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Cite this: DOI: 10.1039/c3cc48026g Received 18th October 2013, Accepted 2nd December 2013
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Co3O4 quantum dots: reverse micelle synthesis and visible-light-driven photocatalytic overall water splitting† Ning Zhang,a Jinwen Shi,a Samuel S. Maoab and Liejin Guo*a
DOI: 10.1039/c3cc48026g www.rsc.org/chemcomm
Co3O4 quantum dots were synthesized by a facile reverse micelle method for the first time, and were capable of splitting pure water into O2 and H2 stoichiometrically under visible-light irradiation (k > 420 nm) without any cocatalyst.
Ever since the first report by Fujishima and Honda,1 photocatalytic H2 production from water splitting has been considered as an ideal solution to the world’s energy problem.2 Up to now, a handful of semiconductors and systems, such as In1 xNixTaO4,3 GaN–ZnO,4 YBiWO65 and Pt/ZrO2/TaON–Pt/WO3 Z-scheme systems,6 capable of overall water splitting into H2 and O2 have been reported. The GaN–ZnO solid solution has been demonstrated to be an excellent photocatalyst for overall water splitting under visible-light irradiation.4 However, it usually required a difficult method and high cost to prepare this highly efficient photocatalyst and to load the cocatalyst. Therefore, design of a novel semiconductor photocatalyst that can be easily synthesized and split pure water under visible-light irradiation without a cocatalyst is extremely urgent. A fundamental bottleneck in photocatalytic overall water splitting is the O2-production half-reaction, which is a multielectron transfer process.6 Owing to the excellent oxidation ability (low valence band maximum (VBM)), nontoxicity, chemical inertness and low cost, Co3O4 with a bandgap of about 2.1 eV has been widely employed as a photocatalyst or cocatalyst for the visible-light-driven photocatalytic O2 evolution reaction.7 Nevertheless, Co3O4 as a single photocatalyst for overall water splitting under visible-light irradiation has not yet been reported due to a lower conduction band minimum (CBM) than the H+ reduction potential.8 Consequently, overall water splitting on a single photocatalyst a
International Research Center for Renewable Energy (IRCRE), State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), Xi’an Jiaotong University (XJTU), 28 West Xianning Road, Xi’an, Shaanxi 710049, P. R. China. E-mail: [email protected] b Lawrence Berkeley National Laboratory, Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, USA † Electronic supplementary information (ESI) available: Experimental section; calculation of the turnover number; supplementary figures and tables. See DOI: 10.1039/c3cc48026g
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of Co3O4 under visible-light irradiation is expected to be achieved if CBM is elevated. Quantum dots with quantum confinement effect have been intensively investigated recently.9 Once the crystal size of the semiconductor is close to its exciton Bohr radius, its bandgap will be enlarged when the crystal size is decreased.10 CdSe quantum dots showed a high quantum yield for photocatalytic H2 evolution under visible-light irradiation.11 In this work, we tried to prepare Co3O4 quantum dots designated as Co3O4-QDs in order to widen its bandgap by the quantum confinement effect, and thus to shift the CBM higher than the H+ reduction potential, and finally expected to realize overall water splitting. Co3O4 quantum dots were prepared via a facile reverse micelle method using a rapid microwave-assisted solvothermal process (see Fig. S1, ESI†). To obtain a highly monodisperse yield of nanocrystals, nucleation must occur rapidly and instantaneously.12 Microwave was thus introduced to