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Supporting Information

1. Experimental section

1.1 Materials

The chemical reagents used for the synthesis of 2D ultrathin TiO2 nanosheets were

commercially available reagents. Titanium isopropoxide (TTIP), hydrochloric acid, ethylene

glycol (EG), absolute ethanol and polyethylene oxide - polypropylene oxide - polyethylene

oxide (PEO20-PPO70-PEO20, P123) were purchased from Sigma-Aldrich. All the chemicals

were directly used as received. Commercial Degussa P25 TiO2 nanoparticles with diameters

around 20 nm were also purchased from Sigma-Aldrich.

1.2 Synthesis of 2D ultrathin TiO2 nanosheets

In a typical synthesis of 2D ultrathin TiO2 nanosheets, in bottle A, 1.05 g TTIP was added

into 0.74 g concentrated HCl solution during vigorous stirring; and in bottle B, 0.2 g P123

was dissolved in 3.0 g ethanol. After stirring for 15 min, the solution in bottle B was added

into bottle A and stirred for another 30 min. Then 2.5 mL of the resultant TTIP solution with

20 mL EG was transferred into a 45 mL autoclave and heated at 100-160 oC for 20 h. The

milk-like products were centrifuged and washed with ethanol 3 times, and the white powders

were collected after drying at 80 oC for 24 h. Some powders were redispersed in ethanol by

ultrasonication for further characterization.

2. Characterization

The morphology of the samples was observed with a scanning electron microscope (SEM,

JSM-7500FA, JEOL, Tokyo, Japan). High resolution transmission electron microscope

observations were carried out using a JEM-2011F (HRTEM, JEOL, Tokyo, Japan) operated

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at 200 kV. The thickness of the 2D ultrathin TiO2 nanosheets was determined by atomic force

microscopy (AFM, MPF-3D, Asylum Research, Santa Barbara, USA). For AFM

observations, the nanosheet specimen was prepared by quickly dipping a single-crystal

silicon substrate that a 100 nm thick amorphous SiO2 top layer into the diluted acetone-

dispersed 2D TiO2 nanosheet solution. The phase composition of the as-prepared powders

were characterized with a powder X-ray diffractometer (XRD, MMA, GBC Scientific

Equipment LLC, Hampshire, IL, USA) with Cu K radiation and by a LabRam-HR 800

Raman microspectrometer (HORIBA JOBIN YVON, Longjumeau Cedex, France). The

chemical states of the nanostructures were determined with an X-ray photoelectron

spectroscope (XPS, PHOIBOS 100 Analyser from SPECS; Al Kα X-rays, Berlin, Germany).

3. Solar cell assembly and performance tests

The ultrathin 2D TiO2 nanosheet photoanode was prepared by mixing TiO2 nanosheet

powders with binder in ethanol to form concentrated slurry, followed by coating the slurry on

fluorine-doped tin oxide (FTO) substrates via doctor blade, then sintering at 400 oC for 2 h.

The thickness of the ultrathin 2D TiO2 nanosheet thin film on the photoanodes was controlled

at around 4-8 m after sintering. It is worth pointing out that the prepared 2D TiO2 nanosheet

thin films are easily to be delaminated from the substrates, especially with a relatively large

thickness, so that special care is needed during the photoanode fabrication. Dye absorption

was carried out by immersing the as-prepared thin films in ethanol-based commercial N719

(Sigma-Aldrich) dye solution at 25 oC for 24 h. The solar cells were prepared by assembling

a Pt counter electrode and a dye-adsorbed ultrathin TiO2 nanosheet photoanode, and then

sealing the cell using a Surlyn (Dupont) thermoplastic frame (25 m thick). A commercial

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electrolyte purchased from Solaronix (Iodolyte AN-50, Switzerland) was poured into the

electrode, and the cell was then sealed again.

The photocurrent density-voltage (J-V) characteristic was collected by exposing the solar

cells to air mass (AM) 1.5 simulated sunlight from a solar simulator (PEL-L12, Peccell

Technologies, Japan) combined with a Keithley 2400 source meter. Incident photon-to-

current quantum conversion efficiency (IPCE) was measured as a reference spectrum, using

an optical fiber (3 mm diameter) for monochromatic irradiation (PEC-S20DC, Peccell

Technologies, Japan). Monochromatic photocurrent was monitored by the continuous

irradiation (dc measurement) method.

4. Computational details

Spin-polarized density functional theory calculations were performed using the QUANTUM

ESPRESSO package to calculate the atomic and electronic structures of the coupled N719

molecule and TiO2 nanosheet/bulk systems. The Kohn–Sham (KS) orbitals and the charge

density were represented using basis sets consisting of plane waves (PWs) up to a maximum

kinetic energy of 40 Ry and 400 Ry, respectively. The Brillouin zone was sampled using the

gamma point only for structural optimization and using a 3×3×1 Monkhorst-Pack grid for

electronic structure analysis. The interaction between the nuclei and the electrons was

modelled using ultrasoft pseudopotentials. The exchange-correlation functional was

approximated using the generalized gradient approximation (GGA) in the parametrization of

Perdew, Burke, and Ernzerhof (PBE). The chosen model system consists of a N719 molecule

anchored to a 2×4 TiO2 anatase nanosheet/bulk surface, exposing the (010) surface (Figure 5).

The coupled N719@TiO2 nanosheet /bulk surface system was separated from its periodic

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images in the z direction by a vacuum space of 30 Å. All atoms were fully relaxed and

optimized until the forces were reduced below 2.6 × 10-2 eV/Å.

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Table S1: Core levels of Ti2p and O1s for ultrathin 2D TiO2 nanosheets measured by XPS

References*

Binding energy(eV)

FWHM(eV)

(eV)

Anatase Rutile Ti3+

Ti 2p Ti 2p1/2 463.19 2.52 465.3 465.3 /Ti 2p3/2 457.51 1.93 459.4 459.3 457.5

O 1s 529.64 2.79 530.95*: (a) Gothelid, M.; Yu, S.; Ahmadi, S.; Sun, C. H.; Zuleta, M. Inter. J. Photoenergy , 2011, 110, 1; (b) Erdem, B.; Hunsicker, R.; Simmons,

G.; Sudol, D.; Dimonie, V.; El-Aasser, M. Langmuir 2001, 17, 2664; (c) Kubo, T.; Sayama, K.; Nozoye, H.; J. Am. Chem. Soc. 2006, 128,4074

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Table S2: Photovoltaic performance (short-circuit current density Jsc, open-circuit voltage Voc, Fill factor FF, and conversion efficiency ) of the solar cells with 2D ultrathin TiO2 nanosheet photoanode and the ones with commercial Degussa P25 TiO2 nanoparticle photoanode.

PhotoanodesPhotocurrent density (Jsc/(mA·cm-2)

Photovoltage(Voc/V)

Fill factor (FF)

Efficiency(/%)

2D TiO2 nanosheet-4 m2D TiO2 nanosheet-

15.73 0.68 0.62 6.63

8 m20.41 0.72 0.56 8.28

Degussa P25 nanoparticles 11.21 0.66 0.69 5.12

Figure S1

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Figure S1: Low magnification (a) and high magnification (b) TEM images of the ultrathin2D TiO2 nanostructures.

Figure S2

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Figure S2: HRTEM images of 2D ultrathin TiO2 nanosheet: (a) anatase phase and (b) rutile phase.

Figure S3

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Figure S3: HRTEM images of 2D ultrathin TiO2 nanosheet after heating at 300 oC for 30 min: (a) the centre of nanosheet and (b) the edge of nanosheet.