Construction of Highly Catalytic Porous TiOPC Nanocomposite

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Construction of Highly Catalytic Porous TiOPC Nanocomposite Counter Electrodes for Dye-Sensitized Solar Cells Ming Chen, Leng-Leng Shao, Yan Xia, Zhongyuan Huang, DongLi Xu, Zong-Wen Zhang, Zhou-Xin Chang, and Wei-Jie Pei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08169 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 13, 2016

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ACS Applied Materials & Interfaces

Construction of Highly Catalytic Porous TiOPC Nanocomposite Counter Electrodes for Dye-Sensitized Solar Cells Ming Chen,*, † Leng-Leng Shao,‡ Yan Xia,§ Zhong-Yuan Huang,†,

#

Dong-Li Xu,†

Zong-Wen Zhang,† Zhou-Xin Chang,† Wei-Jie Pei† †

College of Chemistry and Chemical Engineering, Xinyang Normal University,

Xinyang 464000, China ‡

Grirem Advanced Materials Co., Ltd, General Research Institute for Nonferrous

Metals, Beijing 100088, China §

College of Chemistry, Nankai University, Tianjin 300071, China

#

Department of Chemistry, Xavier University of Louisiana, New Orleans, LA 700125,

USA

ABSTRACT. Developing low-cost, durable and highly catalytic counter electrode (CE) materials based on earth-abundant elements is essential for dye-sensitized solar cells (DSSCs). In this study, we report a highly active nanostructured compositional material, TiOPC, which contains titanium, oxygen, phosphorus and carbon, for efficient CE in I /I electrolyte. The TiOPC nanocomposites are prepared from carbon thermal transformation of TiP2O7 in an atmosphere of nitrogen at high temperature, and their catalytic performance is regulated by changing the carbon content in the nanocomposites. The TiOPC with appropriate 24.6 wt.% carbon and

1

ACS Paragon Plus Environment


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porous structure exhibits an enhanced electrocatalytic activity in the reduction of I , providing a short-circuit current density of 16.64 mA·cm-2, an open-circuit potential of 0.78 V, and an energy conversion efficiency of 8.65%. The photovoltaic performance of TiOPC CE based DSSC is even superior to that of a Pt CE based cell (13.80 mA·cm-2, 0.79 V, and 6.66%). The enhanced catalytic activity of TiOPC is attributed to the presence of predominant Ti-O-P-C structure, along with the continuous conductive carbon network and the porous structure.

KEYWORDS: Dye-sensitized solar cell; Counter electrode; Nanocomposite electrocatalyst; Ti-O-P-C structure; Electrocatalytic activity.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have received considerable attention as next-generation photovoltaic devices due to their advantages of low-cost fabrication, relatively high power conversion efficiency (PCE: 13%), and environmental friendliness, illustrating a splendid future for practical application.1 The typical DSSC has a sandwich-type structure, consisting of a dye-adsorbed mesoporous titania photoanode, an electrolyte containing a triiodide/iodide (I /I ) redox couple, and a platinum (Pt) counter electrode (CE). As an important part in DSSC, the CE serves to collect electrons from the external circuit and catalyze the reduction of redox species for regenerating the dye sensitizer. A desirable CE catalyst requires high electron conductivity, extraordinary catalytic activity, and long-term chemical stability.2 Pt is 2


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currently the most popular CE catalyst and has been widely used in the high-efficiency DSSC. However, the high cost, scarce supply, and insufficient chemical stability limited its scalable application in DSSC.3 Great efforts have been devoted to developing economical, abundant and anticorrosive CE materials, such as metal compounds (metal sulfide, carbide, nitride, and phosphide),4-7 carbon materials,3, 8, 9

conductive polymers,10 as well as their composite materials.11-13 As a promising alternative to Pt, the metal compound/carbon composites, which

often show low cost, high conductivity, and good electrocatalytic activity, have been extensively studied as CE candidates.14 Recently, various metal compound/carbon composite CEs were reported for DSSCs, such as TiN/C,15 CoS/C,16 VC/C,14 TiO2/C,17 Ni5P4/C18 etc., demonstrating marvelous synergistically catalytic property. The series of composites were generally designed based on the principle that highly active metal compound composed of nanosized particles can homogeneously distribute on the interconnected carbon matrix via an in-situ loading approach. However, the composites with the metal compound mechanically anchoring on the carbon solely displayed physically synergistic catalytic behavior for the triiodide reduction,19 and the chemically bonded metal compound and carbon based composite CE,20 which may possess higher synergistic catalytic activity, has not yet been investigated in DSSC. Very recently, the new principle to design composite electrocatalysts was developed in electrocatalysis field, such as oxygen reduction reaction (ORR),21 water splitting,22 etc., by constructing the novel metal-heteroatom-bonded carbon. For 3



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example, Fe-P-C composite exhibited an excellent catalytic activity in ORR due to the synergistic interactions of the metal, heteroatoms and carbon in the presence of Fe-P and P-C species.23 Another typical electrocatalyst is Fe-N-C composite,24 which was comprehensively studied and achieved the inspiring scientific progress. In the Fe-N-C composite, Fe-N and N species are chemically bonded with carbon substrate to form the Fe-N-C structure, meaning that carbon not only supplies an electronic conductive network for Fe-N active sites but reacted with N atoms to form N-C active sites during the synthetic process. The multifunctional Fe-N-C structure contributed to the higher catalytic activity of the compositional catalyst than those of the physically mixed counterpart.25,

26

Therefore, the unique electrocatalyst structure could be

extended to construct the compositional materials in DSSC, and the new structured metal-heteroatom-bonded carbon composites remain to be developed for a high-performance CE. Additionally, carbon material has been fully exploited as CE electrocatalyst with reasonable electrocatalytic activity for the redox couples, as the C=C bonds in the framework are rich in free-flowing π electrons. Nevertheless, the π electrons still play a limited role in reducing the triiodide due to the electroneutrality of the carbon atoms.27, 28 The heteroatom (P, N, B or S) doped carbon, composed by C-C, C=C, and heteroatom-carbon bonds, has been adopted to break the electroneutrality of the carbon and thus improve its charge-transfer ability.29-32 Recent study revealed that the π electrons on the carbon can be activated by conjugating with the lone-pair electrons from P dopants, and the catalytic activity of phosphorus-doped electron-rich carbon 4


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was enhanced by the donorlike C atoms neighboring P.33, 34 The problem is that the dramatic loss of thermally unstable phosphorus species at high-temperature treatment based on the commonly used P sources (H3PO4, (NH4)3PO4, triphenylphosphine etc.) often resulted in a difficulty for preparing high-level P-doping carbon via the traditional synthetic technique.34-36 Thus, the solution by firstly forming the thermo-stable metal phosphates and then chemically bonding P to carbon through a carbothermic reduction process, may create a promising way for constructing efficient P and metal-O-P co-doped carbon (metal-O-P-C) CE compositional materials. Herein, we introduced butyltitanate to H3PO4 contained aqueous solution and synthesized TiOPC nanocomposites by a hydrothermal route followed with a carbothermic reduction process. The TiOPC nanocomposites were characterized to be a mixture of TiP2O7, carbon and Ti-O-P-C species. The submicrometer sized TiP2O7 particles existed in the interlayer of carbon nanosheets, which prevented the stacking of carbon nanosheets caused by the π-π conjugation effect and supplied porous structure for electrolyte diffusion. The new Ti-O-P-C structure formed at the interface of TiP2O7/carbon is particularly responsible for the significantly improved catalytic performance of TiOPC CEs, on the basis of the well-constructed conductive carbon network. The optimum TiOPC CE assembled cell yields a power conversion efficiency of 8.65%, higher than that of DSSC with Pt CE (6.66%), rendering it as the promising candidate for Pt.

