NiTiO3 Nanorod Arrays for Inorganic Sensitized

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Heterostructured TiO2/NiTiO3 Nanorod Arrays for Inorganic Sensitized Solar Cells with Significantly Enhanced Photovoltaic Performance and Stability Yue-Ying Li, Jian-Gan Wang, Huanhuan Sun, and Bingqing Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17044 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

Heterostructured TiO2/NiTiO3 Nanorod Arrays for Inorganic Sensitized Solar Cells with Significantly Enhanced Photovoltaic Performance and Stability †

†

†

Yue-Ying Li, Jian-Gan Wang, * Huan-Huan Sun, and Bingqing Wei †

†‡*

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials

Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Lab of Graphene (NPU), Xi’an 710072, China ‡

Department of Mechanical Engineering, University of Delaware, Newark, DE19716, USA

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT Organic dyes used in the conventional dye-sensitized solar cells (DSSCs) suffer from poor light stability and high cost. In this work, we demonstrate a new inorganic sensitized solar cell based on ordered one-dimensional (1D) semiconductor nanorod arrays of TiO2/NiTiO3 heterostructures prepared via a facile two-step hydrothermal approach. The semiconductor heterostructure arrays are highly desirable and promising for DSSCs due to their direct charge transport capability and slow charge recombination rate. The low-cost NiTiO3 inorganic semiconductor possesses an appropriate band gap that matches well with TiO2, which behaviors like a “dye” to enable efficient light-harvesting and fast electron-hole separation. The solar cells constructed by the ordered TiO2/NiTiO3 heterostructure photoanodes show a significantly improved power conversion efficiency, high fill factor, and more promising, outstanding life stability. The present work will open up an avenue to designing heterostructured inorganics for high-performance solar cells. KEYWORDS: titanium oxide, nickel titanate, solar cell, heterostructure, nanorod array


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INTRODUCTION Dye-sensitized solar cells (DSSCs) have received ever-increasing research interest as one of the promising solar energy conversion systems owing to the high performance/price ratio.

1-2

Semiconductor nanomaterials of titanium dioxide (TiO2) with different morphologies have been considered to be the most successful photoanode candidates in DSSCs in the past decades.

3-5

It is

noted that the charge transport and injection efficiency of the TiO2 photoanodes play an important influence on the photoconversion efficiency (PCE) of DSSCs. 6 Among various TiO2 nanostructures, ordered one-dimensional (1D) arrays of nanotubes, nanorods, and nanowires have been demonstrated to be promising for DSSCs owing to their capability of offering direct electron pathways for efficient charge transfer and collection.

7-11

In addition, the ordered and vertically aligned nanostructures

facilitate light harvesting through multiple internal reflections between adjacent building blocks.

12

Furthermore, the 1D TiO2 nanostructures could construct unique three-dimensional (3D) architecture for easy electrolyte ingress/diffusion. 13 Since TiO2 semiconductor is only active in light adsorption of ultraviolet (UV) owing to its large band gap of 3.2 eV, light sensitizer has become an indispensable component of a typical DSSC, which has a significant influence on the photovoltaic properties. The most commonly used light sensitizer is ruthenium (Ru) organic dye (N719), which, however, suffers from serious problems of high toxicity, complex purification process, expensive, and inferior light stability.

14-15

To address

these issues, one of the promising approaches is to develop a low-cost inorganic sensitizer having good optical absorption as well as excellent chemical stability, as an alternative to the organic N719. 16-21

Inorganic semiconductor quantum dots, such as Cd (Se, S, Te) and Pb (S, Se), have been widely

investigated owing to their good spectral match with the incident solar light.


20-21

However, these


Page 4 of 24

materials contain toxic elements of Cd or Pb and are of low abundance on earth. To this end, it is highly necessary to search for an inexpensive and eco-friendly inorganic sensitizer with a narrow band gap and suitable band structure. Semiconducting perovskites are considered to be an exciting class of visible-light photoactive materials in recent years.

16, 20

Recently, structurally stable inorganics of titanium-based perovskite

oxides of (MTiO3, M = Ca, Ba, Sr, and Ni, etc.) have found widespread applications in the photocatalytic field.

22-27

Of particular note, NiTiO3 (NTO) possesses a narrow band gap of 2.18 eV

with appropriate band structure to enable high photo-response within the visible light spectra.

