Niobium-Doped (001)-Dominated Anatase TiO2 Nanosheets as

Subscriber access provided by University of Newcastle, Australia

Article 2

Niobium-doped (001) Dominated Anatase TiO Nanosheets as Photoelectrode for Efficient Dye-Sensitized Solar Cells Lei Jiang, Lei Sun, Dong Yang, Jian Zhang, Ya-Juan Li, and Wei-Qiao Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14147 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

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

ACS Applied Materials & Interfaces

Niobium-doped (001) Dominated Anatase TiO2 Nanosheets as Photoelectrode for Efficient Dye-Sensitized Solar Cells Lei Jiang,‡ † Lei Sun,‡ † Dong Yang,§ Jian Zhang, § Ya-Juan Li† and Wei-Qiao Deng*† †

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, China. §

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡

These authors contributed equally to this work.

KEYWORDS: niobium-doped titania nanosheets, dye-sensitized solar cells, work functions, density functional theory, first-principles

ABSTRACT: TiO2 nanocrystals with different reactive facets have been attracted extensive interest since their syntheses. The anatase TiO2 nanocrystals with (001) or (100) dominated facets were considered to be excellent electrode materials to enhance the cell performance of dye sensitized solar cells. However, which reactive facet presents the best surface for benefiting photovoltaic effect is still unknown. We report a systematic study of various anatase TiO2 surfaces interacting with N719 dye by means of DFT calculations in combination with microscopic techniques. (001) surface 1 ACS Paragon Plus Environment


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 2 of 30

interacting with N719 would have the lowest work function, leading to the best photovoltaic performances. To further increase the efficiency, Nb dopant was incorporated into the anatase TiO2 nanocrystals. Based on the theoretical prediction, we proposed and demonstrated a novel Nb-doped (001) dominated anatase TiO2 nanosheets as photoelectrode in a dye-sensitized solar cell to further enhance the open-circuit voltage. And a power conversion efficiency of 10% was achieved, which was 22% higher than that of the undoped device (P25 as an electrode).

INTRODUCTION Dye-sensitized solar cells (DSSCs) are promising low-cost alternatives to conventional solar cells such as silicon, GaAs solar cells.1 Significant progresses have been made in past decades for developing DSSCs.2-9 Thus far, the best conversion efficiencies 13% have been obtained with a zinc-porphyrin dye used together with the cobalt (II/III) redox couple.10 In these photoelectrochemical devices, a nanocrystalline film of a wide band gap semiconductor typically TiO2 is sensitized by a dye, which upon optical excitation injects an electron into the TiO2 conduction band resulting in an oxidized dye. The electron diffuses through the TiO2 film to the photoanode and then reaches the cathode by passing through an external circuit. At the cathode, the electron reduces a redox couple, which in turn recovers the oxidized dye. The TiO2 nanocrystalline porous film plays a key role in this cycle, which acts as the electron acceptor and transport layer. To improve the cell performance, some attempt has been made in the preparation of TiO2 electrode, such as introducing dopant.

2 ACS Paragon Plus Environment

Page 3 of 30

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


Many research efforts have been focused on the TiO2 crystal structures (rutile, anatase and brookite) to improve cell efficiencies. Among these main types of TiO2 crystal structures, anatase is the most widely-used crystal type due to its wide band gap. Three most exposing faces are in anatase crystal, i.e. (101), (001) and (100). The formation of the (101) is favoured by its thermodynamic stability, leading to its large exposing area in anatase crystals more than 94%.11 Compared with (101) surface, synthetic crystals of TiO2 with an exposed (001) and (100) surfaces are rarely observed in the past, thus their relevant applications are scarcely reported. Recently, the development of the nanotechnology brings forward new methods to fabricate stable anatase TiO2 nanocrystal with exposed (001)12-14 and (100)15-17 surface. In DSSCs, anatase TiO2 nanocrystals have been suggested as the best photoanode materials and the exploration of different anatase crystal faces provided new technical means for improving the power conversion efficiency (PCE). Lashova et al employed spectro-electrochemical method to measure the shift of flatband potential with (101) and (001) faces sensitized by C101 dye.18 They found that the negative shift of flat-band potential was responsible for the observed enhanced of open-circuit voltage. Wu et al reported an enhancement in DSSCs overall conversion efficiency, through using the synthesis of nano-sized TiO2 single crystals with different percentages of exposed (001) face.19 Wu inferred that the improvement of (001) face was due to not only more dye adsorption but also effective retard in the charge recombination between electrolyte and TiO2. The studies by Jung et al have come to the similar conclusions.20 Besides, Yang et al deduced that 5-fold Ti on the anatase (001) face is 3 ACS Paragon Plus Environment


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 4 of 30

conductive to the electronic coupling between the dye and the TiO2.21 In order to investigate the dynamics of interfacial charge transfer within DSSCs, Wu et al in 2013 used electrochemical impedance spectroscopy for different photoanodes with (101) and (001) faces.22 They concluded that the weaker charge recombination on (001) face is because that the reactive surface face (001) decreased the surface trap sites and the recombination centers, and increased the adsorption amount of dyes on the surface, leading to decreasing the amount of exposed trap sits. Meanwhile they inferred that anatase (001) face can increase the electronic coupling between the dye and TiO2 and favor electron injection from the dye to TiO2 conduction band. At the same time, many groups have begun the theoretical studies on different anatase surfaces to validate the experimental results, such as the theoretical conclusions of conduction band energy upshift for (001) face compared with (101) presented by Angelis et al was in agreement with previous interfacial impedance and recent spectro-electrochemical data.23 However, which reactive facet presents the best surface for benefiting photovoltaic effect is still unknown. To further improve cell performances, several studies have been reported on the incorporation of Nb dopant into TiO2 nanoparticles. 24-29 They found that the better efficiency was demonstrated by appropriate amounts of Nb dopant. It provides valuable insight for designing of high-performance DSSCs. In this work, we have employed a first-principles computational investigation on the

enhancing

mechanism

with

highly

efficient

ruthenium

dye

N719,

bis(tetrabutylammonium)[cis-di(thiocyanato)-bis(2,2'-bipyridyl-4-carboxylate-4'-carb 4 ACS Paragon Plus Environment

