Adsorption Energy Optimization of Co3O4 through Rapid Surface

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Adsorption Energy Optimization of CoO Through Rapid Surface Sulfurization for Efficient Counter Electrode in Dye-Sensitized Solar Cells Shuang Lu, Yinglin Wang, Fei Li, Guochun Yang, Huanying Yang, Xintong Zhang, and Yichun Liu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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The Journal of Physical Chemistry

Adsorption Energy Optimization of Co3O4 Through Rapid Surface Sulfurization for Efficient Counter Electrode in Dye-Sensitized Solar Cells Shuang Lu, Yinglin Wang*, Fei Li, Guochun Yang, Huanying Yang, Xintong Zhang* and Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Lab of UVEmitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China

ABSTRACT

The high-efficiency electrocatalysts have been widely applied in various fields, especially in the counter electrode (CE) of dye-sensitized solar cells (DSSCs). Different from the usual methods for developing high-performance CE materials through searching new materials and designing new nanostructure, we utilized a rapid surface sulfurization treatment to activate the well-known Co3O4 material by increasing the adsorption energy of iodine (I) atom in the electrolyte of DSSCs. The density functional theory (DFT) calculation indicated the adsorption energy of Co3O4 (EIad : 0.374 eV) towards I atom was dramatically increased to 0.835 eV through just transforming Co3O4 at surface into Co3S4. After a short activation time of 30 s, Co3O4 CEs

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showed a superior catalytic performance towards iodide electrolyte comparable with traditional Pt CE, generating a power conversion efficiency of 8.6% in DSSCs. This method of adjusting adsorption energy via surface sulfurization is applicable in the activation of other metal oxides, and provides a convenient but efficient way to modify the simple abundant materials for their different electrocatalytic applications.

INTRODUCTION High-efficiency electrocatalytic materials are essentials in many photonic and electrical devices, including capacitor, hydrolysis system and DSSCs. Especially, the CEs in DSSCs based on electrocatalytic materials play a critical role in facilitating the hole collection by accelerating the reduction of triiodides (I ) in the electrolyte. Nowadays, the superior CE materials are mainly derived from noble metal Pt, which usually suffer from the disadvantages of low abundance and high cost. Hence, researchers paid much effort to search new inexpensive earthabundant CE materials,1-6 such as conductive polymers,7 carbon materials,8-10 inorganic semiconductors11 and hybrid materials.12 However, considering the basic requirements of highefficiency CE, including fast charge transportation in bulk and the swift charge transfer at interface, we believe that targeted optimization of the existed simple materials, by either conductivity improvement or surface activation, is more reasonable in the exploration of alternative CE. P-type spinel Co3O4 has been widely investigated as electrocatalyst in various systems, including oxygen reduction,13-14 carbon dioxide reduction15-16 and electrochemical capacitors,17-18 because of its earth abundance, low cost, low environmental impact and good chemical


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stability.19 However, the activity of Co3O4 as the CE of DSSCs is still far from satisfactory. Considering its acceptable bulk conductivity observed in other electrocatalytic reactions, the dissatisfaction of Co3O4 as CE derives from its large interfacial charge transfer resistance in the triiodides reduction reaction. Many transition metal oxides (TMOs) have been proved to suffer from the low EIad by theoretical calculation, which directly limits the charge transfer from TMOs to I atom. The introduction of oxygen vacancy20 and doping21 have efficiently activated TMOs by increasing their EIad similar to that of Pt (0.52 eV for Pt (111)).22 Since the adsorption of reactant in the electrocatalytic reaction is only associated with the active sites on the surface of catalysts, it’s more convenient to just activate the ultrathin surface layer of inert Co3O4, compared with the complete adjustment of composition in bulk. Herein, we utilized a short-time sulfurization reaction to activate surface Co3O4 for its application as the CE of DSSCs. Through a solution-processed ion-exchange reaction, we purposefully transformed the inert surface of spay-pyrolyzed Co3O4 film to a high-activity ultrathin layer of Co3S4, meanwhile the EIad value of Co3O4 was calculated to increase from 0.347 eV to 0.835 eV. Activation time of 5-10 s was sufficient to dramatically enhance the activity of bare Co3O4 CE for the reduction of triiodides in the electrolyte of DSSCs. And the Co3O4 CE with activation time of 30 s presented comparable catalytic performance as that of traditional pyrolyzed Pt CE, which generated PCE of 8.6% in the DSSCs with iodide electrolyte. Metal oxides are easily converted to metal sulfides at various sulfuration environments, such as sulfide solution, sulphur and H2S vapour, thus this approach of constructing metal-sulfide interface by rapid activation is also applicable for other inert TMOs through rational choice of chemical conversion method. EXPERIMENTAL SECTION


