Tunable Pseudocapacitive Behavior in MetalâOrganic Framework

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Tunable Pseudocapacitive Behavior in Metal-Organic Framework Derived TiO2@Porous Carbon Enabling High-Performance Membrane Capacitive Deionization Meng Ding, Shuang Fan, Shaozhuan Huang, Mei Er Pam, Lu Guo, Yumeng Shi, and Hui Ying Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01839 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 18, 2019

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ACS Applied Energy Materials

Tunable

Pseudocapacitive

Behavior

in

Metal-Organic

Framework

Derived

TiO2@Porous Carbon Enabling High-Performance Membrane Capacitive Deionization

Meng Dinga, Shuang Fana,b, Shaozhuan Huanga, Mei Er Pama, Lu Guoa, Yumeng Shib, Hui Ying Yanga,* a Pillar of Engineering Product Development, Singapore University of Technology and Design,

8 Somapah Road, 487372, Singapore b

Engineering Technology Research Center for 2D Material Information Function Devices and

Systems of Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China *

Corresponding author. E-mail address: [email protected] (H. Y. Yang)

Keywords: metal-organic frameworks, nanoporous carbon, titanium dioxide, membrane capacitive deionization, pseudocapacitance

Abstract Titanium dioxide (TiO2) composites have shown a promising desalination as the electrode materials of capacitive deionization (CDI). However, it remains a significant challenge to explore its pseudocapacitive potential for further enhancement of salt adsorption capacity and long-term stability. Herein, we report a titanium dioxide/porous carbon composite (TiO2@PC) with tunable pseudocapacitance for high-performance membrane CDI (MCDI) based on a metal-organic frameworks (MOFs) derived strategy. By controlling the pyrolysis conditions, the crystalline degree and specific surface areas of TiO2@PC samples have been optimized to improve the salt adsorption performance. A synergy of high pseudocapacitance and good

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oxidation resistant endows the anatase TiO2@PC (annealed at 600 °C) with an improved salt adsorption capacity of 46.7 mg g-1 at 10 mA g-1 and stable cycling performance over 50 cycles. These properties reveal the great potential of anatase TiO2@PC to serve as a promising candidate of electrode materials for MCDI.

1.

Introduction

Capacitive deionization (CDI) is a method of desalination utilizing electronically charged electrodes to immobilize the salt ions from saline solution temporarily.1 The past decade has witnessed the growing interest and great advances in the development of CDI technology due to its promising desalination performance and low impact to the environment.2, 3 In a classic CDI system, there are two porous carbon electrodes which are electronically charged/discharged regularly to realize the adsorption/desorption of the salt ions from feed solutions, which is depicted by the formation of electrical double layer (EDL).1, 4 Compared with commercially available water treatment techniques, such as distillation and reverse osmosis,5-7 CDI has no requirement on the water pressure or heat treatment, which result in low capital and operating cost.8, 9 Energy recovery can be achieved that further reduces the energy consumption.10,

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Although carbon-based CDI electrodes possess such advantages,

their relatively low salt adsorption capacity at high concentration salty water limit their efficacy in industrial application.12, 13

To overcome these limitations, a lot of research efforts have been made to design innovative CDI architectures or search for novel electrode materials.2 Membrane capacitive deionization (MCDI) has demonstrated as an attractive candidate for desalination technique due to its enhanced salt adsorption capacity, improved charge efficiency and reliable long-cycle performance, in contrast to the classic CDI.14 By integrating ion-exchange membranes (IEMs)

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into the typical CDI devices, MCDI permits only anion adsorption on anode and cation adsorption on cathode, so that the counter ions (ions possess the opposite charge with the electrode) are free to migrate in/out of the electrode, while the co-ions (ions with the same charge as the electrode) are blocked. In this way, the co-ion expulsion effect, which generally happens in classic CDI systems without IEMs, is greatly limited.15 By restraining the co-ion expulsion effect, MCDI is able to ideally remove one salt molecule from the bulk solution for each electron transferred between the electrodes, which significantly improves the salt adsorption efficiency.14 Moreover, the long-term performance in MCDI is better than that of classic CDI due to the protection from anode corrosion and the complete depletion of salt ions during regeneration process by applying a reversed electrical potential.16, 17

