View Article Online View Journal
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: K. Li, L. Lin, T. Peng, Y. Guo, R. Li and J. Zhang, Chem. Commun., 2015, DOI: 10.1039/C5CC03812J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/chemcomm
Please do not adjust margins ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C5CC03812J
ChemComm
Asymmetric zinc porphyrin-sensitized nanosized TiO2 for efficient visible-light-driven CO2 photoreduction to CO/CH4 Received 00th January 20xx, Accepted 00th January 20xx
Kan Li, Li Lin, Tianyou Peng*, Yingying Guo, Renjie Li and Jing Zhang
DOI: 10.1039/x0xx00000x www.rsc.org/ 9
positions) available for the dye functionalization, and thus it is feasible to construct an oriented electron transfer channel in their sensitized semiconductors by finely tuning the optical, physical, and electrochemical properties through molecular design. Hence, it is reasonable to think that porphyrin-sensitized TiO2 would be an Owing to the extensive use of fossil fuels and immoderate efficient system for the CO2 photoreduction. Recent studies deforestation, the CO2 level in the atmosphere has been increasing suggested that brookite TiO2 or its mixed phase with anatase may steadily over the past centuries. Hence, the conversion of CO2 into be more efficient in the CO2 photoreduction among the three TiO2 C1/C2 energy compounds become a global issue from the viewpoint polymorphs.10 Herein, we present an investigation on a novel 1 of the sustainable development of human society. Among various asymmetric zinc porphyrin (ZnPy, Fig. S1, ESI†) sensi,zed TiO2 for 2 strategies attempted to CO2 conversion, CO2 photo-reduction over visible-light-driven CO2 photoreduction without noble metal semiconductors by utilizing the inexhaustible solar energy can loading. efficiently reduce the CO2 emission and produce marketable solar Nanosized TiO2 derived from a solvothermal process followed fuels, and thus is considered as the most promising way to solve the by calcination at 500oC has a spongy structure composed of current energy and environmental problems. aggregated nanopar,cles with a mean size of ~10 nm (Fig. S2, ESI†), Since CO2 does not absorb either visible or UV radiation, CO2 and the lattice fringes with d-spacings of ~0.354 nm for those photoreduction is a process requiring suitable catalyst to absorb particles in the HRTEM image (Fig. 1) correspond to the (101) planes 3 UV-vis light and transfer it to CO2. However, TiO2 as the most of anatase TiO2. The XRD pattern (inset in Fig. 1) indicates the extensively used catalyst only absorbs the UV-light due to its wide product is anatase/brookite mixed crystal with anatase as the main 4 bandgap. As an efficient route for harvesting visible light, dye phase.7a,11 The average crystal size calculated from anatase (101) sensitization is widely used to extend the spectral response region peak is 10.6 nm, 11 which is consented with the mean size (~10 nm) of wide bandgap semiconductors in the photocatalytic or observed from the FESEM/TEM images (Fig. S2, ESI†). Raman 5-7 photovoltaic system. Similarly, Co or Zn phthalocyanines were spectrum (Fig. S3, ESI†) indicates weak peaks at 213, 247, 322, 366, used to sensitize TiO2, which exhibited a visible-light-driven activity 460 cm-1 ascribable to the B1g, A1g, B2g vibration modes of the 8 of CO2 photoreduction to HCOOH in a saturated CO2 solution. brookite lattice can be observed from the main Raman peaks of Recently, a metal complex dyad containing a zinc porphyrin as light- anatase, further confirming a small quantity of brookite mixed in harvesting unit and a rhenium bipyridyl complex as catalytic moiety anatase.10a,12 Moreover, the product displays a type IV N2 for the CO2 photoreduction was load on p-type NiO also showed adsorption-desorption isotherm (Fig. S4, ESI†), indicating there is 8 2 -1 visible-light-driven activity of CO2 photoreduction. mesostructure with a BET specific surface area of 128.5 m g and a Mg porphyrin as visible light absorption centre can fix CO2 and Barret-Joyner-Halenda (BJH) pore size distribution in the range of convert it into carbohydrates in the plant photosynthesis, and 2~20 nm cantered at ~5.5 nm, which is attributed to the interporphyrins have some intrinsic advantages such as high extinction crystallite voids among those stacked nanoparticles. coefficient, fluorescence, and quantum yield as well as suitable Asymmetric ZnPy containing three pyridine and one carboxyl energy levels for the semiconductors’ photosensitization. Moreover, groups synthesized through two steps (Fig. S1, ESI†) has a strong porphyrins have many reaction sites (i.e. four meso- and eight β- singlet absorption peak (B-band) at 426 nm, and weak triplet light 13 absorption (Q-band) around 530~630 nm (Fig. 2a). Compared with the pristine TiO2 with an absorption edge at ~400 nm (DRS spectra, College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China, E-mail: [email protected] Fig. 2b), ZnPy-TiO2 shows the light absorption characters of ZnPy Asymmetric zinc porphyrin (ZnPy) was synthesized and used to sensitize nanosized TiO2. A visible-light-driven activity of CO2 photoreduction to CO/CH4 generation in gas phase was observed from the ZnPy-sensitized TiO2 without loading noble metal, and the mechanism was discussed.
†Electronic Supplementary Informa,on (ESI) available: Detailed synthesis procedure and some experimental results and discussions. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
COMMUNICATION
Please do not adjust margins ChemComm
Page 2 of 4 View Article Online
DOI: 10.1039/C5CC03812J
Journal Name
Fig. 1 HRTEM image and XRD pattern (inset) of the synthesized TiO2 nanoparticles.