26-27

Additionally, the NTO material shows prominent advantages of low cost, environmental friendliness, abundant on earth, and good chemical stability. It is, therefore, of great interest to hetero-combining NTO with 1D TiO2 nanostructures to achieve high photovoltaic performance. In this study, we have fabricated an ordered 1D nanorod array of TiO2/NTO heterostructures through a facile two-step hydrothermal process. The NTO is demonstrated to be an excellent inorganic sensitizer to substantially improve the capability of light harvest and charge separation of the TiO2 photoanode. The solar cells assembled with the heterostructured TiO2/NTO nanorod arrays exhibit a significantly enhanced PCE of 1.66 % and outstanding life stability. RESULTS AND DISCUSSION Figure 1a illustrates the typical two-step hydrothermal fabrication processes of heterostructured TiO2/NTO nanorod arrays. Quasi-vertically aligned TiO2 nanorod array is hydrothermally deposited grown on a conductive FTO glass through the process (I). The diameter of the TiO2 nanorods is ranging from 150 to 200 nm (Figure 1b), and the length is estimated to be about 3 µm according to the cross-section view (Figure S1a, Supporting Information). The subsequent hydrothermal treatment


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(II) enables the formation of NTO nanostructures through a dissolution-precipitation process

29

,

which can be written as: TiO2+2H2O→Ti(OH)4

(1)

Ni2+ + Ti(OH)4 → NiTiO3 + 2H+ + H2O

(2)

(a)

(b)

(c)

(d)

(e)

Figure 1. (a) Schematic illustration for the fabrication of TiO2/NTO; FE - SEM images of (b) TiO2 and (c) TiO2/NTO; (d)) TEM and (e) HR-TEM images of TiO2/NTO. The top view of the resulting TiO2/NTO in Figure 1c exhibits hybrid nanorod arrays without



noticeable morphology change. It is noted that the film color evolves from gray to yellow (inset), suggesting a significant difference in the light absorption. The energy-disperse X-ray (EDX) spectra (Figure S2) and the corresponding elemental mappings of a single hybrid nanorod (Figure S3) manifest the existence and homogeneous distribution of Ni, Ti, and O elements. The microstructure of the TiO2/NTO nanorod scraped from the FTO substrate is further investigated by transmission electron microscopy (TEM) imaging. As shown in Figure 1d, the TiO2/NTO is typical of nanorod morphology with a diameter of about 190 nm, which is in line with the SEM observation. It is noted that the NTO nanoshells are uniformly sheathed onto the TiO2 cores, constructing unique core-shell configuration with clear interfaces. Figure 1e exhibits the high-resolution TEM (HRTEM) image of a TiO2/NTO nanorod. The lattice distances of 0.32 and 0.29 nm correspond to the planes of (110) and (001) of the rutile phase, respectively. 23 In addition, the lattice fringes at the nanorod edge display a clear distance of 0.25 nm, which belongs to the (110) crystal plane of NTO, evidently indicating the formation of NTO. 26 The crystallographic structures of the pristine TiO2 and TiO2/NTO hybrid are investigated using X-ray diffraction (XRD), as shown in Figure 2a. For the pristine TiO2 nanorods, three well-defined XRD diffraction peaks centered at around 27.4°, 36.1°, and 54.5° are characteristic of the rutile phase, which are readily indexed to the crystal planes of (110), (101), and (211) (JCPDS # 21-1276). After the subsequent hydrothermal treatment, a new set of diffraction peaks appear at 2θ = 41.2° and 43.8°, which correspond well to the (113) and (202) planes of the hexagonal NTO (JCPDS # 33-0960). Raman spectroscopy provides more insights into the structural information of the TiO2/NTO heterostructures. As shown in Figure 2b, three Raman peaks of the TiO2 nanorods are observed with typical bands locating at 144, 445, and 612 cm-1, which can be assigned to the B1g, Eg,


Page 6 of 24

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and A1g vibrational modes of rutile TiO2, respectively. characteristic of a multiphonon peak of rutile phase.

32

30-31

The broadband at 233 cm-1 (Eg) is the

A small Raman peak is observed at around

706 cm-1 in the TiO2/NTO heterostructures, which can be well ascribed to the NTO phase with hexagonal structure. 33

(a)

♦ NTO • TiO2

• •

♦

TiO2/NTO

♦ •♦ •

♦

(b) • Intensity (a.u.)

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


TiO2

•

•

♦ NTO • TiO2

• ♦

TiO2/NTO

TiO2 10

20

30

40

2 theta (deg.)

