Investigation of Regeneration Kinetics of a Carbon-Dot-Sensitized

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Investigation of Regeneration Kinetics of Carbon Dots-Sensitized Metal Oxide Semiconductor With Scanning Electrochemical Microscopy Naiyun Liu, Yunlong Qin, Mumei Han, Hao Li, Yue Sun, Siqi Zhao, Hui Huang, Yang Liu, and Zhenhui Kang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00292 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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ACS Applied Energy Materials

Investigation of Regeneration Kinetics of Carbon Dots-Sensitized Metal Oxide Semiconductor with Scanning Electrochemical Microscopy Naiyun Liu, Yunlong Qin, Mumei Han, Hao Li, Yue Sun, Siqi Zhao, Hui Huang,* Yang Liu* and Zhenhui Kang* Jiangsu Key Laboratory for Carbon-based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu, China. KEYWORDS: Carbon dots, metal oxide semiconductor, regeneration kinetics, scanning electrochemical microscopy, interfacial charge transfer

ABSTRACT: Carbon dots (CDs) have been widely studied as sensitizers for metal oxide semiconductor electrodes. CDs/TiO2 photoanodes were fabricated and the regeneration kinetics of CDs were examined by scanning electrochemical microscopy in feedback mode. Regeneration rate constants of the CDs were obtained by using different concentrations of redox mediators and light intensities. Testing the regeneration rate of CDs within a single sensitized electrode, provides some new insight into the analysis of the performance of CDs sensitized metal oxide semiconductor electrodes.

1. Introduction

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Photosensitized reactions at metal oxide semiconductor (MOS) electrodes have recently attracted considerable attentions, because of their great potential in new energy and photoelectronic device applications.1-5 Suitable electrodes should be easy to prepare and inexpensive, have stable behavior and a wide absorption range to allow efficient utilization of the solar energy spectrum. A variety of MOS have been examined and among those TiO2 electrodes have been demonstrated to be efficient for various applications.4-10 However, the pure TiO2 electrodes are still limited by the weak responses to the visible light region owing to their wide band gap (3.0 and 3.2 eV for rutile and anatase phase, respectively)6. To improve light harvesting efficiency of TiO2 electrodes under visible light, many strategies have been utilized, such as doping with foreign atoms to modify the electronic structure of TiO28-9 and the dye sensitization of TiO2 electrode11-12. Among these methods, the sensitization approach utilizing dye or highabsorption semiconductors as sensitizers is easily controllable and considerably convenient. Carbon dots (CDs) defined as small carbon nanoparticles exhibit a wide absorption range, unique electronic properties, chemical stability, low cost and nontoxicity.13-22 With these advantages, CDs have been extensively studied and used in many domains. The high optical absorptivity of CDs at visible wavelengths makes them candidates for inexpensive sensitizer in sensitized MOS based devices, typically referred to as dye-sensitized solar cells (DSSC).20-27 Long term studies have shown that the photoexcited CDs can act as electron donors and acceptors.16-17 In addition, the conduction band edge difference between CDs and MOS electrodes will drive charge separation and electron transfer processes at the interface.23-26 Since the regeneration rate of CDs is of crucial importance to photovoltaic performance of the CDssensitized MOS electrodes, a study of the kinetics of the CDs regeneration is necessary. In the past years, scanning electrochemical microscopy (SECM) has received attention for investigating


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the dye regeneration kinetics under work condition, due to its high time, spatial resolution and sensitively.28-37 However, there is little research on the CDs regeneration reaction. Herein, we investigate the kinetics of CDs regeneration at CDs-sensitized TiO2 electrode using the SCEM technique. CDs were fabricated by the electrochemical etching of graphite rods. The conduction band (CB) value of CDs is approximately -1.84 V (versus NHE), which is lower than the CB edge of TiO2.6 This indicates that CDs can be utilized as the sensitizer in MOS devise owing to the fact that the CDs should be capable to inject electrons across the CDs/semiconductor interface into TiO2. The CDs regeneration kinetics are measured by examining the ultramicroelectrode feedback current in relation to the variation of active species concentrations with short-circuit condition. The active species here are chosen as I3-/I- in acetonitrile. The valence band (VB) value of the CDs (1.74 V versus NHE) ensures that the oxidized CDs is able to be reduced through an electron donation from the electrolyte (I3-/I-)11, 23. The relevant description of the SECM model is provided in the Supporting Information, which has been mentioned in many previous literature.34,