Page 5 of 30

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


oxylic acid)-ruthenium(II)], on the anatase surfaces (101), (100) and (001). Based on the theoretical understandings, we proposed and demonstrated a new strategy by using Nb-doped (001) oriented TiO2 nanosheets as photoelectrode for DSSCs. We firstly synthesized the highly crystallized Nb-doped anatase (001) and (100) oriented TiO2 nanosheets. The photovoltaic performances of the DSSCs based on Nb-doped (001) oriented TiO2 was significantly improved, which has been boosted as high as 10.0%, by about 1.22 times compared to that of a cell based on P25 (commercialization TiO2). EXPERIMENTAL SECTION Synthesis of Nb-doped (001)-oriented TiO2 nanosheets and (001)-oriented TiO2 nanosheets. The experimental procedure of Nb-doped (001)-oriented TiO2 nanosheets, 12.50 mL of titanium butoxide (Ti(OBu)4), Aladdin, 99%, AR) and 0.23 mL of niobium ethoxide (Nb(OEt)5, ACROS, > 99.95%) and 1.25 mL of hydrofluoric acid (HF, Aladdin, 47%) were mixed in a Teflon autoclave with a capacity of 50 mL, and then kept at 170 ºC for 24 hours. After being cooled to room temperature, the resulting white power was separated by centrifuging at 11000 r.p.m. and washed with ethanol and deionized water for several times. The (001) nanosheets (Nb 0 mol%) were also prepared by following the similar steps, 12.50 mL of titanium butoxide (Ti(OBu)4, Aladdin, 99%, AR) and 1.25 mL of hydrofluoric acid (HF, Aladdin, 47%) were mixed in a Teflon autoclave with a capacity of 50 mL, and then kept at 170 ºC for 24 hours.



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 6 of 30

Synthesis of Nb-doped (100)-oriented TiO2 nanosheets and (100)-oriented TiO2 nanosheets. The Nb-doped (100)-oriented TiO2 nanosheets were prepared by following steps, 19.5 mL of titanium butoxide and 0.36 mL of niobium ethoxide were mixed in a Teflon autoclave with a capacity of 100 mL, stirred 15 mins and then add 28 mL N,N-dimethylethanolamine

(DMEA, Aladdin), ultrasonic cleaning 10 mins.

After adding 18 mL deionized water rapidly, and heated at 150 ºC for 18 hours. The (100)-oriented TiO2 nanosheets were synthesized by following the similar steps, 19.5 mL of titanium butoxide was added into a 100 mL Teflon autoclave, stirred 15 mins and then add 28 mL DMEA, ultrasonic cleaning 10 mins. After adding 18 mL deionized water rapidly, and heated at 150 ºC for 18 hours. Synthesis of Nb-doped (101)-oriented TiO2 nanoparticle. The Nb-doped nanoparticles were synthesized by following a previously reported procedure,30 0.0738 mM of Nb(OEt)5 and 1.44 mM TiCl4 were added into 3.6 mM tert-Butyl alcohol. The solution was transferred in a Teflon sealed autoclave and kept at 180 ºC for 16 hours. Preparation of photoelectrodes and solar cells. The Nb-doped (001) TiO2 nanosheets paste was prepared following a previously reported procedure.31 F-doped tin oxide (FTO, TCO-15, 14ohm/square, NSG, Japan) was used as transparent conducting substrate. Mesoscopic Nb-doped TiO2 films were prepared by screen printing Nb-doped (001) TiO2 nanosheets paste on the FTO, followed by sintering at 500 ºC for 45 min. The thickness of the film was about 10 µm. Then the film was coated by a 5-µm-thick layer of scattering TiO2 nanoparticles (WER-2, Dyesol, 6 ACS Paragon Plus Environment

Page 7 of 30

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


Australia), final thickness of the film was about 15 µm. The sintered electrode was immersed

in

a

solution

of

cis-di(thiocyanato)-N-N'-bis(2,2'-bipyridyl-4-carbo–xylicacid-4'-tetrabutylammonium carboxylate) ruthenium (II) dye (N719, Dyesol) (0.3 mM in a mixture of acetonitrile and tert-butyl alcohol (volume ratio, 1:1) with 1 mM chenodeoxycholic acid and 0.6 mM Tetrabutylammonium hydroxide, TBAOH) for 20 h, rinsed with acetonitrile, and then assembled using a thermally platinized FTO counter electrode through a 50 µm thick sealing material (Bynel, Dupont). The electrolyte consists of 1.0 M 1,3-dimethylimidazolium iodide, 0.05 M lithium iodide (LiI), 0.03 M iodine, 0.5 M tert-butylpyridine and 0.1 M guanidinium thiocyanate in an 85:15 (v/v) acetonitrile/valeronitrile mixture (Z960) (all the chemicals were purchased from Aldrich Chemical). Other devices were also prepared by following the similar steps. Photovoltaic Characterization. The composition and phase of the products were measured by X-ray powder diffraction (XRD, Rigaku D/max-2500/PC) with a monochromatized source of Cu Kα radiation (λ = 0.15406 nm) at a scan rate of 2º min-1. The morphology and structure of products were observed by transmission electron microscopy (TEM, TECNAI G2 SPIRIT) and scanning electron microscopy (SEM, QUANTA 200 FEG). UV-vis absorption spectra were recorded on a Persee TU-1810 UV-vis spectrometer. A Sun 2000 solar simulator (Abet-technologies, USA) was used to give an irradiance of 100 mW/cm2 (AM 1.5 G) on the surface of the solar cell. The current-voltage characteristics of the cell were measured with a Keithley 2400 source meter (Keithley, USA). The tested cells were masked to a working area 7 ACS Paragon Plus Environment