with significantly enhanced Q-band absorption, which is the outcome of a mixture of metal-to-ligand charge transfer (MLCT), 14 ion-to-ligand, and intraligand transitions (ILCT). The increment in Q-band absorption implies the electron transfer from ZnPy to TiO2. -1 In addition, ZnPy-TiO2 shows a strong IR peak at ~1600 cm , which -1 is the superposition of 1603 and 1629 cm (Fig. S6a, ESI†), and can 13,15 due to the COOH of ZnPy and Ti-O bond of TiO2, respectively. This result indicates the connection between ZnPy and TiO2. The optical energy gap (E0-0) of ZnPy is calculated to be 2.06 eV from the intersection of its normalized absorption and emission spectra (Fig. 5 S6b, ESI†), and its HOMO (Eox) and LUMO (E*=Eox-Eo–o) obtained 5 from cyclic voltammograms are 0.72 and -1.63 eV (Table S1, ESI†). Density function theory (DFT) calculation was carried out on ZnPy at the B3LYP/6-31G(d) level to learn more about the frontier molecular orbitals (MO) and the charge transfer from ZnPy to 16 TiO2. The calculated LUMO/HOMO and frontier MO (Table S1 and Fig. S5, ESI†) demonstrate that the electrons of ZnPy’s HOMO-2 are mostly dispersed around Zn, while HOMO-1 is delocalized over Py rings. This suggested the ZnPy’s MLCT. Moreover, the HOMO and LUMO of ZnPy are also delocalized over the Py rings, and then the LUMO-1 and LUMO-2 are respectively delocalized over the Py rings and further move to the carboxyl groups step by step. The calculated energy gap between these orbitals basically agree with of Q-band absorption (Fig. 2a), confirming the above MLCT and ILCT 13,14 assumption of ZnPy. It indicates that the photoexcited electrons can transfer from the Py skeleton to the carboxyl groups, which is a benefit to the electron injection of the excited ZnPy to TiO2, and then causing the visible-light-driven CO2 photoreduction activity. Control experiments showed that both photocatalyst and irradiation are necessary for the present gaseous CO2 photo-
Fig. 2 (a) UV-Vis absorption spectrum of ZnPy solution (5×10-6 mol L-1); (b) UV-Vis diffuse reflection absorption spectra (DRS) of the synthesized TiO2 and its ZnPy-sensitized products.
reduction system. The primary results showed that CO2 can be reduced to CO and CH4 in the presence of H2O vapour and ZnPyTiO2, and no other reduced product such as CH3OH, HCHO or HCOOH is detected in gas or liquid phase by using GC-FID method. Moreover, neither CO nor other carbon-containing organic matters can be detected with N2 instead of CO2, demonstrating that the CO/CH4 generation was stemmed from the CO2 photoreduction process. Fig. 3a shows the effects of the ZnPy-loading level on the CO/CH4 production activities over ZnPy-TiO2 during the initial 2 h irradiation, and the respective data are summarized in Table 1. The pristine TiO2 shows no visible-light-driven photoreduction activity due to the inefficient visible light harvesting. CO produced rate shows an obvious increasing trend when the ZnPy-loading level is enhanced from 0.1% to 1.0%, and then goes downhill with further enhancing ZnPy (Fig. 3a). Once 0.8% ZnPy is loaded, obvious CH4 is generated, and 1.0% ZnPy-TiO2 shows a maximum CO/CH4 -1 -1 production activity of 8.07/1.01 μmol g h . The corresponding overall activity for CO/CH4 generation can be estimated with the total consumed electron number (TCEN, ESI†, for details). The -1 -1 highest TECN of 1.0% ZnPy-TiO2 is 24.17 μmol g h , which is 11.2
Fig. 3 (a) Effects of the ZnPy-loading level on the CO/CH4 production activity and the TCEN value for the CO2 photoreduction over the synthesized TiO2 during the initial 2 h light irradiation (λ≥420 nm); (b) typical time course of CO/CH4 production amount over 1.0% ZnPy-TiO2 under λ≥420 nm light irradiation.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
COMMUNICATION
Please do not adjust margins ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C5CC03812J
Journal Name
COMMUNICATION
Table 1 CO2 photoreduction activity and the TCEN value of ZnPy-TiO2. Catalysts Pristine TiO2 0.1% ZnPy-TiO2 0.5% ZnPy-TiO2 0.8% ZnPy-TiO2 1.0% ZnPy-TiO2 1.5% ZnPy-TiO2 2.0% ZnPy-TiO2
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
b.
CO produced rate / μmol g-1 h-1 0 1.07 3.55 6.89 8.07 7.74 5.96
CH4 produced rate b / μmol g-1 h-1 0 0 Trace 0.87 1.01 0.48 Trace
TCEN / μmol g-1 h-1 0 2.15 7.09 20.70 24.17 19.28 11.92
Dye-sensitized level is determined by the concentration difference of the ZnPy solution before/after sensitization. Trace means that CH4 produced rate cannot measure quantitatively.
times higher than that of 0.1% ZnPy-TiO2. Further enhancing ZnPy leads to a significant decrease in the overall activity (TCEN). It can be ascribed to the inefficient light harvesting and electron injection due to the π-π stacking of ZnPy on TiO2, which is verified by the widen and weaken B-band absorption of 2.0% ZnPy-TiO2 (Fig. 2b).13 Generally, CO2 photoreduction is conducted in liquid or gas phase system. The catalyst in liquid phase system can be dispersed uniformly and well attached with the dissolved CO2 species such as CO32-, HCO3-, CO2 and H2CO3, and thus the photoactivity is usually higher than that of the gaseous system.1,8b However, it is difficult to determine the initial reactants due to the ionization equilibrium among CO32-, HCO3-, CO2 and H2CO3 in the liquid phase system, and thus gaseous photoreduction system has increasingly got recognition lately. Although the overall activity (TCEN) is much lower than the similar dye-sensitized catalyst in liquid phase system, the present TCEN value is relatively high among those gaseous systems reported previously (Table S2, ESI†, for details).10c,17,18 For the photostability test, the recycled 1.0% ZnPy-TiO2 was used in the subsequent photoreduction process (ESI†, for details). As can be seen in Fig. 