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The approach allows us to study the

regeneration rate of the CDs at CDs-sensitized TiO2 electrode. This is important for designing efficient CDs-sensitized MOS based devices, enabling further performance enhancements of sensitized MOS electrodes by testing the regeneration rate of different sensitizer with a single sensitized electrode. 2. Experimental Preparation of CDs-sensitized TiO2 photo-anodes In the experiments, CDs were fabricated by the electrochemical etching of graphite rods. The preparation processes have been introduced in our previous works in detail.18, 38 TiO2 nanowire arrays deposited on the FTO glass substrates were grown by a hydrothermal method. The


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detailed strategy has been given elsewhere.10 The CDs-sensitized TiO2 photo-anodes were fabricated by immersing the FTO glass substrates coated with the TiO2 nanowire film into the CDs solution at room temperature overnight. After being washed with DI water, the CDssensitized TiO2 photo-anodes were obtained. SECM measurement A model 920C Scanning Electrochemical Microscope (CH Instruments, Shanghai, China) was employed to study the CDs-regeneration progress with a feedback model in iodine-based electrolyte. The sensitized electrode was placed in the bottom of a homemade Teflon cell with an O-ring (exposed area: ~0.5 cm2) with a Pt auxiliary electrode and a Ag/AgCl reference electrode (saturated KCl). The potential of Ag/AgCl electrode vs. reversible hydrogen electrode (RHE) is 0.197 V. Acetonitrile solution with 0.1 M LiTFSI containing I3- only was used as the electrolyte. This requires titrating the KI solution with I2 until the cyclic voltammetry curves obtained at the Pt ultramicroelectrode have the plateau and zero current. The procedure was adopted from previous literature30, 34 and a detailed description of this SECM model is described in Supporting Information. After titration, the solution with exclusive I3- was obtained. Here the concentration of the I3- stock solution was 2.11 mM. In the experiments, the effect of both the concentrations of I3- and the photon flux on the approach curves were investigated. By diluting the I3- stock solution with 0.1 M LiTFSI in acetonitrile, series of electrolytes with different I3- concentrations used were prepared: 0.06, 0.1, 0.3, 0.6, 0.8 and 1 mM. The xenon lamp with band-pass filters optical fiber (with 20 nm bandwidth centered at 475 nm) was used as the illuminator and operated on the back side of the photoanode. The optical fiber is used to avoid the activation of the semiconductor TiO2. The real densities were measured with an optical power meter at the location of the sample. While the illuminator has the density P = 2.6 mW·cm-2, the resulting


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photon flux expressed by Jhv=P/(Ehv× NA) is determined to be 1.03×10-8 mol·cm-2·s-1 with the photon energy Ehv = hc/λ = 4.19×10-19 J. The illuminator used was employed with the following intensities: 0.59, 0.88, 2.06, 3.24, 5.3, 7.66 and 10.31×10-9 mol·cm-2·s-1. Characterization Methods A FEI/Philips Tecnai 12 BioTWIN TEM and a FEI Titan G2 80-200 ChemiSTEM were used to obtain the Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images, respectively. A Varian Spectrum GX spectrometer was used to record the Fourier Transform Infrared (FTIR) spectrum of the CDs. We also obtained the UVvisible absorption spectra of CDs by an Agilent 8453 UV-VIS Diode Array Spectrophotometer. Morphology of the as-synthesized TiO2 nanowire film was observed by the scanning electron microscopy (SEM, FEI-Quanta 200). Raman spectra were taken using 514 nm excitation with a Jobin Yvon Model HR 800 Raman spectroscope. 3. Results and discussion In this experiment, the CDs were synthesized through electrochemical etching of graphite rods.14-15, 18 From the transmission electron microscopy (TEM) image (Figure. 1a), we can find that the synthesized CDs are graphitic nanocrystals with a narrow size distribution. Observed with the high resolution TEM (HRTEM) image (the upper insert in Figure. 1a), the crystal lattice spacing of the CDs is calculated as 0.34 nm, corresponding to the lattice planes of graphite carbon.39-40 The particle size distribution data (the bottom inset in Figure. 1a) provide the CDs having the diameter ranging from 3 to 8 nm, with an average size of 6 nm. Fourier transform infrared (FT-IR) spectrum (Figure. 1b) gives information that lots of functional groups, such as hydroxyl (-OH), carboxyl (C-O) and carbonyl (C=O) present on the surface of CDs.19 The C 1s XPS spectrum (Figure. S1) shows that the carbon/oxygen bonding (C-O and


