NiO nanorod hierarchical nanostructures: p–n heterojunctions towards efficient photocatalysis

Journal Pre-proofs TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p–n heterojunctions towards efficient photocatalysis Jie Chen, Minggui Wang, Jie Han, Rong Guo PII: DOI: Reference:

S0021-9797(19)31490-0 https://doi.org/10.1016/j.jcis.2019.12.031 YJCIS 25771

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

29 September 2019 5 December 2019 7 December 2019

Please cite this article as: J. Chen, M. Wang, J. Han, R. Guo, TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p–n heterojunctions towards efficient photocatalysis, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.031

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© 2019 Published by Elsevier Inc.

TiO2 nanosheet/NiO nanorod hierarchical nanostructures: p–n heterojunctions towards efficient photocatalysis Jie Chen a, Minggui Wang b, Jie Han a*, Rong Guo a * a

School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,

Jiangsu 225002, PR China b

Guangling College, Yangzhou University, Yangzhou, Jiangsu 225009, PR China

* Corresponding author. E-mail address: [email protected]; [email protected]

Abstract: TiO2 nanosheet/NiO nanorod heterojunction hybrids have been developed through a hydrothermal route, where NiO nanorods (size: 5 nm in diameter and 20~40 nm in length) are deposited at the {001} facet of anatase TiO2 nanosheets. The photocatalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N2 adsorption-desorption analysis, UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy and time-resolved fluorescence. The TiO2/NiO photocatalysts exhibited good photocatalytic activities towards the degradation of methyl blue (MB) and phenol, and hydrogen generation efficiency under visible light irradiation. The maximum rate constant can be reached 0.0279 min-1 and 0.0135 min-1 respectively, which are about 12 and 10 times higher than that of TiO2 nanosheets. And the hydrogen generation efficiency is 10 times higher than physical mixing of TiO2 and NiO. Photocatalytic degradation efficiency remains more than 90% after 6 times cycle dye degradation, and the H2 production efficiency is almost the same after four cycles, suggesting good stability and reusability. The enhanced photocatalytic activities are associated with the rational design of TiO2/NiO hierarchical heterojunctions which ensues high photogenerated charge separation efficiency.

With

the

improved photocatalytic

performance,

the

TiO2/NiO

heterojunction hybrids are expected to be potential photocatalysts in environmental and energy related areas.

Keywords: TiO2; NiO; heterojunction; photocatalyst

1. Introduction Titanium dioxide (TiO2) has received wide attention in field of photocatalysis due to its excellent catalytic activity, nontoxicity, low price, and high stability against corrosion [1-11]. However, its wide band gap (~3.2 eV) and low electron-hole separation efficiency limited its practical applications. To make full usage of the sunlight, designed synthesis of TiO2 based photocatalysts with visible light activity is highly desirable. Many strategies have been developed to narrow the band gap of TiO2, including doping with non-metallic elements (such as C, N, S, F) [12-16], and coupling with noble metal (such as Au, Ag, Pt) nanoparticles [17-21] and narrowed band-gap semiconductor [22-26] to expand light absorption range and improve the photoinduced charges generation, separation and transmission. However, doping non-metallic element normally requires high temperature with low and uncontrolled elemental doping amount and state. Coupling with noble metal nanoparticles may result in dissolution of noble metal nanoparticles during the photocatalytic reactions, and noble metal are too expensive, rare and harmful to the environment for practical applications [27-29]. Therefore, it is crucial to narrow the band gap of TiO2 by forming the heterojunction hybrids through coupling with other semiconductors for promising enhancement of photocatalytic activity. For instances, when TiO2 is coupled with CdS, Cu2O, or NiO to form heterojunction hybrids, the recombination of photo-generated electron-hole pairs can be obviously reduced, leading to improved photocatalytic activity [22-24]. Among these various TiO2 based heterojunction hybrids, TiO2/NiO heterojunction hybrids have aroused considerable attention in particular [30-32]. As a typical p-type semiconductor, NiO has excellent catalytic, electric and magnetic properties, and particular high p-type concentration and hole mobility. Heterojunction hybrids can be formed while n-type TiO2 and p-type NiO are contacted, and an inner electric field is formed at the interfaces of TiO2/NiO heterojunctions. The n-type TiO2 regions are accumulated with positive charges, whereas the p-type NiO regions are enriched with negative charges. As a result, the establishment of charge equilibrium

