [7

7. X. Wang, L. Zhi, and K. Müllen, “Transparent, conductive graphene electrodes for dye-sensitized solar cells,” Nano Lett. 8(1), 323–327 (2008). [CrossRef] [PubMed]

], light-emitting diodes [8

8. J. Wu, M. Agrawal, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, and P. Peumans, “Organic light-emitting diodes on solution-processed graphene transparent electrodes,” ACS Nano 4(1), 43–48 (2010). [CrossRef] [PubMed]

], biological imaging [4

4. X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric, and H. Dai, “Nano-graphene oxide for cellular imaging and drug delivery,” Nano Res 1(3), 203–212 (2008). [CrossRef] [PubMed]

], and biosensing [9

9. C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen, and G. N. Chen, “A graphene platform for sensing biomolecules,” Angew. Chem. Int. Ed. Engl. 48(26), 4785–4787 (2009). [CrossRef] [PubMed]

]. Moreover, due to its chemical processability, GO can be easily doped [10

10. X. Wang, X. Li, L. Zhang, Y. Yoon, P. K. Weber, H. Wang, J. Guo, and H. Dai, “N-doping of graphene through electrothermal reactions with ammonia,” Science 324(5928), 768–771 (2009). [CrossRef] [PubMed]

], bonded with fluorescent ions [11

11. Z. X. Gan, S. J. Xiong, X. L. Wu, C. Y. He, J. C. Shen, and P. K. Chu, “Mn2+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer,” Nano Lett. 11(9), 3951–3956 (2011). [CrossRef] [PubMed]

], or integrated into other optical materials [1

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

, 2

2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

] 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

12. T. Sasaki and M. Watanabe, “Semiconductor nanosheet crystallites of quasi-TiO2 and their optical properties,” J. Phys. Chem. B 101(49), 10159–10161 (1997). [CrossRef]

, 13

13. N. D. Abazović, M. I. Čomor, M. D. Dramićanin, D. J. Jovanović, S. P. Ahrenkiel, and J. M. Nedeljković, “Photoluminescence of anatase and rutile TiO2 particles,” J. Phys. Chem. B 110(50), 25366–25370 (2006). [CrossRef] [PubMed]

]. Recently, much research effort has been devoted to the construction of TiO2 with nanostructured carbonaceous materials [14

14. K. Woan, G. Pyrgiotakis, and W. Sigmund, “Photocatalytic carbon-nanotube-TiO2 composites,” Adv. Mater. (Deerfield Beach Fla.) 21(21), 2233–2239 (2009). [CrossRef]

]. In particular, the light responsive activity of GO sheets coated with TiO2 nanoparticles can be greatly enhanced along with the improved photocatalytic activity [15

15. Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Lett. 11(7), 2865–2870 (2011). [CrossRef] [PubMed]

, 16

16. H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene composite as a high performance photocatalyst,” ACS Nano 4(1), 380–386 (2010). [CrossRef] [PubMed]

]. 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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

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

In Figs. 2(a)
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.
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

12. T. Sasaki and M. Watanabe, “Semiconductor nanosheet crystallites of quasi-TiO2 and their optical properties,” J. Phys. Chem. B 101(49), 10159–10161 (1997). [CrossRef]

, 13

13. N. D. Abazović, M. I. Čomor, M. D. Dramićanin, D. J. Jovanović, S. P. Ahrenkiel, and J. M. Nedeljković, “Photoluminescence of anatase and rutile TiO2 particles,” J. Phys. Chem. B 110(50), 25366–25370 (2006). [CrossRef] [PubMed]

, 15

15. Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Lett. 11(7), 2865–2870 (2011). [CrossRef] [PubMed]

]. This kind of defect-mediated transitions is normally associated with an extremely low quantum yield (<1%) [13

13. N. D. Abazović, M. I. Čomor, M. D. Dramićanin, D. J. Jovanović, S. P. Ahrenkiel, and J. M. Nedeljković, “Photoluminescence of anatase and rutile TiO2 particles,” J. Phys. Chem. B 110(50), 25366–25370 (2006). [CrossRef] [PubMed]

], as confirmed by the PL decay curve (blue) shown in Fig. 3
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.
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
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.
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.