2. EXPERIMENTAL SECTION 5



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2.1 Preparation of TiOPC Nanocomposites TiOPC nanocomposites were synthesized by a typical procedure. In brief, 5.0 g tetrabutyl titanate was added dropwise into the phosphoric acid aqueous solution (2.3 g in 30 ml) under continuous magnetic stirring. The reaction mixture was magnetically stirred over a period of 30 min, transferred into a 50 ml teflon-lined autoclave, and heated at 180 oC for 12 h in a furnace. The resultant colloidal solution with the white precipitates at the bottom was further mixed with 3.0 g (1.0, 5.0 or 9.0 g) of sucrose and continually stirred for 1 h. To obtain TiOPC nanocomposites, the mixture was firstly dried at 120 oC for 12 h and subsequently annealed at 900 oC for 3 h in flowing N2 with a heating rate of 2 oC/min. The obtained black powder was manually grounded for 15 min to get the final product. For convenience, the TiOPC nanocomposites synthesized by using different mass ratios of sucrose were designated as TiOPCx (x = 1, 3, 5, 9), where x represents the initially added amount of sucrose in the synthetic procedure. The pristine carbon, P-doped carbon, TiO2/C and TiOP were prepared with the absence of Ti/P, Ti, P and C source, respectively. The oxidization of the carbon in TiOPC3 was carried out in air condition at 700 oC for 6 h. The heat treatment on TiOPC3 was accomplished by annealing the sample at 1000 oC or 1300 o

C for 1 h in N2 atmosphere.

2.2 Construction of CEs and Assembly of DSSCs TiOPC CEs were prepared in the following steps: firstly, 0.26 g of TiOPC catalyst, 40 mg of TiO2 (P25, size: 200 nm, Degussa) and 30 mg of polyethylene glycol (molecular weight: 2000) were ultrasonically dispersed in 1.5 ml of deionized 6


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water and then continuously kept magnetic stirring for 2 h. The as-prepared slurry was then directly coated onto the cleaned FTO glasses (14 Ω □-1, Nippon Sheet Glass) and dried in a vacuum oven at 120 ºC for 10 h to obtain the final TiOPC CEs. Other CEs were fabricated with the similar procedure to TiOPC CEs. As references, Pt CEs were also prepared via thermal pyrolysis of 30 mM H2PtCl6·6H2O solution on the FTO substrates at 400 ºC for 30 min. The dye-sensitized bilayer TiO2 photoanodes with the active area of 0.5 × 0.5 cm2 were fabricated according to our previous report.37 The DSSC device was assembled by sandwiching redox electrolyte between a dye-sensitized TiO2 photoanode and a CE. The photoanode and CE were spaced by using 50 µm scotch tapes between them, and then hot pressed together using thermoplastic surlyn to form a seal. The redox electrolyte was injected into the space between photoanode and CE by an injector, and the electrolyte consists of 0.05 M I2, 0.5 M LiI, 0.3 M 1,2-dimethyl-3-propylimidazoliumiodide(DMPII), and 0.5 M 4-tert-butylpyridine with acetonitrile as the solvent. 2.3 Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku SmartLab9 diffractometer, with Cu Kα radiation (λ = 1.5406 Å) operated at 40 mA and 40 kV. Scanning electron microscopy (SEM) was conducted on a Hitachi S-4800 microscope at 5 kV. Transmission electron microscopy (TEM) was operated on a Tecnai G2 F20 microscope at 200 kV. Solid-state

31

P magic angle spinning nuclear magnetic

resonance (31P MAS NMR) measurements were performed on a Varian Unity 7



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plus-400 spectrometer at spinning rate of 12 kHz and resonance frequency of 161.9 MHz with a recycle delay of 5 s. Chemical shifts were indicated using an external 85% H3PO4 reference. XPS was recorded on a Kratos Axis Ultra DLD (delay line detector) spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Raman

spectroscopy

was

performed

on

a

Renishaw-1000

spectrometer.

Thermogravimetric analysis (TGA) were conducted on a TA SDT Q600 analyser in air or nitrogen atmosphere with a heating rate of 5 K/min. N2 adsorption and desorption isotherms were measured on a ASAP2460 instrument (micromeritics instrument corp, USA) at 77 K. The photocurrent density-voltage characteristics of DSSCs were measured under standard AM 1.5 G illumination (100 mW cm-2) of a solar simulator (Oriel Sol 2A, Newport) with an active area of 0.25 cm2. The chemical stability measurement was conducted by measuring the photovoltaic parameters at defined time intervals of 12 h during 72 h illumination. The electrochemical impedance spectroscopy (EIS) measurements were conducted on a Zahner Zennium electrochemical workstation (IM6e) over the frequency range of 0.1Hz-100 kHz at -0.7 V bias potential with 10 mV of amplitude. Tafel polarization curves were recorded at the scan rate of 10 mV s-1 by using symmetrical dummy cells. Cyclic voltammetry (CV) was conducted in the three-electrode system in acetonitrile solution consisting of 10 mM LiI, 1 mM I2 and 0.1 M LiClO4. The different CEs serve as working electrode, a pure Pt foil as counter electrode, and Ag/Ag+ as reference electrode.

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3. RESULTS AND DISCUSSION 3.1 Sample Characterization

(a)

(600) (630) (511)

(721) (660) (933)

TiOPC3

Intensity (a.u.)

TiOP (101)

TiO2/C3

(200) (105) (211) (213)

(004) (002)

PC3

(101)

C

10

20

30

40

50

60

70

80

2θ (degree)

(b)

TiOPC1 Intensity (a.u.)

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TiOPC3 TiOPC5 TiOPC9 10

20

30

40

50

60

70

80

2θ (degree)

Figure 1. XRD patterns of (a) TiOPC3, TiOP, TiO2/C3, PC3, C, and (b) TiOPCx (x = 1, 3, 5, 9) composites.

Figure 1 shows the XRD results of all the prepared samples. In Figure 1a, both 9



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the carbon and phosphorous-doped carbon show two broad peaks at around 23.0o and 43.8o (2θ), which are attributed to the characteristic peak of the amorphous carbon and (101) reflection of the graphitic carbon, respectively. The partially graphitic carbons were obtained after carbonizing sucrose at 900 °C for 3 h. The peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, and 62.1° are correspondingly indexed to (101), (004), (200), (105), (211) and (213) planes of TiO2 with anatase phase (JCPDS No. 21-1272). Noticeably, the TiOP and all the TiOPCx show well-defined and intense diffraction peaks at 19.5°, 22.6°, 25.3°, 27.7°, 32.1° and 37.9°, which are assigned to (511), (600), (630), (721), (660) and (933) diffractions of TiP2O7 (JCPDS No. 52-1470), respectively. In the case of TiOPCx, no peaks assigned to titanium carbide/phosphide were identified, and the peaks belonged to carbon were not obvious due to the weak carbon signal depressed by that of high-crystalized titanium pyrophosphate. The XRD results reveal that the prepared samples are carbon (C), P-doped carbon (PC3), TiO2/carbon (TiO2/C3), TiP2O7 (TiOP) and TiP2O7/carbon (TiOPC). In Figure 1b, the XRD results of TiOPCx (x = 1, 3, 5, 9) composites show that the intensity of diffraction peaks decreased with the increment of carbon amount in the TiOPCx (x = 1, 3, 5, 9) composites, due to the lower mass ratio of TiP2O7 to carbon and the smaller particle size resulting from the enhanced space constraints of the carbon.

10


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. SEM images of TiOP (a-b), TiOPC3 (c-d) and oxidized TiOPC3 (e-f).