resulting in better interfacial charge separation and transfer to enhance the lifetimes of the charge carries [29-31]. It has been established that the nanostructure has great effect on the photocatalytic performance of TiO2 photocatalysts. The correlation between TiO2 nanostructure and band structure as well as their effects on the behavior of photo-generated charge carriers have been disclosed theoretically and experimentally, where the photocatalytic activity ranks the following order: nanosheet > nanotube > nanoparticle [33]. It has been reported that the {001} and {101} facets of TiO2 nanosheets tend to accommodate holes and electrons, respectively [34]. More recently, CuO nanoparticles (~3.5 nm) that loaded on the {101} facet of TiO2 nanosheets have been developed, which show excellent photocatalytic performance in conversion of methanol [35]. It is believed that the rational design of TiO2 nanosheets/NiO heterojunctions via selective deposition of NiO nanostructures on the desired facets of TiO2 can enhance the separation efficiency of photo-generated electrons and holes and therefore leading to advanced photocatalysts. However, the construction of TiO2 nanosheet/NiO heterojunction hybrids as efficient photocatalysts has not yet been reported. Herein, we report the synthesis of TiO2 nanosheets heterojunction loaded with NiO nanorods mostly on the {001} facets of TiO2 nanosheets via a hydrothermal route. The introduction of rod-like NiO on TiO2 nanosheets has the following advantages: (1) the formation of TiO2 nanosheet/NiO heterojunction can broaden the visible light response of TiO2 and prolong the lifetime of charge carriers; (2) the favorable morphological characteristics leading to increased specific surface area and the active sites. The photocatalytic degradation of methyl blue (MB) and phenol, as well as photocatalytic H2 production reveal the excellent photocatalytic activities of TiO2 nanosheet/NiO nanorod photocatalysts, which are considerably higher than that of TiO2 nanosheets and NiO nanorods as well as conventional photocatalysts.

2. Experimental 2.1 Chemicals.

Tetrabutyl orthotitanate (TBOT, 97%) was obtained from Fluka. Hydrofluoric acid (HF, 40%), Ni(NO3)26H2O, urea (CO(NH2)2), Anhydrous ethanol, and all other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). The water used in this study was deionized and purified through a Millipore system. 2.2 Synthesis of TiO2 nanosheets. TiO2 nanosheets were prepared according the previous report [36]. Typically, 25 ml TBOT and 3.5 ml HF was mixed and stirred for 1 h, then the mixture was sealed and hydrothermal treated in a reaction kettle at 180 °C for 24 h. While cooling to room temperature, the obtained products were washed with ethanol and water, and then dried for further use. 2.3 Synthesis of TiO2 nanosheet/NiO nanorod heterojunction hybrids. The typically, 40.0 mg of as-prepared TiO2 nanosheets were dispersed in different concentration Ni(NO3)26H2O and urea (the mass ratio of Ni(NO3)26H2O/urea is 7:2) aqueous solution using ultrasonication and heated at 120 °C for 5 h through the hydrothermal reaction. The products were washed and dried, and finally were calcined at 300 °C for 2 h under air condition, leading to the formation of TiO2 nanosheet/NiO nanorod heterojunction hybrids. When the mass of Ni(NO3)26H2O used is 0.35, 0.7, and 1.4 g, the as-prepared TiO2/NiO heterojunction hybrids were denoted as TiO2/NiO1, TiO2/NiO-2 and TiO2/NiO-3, respectively. For comparison, NiO nanorods were prepared through a hydrothermal route. Briefly, 0.7 g Ni(NO3)26H2O and 0.4 g urea were dispersed in 25 mL of deionized water under ultrasound, then the solution was reacted at 120 °C for 5 h, and finally calcined at 300 °C for 2 h under air condition. Physical mixing of TiO2 nanosheets and NiO nanoparticles were also employed and denoted as TiO2/NiO-(PM). 2.4 Photocatalytic activity Briefly, 5.0 mg TiO2/NiO catalyst was dispersed in 25.0 mL MB (2.0×10-5 mol/L) or phenol aqueous solution (10 ppm), and then first stirred in dark for 30 min to reach the equilibrium adsorption. After that the photocatalytic reaction was carried out in photoreactor system (Xujiang XPA-7) under 400 W metal halide (with a filter of 400 nm ) irradiation. The MB (or phenol) concentration was detected with UV-vis

spectrophotometer (Lambada 650, PerkinElmer). 2.5 Photocatalytic H2 production 10.0 mg TiO2/NiO catalyst was dispersed in 100.0 mL of solution containing 70.0 mL water and 30.0 mL 3-ethanolamine under stirring. The system was evacuated to completely remove the air before the irradiation. A 300 W Xe arc lamp with a filter of 400 nm was used as light source during the photocatalytic reaction. The hydrogen evolution was analyzed by online gas chromatography (CEAULIGHT, GC-7920).