Further improvement of MCDI device can be achieved by designing novel electrode materials. Various carbon materials have been studied as the MCDI electrode, such as carbon cloth,15 carbon nanotubes,18 carbon nanofibers, carbon xerogel,19 modified activated carbon,20 graphene,21 and their composites.22 High specific surface area, good electronical conductivity and better wettability are the desired properties for better salt adsorption capacity. Good stability and affordable cost of the MCDI electrode materials are also required to bring it into practical desalination applications.1, 13 However, the expanded surface area does not always bring a better ion adsorption capacity because only the ion-accessible surface area can contribute to the salt adsorption in EDLs.1 Alternatively, the non-carbon materials, such as manganese oxide (MnO2),23 iron(II, III) oxide (Fe3O4),24 sodium manganese oxide (Na4Mn9O18),12 Prussian blue analogues25,

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have been extensively studied in the recent

decade owing to their intrinsic pseudocapacitive properties and material diversities. The pseudocapacitive properties can offer additional redox reactions and ion intercalation, so that

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the salt adsorption capacity is no longer limited by the surface area.27 Thus, the ion adsorption based on such materials has been significantly improved in various CDI systems.

As a pseudocapacitive material, titanium dioxide (TiO2) possesses a series of advantages, such as high capacity, abundant resource, non-toxicity, excellent stability, and environmental friendliness, making it highly suitable for electrode materials of (M)CDI.28, 29, 30 To develop TiO2 electrode for promising desalination application, several limitations should be resolved, including the poor electronical conductivity and low ion diffusion rate. One effective method is to confine TiO2 nanoparticles in carbonaceous materials, such as activated carbon,31,

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carbon nanotubes,31 carbon nanofibers,32 graphene,29 and reduced graphene oxide.28 For example, Kim et al. reported a TiO2 coated carbon electrode that achieved a salt adsorption capacity of 17 mg g-1 in a single-pass CDI system, a twice enhancement of activated carbon electrodes.35 Srimuk et al. demonstrated an improvement of CDI stability by utilizing oxygenresistant TiO2/activated carbon composites electrodes, showing a much slower decrease of salt adsorption capacity than activated carbon electrode.17 However, most of work requires for complex multistep synthesis processes with weak internal bonding between TiO2 and carbon, which shows relatively low salt adsorption capacity.36 Therefore, it is desirable to develop a facile approach for the fabrication of TiO2/C composite electrode with strong internal bonding between TiO2 and carbon, as well as with high salt adsorption capacity.

Herein, we develop a facile pyrolysis process to prepare the TiO2@PC composites with TiO2 nanocrystals confined within porous carbon scaffold using a titanium-based metal-organic framework (Ti-MOF) as the precursor. By tuning the thermal treatment temperature, we have obtained a series of TiO2@PC composites with TiO2 polymorphs (anatase, rutile and mixed), crystalline degree, and carbon texture. The crystal structures are formed by stacking of edge-

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sharing TiO6 octahedra with different tunnels. The TiO2 polymorphs with varied tunnel structures produce different pseudocapacitive properties that affects their desalination results.37 The growth mechanism of the several TiO2@PC composites has been carefully studied through various characterization methods. Meanwhile, the electrochemical and desalination performance have been evaluated to determine the relationship between material structures and properties. Especially, ion adsorption mechanism and surface chemistry change after the long-time cycling of different TiO2@PC samples has been carefully studied to analyze the effect of pseudocapacitance on desalination performance. This is the first study of salt adsorption mechanism of TiO2-based MCDI electrodes. The results reveal that a synergy of high specific surface area and superior pseudocapacitive properties endows the anatase TiO2@PC composites with a highest salt adsorption capacity of 46.7 mg g-1 and reliable cyclic performance over 50 cycles.