The morphology of TiOP, TiOPC3 and oxidized TiOPC3 was characterized by SEM images. As shown in Figure 2(a-b), the TiOP was composed of polyhedral and irregular particles with the size ca. 400-600 nm, and the TiOPC3 nanocomposite with rod-like particles displayed a smaller size of ca. 200-400 nm. The particles for both TiOP and TiOPC3 aggregated together to form the microsized secondary particles, resulting in considerable voids. Moreover, there are numerous ultrathin sheets with the thickness of 10-20 nm uniformly distributed among TiP2O7 particles in TiOPC3, and the sheet-like structure disappeared for the oxidized TiOPC3 (Figure 2(e-f)), which are only composed of ca. 200-400 nm sized coral-like particles. By comparing the morphologies of TiOP, TiOPC3 and oxidized TiOPC3, the sheet-like structure in 11



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TiOPC3 is deduced to be the carbon network. The secondary electron (SE) and corresponding elemental mapping images in Figure S1 (Supporting Information) further confirmed the presence of carbon network in TiOPC3, and the relatively uniform carbon distribution inter and intra TiOPC3 particles. It can be concluded that the existence of TiP2O7 particles avoids the stacking of carbon nanosheets caused by the π-π conjugation effect.38 The ultrathin carbon nanosheets with distinctly wrinkled and folded features, which probably originated from the intercalated P defects and the structural distortion in the TiOPC3, are likely linked each other to form the three-dimensional interconnected network. Abundant edge sites were also exposed due to the characteristic of surface wrinkling and folding, which are demonstrated to be favorable for electrocatalytic application.39 In addition, the carbon morphology in TiOPCx composite, as displayed in Figure S2, changed from discontinuous ultrathin nanosheets (5-10 nm) for TiOPC1 to interconnected nanosheets for TiOPC3/TiOPC5, and to aggregated monolithic blocks for TiOPC9 with the increment of carbon content.

12


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(a)

(c)

(b)

(d)

(e)

Figure 3. TEM images (a-b) and selected area electron diffraction (SAED) patterns (c-e) of TiOPC3 nanocomposite. TEM images are used to further investigate the micromorphology of representative TiOPC3 nanocomposite. In Figure 3, the 200-300 nm sized TiOPC3 particles with porous structure are randomly packed each other, and the HRTEM image confirmed that the TiOPC3 nanocomposite is composed by two parts, including the alternating crystal and amorphous structure. The alternating crystal phase with the interplanar spacing of 0.40 nm, are consistent with d-spacing of the (600) plane of TiP2O7. The amorphous district most probably arises from the carbon nanosheets. The results demonstrate that some TiP2O7 particles have incorporated into the amorphous carbon nanosheets or intergrown with them during the high-temperature treatment, which is beneficial for the formation of Ti-O-P-C structure at the interface of TiP2O7 and carbon. The SAED patterns in Figure 3(c-e) show a varied interplanar spacing, corresponding to the three different districts in Figure 3b for TiOPC3: carbon (hollow rings pattern), interface of TiP2O7/carbon (hollow rings dotted with irregular bright 13



spots) and TiP2O7 (bright spot matrix of well-defined diffraction pattern), respectively, further providing a solid evidence for the incorporation of TiP2O7 into the graphitic carbon framework and the formation of Ti-O-P-C structure.

(a)

TiOPC3

Intensity (a.u.)

O1s

Ti2p

P2p

C1s

600

500

400

300

200

100

0

Binding energy (eV)

(b) Ti2p TiOP Intensity (a.u.)

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Ti in octahedral coordination

Ti in tetrahedral coordination

TiOPC3

465 464 463 462 461 460 459 458 457 Binding energy (eV)

14


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P-O-Ti 134.3

OTi

Intensity (a.u.)

(c) P2p P-O 133.7

TiOPC3 PC3

P-C 133.1

C P O O

140

138

136

134

132

130

Binding energy (eV)

(d) C1s C-O 287.3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiOPC3 C-P 285.5

C-C 284.6

PC3

288

287

286

285

284

283

282

Binding energy (eV)

Figure 4. XPS survey spectrum (a) and high-resolution XPS spectra of TiOPC3 nanocomposite: (b) Ti 2p peaks, (c) P 2p peaks, and (d) C1s peaks.

To further study the composition and structure of TiOPC3 nanocomposite, the X-ray photoelectron spectroscopy (XPS) measurements were carried out. The XPS survey spectrum of TiOPC3 shows four main peaks of Ti2p, O1s, P2p, and C1s at ~460.7 eV, ~531.2 eV, ~133.7 eV and ~284.9 eV, respectively (Figure 4a), with the 15



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corresponding atomic concentration of 1.00: 6.32: 1.76: 0.24 that deviated slightly from the ideal element composition of TiP2O7. The high-resolution Ti 2p XPS spectra derived from the TiOP and TiOPC3 samples were shown in Figure 4b. The Ti 2p3/2 region of TiOP is composed of a single peak at ca. 460.1 eV and shows the Ti(IV) state.40 However, with the introduction of carbon into TiOP for TiOPC3, the Ti 2p3/2 peak shifted to higher binding energy of 460.4 eV, indicating the chemical interaction of TiOP and carbon occurred. The deconvoluted two peaks at 460.2 eV and 461.1 eV from the broad Ti 2p3/2 imply two different chemical environments of Ti ions in the TiOPC3 structure. The former Ti ions are assigned to an octahedral coordination with oxygen, and the latter Ti ions are in a tetrahedral environment with unsaturated four-coordination state.41 The Ti-O-P-C structure formed by the reduction of TiP2O7 with carbon may be responsible for the unsaturated Ti ions and nonstoichiometry of 900 ℃

TiP2O7 + 2x C TiP2O7-xCx + x CO ↑

(1)

the TiP2O7 in TiOPC3. For high-resolution P 2p spectra in Figure 4c, the P 2p signal for the PC3 displays a nearly horizontal line, indicating the low concentration of P species in carbon due to the dramatic loss of phosphorus during the high-temperature treatment process.42 After the introduction of Ti4+ for TiOPC3, an intense peak appeared, which can be deconvoluted into three major components of P-C (133.1 eV), P-O (P2, 133.7 eV), and P-O-Ti (134.3 eV).43, 44 The results demonstrate that a certain part of P atoms in TiP2O7 has been incorporated into the carbon lattice and bonded with carbon in the predominant form of Ti-O-P-C during the carbothermic reduction process. The calculated P content in carbon reaches 6.05 at% for TiOPC3 and only 16


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0.54 at% for PC3, demonstrating higher thermal stability of the Ti-O-P-C structure than other bonding forms of P species (eg. P-C, POC3, PO2C2, and PO3C, etc).45 Additionally, no characteristic peak at 129.0 eV was observed, also confirming the absence of Ti-P bonds, which agrees well with the XRD results. The C 1s XPS spectra for the PC3 and TiOPC3 were fitted into three peaks centered at ca. 287.3, 285.5 and 284.6 eV, which are correspondingly assigned to C-O, P-C and C-C bonding,46 and further support the successful P doping. Moreover, the P-C abundance in TiOPC3 was also maintained at a higher level than that of PC3, due to the presence of Ti-O-P-C structure.

P-C P-O

TiOPC3 Intensity (a.u.)

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-200

P-O

TiOP

-100

0 δ (ppm)

100

200

Figure 5. 31P MAS NMR spectra of TiOP and TiOPC3.

The

31

P NMR spectra of solid TiOP and TiOPC3 are shown in Figure 5. Only a

sharp singal at -39 ppm was observed for the TiOP, which is assigned to the pyrophosphate species that correspond to the tetrahedral phosphorus environments 17



connected with two O-Ti bands.47, 48 While the TiOPC3 shows a broad feature with the overlapping peaks, indicating that the different chemical environment of P atoms consists of terminal oxygens or carbons. In addition, the chemical shifts of P atoms change toward low magnetic fields, with the decrease in the number of metal atoms bonded to PO4 units.49 As a result, the changes of 31P NMR spectra in TiOPC3 can be attributed to the existence of unsaturated Ti ions and the formation of P-C bonds, supporting the conclusion obtained from XPS analysis.