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Rapid surface sulfurization of Co3O4 film. The Co3O4 film was obtained on the well-cleaned SnO2: F (FTO) glass (2.25 cm2) by the spray pyrolysis method at 400 °C, using the ethanol solution of CoCl2·6H2O (0.01 M). For rapid chemical surface sulfurization, the obtained Co3O4 films were soaked in Na2S aqueous solution (0.01 M) at 90 °C for different times, then washed with deionized water and dried at 60 °C. The preparation and surface sulfurization of NiO film were similar with that of Co3O4 film. Fabrication of DSSCs. The well-cleaned FTO was first treated with an aqueous solution of TiCl4 (0.04 M) at 70 °C for 35 min, and then TiO2 mesoporous film was obtained by screenprinting method.23 After sintered at 500 °C for 30 min and cooled to room temperature, the electrode was dipped in a dry ethanol solution (0.3 mM) of N719 at room temperature for 24 h to complete dye absorption. The Co3O4 film with different activation times and the Pt-coated FTO substrate made by thermal deposition of H2PtCl6 were used as CEs to fabricate the DSSCs. A mixture of 0.6 M 1,3-dimethylimidazolium iodide, 0.1 M guanidinium thiocyanate, 0.03 M iodine, 50 mM lithium iodide and 0.5 M 4-tert-butylprine in acetonitrile/valeronitrile (85:15, v/v) was used as the electrolyte (I /I ) which injected into the space between the two electrodes. Characterizations. Morphology of samples was observed via a FEI Quanta 250 fieldemission scanning electron microscope. Transmission electron microscope images were acquired using a JEOL JEM-2100 instrument working at an acceleration voltage of 200 kV. X-ray diffraction patterns were recorded on a Rigaku, D/max-2500 X-ray diffractometer using Cu Kα radiation (λ = 1.542 Å) operated at 40 kV and 100 mA. X-ray photoelectron spectroscopy experiments were performed on a VGESCA-LAB MKII instrument with an Al Kα ADES (hν=1486.6 eV) source. Photocurrent density-voltage (J–V) characteristics of the solar cell were


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measured with a Keithley 2400 source meter under AM 1.5G simulated solar light (ABET Technology, 100 mW/cm2) calibrated with a standard silicon reference cell. Tafel polarization curves and electrochemical impedance spectroscopy were obtained with symmetrical cells consisting of two identical electrodes by using a Princeton PARSTAT 2273 Potentiostat/Galvanostst. Tafel polarization curves were carried out under the bias between −1 V and 1 V, with step height of 1 mV. Electrochemical impedance spectroscopy was recorded over a frequency range from 100 mHz to 600 KHz, under forward bias of 0.4 V. Cyclic voltammetry (CV) was performed using a three-electrode system at a scan rate of 50 mV s-1. A Pt foil and an Ag/Ag+ electrode was served as the counter and reference electrode, respectively. The electrolyte solution contained 100 mM LiClO4, 10 mM LiI and 1 mM I2 in acetonitrile. Computational Methods. Structural relaxations and total energy calculations were performed in the framework of DFT within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation program package (VASP).24-26 Electron-ion interactions were described using standard PAW potentials,27-28 with valence configurations of 3s23p64s13d8 for Co, 5s25p5 for I, 2s22p4 for O, and 3s22p4 for S. Cut-off energy of 520 eV was used, which has been tested in previous study.22 Herein Perdew-Burke-Ernzerhof (PBE)29 functional were used for the GGA-level DFT calculations. Due to the insufficient consideration of the on-site Columbic repulsion between the Co d electrons, DFT may fail to describe the electronic structure of Co3O4. To overcome this shortcoming, the GGA+U approach was adopted.30 U-J = 6.0 eV for the Co atoms was used. To accurately simulate the Co3O4 (111) surface, a 10-layer slab was enclosed in a supercell with a sufficient large vacuum region of 15 Å to ensure the periodic images to be well separated. During the structural relaxations, the atoms in the bottom three layers were fixed, and the rest atoms were allowed to relax until the Hellmann-Feynman forces


