sensitized solar cells. TiO for high-photoactivity in dye- Large-area(001) faceted anatase

Large-area (001) faceted anatase TiO ₂ for high-photoactivity in dye-

sensitized solar cells.

Akrajas Ali Umar and Siti Khatijah Md Saad Institute of Microengineering and

Nanoelectronics, National University of Malaysia

Outline

• Introduction

• Photophysical process in (001) faceted anatase TiO2.

-Charge transfer properties.

-Transport properties.

-Exciton dynamic (Carrier recombination properties, Electron lifetime).

-V_oc properties.

-Light-scattering effect.

• Synthetic Approach.

• Future outlook.

Journal of Materials Chemistry A 2015, 3(10):5692-5700.

DSSC system

(001) Face of anatase

Selloni, Nature Materials 7, 613 - 615 (2008); Phys. Rev. B 2002, 65,119901.

(001) (101)

The case of Anatase surface

The surface energy () follow the

order of: (110) > (001) > (100) > (101)

(101) (100) (001) (110)

 (J m^-2) 0.44 0.53 0.90 1.09

(110) face diminish during the growth. (101) faces are thermodynamically stable, highly abundant (typically around 95% of the

crystal surface), and nonreactive.

Observable highest energy surface is (001).

Photophysical process

Charge transfer properties

LDOS on the surface

E is LUMO energy injection

Surface-dye coupling

Where ^𝛾 is surface energy function, influenced by properties of surface atom

High-energy facet promises better coupling with dye and facile charge transfer/injection, adiabatic.

Two-state injection

model. (¹MLCT) injects

electrons to the conduction band of TiO₂

Single-crystalline ultrathin 2D TiO2 nanosheets: A bridge towards superior photovoltaic devices. Materials Today Energy 3 (2017) 32e39

Enhanced carrier transport via oriented- attachment.

Minimum interface resistance: Efficient oriented-attachment process.

Naturally Aligned Nanocrystals.Science 2000, 289, 736-737

highly efficient oriented attachment effect between the high-energy facets,

minimizing grain interface effect for efficient carrier transport

Three fundamental direct optical excitation in anatase TiO₂

• Wannier–Mott and Frenkel regimes in nature.

• Displays a peculiar two- dimensional wavefunction in the three-dimensional lattice.

• Screen Coulombic interaction, for long- lifetime.

Excitonic property

Lowest direct excitation (exc I)

Nature Communicationsvolume 8, Article number: 13 (2017)

ARPES and 2D transient UV spectroscopy reveal the

existence of bound exciton of direct excitation in the (001) faceted anatase TiO2

3.79 eV

Enhanced-Electron Lifetime

Xiong He, Xin Li a, Menghua Zhu Journal of Power Sources 333 (2016) 10e16

Reduced Recombination

Rapid Electron Injection in Nitrogen- and Fluorine-Doped Flower- Like Anatase TiO2 with {001} Dominated Facets and Dye-Sensitized Solar Cells with a 52% Increase in Photocurrent Jia Yu, Yulin Yang,* Ruiqing Fan,* Liang Li, and Xinyuan Li. J. Phys. Chem. C 2014, 118, 8795−8802

V

_oc

Properties

E_fb = E₀ – 0.059pH

Optical absorbance (640 nm) of TiO2 films nanosheets vs applied electrochemical potential in 0.2 M

NaClO4. Trap charge density defined via transient photocurrent/

photovoltage of DSSC with electrolyte of LiI(red), NaI(blue), and LiIm(green) NS (filled) NP (opened)

V

_oc

Properties

Light Scattering properties

Hierarchically assembled microspheres consisting of nanosheets of highly exposed (001)-facets TiO2 for dye-sensitized solar cells† Jia-De Peng,a Hsi-Hsin Lin,a Chi-Ta Lee,a Chuan-Ming Tseng,b V. Suryanarayanan,cR. Vittala and Kuo-Chuan Ho*ad RSC Adv., 2016, 6, 14178

Remarks

• (001) faceted anatase TiO2 facilitates facile charge transfer/injection (via effective dye-surface

coupling).

• Strong photocurrent, long life time, limited

recombination due to strong exciton bond and two- dimensional excitonic wavefunction.

• Facile transport via 2D exciton and effective oriented attachment.

• Higher Voc due to less trap density

• Etc.

How to grow?

Key surfactants:

Amines, thiol, carboxylic, phosphine, sulphate, etc.

(i) Surfactant effect, surfactant molecule has functional ligand that is easily

bounded to crystallite surface via metal-ligand bonding. (Passivating the attached- surface).

(ii) Depending on the kinetic condition of the reaction, different crystallization and growth behaviour can be projected.