D band G band TiOPC9

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TiOPC5 TiOPC3 TiOPC1 PC3 C

300

600

900 1200 1500 1800 2100 2400 -1

Wave number (cm )

Figure 6. Raman spectra of C, PC3 and TiOPCx composites.

The structure of TiOPCx composites was further studied by Raman spectra in Figure 6. In the case of TiOPC1, a broad band at 200-400 cm-1 is attributed to the Ti-O stretching modes in the network of TiO6 octahedral. Two obvious bands at ca. 700 and 1050 cm-1 are also observed, which correspond to the symmetric P-O and P-O-P stretching mode associated with the PO43- tetrahedral and can be ascribed to the 18


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presence of TiP2O7.50, 51 However, no peaks at ca. 700 and 1050 cm-1 were observed for TiOPCx (x >1) composites, which can be in part due to the enhanced amorphizing effect of the thick carbon nanosheets around TiP2O7 particles. In addition, two broad peaks at approximately 1340 cm-1 and 1590 cm-1, which are correspondingly ascribed to the D-band of sp3-hybridized carbon and G-band of sp2 carbon, were presented for all the TiOPCx composites. The intensity ratio of D-band to G-band (ID/IG) is generally used as a measure of the carbon disorder.9 The ID/IG is 0.88 for the pristine carbon, and the values are increased to 1.42 for PC3 and 1.31 for TiOPC3, suggesting that considerable defective sites were introduced into the partly graphitic carbon due to the P doping.42

105 TiOPC3

100

30

95 90

0

85 80

Heat flow (mW)

Weight percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-30 75 70

100 200 300 400 500 600 700 800 900 o

Temperature ( C)

Figure 7. TG-DSC curves of TiOPC3 nanocomposite carried out in air condition.

TG-DSC analysis was also performed to have a comprehensive understanding on the composition features of the representative TiOPC3 nanocomposite. In Figure 7, an 19



obvious weight loss with the corresponding exothermic peak at 600 oC is due to the oxidation of carbon in the air atmosphere, revealing that approximately 24.6 wt% of carbon is contained in the TiOPC3 nanocomposite. In addition, the carbon content, as shown in Figure S3, increased from 8.2 wt.% for TiOPC1 to 46.5 wt.% for TiOPC5 and 73.8 wt.% for TiOPC9 that prepared with larger sucrose amount. However, excessive carbon in TiOPC composite may block the voids formed by the package of the TiP2O7 particles, thus hindering the electrolyte diffusion and making less

3

3

240

Volume Adsorbed(cm /g STP)

Ti-O-P-C structure exposed, which is adverse for electrocatalysis.

Volume Adsorbed (cm /g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 45

200 160

TiOPC1 TiOPC3 TiOPC5 TiOPC9

300 280 carbon

260 240 220 200

0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/Po)

120 80 40 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po)

Figure 8. N2 adsorption-desorption isotherms of TiOPCx composites and carbon. The surface area and porous structure of TiOPCx composites were confirmed with N2 adsorption-desorption analysis at 77 K. As shown in Figure 8, the BET surface area of pristine carbon (852 m2 g-1) is very high due to the presence of numerous micropores that derived from the burn-out of C, H, and O in carbon framework during pyrolysis at 900 oC.52 However, the surface areas of TiOPCx (x = 1, 20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3, 5, 9) composites were dramatically decreased to 97, 227, 276, and 397 m2g-1, respectively. Evidently, the nitrogen isotherms of TiOPCx composites show the characteristics of meso/microporous structure. The suitable surface area would provide abundant catalytic active sites, and the hierarchical porous structure paves a way for the electrolyte accessing to the active sites, which is helpful to enhance the electrocatalytic activity towards the triiodide reduction.

3.2 The Formation Mechanism of TiOPCx Composites As discussed above, the TiOPCx composites have been successfully prepared from carbothermic reduction of the hydrothermal products of butyltitanate in H3PO4 solution, using sucrose as organic reducer (Scheme 1). The possible formation route of TiOPCx composites was investigated by analyzing the XRD results and TG curves of specific samples during the synthetic procedure. Figure S4 shows the XRD pattern of the hydrothermal product, the precursor of TiOPC, and the peaks fit well with the Ti(OH)PO4 (JCPDS No. 36-0697). Based on the XRD analysis, the reaction routes for the formation of TiP2O7 in TiOPC3 were proposed as follows: 25 ℃

Ti(OC4H9)4 + H2O Ti(OH)4+ 4C4H9OH 180 ℃

Ti(OH)4 + H3PO4 Ti(OH)PO4 + 3H2O 900 ℃

Ti(OH)PO4 + H3PO4 TiP2O7 + 2H2O

(2) (3) (4)

To further examine the reaction routes of carbonthermal reduction process, the TG curves of specific samples, such as H3PO4, sucrose, PC3-precursor, TiOP-precursor and TiOPC3-precursor were recorded and presented in Figure 9. In the temperature range of 30-120 oC, the weight loss of all the samples except sucrose 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

is caused by the vaporization of the adsorbed water. For the precursor of TiOP, the weight gradually decreased as the temperature increased to 700 oC, due to the dimerization of Ti(OH)PO4 to TiP2O7. No more weight loss was observed after 700 oC for the precursor of TiOP. As for the precursor of PC3, the weight loss in the TG curve can be divided into three temperature regions. One is in the temperature range of 120-490 oC (20 wt%), which can be ascribed to the condensation and pyrolyzation of the sucrose in the PC3-precursor, as evidenced by the weight loss curve of pristine sucrose. The second obvious weight loss (40 wt%) between 490 oC and 700 °C is mainly due to the condensation and vaporization of H3PO4, which was confirmed by the TG curve of pure H3PO4. The small weight loss of 5 wt% after 700 oC is probably associated with the loss of P species, the oxygen containing groups anchoring on the surface of carbon, and the new formed P-doped carbon species, such as POC3, PO2C2 and PO3C. With regard to the precursor of TiOPC3, there is a similar weight loss between 150-550 oC (20 wt%) to the case of PC3-precursor, corresponding to the polymerization and pyrolyzation of sucrose, and the dimerization of Ti(OH)PO4 to TiP2O7. In the section of 550-900 oC, the small weight decrease (6 wt%) is due to the vaporization of P species, and the release of CO generated by the formation of Ti-O-P-C (eq. 1). An interesting phenomenon for TiOPC3-precursor is that there is a dramatic weight loss after 900 oC, which is not observed for TiOP. It is probably related to the thermolysis and vaporization of the P species in carbon and the further carbothermic reduction of TiP2O7-xCx with carbon. The redox reaction of TiP2O7-xCx with carbon after 900 oC could be confirmed as reaction equation 5-6 by further 22


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extending the TG temperature to 1300 oC and analyzing the XRD results of heat-treated TiOPC3 at 1000 °C and 1300 oC (Figure S5-6). 900 ℃

9TiP2O7-xCx + (27-18x) C Ti9P8O36 + (27-9x) CO ↑ + 10P ↑ 900 ℃

TiP2O7-xCx + (5-2x) C TiO2 + (5-x) CO ↑ + 2P ↑

(5) (6)

In brief, the TG results demonstrate that most of phosphorus was lost for the sample of PC3 before 900 oC and was well maintained for the samples of TiOP and TiOPC3. The addition of Ti4+ contributed to the formation of more stable TiP2O7, which avoid the loss of P sources. Especially in TiOPC3, the thermally stable Ti-O-P-C structure was constructed at the interface of TiP2O7 and carbon at 900 oC, as evidenced by the XPS analysis. It is worth mentioning that the further carbonthermal reduction of TiP2O7-xCx with carbon when the heat-treatment temperature is higher than 900 oC, would lead to the breakage of the Ti-O-P-C structure.