3. Results and discussion 3.1. Morphology and characterization of TiO2/NiO heterojunction hybrids

Fig. 1 (a) The schematic diagram of the synthesis processes of TiO2/NiO heterojunction hybrids. TEM images of (b) TiO2 nanosheets; (c) TiO2@Ni(OH)2 hybrids; (d) TiO2/NiO-2 hybrids after 300 oC

calcination. (e) HRTEM; (f) HAADF-STEM; (g-h) the elemental mappings of Ti, O and Ni; and

(j) overlayered element mapping of Ti and Ni from TiO2/NiO-2 hybrids.

The synthesis processes of 2D/1D TiO2 nanosheet/NiO nanorod heterojunction hybrids are given in Fig. 1a. Firstly, ultrathin uniform TiO2 nanosheets are prepared according to the previous report [36]. After the hydrothermal treatment of colloidal aqueous solution containing TiO2 nanosheets, Ni(NO3)2 and urea, the as-formed Ni(OH)2 nanorods can be rooted on surfaces of TiO2 nanosheets, realizing the formation of 2D/1D TiO2 nanosheets/Ni(OH)2 nanorods hybrids. Finally, the as-formed 2D/1D TiO2 nanosheets/Ni(OH)2 nanorods hybrids are calcined at 300 °C to realize the transformation of Ni(OH)2 to NiO, leading to the formation of 2D/1D TiO2 nanosheet/NiO nanorod heterojunction hybrids. The typical TEM image of TiO2 nanosheets is shown in Fig. 1b, exhibiting a rectangular outline, side length of ∼40 nm, and thickness of ∼6 nm. Fig. 1c displays the TEM image of TiO2/Ni(OH)2 hybrids (the formation of Ni(OH)2 has been confirmed by XRD pattern as shown in Fig. S1). Ni(OH)2 nanorods with the diameter of 10 nm and length of 50~100 nm rooted on both top and bottom {001} facets of TiO2 nanosheets can be evidenced. Each TiO2 nanosheet has tens of Ni(OH)2 nanorods mainly supported on both facets of TiO2 nanosheet. After the calcination treatment, Ni(OH)2 nanorods are transferred into NiO nanorods, leading to the formation of 2D/1D TiO2 nanosheet/NiO nanorod heterojunction hybrids (Fig. 1d). The NiO nanorods are 5 nm in diameter and 20~40 nm in length. As shown from the HRTEM image in Fig. 1e, the lattice fringes about 0.35 nm and 0.21 nm match the planes of anatase TiO2 {101} and NiO {200}, respectively [32], suggesting the formation of well-defined TiO2/NiO heterojunction hybrids. The HAADF-STEM image in Fig. 1f and the elemental mappings of Ti (Fig. 1g), O (Fig. 1h), Ni (Fig. 1i) and their overlayered elemental mapping (Fig. 1j) further proves the formation of TiO2/NiO heterojunction hybrids. It was interesting to found that the density of NiO nanorods on surface of TiO2 nanosheets could be turned by changing the Ni(NO3)2 concentration during the hydrothermal route. As shown in Fig. S2, the amount of supported NiO nanorods is increasing with the Ni(NO3)2 concentration. At low, medium and high concentration of Ni(NO3)2, the resulting 2D/1D TiO2 nanosheets/NiO nanorods heterojunction hybrids are denoted as TiO2/NiO-1, TiO2/NiO-2, and TiO2/NiO-3, respectively. The amount of

NiO in TiO2/NiO-1, TiO2/NiO-2 and TiO2/NiO-3 hybrids were measured to be 42.1

N(200)

TiO2/NiO-2 TiO2/NiO-3

N(222)

T(215) N(311)

T(220)

N(220)

T(204)

(211)

T(200)

NiO

NiO

TiO2 TiO2/NiO-1

T(105)

N(111) T(004)

TiO2

T(101)

Intensity (a.u.)

wt%, 61.5 wt% and 71.9 wt%, respectively, by energy dispersive spectrometer (EDS).

PDF#47-1049 (NiO)

10

20

30

40

50

2 (degree)

60

70

80

Fig. 2 XRD patterns of TiO2, TiO2/NiO-1, TiO2/NiO-2, TiO2/NiO-3 heterojunction hybrids.