2.

Experimental section

2.1

Chemicals and reagents

Terephthalic acid (H2BDC), titanium isopropoxide, N,N-dimethylmethanamide (DMF), methanol, polyvinylidene fluoride (PVDF, Mw ~ 543 000), acetylene black, and N-methyl-2pyrrolidone (NMP, 99.5%) were purchased from Sigma-Aldrich. All the chemicals and reagents were of analytical grade and used as received. 2.2

Synthesis of MIL-125 (Ti)

MIL-125 (Ti) was synthesized based on a modified method.40 In general, 0.60 g H2BDC was mixed in a solution of 9.0 mL DMF and 1.2 mL methanol under vigorous stirring for 30 minutes in room temperature (295 K). Then, 0.31 mL titanium isopropoxide was dropped into the mixture within 5 seconds and kept stirring for another 10 minutes. After that, the mixed solution was transferred to a 50 mL Teflon autoclave and treated at 150 °C for 13 hours in a

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heating oven starting at room temperature (UN750, Memmert). Eventually, the white product was collected by centrifugation with the speed of 7000 rpm and a duration of 5 minutes. After that, the white product was dispersed in 35 mL methanol well and further centrifuged to remove the residues of reaction. The washing process by centrifugation was repeated for 3 times, followed by a drying process at 60 °C overnight in an oven. The yield of MIL-125 is around 400 mg. 2.3

Synthesis of TiO2@PC composites

TiO2@PC composites were obtained from carbonizing the MIL-125 (Ti) under different thermal conditions. To offer TiO2@PC-600 as an example, 200 mg of the as-prepared MIL125 (Ti) nanoparticles were carbonized in the argon atmosphere by increasing the annealing temperature to 600 °C with a heating rate of 5° min-1 and kept for 2 hours. The argon flow rate is 10 sccm. The black powder was collected after cooling down naturally with the yield around 73 mg. Similarly, TiO2@PC-800 and TiO2@PC-1000 were obtained from carbonization in the argon atmosphere under 800 °C and 1000 °C, respectively. 2.4

Fabrication of electrodes

Electrodes for electrochemical tests and electrosorption experiments were fabricated based on our previous work.13 Generally, TiO2@PC composites (56 mg) were mixed with carbon black (7 mg) and PVDF (7 mg) in a mass ratio of 8:1:1 and ground gently. Then, 4 mL NMP were added to the mixture and stirred for 6 hours to form a uniform slurry. For electrochemical tests, the slurry was cast on small pieces of graphite sheets (10 mm × 2 mm). For the electrodes for electrosorption experiments, the slurry was sprayed by an air brush (Model 150, Badger) to the graphite sheet. A hollow square-shaped mask was placed on the graphite sheet to ensure the slurry could be only applied on the central part of the graphite sheets within a well-defined 50 mm x 50 mm square. A hotplate was placed under the graphite sheet and maintained at 100 °C. The as-prepared electrodes were dried at 60 °C overnight in an oven.

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2.5

Characterization

A field-emission scanning electron microscopy (FESEM, JEOL JSM-7600) and a transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN) were utilized to investigate the morphologies, structure, and particle sizes. The lattice structure was determined by the high-resolution TEM (HRTEM) with an acceleration voltage of 200 kV. The crystallinity and phases of the synthesized samples were studied by powder X-ray diffraction (XRD, Bruker D8 Advance diffractometer) equipped with a Ni filtered Cu K𝛼 radiation (𝜆 = 1.5406 Å, 40 kV, and 40 mA) and by X-ray photoelectron spectroscopy (XPS, PHI-5400) with an Al K𝛼 beam source (250 W). Raman spectra were obtained by a confocal microprobe Raman system (WITec 410) with the laser excitation of 532 nm. Thermogravimetric analysis was performed by a differential thermal thermogravimetry analyzer (TGA, Shimadzu DTG-60) in a compressed air atmosphere from 25 to 1000 °C. The specific surface area (SSA) and porosity profiles were determined from the nitrogen adsorption/desorption measurements by a gas sorption analyzer (Autosorb-iQ-MP-XR, Quantachrome). The SSA was calculated based on the Brunauer-Emmett-Teller (BET) method and the pore size distribution was analyzed based on Barrett-Joyner-Halenda (BJH) method. 2.6