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

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

], which has been reported to work more efficiently on the long-wavelength side of the TiO2 PL spectrum [15

15. Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Lett. 11(7), 2865–2870 (2011). [CrossRef] [PubMed]

]. 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

18. D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,” Adv. Mater. (Deerfield Beach Fla.) 22(6), 734–738 (2010). [CrossRef] [PubMed]

], however, there are still limited reports on its broad fluorescence spanning the red to near-infrared range [1

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

, 2

2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

, 4

4. X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric, and H. Dai, “Nano-graphene oxide for cellular imaging and drug delivery,” Nano Res 1(3), 203–212 (2008). [CrossRef] [PubMed]

, 5

5. Z. Luo, P. M. Vora, E. J. Mele, A. T. C. Johnson, and J. M. Kikkawa, “Photoluminescence and band gap modulation in graphene oxide,” Appl. Phys. Lett. 94(11), 111909 (2009). [CrossRef]

], which is proposed to come directly from the localized sp2 domains owing to the quantum confinement effect [1

1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

]. In contrast, blue PL was observed from GO only after it received specific treatments, such as appropriate control of the sp2 cluster concentration [3

3. G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, and M. Chhowalla, “Blue photoluminescence from chemically derived graphene oxide,” Adv. Mater. (Deerfield Beach Fla.) 22(4), 505–509 (2010). [CrossRef] [PubMed]

], surface passivation of the reactive sites [19

19. Q. Mei, K. Zhang, G. Guan, B. Liu, S. Wang, and Z. Zhang, “Highly efficient photoluminescent graphene oxide with tunable surface properties,” Chem. Commun. (Camb.) 46(39), 7319–7321 (2010). [CrossRef] [PubMed]

], chemical bonding with fluorescent ions [11

11. Z. X. Gan, S. J. Xiong, X. L. Wu, C. Y. He, J. C. Shen, and P. K. Chu, “Mn2+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer,” Nano Lett. 11(9), 3951–3956 (2011). [CrossRef] [PubMed]

], or hydrothermal cutting into nanometer sized quantum dots [18

18. D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,” Adv. Mater. (Deerfield Beach Fla.) 22(6), 734–738 (2010). [CrossRef] [PubMed]

]. 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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

].

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

2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

, 18

18. D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,” Adv. Mater. (Deerfield Beach Fla.) 22(6), 734–738 (2010). [CrossRef] [PubMed]

], 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

6. T. Yeh, F. Chan, C. Hsieh, and H. Teng, “Graphite oxide with different oxygenated levels for hydrogen and oxygen production from water under illumination: The band positions of graphite oxide,” J. Phys. Chem. C 115(45), 22587–22597 (2011). [CrossRef]

, 11

11. Z. X. Gan, S. J. Xiong, X. L. Wu, C. Y. He, J. C. Shen, and P. K. Chu, “Mn2+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer,” Nano Lett. 11(9), 3951–3956 (2011). [CrossRef] [PubMed]

]. 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

23. S. H. Elder, F. M. Cot, Y. Su, S. M. Heald, A. M. Tyryshkin, M. K. Bowman, Y. Gao, A. G. Joly, M. L. Balmer, A. C. Kolwaite, K. A. Magrini, and D. M. Blake, “The discovery and study of nanocrystalline TiO2-(MoO3) core-shell materials,” J. Am. Chem. Soc. 122(21), 5138–5146 (2000). [CrossRef]

], 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

24. V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature 447(7143), 441–446 (2007). [CrossRef] [PubMed]

].