50

60

200

400

600

800

1000

Raman shift (cm-1)

Figure 2. (a) XRD patterns and (b) Raman spectra of TiO2 and TiO2/NTO. X-ray photoelectron spectroscopy (XPS) technique was performed to probe the surface chemistry and the elemental states of the TiO2/NTO hybrids. The survey spectrum in Figure 3a demonstrates the existence of elemental Ti, Ni, and O components. The core-level Ti 2p XPS spectrum of the TiO2/NTO nanorods (Figure 3b) exhibits shoulder peaks at 458.4 and 464.1 eV, which belong to the Ti 2p3/2 and Ti 2p1/2, respectively. The binding energies of Ti 2p having a typical spin-energy separation of 5.7 eV is featuring the Ti4+ species in the rutile TiO2.

10

Figure 3c shows

the core-level O 1s spectrum, which is decomposed of two subpeaks at 529.6 and 531.3 eV, corresponding to the Ti-O bonds in the lattice oxygen and the hydroxyl Ti-OH groups on the surface, respectively.

34

Figure 3d presents the core-level XPS spectrum of Ni 2p in TiO2/NTO. The

characteristic peaks at 856.2 and 874.2 eV can be attributable to the binding energies of Ni 2p3/2 and Ni 2p1/2, respectively.

33

It is worth to note that, compared with the pristine TiO2 and pure NTO, the



binding energies of both Ti 2p and O 1s show a negative shift, whilst the binding energies of the Ni 2p have a positive shift. The binding energies with negative/positive shifts indicate that the TiO2 and

(b) Intensity (a.u.)

Ti 2p

Intensity (a.u.)

C 1s

O 1s

(a)

Ni 2p

NTO have strong electronic interactions. 30, 35

TiO2/NTO

TiO2

Ti 2p Ti 2p3/2 458.4 eV Ti 2p1/2 464.1 eV

TiO2/NTO

464.3 eV

300

450

600

750

900

456

Binding energy (eV) Ti-O 529.6

TiO2/NTO

460

464

468

Binding energy (eV) O 1s

(d)

Ni 2p Ni 2p3/2

Ni 2p1/2

855.0

Ti-OH 531.3

Intensity (a.u.)

(c)

529.7 531.4

TiO2 528

458.6 eV

TiO2

150

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

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Satellite peak

872.8

Statellite peak

NTO

856.2

874.2

TiO2/NTO

530

532

Binding energy (eV)

534

852

858

864

870

876

882

Binding energy (eV)

Figure 3. (a) Full scan XPS spectra of TiO2/NTO and TiO2; Core-level XPS spectra of (b) Ti 2p, (c) O 1s and (d) Ni 2p. Solar cells based on the TiO2 and heterostructured TiO2/NTO nanorod arrays were assembled to evaluate the photovoltaic performance. Figure 4a displays the resulting current density-voltage (J-V) curves of the solar devices. The corresponding photovoltaic performance parameters of the cells are listed in Table 1. The TiO2/NTO-based device manifests an outstanding photovoltaic conversion efficiency (PCE) of 1.66 %. More specifically, the open-circuit voltage (Voc), the short-circuit current


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density (Jsc)

and the fill factor (FF) of the device are 660 mV, 3.50 mA cm-2, and 72 %,

respectively. For comparison, the pure TiO2-based device shows much inferior performance of Voc = 630 mV, Jsc = 1.68 mA cm-2, FF = 71 %, and PCE = 0.75 %. It is worth to mention that the heterostructured TiO2/NTO-based cell is not only superior to the pure TiO2-based cell, but also surpasses many previously-reported inorganic sensitized solar cells, such as BiFeO3, MOF,

18-19

quantum dots,

36-37

16

MoS2,

17

and even some organic N719-sensitized TiO2 nanorods-based solar

cells (details see the Table 1). 38-40 Incident photon-to-electron conversion efficiency (IPCE) spectra of the cells assembled by TiO2 and TiO2/NTO were conducted to investigate the capability of NTO on light utilization. It is evident from the Figure 4b that the heterostructured TiO2/NTO gives rise to a substantial improved IPCE in the spectral region of 400-550 nm, suggesting that the introduction of NTO improves the light utilization. UV-vis absorption is further measured to confirm the improved light harvesting. As displayed in Figure S4, the light absorption range of the heterostructured TiO2/NTO extends to the visible region with substantially enhanced absorption intensity. The improved light harvesting capability demonstrates that the NTO can function as “light sensitizer” to yield a higher Jsc. Electrochemical impedance spectra (EIS) was employed to obtain an in-depth understanding of the enhanced photovoltaic performance of the TiO2/NTO-based solar cell. Figure 4c compares the resulting Nyquist plots of the two devices. It is observed that the plots are composed of two semicircles corresponding to the resistance of the electrolyte/counter electrode interface (Rpt) within the high-frequency range and the charge transfer resistance (Rct) at the interface of electrolyte/photoanode in the lower frequency range, respectively.