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C=O) content of CDs is 50.0%. The UV-visible absorption spectrum of the CDs presented in Figure. 1c displays obvious light absorption band in the UV-visible range. This result endows them with an advantage in serving as photosensitizers. The PL spectrum is shown in Figure. 1c as well, displaying that with the excitation wavelength of 340 nm the CDs has a PL emission peak at around 453 nm. Furthermore, the bandgap structure of the CDs was investigated as well by an electrochemical method.13, 41 The studies were performed by linear sweep voltammetry (LSV) in acetonitrile with 0.1 M TBAPF6 at a scan rate of 10 mV/s at room temperature. A glassy carbon electrode modified with CDs was used as the work electrode with a Pt auxiliary electrode and an Ag/AgCl reference electrode. The cathodic scan and anodic scan (Figure. 1d and inset) show that the conduction band (CB) and valence band (VB) values of CDs are approximately -1.84 V and 1.74 V (versus NHE), respectively. The CB level of the CDs sensitizers is more negative than the CB edge of TiO26, which can ensure efficient electron injection from the CDs sensitizers towards TiO2. Meanwhile, the VB of the CDs is more positive than the redox potential of I3-/I-, leading to the rapid regeneration of CDs by electron donation from the redox shuttle.11, 23


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Figure 1. (a) TEM and HRTEM (the upper inset; scale bar, 2 nm) images of the synthesized CDs. The bottom inset is the particle size distribution. (b) The FT-IR, (c) UV-visible absorption and PL spectra, and (d) Cathodic and anodic (the inset) scans at 10 mV s-1 of CDs. TiO2 nanowire films on fluorine-doped tin oxide (FTO) substrate were fabricated by the hydrothermal growth (Experimental Section).10 Scanning electron microscopy (SEM) images of the TiO2 film (Figure. 2a) shows the nanowire arrays having a rectangular cross section and a dense vertical growth. The average diameter and length of these nanowires are uniform with diameters of 150-200 nm and length of 2.5-3 µm (upper inset in Figure. 2a), respectively. In the X-ray diffraction (XRD) pattern (bottom inset in Figure. 2a), the diffraction peaks marked by a diamond sign (◇) correspond to the (110), (200), (220) and (002) planes of the tetragonal rutile TiO2 (JCPDS No. 21-1276), respectively, indicating that the as-prepared nanowires are rutile TiO2.8 The asterisks represent the diffraction peaks of FTO substrate.8 The CDs-sensitized TiO2 films were fabricated by immersing the FTO substrates coated with the TiO2 nanowire film into the CDs solution at room temperature overnight. The TEM of the TiO2 nanowires loaded with CDs (Figure. 2b) exhibits that the nanowires have diameters of 150-200 nm, in accord with that observed by the SEM image. The HRTEM image (Figure. 2c) displays that CDs with lattice spacing of 0.34 nm ( lattice plane) were in close contact with the TiO2 nanowire with the (lattice spacing, 0.22 nm)42 and (lattice spacing, 0.30 nm)43 crystal planes exposed. As shown in Figure. 2d, Raman spectrum of CDs-sensitized TiO2 film show three characteristic peaks centred at 222, 444 and 606 cm-1 corresponding to a rutile crystalline structure.44-45 In addition, the Raman-active modes of the CDs can be clearly observed as well: the D (disordered) peak centred at 1364 cm-1 and G (graphitic) peak at 1606 cm-1.19 The previous results provided that CDs were loaded on the surface of the TiO2 nanowires. The concentration of the CDs (1.95


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× 10-6 mol·cm-3) loading on the TiO2 nanowire film was measured as well. The detailed description is described in the Supporting Information.