100 Weight percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

174

200 700

120 150 550

60 40

H3PO4

20

sucrose PC3-precursor TiOP-precursor TiOPC3-precursor

0

900

490

200

375

700 655

400

600

800

1000

o

Temperature ( C)

Figure 9. TG curves of specific samples during the synthetic procedure under N2 atmosphere.

23



Scheme 1. The illustration of the synthetic route for TiOPC composites.

3.3 Electrocatalytic Activity and Photovoltaic Performance

40

1000

(a)

Z" (Ohm)

TiOP

30 Z" (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

0

Pt C PC3 TiO2/C3

20

10

0

0

5

10

15

20

0

25

500 Z' (Ohm)

30

Z' (Ohm)

24


1000

35

40

Page 25 of 45

5 (b)

TiOPC1 TiOPC3 TiOPC5 TiOPC9

4

Z" (Ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 2 1

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Z' (Ohm)

Figure 10. Nyquist plots of Pt, C, PC3, TiO2/C3, TiOP (a), and TiOPCx CEs (b). The electrocatalytic properties of the CEs were evaluated by electrochemical impedance spectroscopy (EIS) using the symmetric dummy cells assembled with two identical CEs (CE//electrolyte//CE). Figure 10 shows the obtained Nyquist plots of various CEs. The corresponding EIS parameters of series resistance (Rs) and charge-transfer resistance (Rct) derived from the equivalent circuit (Figure 10a (inset)) are summarized in Table 1. In a dummy cell, the Rs value represents the ability of collecting electrons from the external circuit, and the Rct value reveals the intrinsic electrocatalytic activity of CEs towards the triiodide reduction.53 The EIS data in Table 1 shows that the Rs of TiOPC3, TiOP, TiO2/C3, P-doped carbon and pure carbon are 0.30, 3.23, 0.25, 0.12 and 0.05 Ω, respectively. As the same addition of binding agents and electrolytes, the lower Rs for TiOPC3 than that of TiOP originated from the improved electrical conductivity provided by intimate networking of interconnected carbon nantosheets among TiP2O7 particles. In terms of Rct, the TiOPC3 electrode displays a Rct value of 0.35 Ω. Contrarily, the Rct of TiOP electrode exhibits an 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

extremely large value of 485.25 Ω, and the TiO2/C3, P-doped carbon and pristine carbon electrodes delivered Rct values of 27.86, 35.76 and 40.01 Ω, respectively. The biggest Rct and Rs of TiOP electrode can be ascribed to the poor electrocatalytic activity and conductivity of TiP2O7. However, with the introduction of carbon into TiOP for TiOPC3, the lowest Rct and appropriate Rs were achieved, demonstrating its superior catalytic activity to those of reference CEs. Besides, the TiOPC3 CE delivered a much lower Rct than that of Pt CE (5.56 Ω), indicating the TiOPC3 can act as a superior catalyst to Pt. The good electrocatalytic performance of TiOPC3 CEs may arise from the newly formed Ti-O-P-C structure, which serves as the highly catalytic active sites. In the catalytic active center of Ti-O-P-C, unsaturated four-coordinated Ti ions with positive charge can function as the adsorption sites for

I and the bonded carbon transports the electrons to the Ti ions for reducing I into I via conjugation effect with Ti-O-P. Besides, the large surface area and hierarchical porous structure are in favor of the catalytic behavior by offering abundant ion-accessible catalytically active sites for the triiodide reduction. More importantly, it is found that the electrocatalytic activity of TiOPCx CEs is closely related to the carbon amount in the composite, as supported in Figure 10b. The Rct of TiOPCx (x = 1, 3, 5, 9) firstly decreased dramatically from 3.03 Ω to 0.35 Ω and then increased to 2.82 Ω with the increment of carbon content. The largest Rct of TiOPC1 may result from the lack of interconnected conductive carbon network and the inadequate catalytically active sites of Ti-O-P-C due to the relative low carbon content. The smallest Rct was reached for TiOPC3 among all the TiOPCx CEs, 26


Page 26 of 45

Page 27 of 45

resulting in a superior Pt-free CE candidate. The outstanding catalytic behavior of TiOPC3 can be attributed to the good establishment of Ti-O-P-C structure and conductive carbon network among the particles. Besides, the enhanced surface area and meso/microporous structure can ensure the fast penetration of electrolyte in and out of TiOPC3 and supply more ion-accessible active sites of Ti-O-P-C. However, the increased Rct for the TiOPC5 and TiOPC9 may illustrate that the excessive carbon would block the electrolyte diffusion channels and cause less I accessible to the Ti-O-P-C active sites.

2.0 lgJlim

1.5 -2

Log J ( log mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lgJ0

1.0 0.5 0.0 -0.5 -1.0 -1.5 -0.6

Pt C PC3 TiO2/C3 TiOPC3 -0.4

-0.2

0.0

0.2

0.4

0.6

Voltage (V)

Figure 11. Tafel polarization curves of various CEs. Tafel polarization measurements were used to reconfirm the catalytic activity of TiOPC electrodes. Figure 11 displays the Tafel polarization curves for symmetrical cells similar to the ones used in EIS measurements. The curves show logarithmic current density (logJ) as a function of voltage (V), which can be divided into three zones: polarization zone (low potential, |V| < 120 mV), Tafel zone (intermediate potential with a sharp slope), and diffusion zone (high potential). The latter two zones 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 45

provide useful information about the exchange current density (J0) and the limiting diffusion current density (Jlim), which are positively correlated with the catalytic activity and the diffusion properties of the redox couple, respectively.8 In Figure 11, the TiOPC3 exhibited a higher J0 of 8.32 mAcm-2 in comparison with that of Pt electrode (6.85 mAcm-2), indicating the superior catalytic activity. On the other hand, the larger J0 of TiOPC3 than those of pristine carbon (3.18 mAcm-2), P-doped carbon (4.24 mAcm-2) and TiO2/C3 (5.64 mAcm-2) further confirmed the decisive roles of as-formed Ti-O-P-C structure for the enhanced catalytic activity. In addition, the J0 is theoretically in inverse proportion to Rct by the eq. 7.54

=

(7)

where Rct is the charge-transfer resistance obtained from EIS spectra, R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the reduction of I to I , and F is the Faraday’s constant. The change tendency of J0 that extracted from Tafel curves is in good accordance with that of EIS results. Another important parameter of Jlim reflects the diffusion characteristics of the redox couple according to the eq. 8.55

=

(8)

where is the diffusion coefficient of triiodide, l is the spacer thickness, C is a triiodide concentration. The larger Jlim of TiOPC3 relative to those of Pt, pristine carbon, P-doped carbon and TiO2/C3 demonstrates a higher diffusion velocity for the electrolyte containing redox couple, which benefit from its hierarchical porous structure. Large J0 and Jlim render the TiOPC3 as the potential Pt-free CE alternative. 28


Page 29 of 45

1.0 -2

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.4

Pt C PC3 TiO2/C3

0.2

TiOPC3

0.6

0.34 V ∆Ep

-

0.0

3I2 + 2e → 2I3

-0.2 -0.4

-

-

I3 + 2e → 3 I

-0.6 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

+

Potential vs Ag/Ag (V)

Figure 12. CV curves of different CEs.