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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

], 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

24. V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature 447(7143), 441–446 (2007). [CrossRef] [PubMed]

], 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

3. G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, and M. Chhowalla, “Blue photoluminescence from chemically derived graphene oxide,” Adv. Mater. (Deerfield Beach Fla.) 22(4), 505–509 (2010). [CrossRef] [PubMed]

]. 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

3. G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, and M. Chhowalla, “Blue photoluminescence from chemically derived graphene oxide,” Adv. Mater. (Deerfield Beach Fla.) 22(4), 505–509 (2010). [CrossRef] [PubMed]

]. The residual fluorescence observed in rGO/TiO2 may reflect the fact that GO cannot be fully reduced to graphene [25

25. A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B. Shenoy, “Structural evolution during the reduction of chemically derived graphene oxide,” Nat. Chem. 2(7), 581–587 (2010). [CrossRef] [PubMed]

]. 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

22. C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl. 51(27), 6662–6666 (2012). [CrossRef] [PubMed]

, 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

22. C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl. 51(27), 6662–6666 (2012). [CrossRef] [PubMed]

. 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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

, 26

26. K. K. Manga, Y. Zhou, Y. Yan, and K. P. Loh, “Multilayer hybrid films consisting of alternating graphene and titania nanosheets with ultrafast electron transfer and photoconversion properties,” Adv. Funct. Mater. 19(22), 3638–3643 (2009). [CrossRef]

] 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

22. C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl. 51(27), 6662–6666 (2012). [CrossRef] [PubMed]

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

19. Q. Mei, K. Zhang, G. Guan, B. Liu, S. Wang, and Z. Zhang, “Highly efficient photoluminescent graphene oxide with tunable surface properties,” Chem. Commun. (Camb.) 46(39), 7319–7321 (2010). [CrossRef] [PubMed]

]. 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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

, 25

25. A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B. Shenoy, “Structural evolution during the reduction of chemically derived graphene oxide,” Nat. Chem. 2(7), 581–587 (2010). [CrossRef] [PubMed]

], which is not too much less than those measured in our GO nanosheets and reported in other GO samples [6

6. T. Yeh, F. Chan, C. Hsieh, and H. Teng, “Graphite oxide with different oxygenated levels for hydrogen and oxygen production from water under illumination: The band positions of graphite oxide,” J. Phys. Chem. C 115(45), 22587–22597 (2011). [CrossRef]

, 22

22. C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl. 51(27), 6662–6666 (2012). [CrossRef] [PubMed]

]. 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

17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

, 26

26. K. K. Manga, Y. Zhou, Y. Yan, and K. P. Loh, “Multilayer hybrid films consisting of alternating graphene and titania nanosheets with ultrafast electron transfer and photoconversion properties,” Adv. Funct. Mater. 19(22), 3638–3643 (2009). [CrossRef]

]. 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

2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

].

Acknowledgments

The authors would like to thank the financial support from State Key Program for Basic Research of China (Nos. 2012CB921801 and 2011CBA00205), the National Natural Science Foundation of China (Nos. 91021013, 11021403 and 11274161), Fundamental Research Funds for the Central Universities, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

References and links

1.

K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem. 2(12), 1015–1024 (2010). [CrossRef] [PubMed]

2.

G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.) 22(22), 2392–2415 (2010). [CrossRef] [PubMed]

3.

G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, and M. Chhowalla, “Blue photoluminescence from chemically derived graphene oxide,” Adv. Mater. (Deerfield Beach Fla.) 22(4), 505–509 (2010). [CrossRef] [PubMed]

4.

X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric, and H. Dai, “Nano-graphene oxide for cellular imaging and drug delivery,” Nano Res 1(3), 203–212 (2008). [CrossRef] [PubMed]

5.

Z. Luo, P. M. Vora, E. J. Mele, A. T. C. Johnson, and J. M. Kikkawa, “Photoluminescence and band gap modulation in graphene oxide,” Appl. Phys. Lett. 94(11), 111909 (2009). [CrossRef]

6.

T. Yeh, F. Chan, C. Hsieh, and H. Teng, “Graphite oxide with different oxygenated levels for hydrogen and oxygen production from water under illumination: The band positions of graphite oxide,” J. Phys. Chem. C 115(45), 22587–22597 (2011). [CrossRef]

7.

X. Wang, L. Zhi, and K. Müllen, “Transparent, conductive graphene electrodes for dye-sensitized solar cells,” Nano Lett. 8(1), 323–327 (2008). [CrossRef] [PubMed]

8.