accelerate the nucleation process, and the reaction time was precisely controlled to suppress the overgrowth of nanocrystals. For comparison, the sample designated as Co3O4-SSR was prepared by a solid-state reaction. Powder X-ray diffraction (PXRD) patterns (see Fig. S2, ESI†) revealed that both Co3O4-QDs and Co3O4-SSR were in cubic phase (JCPDS No. 00-001-1152). It can be seen that the diffraction peaks of Co3O4-QDs broadened compared with those of Co3O4-SSR, thus indicating that the crystal size of Co3O4-QDs was much smaller than that of Co3O4-SSR. Survey-scan X-ray photoelectron spectroscopy (XPS) (Fig. S3a, ESI†) proved that only Co, O and C elements could be found in both Co3O4-QDs and Co3O4-SSR. High-resolution XPS spectra of Co 2p (Fig. S3b, ESI†) showed that peaks at 780.3, 782.1, 795.3, 798.0, 787.9, and 803.6 eV corresponded to Co2+ (2p1/2), Co3+ (2p1/2), Co2+ (2p3/2), Co3+ (2p3/2), shake-up satellites of Co2+ (2p1/2) and Co2+ (2p3/2), respectively,13 and that the calculated area ratio of Co3+ (2p3/2) to Co2+ (2p3/2) for both samples approximated to 2. It was thus deduced that both samples were in the chemical form of Co2+(Co3+)2O4. As shown by the transmission electron microscopy (TEM) image (see Fig. 1a), irregular polyhedron nanoparticles with a size of around 20–60 nm could be seen in Co3O4-SSR, which was
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Fig. 2 (a) Raman spectra, (b) UV-Vis spectra, and (c) PL spectra (excitation wavelength, 420 nm) of Co3O4-QDs and Co3O4-SSR. Inset of (b): photographs of Co3O4-QDs and Co3O4-SSR dispersed in absolute ethanol (bottom left) and valence-band XPS spectra of Co3O4-QDs and Co3O4SSR (top right).
Fig. 1 (a and b) TEM images, (c) HRTEM image, and (d) selected-area electron diffraction pattern of (a) Co3O4-SSR; (b, c and d) Co3O4-QDs. The squared region in (b) was used for HRTEM, and the circled region in (b) was used for selected-area electron diffraction.
consistent with the literature.14 In contrast, nanospheres assembled by nanocrystals with smaller size could be found in Co3O4-QDs (see Fig. 1b). Selected-area electron diffraction (see Fig. 1d) confirmed the polycrystalline characteristic of nanospheres. Diffraction rings with diameters of 2.34, 1.43, 1.24, and 0.93 Å corresponded to the lattice planes of (222), (440), (533) and (662) for cubic-phase Co3O4, respectively. It can be seen from the high-resolution transmission electron microscopy (HRTEM) image (see Fig. 1c) that the nanocrystals forming nanospheres all had a size of 3–4 nm, which was close to the exciton Bohr radius, thus verifying the successful preparation of Co3O4 quantum dots. The agglomeration of nanocrystals into nanospheres rendered Co3O4-QDs stable by reducing the surface energy without destroying the individual properties of quantum dots. Moreover, the agglomerated quantum dots could be easily dispersed in ethanol by ultrasonic treatment to form a metastable (more than 3 days) colloid, while Co3O4-SSR formed an unstable (less than 1 hour) suspension after ultrasonic treatment. The Raman spectrum of Co3O4-SSR (see Fig. S4, ESI†) showed four peaks at around 477, 518, 614, and 685 cm 1 that corresponded to Eg, F2g1, F2g2, and A1g modes of cubic-phase Co3O4, respectively.14 Compared with those of Co3O4-SSR, Raman peaks (see Fig. 2a and Fig. S4, ESI†) of Co3O4-QDs exhibited a distinct redshift and were broadened, which could be attributed to the quantum confinement effect of phonon modes.15,18 Besides, as listed in Table S1 (see ESI†), the Brunauer–Emmett–Teller (BET) surface area of Co3O4-QDs (147.8 m2 g 1) amounted to around 15 times of that of Co3O4-SSR (9.6 m2 g 1) due to the smaller particle size of Co3O4-QDs. It could be seen from UV-Vis spectra (see Fig. 2b) that the absorption edge of Co3O4-QDs exhibited a clear blueshift from 950 to 850 nm compared with that of Co3O4-SSR, which agreed with the change of color (see inset in Fig. 2b) from black (Co3O4-SSR) to yellow (Co3O4-QDs) and further confirmed the quantum confinement effect. Moreover, it was deduced that Co3O4-SSR showed two Eg with values of 1.57 (Eg1) and 2.15 eV (Eg2) (see Fig. S5, ESI†), which agreed with the band structure of