Figure 2. (a) The top-view SEM image, XRD pattern (shown as the overlay) and side-view SEM image (the upper inset) of the as-synthesized TiO2 nanowire arrays. The asterisks in the XRD pattern represent the diffraction peaks of the FTO substrate. Scale bar in the inset image is 2 µm. (b) TEM and (c) HR-TEM image of CDs/TiO2 nanowire arrays. (d) Raman spectra of CDs (purple line), TiO2 nanowire arrays (orange line) and CDs/TiO2 nanowire arrays (green line). We further investigated the regeneration kinetics of CDs ( ) by scanning electrochemical microscopy (SECM) in the feedback mode (Figure. 3a).29-31 The ultramicroelectrode (UME) current will be effected by the substrate type and the used experimental conditions. Therefore, processes occurred on the substrate can be analysed through the UME current response (see Support Information for details). The UME potential was set to ET = -0.7 V vs. Ag/AgCl. At this point, diffusion-controlled reductions of I3- occurred on the UME with I- formation (Figure. S2). Upon photoexcitation of the CDs, the electrons will be injected into the CB of TiO2. The band-


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pass filters optical fiber (with 20 nm bandwidth centered at 475 nm) is used to avoid the activation of the semiconductor TiO2. When the UME is approached to the substrate, the resulting CDs oxidized state (CDs+) is regenerated by the produced I- in the electrolyte. The schematic of energy level and electron transfer processes is shown in Figure. 3b. The regeneration kinetics of the CDs were investigated in the feedback mode through the heterogeneous reaction at the CDs sensitized MOS/electrolyte interface under short-circuit condition.

Figure 3. (a) Schematic illustration of the SECM setup operated in the feedback mode for the investigation the heterogeneous reaction at the CDs-sensitized TiO2 interface in an acetonitrile solution containing I3-/I-. (b) The schematic of energy levels and electron-transfer processes of the TiO2/CDs electrode. The sensitizing CDs absorb photons (energy hν) and inject electrons into the CB of TiO2, and then the electrons travel to FTO glass (not shown). The oxidized CDs is reduced by the redox shuttle (I3-/I-). is the regeneration rate constant of the CDs. To study the effect of concentrations of I3- ([I3-]) on the approach curves (tip current-distance relationships), approach curves under illumination at a constant intensity (1.03×10-8 mol·cm-2·s1

) with various [I3-] were carried out (Figure. 4a). The tip height d/ is normalized by


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dividing the distance between the UME and substrate (d) by the radius of the active area of the UME ( ). / , , is defined as the normalized tip current, obtained by the ratio of the tip current ( ) to the steady state current ( , ).29 As shown in Figure. 4a, recorded above an illuminated electrode (open symbols with different colours) is much higher than that recorded above the insulating surface (black curve), indicating the presence of a positive feedback. Through fitting the normalized experimental approach curves with Eq. S1-S5,30-31 the normalized heterogeneous rate constant can then be extracted. The values of obtained decrease with the increase of I3- concentrations because, compared with the diffusion of I3- produced by the regeneration of CDs, the flux from the electrolyte is dominant. While the diffusion coefficient D of I3- in acetonitrile electrolyte is 1.37×10-5 cm-2·s-1,29 the effective heterogeneous rate constant [cm·s-1] for CDs regeneration can be evaluated with Eq. 1, / 1 The values were plotted against various concentrations of I3- (Figure. 4b), and the data are summarized in Table S1. The experimental data were fitted to Eq. 2,30

3[ ]∅ 2 6 [] + 2∅ "

Herein, the TiO2 film thickness is 2.7×10-6 cm, the CDs concentration [D0] is 1.95×10-6 mol·cm-3. [C] is the I3- concentration in acetonitrile [mol·cm-3] and " is the incident photon flux [mol·cm-2·s-1]. Through fitting with the different I3- concentrations by Eq. 2 (solid line in Figure. 4b), the CDs regeneration rate constants ( ) and the absorption cross-section of the CDs (∅ ) are evaluated simultaneously to be 2.07×109 mol-1·cm3·s-1 and 2.72×1010 cm2·mol-1, respectively.