1-2, 41

The corresponding

electrochemical parameters can be simulated by fitting the resulting EIS curve using the equivalent



circuit model (Figure S5). As shown in Figure 4c and Table 2, both DSSC devices show similar Rpt value (20 Ω) owing to their identical electrolyte/counter electrode. In sharp contrast, the Rct of the TiO2/NTO device is smaller (66.7 Ω) than that of the TiO2 device (72.2 Ω), indicating faster charge transfer kinetics and a reduced recombination rate.

(b)60

TiO2/NTO

TiO2/NTO 3

TiO2

50

IPCE (%)

Current density (mA cm-2)

(a)

2 TiO2

40 30 20

1 10

0 0.0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400

(c) 24

500

600

700

800

Wavelength (nm)

Voltage (V)

(d)

TiO2/NTO

TiO2/NTO

16

Phase (theta)

TiO2

18

-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

Page 10 of 24

12

TiO2 12

8

6 4

0

40

60

80

Z' (ohm)

100

120

101

102

103

104

105

Frequency (Hz)

Figure 4. (a) J - V curves, (b) IPCE characteristics, (c) EIS and (d) Bode plots of the solar cells based on TiO2 and TiO2/NTO. Additionally, EIS was conducted to reveal the electron lifetime (τn) in the process of the photo-to-electron conversion. Figure 4d shows the resulting bode phase plots based on the equation of τn = 1/(2πfmax), in which fmax denotes the corresponding frequency when the phase angle reaches the maximum value. 42 The τn value of photoanodes based on TiO2 and TiO2/NTO are determined to be 1.07 and 1.66 ms, respectively (Table 2). The prolonged τn will lead to a slower interfacial


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electron recombination and higher trapping density, which can be further validated by Mott-Schottky plots (Figure S6).

43

Compared to the pristine TiO2, the TiO2/NTO possesses a smaller slope and a

higher carrier density, leading to the upshift of the TiO2 quasi-Fermi level and the increase of Voc. Meanwhile, the dark current (Figure S7a) and photoluminescence (PL) spectra (Figure S7b) were measured to evaluate the charge recombination process. The results reveal that the TiO2/NTO-based device could achieve more efficient charge separation and less electron recombination than the TiO2 based cell, which agrees well with the EIS analysis and thus is responsible for the significant increase of Jsc. Table 1. Comparison of photovoltaic performance based on TiO2 with different sensitizers. Device configuration

Jsc (mA cm-2)

Voc (V)

FF (%)

PCE (%)

Ref.

FTO/TiO2/MoS2//I--I3-/Pt-FTO

3.55

0.68

45

1.08

16

FTO/TiO2/BiFeO3/NiO/Au

0.51

0.67

55

0.19

15

FTO/TiO2 - MOF/I- - I3-/Pt - FTO

0.44

0.48

55

0.12

17

FTO/TiO2/TiO2 + MWCNT/Cu-MOF/I-

1.95

0.48

51

0.46

18

4.95

0.50

42.0

1.04

35

7.56

0.56

24.5

1.03

36

FTO/TiO2/N719/I- - I3-/Pt FTO

4.08

0.67

34

0.93

37


3.32

0.64

72

1.32

38


2.51

0.68

53

0.90

39

FTO/TiO2/I--I3-/Pt - FTO

1.68

0.63

71

0.75

Our study

FTO/TiO2-NTO/I--I3-/Pt - FTO

3.50

0.66

72

1.66

Our study

- I3-/Pt FTO FTO/TiO2/CdS QDs/polysulfide electrolyte/Pt FTO FTO/TiO2/CdSe QDs/polysulfide electrolyte/Pt FTO

Table 2. Simulative values of resistance from EIS spectra calculated by equivalent circuit Sample

RS [Ω]

Rpt [Ω]

Rct [Ω]

Frequency [Hz]


τn [ms]


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TiO2

38.5

19.6

72.2

148.7

1.07

TiO2/NTO

38.6

21.1

66.7

95.7

1.66

The charge transfer pathways and working mechanism of the solar cells sensitized with NTO were investigated through band structure analysis. The optical band gaps (Eg) of TiO2 and NTO are determined using the Tauc plot ((Ahν)1/2 = hν-Eg) (Figure 5a), where the h, ν, A, and Eg denote Planck’s constant, the light frequency, the absorbance, and the band gap energy, respectively.