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Co3O4 with Co3+ t2g - Co2+ t2g (Eg1), O 2p - Co2+ t2g (Eg2), and O 2p - Co3+ eg (Eg3, not observed due to the low signal-to-noise ratio in the UV region) charge-transfer transitions, respectively.8,16 In contrast, the bandgaps of Co3O4-QDs were 1.74 (Eg1) and 2.26 eV (Eg2), respectively, and were obviously wider than those of Co3O4SSR. Furthermore, valence-band XPS spectra (see Fig. 2c) showed that VBMs of Co3O4-QDs and Co3O4-SSR are almost at the same position (O 2p, 0.718 eV), therefore implying that bandgap widening of Co3O4-QDs was mainly ascribed to the CBM upshift, which would provide higher potential for H+ reduction beneficial for photocatalytic H2 evolution. In addition, compared with that of Co3O4-SSR, the fluorescence peak of Co3O4-QDs in the photoluminescence (PL) spectrum narrowed and blueshifted (see Fig. 2c), which was consistent with the change of optical absorption properties originating from the quantum confinement effect.9 The photocatalytic reaction was carried out without loading the cocatalyst. In order to eliminate the mechano-catalytic H2 production effect,17 the photocatalytic system was run without stirring. Control experiments showed that no H2 was formed when the reaction proceeded without a photocatalyst or in the dark while other conditions remained unchanged. Fig. 3a shows the UV-driven photocatalytic H2 evolution on Co3O4-QDs and Co3O4-SSR using ethanol as the sacrificial reagent. It can be seen that Co3O4-SSR showed the ability for photocatalytic H2 production (3.03 mmol h 1), and that Co3O4-QDs showed a higher (around 1.8 times) photocatalytic activity for H2 evolution (5.46 mmol h 1). The improvement of activity of Co3O4-QDs could be attributed to the smaller crystal size that resulted in higher specific surface area (providing more reactive sites), shorter migration distance (reducing the recombination of photogenerated charge carriers) and stronger redox ability of photogenerated charge carriers beneficial for the photocatalytic reaction. Fig. 3b shows the visible-light-driven (l > 420 nm) photocatalytic H2 evolution on Co3O4-QDs and Co3O4-SSR using ethanol as the sacrificial reagent. Co3O4-QDs showed activity for H2 evolution (1.10 mmol h 1) but Co3O4-SSR did not. A series of photocatalytic tests were supplemented by the same method over Co3O4-SSR and Co3O4-QD photocatalysts except that different UV cutoff filters (i.e., 380 and 400 nm) were employed. The results are presented in Table S2 (ESI†). It can be seen that Co3O4-SSR turned on for water splitting when the cutoff wavelength of the filter was 380 nm (3.26 eV), while Co3O4-SSR turned off when the cutoff wavelength of the filter was 400 nm (3.10 eV). Eg3 transition could result in
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Fig. 3 (a and b) Photocatalytic H2 evolution on Co3O4-QDs and Co3O4SSR under (a) UV irradiation, and (b) visible-light irradiation (l > 420 nm) using 0.010 g photocatalyst suspended in 100 mL 50% ethanol aqueous solution in a Pyrex glass cell; (c) schematic band structures of Co3O4-SSR and Co3O4-QDs; (d) photocatalytic overall water splitting on Co3O4-QDs under visible-light irradiation (l > 420 nm) using 0.010 g photocatalyst suspended in 100 mL pure water in a Pyrex glass cell.