Electrochemical tests

The electrochemical tests were carried out in a three-electrode setup, consisting of a working electrode of the TiO2@PC samples on a graphite sheet, a counter electrode of platinum foil, and a saturated calomel reference electrode (SCE). Cyclic voltammetry (CV) was conducted in 1 M NaCl aqueous solution at a series of scanning rate from 1 to 100 mV s-1, within a voltage window of -0.5 to 0.5 V. Galvanostatic charge-discharge (GCD) profiles were obtain at a series of current densities from 0.1 to 10 A g-1, within the same voltage window as in the CV experiments. The specific capacitance, 𝐶 was calculated using the following equation (1): 𝐼 ∙ ∆𝑡

𝐶 = 𝑚 ∙ ∆𝑉

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(1)


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where 𝐶 is the specific capacitance (F g-1), 𝐼 is the constant discharge current (A), ∆𝑡 is the discharge time (s), 𝑚 is mass of active material on the working electrode (g), and ∆𝑉 is the potential change during discharge after IR drop (Ohmic drop). All of the CV, GCD, and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (VMP3, Bio-Logic). 2.7

Electrosorption experiments

The electrosorption experiments were conducted in a batch-mode setup as depicted in our previous work.39 In each experiment, the electrodes were tested in 50 mL NaCl aqueous solution with a varying concentration (500 to 1500 mg L-1). The flow rate of the feed solution kept at 60 mL min-1 by a peristaltic pump (BT301L, Lead Fluid). The effluent solution conductivity was measured by a conductivity meter (DDSJ-308F, Leici). All of the electrosorption experiments were performed at room temperature (298 K). The testing cell was operated in a constant current mode with an applied constant current density (10 to 100 mA g-1) across the two electrodes. The cell was charged to 1.4 V during the adsorption process and then discharged to -1.4 V for ion release using a battery tester (Neware). The salt adsorption capacity (SAC), Γ of TiO2@PC electrodes was determined by equation (2): Γ=

(𝐶𝑜 ― 𝐶𝑒) ∙ 𝑉 𝑚

(2)

where 𝐶0 and 𝐶𝑒 are the solution concentration at initial and equilibrium states respectively (mg L-1), 𝑉 stands for the volume of NaCl solution used in the batch-mode test, and 𝑚 is the total mass of the active material from electrodes (g). The charge efficiency, Λ of the electrode materials was defined as equation (3): 𝛤∙𝑚∙𝐹

Λ = 1000 ∙ 𝛴

(3)

where 𝐹 is the Faraday constant of 96,485 (C mol-1), and Σ is the specific charge accumulated during charging process (C g-1). The total energy consumption, 𝐸𝑡𝑜𝑡 was equal to the sum of the energy consumed and the energy recovered, as shown in equation (4): 8


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𝑡

𝑡

𝐸𝑡𝑜𝑡 = 𝐼𝑎𝑑𝑠 ∙ ∫𝑡𝑎𝑑𝑠𝑉 ∙ d𝑡 + 𝐼𝑑𝑒𝑠 ∙ ∫𝑡𝑡𝑜𝑡 𝑉 ∙ d𝑡 0

𝑎𝑑𝑠

(4)

where 𝑡0 and 𝑡𝑡𝑜𝑡 are the time at start and end for one cycle, 𝐼𝑎𝑑𝑠, 𝑡𝑎𝑑𝑠 are current and the end time of the adsorption process, and 𝐼𝑑𝑒𝑠 is the current of the desorption process.10, 40 And the energy consumption per ion removed is defined as the ratio of the total energy consumption during one cycle to the number of ions removed from the feed solution.