3b, 1.0% ZnPy-TiO2 has relatively good stability in the first three runs (6 h) with only ~5% overall activity loss, and the recycled catalyst kept in dark for 42 days still exhibits some activity even after eight runs of accumulative 16 h irradiation. Moreover, the recycled catalyst after 16 h irradiation shows synchronous decreases in the absorbance of both B-band and Q-band of ZnPy (Fig. S7, ESI†), indicating some ZnPy molecules are possibly separated from TiO2. In addition to the cycling stability, typical CO/CH4 produced amount over 1.0% ZnPy-TiO2 for 1, 2, 4, 6, 8, and 10 h continuous irradiation are conducted (Fig. S8, ESI†). Both of CO/CH4 produced amounts keep an increasing trend when the irradiation is not longer than 6 h. With further enhancing to 8 and 10 h, CH4 produced amount is reduced although CO produced amount shows a limited increasing trend. Since the present system is kept gas-closed without sacrificial reagent, the resultants would accumulate in the system and the oxidized ZnPy after the electron injection cannot be regenerated efficiently, which prevents the further shifts of chemical equilibrium for the CO2 photoreduction. Therefore, the activity losses after the second runs (Fig. 3b) can be related to the resultants’ accumulation, some ZnPy molecules separated from TiO2 and/or oxidized synergistically. Possibly, CO2/CH4 (-0.24 V) has slightly more positive reduction potential than CO2/CO (-0.48 V),1,17 thus the oxidized ZnPy molecules trend to be regenerated by the accumulated CH4 rather than CO, and then causing the CH4 produced amount decreases more easily. Although the CO2 photoreduction mechanism is complex and the detail information is not well understood, it is a consensus that the
protons from the water splitting and the adsorption states of carbonates (or its hydrolytes) serve as important intermedias of the reaction.2,17 Hence, in situ DRIFT spectra (Fig. S9, ESI†) are used to detect the adsorption of carbonates. Due to the high CO2 concentration in the present photoreaction system, the pristine TiO2 adsorbs bidentate carbonate (b-CO32−) at ~1296 cm-1 and monodentate carbonate (m-CO32−) at ~1378 cm-1;10,18 no peak of bicarbonate (HCO3−) or hydroxy is observed. Hence, most TiO2 is trend to convert CO2 to CO rather than CH4 under UV irradiation.10 With enhancing ZnPy-loading level from 0.1% to 2.0%, the product shows a slightly decreasing trend in the CO2 adsorption capacity, which can be due to the TiO2 surface sites partly occupied by ZnPy, and thus the carbonates trend to form m-CO32− on TiO2, which causes a new peak of m-CO32− occurred at ~1555 cm-1 and the peak of b-CO32− shifts from 1296 to 1303 cm-1 (Fig. S9, ESI †).10,18 Meanwhile, the IR peak of m-CO32− shifts from 1378 to 1362 cm-1 also imply the decreased b-CO32− adsorption, and the peak at ~1652 and ~1430 cm-1 can be ascribed to H2O (hydroxy) and HCO3− adsorption,10 respectively. Moreover, the enhanced CH4 produced activity of 0.5%~1.0% ZnPy-TiO2 can be ascribed to the additionally increased adsorption of HCO3− (at 1430 cm-1) and H2O (at 1652 cm-1) as shown in Fig. S9 (ESI†), while the reduced activity of 1.5% and 2.0% ZnPy-TiO2 is due to the more sites occupied by ZnPy, which competes with the HCO3− and H2O adsorption. On the basis of the above experimental results and discussions, a possible mechanism for the CO2 photoreduction over ZnPy-TiO2 is proposed in Fig. 4. The asymmetric structure of ZnPy provides a directional electron transfer channel (Fig. S5, ESI†) and a sufficiently negative LUMO as compared with TiO2’s CB (-0.53 V vs. NHE),5 thus the electrons of the excited dye (marked as ① in Fig. 4) can inject into the TiO2’s CB, oriented from prophyrin ring to TiO2 connected via carboxyl group (marked as ②). Since the TiO2’s CB (-0.53 V) is slightly higher than the reduction potentials of CO2/CO (-0.48 V) and CO2/CH4 (-0.24 V),1,17 and the CO2 and H2O are adsorbed on
Fig. 4 The possible mechanism of CO2 photoreduction to CO/CH4 generation over the ZnPy-sensitized TiO2.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins
ChemComm Accepted Manuscript
a.
Dye-sensitized level a/ % 0 0.08 0.42 0.70 0.86 1.23 1.51
Please do not adjust margins ChemComm
Page 4 of 4 View Article Online
DOI: 10.1039/C5CC03812J
COMMUNICATION
Journal Name
Acknowledgements This work was supported by the Natural Science Foundation of China (21271146, 21271144, 21171134, 20973128, and 20871096),
the Funds for Creative Research Groups of Hubei Province (2014CFA007) of China.
Notes and references 1 (a) J. Mao, K. Li and T. Y. Peng, Catal. Sci. Technol., 2013, 3, 2481; (b) J. Mao, T. Y. Peng, X. H. Zhang, K. Li, L. Q. Ye and L. Zan, Catal. Sci. Technol., 2013, 3, 1253. 2 (a) W. G. Tu, Y. Zhou and Z. G. Zhou, Adv. Mater., 2014, 26, 4607; (b) K. F. Li, X. Q. An, K. H. Park, M. Khraisheh and J. W. Tang, Catal. Today, 2014, 224, 3; (c) E.V. Kondratenko, G. Mul, J. Baltrusaitis, G. O. Larrazabal and J. Perez-Ramirez, Energ. Environ. Sci., 2013, 6, 3112. 3 (a) Y. P. Yuan, S. W. Cao, Y. S. Liao, L. S. Yin and C. Xue, Appl. Catal. B-Environ., 2013, 140, 164; (b) Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu and Z. H. Li, Angew. Chem. Int. Ed., 2012, 51, 3364; (c) W. G. Tu, Y. Zhou, Q. Liu, S. C. Yan, S. S. Bao, X. Y. Wang, M. Xiao and Z. G. Zhou, Adv. Funct. Mater., 2013, 23, 1743; (d) F. Sastre, A.V. Puga, L.C. Liu, A. Corma and H. Garcia, J. Am. Chem. Soc., 2014, 136, 6798. 4 L. Z. Wang and T. Sasaki, Chem. Rev., 2014, 114, 9455. 5 (a) X. H. Zhang, L. J. Yu, R. J. Li, T. Y. Peng and X. G. Li, Catal. Sci. Technol., 2014, 4, 3251; (b) X. H. Zhang, U. Veikko, J. Mao, P. Cai, and T. Y. Peng, Chem.-Eur. J., 2012, 18, 12103. 6 J. He, J. Q. Wang, Y. J. Chen, J. P. Zhang, D. L. Duan, Y. Wang and Z. Y. Yan, Chem. Commun., 2014, 50, 7063. 