photogenerated electrons on Co2+ eg and thus enable Co3O4-SSR for water splitting. It was thus demonstrated that the Eg3 value of Co3O4SSR was between 3.10 and 3.26 eV. By combining the results of UV-Vis spectra, valence-band XPS spectra and the above photocatalytic activities, the band structures of Co3O4-QDs and Co3O4-SSR are schematically illustrated in Fig. 3c. Co3O4-SSR turned off for water splitting when the cutoff wavelength of the filter was 420 nm (2.95 eV), while Co3O4-QDs turned on when the cutoff wavelength of the filter was 420 nm (2.95 eV). On the one hand, Eg3 transition could not happen on both Co3O4-SSR and Co3O4-QDs when the cutoff wavelength of the filter was 400 nm, thus resulting in no photogenerated electrons on the energy level of Co3+ eg to participate in the H+ reduction half-reaction. On the other hand, Eg1 and Eg2 transitions happened on both Co3O4-SSR and Co3O4-QDs when the cutoff wavelength of the filter was 400 nm, thus resulting in photogenerated electrons on Co2+ t2g. However, the photogenerated electrons on Co2+ t2g for Co3O4-SSR could not take part in the H+ reduction half-reaction to evolve H2 because of their insufficient reduction ability. In contrast, the photogenerated electrons on Co2+ t2g for Co3O4-QDs reduced H+ to H2 because of the upshift of Co2+ t2g to a position higher than the H+ reduction potential caused by the quantum confinement effect. Both Co3O4-SSR and Co3O4-QDs showed UV-driven photocatalytic activity for H2 evolution mainly due to the participation of the electrons photogenerated from the Eg3 transition in H+ reduction half-reaction. In addition, electrons photogenerated from the Eg1 and Eg2 transitions on Co3O4-QDs also contributed to the H2 evolution reaction, which thus further improves the photocatalytic activity. In view of the optimized band structure of Co3O4-QDs compared with that of Co3O4-SSR, the visible-light-driven photocatalytic overall water splitting was carried out without loading the cocatalyst. It was encouraging that Co3O4-QDs exhibited visible-light-driven
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photocatalytic activity with stable H2 (0.79 mmol h 1) and O2 (0.40 mmol h 1) evolution rates (see Fig. 3d) and with the ratio of H2 to O2 approximately equal to 2 over 244 h of test (see Fig. 3d), and the corresponding turnover number (TON, see ESI†) for H2 (or O2) evolution was 9.3 (or 9.4), thereby confirming the process of photocatalytic overall water splitting.3,19 To the best of our knowledge, this is the first report of Co3O4 functioning as a stable photocatalyst for overall water splitting under visible-light irradiation without any cocatalyst. Besides, the gradually increased rates in the initial stages of both H2 and O2 evolution processes were caused by gas diffusion in the free volume of the reactor. Moreover, XPS spectra (Fig. S6, ESI†) showed that the surface components and chemical states of Co3O4-QDs did not change before and after reaction, thus confirming the excellent stability. In conclusion, we presented a reverse micelle method using a microwave-assisted solvothermal reaction for simply and rapidly synthesizing 3–4 nm Co3O4 quantum dots for the first time. Under visible-light irradiation, Co3O4 quantum dots were capable of photocatalytic overall water splitting, which is the first report to date and is attributed to the shift of the conduction band to the level negative than the reduction potential of H+. It provided a potential strategy for applying narrow bandgap semiconductors in pure water splitting. We are grateful for financial support by the National Natural Science Foundation of China (No. 51302212, 51236007 and 51121092), the National Basic Research Program of China (No. 2009CB220000), the China Postdoctoral Science Foundation (No. 2013M540745), and the Fundamental Research Funds for the Central Universities (No. 2013jdhz20).
Notes and references 1 A. Fujishima and K. Honda, Nature, 1972, 238, 37–38. 2 X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503–6570. 3 Z. G. Zou, J. H. Ye, K. Sayama and H. Arakawa, Nature, 2001, 414, 625–627. 4 K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue and K. Domen, Nature, 2006, 440, 295. 5 H. Liu, J. Yuan, W. F. Shangguan and Y. Teraoka, J. Phys. Chem. C, 2008, 112, 8521–8523. 6 K. Maeda, M. Higashi, D. L. Lu, R. Abe and K. Domen, J. Am. Chem. Soc., 2010, 132, 5858–5868. 7 S. Yusuf and F. Jiao, ACS Catal., 2012, 2, 2753–2760. 8 M. C. Long, W. M. Cai, J. Cai, B. X. Zhou, X. Y. Chai and Y. H. Wu, J. Phys. Chem. B, 2006, 110, 20211–20216. 9 C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102. 10 A. P. Alivisatos, Science, 1996, 271, 933–937. 11 M. A. Holmes, T. K. Townsend and F. E. Osterloh, Chem. Commun., 2012, 48, 371–373. 12 A. R. Tao, S. Habas and P. D. Yang, Small, 2008, 4, 310–325. 13 L. Fu, Z. M. Liu, Y. Q. Liu, B. X. Han, P. G. Hu, L. C. Cao and D. B. Zhu, Adv. Mater., 2005, 17, 217–221. 14 F. Gu, C. Z. Li, Y. J. Hu and L. Zhang, J. Cryst. Growth, 2007, 304, 369–373. 15 M. J. Seong, O. I. Micic, A. J. Nozik, A. Mascarenhas and H. M. Cheong, Appl. Phys. Lett., 2003, 82, 185–187. 16 K. J. Kim and Y. R. Park, Solid State Commun., 2003, 127, 25–28. 17 S. Ikeda, T. Takata, T. Kondo, G. Hitoki, M. Hara, J. N. Kondo, K. Domen, H. Hosono, H. Kawazoe and A. Tanaka, Chem. Commun., 1998, 2185–2186. 18 J. W. Shi, L. J. Ma, P. Wu, Z. H. Zhou, P. H. Guo, S. H. Shen, D. W. Jing and L. J. Guo, Nano Res., 2012, 5, 576–583. 19 A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253–278.