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Figure 4. (a) Experimental and theoretical feedback approach curves of Pt UME ( =5 µm, RG=10) on a FTO/TiO2/CDs film under illumination at a constant intensity Jhv of 1.03×10-8 mol·cm-2·s-1 in acetonitrile with the following I3- concentrations in mM (1) 0.06, (2) 0.1, (3) 0.3, (4) 0.6, (5) 0.8 and (6) 1. The tip potential was set at -0.7 V vs. Ag/AgCl (scan rate, 1 µm·s-1). Curve (7) is the approaching curve of a UME towards an inert insulating surface. By fitting the experimental approach curves (open symbols) to theoretical approach curves (colourful curves), the values of the normalized rate constant were obtained (1) 0.27, (2) 0.15, (3) 0.11, (4) 0.084, (5) 0.075 and (6) 0.069, respectively. (b) Plot of versus different I3- concentrations in acetonitrile on a FTO/TiO2/CDs film under illumination. Furthermore, we proceeded to study the effects of different light intensities on the CDs regeneration kinetics with a fixed I3- concentration (0.06 mM). The illuminator used with different photon flux densities was employed. The experimental approach curve results were displayed in Figure. 5a, the normalized tip current shows much higher values with higher intensity of light and the feedback current tends to saturation with the light intensity increase, because of the gradually saturated number of photons absorbed by the CDs on the electrode. The extracted of each curve and the calculated are summarized in Table S2. values recorded


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increase with the intensity of illumination increasing. Based on the analyticl approximation Eq. 2, can be expressed as function of " (Figure. 5b), yielding = 1.30×109 mol-1·cm3·s-1 and ∅hv = 6.92×1010 cm2·mol-1. The value obtained here is, however, somewhat smaller than the value measured through using different I3- concentrations. The analysis method of the CDs regeneration rate using the SECM model introduces uncertainty, and the preparation of CDs-sensitized TiO2 electrode will add this value as well. Different D° values will lead to different , due to high coupling between the parameter D° and (Eq. 2). The concetration of CDs loading determined experimentally is an average value from a limited number of samples. The ∅ values obtained will be affected by the uncertainty in the preparation processes as well.

Figure 5. (a) Experimental and theoretical feedback approach curves of the Pt UME ( =5 µm, RG=10) on a FTO/TiO2/CDs film in acetonitrile solution containing 0.06 mM I3-. The illuminator used was employed with the following intensities of light in 10-9 mol·cm-2·s-1 (1) 0.59, (2) 0.88, (3) 2.06, (4) 3.24, (5) 5.3, (6) 7.66 and (7) 10.31. The tip potential was set at -0.7 V vs. Ag/AgCl (scan rate, 1 µm·s-1). Curve (8) is a calculated approaching curve of a UME towards an inert insulating surface. Through fitting the experimental approach curves (open symbols) to theoretical approach curves (colorful curves), the values of the normalized rate


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constant were obtained (1) 0.05, (2) 0.09, (3) 0.14, (4) 0.18, (5) 0.23, (6) 0.25 and (6) 0.28, respectively. (b) Plot of versus different light intensities for CDs-sensitized TiO2 photoelectrochemical electrodes in acetonitrile. 4. Conclusions CDs have shown great promise in a wide range of photovoltaic devices as inexpensive sensitizer candidates. The regeneration kinetics of CDs at CDs-sensitized TiO2 electrode were investigated by the feedback mode of scanning electrochemical microscopy technique for the first time. Regeneration rate of the CDs were obtained by measuring the feedback current at the ultramicroelectrode, which is in relation to the variation of the active species concentrations under short-circuit condition. This work provides insights into the study of the CDs regeneration kinetics on the CDs sensitized metal oxide semiconductors, so that the design of highperformance photoelectrodes for future energy and photoelectronic applications can be realized. ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website. Additional details about the measurements of the amount of CDs-loading, the SECM feedback mode and fitting routine, derivation of the used equations, XPS spectra, schematic of energy level and electron transfer processes, cyclic voltammograms, the calibration curve and UV-vis absorption spectra of CDs, schematics of SECM feedback mode and tabulated data. (PDF). AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] ACKNOWLEDGMENT This work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology, the National Natural Science Foundation of China (51725204, 21771132, 51572179, 21471106, 51422207, 21501126), the Natural Science Foundation of Jiangsu Province (BK20161216) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1)

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Investigation of Regeneration Kinetics of a Carbon-Dot-Sensitized

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