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were smaller than 0.01 eV/Å. Due to the inherent magnetism of Co3O4, spin-polarization calculation was performed. For Brillouin-zone integrations, gamma-centered k-point grids of special points with a (5×5×1) mesh were used for the (1×1) surface cell, and a (3×5×1) mesh were used for the calculation of I adsorption on S-replaced Co3O4 (111) surface.21 RESULTS AND DISCUSSION By virtue of DFT calculations, we firstly investigated the low EIad of original Co3O4 as the CE, and further predicted the feasibility of adsorption energy adjustment of inert Co3O4 through surface sulfurization. The EIad value of Co3O4 was calculated to be 0.347 eV (Figure S1a), which was much smaller than the reported of Pt (0.52 eV), leading to the bad performance of Co3O4 CE. However, the EIad of Co3O4 was gradually increased by substituting surface O atom through S atom step by step (Figure 1), and it rose to 0.835 eV when the surface Co3O4 was fully transformed into Co3S4. While a further transformation of inner Co3O4 into Co3S4 did not increase the EIad obviously (Figure S1b). Moreover, the value of calculated Hirshfeld charge31-32 between Co and redox species atom increased by 4 times after activation (Co3O4: 0.01 e, Co3S4/Co3O4: 0.05 e), this change was in accordance with that of adsorption energy. The DFT results confirmed that surface activation by sulfurization treatment could efficiently adjust the EIad and Hirshfeld charge values of Co3O4, thus improved the electrocatalytic ability of inert Co3O4 through adsorption energy adjustment. Based on the above theoretical prediction, we prepared the p-type Co3O4 films (thickness: ~60 nm, Figure S2) by the spray pyrolysis method on the FTO conductive substrate at 400 °C, and further activated them in an aqueous solution of Na2S (0.01 M) at 90 °C for 5-30 s. The detailed processes were described in the Experimental Section. The sulfured samples were named by Co3S4/Co3O4-5 s, Co3S4/Co3O4-10 s, Co3S4/Co3O4-30 s, respectively, according to the


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sulfurization time. The top-view scanning electron microscope (SEM) image of Co3O4 in Figure 2a indicated that the spray pyrolyzed film was composed of spherical aggregates. And the shorttime surface sulfurization process induced negligible change in the morphology of initial Co3O4 film (Figure 2a and 2b). The elemental mapping (Figure S3) showed additional S signal in activated Co3O4 except for Co and O signals. Utilizing energy dispersive X-ray (EDX) spectra (Figure S4 and Table S1), we calculated the atom ratio of oxygen to sulfide (O: S) of activated Co3O4 was close to 13:1. Another, the atom ratio of cobalt to oxygen and sulfide (Co: (O+S)) was 3:4.14, which was close to that of Co3O4 or Co3S4 (3:4). According to the EDX data, we suspected that the process of rapid sulfurization successfully transformed part of Co3O4 into Co3S4 film. Furthermore, we confirmed the formation of Co3S4 after activation by X-ray diffraction (XRD) and transmission electron microscope (TEM) measurements. The XRD measurement (Figure 2c and Figure S5) confirmed the structure of the synthesized Co3O4 samples. The diffraction peaks at 19.00°, 31.27°, 36.85°, 38.54°, 44.81°, 55.66°, 59.36° and 65.24°, corresponded to crystal planes of (111), (220), (311), (222), (400), (422), (511) and (440) for the cubic spinel Co3O4 (JCPDS card #42-1467).33-34 After sulfurization process, the samples showed additional diffraction peaks at 47.44° and 50.62°, apart from the diffraction peaks of Co3O4, which were assigned to (422) and (511) crystal planes of the cubic Co3S4 (JCPDS card #19-0367). Also, the high-resolution transmission electron microscope (HR-TEM) image of sulfurized Co3O4 film clearly depicted the distinct lattice fringes (422) of the cubic Co3S4 with a lattice spacing of 0.19 nm, except for the lattice fringes (311) of spinel Co3O4 with 0.24 nm lattice distance (Figure 2d). And the selected area electron diffraction (SAED) pattern (Figure 2e) confirmed that the product contained (220), (422) planes of Co3S4 and (311), (511) planes of Co3O4. The corresponding spots from the fast Fourier transform (FFT) patterns (Figure S6) could