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 8 of 30

of 0.160 cm2. The measurement of the incident photon-to-current conversion efficiency (IPCE) was performed by a QE/IPCE Measurement Kit (Oriel, USA, M66901). Scanning Kelvin Probe Microscopy (SKPM) measurements were carried out on a Bruker Metrology Nanoscope VIII atomic force microscope in ambient atmosphere. Conducting atomic force microscopy (AFM) tips (SCM-PIT/PtIr, Bruker, USA) used for this study had a typical spring constant of 2.8 N/m and a resonance frequency of 75 kHz. The highly oriented pyrolytic graphite (HOPG) was used as reference sample, which work function is 4.6 eV.

Figure 1. Fully optimized geometries of the N719 on anatase (101), (100) and (001) surfaces. The Nb dopant was randomly doped in the top and bottom of the TiO2 surfaces (green color for top doping and blue color for bottom doping).

THEORETICAL CALCULATIONS The DFT calculation was performed using the Vienna Ab initio Simulation Package (VASP) code,32,33 with exchange correlation effects being described by the Perdew−Burke−Ernzerhof (PBE) version of the generalized gradient approximation (GGA).34 In the choice of supercell dimensions, firstly the surface of supercells must 8 ACS Paragon Plus Environment

Page 9 of 30

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


be large enough to prevent the interaction of the dye molecules with its periodic image; secondly the supercells must be thick enough to reasonably reproduce most of the TiO2 bulk properties. In order to obtain similar surface coverage, the anatase (101), (100) and (001) surface has been modeled by periodic (4×5) (21.772 Å and 18.880 Å), (2×5) (19.077 Å and 18.855 Å) and (5×5) (18.880 Å and 18.880 Å) surface unit cells. Therefore, it is hard to use much thicker slab. It is notable that a “layer” here means an atomic layer. For these surface models, (101) surface contains twelve-layer O atoms and six-layer Ti atoms and (001) contains eight-layer O atoms and four-layer Ti atoms, respectively, as the values in other theoretical studies before.35,36 But (100) surface adopts five-layer structure for both O and Ti atoms to assure the similar number of atoms in one supercell. In all calculations, the atoms in the top of (TiO2) slab have been relaxed to their minimum energy configurations where total energy and atomic forces are minimized, while fixing all other atoms to bulk truncated positions. The vacuum between the bottom of the slab and the top the adsorbed molecule has been taken at least 8 Å. The density of the Gamma k-mesh was 1×1×1 in the geometry optimization. Maximum force magnitude that remained on each atom has been limited to 0.06 eV/Å. A plane-wave basis set with kinetic-energy cutoff of 400 eV has been used. For the energy calculations, a finer k-mesh (2×2×1) has been used. Three Nb cations were doped TiO2 (101) surface with the content of 2.5% mol%. Because the total Ti atomic number of the chosen (100) and (001) surfaces is twenty less than the one of (101) surface, two Nb cations were doped respectively in TiO2 (100) and (001) surfaces. As shown in Figure 1, two doping patterns (top and bottom) 9 ACS Paragon Plus Environment


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 10 of 30

were chosen to seperately represent surface doping and inside doping. Due to the numerous doping sites on each layer, we randomly selected one doping position in our studies. In the top doping mode, the positions of Nb cations avoided placing on the site of Ti-O bond. The open-circuit voltage of a DSSC is determined by the energy difference between the quasi Fermi levels of the TiO2 (EF,n) and the redox species (Eredox),37,38 equation (1). qVoc = EF ,n − Eredox (1)

Considering that the redox species would not change, the quasi Fermi level is crucial for improving the open-circuit voltage. Although the quasi Fermi level under steady state operation cannot be obtain directly from the DFT calculations, there is some relationship between EF,n and the calculated Fermi level (EF) of N719-TiO2 system. For non-intrinsic semiconductor, the electronic Fermi level can be given in terms of electron local equilibrium concentration by

EF = Ec ,m + kT ln

ne (2) Nc

where Ec,m is the bottom of conduction band, ne is the free electron density and Nc is the state density. When the cell is running, additional electrons inject into the conduction band of TiO2 with an increment of ∆ne. Then EF,n can be given in term as

EF ,n = Ec ,m + kT ln

ne + ∆ne (3) Nc

as shown in Figure 2. Supposing that the amount of injection photoelectrons and recombination electron are the same for each TiO2 surface, EF,n will increase along 10 ACS Paragon Plus Environment

Page 11 of 30

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


with EF growth. Therefore, we used the calculated Fermi level of N719-TiO2 system to replace the quasi Fermi level under steady state operation to reflect the open-circuit voltage under the ideal conditions. Because the zero total energy of different surface – dye combined systems is not uniform, the Fermi levels, which are directly obtained in the energy calculation, cannot be compared with each other. We investigated the Fermi energy of different combined system through work function. The value of work function can be obtained from the output file named as LOCPOT. Because the work function was defined by removing the electron from the Fermi level of the neutral combined system into the level of the vacuum nearby the surface, it can be calculated as the difference between the Fermi level and the vacuum level, as shown in Figure S1. The vacuum level was defined as the potential in the vacuum region where it reached a maximum. If we take vacuum level at infinity as the zero total energy, -EF is the thermodynamic work required to remove an electron from the material to a state of zero total energy (-EF = WF). Therefore, we estimated the quasi Fermi levels of anatase surfaces (101), (100)

and (001) structures with N719 dye by calculating the work functions (WF) of the combination systems, as shown in Figure 2.