(iii) At certain concentration, the surfactant molecules may self- assemble each other forming supra-structure with varieties of shapes, such as spherical, rod, etc., thanks to their amphiphilic

behaviour which may function as scaffold for nanostructure growth.

This concentration is called as critical micelle concentration (cmc).

(iv) The morphology of micelles can be modified by using binary, ternary or quarternary surfactant system.

What happens when more than one surfactant present?

• Surface passivation effect with more dynamic attachment due to different chemical properties.

• Short- and long-interaction amongst the surfactant may induce anisotropic and exotic growth.

• In more extreme case, they may modify the nature of atom-atom bonding in the nanocrystals, such as change from non-directional to direction bonding in metals, leads to allotropism or polymorphism in the nanocrystals.

(001) Faceted-anatase

Tio ₂ : Recent reported

results

TiO₂ hollow box obtained by decomposition of TiOF₂ cubes from one-step solvothermal reaction of tetrabutyl titanate, HF & acetic acid

{001} facets >83%

High photocatalytic H₂ evolution rate of 7.55 mmol g¹h¹

TiO₂nanosize (30–85 nm) prepared using Ti(SO4)2 in HF hydrothermally treated at 180 oC

{001} facets >17%

High photocatalytic activity due TiO₂ in nanosize compared to micron size TiO₂with 72%

of {001}

TTIP, [bmim][BF 4 ] and Hac autoclave for 24 h at 200 oC

{001} facets >83% with S_BET 53.8 m 2 g − 1

normalized photoreactivity of clean cuboids is ≈ 3 times higher

Ti powder in HF autoclave for 10 h at 120 oC

Flower-like TiO₂ nanostructures with exposed {001} facets

Ti metal foil in 0.5% HF solution(pH = 1.3) at 180 oC for 3 h

Microsphere TiO₂(16.5 mm) exhibit J_scof 15.46 mA cm²and V_oc of 729 mV

FF of 70.2% and η of 7.91%

Acetic acid added in TiF4–DEG stirred for 3 h at room temp., then autoclave for 8 h at 180 oC

S_BET of 63.5 m² g^-1 and 90% {001} facets

J_scof 17.9 mA cm², V_oc of 0.65 and η of of 7.51%

TIP in DETA and IPA stirred for 10 min heated then 200 °C for 24 h

S_BET of 245.1 m² g^-1 and nearly 90% {001}

facets

J_sc of 13.1 mA cm², V_oc of 0.76 and η of of 6.01%

Ti(OC₄H₉)₄ and HF autoclaved mixed at room temp. and later keep at 180 C for 24 h

S_BET of 44 m² g^-1 and nearly 75% {001} facets

J_sc of 12.5 mA cm², V_oc of 0.58 V with FF of 0.63 and η of of 4.56 %

Mesoporous TiO2 single crystals delivering enhancedmobility and optoelectronic device performance Edward J. W. Crossland1, Nakita Noel1, Varun Sivaram1, Tomas Leijtens1, Jack A. Alexander-Webber1 & Henry J. Snaith 2 0 1 3 | VO L 4 9 5 | N AT U R E | 2 1 5

Template assisted of anatase {001}

Crystal Facets growth

Surface fluorination effect

1. Surface fluorination is also

assumed as the driving factor for

the formation of {001} faces.

Synthetic method

TiO2 microtablet

b

D

D

100 nm

d C

1 µm

a

Growth condition over time

^{A B}

B

D C

D

1 h

15 h

A. A. Umar,, et al. Sol. Energ. Mater. Sol. Cell. 122 (2014)174-182

POROUS TIO2 NANOWALL via replacing the F scavenger with dopant precursor

Md Saad, S.K., Umar A A et al., Porous (001)-faceted Zn-doped anatase TiO2

nanowalls and their heterogeneous photocatalytic characterization. RSC Advances, 2014. 4(100): p. 57054-57063.

T i

Z n

Reaction: potassium hexachloro titanate, Hexamethylenetetramine and zinc nitrate.

1 mm

500 nm 100 nm

2.5 nm 20 nm

10 nm

200 nm

Nanobelt of (001) anatase TiO

₂

10 nm Reaction: potassium hexachloro titanate,

Hexamethylenetetramine and boric acid.

Summary and Outlook

• Two-dimensional excitonic character in (001) faceted anatase TiO2 promises peculiar

photophysical process for outstanding performance in applications.

• Preparing single-crystal, ultimate thin (001) faceted anatase TiO2 nanostructures, resembling 2D atomic crystal properties, may further enhances the

existing properties.

Acknowledgment

• SMIC organizing committee.

• Ministry of Higher Education of Malaysia.

• Ministry of Science, Technology and Innovation

• Universiti Kebangsaan Malaysia.

• Postgraduate students and postdoctoral fellows.

Thank you for your attention