44-45

The Eg of TiO2 and NTO are characterized as 2.95 and 2.14 eV, respectively. Mott-Schottky (MS) theory is commonly used to evaluate the flat-band potential.

46-47

As shown in Figure 5b, the

flat-band potentials corresponds to -0.28 and -0.37 V vs. NHE for the TiO2 and NTO, respectively. Accordingly, the valance band (VB) positions of TiO2 and NTO can be determined to be 2.67 and 1.77 V vs. NHE on the basis of the band gap energy, respectively.

48

The VB positions of TiO2 and

NTO were further validated by the VB XPS. As exhibited in Figure S8, the VB of TiO2 and NTO are determined to be 2.56 and 0.74 eV, respectively. The higher VB potential of TiO2 relative to NTO is in good agreement with the Mott-Schottky results. Consequently, the energy band illustration of TiO2 and NTO can be schematically drawn in the Figure 5c. The energy bands of the NTO have a good alignment with the TiO2 to construct a heterojunction. Upon excitation of NTO and TiO2 by light illumination, the heterojunction promotes the photogenerated electrons of the conduction band (CB) migrating from NTO to

TiO2 and a simultaneous flow of holes from TiO2 to NTO. The separate

electron/hole transport across the junction to different sites could facilitate a reduced recombination of electron-hole carriers and an enhanced charge separation efficiency. And finally, the electrons are collected by the current collector (FTO) while the holes are reduced by the I-/I3- couples. Life stability is a critical point for the potential implementation of solar devices. The stability of


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the inorganic NTO and the control organic N719 sensitized TiO2 nanorods-based solar cells without encapsulation are evaluated under ambient conditions for comparison. Figure S9 displays the resulting J-V profiles and Table S1 summarize the corresponding photovoltaic parameters. Figure 5d exhibits the PCE retention of both cells upon the duration time. Encouragingly, the NTO-sensitized solar cell manifests excellent life stability with 81.7 % of PCE retention after 35 days. In sharp contrast, the organic N719 dye-sensitized cell shows a substantial PCE degradation of 67.2 % under the same conditions. The comparison indicates that the inorganic NTO possesses superior ambient stability than the organic N719 dye under light illumination condition.

(a)

(b)

TiO2

1.8

NTO

C-2 (F-2 ×105) 2.0

TiO2

8.0

NTO

(Αhν)1/2 ((eV)1/2)

2.2

2.4

2.6

2.8

6.0

4.0

2.0

0.0 -0.5

3.0

-0.4

hν (eV)

-0.3

-0.2

-0.1

0.0

Potential (V vs NHE)

(d)100 PCE retention (%)

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

(II) 60

40

(I)

TiO2/NTO

20

TiO2-N719 dye 0

0

5

10

15

20

25

30

35

Duration (days) Figure 5. (a) Tauc plots and (b) Mott - Schottky curves of TiO2 and NTO; (c) schematic diagram of



Page 14 of 24

the electronic band structures, and (d) PCE retention as a function of time for solar cells sensitized with (I) organic N719 dye and (II) NTO. CONCLUSIONS In summary, ordered 1D nanorod arrays of NiTiO3@TiO2 heterostructures have been successfully grown on the FTO substrate by a facile hydrothermal approach. The NiTiO3 with suitable band gap can function as an ideal inorganic dye to facilitate the light harvest and electron migration of the TiO2 photoanode. As a result, the heterostructured NiTiO3@TiO2-based solar cells manifest a significantly enhanced PCE of 1.66 %, which is 2.2 times larger than that based on the bare TiO2. Moreover, the inorganic sensitizer bestows the cell with excellent life stability. The present study provides a new design strategy of incorporating eco-friendly and low cost inorganic semiconductors into 1D nanostructure arrays for high-performance photovoltaic devices. METHODS Materials Synthesis. Conductive FTO was used as the substrate for the growth of TiO2 nanorod arrays through a hydrothermal route according to the previous study.