7 (a) J. L. Xu, K. Li, W. Y. Shi, R. J. Li and T. Y. Peng, J. Power Sources, 2014, 260, 233; (b) J. N. Clifford, E. Martinez-Ferrero, A. Viterisi and E. Palomares, Chem. Soc. Rev., 2011, 40, 1635. 8 (a) Z. H. Zhao, J. M. Fan, S. H. Liu and Z. Z. Wang, Chem. Eng. J., 2009, 151, 134; (b) Z.H. Zhao, J.M. Fan and Z.Z. Wang, J. Clean. Prod., 2007, 15, 1894; (c) Y. Kou, S. Nakatani, G. Sunagawa, Y. Tachikawa, D. Masui, T. Shimada, S. Takagi, D.A. Tryk, Y. Nabetani, H. Tachibana and H. Inoue, J. Catal., 2014, 310, 57. 9 (a) C. Hsieh, H. Lu, C. Chiu, C. Lee, S. Chuang, C. Mai, W. Yen, S. Hsu, E. W. Diau and C. Yeh, J. Mater. Chem., 2010, 20, 1127; (b) S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazzeeruddin and M. Gratzell, Nat. Chem., 2014, 6, 242. 10 (a) L. J. Liu, H. L. Zhao, J. M. Andino and Y. Li, ACS Catal., 2012, 2, 1817; (b) L. J. Liu, D. T. Pitts, H. Zhao, C. Y. Zhao and Y. Li, Appl. Catal. A-Gen. 2013, 467, 474; (b) H. L. Zhao, L. J. Liu, J. M. Andino and Y. Li, J. Mater. Chem. A, 2013, 1, 8209; (c) J. G. Yu, J. X. Low, W. Xiao, P. Zhou and M. Jaroniec, J. Am. Chem. Soc., 2014, 136, 8839. 11 K. Li, J. L. Xu, X. H. Zhang, T. Y. Peng and X. G. Li, Int. J. Hydrogen Energ., 2013, 38, 15965. 12 S. Zhou, Y. Liu, J. M. Li, Y. J. Wang, G. Y. Jiang, Z. Zhao, D. X. Wang, A. J. Duan, J. Liu and Y. C. Wei, Appl. Catal. B-Environ., 2014, 158, 20. 13 (a) J. Akhigbe, M. Luciano, M. Zeller and C. Bruchner, J. Org. Chem., 2015, 80, 499; (b) S. Kim, T. Tachikawa, M. Fujitsuka and T. Majima, J. Am. Chem. Soc., 2014, 136, 11707. 14 W. Sinha, M. G. Sommer, N. Deibel, F. Ehret, B. Sarkar and S. Kar, Chem.-Euro. J., 2014, 20, 15920. 15 Y. S. Li, F. L. Jiang, Q. Xiao, R. Li, K. Li, M. F. Zhang, A. Q. Zhang, S. F. Sun and Y. Liu, Appl. Catal. B-Environ., 2010, 101, 118. 16 (a) L. J. Yu, W. Y. Shi, L. Lin, Y. Y. Guo, R. J. Li and T. Y. Peng, Dyes Pigments, 2015, 114, 231; (b) L. J. Yu, W. Y. Shi, L. Lin, Y. W. Liu, R. J. Li, T. Y. Peng and X.G. Li, Dalton T., 2014, 43, 8421. 17 H. Zhou, P. Li, J. J. Guo, R. Y. Yan, T. X. Fan, D. Zhang and J. H. Ye, Nanoscale, 2015, 7, 113. 18 (a) J. Mao, L. Q. Ye, K. Li, X. H. Zhang, J. Y. Liu, T. Y. Peng and L. Zan, App. Catal. B-Environ., 2014, 144, 855; (b) J. Mao, T. Y. Peng, X. H. Zhang, K. Li and L. Zan, Catal. Commun., 2012, 28, 38.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
1,2,17
TiO2, it is promised to photocatalytically convert CO2 to CO/CH4 on the TiO2 (marked as ③). Since the present gaseous system is kept gas-closed without sacrificial reagent, the resultants would accumulate in the photoreaction system and the oxidized ZnPy cannot be regenerated efficiently. Therefore, the backward electron transfers such as the photogenerated carriers’ self-recombination of ZnPy (marked as ④) and the injected electrons in TiO2’s CB recombination with the oxidized ZnPy (marked as ⑤) would occur inevitably, and thus is harmful to the photoactivity of ZnPy-TiO2. Along with the accumulations of the oxidized ZnPy in the system, the CO2 photoreduced products might act as the electron donor to regenerate the oxidized ZnPy (marked as ⑥), which would lead to the reduced activity of ZnPy-TiO2 under longer irradiation as shown in Fig. 3b. In the present system, the protons for the CH4 generation are derived from the limited H2O molecule adsorbed on TiO2, which is unfavourable for the CH4 production reaction involving 8 protons. Hence, the main products in the present system are CO/CH4 rather 17 than other hydrocarbons such as CH3OH, HCHO, HCOOH. Moreover, the reduction potential of CO2/CH4 is slightly more positive than that of CO2/CO, thus the backward electron transfer of CH4 to the oxidized ZnPy is more easily. Hence, the CH4 produced rate (Fig. S8, ESI†) is more easily affected due to the more complex reaction and easier backward electron transfer processes. The effects of ZnPy-loading levels on the charge recombination process can be further validated from the photocurrent responses, the time-resolved PL spectra and EIS spectra. The photocurrent response of 1.0% ZnPy-TiO2 is much stronger than the pristine one (Fig. S10a, ESI†), ascribable to the fast charge transfer from ZnPy to TiO2 and the efficient visible light absorption of ZnPy. Time-resolved PL spectra also support the charge transfer from ZnPy to TiO2 (Fig. S10b, ESI†). ZnPy and TiO2 are both excited under 375 nm laser ’ exciter, and the lifetime (τn ) of ZnPy-TiO2 is 7.78 ns, much longer than that (1.55 ns) of the pristine one (Table S3, ESI†). The prolonged lifetime is due to the ZnPy excitation and the charge 5 transfer from ZnPy to TiO2. Moreover, EIS spectra also confirm the frequent charge transfer from ZnPy to TiO2 (Fig. S11, and Table S3, 7b,16a ESI†, for details). Hence, it can be concluded that the charge transfer of ZnPy-TiO2 is that the excited ZnPy inject electrons to TiO2 and then convert CO2 over the TiO2 surface. In conclusion, asymmetric zinc porphyrin (ZnPy) with a carboxyl group is successfully synthesized and applied as a novel sensitizer of nanosized TiO2, and the obtained ZnPy-TiO2 exhibits a visible-lightdriven activity of CO2 photoreduction to CO/CH4 in a gaseous system without noble metal loading. The optimal ZnPy amount is determined to be 1.0% with the best CO/CH4 production rate. The molecular structural asymmetry and the connection of the carboxyl group produce an oriented electron transfer channel of the excited electrons from ZnPy to TiO2, which make CO2 conversion possible under visible light irradiation. Although the photocatalytic activity and stability of ZnPy-TiO2 needs further improvement, the present results provide an important indication about the effect of dyesensitization on the photocatalytic CO2 conversion, and fulfil the deficiency of the research in gaseous dye-sensitized CO2 conversion.