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 12 of 30

Figure 2. Definitions of energy levels in dye-sensitized solar cells.

RESULTS AND DISCUSSION Firstly, WF of (101), (100) and (001) anatase surfaces were investigated through DFT calculations. As shown in Figure S2, the calculated WF of (001) surface is lower than the other two TiO2 anatase surfaces. The sequence of WF is as follow: (101) > (100) > (001). The calculation results indicated that EF of (001) surface will be the highest one among three anatase surfaces. The difference is originated from the different atomic configurations on each surface, which leads to the lowest relative trapping energy of electrons in these three surfaces.39 Although WF depends on the molecular conformation and surface concentration of dye molecules, we mainly focused on the changes of TiO2 electrode caused by the Nb dopant. The absorption orientation of N719 on TiO2 has been studied and discussed by several groups using vibrational spectroscopy, X-ray absorption spectroscopy and computational model studies. However, the binding mechanism of N719 dye onto TiO2 surface is not yet fully understood and remains a subject of debate. Therefore, we chose one absorption model as shown in Figure 1, which was proposed on the basis of molecular dynamics 12 ACS Paragon Plus Environment

Page 13 of 30

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


simulations and electronic structure calculations by Angelis et. al.40 For the optimized TiO2-N719 dye systems, the calculated bond length of Ti-O between dye molecule and TiO2 surface are all around 2.00 Å (Table S1), which indicates that the dye bonding in the TiO2 slabs with N719 molecule occurs via three carboxylic groups. The oxygen atom of the carboxylic group is attached to the fivefold coordinated Ti atom in the surface. The protons are bound to the surface oxygen close to the carboxylic groups. Meanwhile, the similar coverage densities of N719 on different TiO2 surfaces were set (Table S2) based on the similar-sized unit cells. Thus, the calculated WF will directly reflect the difference of each TiO2 surface with or without Nb dopant. As shown in Table 1, the calculated WF of (001) surface – N719 dye (4.57 eV) is lower than the other two TiO2-dye systems (4.85 eV for (101) surface and 4.66 eV for (100) surface). Note that the units of WF and EF are eV, but the open-circuit voltage of solar cell normally uses a unit of mV. Only 0.1 eV difference of Fermi level may lead to 100 mV difference for open-circuit voltage, which is remarkable in solar cell. In order to verify our calculation, the work functions of anatase surfaces (101), (100) and (001) were determined from the contact potential difference (CPD) measurements using Scanning Kelvin Probe Microscopy (SKPM). With respect to the vacuum level, the work functions of these three TiO2-dye systems were calculated as 4.42, 4.18 and 4.12 eV, respectively. Since the theoretical coverage density dose not equal to the experimental one as shown in Table S2, the calculated WF cannot be directly compared with the measured result. We shifted the calculated WF by minus 0.17 eV. The obtained results are shown in Table 1, which are basically in accordance 13 ACS Paragon Plus Environment


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 14 of 30

with the experimental measurement. However, both of the theoretical and experimental results reveal that EF of anatase (001) surface is the higher than the others. Table 1. Work functions of different electrodes absorbed with N719 dyes, independently measured by Kelvin probe and predicted by DFT calculation. The values in the parentheses were corrected by shifting 0.17 eV. No doping

Doping Nb Cal.

Work function (eV) Exp.

Cal.

Exp. Top

Bottom

N719+(101)

4.42

4.85 (4.68)

4.36

4.69 (4.52)

4.51 (4.34)

N719+(100)

4.18

4.66 (4.49)

4.14

4.27 (4.10)

4.24 (4.07)

N719+(001)

4.12

4.57 (4.40)

4.07

4.13 (3.96)

3.98 (3.81)

According to equation (1), one way to increase the Voc is to use a new electrolyte with negative-shift Eredox compared to I3-/I-, and make sure that the dyes’ ground state potential is positive enough to efficiently complete the electron regeneration process between cationic dyes and reduced electrolyte ions. Another way is to increase the electron density of the photoanode, thereby raising the fermi lecel. Without dye changed, the right amount dopant of Nb element may be an efficient method for increasing EF. Three structures were constructed for the fully optimization: N719 molecule on Nb doped anatases TIO2 (101), (100) and (001) surfaces, as shown in Figure 1. Based on the optimized geometries, the work functions and vacuum levels of three Nb doped nanocrystals were evaluated. As shown in Table 1，the comparative analyses before and after doping display that WF for Nb-doped TiO2 surfaces are 14 ACS Paragon Plus Environment