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Cite this: DOI: 10.1039/c3cc48026g Received 18th October 2013, Accepted 2nd December 2013
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Co3O4 quantum dots: reverse micelle synthesis and visible-light-driven photocatalytic overall water splitting† Ning Zhang,a Jinwen Shi,a Samuel S. Maoab and Liejin Guo*a
DOI: 10.1039/c3cc48026g www.rsc.org/chemcomm
Co3O4 quantum dots were synthesized by a facile reverse micelle method for the first time, and were capable of splitting pure water into O2 and H2 stoichiometrically under visible-light irradiation (k > 420 nm) without any cocatalyst.
Ever since the first report by Fujishima and Honda,1 photocatalytic H2 production from water splitting has been considered as an ideal solution to the world’s energy problem.2 Up to now, a handful of semiconductors and systems, such as In1 xNixTaO4,3 GaN–ZnO,4 YBiWO65 and Pt/ZrO2/TaON–Pt/WO3 Z-scheme systems,6 capable of overall water splitting into H2 and O2 have been reported. The GaN–ZnO solid solution has been demonstrated to be an excellent photocatalyst for overall water splitting under visible-light irradiation.4 However, it usually required a difficult method and high cost to prepare this highly efficient photocatalyst and to load the cocatalyst. Therefore, design of a novel semiconductor photocatalyst that can be easily synthesized and split pure water under visible-light irradiation without a cocatalyst is extremely urgent. A fundamental bottleneck in photocatalytic overall water splitting is the O2-production half-reaction, which is a multielectron transfer process.6 Owing to the excellent oxidation ability (low valence band maximum (VBM)), nontoxicity, chemical inertness and low cost, Co3O4 with a bandgap of about 2.1 eV has been widely employed as a photocatalyst or cocatalyst for the visible-light-driven photocatalytic O2 evolution reaction.7 Nevertheless, Co3O4 as a single photocatalyst for overall water splitting under visible-light irradiation has not yet been reported due to a lower conduction band minimum (CBM) than the H+ reduction potential.8 Consequently, overall water splitting on a single photocatalyst a
International Research Center for Renewable Energy (IRCRE), State Key Laboratory of Multiphase Flow in Power Engineering (MFPE), Xi’an Jiaotong University (XJTU), 28 West Xianning Road, Xi’an, Shaanxi 710049, P. R. China. E-mail: [email protected] b Lawrence Berkeley National Laboratory, Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA 94720, USA † Electronic supplementary information (ESI) available: Experimental section; calculation of the turnover number; supplementary figures and tables. See DOI: 10.1039/c3cc48026g
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of Co3O4 under visible-light irradiation is expected to be achieved if CBM is elevated. Quantum dots with quantum confinement effect have been intensively investigated recently.9 Once the crystal size of the semiconductor is close to its exciton Bohr radius, its bandgap will be enlarged when the crystal size is decreased.10 CdSe quantum dots showed a high quantum yield for photocatalytic H2 evolution under visible-light irradiation.11 In this work, we tried to prepare Co3O4 quantum dots designated as Co3O4-QDs in order to widen its bandgap by the quantum confinement effect, and thus to shift the CBM higher than the H+ reduction potential, and finally expected to realize overall water splitting. Co3O4 quantum dots were prepared via a facile reverse micelle method using a rapid microwave-assisted solvothermal process (see Fig. S1, ESI†). To obtain a highly monodisperse yield of nanocrystals, nucleation must occur rapidly and instantaneously.12 Microwave was thus