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: K. Li, L. Lin, T. Peng, Y. Guo, R. Li and J. Zhang, Chem. Commun., 2015, DOI: 10.1039/C5CC03812J.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/chemcomm
Please do not adjust margins ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C5CC03812J
ChemComm
Asymmetric zinc porphyrin-sensitized nanosized TiO2 for efficient visible-light-driven CO2 photoreduction to CO/CH4 Received 00th January 20xx, Accepted 00th January 20xx
Kan Li, Li Lin, Tianyou Peng*, Yingying Guo, Renjie Li and Jing Zhang
DOI: 10.1039/x0xx00000x www.rsc.org/ 9
positions) available for the dye functionalization, and thus it is feasible to construct an oriented electron transfer channel in their sensitized semiconductors by finely tuning the optical, physical, and electrochemical properties through molecular design. Hence, it is reasonable to think that porphyrin-sensitized TiO2 would be an Owing to the extensive use of fossil fuels and immoderate efficient system for the CO2 photoreduction. Recent studies deforestation, the CO2 level in the atmosphere has been increasing suggested that brookite TiO2 or its mixed phase with anatase may steadily over the past centuries. Hence, the conversion of CO2 into be more efficient in the CO2 photoreduction among the three TiO2 C1/C2 energy compounds become a global issue from the viewpoint polymorphs.10 Herein, we present an investigation on a novel 1 of the sustainable development of human society. Among various asymmetric zinc porphyrin (ZnPy, Fig. S1, ESI†) sensi,zed TiO2 for 2 strategies attempted to CO2 conversion, CO2 photo-reduction over visible-light-driven CO2 photoreduction without noble metal semiconductors by utilizing the inexhaustible solar energy can loading. efficiently reduce the CO2 emission and produce marketable solar Nanosized TiO2 derived from a solvothermal process followed fuels, and thus is considered as the most promising way to solve the by calcination at 500oC has a spongy structure composed of current energy and environmental problems. aggregated nanopar,cles with a mean size of ~10 nm (Fig. S2, ESI†), Since CO2 does not absorb either visible or UV radiation, CO2 and the lattice fringes with d-spacings of ~0.354 nm for those photoreduction is a process requiring suitable catalyst to absorb particles in the HRTEM image (Fig. 1) correspond to the (101) planes 3 UV-vis light and transfer it to CO2. However, TiO2 as the most of anatase TiO2. The XRD pattern (inset in Fig. 1) indicates the extensively used catalyst only absorbs the UV-light due to its wide product is anatase/brookite mixed crystal with anatase as the main 4 bandgap. As an efficient route for harvesting visible light, dye phase.7a,11 The average crystal size calculated from anatase (101) sensitization is widely used to extend the spectral response region peak is 10.6 nm, 11 which is consented with the mean size (~10 nm) of wide bandgap semiconductors in the photocatalytic or observed from the FESEM/TEM images (Fig. S2, ESI†). Raman 5-7 photovoltaic system. Similarly, Co or Zn phthalocyanines were spectrum (Fig. S3, ESI†) indicates weak peaks at 213, 247, 322, 366, used to sensitize TiO2, which exhibited a visible-light-driven activity 460 cm-1 ascribable to the B1g, A1g, B2g vibration modes of the 8 of CO2 photoreduction to HCOOH in a saturated CO2 solution. brookite lattice can be observed from the main Raman peaks of Recently, a metal complex dyad containing a zinc porphyrin as light- anatase, further confirming a small quantity of brookite mixed in harvesting unit and a rhenium bipyridyl complex as catalytic moiety anatase.10a,12 Moreover, the product displays a type IV N2 for the CO2 photoreduction was load on p-type NiO also showed adsorption-desorption isotherm (Fig. S4, ESI†), indicating there is 8 2 -1 visible-light-driven activity of CO2 photoreduction. mesostructure with a BET specific surface area of 128.5 m g and a Mg porphyrin as visible light absorption centre can fix CO2 and Barret-Joyner-Halenda (BJH) pore size distribution in the range of convert it into carbohydrates in the plant photosynthesis, and 2~20 nm cantered at ~5.5 nm, which is attributed to the interporphyrins have some intrinsic advantages such as high extinction crystallite voids among those stacked nanoparticles. coefficient, fluorescence, and quantum yield as well as suitable Asymmetric ZnPy containing three pyridine and one carboxyl energy levels for the semiconductors’ photosensitization. Moreover, groups synthesized through two steps (Fig. S1, ESI†) has a strong porphyrins have many reaction sites (i.e. four meso- and eight β- singlet absorption peak (B-band) at 426 nm, and weak triplet light 13 absorption (Q-band) around 530~630 nm (Fig. 2a). Compared with the pristine TiO2 with an absorption edge at ~400 nm (DRS spectra, College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China, E-mail: [email protected] Fig. 2b), ZnPy-TiO2 shows the light absorption characters of ZnPy Asymmetric zinc porphyrin (ZnPy) was synthesized and used to sensitize nanosized TiO2. A visible-light-driven activity of CO2 photoreduction to CO/CH4 generation in gas phase was observed from the ZnPy-sensitized TiO2 without loading noble metal, and the mechanism was discussed.
†Electronic Supplementary Informa,on (ESI) available: Detailed synthesis procedure and some experimental results and discussions. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
COMMUNICATION
Please do not adjust margins ChemComm
Page 2 of 4 View Article Online
DOI: 10.1039/C5CC03812J
Journal Name
Fig. 1 HRTEM image and XRD pattern (inset) of the synthesized TiO2 nanoparticles.