Fig. 2 gives the XRD patterns of TiO2 nanosheets,and TiO2/NiO heterojunction hybrids. The sharp diffraction peaks at 25, 38, 48, 54, and 55o are ascribed to the {101}, {004}, {200}, {105}, and {211} facets of anatase TiO2, respectively (JCPDS card No. 21-1272) [35]. When TiO2 nanosheets are decorated with NiO nanorods, the typical peaks of NiO (JCPDS card No. 47-1049, as given in Fig. 2) can also be identified, indicating the successful formation of hybrids. As seen, the diffraction peak of {004} for TiO2 is consistent with that of NiO. However, there is no enhancement for the intensity of {004} peak after NiO loading, especially for sample TiO2/NiO-2, where the intensity decreases significantly. The result is consistent with the morphology observation, where the intensity for TiO2 is shielded by NiO, showing a mostly NiO growth along the {001} direction [34]. For sample TiO2/NiO-3, the intensity increases mainly from the contribution of NiO.

300

(a)

0.24

dV/dD (cm 3 nm -1g -1)

Volume (mL/g )

360

TiO2 TiO2/NiO-1

240

TiO2/NiO-2

180

TiO2/NiO-3 TiO2/NiO-(PM)

120 60

0.20

(b)

TiO2 TiO2/NiO-1

0.16

TiO2/NiO-2

0.12

TiO2/NiO-3 TiO2/NiO-(PM)

0.08 0.04 0.00

0 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

0

3

6

9

12

Pore Diameter D (nm)

15

Fig. 3 (a) N2 adsorption-desorption isotherms, (b) pore size distribution obtained by using the BJH method. Tab. 1 BET specific surface area and BJH pore size for pure TiO2 nanosheet and TiO2/NiO heterojunction hybrids. Specific surface area (m2 g-1)

Pore volume (m3 g-1)

TiO2

18.67

0.1512

TiO2/NiO-1

121.4

0.4559

TiO2/NiO-2

146.8

0.7274

TiO2/NiO-3

132.5

0.5287

TiO2/NiO-(PM)

86.47

0.3167

Catalysts

Fig. 3a and 3b give the N2 adsorption-desorption isotherms and pore size distribution of samples respectively, and the corresponding data are summarized in Tab. 1. The corresponding N2 adsorption-desorption isotherms and pore size distribution of pure NiO are given in Fig. S4, where the specific surface area of NiO nanorods as measured is 81.64 m2 g-1. It can be seen that specific surface areas and the pore volume of the hybrids show a trend of first increase and then decrease with the increasing amount of NiO. The pore sizes of the TiO2/NiO are approximately 3-6 nm. The TiO2/NiO catalysts with large surface area and optimized pore size distributions are beneficial for effective reactors diffusion during the photocatalytic processes.

2

0.8

TiO2 TiO2/NiO-1

0.6 0.4

TiO2/NiO-2 TiO2/NiO-3

0.2 0.0 300

400

500

600

(c) Intensity (a.u.)

700

15

TiO2

10

800

Wavelength (nm)

(b)

20

TiO2/NiO-1

5

TiO2/NiO-2

0

TiO2/NiO-3

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Photo Energy (eV)

(d)

Ti 2p Intensity (a.u.)

200

25

2

1.0

30

(a)

(Ephoton) (eV/nm)

Intensity (a.u.)

1.2

TiO2 TiO2/NiO

Ni 2p

2p3/2 2p3/2 sat

2p1/2

TiO2/NiO 2p1/2 sat

NiO 450

455

460

465

470

Binding energy (eV)

475

850

855

860

865

870

875

Binding energy (eV)

880

885

Fig. 4 (a) UV-vis diffusion reflectance and (b) the corresponding (αEphoton)2 vs. photon energy curves. (c, d) XPS spectra of (c) Ti 2p and (d) Ni 2p for TiO2, NiO and TiO2/NiO hybrids.

The UV-Vis diffuse reflectance absorption spectra and corresponding (αEphoton)2 vs. photon energy curves of TiO2 and TiO2/NiO samples are given in Fig. 4a and 4b. The UV-Vis diffuse reflectance absorption spectrum of NiO was given in Fig. S3 for comparison. The TiO2 nanosheets exhibit a strong photoresponse in UV region, corresponding to narrowed band gap. After coupling with NiO nanorods, the TiO2/NiO hybrids exhibit strong visible light absorption as compared with TiO2. It has been reported that NiO may distinctively alter the absorption of the hybrids owing to the overlap of Ti d and Ni d orbitals in the junction region [32, 37]. To further investigate the surface elemental composition and chemical states in TiO2/NiO heterostructured photocatalyst, XPS was further carried out. Fig. 4c and 4d give the XPS curves of Ti 2p and Ni 2p for TiO2, NiO and TiO2/NiO hybrids. The characteristic binding energies at 458.5 eV and 464.2 eV can be attributed to Ti4+ in anatase TiO2 [38-41]. The binding energies at 855.0 eV and 861.8 eV are assigned to Ni 2p3/2, and that at 872.8 eV is ascribed to Ni 2p1/2, indicating the formation of NiO