introduced to accelerate the nucleation process, and the reaction time was precisely controlled to suppress the overgrowth of nanocrystals. For comparison, the sample designated as Co3O4-SSR was prepared by a solid-state reaction. Powder X-ray diffraction (PXRD) patterns (see Fig. S2, ESI†) revealed that both Co3O4-QDs and Co3O4-SSR were in cubic phase (JCPDS No. 00-001-1152). It can be seen that the diffraction peaks of Co3O4-QDs broadened compared with those of Co3O4-SSR, thus indicating that the crystal size of Co3O4-QDs was much smaller than that of Co3O4-SSR. Survey-scan X-ray photoelectron spectroscopy (XPS) (Fig. S3a, ESI†) proved that only Co, O and C elements could be found in both Co3O4-QDs and Co3O4-SSR. High-resolution XPS spectra of Co 2p (Fig. S3b, ESI†) showed that peaks at 780.3, 782.1, 795.3, 798.0, 787.9, and 803.6 eV corresponded to Co2+ (2p1/2), Co3+ (2p1/2), Co2+ (2p3/2), Co3+ (2p3/2), shake-up satellites of Co2+ (2p1/2) and Co2+ (2p3/2), respectively,13 and that the calculated area ratio of Co3+ (2p3/2) to Co2+ (2p3/2) for both samples approximated to 2. It was thus deduced that both samples were in the chemical form of Co2+(Co3+)2O4. As shown by the transmission electron microscopy (TEM) image (see Fig. 1a), irregular polyhedron nanoparticles with a size of around 20–60 nm could be seen in Co3O4-SSR, which was
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Fig. 2 (a) Raman spectra, (b) UV-Vis spectra, and (c) PL spectra (excitation wavelength, 420 nm) of Co3O4-QDs and Co3O4-SSR. Inset of (b): photographs of Co3O4-QDs and Co3O4-SSR dispersed in absolute ethanol (bottom left) and valence-band XPS spectra of Co3O4-QDs and Co3O4SSR (top right).
Fig. 1 (a and b) TEM images, (c) HRTEM image, and (d) selected-area electron diffraction pattern of (a) Co3O4-SSR; (b, c and d) Co3O4-QDs. The squared region in (b) was used for HRTEM, and the circled region in (b) was used for selected-area electron diffraction.
consistent with the literature.14 In contrast, nanospheres assembled by nanocrystals with smaller size could be found in Co3O4-QDs (see Fig. 1b). Selected-area electron diffraction (see Fig. 1d) confirmed the polycrystalline characteristic of nanospheres. Diffraction rings with diameters of 2.34, 1.43, 1.24, and 0.93 Å corresponded to the lattice planes of (222), (440), (533) and (662) for cubic-phase Co3O4, respectively. It can be seen from the high-resolution transmission electron microscopy (HRTEM) image (see Fig. 1c) that the nanocrystals forming nanospheres all had a size of 3–4 nm, which was close to the exciton Bohr radius, thus verifying the successful preparation of Co3O4 quantum dots. The agglomeration of nanocrystals into nanospheres rendered Co3O4-QDs stable by reducing the surface energy without destroying the individual properties of quantum dots. Moreover, the agglomerated quantum dots could be easily dispersed in ethanol by ultrasonic treatment to form a metastable (more than 3 days) colloid, while Co3O4-SSR formed an unstable (less than 1 hour) suspension after ultrasonic treatment. The Raman spectrum of Co3O4-SSR (see Fig. S4, ESI†) showed four peaks at around 477, 518, 614, and 685 cm 1 that corresponded to Eg, F2g1, F2g2, and A1g modes of cubic-phase Co3O4, respectively.14 Compared with those of Co3O4-SSR, Raman peaks (see Fig. 2a and Fig. S4, ESI†) of Co3O4-QDs exhibited a distinct redshift and were broadened, which could be attributed to the quantum confinement effect of phonon modes.15,18 Besides, as listed in Table S1 (see ESI†), the Brunauer–Emmett–Teller (BET) surface area of Co3O4-QDs (147.8 m2 g 1) amounted to around 15 times of that of Co3O4-SSR (9.6 m2 g 1) due to the smaller particle size of Co3O4-QDs. It could be seen from UV-Vis spectra (see Fig. 2b) that the absorption edge of Co3O4-QDs exhibited a clear blueshift from 950 to 850 nm compared with that of Co3O4-SSR, which agreed with the change of color (see inset in Fig. 2b) from black (Co3O4-SSR) to yellow (Co3O4-QDs) and further confirmed the quantum confinement effect. Moreover, it was deduced that Co3O4-SSR showed two Eg with values of 1.57 (Eg1) and 2.15 eV (Eg2) (see Fig. S5, ESI†), which agreed with the band structure of