with significantly enhanced Q-band absorption, which is the outcome of a mixture of metal-to-ligand charge transfer (MLCT), 14 ion-to-ligand, and intraligand transitions (ILCT). The increment in Q-band absorption implies the electron transfer from ZnPy to TiO2. -1 In addition, ZnPy-TiO2 shows a strong IR peak at ~1600 cm , which -1 is the superposition of 1603 and 1629 cm (Fig. S6a, ESI†), and can 13,15 due to the COOH of ZnPy and Ti-O bond of TiO2, respectively. This result indicates the connection between ZnPy and TiO2. The optical energy gap (E0-0) of ZnPy is calculated to be 2.06 eV from the intersection of its normalized absorption and emission spectra (Fig. 5 S6b, ESI†), and its HOMO (Eox) and LUMO (E*=Eox-Eo–o) obtained 5 from cyclic voltammograms are 0.72 and -1.63 eV (Table S1, ESI†). Density function theory (DFT) calculation was carried out on ZnPy at the B3LYP/6-31G(d) level to learn more about the frontier molecular orbitals (MO) and the charge transfer from ZnPy to 16 TiO2. The calculated LUMO/HOMO and frontier MO (Table S1 and Fig. S5, ESI†) demonstrate that the electrons of ZnPy’s HOMO-2 are mostly dispersed around Zn, while HOMO-1 is delocalized over Py rings. This suggested the ZnPy’s MLCT. Moreover, the HOMO and LUMO of ZnPy are also delocalized over the Py rings, and then the LUMO-1 and LUMO-2 are respectively delocalized over the Py rings and further move to the carboxyl groups step by step. The calculated energy gap between these orbitals basically agree with of Q-band absorption (Fig. 2a), confirming the above MLCT and ILCT 13,14 assumption of ZnPy. It indicates that the photoexcited electrons can transfer from the Py skeleton to the carboxyl groups, which is a benefit to the electron injection of the excited ZnPy to TiO2, and then causing the visible-light-driven CO2 photoreduction activity. Control experiments showed that both photocatalyst and irradiation are necessary for the present gaseous CO2 photo-
Fig. 2 (a) UV-Vis absorption spectrum of ZnPy solution (5×10-6 mol L-1); (b) UV-Vis diffuse reflection absorption spectra (DRS) of the synthesized TiO2 and its ZnPy-sensitized products.
reduction system. The primary results showed that CO2 can be reduced to CO and CH4 in the presence of H2O vapour and ZnPyTiO2, and no other reduced product such as CH3OH, HCHO or HCOOH is detected in gas or liquid phase by using GC-FID method. Moreover, neither CO nor other carbon-containing organic matters can be detected with N2 instead of CO2, demonstrating that the CO/CH4 generation was stemmed from the CO2 photoreduction process. Fig. 3a shows the effects of the ZnPy-loading level on the CO/CH4 production activities over ZnPy-TiO2 during the initial 2 h irradiation, and the respective data are summarized in Table 1. The pristine TiO2 shows no visible-light-driven photoreduction activity due to the inefficient visible light harvesting. CO produced rate shows an obvious increasing trend when the ZnPy-loading level is enhanced from 0.1% to 1.0%, and then goes downhill with further enhancing ZnPy (Fig. 3a). Once 0.8% ZnPy is loaded, obvious CH4 is generated, and 1.0% ZnPy-TiO2 shows a maximum CO/CH4 -1 -1 production activity of 8.07/1.01 μmol g h . The corresponding overall activity for CO/CH4 generation can be estimated with the total consumed electron number (TCEN, ESI†, for details). The -1 -1 highest TECN of 1.0% ZnPy-TiO2 is 24.17 μmol g h , which is 11.2
Fig. 3 (a) Effects of the ZnPy-loading level on the CO/CH4 production activity and the TCEN value for the CO2 photoreduction over the synthesized TiO2 during the initial 2 h light irradiation (λ≥420 nm); (b) typical time course of CO/CH4 production amount over 1.0% ZnPy-TiO2 under λ≥420 nm light irradiation.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
COMMUNICATION
Please do not adjust margins ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C5CC03812J
Journal Name
COMMUNICATION
Table 1 CO2 photoreduction activity and the TCEN value of ZnPy-TiO2. Catalysts Pristine TiO2 0.1% ZnPy-TiO2 0.5% ZnPy-TiO2 0.8% ZnPy-TiO2 1.0% ZnPy-TiO2 1.5% ZnPy-TiO2 2.0% ZnPy-TiO2
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
b.
CO produced rate / μmol g-1 h-1 0 1.07 3.55 6.89 8.07 7.74 5.96
CH4 produced rate b / μmol g-1 h-1 0 0 Trace 0.87 1.01 0.48 Trace
TCEN / μmol g-1 h-1 0 2.15 7.09 20.70 24.17 19.28 11.92
Dye-sensitized level is determined by the concentration difference of the ZnPy solution before/after sensitization. Trace means that CH4 produced rate cannot measure quantitatively.
times higher than that of 0.1% ZnPy-TiO2. Further enhancing ZnPy leads to a significant decrease in the overall activity (TCEN). It can be ascribed to the inefficient light harvesting and electron injection due to the π-π stacking of ZnPy on TiO2, which is verified by the widen and weaken B-band absorption of 2.0% ZnPy-TiO2 (Fig. 2b).13 Generally, CO2 photoreduction is conducted in liquid or gas phase system. The catalyst in liquid phase system can be dispersed uniformly and well attached with the dissolved CO2 species such as CO32-, HCO3-, CO2 and H2CO3, and thus the photoactivity is usually higher than that of the gaseous system.1,8b However, it is difficult to determine the initial reactants due to the ionization equilibrium among CO32-, HCO3-, CO2 and H2CO3 in the liquid phase system, and thus gaseous photoreduction system has increasingly got recognition lately. Although the overall activity (TCEN) is much lower than the similar dye-sensitized catalyst in liquid phase system, the present TCEN value is relatively high among those gaseous systems reported previously (Table S2, ESI†, for details).10c,17,18 For the photostability test, the recycled 1.0% ZnPy-TiO2 was used in the subsequent photoreduction process (ESI†, for details). As can be seen in Fig. 3b, 1.0% ZnPy-TiO2 has relatively good stability