To verify the different catalytic kinetics of various CEs, CV measurements were carried out. In Figure 12, two pairs of redox peaks were observed for all the CEs except for the pristine carbon, in which the left pair of peaks at low potential is attributed to the redox reaction 9, and the right one at high potential is assigned to the redox reaction 10.56

I + 2e ↔ 3I

(9)

3I + 2e ↔ 2I

(10)

As shown in Figure 12, the TiOPC3 exhibited a higher reduction current density than those of pristine carbon, PC3 and TiO2/C3 CEs, and even Pt CE, which shows a higher electrochemical catalytic activity for the reduction of I to I . In addition, the peak-to-peak separation (∆Ep) was 0.34 V for TiOPC3 and was the lowest among all the CEs. In theory, ∆Ep varies inversely with charge transfer rate (ks), and thus the 29



lowest ∆Ep reflects the fastest redox reaction of I to I .14 With a comprehensive analysis of current densities and ∆Ep of the redox peaks, TiOPC3 shows an outstanding electrocatalytic activity and kinetics for the reduction of I to I . In contrary, the pristine carbon displayed only one pair of redox peaks assigned to redox reaction 8, demonstrating its insufficient intrinsic catalytic activity. The CV results confirm again that the combination of highly catalytic Ti-O-P-C structure and interconnected conductive carbon films as well as the hierarchical porous system into the composite is a feasible way for producing a superior catalyst (TiOPC3).

18 16 2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 45

(a)

14 12 Pt C PC3 TiO2/C3

10 8 6

TiOP TiOPC3

4 2 0 0.0

0.2

0.4

0.6

Voltage (V)

30


0.8

Page 31 of 45

16

(b)

2

Current density (mA/cm )

18

14 12 TiOPC1 TiOPC3 TiOPC5 TiOPC9

10 8 6 4 2 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Voltage (V)

15

0.8

-2

Jsc (mA cm )

20

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

Pt TiOPC3

10 5

0.6

0

15

30

45

60

75

Time (h)

Figure 13. Photocurrent density-voltage curves of DSSCs with Pt, C, PC3, TiO2/C3, TiOP CEs (a) and TiOPCx CEs (b), and the photovoltaic parameters of DSSCs based on TiOPC3 and Pt CEs within 72 h (c).

The TiOPCx composites were further evaluated as CEs in DSSCs through the measurements of photocurrent density-voltage (J-V) characteristics. The J-V curves of 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

DSSCs are shown in Figure 13, and the J-V curves of the reference DSSCs with Pt, pristine carbon, PC3, TiO2/C3 and TiOP CEs are also presented. The detailed photovoltaic parameters, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η), are listed in Table 1, and each value is taken as an average of three DSSCs fabricated in the same batch. As for the reference CEs, the DSSC with the pristine carbon CE gets a η of 4.65%, whereas the cell with PC3 CE exhibits a higher η of 5.78%, due to the enhanced catalytic activity benefiting from the increased defective sites by P-doping. The device using TiOP as CE shows a lowest η of 0.20%, further confirming its poor catalytic performance, and the TiO2/C3 CE based DSSC exhibits a relative higher η value of 5.79%. Noticeably, the η for the cells with TiOPC1, TiOPC3, TiOPC5 and TiOPC9 CEs are 7.08%, 8.65%, 7.99% and 7.59%, respectively, showing extraordinary photovoltaic performance. The statistical photovoltaic parameters of 30 individual cells with TiOPC3 CEs show a small standard deviation, indicating the good repeatability of the devices. (Figure S7). Besides, all TiOPC CE based cells display higher η, FF and Jsc than those of the cells with the pristine carbon, PC3, TiO2/C3 and Pt CEs. Three reasons may account for the outstanding catalytic performance of TiOPC CEs. One is that the newly formed active sites of Ti-O-P-C structure at the interface of TiP2O7 and carbon would dramatically enhance the intrinsic catalytic activity of the composite. The second reason is related to the good electrical conductivity of the TiOPC CEs, which benefits from the interconnected conductive carbon network. The last one is the lower diffusion impedance of the 32


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


redox species in TiOPC3 due to the hierarchical porous architecture. In particular, the TiOPC3 CE based cell shows the optimum photovoltaic performance by the combination of highest intrinsic catalytic activity, optimum electron conductivity and the fastest electrolyte diffusion. For evaluation of the electrochemical stability, freshly assembled DSSCs based on TiOPC3 and Pt CEs were continuously illuminated under AM 1.5 G lighter at room temperature for 72 h, and then the cells were subjected to J-V measurements. Figure 13c shows the electrochemical stability of TiOPC3 and Pt tested per 12 h during the 72 h illumination. No noticeable changes in both Jsc and Voc were observed, suggesting that the cycling time had negligible influence on the catalytic performance of the CE and the η of the cells. The Jsc for the Pt based cell was 13.75 mA cm-2, but it was gradually decreased to 10.88 mA cm-2 at the final illumination time. In contrast, the freshly TiOPC3 assembled device exhibited a Jsc of 16.52 mA cm-2 in initial time, and can still maintain at 16.35 mA cm-2 after 72 h, demonstrating a superior electrochemical stability in acidic I /I medium.

Table 1. Photovoltaic parameters of DSSCs and EIS parameters of symmetrical dummy cells based on different CEs.

Voc (V)

Jsc(mAcm-2)

η (%)

FF

Rs (Ω)

Rct (Ω)

Pt

0.79

13.80

6.66

0.61

0.31

5.56

C

0.75

12.83

4.65

0.48

0.05

40.01

PC3

0.75

13.04

5.78

0.59

0.12

35.76

TiO2/C3

0.74

13.28

5.79

0.59

0.25

27.86

Counter electrode

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 45

TiOP

0.64

3.10

0.20

0.10

3.23

485.25

TiOPC1

0.76

14.86

7.08

0.63

0.39

3.03

TiOPC3

0.78

16.64

8.65

0.67

0.30

0.35

TiPOC5

0.78

15.36

7.99

0.67

0.24

1.55

TiPOC9

0.79

15.16

7.59

0.63

0.15

2.82

4. CONCLUSIONS In conclusion, we present a low-cost TiOPC counter electrode containing earth-abundant elements and operating efficiently in reducing I for DSSC with appreciable charge transfer resistance of only ∼0.35 Ω. The novel TiOPC composites were prepared by facile carbon thermal transformation of TiP2O7 at high-temperature treatment in an atmosphere of nitrogen, and the TiOPC composites possess the high catalytic activity of Ti-O-P-C structure and the good electronic conductivity of the interconnected carbon network, as well as the hierarchical porous architecture for the fast electrolyte diffusion. Under an optimal condition, the DSSC based on TiOPC CE delivers a power conversion efficiency of 8.65%, which is increased by 29.8% compared with the cell with Pt CE (6.66%). With its lower cost and superior performance over Pt, the TiOPC can be potentially applied as an extraordinary Pt-free CE electrocatalyst in the DSSC.

ASSOCIATED CONTENT Supporting Information SEM images of TiOPC composites, SE and elemental mapping images of TiOPC3 composites, TG curves of TiOPC composites in air condition, XRD pattern and 34


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TG-DSC curves of the TiOPC3 precursor, XRD patterns of the heat treated TiOPC3 nanocomposite, and the statistical photovoltaic parameters of 30 devices. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was financially supported by the Foundation of Henan Educational Committee (No.16A150023), Nanhu Scholars Program for Young Scholars of XYNU, the Doctoral Start-up Research Fund of Xinyang Normal University (15006) and the College of Chemistry and Chemical Engineering of Xinyang Normal University.