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Co3O4 with Co3+ t2g - Co2+ t2g (Eg1), O 2p - Co2+ t2g (Eg2), and O 2p - Co3+ eg (Eg3, not observed due to the low signal-to-noise ratio in the UV region) charge-transfer transitions, respectively.8,16 In contrast, the bandgaps of Co3O4-QDs were 1.74 (Eg1) and 2.26 eV (Eg2), respectively, and were obviously wider than those of Co3O4SSR. Furthermore, valence-band XPS spectra (see Fig. 2c) showed that VBMs of Co3O4-QDs and Co3O4-SSR are almost at the same position (O 2p, 0.718 eV), therefore implying that bandgap widening of Co3O4-QDs was mainly ascribed to the CBM upshift, which would provide higher potential for H+ reduction beneficial for photocatalytic H2 evolution. In addition, compared with that of Co3O4-SSR, the fluorescence peak of Co3O4-QDs in the photoluminescence (PL) spectrum narrowed and blueshifted (see Fig. 2c), which was consistent with the change of optical absorption properties originating from the quantum confinement effect.9 The photocatalytic reaction was carried out without loading the cocatalyst. In order to eliminate the mechano-catalytic H2 production effect,17 the photocatalytic system was run without stirring. Control experiments showed that no H2 was formed when the reaction proceeded without a photocatalyst or in the dark while other conditions remained unchanged. Fig. 3a shows the UV-driven photocatalytic H2 evolution on Co3O4-QDs and Co3O4-SSR using ethanol as the sacrificial reagent. It can be seen that Co3O4-SSR showed the ability for photocatalytic H2 production (3.03 mmol h 1), and that Co3O4-QDs showed a higher (around 1.8 times) photocatalytic activity for H2 evolution (5.46 mmol h 1). The improvement of activity of Co3O4-QDs could be attributed to the smaller crystal size that resulted in higher specific surface area (providing more reactive sites), shorter migration distance (reducing the recombination of photogenerated charge carriers) and stronger redox ability of photogenerated charge carriers beneficial for the photocatalytic reaction. Fig. 3b shows the visible-light-driven (l > 420 nm) photocatalytic H2 evolution on Co3O4-QDs and Co3O4-SSR using ethanol as the sacrificial reagent. Co3O4-QDs showed activity for H2 evolution (1.10 mmol h 1) but Co3O4-SSR did not. A series of photocatalytic tests were supplemented by the same method over Co3O4-SSR and Co3O4-QD photocatalysts except that different UV cutoff filters (i.e., 380 and 400 nm) were employed. The results are presented in Table S2 (ESI†). It can be seen that Co3O4-SSR turned on for water splitting when the cutoff wavelength of the filter was 380 nm (3.26 eV), while Co3O4-SSR turned off when the cutoff wavelength of the filter was 400 nm (3.10 eV). Eg3 transition could result in
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Fig. 3 (a and b) Photocatalytic H2 evolution on Co3O4-QDs and Co3O4SSR under (a) UV irradiation, and (b) visible-light irradiation (l > 420 nm) using 0.010 g photocatalyst suspended in 100 mL 50% ethanol aqueous solution in a Pyrex glass cell; (c) schematic band structures of Co3O4-SSR and Co3O4-QDs; (d) photocatalytic overall water splitting on Co3O4-QDs under visible-light irradiation (l > 420 nm) using 0.010 g photocatalyst suspended in 100 mL pure water in a Pyrex glass cell.