3.

Results and discussion

3.1

Morphology and structure

The fabrication procedure for the TiO2@PC samples is schematically illustrated in Figure 1. Tablet-like Ti-MOF (MIL-125) were synthesized via a hydrothermal method and served as the template or precursor for fabricating porous materials. The resultant white powder was then annealed in an argon atmosphere at 600 °C, 800 °C, and 1000 °C, which are denoted as TiO2@PC-600, TiO2@PC-800, and TiO2@PC-1000, respectively. The thermal pyrolysis creates a continuous conductive network and a great number of channels in the carbon matrix that is expected to benefit the improvement of electronic conductivity and the salt ions diffusion rate of salt ions from the solution to the interface.38, 41

Figure 2a-c display the scanning electron microscope (SEM) images of the three TiO2@PC samples. Before thermal pyrolysis, the Ti-MOF precursor shows well-defined rounded square shape with an average diameter of 480 nm and a height of 300 nm (Figure S1a,b). After annealed in 600 °C, the TiO2@PC-600 remains the same morphology and size with that of MIL-125 (Figure 2a). While as the annealing temperature increases to 800 °C and 1000 °C, the size of TiO2@PC nanoparticles gradually shrinks due to the further decomposition of the carbon matrix at higher temperature (Figure 2b,c).38 Moreover, the higher temperature induces further grain growth, leading to bigger TiO2 crystal size on the surface of TiO2@PC samples.42

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Additionally, energy-dispersive X-ray (EDX) mapping results confirm the uniform distribution of TiO2 on the carbon structure in all of the three samples (Figure S2). As shown in transmission electron microscope (TEM) images (Figure 2d-f), the morphology and structure of TiO2@PC samples are consistent with the SEM results. And a large number of ultrafine TiO2 nanocrystals can be observed in the porous carbon networks with an increased size from 4 nm to 60 nm as temperature grows from 600 to 1000 °C. It is noteworthy that the ultrafine TiO2 nanoparticles distribute uniformly in the porous carbon matrix which greatly reduce the ion diffusion length and the carbon network matrix offers abundant pores and channels, improving the salt adsorption capacity.

3.2

Crystallographic studies

The powder X-ray diffraction (XRD) patterns as shown in Figure S3 confirms the phase purity of the synthesized MIL-125.43 After the annealing process, the MIL-125 was converted to a porous carbon matrix integrated with TiO2 nanoparticles. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were conducted in air to study the material transformations during the annealing process, as shown in Figure S4a. The first weight loss occurred between 25 and 200 °C is attributed to the evaporation of physically adsorbed H2O. The second drop of weight from 420 to 450 °C is around 26.1%, suggesting the decomposition of organic groups.44, 45 The exothermic peak observed at around 470 °C was ascribed to the crystallization of anatase phase of TiO2. The exothermic peak from 615 to 642 °C is related to the appearance of rutile phase.46, 47

In Figure 3a, the XRD patterns of carbonized samples exhibit hybrid phases of anatase and rutile TiO2. For TiO2 with the anatase phase, the XRD pattern exhibits strong diffraction peaks at 25° and 48°.48 And the strong diffraction peaks at 27°, 36°, and 55° shows TiO2 in the rutile

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phase.49 As shown in XRD patterns of TiO2@PC samples, anatase phase was observed in TiO2@PC-600, both anatase and rutile phases were shown in TiO2@PC-800, while only rutile phase was presented in TiO2@PC-1000, confirming the percentage of the two phases is adjustable by varying annealing temperature. As listed in Table 1 of the calculated weight fraction of anatase to rutile based on K-value method, the composition ratio of anatase to rutile decreased from 2.80 to 0.18, suggesting anatase TiO2 gradually converted to rutile TiO2 with the increased temperature.50 The mass ratio between carbon and TiO2 contents in TiO2@PC samples are quntified by TGA as shown in Figure S4b and Table S1. Derived from the same Ti-based MOF precursor, the three TiO2@PC samples have similar TiO2/carbon mass ratio (1.9, 2.2, and 2.1 for TiO2@PC-600, -800, and -1000).