in the first three runs (6 h) with only ~5% overall activity loss, and the recycled catalyst kept in dark for 42 days still exhibits some activity even after eight runs of accumulative 16 h irradiation. Moreover, the recycled catalyst after 16 h irradiation shows synchronous decreases in the absorbance of both B-band and Q-band of ZnPy (Fig. S7, ESI†), indicating some ZnPy molecules are possibly separated from TiO2. In addition to the cycling stability, typical CO/CH4 produced amount over 1.0% ZnPy-TiO2 for 1, 2, 4, 6, 8, and 10 h continuous irradiation are conducted (Fig. S8, ESI†). Both of CO/CH4 produced amounts keep an increasing trend when the irradiation is not longer than 6 h. With further enhancing to 8 and 10 h, CH4 produced amount is reduced although CO produced amount shows a limited increasing trend. Since the present system is kept gas-closed without sacrificial reagent, the resultants would accumulate in the system and the oxidized ZnPy after the electron injection cannot be regenerated efficiently, which prevents the further shifts of chemical equilibrium for the CO2 photoreduction. Therefore, the activity losses after the second runs (Fig. 3b) can be related to the resultants’ accumulation, some ZnPy molecules separated from TiO2 and/or oxidized synergistically. Possibly, CO2/CH4 (-0.24 V) has slightly more positive reduction potential than CO2/CO (-0.48 V),1,17 thus the oxidized ZnPy molecules trend to be regenerated by the accumulated CH4 rather than CO, and then causing the CH4 produced amount decreases more easily. Although the CO2 photoreduction mechanism is complex and the detail information is not well understood, it is a consensus that the
protons from the water splitting and the adsorption states of carbonates (or its hydrolytes) serve as important intermedias of the reaction.2,17 Hence, in situ DRIFT spectra (Fig. S9, ESI†) are used to detect the adsorption of carbonates. Due to the high CO2 concentration in the present photoreaction system, the pristine TiO2 adsorbs bidentate carbonate (b-CO32−) at ~1296 cm-1 and monodentate carbonate (m-CO32−) at ~1378 cm-1;10,18 no peak of bicarbonate (HCO3−) or hydroxy is observed. Hence, most TiO2 is trend to convert CO2 to CO rather than CH4 under UV irradiation.10 With enhancing ZnPy-loading level from 0.1% to 2.0%, the product shows a slightly decreasing trend in the CO2 adsorption capacity, which can be due to the TiO2 surface sites partly occupied by ZnPy, and thus the carbonates trend to form m-CO32− on TiO2, which causes a new peak of m-CO32− occurred at ~1555 cm-1 and the peak of b-CO32− shifts from 1296 to 1303 cm-1 (Fig. S9, ESI †).10,18 Meanwhile, the IR peak of m-CO32− shifts from 1378 to 1362 cm-1 also imply the decreased b-CO32− adsorption, and the peak at ~1652 and ~1430 cm-1 can be ascribed to H2O (hydroxy) and HCO3− adsorption,10 respectively. Moreover, the enhanced CH4 produced activity of 0.5%~1.0% ZnPy-TiO2 can be ascribed to the additionally increased adsorption of HCO3− (at 1430 cm-1) and H2O (at 1652 cm-1) as shown in Fig. S9 (ESI†), while the reduced activity of 1.5% and 2.0% ZnPy-TiO2 is due to the more sites occupied by ZnPy, which competes with the HCO3− and H2O adsorption. On the basis of the above experimental results and discussions, a possible mechanism for the CO2 photoreduction over ZnPy-TiO2 is proposed in Fig. 4. The asymmetric structure of ZnPy provides a directional electron transfer channel (Fig. S5, ESI†) and a sufficiently negative LUMO as compared with TiO2’s CB (-0.53 V vs. NHE),5 thus the electrons of the excited dye (marked as ① in Fig. 4) can inject into the TiO2’s CB, oriented from prophyrin ring to TiO2 connected via carboxyl group (marked as ②). Since the TiO2’s CB (-0.53 V) is slightly higher than the reduction potentials of CO2/CO (-0.48 V) and CO2/CH4 (-0.24 V),1,17 and the CO2 and H2O are adsorbed on
Fig. 4 The possible mechanism of CO2 photoreduction to CO/CH4 generation over the ZnPy-sensitized TiO2.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins
ChemComm Accepted Manuscript
a.
Dye-sensitized level a/ % 0 0.08 0.42 0.70 0.86 1.23 1.51
Please do not adjust margins ChemComm
Page 4 of 4 View Article Online
DOI: 10.1039/C5CC03812J
COMMUNICATION
Journal Name
Acknowledgements This work was supported by the Natural Science Foundation of China (21271146, 21271144, 21171134, 20973128, and 20871096),
the Funds for Creative Research Groups of Hubei Province (2014CFA007) of China.
Notes and references 1 (a) J. Mao, K. Li and T. Y. Peng, Catal. Sci. Technol., 2013, 3, 2481; (b) J. Mao, T. Y. Peng, X. H. Zhang, K. Li, L. Q. Ye and L. Zan, Catal. Sci. Technol., 2013, 3, 1253. 2 (a) W. G. Tu, Y. Zhou and Z. G. Zhou, Adv. Mater., 2014, 26, 4607; (b) K. F. Li, X. Q. An, K. H. Park, M. Khraisheh and J. W. Tang, Catal. Today, 2014, 224, 3; (c) E.V. Kondratenko, G. Mul, J. Baltrusaitis, G. O. Larrazabal and J. Perez-Ramirez, Energ. Environ. Sci., 2013, 6, 3112. 3 (a) Y. P. Yuan, S. W. Cao, Y. S. Liao, L. S. Yin and C. Xue, Appl. Catal. B-Environ., 2013, 140, 164; (b) Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, X. Z. Fu and Z. H. Li, Angew. Chem. Int. Ed., 2012, 51, 3364; (c) W. G. Tu, Y. Zhou, Q. Liu, S. C. Yan, S. S. Bao, X. Y. Wang, M. Xiao and Z. G. Zhou, Adv. Funct. Mater., 2013, 23, 1743; (d) F. Sastre, A.V. Puga, L.C. Liu, A. Corma and H. Garcia, J. Am. Chem. Soc., 2014, 136, 6798. 4 L. Z. Wang and T. Sasaki, Chem. Rev., 2014, 114, 9455. 5 (a) X. H. Zhang, L. J. Yu, R. J. Li, T. Y. Peng and X. G. Li, Catal. Sci. Technol., 2014, 4, 3251; (b) X. H. Zhang, U. Veikko, J. Mao, P. Cai, and T. Y. Peng, Chem.-Eur. J., 2012, 18, 12103. 6 J. He, J. Q. Wang, Y. J. Chen, J. P. Zhang, D. L. Duan, Y. Wang and Z. Y. Yan, Chem. Commun., 2014, 50, 7063. 