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be labeled with planes of (311), (023) of Co3O4 and (422) planes of Co3S4, respectively. In the X-ray photoelectron spectroscopy (XPS) (Figure 3), after the sulfurization treatment, a new peak emerged at a lower binding energy in Co 2p3/2 spectrum (Figure 3a and S7), characteristic of CoS bonding in Co3S4. And the peak in the O 1s spectrum at 529.6 eV was assigned to the lattice oxygen (Olat) of Co3O4.21 In addition, two peaks were noticed in the region of S 2p spectrum.35-36 Consequently, all results above indicated that the process of rapid sulfurization successfully transformed part of Co3O4 into Co3S4 film. We further used XPS depth profiling of O 1s and S 2p signals to confirm the distribution of Co3S4 on the Co3O4 film (Figure 3b). With the etching time increasing, the intensity of Olat signal increased firstly and maintained constantly after 240 s, but the intensity of S signal decreased gradually and disappeared completely at 400 s. This change tendency of signal intensity for O and S elements with etching time was shown in Figure S8. The XPS measurement demonstrated that Co3S4 distributed only on the surface of Co3O4 film, rather than in the bulk. Therefore, we successfully constructed an active Co3S4 thin layer with suitable adsorption energy on the surface of inert Co3O4 film for its application as the CE of DSSCs. We fabricated symmetric dummy cells by Co3O4 CEs with different sulfurization times, and investigated the effect of the short-time surface activation on the electrocatalytic performance of Co3O4 CEs to reduce triiodides in the traditional I /I electrolyte through electrochemical impedance spectroscopy (EIS). The arcs at high frequency (left) of EIS spectra (Figure 4a) were directly related to the electrocatalytic activity of CE, which could be fitted to estimate the series resistance (Rs) and charge-transfer resistance (Rct) of CE using the equivalent circuit in the inset. As shown in Table 1, the Rs values of Co3O4 CE were not changed apparently by the surface activation, which were consistent with that of traditional pyrolyzed Pt CE. However, the Rct of


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bare Co3O4 was too large to be calculated, indicating Co3O4 had no electrocatalytic activity for the reduction of triiodides. With the time of surface activation prolonging from 5 s to 30 s, the Rct values of Co3S4/Co3O4 complex films were largely reduced, which was consistent with the change trend of EIad value in Figure 1. Especially, the sample with activation time of 30 s (Co3S4/Co3O4-30 s) presented a similar Rct value with Pt CE (4.8 Ω/cm2 for Co3S4/Co3O4-30 s and 5.5 Ω/cm2 for Pt), suggesting the high catalytic activity of activated Co3O4 CE. The Tafel-polarization curves (Tafel) (Figure 4b) of different symmetric cells further elucidated the catalytic activity of Co3O4 films. The exchange current density (J0) of electrodes could be obtained from the intercept of a tangent to Tafel curves, which was well correlated with Rct values from EIS results in terms of the following equation (1):

=

(1)

Where R is the gas constant, T is the temperature, F is Faraday’s constant and n is the electron number involved in the electrochemical reduction of triiodides at the electrode. As observed in the EIS tests, the bare Co3O4 film showed no electrocatalytic activity as the CE of DSSCs. As activation time was prolonged from 5 s to 30 s, the variation of J0 was generally in accordance with the change tendency of Rct obtained from EIS measurements, and the Co3S4/Co3O4-30 s film showed comparable electrocatalytic performance with Pt film. When the activation time was more than 30s, the activity of the samples was not improved obviously (Figure S9), which was also in accordance with the variation trend of EIad value in Figure 1 and Figure S1. Figure 5 showed the J–V curves of DSSCs with Pt, inert Co3O4 and activated Co3O4 CEs, which were combined with N719-sensitized TiO2 photoanode and I /I electrolyte. The detailed photovoltaic parameters were shown in the inset. The Pt-based DSSC exhibited short-circuit current density (Jsc) of 18.36 mA cm-2, open-circuit voltage (Voc) of 0.68 V and fill factor (FF) of