REFERENCES (1) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Gratzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved through The Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247. (2) Wu, M.; Ma, T. Recent Progress of Counter Electrode Catalysts in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 16727-16742. 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3) Kay, A.; Graitzel, M. Low Cost Photovoltaic Modules Based on Dye-Sensitized

Nanocrystalline Titanium Dioxide and Carbon Powder. Sol. Energy

Mater. Sol. Cells 1996, 44, 99-117. (4) Wang, M. K.; Anghel, M. A.; Marsan, B.; Ha, N. C.; Nuttapol Pootrakulchote, N.; Zakeeruddin, S.M.; Gratzel, M. CoS Supersedes Pt as Efficient Electrocatalyst for Triiodide Reduction in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 15976–15977. (5) Wu, M.; Lin, X.; Hagfeldt, A.; Ma, T. Low-Cost Molybdenum Carbide and Tungsten Carbide Counter Electrodes for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2011, 50, 3520-3524. (6) Jiang, Q. W.; Li, G. R.; Gao, X. P. Highly Ordered TiN Nanotube Arrays as Counter Electrodes for Dye-Sensitized Solar Cells. Chem. Commun. 2009, 6720-6722. (7) Dou, Y. Y.; Li, G. R.; Song, J.; Gao, X. P. Nickel Phosphide-Embedded Graphene as Counter Electrode for Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2012, 14, 1339-1342. (8) Wu, M. X.; Lin, X.; Wang, T. H.; Qiu, J. S.; Ma, T. L. Low-Cost Dye-Sensitized Solar Cell Based on Nine Kinds of Carbon Counter Electrodes. Energy Environ. Sci. 2011, 4, 2308-2315. (9) Chen, M.; Shao, L.-L.; Qian, X.; Liu, L.; Ren, T.-Z.; Yuan, Z.-Y. Mesoporous Carbon Counter Electrode Materials for Dye-Sensitized Solar Cells: The Effect of Structural Mesopore Ordering. Chem. Eng. J. 2014, 256, 23-31. 36


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(10) Yue, G.; Wu, J.; Xiao, Y.; Lin, J.; Huang, M.; Lan, Z. Application of Poly(3,4-ethylenedioxythiophene):Polystyrenesulfonate/Polypyrrole

Counter

Electrode for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 18057-18063. (11) He, B.; Tang, Q.; Liang, T.; Li, Q. Efficient Dye-Sensitized Solar Cells from Polyaniline–Single Wall Carbon Nanotube Complex Counter Electrodes. J. Mater. Chem. A 2014, 2, 3119-3126. (12) He, B.; Tang, Q.; Luo, J.; Li, Q.; Chen, X.; Cai, H. Rapid Charge-Transfer in Polypyrroleesingle Wall Carbon Nanotube Complex Counter Electrodes: Improved Photovoltaic Performances of Dye-Sensitized Solar Cells. J. Power Sources 2014, 256, 170-177. (13) Sudhagar, P.; Nagarajan, S.; Lee, Y. G.; Song, D.; Son, T.; Cho, W.; Heo, M.; Lee, K.; Won, J.; Kang, Y. S. Synergistic Catalytic Effect of A Composite (CoS/PEDOT:PSS) Counter Electrode on Triiodide Reduction in Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 1838-1843. (14)Wu, M.; Lin, X.; Wang, Y.; Wang, L.; Guo, W.; Qi, D.; Peng, X.; Hagfeldt, A.; Gratzel, M.; Ma, T. Economical Pt-free Catalysts for Counter Electrodes of Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2012, 134, 3419-3428. (15) Ramasamy, E.; Jo, C.; Anthonysamy, A.; Jeong, I.; Kim, J. K.; Lee, J. Soft-Template Simple Synthesis of Ordered Mesoporous Titanium Nitride-Carbon Nanocomposite for High Performance Dye-Sensitized Solar Cell Counter Electrodes. Chem.Mater. 2012, 24, 1575-1582. (16) Bi, E.; Chen, H.; Yang, X.; Peng, W.; Grätzel, M.; Han, L. A Quasi Core– 37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

Shell Nitrogen-Doped Graphene/Cobalt Sulfide Conductive Catalyst for Highly Efficient Dye-Sensitized Solar Cells. Energy Environ. Sci. 2014, 7, 2637-2641. (17) Wang, R.; Wu, Q. D.; Lu, Y.; Liu, H. W.; Xia, Y. Z.; Liu, J. Q.; Yang, D. J.; Huo, Z. Y.; Yao, X. D.; Preparation of Nitrogen-Doped TiO2/Graphene Nanohybrids and Application as Counter Electrode for Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2014, 6, 2118-2124. (18) Wu, M.; Bai, J.; Wang, Y.; Wang, A.; Lin, X.; Wang, L.; Shen, Y.; Wang, Z.; Hagfeldt, A.; Ma, T. High-Performance Phosphide/Carbon Counter Electrode for Both Iodide and Organic Redox Couples in Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 11121-11127. (19) Yun, S.; Zhang, H.; Pu, H.; Chen, J.; Hagfeldt, A.; Ma, T. Metal Oxide/Carbide/Carbon

Nanocomposites:

In

Situ

Synthesis,

Characterization,

Calculation, and their Application as an Efficient Counter Electrode Catalyst for Dye-Sensitized Solar Cells. Adv. Energy Mater. 2013, 3, 1407-1412. (20) Artyushkova, K.; Serov, A.; Rojas-Carbonell, S.; Atanassov, P. Chemistry of Multitudinous Active Sites for Oxygen Reduction Reaction in Transition Metal– Nitrogen–Carbon Electrocatalysts. J. Phys. Chem. C 2015, 119, 25917-25928. (21) Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S. G. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882-10885. (22) Shan, Z.; Archana, P. S.; Shen, G.; Gupta, A.; Bakker, M. G.; Pan, S. 38


Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NanoCOT: Low-Cost Nanostructured Electrode Containing Carbon, Oxygen, and Titanium for Efficient Oxygen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 11996-12005. (23) Singh, K. P.; Bae, E. J.; Yu, J. S. Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 3165-3168. (24) Kramm, U. I.; Lefevre, M.; Larouche, N.; Schmeisser, D.; Dodelet, J. P. Correlations Between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for The ORR in PEM Fuel Cell via 57Fe Mossbauer Spectroscopy and Other Techniques. J. Am. Chem. Soc. 2014, 136, 978-985. (25) Tang, Y. F.; Allen, B. L.; Kauffman, D. R.; Star, A. Electrocatalytic Activity of Nitrogen-Doped Carbon Nanotube Cups. J. Am. Chem. Soc. 2009, 131, 13200-13201. (26) Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657. (27) Yu, D. S.; Zhang, Q.; Dai, L. M. Highly Efficient Metal-Free Growth of Nitrogen-Doped Single-Walled Carbon Nanotubes on Plasma-Etched Substrates for Oxygen Reduction. J. Am. Chem. Soc. 2010, 132, 15127-15129. (28) Liu, J.; Liu, H.; Zhang, Y.; Li, R.; Liang, G.; Gauthier, M.; Sun, X. Synthesis and Characterization of Phosphorus–Nitrogen Doped Multiwalled Carbon Nanotubes. Carbon 2011, 49, 5014-5021. (29) Wang, Z.; Li, P.; Chen, Y.; He, J.; Liu, J.; Zhang, W.; Li, Y. Phosphorus-Doped Reduced Graphene Oxide as an Electrocatalyst Counter Electrode 39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