7 (a) J. L. Xu, K. Li, W. Y. Shi, R. J. Li and T. Y. Peng, J. Power Sources, 2014, 260, 233; (b) J. N. Clifford, E. Martinez-Ferrero, A. Viterisi and E. Palomares, Chem. Soc. Rev., 2011, 40, 1635. 8 (a) Z. H. Zhao, J. M. Fan, S. H. Liu and Z. Z. Wang, Chem. Eng. J., 2009, 151, 134; (b) Z.H. Zhao, J.M. Fan and Z.Z. Wang, J. Clean. Prod., 2007, 15, 1894; (c) Y. Kou, S. Nakatani, G. Sunagawa, Y. Tachikawa, D. Masui, T. Shimada, S. Takagi, D.A. Tryk, Y. Nabetani, H. Tachibana and H. Inoue, J. Catal., 2014, 310, 57. 9 (a) C. Hsieh, H. Lu, C. Chiu, C. Lee, S. Chuang, C. Mai, W. Yen, S. Hsu, E. W. Diau and C. Yeh, J. Mater. Chem., 2010, 20, 1127; (b) S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazzeeruddin and M. Gratzell, Nat. Chem., 2014, 6, 242. 10 (a) L. J. Liu, H. L. Zhao, J. M. Andino and Y. Li, ACS Catal., 2012, 2, 1817; (b) L. J. Liu, D. T. Pitts, H. Zhao, C. Y. Zhao and Y. Li, Appl. Catal. A-Gen. 2013, 467, 474; (b) H. L. Zhao, L. J. Liu, J. M. Andino and Y. Li, J. Mater. Chem. A, 2013, 1, 8209; (c) J. G. Yu, J. X. Low, W. Xiao, P. Zhou and M. Jaroniec, J. Am. Chem. Soc., 2014, 136, 8839. 11 K. Li, J. L. Xu, X. H. Zhang, T. Y. Peng and X. G. Li, Int. J. Hydrogen Energ., 2013, 38, 15965. 12 S. Zhou, Y. Liu, J. M. Li, Y. J. Wang, G. Y. Jiang, Z. Zhao, D. X. Wang, A. J. Duan, J. Liu and Y. C. Wei, Appl. Catal. B-Environ., 2014, 158, 20. 13 (a) J. Akhigbe, M. Luciano, M. Zeller and C. Bruchner, J. Org. Chem., 2015, 80, 499; (b) S. Kim, T. Tachikawa, M. Fujitsuka and T. Majima, J. Am. Chem. Soc., 2014, 136, 11707. 14 W. Sinha, M. G. Sommer, N. Deibel, F. Ehret, B. Sarkar and S. Kar, Chem.-Euro. J., 2014, 20, 15920. 15 Y. S. Li, F. L. Jiang, Q. Xiao, R. Li, K. Li, M. F. Zhang, A. Q. Zhang, S. F. Sun and Y. Liu, Appl. Catal. B-Environ., 2010, 101, 118. 16 (a) L. J. Yu, W. Y. Shi, L. Lin, Y. Y. Guo, R. J. Li and T. Y. Peng, Dyes Pigments, 2015, 114, 231; (b) L. J. Yu, W. Y. Shi, L. Lin, Y. W. Liu, R. J. Li, T. Y. Peng and X.G. Li, Dalton T., 2014, 43, 8421. 17 H. Zhou, P. Li, J. J. Guo, R. Y. Yan, T. X. Fan, D. Zhang and J. H. Ye, Nanoscale, 2015, 7, 113. 18 (a) J. Mao, L. Q. Ye, K. Li, X. H. Zhang, J. Y. Liu, T. Y. Peng and L. Zan, App. Catal. B-Environ., 2014, 144, 855; (b) J. Mao, T. Y. Peng, X. H. Zhang, K. Li and L. Zan, Catal. Commun., 2012, 28, 38.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins
ChemComm Accepted Manuscript
Published on 26 June 2015. Downloaded by Freie Universitaet Berlin on 01/07/2015 06:52:05.
1,2,17
TiO2, it is promised to photocatalytically convert CO2 to CO/CH4 on the TiO2 (marked as ③). Since the present gaseous system is kept gas-closed without sacrificial reagent, the resultants would accumulate in the photoreaction system and the oxidized ZnPy cannot be regenerated efficiently. Therefore, the backward electron transfers such as the photogenerated carriers’ self-recombination of ZnPy (marked as ④) and the injected electrons in TiO2’s CB recombination with the oxidized ZnPy (marked as ⑤) would occur inevitably, and thus is harmful to the photoactivity of ZnPy-TiO2. Along with the accumulations of the oxidized ZnPy in the system, the CO2 photoreduced products might act as the electron donor to regenerate the oxidized ZnPy (marked as ⑥), which would lead to the reduced activity of ZnPy-TiO2 under longer irradiation as shown in Fig. 3b. In the present system, the protons for the CH4 generation are derived from the limited H2O molecule adsorbed on TiO2, which is unfavourable for the CH4 production reaction involving 8 protons. Hence, the main products in the present system are CO/CH4 rather 17 than other hydrocarbons such as CH3OH, HCHO, HCOOH. Moreover, the reduction potential of CO2/CH4 is slightly more positive than that of CO2/CO, thus the backward electron transfer of CH4 to the oxidized ZnPy is more easily. Hence, the CH4 produced rate (Fig. S8, ESI†) is more easily affected due to the more complex reaction and easier backward electron transfer processes. The effects of ZnPy-loading levels on the charge recombination process can be further validated from the photocurrent responses, the time-resolved PL spectra and EIS spectra. The photocurrent response of 1.0% ZnPy-TiO2 is much stronger than the pristine one (Fig. S10a, ESI†), ascribable to the fast charge transfer from ZnPy to TiO2 and the efficient visible light absorption of ZnPy. Time-resolved PL spectra also support the charge transfer from ZnPy to TiO2 (Fig. S10b, ESI†). ZnPy and TiO2 are both excited under 375 nm laser ’ exciter, and the lifetime (τn ) of ZnPy-TiO2 is 7.78 ns, much longer than that (1.55 ns) of the pristine one (Table S3, ESI†). The prolonged lifetime is due to the ZnPy excitation and the charge 5 transfer from ZnPy to TiO2. Moreover, EIS spectra also confirm the frequent charge transfer from ZnPy to TiO2 (Fig. S11, and Table S3, 7b,16a ESI†, for details). Hence, it can be concluded that the charge transfer of ZnPy-TiO2 is that the excited ZnPy inject electrons to TiO2 and then convert CO2 over the TiO2 surface. In conclusion, asymmetric zinc porphyrin (ZnPy) with a carboxyl group is successfully synthesized and applied as a novel sensitizer of nanosized TiO2, and the obtained ZnPy-TiO2 exhibits a visible-lightdriven activity of CO2 photoreduction to CO/CH4 in a gaseous system without noble metal loading. The optimal ZnPy amount is determined to be 1.0% with the best CO/CH4 production rate. The molecular structural asymmetry and the connection of the carboxyl group produce an oriented electron transfer channel of the excited electrons from ZnPy to TiO2, which make CO2 conversion possible under visible light irradiation. Although the photocatalytic activity and stability of ZnPy-TiO2 needs further improvement, the present results provide an important indication about the effect of dyesensitization on the photocatalytic CO2 conversion, and fulfil the deficiency of the research in gaseous dye-sensitized CO2 conversion.