J. Wu, M. Agrawal, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, and P. Peumans, “Organic light-emitting diodes on solution-processed graphene transparent electrodes,” ACS Nano 4(1), 43–48 (2010). [CrossRef] [PubMed]

9.

C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen, and G. N. Chen, “A graphene platform for sensing biomolecules,” Angew. Chem. Int. Ed. Engl. 48(26), 4785–4787 (2009). [CrossRef] [PubMed]

10.

X. Wang, X. Li, L. Zhang, Y. Yoon, P. K. Weber, H. Wang, J. Guo, and H. Dai, “N-doping of graphene through electrothermal reactions with ammonia,” Science 324(5928), 768–771 (2009). [CrossRef] [PubMed]

11.

Z. X. Gan, S. J. Xiong, X. L. Wu, C. Y. He, J. C. Shen, and P. K. Chu, “Mn2+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer,” Nano Lett. 11(9), 3951–3956 (2011). [CrossRef] [PubMed]

12.

T. Sasaki and M. Watanabe, “Semiconductor nanosheet crystallites of quasi-TiO2 and their optical properties,” J. Phys. Chem. B 101(49), 10159–10161 (1997). [CrossRef]

13.

N. D. Abazović, M. I. Čomor, M. D. Dramićanin, D. J. Jovanović, S. P. Ahrenkiel, and J. M. Nedeljković, “Photoluminescence of anatase and rutile TiO2 particles,” J. Phys. Chem. B 110(50), 25366–25370 (2006). [CrossRef] [PubMed]

14.

K. Woan, G. Pyrgiotakis, and W. Sigmund, “Photocatalytic carbon-nanotube-TiO2 composites,” Adv. Mater. (Deerfield Beach Fla.) 21(21), 2233–2239 (2009). [CrossRef]

15.

Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Lett. 11(7), 2865–2870 (2011). [CrossRef] [PubMed]

16.

H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene composite as a high performance photocatalyst,” ACS Nano 4(1), 380–386 (2010). [CrossRef] [PubMed]

17.

W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater. 22(6), 1215–1221 (2012). [CrossRef]

18.

D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,” Adv. Mater. (Deerfield Beach Fla.) 22(6), 734–738 (2010). [CrossRef] [PubMed]

19.

Q. Mei, K. Zhang, G. Guan, B. Liu, S. Wang, and Z. Zhang, “Highly efficient photoluminescent graphene oxide with tunable surface properties,” Chem. Commun. (Camb.) 46(39), 7319–7321 (2010). [CrossRef] [PubMed]

20.

J. Ito, J. Nakamura, and A. Natori, “Semiconducting nature of the oxygen-adsorbed graphene sheet,” J. Appl. Phys. 103(11), 113712 (2008). [CrossRef]

21.

J. Hensel, G. Wang, Y. Li, and J. Z. Zhang, “Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation,” Nano Lett. 10(2), 478–483 (2010). [CrossRef] [PubMed]

22.

C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl. 51(27), 6662–6666 (2012). [CrossRef] [PubMed]

23.

S. H. Elder, F. M. Cot, Y. Su, S. M. Heald, A. M. Tyryshkin, M. K. Bowman, Y. Gao, A. G. Joly, M. L. Balmer, A. C. Kolwaite, K. A. Magrini, and D. M. Blake, “The discovery and study of nanocrystalline TiO2-(MoO3) core-shell materials,” J. Am. Chem. Soc. 122(21), 5138–5146 (2000). [CrossRef]

24.

V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature 447(7143), 441–446 (2007). [CrossRef] [PubMed]

25.

A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B. Shenoy, “Structural evolution during the reduction of chemically derived graphene oxide,” Nat. Chem. 2(7), 581–587 (2010). [CrossRef] [PubMed]

26.