Page 15 of 30

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


lower than those undoped surfaces, and the sequence of WF for three TiO2 surface ((101) > (100) > (001)) is not influenced by the doping of Nb element. Then, the work functions of Nb-doped (101), (100) and (001) TiO2 surfaces were also determined from the contact potential difference measurements using SKPM. With respect to the vacuum level, the work functions of these three Nb-doped TiO2-dye systems were calculated as 4.36, 4.14, and 4.07 eV, respectively, which were in good agreement with the DFT calculation results (shown in Table 1). The energy change reflected the number of free electrons emitted from the Nb dopant. Experimental measurement shows electron densities of only up to 1018 cm-3 for pure TiO2. Nb dopant exhibits a high 4d atomic orbital energy, leading to more likely to transfer their electron to the conduction band than Ti with a lower energy of 3d orbital.41 As shown in Figure S3, the partial densities of states show that the conduction band of TiO2-dye system consists of the hybridization of Ti 3d and Nb 4d orbitals. The free electron density of pure TiO2 can be raised via Nb dopant. Therefore, the Fermi levels of the TiO2 can be increased by the excess free electron density. The theoretical and experimental results indicate that Nb dopant will increase the quasi Fermi level of TiO2 – N719 dye system, leading to a higher open-circuit potential in DSSCs employing Nb-doped (001) oriented TiO2. Encouraged by the calculation results, we carried out the synthesis procedure for Nb-doped (101), (100) and (001) oriented TiO2 nanocrystal samples. By a simple hydrothermal method using tertrabutyl titanate, Ti(OBu)4, niobium ethoxide, Nb(OEt)5, as sources and hydrofluoric acid solution as the solvent, Nb-doped (001) 15 ACS Paragon Plus Environment


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 16 of 30

oriented TiO2 nanosheets (named as Nb-(001)-NS) and Nb-doped (100) oriented TiO2 nanosheets (named as Nb-(100)-NS) were first successful synthesized (experimental details are provided in the Experimental section). The crystalline structures of the as-prepared samples were examined by X-ray powder diffraction (XRD), as shown in Figure 3. All peaks of the as-synthesized (100), (001), Nb-(101), Nb-(100) and Nb-(001) can be assigned to the anatase phase (JPCDS No. 21-1272, space group: I41/amd (141); a=3.785 Å, c=9.514 Å), indicating that the anatase nanocrystalline structure is retained after Nb doping.

Figure 3. (a) XRD patterns of as-prepared samples. (b) XRD patterns of P25, (001) and Nb-doped (001)-oriented TiO2 nanosheets. The inset shows details of the position of the (101) peak. R for rutile TiO2 in P25. As shown in Figure 3b, the (101) diffraction peak of Nb-(001)-NS shifts to lower angles with Nb doping because of the larger radius of Nb5+ ions (64 pm) compared to that of Ti4+ ions (60.5 pm), according to Vegard´s law. Figure 4a shows the transmission electron microscope (TEM) image of Nb-(001)-NS, that consisted of well-defined sheet-shaped structures having a rectangular outline, side length is ~50 16 ACS Paragon Plus Environment

Page 17 of 30

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


nm and thickness is ~5 nm (Table S3, Figure S4). The High-resolution transmission electron microscope (HRTEM) images (Figure 4b, Figure S4f) directly show that the lattice spacing parallel to the top and bottom facets is ~0.238 nm, corresponding to the (001) planes of anatase TiO2, indicating that the Nb-(001)-NS have a high percentage of exposed (001) facets. For the above structural information, the percentage of (001) facets in Nb-(001)-NS was ~83.3%, respectively (the calculation details in Table S3). Figure 4e and f show the TEM and HRTEM images of Nb-(100)-NS, consisted of well-defined sheet-shaped rod-like structures, with length of ~150 nm, width of ~35 nm, and the thickness of ~4 nm. The HRTEM image of Nb-(100)-NS provides three lattice fringes with spaces of 0.35, 0.35 and 0.48 nm, which was corresponding to -

(101), (101 ) and (002) of anatase phase, indicating that the Nb-(100)-NS are dominated by the (100) facets. The percentage of (100) facets in Nb-(100)-NS was also calculated to be ~87.6% (see Table S3). The TEM images of (001)-NS, (100)-NS and Nb-(101)-NP are shown in Figure S4. In order to investigate the distribution of Nb in the doped nanosheets, scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDS) elemental mapping were performed. Figure 4c shows the SEM image of Nb-(001)-NS and the corresponding elemental mapping for Nb atoms is shown in Figure 4d, which reveals the expected homogeneous distribution of dopant Nb atoms. In addition, the dark-field STEM elemental analysis of the Nb-doped TiO2 nanosheets sample is presented in Figure S5. The Nb content (Nb mol/(Nb mol + Ti mol)) in the Nb-(001)-NS was determined by using inductively coupled plasma-optical emission spectroscopy (ICP-OES) and EDS (Figure S6). As 17 ACS Paragon Plus Environment


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 18 of 30

listed in Table S4, the Nb content in the Nb-(001)-NS, Nb-(101)-NP and Nb-(100)-NS was 2.52, 2.51 and 2.1 mol%, respectively.

Figure 4. (a) TEM image of Nb-doped (001) TiO2 nanosheets. (b) HRTEM image from the vertical nanosheets. (c) SEM image of Nb-(001) nanosheets and (d) corresponding EDS Nb mapping. (e) TEM image of Nb-doped (100) TiO2 nanosheets. (f) HRTEM image of Nb-doped (100) TiO2 nanosheets. (g) SEM image of Nb-(100) nanosheets and (h) corresponding EDS Nb mapping.

DSSCs were fabricated by using the Nb-(001)-NS as electrode. For comparison, five other types of DSSCs were fabricated with electrode materials based on P25 ((101) TiO2 nanoparticles), (001) oriented TiO2 nanosheets (named as (001)-NS), (100) oriented TiO2 nanosheets (named (100)-NS), Nb-doped (101) oriented TiO2 nanoparticles (named as Nb-(101)-NP) and Nb-doped (100) oriented TiO2 nanosheets, respectively. The fabrication processes of these DSSCs are same. Under one sun solar


Page 19 of 30

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


illumination (AM 1.5G, 100 mW/cm2), the current density-voltage curves of the DSSCs based on different TiO2 electrodes were shown in Figure 5.

Figure 5. J-V curves of dye-sensitized solar cells based on different electrodes.