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0.69, generating a power conversion efficiency (PCE) of 8.6%. The inert Co3O4-based solar cell had poor performance, while PCE of the solar cell with Co3S4/Co3O4-30 s CE was the same with that of Pt-based DSSCs, which was derived from the Jsc of 17.99 mA cm-2, Voc of 0.67 V and FF of 0.71. The standard deviation values of Jsc, Voc, FF and PCE were calculated to be 0.1992, 0.0048, 0.0127 and 0.1500, respectively, from eight DSSCs with Co3S4/Co3O4 CE (Figure S10 and Table S2), which indicated that the activated Co3S4/Co3O4 CEs had good consistency. The electrochemical and photovoltaic data confirmed that the adjusting adsorption energy of inert Co3O4 through surface activation was efficient for improving its electrocatalytic ability, as has been predicted by the theoretical calculation. This rapid and efficient activation method is also suitable for other inert TMOs. For example, an ultrathin NiS layer was also obtained on the surface of spay-pyrolyzed NiO film, and improved the electrocatalytic activity of original bare NiO film as CE. The corresponding Tafel and EIS results were shown in Figure S11. The stability is one of the most important factors to influence the practical application of the CE.31-32,37-40 We used successive CV, Tafel polarization and current-time tests to examine the stability of the activated Co3O4 CE. The CV and Tafel polarization curves displayed good repeatability during successive test (CV: 35 cycles, Tafel: 100 cycles), and the decreases of corresponding current density in two tests were less than 10% (Figure 6a and 6b). During the current-time test, the current density maintained 99% and 78% of the original ones after 6000-s test at −0.25 V and −0.50 V, respectively (Figure 6c). We further monitored the photovoltaic parameters of the DSSCs during 96 h to evaluate the stability of the CE. As shown in Figure 6d, the Voc and FF values of Co3S4/Co3O4- and Pt-based solar cells were approximatively unchanged. And the Jsc and PCE values of two solar cells were decreased by 18-20% during 96 h, which may


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be related to the corrosion of the photoanode. These results from different stability tests suggested that the activated Co3O4 CE exhibited comparable stability with Pt CE. CONCILUSIONS In conclusion, we proved that adsorption energy adjustment through rapid surface sulfurization was a convenient but efficient method to improve electrocatalytic ability of Co3O4 as the CE of DSSCs. The Co3O4 CE with a short activation time of 30 s showed a superior catalytic performance towards iodide electrolyte comparable with traditional Pt film, generating a PCE of 8.6% in DSSCs. The improved performance could be attributed to the increased Hirshfeld charge and EIad of Co3O4 from 0.347 to 0.835 eV by just transformation of Co3O4 at surface to Co3S4. This approach based on the adjustment of adsorption energy provides a new idea for improving the catalytic activity of inert materials. The method can improve the performance as long as we can find the suitable method to combine surface activation with the unique properties of TMOs film, such as surface nitridation, carbonization, phosphating and selenylation. It is beneficial to develop the superior and low-cost electrocatalysts in different applications, such as DSSCs, quantum dot sensitized solar cells, water decomposition, supercapacitors, lithium-ion batteries and oxygen evolution reactions.


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Figure 1. The atom arrangements diagrams of different degrees of substituting surface O atom of Co3O4 ((111) surface) through S atom and model diagrams of interfacial reaction on inert and activated Co3O4 films. Atoms in blue, red, yellow and purple colors represent Co, O, S and I, respectively.


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Figure 2. (a and b) SEM images and (c) XRD patterns of the original and activated Co3O4 samples; (d and e) HR-TEM images and SAED pattern of the Co3S4/Co3O4 samples.