K. K. Manga, Y. Zhou, Y. Yan, and K. P. Loh, “Multilayer hybrid films consisting of alternating graphene and titania nanosheets with ultrafast electron transfer and photoconversion properties,” Adv. Funct. Mater. 19(22), 3638–3643 (2009). [CrossRef]

OCIS Codes
(300.2530) Spectroscopy : Fluorescence, laser-induced
(300.6470) Spectroscopy : Spectroscopy, semiconductors

ToC Category:
Spectroscopy

History
Original Manuscript: August 30, 2012
Revised Manuscript: November 28, 2012
Manuscript Accepted: December 3, 2012
Published: December 12, 2012

Citation
Shanshan Bao, Zheng Hua, Xiaoyong Wang, Yong Zhou, Chunfeng Zhang, Wenguang Tu, Zhigang Zou, and Min Xiao, "Indirect optical transitions in hybrid spheres with alternating layers of titania and graphene oxide nanosheets," Opt. Express 20, 28801-28807 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-27-28801


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References

  1. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nat. Chem.2(12), 1015–1024 (2010). [CrossRef] [PubMed]
  2. G. Eda and M. Chhowalla, “Chemically derived graphene oxide: Towards large-area thin-film electronics and optoelectronics,” Adv. Mater. (Deerfield Beach Fla.)22(22), 2392–2415 (2010). [CrossRef] [PubMed]
  3. G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, and M. Chhowalla, “Blue photoluminescence from chemically derived graphene oxide,” Adv. Mater. (Deerfield Beach Fla.)22(4), 505–509 (2010). [CrossRef] [PubMed]
  4. X. Sun, Z. Liu, K. Welsher, J. T. Robinson, A. Goodwin, S. Zaric, and H. Dai, “Nano-graphene oxide for cellular imaging and drug delivery,” Nano Res1(3), 203–212 (2008). [CrossRef] [PubMed]
  5. Z. Luo, P. M. Vora, E. J. Mele, A. T. C. Johnson, and J. M. Kikkawa, “Photoluminescence and band gap modulation in graphene oxide,” Appl. Phys. Lett.94(11), 111909 (2009). [CrossRef]
  6. T. Yeh, F. Chan, C. Hsieh, and H. Teng, “Graphite oxide with different oxygenated levels for hydrogen and oxygen production from water under illumination: The band positions of graphite oxide,” J. Phys. Chem. C115(45), 22587–22597 (2011). [CrossRef]
  7. X. Wang, L. Zhi, and K. Müllen, “Transparent, conductive graphene electrodes for dye-sensitized solar cells,” Nano Lett.8(1), 323–327 (2008). [CrossRef] [PubMed]
  8. J. Wu, M. Agrawal, H. A. Becerril, Z. Bao, Z. Liu, Y. Chen, and P. Peumans, “Organic light-emitting diodes on solution-processed graphene transparent electrodes,” ACS Nano4(1), 43–48 (2010). [CrossRef] [PubMed]
  9. C. H. Lu, H. H. Yang, C. L. Zhu, X. Chen, and G. N. Chen, “A graphene platform for sensing biomolecules,” Angew. Chem. Int. Ed. Engl.48(26), 4785–4787 (2009). [CrossRef] [PubMed]
  10. X. Wang, X. Li, L. Zhang, Y. Yoon, P. K. Weber, H. Wang, J. Guo, and H. Dai, “N-doping of graphene through electrothermal reactions with ammonia,” Science324(5928), 768–771 (2009). [CrossRef] [PubMed]
  11. Z. X. Gan, S. J. Xiong, X. L. Wu, C. Y. He, J. C. Shen, and P. K. Chu, “Mn2+-bonded reduced graphene oxide with strong radiative recombination in broad visible range caused by resonant energy transfer,” Nano Lett.11(9), 3951–3956 (2011). [CrossRef] [PubMed]
  12. T. Sasaki and M. Watanabe, “Semiconductor nanosheet crystallites of quasi-TiO2 and their optical properties,” J. Phys. Chem. B101(49), 10159–10161 (1997). [CrossRef]