The photovoltaic parameters, short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill factor (FF), and solar-to-electric power conversion efficiencies (η) were listed in Table 2. The DSSC based on Nb-(101)-NP increases the Jsc to 18.89 mA/cm2, improved 2.29 mA/cm2 compared with a DSSC based on P25. The DSSC based on (001)-NS increases the Voc to 793 mV, which was about 74 mV higher than that of P25 one. Comparing with the DSSC based on P25, the DSSC based on Nb-(101)-NP had higher photocurrent density and the DSSC based on (001)-NS had higher photovoltage, which were consistent with the previous ref.19,24 The cell based on Nb-(001)-NS was found to give the best performance in this series. The power conversion efficiency of 10.0% was achieved for the DSSCs based on Nb-(001)-NS, which was 11.1% higher than that of the undoped (001)-NS, and was 22% higher than that of P25. The most significant result is that the DSSCs based on Nb-(001)-NS had both improved the Jsc (improved 2.04 mA/cm2) and the Voc (improved 101 mV). To 19 ACS Paragon Plus Environment


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 30

further analyze the enhanced performances of DSSCs based on Nb-(001)-NS, the incident photon to current conversion efficiency (IPCE) measurements were conducted, as shown in Figure 6. The IPCE of the cell based on Nb-(001)-NS is above 88% at 580 nm, comparing to 77% at 560 nm for the cell based on P25 electrode, which describes an 11% enhancement. And the absorption edges of the IPCE spectra are much greater for the DSSCs based on Nb-(001)-NS electrode. These results indicate that the injection of excited electrons of DSSCs based on Nb-(001)-NS electrode is much more efficient than that of P25 one. The summary of all fabricated DSSCs has been shown in Figure S7. Table 2. Photovoltaic parameters of DSSCs based on different electrodes.

a

DSSCsa

Jsc (mA/cm2)

Voc (mV)

FF

η (%)

P25

16.60

719

0.68

8.2

(100)-NS

16.74

727

0.69

8.4

(001)-NS

17.36

793

0.65

9.0

Nb-(101)-NP

18.89

696

0.61

8.1

Nb-(100)-NS

17.73

738

0.65

8.6

Nb-(001)-NS

18.64

820

0.65

10.0

The tested cells were masked to a working area of 0.160 cm2.

Through comparison of theoretical and experimental results, the changing of the experimental open-circuit voltages is in general accordance with the theoretical predictions of the work functions of different TiO2 surface – N719 dye systems, except for TiO2 (101) surfaces. This may be attributed to the idealized model used in DFT calculations. The Nb doping will lead to a higher defect concentration, which 20 ACS Paragon Plus Environment

Page 21 of 30

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


will increase the electron recombination between the electrolyte space and the uncovered TiO2 surface. Since the same electrolyte was used in the preparation of solar cell, the effect of electrolyte would be similar for the similar-sized theoretical models. The calculations carried out in vacuum without electrolyte. But in fact, the specific surface area (BET) of synthetic photoelectrodes are different from each other, especially for the doping ones, which would cause some deviation of theoretical predictions. According to Table S2, specific surface area (BET) and dye coverage density (NA) after doping produced apparent change. In practice, the greater BET and less NA will exacerbate the voltage losses originated from electron recombination between the electrolyte and TiO2 surface, especially for TiO2 (101) surface with the increment of 1.38 times for BET and the decrease of 5.1% for NA. It explains why the open-circuit voltage does not increase obviously with the decline of work function, but drops from 719 mV to 696 mV for TiO2 (101) surfaces after Nb doping. For the same reason, the increase of 10 mV for the open-circuit voltage of TiO2 (100) with the dopant was not significant.

Figure 6. IPCE spectrum of dye-sensitized solar cells based on different electrodes. 21 ACS Paragon Plus Environment


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 22 of 30

In addition, to investigate the influence of Nb dopant content on the DSSCs’ performance, a 5.0 mol% doping level Nb-(001)-NS have been made and applied as the photoanode material in DSSCs. A significant decrease of the cell performance has been observed (Figure S8). Two possible reasons can explain this result. First, heavy dopant is unfavorable for electrochemical applications due to the increased electron recombination at the semiconductor electrolyte interface.42 Second, the defect concentration of the 5.0 mol% Nb-(001)-NS is much higher than that of the 2.5 mol% Nb-(001)-NS. The electron mobility decreases rapidly with high defect concentration due to electron scattering by the defects,43 leading to a low Jsc and Voc. The work function of 5.0 mol% Nb-(001)-NS was 4.35eV measured by SKPM, which was much higher than that of 2.5 mol% Nb-(001)-NS (4.07eV), leading to a low Voc. Therefore, appropriate amounts of Nb dopant should be incorporated into anatase (001) surface to avoid the degradation of cell performance. CONCLUSIONS

In summary, we used DFT calculations to address the best TiO2 surface for benefiting photovoltaic effect in dye sensitized solar cells. Based on the theoretical understandings, we firstly synthesized the highly crystallized Nb-doped anatase (001) and (100) oriented TiO2 nanosheets and demonstrated a Nb-doped (001) dominated TiO2 nanosheets as a photoanode for efficient DSSCs. Both photocurrent and open voltage was significantly improved, leading to boosted cell efficiency as high as 10.0%. Based on the strategy in our studies, more candidates of photoanode materials can be screened in futrue. 22 ACS Paragon Plus Environment

Page 23 of 30

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


ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.” Percentage of active facets on TiO2 nanosheets, ICP-OES and EDS of Nb-doped TiO2 nanosheet, surface properties of different TiO2 surface – N719 dye systems, work function of different TiO2 surface – N719 dye combined systems, dark-field STEM image and EDS spectra of Nb-doped (001) TiO2 nanosheets, PCE distribution histogram of DSSCs

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by NSFC 91333116, 21525315, and 21403211.