photogenerated electrons on Co2+ eg and thus enable Co3O4-SSR for water splitting. It was thus demonstrated that the Eg3 value of Co3O4SSR was between 3.10 and 3.26 eV. By combining the results of UV-Vis spectra, valence-band XPS spectra and the above photocatalytic activities, the band structures of Co3O4-QDs and Co3O4-SSR are schematically illustrated in Fig. 3c. Co3O4-SSR turned off for water splitting when the cutoff wavelength of the filter was 420 nm (2.95 eV), while Co3O4-QDs turned on when the cutoff wavelength of the filter was 420 nm (2.95 eV). On the one hand, Eg3 transition could not happen on both Co3O4-SSR and Co3O4-QDs when the cutoff wavelength of the filter was 400 nm, thus resulting in no photogenerated electrons on the energy level of Co3+ eg to participate in the H+ reduction half-reaction. On the other hand, Eg1 and Eg2 transitions happened on both Co3O4-SSR and Co3O4-QDs when the cutoff wavelength of the filter was 400 nm, thus resulting in photogenerated electrons on Co2+ t2g. However, the photogenerated electrons on Co2+ t2g for Co3O4-SSR could not take part in the H+ reduction half-reaction to evolve H2 because of their insufficient reduction ability. In contrast, the photogenerated electrons on Co2+ t2g for Co3O4-QDs reduced H+ to H2 because of the upshift of Co2+ t2g to a position higher than the H+ reduction potential caused by the quantum confinement effect. Both Co3O4-SSR and Co3O4-QDs showed UV-driven photocatalytic activity for H2 evolution mainly due to the participation of the electrons photogenerated from the Eg3 transition in H+ reduction half-reaction. In addition, electrons photogenerated from the Eg1 and Eg2 transitions on Co3O4-QDs also contributed to the H2 evolution reaction, which thus further improves the photocatalytic activity. In view of the optimized band structure of Co3O4-QDs compared with that of Co3O4-SSR, the visible-light-driven photocatalytic overall water splitting was carried out without loading the cocatalyst. It was encouraging that Co3O4-QDs exhibited visible-light-driven
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photocatalytic activity with stable H2 (0.79 mmol h 1) and O2 (0.40 mmol h 1) evolution rates (see Fig. 3d) and with the ratio of H2 to O2 approximately equal to 2 over 244 h of test (see Fig. 3d), and the corresponding turnover number (TON, see ESI†) for H2 (or O2) evolution was 9.3 (or 9.4), thereby confirming the process of photocatalytic overall water splitting.3,19 To the best of our knowledge, this is the first report of Co3O4 functioning as a stable photocatalyst for overall water splitting under visible-light irradiation without any cocatalyst. Besides, the gradually increased rates in the initial stages of both H2 and O2 evolution processes were caused by gas diffusion in the free volume of the reactor. Moreover, XPS spectra (Fig. S6, ESI†) showed that the surface components and chemical states of Co3O4-QDs did not change before and after reaction, thus confirming the excellent stability. In conclusion, we presented a reverse micelle method using a microwave-assisted solvothermal reaction for simply and rapidly synthesizing 3–4 nm Co3O4 quantum dots for the first time. Under visible-light irradiation, Co3O4 quantum dots were capable of photocatalytic overall water splitting, which is the first report to date and is attributed to the shift of the conduction band to the level negative than the reduction potential of H+. It provided a potential strategy for applying narrow bandgap semiconductors in pure water splitting. We are grateful for financial support by the National Natural Science Foundation of China (No. 51302212, 51236007 and 51121092), the National Basic Research Program of China (No. 2009CB220000), the China Postdoctoral Science Foundation (No. 2013M540745), and the Fundamental Research Funds for the Central Universities (No. 2013jdhz20).
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