13

Briefly, the FTO substrate

was loaded into a sealed Teflon-lined stainless-steel autoclave, which is filled with a mixture solution of titanium n-butoxide (AR, Macklin, 0.5 ml), deionized water (12 ml) and hydrochloric acid (AR, 36.5 %-38 wt.%, Sinopharm, 12 ml)). The autoclave was placed in a furnace and heated to 150 °C for an incubation time of 5 h. When the autoclave was cooled naturally, rutile TiO2 nanorods were grown on the FTO substrate, followed by water rinse for three times. Heterostructured TiO2/NTO nanorods were fabricated by a second hydrothermal treatment. In a typical process, the as-grown TiO2 nanorods-FTO substrate was placed into a solution of 0.2 M Ni(CH3COO)2 (AR, 99%, Aladdin), which was then sealed in a Teflon-lined stainless-steel autoclave for hydrothermal treat at 200 °C for


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6 h. The as-prepared samples were rinsed with deionized water, followed by an annealing treatment of 600 °C/2 h. Device Assembly. The solar cells were assembled according to the Grätzel model, which employed the as-prepared TiO2/NTO nanorods/FTO and a Pt-modified FTO glass as the working electrode and counter electrode, respectively. The liquid I-/I3- redox solution was used as the electrolyte. All devices were not specifically encapsulated for the following characterization. Materials Characterization and Photovoltaic Evaluation. The crystal structure was examined through powder XRD technique (X’Pert PRO). The microstructure was characterized by a TEM (FEI Tecnai F30G2) and the morphology was observed by SEM (FEI Nano SEM 450) equipped with EDX system. Raman microscopy (Renishaw Invia) was employed to record the Raman spectra using a laser wavelength of 532 nm at the ambient temperature. Photoluminescence (PL) spectra were obtained from a PL spectrometer (FLS 980). The valence states of the elements were obtained using a Physical Electronics PHI-5802 XPS equipment in an ultrahigh vacuum environment. The contaminant carbon (C 1s) of 284.8 eV was used as a reference to calibrate the binding energies. The absorption spectra were measured via ultraviolet-visible (UV-vis) spectrophotometer (Perkin-Elmer Lambda 35 UV-VIS-NIR). J-V measurements were tested using a Keithley digital source meter at 25 °C (2420 model), which is modulated by Test point software under an Xenon lamp (AM 1.5G, 100 mW·cm-2). The light intensity in the range of 350-700 nm was calibrated by a standard solar cell to minimize the mismatch of the simulated light to < 2% relative to the standard AM 1.5G solar light. The effective area of the working electrode was controlled to be 0.25 cm2 by wrapping a black mask onto the electrode. Five devices have been tested of each set. IPCE was recorded using monochromatic



Page 16 of 24

incident light offered by a Newport xenon lamp (2936-R, 300 W). EIS was obtained at the open-circuit voltage under light irradiation on the CHI660C electrochemical workstation (Shanghai, China) in the frequency region of 0.1 to 105 Hz with a perturbation amplitude of 10 mV. The resulting Nyquist plots were fitted by a ZSimDemo software using an equivalent circuit.

28

Mott-Schottky measurements were carried out in a typical configuration of three-electrode cell using TiO2 or NTO film as the working electrode,, Pt plate as the counter electrode, and Ag/AgCl as the reference electrode. All measurements were carried out under dark condition at ambient temperature. Supporting Information Additional data, including: Cross section images; EDS spectrum; Elemental mapping image of a single TiO2/NTO nanorod; UV - vis absorption spectra; Equivalent circuit for fitting the Nyquist plots; Mott-Schottky plots; dark J - V curves of solar cells; PL spectra; VB-XPS plots; J-V curves of solar cells after different durations; Photovoltaic parameters of sensitized solar cells after different times duration. Corresponding Authors *E-mail: [email protected] (J.-G. Wang); [email protected] (B. Wei) ACKNOWLEDGMENTS The authors would like to thank the funds of the National Natural Science Foundation of China (51772249, 51472204, 51521061), the Hong Kong Scholars Program (XJ2017012), the Fundamental Research Funds for the Central Universities (G2017KY0308), the Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University, and the Program of Introducing Talents of Discipline to Universities (B08040). We also appreciate the Research Fund from the State Key Laboratory of Solidification Processing (NWPU, 123-QZ-2015) and Control and Simulation of


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Power System and Generation Equipment (Tsinghua University, SKLD17KM02) ) ,

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NiTiO3 Nanorod Arrays for Inorganic Sensitized

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