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Supplementary Information

High-performance self-powered ultraviolet photodetectors based on

mixed-dimensional heterostructure arrays formed from NiO

nanosheets and TiO2 nanorods

Rui Cao,a Jianping Xu,*b Shaobo Shi,c Jing Chen,a Ding Liu,a Yichen Bu,a Xiaosong Zhang,a Shougen Yin*a,

and Lan Li*a

a. School of Materials Science and Engineering, Key Laboratory of Display Materials

and Photoelectric Devices, Ministry of Education and Tianjin Key Laboratory for

Photoelectric Materials and Devices, Tianjin University of Technology, Tianjin

300384, China.b. School of Science, Tianjin University of Technology, Tianjin 300384, China.c. School of Science, Tianjin University of Technology and Education, Tianjin 300222,

China.

*E-mail addresses: xjp0335@163.com sgyin@tjut.edu.cn lilan@tjut.edu.cn

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry C.This journal is © The Royal Society of Chemistry 2020

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ExperimentalPreparation of TiO2 NRs: TiO2 NRs were grown on FTO substrates by hydrothermal

method. After cleaned by deionized water, acetone, isopropanol and absolute ethanol, FTO

surface is treated by UV/ozone to remove the organic matter. FTO was put into the autoclave

with the conductive surface facing down in the transparent precursor solution for TiO2 NRs

including 15 ml H2O, 15 ml HCl and 0.5 ml C16H36O4Ti. After the growth at 170 °C for 3 h,

TiO2 NRs film was rinsed with deionized water followed by the annealing process in ambient

air at 450 °C for 2 h.

Preparation of mixed dimensional 2D/1D heterojunctions: NiO nanosheets were

deposited on TiO2 NRs films by hydrothermal method. This process consists of the following

three parts: (i) 1 mmol NiSO4·6H2O, 2 mmol NH4F and 5 mmol CO(NH2)2 were dissolved in

100 ml high purity water under stirring for 20 minutes. The homogeneous transparent solution

was transferred into the autoclave with the TiO2 NRs facing down. NiO nanosheets were grown

at 105 °C for 3 h, 5 h and 7 h. After rinsed with deionized water, NiO nanosheets were annealed

in ambient at 350 °C for 2 h. (ii) NiO nanosheets with different precursor concentrations were

grown for 3 h. The molar ratio of NiSO4.6H2O: NH4F: CO(NH2)2 were kept 1:2:5. The molar

amount of NiSO4.6H2O is 0.6 and 0.2 mmol. (iii) The thick NiO nanosheet layers with the film

thickness (~1.8 µm) was obtained using 1 mmol NiCl·6H2O, 2 mmol NH4F and 5 mmol

CO(NH2)2 grown for 3 h. The thin NiO nanosheet layers with the same film thickness were

obtained using NiSO4.6H2O grown for 7 h. The NiO nanosheets were grown on the FTO

substrates under the above experimental conditions.

Preparation of the device: Au metal with a thickness of about 40 nm was sputted on NiO

nanosheets/TiO2 NRs. The working area is approximately 0.04 cm2. The preparation process of

devices is shown in Fig. S1.

Characterization Methods: The morphologies of the samples were characterized by a field-

emitting scanning electron microscopy (FESEM, Hitachi S-8010). X-ray diffraction (XRD,

Rigaku, 2500 V/PC) patterns and Raman spectra (RENISHAO in Via Raman Microscope)

investigated the crystal structure. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB-

250) was used to analyze the chemical state of the element and Fermi level. Absorption spectra

of samples were obtained using a UV-vis spectrometer (Hitachi UV 4100). The electrochemical

impedance spectroscopy (EIS) and Mott-Schottky (M-S) analysis were performed via the

electrochemical workstation (CHI660D, Shanghai, China).

Measurement methods: A Keithley 2450 Source Meter was used to measure steady-state

and transient photocurrent. LED-365 nm lamp and Hitachi F-4500 provide the monochromatic

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light. Transient photocurrent (TPC) was measured using a Tektronix MSO 2012B Mixed Signal

Oscilloscope to determine the response (rise and decay) time of the self-powered UV PDs.