in Dye-Sensitized Solar Cells. J. Power Sources 2014, 263, 246-251. (30) Yang, D.-S.; Kim, C.; Song, M. Y.; Park, H.-Y.; Kim, J. C.; Lee, J.-J.; Ju, M. J.; Yu, J.-S. N-Doped Hierarchical Hollow Mesoporous Carbon as Metal-Free Cathode for Dye-Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 16694-16702. (31) Yu, C.; Fang, H.; Liu, Z.; Hu, H.; Meng, X.; Qiu, J. Chemically Grafting Graphene Oxide to B,N Co-Doped Graphene via Ionic Liquid and Their Superior Performance for Triiodide Reduction. Nano Energy 2016, 25, 184-192. (32) Yang, W.; Ma, X.; Xu, X.; Li, Y.; Raj, S. I.; Ning, G.; Wang, A.; Chen, S. Sulfur-Doped Porous Carbon as Metal-Free Counter Electrode for High-Efficiency Dye-Sensitized Solar Cells. J. Power Sources 2015, 282, 228-234. (33) Cruz-Silva, E.; Lopez-Urıas, F.; Munoz-Sandoval, E.; Sumpter, B. G.; Terrones, H.; Charlier, J. C.; Meunier, V.; Terrones, M. Electronic Transport and Mechanical Properties of Phosphorus- and Phosphorus-Nitrogen-Doped Carbon Nanotubes. ACS Nano 2009, 3, 1913-1921. (34) Eduardo Cruz-Silva, E.; Cullen,D. A.; Gu, L.; Romo-Herrera, J. M.; Muñoz-Sandoval, E.; López-Urías, F.; Sumpter, B. G.; Meunier, V.; Charlier, J. C.; Smith, D. J.; Terrones, H.; Terrones, M. Heterodoped Nanotubes: Theory, Synthesis, and Characterization of Phosphorus-Nitrogen Doped Multiwalled Carbon Nanotubes. ACS Nano 2008, 2, 441-448. (35) Wang, C.; Sun, L.; Zhou, Y.; Wan, P.; Zhang, X.; Qiu, J. P/N Co-Doped Microporous Carbons from H3PO4-Doped Polyaniline by In Situ Activation for Supercapacitors. Carbon 2013, 59, 537-546. 40


Page 40 of 45

Page 41 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Yang, D. S.; Bhattacharjya, D.; Inamdar, S.; Park, J.; Yu, J. S. Phosphorus-Doped Ordered Mesoporous Carbons with Different Lengths as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media. J. Am. Chem. Soc. 2012, 134, 16127-16130. (37) Chen, M.; Shao, L.-L.; Qian, X.; Ren, T.-Z.; Yuan, Z.-Y. Direct Synthesis of Cobalt

Nanoparticle-Imbedded

Mesoporous

Carbons

for

High-Performance

Dye-Sensitized Solar Cell Counter Electrodes. J. Mater. Chem. C 2014, 2, 10312-10321. (38) Krishnan, D.; Kim, F.; Luo, J.; Cruz-Silva, R.; Cote, L. J.; Jang, H. D.; Huang, J. Energetic Graphene Oxide: Challenges and Opportunities. Nano Today 2012, 7, 137-152. (39) Choi,C. H.; Park, S.H.; Woo, S. I. Binary and Ternary Doping of Nitrogen, Boron, and Phosphorus into Carbon for Enhancing Electrochemical Oxygen Reduction Activity. ACS Nano 2012, 6, 7084-7091. (40) Ren, T. Z.; Yuan, Z.Y.; Azioune, A.; Pireaux, J. J.; Su, B. L. Tailoring the Porous Hierarchy of Titanium Phosphates. Langmuir 2006, 22, 3886-3894. (41) Yu, J. C.; Zhang, L. Z.; Zheng, Z.; Zhao, J. C. Synthesis and Characterization of Phosphated Mesoporous Titanium Dioxide with High Photocatalytic Activity. Chem. Mater. 2003, 15, 2280-2286. (42) Chen, M.; Shao, L.-L.; Guo, Y.-X.; Cao, X.-Q. Nitrogen and Phosphorus Co-Doped Carbon Nanosheets as Efficient Counter Electrodes of Dye-Sensitized Solar Cells. Chem. Eng. J. 2016, 304, 303-312. 41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

(43) Liu, J.; Wei, X. Supercritical Synthesis of Layered Elongated Hexagonal Titanium Phosphate Nanoplates. RSC Adv. 2015, 5, 7798-7802. (44) Wang, Q.; Jiang, H.; Zang, S.; Li, J.; Wang, Q. Gd, C, N and P Quaternary Doped Anatase-TiO2 Nano-Photocatalyst for Enhanced Photocatalytic Degradation of 4-chlorophenol under Simulated Sunlight Irradiation. J. Alloys Compd. 2014, 586, 411-419. (45) Wu, J.; Yang, Z.; Sun, Q.; Li, X.; Strasser, P.; Yang, R. Synthesis and Electrocatalytic Activity of Phosphorus-Doped Carbon Xerogel for Oxygen Reduction. Electrochim. Acta 2014, 127, 53-60. (46) Yang, D.-S.; Bhattacharjya, D.; Song, M. Y.; Yu, J.-S. Highly Efficient Metal-Free

Phosphorus-Doped

Platelet

Ordered

Mesoporous

Carbon

for

Electrocatalytic Oxygen Reduction. Carbon 2014, 67, 736-743. (47) Sayari, A.; Moudrakovski, I.; Reddy, J. S. Synthesis of Mesostructured Lamellar Aluminophosphates Using Supramolecular Templates. Chem. Mater. 1996, 8, 2080-2088. (48) Alfaya, A. A. S.; Gushikem, Y. Highly Dispersed Phosphate Supported in a Binary Silica Titania Matrix: Preparation and Characterization. Chem. Mater. 1998, 10, 909-913. (49) Wang, L.; Tian, B.; Fan, J.; Liu, X.; Yang, H.; Yu, C.; Tu, B.; Zhao, D. Block Copolymer Templating Syntheses of Ordered Large-pore Stable Mesoporous Aluminophosphates and Fe-aluminophosphate Based on an “Acid-Base Pair” Route. Microporous Mesoporous Mater. 2004, 67, 123-133. 42


Page 43 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Imanaka, N.; Masui, T.; Hirai, H.; Adachi, G. Amorphous Cerium-Titanium Solid Solution Phosphate as a Novel Family of Band Gap Tunable Sunscreen Materials. Chem. Mater. 2003, 15, 2990-2991. (51) Stanghellini, P. L.; Boccaleri, E.; Diana, E.; Alberti, G.; Vivani, R. Vibrational Study of Some Layered Structures Based on Titanium and Zirconium Phosphates. Inorg. Chem. 2004, 43, 5698-5703. (52) Sun, Z.; Sun, B.; Qiao, M.; Wei, J.; Yue, Q.; Wang, C.; Deng, Y.; Kaliaguine, S.;

Zhao,

D.

A

General

Nanoparticles-Incorporated

Chelate-Assisted

Ordered

Co-Assembly

Mesoporous

Carbon

to

Metallic

Catalysts

for

Fischer-Tropsch Synthesis. J. Am. Chem. Soc. 2012, 134, 17653-17660. (53) Yue, G.; Wu, J.; Lin, J.-Y.; Xiao, Y.; Tai, S.-Y.; Lin, J.; Huang, M.; Lan, Z. A Counter Electrode of Multi-Wall Carbon Nanotubes Decorated with Tungsten Sulfide Used in Dye-Sensitized Solar Cells. Carbon 2013, 55, 1-9. (54) Tai, Q.D.; Chen, B. L.; Guo, F.; Xu, S.; Hu, H.; Sebo, B.; Zhao, X. Z. In Situ Prepared Transparent Polyaniline Electrode and Its Application in Bifacial Dye-Sensitized Solar Cells. ACS Nano 2011, 5, 3795-3799. (55) Shao, L.-L.; Chen, M.; Yuan, Z.-Y. Hierarchical Porous Carbons as A Metal-Free Electrocatalyst of Triiodide Reduction for Dye-Sensitized Solar Cells. J. Power Sources 2014, 272, 1091-1099. (56) Soo Kang, J.; Park, M. A.; Kim, J. Y.; Ha Park, S.; Young Chung, D.; Yu, S. H.; Kim, J.; Park, J.; Choi, J. W.; Jae Lee, K.; Jeong, J.; Jae Ko, M.; Ahn, K. S.; Sung, Y. E. Reactively Sputtered Nickel Nitride as Electrocatalytic Counter Electrode for 43



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Dye- and Quantum Dot-Sensitized Solar Cells. Sci. Rep. 2015, 5, 10450.

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Table of Contents Graphic


Construction of Highly Catalytic Porous TiOPC Nanocomposite

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