REFERENCES (1) Oregan, B.; Grätzel, M. A Low-Cost High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740.



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 24 of 30

(2) Wang, P.; Zakeeruddin, S. M.; Moser, J. E.; Nazeeruddin, M. K.; Sekiguchi, T.; Grätzel, M. A Stable Quasi-Solid-State Dye-Sensitized Solar Cell with an Amphiphilic Ruthenium Sensitizer and Polymer Gel Electrolyte. Nat. Mater. 2003, 2, 402-407. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455-459. (4) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. High-Performance Dye-Sensitized Solar Cells Based on Solvent-Free Electrolytes Produced from Eutectic Melts. Nat. Mater. 2008, 7, 626-630. (5) Chung, I.; Lee, B.; He, J. Q.; Chang, R. P. H.; Kanatzidis, M. G. All-Solid-State Dye-Sensitized Solar Cells with High Efficiency. Nature 2012, 485, 486-494. (6) Lin, Y.; Li, C.; Lee, C. P.; Leu, Y.; Ezhumalai, Y.; Vittal, R.; Chen, M.; Lin J.; Ho, K. Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid-State Dye-Sensitized Solar Cells with a High Open-Circuit Voltage. ACS Appl. Mater. Interfaces 2016, 8, 15267–15278.

(7) Feldt, S. M.; Gibson, E. A.; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714-16724. (8) Xie,Y.; Tang,Y.; Wu, W.; Wang, Y.; Liu, J.; Li, X.; Tian, H.; Zhu, W. Porphyrin Cosensitization for a Photovoltaic Efficiency of 11.5%: A Record for Non-Ruthenium Solar Cells Based on Iodine Electrolyte. J. Am. Chem. Soc. 2015, 137, 14055–14058. 24 ACS Paragon Plus Environment

Page 25 of 30

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


(9) Rani, A.; Chung, K.; Kwon, J.; Kim, S. J.; Jang, Y. H.; Jang, Y.; Quan, L.; Yoon, M.; Park, J. H.;

Kim, D. H. Layer-by-Layer Self-Assembled Graphene Multilayers

as Pt-Free Alternative Counter Electrodes in Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces, 2016, 8, 11488–11498.

(10) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247. (11) Yang, H.; Sun, C.; Qiao, S.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H.; Lu, G. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature 2008, 453, 638-641. (12) Ma, X.; Chen, Z.; Hartono, S. B.; Jiang, H.; Zou, J.; Qiao, S.; Yang, H. Fabrication of Uniform Anatase TiO2 Particles Exposed by Facets. Chem. Commun. 2010, 46, 6608-6610. (13) Han, X.; Kuang, Q.; Jin, M.; Xie, Z.; Zheng, L. Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152-3153. (14) Maisano, M.; Dozzi, M. V.; Coduri, M.; Artiglia, L.; Granozzi, G.; Selli, E. Unraveling the Multiple Effects Originating the Increased Oxidative Photoactivity of Facet Enriched Anatase TiO2. ACS Appl. Mater. Interfaces 2016, 8, 9745–9754.



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 26 of 30

(15) Zhao, X.; Jin, W.; Cai, J.; Ye, J.; Li, Z.; Ma, Y.; Xie, J.; Qi, L. Shape- and Size-Controlled Synthesis of Uniform Anatase TiO2 Nanocuboids Enclosed by Active (100) and (001) Facets. Adv. Funct. Mater. 2011, 21, 3554-3563. (16) Pan, J.; Wu, X.; Wang, L.; Liu, G.; Lu, G.; Cheng, H. Synthesis of Anatase TiO2 Rods with Dominant Reactive {010} Facets for the Photoreduction of CO2 to CH4 and Use in Dye-Sensitized Solar Cells. Chem. Commun. 2011, 47, 8361-8363. (17) Pan, J.; Liu, G.; Lu, G.; Cheng, H. On the True Photoreactivity Order of (001), (010), and (101) Facets of Anatase TiO2 Crystals. Angew. Chem., Int. Ed. 2011, 50, 2133-2137. (18) Laskova, B.; Zukalova, M.; Kavan, L.; Chou, A.; Liska, P.; Wei, Z.; Bin, L.; Kubat, P.; Ghadiri, E.; Moser, J. E.; Grätzel, M. Voltage Enhancement in Dye-Sensitized Solar Cell Using (001)-Oriented Anatase TiO2 Nanosheets. J. Solid State Electr 2012, 16, 2993-3001.

(19) Wu, X.; Chen, Z.; Lu, G. Q.; Wang, L. Nanosized Anatase TiO2 Single Crystals with Tunable Exposed (001) Facets for Enhanced Energy Conversion Efficiency of Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2011, 21, 4167-4172. (20) Jung, M. H.; Chu, M.; Kang, M. G. TiO2 Nanotube Fabrication with Highly Exposed (001) Facets for Enhanced Conversion Efficiency of Solar Cells. Chem. Commun. 2012, 48, 5016-5018.