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Figure 3. (a) The XPS data: Co 2p3/2, O 1s and S 2p spectra of Co3O4 and Co3S4/Co3O4-30 s films; (b) the XPS depth profiles: O 1s and S 2p spectra of Co3S4/Co3O4-30 s films with different etching times. The binding energy is corrected referencing C 1s (284.60 eV). The peaks at 531.0 eV and 166.8 eV are attributed to the adsorbed oxygen and sulfur (Sads), respectively.21,41


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Figure 4. (a) Nyquist plots and (b) Tafel polarization curves of Pt and Co3O4 films with different times of surface activation. Both experiments were performed with the symmetrical dummy cells with two identical electrodes (CE//iodide//triiodides electrolyte//CE). The inset shows the equivalent circuit model of the symmetrical cells for fitting EIS results.


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Figure 5. J–V curves of DSSCs based on Co3O4, Co3S4/Co3O4-30 s and Pt films, measured under AM 1.5G solar simulator illumination. The active area of DSSC was 0.126 cm2. Inset: the parameters summary of the DSSCs.


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Figure 6. (a) Successive CV scanning, (b) the change of the current density under bias voltage of −0.25 V for different cycle times and (c) the current density vs. time plots under bias voltages of Pt and Co3S4/Co3O4 CEs; the bias voltages of −0.25 V was chosen from Tafel zone, and −0.50 V was near to the voltage value (0.52 V) at the maximum efficiency spots of DSSCs. (d) The normalized Jsc, Voc, FF, and PCE of Pt- and Co3S4/Co3O4-based solar cells with different times. The inset in 6a and 6b showed anodic, cathodic peak current densities of the Co3S4/Co3O4 for different cycles and the origin Tafel polarization curves of Pt and Co3S4/Co3O4 CEs.


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Table 1. Summary of Rs and Rct fitted from the EIS results of Pt and Co3O4 films with different times of surface activation in Figure 4a. Resistance

Pt

Co3O4

Co3S4/Co3O4-5 s

Co3S4/Co3O4-10 s

Co3S4/Co3O4-30 s

Rs (Ω/cm2)

20.4

19.4

19.4

19.7

20.3

Rct (Ω/cm2)

5.5

---

3131.3

43.8

4.8

ASSOCIATED CONTENT Supporting Information. The atom arrangement of inert and long-time activated Co3O4 and the corresponding EIad . Cross sectional SEM image of the obtained Co3O4 film. The elemental mapping of the activated Co3O4 film. The energy dispersive X-ray spectra and the elemental atomic ratio of the Co3S4/Co3O4-30 s films. The XRD patterns of Co3O4 film with different activation times. The FFT patterns of Co3S4 and Co3O4 in Figure 2d. The XPS data: Co 2p spectra of Co3O4 and Co3S4/Co3O4-30 s films. Change tendency of signals intensity for O and S elements with etching time. Tafel polarization curves of long-time activated Co3O4 films. The JV curves of eight DSSCs based on activated Co3O4 CE. The corresponding average photovoltaic parameters and standard deviation of the eight cells. Nyquist plots and Tafel polarization curves of inert and activated NiO films. This material is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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*E-mail: [email protected]. Fax: +86 431 85099772. Tel.: +86 431 85099772 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the Natural Science Foundation of China (Grant Nos. 51372036, 51602047, and 91233204), the Key Project of China Ministry of Education (NO. 113020A). And the authors sincerely acknowledge Dr. Hancheng Zhu and Dr. Haixia Wang for providing technology supporting on characterization of the samples. REFERENCES (1) Ahmad, S.; Guillén, E.; Kavan, L.; Grätzel, M.; Nazeeruddin, M. K. Metal free sensitizer and catalyst for dye sensitized solar cells. Energy Environ. Sci. 2013, 6, 3439-3466. (2) Yun, S.; Liu, Y.; Zhang, T.; Ahmad, S. Recent advances in alternative counter electrode materials for Co-mediated dye-sensitized solar cells. Nanoscale 2015, 7, 11877–11893. (3) Yun, S.; Freitas, J. N.; Nogueira, A. F.; Wang, Y.; Ahmad, S.; Wang, Z.-S. DyeSensitized Solar Cells Employing Polymers. Prog. Polym. Sci. 2016, 59, 1−40.


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TOC Graphic


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Adsorption Energy Optimization of Co3O4 through Rapid Surface

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