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Fig. S1 Preparation process of mixed-dimensional heterostructure arrays formed from 2D NiO

nanosheets and 1D TiO2 nanorods.

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Fig. S2 (a)-(c) SEM images of NiO nanosheets grown for different time. (d)-(f) SEM images

of NiO/TiO2 heterojunctions with NiO nanosheets grown in different concentrations solution.

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20 40 60 804.8k

5.2k

5.6k

6.0k

6.4k(a)

(111

)

(200

)

XRD

Inte

nsity

(a.u

.)

2 Theta (Degree)

(220

)

NiO

1k 900 600 300 00

200k

400k

600k

NiO(3h)

NiO(5h)

NiO(7h)

(b)

Ti 2

p

Ni 2

s

Ni 3

s

XPS

Inte

nsity

(a.u

.)

Binding Energy (eV)

C 1s

O 1

s

OKL

L

Ni 3

pLine

sAu

ger

NiNi

2pTiO2

2.0 2.5 3.0 3.5 4.00

1

2

3

4 (c)

3.0 eV

TiO2

(F(R

)h)1/

2 (eV

nm-1)1/

2

h (eV)-1.0 -0.5 0.0 0.5 1.00

200

400

600 (d)

1/C2 (

109 F-2

cm4 ) p-NiO

Potential (V) vs Ag/AgCl

NiO/TiO2

n-TiO2

Fig. S3 (a) XRD patterns of NiO nanosheets grown on glass substrate. (b) The full XPS spectra

for NiO nanosheets and TiO2 nanorods. (c) TiO2 band-gap estimated from absorption spectrum.

(d) M-S curve for NiO/TiO2 heterojunction film.

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-1.0 -0.5 0.0 0.5 1.0-10

-5

0

5

10 (a) NiO/Au

Curr

ent (

nA)

Voltage (V)-1.0 -0.5 0.0 0.5 1.0

-2

0

2

4(b) TiO2/FTO

Curr

ent (

mA)

Voltage (V)

0.0 5.0x10-4 1.0x10-3

0.2

0.4

0.6

0.8

1.0

Au/NiO/TiO2/FTO

(c) NiO(3h)/TiO2 NiO(5h)/TiO2 NiO(7h)/TiO2

Rs=0.6 Rs=7.1 K

Rs=7.7 K

dV/d

(lnI

) (V)

Current (A)0.06 0.08 0.10 0.12 0.14

-18

-17

-16

-15

Au/NiO/TiO2/FTO NiO(3h)/TiO2 NiO(5h)/TiO2 NiO(7h)/TiO2

(d)

n=5.7

n=4.6

n=3.7

lnI (

A)

Voltage (V)

Fig. S4 I-V curves for devices of (a) NiO/Au and (b) TiO2/FTO in the dark. (c) dV/d (lnI) -I

curves of heterojunction devices. (d) lnI-V curves of heterojunction devices.

The calculation equations for series resistance (Rs) and ideality factor (n) can be transformed

into the differential formula (1):1

(1)

ln SdV nkT IR

d I q

From the Figure S4c, the dV/d (lnI) -I curves are straight lines. The slope of the straight line is

the Rs. The calculated Rs is 7.7, 0.6 and 7.1 KΩ for the heterojunction devices with NiO

nanosheets grown for 3 h, 5 h and 7 h, respectively. Rs first decreases and then increases with

NiO growth time increase. The n can be obtained from the Y-axis intercept of dV/d (lnI) -I

curves, which can be deduced according equation (2):1

(2)

lnq dVn

kT d I

The lnI-V curves in Fig. S4d show the linear behavior. The n is 3.7, 4.6 and 5.7 for the

heterojunction devices with NiO nanosheets grown for 3 h, 5 h and 7 h, respectively. The n

increases with NiO growth time increase.