  13. N. D. Abazović, M. I. Čomor, M. D. Dramićanin, D. J. Jovanović, S. P. Ahrenkiel, and J. M. Nedeljković, “Photoluminescence of anatase and rutile TiO2 particles,” J. Phys. Chem. B110(50), 25366–25370 (2006). [CrossRef] [PubMed]
  14. K. Woan, G. Pyrgiotakis, and W. Sigmund, “Photocatalytic carbon-nanotube-TiO2 composites,” Adv. Mater. (Deerfield Beach Fla.)21(21), 2233–2239 (2009). [CrossRef]
  15. Y. T. Liang, B. K. Vijayan, K. A. Gray, and M. C. Hersam, “Minimizing graphene defects enhances titania nanocomposite-based photocatalytic reduction of CO2 for improved solar fuel production,” Nano Lett.11(7), 2865–2870 (2011). [CrossRef] [PubMed]
  16. H. Zhang, X. Lv, Y. Li, Y. Wang, and J. Li, “P25-graphene composite as a high performance photocatalyst,” ACS Nano4(1), 380–386 (2010). [CrossRef] [PubMed]
  17. W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, and Z. Zou, “Robust hollow spheres consisting of alternating titania nanosheets and graphene nanosheets with high photocatalytic activity for CO2 conversion into renewable fuels,” Adv. Funct. Mater.22(6), 1215–1221 (2012). [CrossRef]
  18. D. Pan, J. Zhang, Z. Li, and M. Wu, “Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots,” Adv. Mater. (Deerfield Beach Fla.)22(6), 734–738 (2010). [CrossRef] [PubMed]
  19. Q. Mei, K. Zhang, G. Guan, B. Liu, S. Wang, and Z. Zhang, “Highly efficient photoluminescent graphene oxide with tunable surface properties,” Chem. Commun. (Camb.)46(39), 7319–7321 (2010). [CrossRef] [PubMed]
  20. J. Ito, J. Nakamura, and A. Natori, “Semiconducting nature of the oxygen-adsorbed graphene sheet,” J. Appl. Phys.103(11), 113712 (2008). [CrossRef]
  21. J. Hensel, G. Wang, Y. Li, and J. Z. Zhang, “Synergistic effect of CdSe quantum dot sensitization and nitrogen doping of TiO2 nanostructures for photoelectrochemical solar hydrogen generation,” Nano Lett.10(2), 478–483 (2010). [CrossRef] [PubMed]
  22. C. T. Chien, S. S. Li, W. J. Lai, Y. C. Yeh, H. A. Chen, I. S. Chen, L. C. Chen, K. H. Chen, T. Nemoto, S. Isoda, M. Chen, T. Fujita, G. Eda, H. Yamaguchi, M. Chhowalla, and C. W. Chen, “Tunable photoluminescence from graphene oxide,” Angew. Chem. Int. Ed. Engl.51(27), 6662–6666 (2012). [CrossRef] [PubMed]
  23. S. H. Elder, F. M. Cot, Y. Su, S. M. Heald, A. M. Tyryshkin, M. K. Bowman, Y. Gao, A. G. Joly, M. L. Balmer, A. C. Kolwaite, K. A. Magrini, and D. M. Blake, “The discovery and study of nanocrystalline TiO2-(MoO3) core-shell materials,” J. Am. Chem. Soc.122(21), 5138–5146 (2000). [CrossRef]
  24. V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. McGuire, and A. Piryatinski, “Single-exciton optical gain in semiconductor nanocrystals,” Nature447(7143), 441–446 (2007). [CrossRef] [PubMed]
  25. A. Bagri, C. Mattevi, M. Acik, Y. J. Chabal, M. Chhowalla, and V. B. Shenoy, “Structural evolution during the reduction of chemically derived graphene oxide,” Nat. Chem.2(7), 581–587 (2010). [CrossRef] [PubMed]
  26. K. K. Manga, Y. Zhou, Y. Yan, and K. P. Loh, “Multilayer hybrid films consisting of alternating graphene and titania nanosheets with ultrafast electron transfer and photoconversion properties,” Adv. Funct. Mater.19(22), 3638–3643 (2009). [CrossRef]

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