Together with the XRD results, the HRTEM images of lattice fringe spacings also varies as the temperature changes, further confirming the phase transformation (Figure S5a-c). In TiO2@PC-600 samples, the spacing is 0.35 nm indexing to the (101) plane of anatase TiO2 (a = b = 3.7710 Å, c = 9.430 Å). While in TiO2@PC-1000 samples, the (110) plane of rutile TiO2 (a = b = 4.5933 Å, c = 2.9592 Å) with spacing of 0.32 nm has a large majority. TiO2@PC800 sample shows a mixture of (101) plane of anatase and (110) plane of rutile, indicating both phases of TiO2 coexist in the carbon structures.37 The difference of crystalline phase in TiO2@PC nanoparticles is related to their different electrochemical behavior which will be analyzed soon.

The graphitic degree of carbon matrix in TiO2@PC samples was studied by Raman spectroscopy. As shown in Figure 3b, the Raman spectra of TiO2@PC samples present two distinct peaks at 1339 cm-1 and 1599 cm-1, representing the defect band and graphite band, respectively. And the broad peak around 2700 cm-1 indicates the second order of the D band

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(2D band). The D band is due to the hybridized vibrational mode related to graphene edges proving the existence of defects in the carbon structure of TiO2@PC samples. The G band is assigned to E2g mode.51 The intensity ratio of D band to G band (ID/IG) is calculated to evaluate the graphitic degree of carbon materials. It is seen from Figure 3b that the intensity difference between D band and G band grows larger as the annealing temperature increases, showing a decreased ID/IG value of 0.990, 0.949, and 0.865 for TiO2@PC-600, -800, and -1000, respectively. The decreasing ID/IG values indicate larger graphitic domains and less disorder in the carbon matrix treated in higher annealing temperature that are responsible for better electron conductivity.52 The increased intensity of 2D band further confirms the improving graphic crystallization of samples at the higher annealing temperature.

X-ray photoelectron spectroscopy (XPS) analysis was performed on the three TiO2@PC samples for comparison of elemental composition. Figure S6a shows the survey spectra with similar peaks of Ti, O, and C elements for all the three samples.53 In Figure 3d, The Ti 2p spectra shows two peaks at 458.6 and 464.4 eV, indicating Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively.54 It is notable that the Ti 2p peaks are shifted to lower energy states for the samples treated in lower temperature, indicating more defects are presented in the TiO2 nanocrystal of TiO2@PC-600 and -800 samples.55 The reduced defects of TiO2@PC-1000 may be due to the high annealing temperature. In general, the increasing defects is responsible for a faster ion diffusion rate, which would improve the ion adsorption capacity.56 The O 1s spectra of all the samples show two peaks at 530.5 and 532 eV, corresponding to Ti – O and O – C – O or C – OH bonds (Figure S6b). The chemical shift of peaks to higher binding energy with the increasing pyrolysis temperature describe the change of chemical environments and phase varieties.57 And the proportion of O – C – O/C – OH peak grows higher at higher annealing temperature. The results demonstrate a higher positive oxidation state.58, 59

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3.3

Porosity and specific surface area

To investigate the specific surface area and pore size distribution, nitrogen adsorptiondesorption tests have been conducted. The nitrogen sorption isotherms and porosity measurements of TiO2@PC samples are presented in Figure 3d. The sharp uptakes at low relative pressure (

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