(21) Yang, W.; Li, J.; Wang, Y.; Zhu, F.; Shi, W.; Wan, F.; Xu, D. A Facile Synthesis of Anatase TiO2 Nanosheets-Based Hierarchical Spheres with over 90% {001} Facets for Dye-Sensitized Solar Cells. Chem. Commun. 2011, 47, 1809-1811. 26 ACS Paragon Plus Environment

Page 27 of 30

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


(22) Wu, W.; Rao, H.; Xu, Y.; Wang, Y.; Su, C.; Kuang, D. Hierarchical Oriented Anatase TiO2 Nanostructure Arrays on Flexible Substrate for Efficient Dye-Sensitized Solar Cells. Sci. Rep. 2013, 3, 1892. (23) De Angelis, F.; Vitillaro, G.; Kavan, L.; Nazeeruddin, M. K.; Grätzel, M. Modeling Ruthenium-Dye-Sensitized TiO2 Surfaces Exposing the (001) or (101) Faces: A First-Principles Investigation. J. Phys. Chem. C 2012, 116, 18124-18131. (24) Lue, X.; Mou, X.; Wu, J.; Zhang, D.; Zhang, L.; Huang, F.; Xu, F.; Huang, S. Improved-Performance Dye-Sensitized Solar Cells Using Nb-Doped TiO2 Electrodes: Efficient Electron Injection and Transfer. Adv. Funct. Mater. 2010, 20, 509-515. (25) Kozlov, S.; Nikolskaia, A.; Larina, L.; Vildanova, M.; Vishnev, A.; Shevaleevskiy, O. Rare-Earth and Nb Doping of TiO2 Nanocrystalline Mesoscopic Layers for High-Efficiency Dye-Sensitized Solar Cells. Phys. Status Solidi A. 2016, 213, 1801-1806.

(26) Yao, Q.; Liu, J.; Peng, Q.; Wang, X.; Li, Y. Nd-Doped TiO2 Nanorods: Preparation and Application in Dye-Sensitized Solar Cells. Chem. Asian J. 2006, 1, 737-741. (27) Fontana, S. M.; Dadmun, M. D.; Lowndes, D. H. Growth of Vertically Aligned Carbon Nanofibers from Nickel Nanodot Arrays Produced from Diblock Copolymer Thin Film Templates. J. Nanosci. Nanotechnol. 2006, 6, 3756-3762. (28) Nikolay, T.; Larina, L.; Shevaleevskiy, O.; Ahn, B. T. Electronic Structure Study of Lightly Nb-doped TiO2 Electrode for Dye-Sensitized Solar Cells. Energy Environ. Sci. 2011, 4, 1480-1486.



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 30

(29) Archana, P. S.; Jose, R.; Yusoff, M. M.; Ramakrishna, S. Near Band-Edge Electron Diffusion in Electrospun Nb-doped Anatase TiO2 Nanofibers Probed by Electrochemical Impedance Spectroscopy. Appl. Phys. Lett. 2011, 98, 152106. (30) Liu,Y.; Szeifert, J. M.; Feckl, J. M.; Mandlmeier, B.; Rathousky, J.; Hayden, O.; Fattakhova-Rohlfing, D.; Bein, T. Niobium-Doped Titania Nanoparticles: Synthesis and Assembly into Mesoporous Films and Electrical Conductivity. ACS Nano 2010, 4, 5373-5381.

(31) Ito, S.; Murakami, T. -N.; Comte, P.; Liska, P.; Grätzel, C.; Nazeeruddin, M. -K.; Grätzel, M. Fabrication of Thin Film Dye Sensitized Solar Cells with Solar to Electric Power Conversion Efficiency over 10%. Thin Solid Films 2008, 516, 4613-4619. (32) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558-561.

(33) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comp. Mater. Sci. 1996, 6, 15-50. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396-1396. (35) Cakir, D.; Gulseren, O.; Mete, E.; Ellialtioglu, S. Dye Adsorbates BrPDI, BrGly, and BrAsp on Anatase TiO2 (001) for Dye-Sensitized Solar Cell Applications. Phys. Rev. B, 2009, 80, 035431.


Page 29 of 30

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) Martsinovich, N.; Troisi, A. How TiO2 Crystallographic Surfaces Influence Charge Injection Rates from a Chemisorbed Dye Sensitiser. Phys. Chem. Chem. Phys. 2012, 14, 13392-13401. (37) Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819-1826. (38) Yum, J. H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.; Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J. E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. A Cobalt Complex Redox Shuttle for Dye-Sensitized Solar Cells with High Open-Circuit Potentials. Nat. Commun. 2012, 3, 631. (39) Ma, X. C.; Dai, Y.; Guo M.; Huang, B. B. Relative Photooxidation and Photoreduction Activities of the (100), (101), and (001) Surfaces of Anatase TiO2. Langmuir 2013, 29, 13647-13654.

(40) De Angelis, F.; Fantacci, S.; Selloni, A.; Nazeeruddin, M. K.; Grätzel, M. First-Principles Modeling of the Adsorption Geometry and Electronic Structure of Ru(II) Dyes on Extended TiO2 Substrates for Dye-Sensitized Solar Cell Applications. J. Phys. Chem. C 2010, 114, 6054-6061.

(41) Visscher, L.; Dyall, K. G. Dirac-Fock Atomic Electronic Structure Calculations Using Different Nuclear Charge Distributions. At. Data Nucl. Data Tables 1997, 67, 207-224. (42) Memming, R. Electron Complexes at

Semiconductor

Transfer

Reactions

of

Excited Ruthenium(II)

Electrodes. Surf. Sci. 1980, 101, 551-563.



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 30

(43) Fahey, P. M.; Griffin, P. B.; Plummer, J. D. Point Defects and Dopant Diffusion in Silicon. Rev. Mod. Phys. 1989, 61, 289-384.

Based on density functional theory calculations, the best oriented TiO2 surface has been addressed for benefiting photovoltaic effect in dye sensitized solar cells. Thus Nb-doped TiO2 (001) oriented nanosheets were first synthesized and applied as photoelectrode for a dye sensitized solar cell with efficiency as high as 10.0%, which is 22% better than that of a cell based on P25.


Niobium-Doped (001)-Dominated Anatase TiO2 Nanosheets as

Recommend Documents