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0.000 0.002 0.004 0.006 0.008 0.010

NiO(3h)/TiO2 (a)

10%

90%

rise=3 ms

Volta

ge (V

)

Time (s)0.000 0.002 0.004 0.006 0.008 0.010

(b) 90%

10%

rise=3 ms

NiO(5h)/TiO2

Volta

ge (V

)

Time (s)0.000 0.002 0.004 0.006 0.008 0.010

(c) 90%

10%

NiO(7h)/TiO2

rise=3 ms

Volta

ge (V

)

Time (s)

0.06 0.08 0.10 0.12 0.14 0.16 0.18

(d) 90%

10%

decay=64 ms

NiO(3h)/TiO2

Volta

ge (V

)

Time (s)0.14 0.16 0.18 0.20 0.22 0.24 0.26

NiO(5h)/TiO2 (e) 90%

decay=54 ms

Volta

ge (V

)Time (s)

10%

0.12 0.14 0.16 0.18 0.20 0.22 0.24

NiO(7h)/TiO2 (f) 90%

10%

decay=60 ms

Volta

ge (V

)

Time (s)

Fig. S5 Photoresponse time of NiO/TiO2 heterojunction devices under 365 nm illumination at

zero bias: (a)-(c) the photocurrent response time and (d)-(f) the photocurrent decay time for the

devices with NiO nanosheets grown for 3 h, 5 h and 7 h.

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300 400 500 600 700 8000.0

0.4

0.8

1.2 NiO(3h)/TiO2

NiO(5h)/TiO2

NiO(7h)/TiO2

Au/NiO/TiO2/FTO

Resp

onsi

vity

(A/W

)

Wavelength (nm)

Vbias=-0.5 VP=0.2 mW/cm2

370 nm

380 nm

TiO2 NiO(3h)

E00

-3

-4

-5

-6

-7

-8

Eg=3.0 eV

Eg=3.55 eVe

e e

h

hh

EFEner

gy (e

V)

365 nm

e e

h

qVD=1.33 eV

h

e

(b)

h

TiO2 NiO(5h)

E00

-3

-4

-5

-6

-7

-8

Eg=3.0 eV

Eg=3.55 eVe

e e

h

hhEFEn

ergy

(eV

)

365 nm

e e

h h

qVD=1.40 eV

e

h

e

h

e

h

(c)

TiO2 NiO(7h)

E0

0

-3

-4

-5

-6

-7

-8

Eg=3.0 eV

Eg=3.55 eVe

e e

h

hhEFEn

ergy

(eV

)

365 nm

qVD=1.45 eV

e

e

h h

h

e

h

e

(d)

(a)

Fig. S6 (a) Responsivity as a function of wavelength in the range from 300 to 800 nm at -0.5 V

reverse bias under 365 nm illumination with the power density of 0.2 mW/cm2. (b)-(d) The

band diagrams for the heterojunctions with NiO nanosheets grown for 3 h, 5 h and 7 h under -

0.5 V reverse bias.

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TiO2 NiO

0

-3

-4

-5

-6

-7

-8

Eg=3.0 eV

Eg=3.55 eV

e e e

h

hh

EF

Ene

rgy

(eV

)

365 nm

e e

h h

qVD=0.84 eV

NiCl/TiO2

(e)

TiO2 NiO

E0

365 nm

h

qVD=0.95 eV

e

e

h

h h

NiSO4/TiO2

(f)

e

e

0

-3

-4

-5

-6

-7

-8

Eg=3.0 eV

Eg=3.55 eV

ee

h

h

EF

Ener

gy (e

V)

Fig. S7 SEM images of NiO/TiO2 heterojunctions with NiO nanosheets by using (a) NiCl and

(b) NiSO4 nickel reactants. The thickness of the heterojunction films is 1.8 μm. (c) The transient

photocurrent response of the devices with NiO nanosheets by using NiCl and NiSO4 nickel

reactants. (d) Responsivity as a function of wavelength in the range from 300 to 800 nm at zero

bias under UV light (365 nm, 0.2 mW/cm2) illumination. (e) and (f) Energy band diagrams of

NiO/TiO2 heterojunction devices with NiO nanosheets by using NiCl and NiSO4 nickel

reactants.

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Reference1 G. Turgut, E. F. Keskenler, S. Aydın, S. Dogan, S. Duman, S. Özçelik, B. Gürbulak, B.

Esen, phys. Status. Solidi. A, 2014, 211, 580-586.