[30, 42]. Moreover, it can be found that the peaks of Ti 2p and Ni 2p in TiO2/NiO exhibit obvious binding energy shift while compared with pure TiO2 and NiO, which indicates the strong interaction between TiO2 and NiO in the TiO2/NiO heterojunction hybrids. 3.2. Photocatalytic performance

1.0

3.5

(a)

3.0

0.8

Ct /C0

-ln(Ct /C0)

2.5

0.6 0.4

TiO2 TiO2/NiO-1 TiO2/NiO-2

0.2

TiO2/NiO-(PM)

0

20

40

1.5

t / min

80

1.0

100

0

2.5

(c)

-ln(Ct /C0)

1.5

0.6

1.0

TiO2 TiO2/NiO-1 TiO2/NiO-2

0.2

20

(d)

2.0

0.4

R

0.9853 0.9994 0.9890 0.9799 0.9768

0.0 60

0.8

Ct /C0

2.0

2

k 0.00226 0.02250 0.02790 0.01140 0.00735

0.5

TiO2/NiO-3

0.0

1.0

(b)

40

60

t / min

80

100

2

k

R

0.0013 0.0099 0.0135 0.0067 0.0041

0.9819 0.9948 0.9994 0.9908 0.9732

0.5

TiO2/NiO-3 TiO2/NiO-(PM)

0.0 0

30

60

0.0 90

t / min

120

150

0

30

60

90

t / min

120

150

Fig. 5 Evolution of (a) MB and (c) phenol concentration with reaction time and (b, d) their corresponding apparent reaction rates under visible light in the presence of TiO2 and TiO2/NiO hybrids.

The photocatalytic activity of TiO2 and TiO2/NiO samples was firstly evaluated by the photocatalytic degradation of organic pollutants MB and phenol. As seen from Fig. 5a, the TiO2 nanosheets show low photocatalytic activity. It can be seen that TiO2/NiO heterojunction hybrids show remarkably enhanced phtotocatalytic activity, and show a trend of first increase and then decrease with the increasing amount of NiO, which is associated with the results of BET measurement. A higher loading of NiO nanorods on surfaces of TiO2 nanosheets that possibly becomes the recombination centers of photo-

generated electrons/holes which result in the decreased photocatalytic activity. The TiO2/NiO-2 hybrids, which show the highest photocatalytic activity, can decompose almost 100% MB after 100 min irradiation under visible light. In addition, physical mixing TiO2 nanosheets and NiO nanorods as photocatalyst for degradation MB was also studied, where the photocatalytic activity are better than that of pure TiO2, but worse than that of TiO2/NiO heterojunction hybrids. The MB photodegradation kinetics was fitted by applying a first-order model and the -ln(Ct/C0) plot as shown in Fig. 5b. Similarly, the photocatalytic degradation of phenol was showed in Fig. 5c and 5d. As seen, the catalytic behavior is consistent with that of MB. The TiO2/NiO-2 hybrids also exhibit the highest photocatalytic activity, and almost 90% phenol is decomposed after irradiated 120 min under visible light. In addition, the activity sequence of TiO2/NiO hybrids is the same while the photocatalytic activity is normalized to the specific

(a)

MB Phenol

100

97 %

80

87 %

2

60

Degr ad at io n

Rate constant (10

92 % 84 %

-2

3

Phenol MB

(b)

ratio (%)

-1

min )

surface area (Fig. S5).

40 20

1

0

6

0

Cy5 4 cl e Ti O 2

) -3 -1 -2 -(PM /N i O O /N i O O /N i O Ni O / Ti 2 Ti O 2 Ti 2 Ti O 2

3 2 1

Fig. 6 (a) Comparison of rate constants of TiO2 and TiO2/NiO hybrids for organic pollution degradation (MB and phenol) under visible light irradiation. (b) Recyclability measurements of the MB and phenol degradation under visible light irradiation by TiO2/NiO-2 hybrids.

The reaction rate constants of TiO2 and TiO2/NiO hybrids on MB and phenol degradation are summarized in Fig. 6a. The degradation rate constants of TiO2/NiO-2 hybrids are 0.0279 min-1 and 0.0135 min-1 toward MB and phenol degradation, respectively, about 12.3 and 10.4 times higher than that of TiO2 nanosheets. The stability of the as-prepared catalysts was also studied as shown in Fig. 6b. It was found that the degradation efficiency is almost unchanged after 6 times recycling, which

confirms the excellent stability of TiO2/NiO hybrids. Furthermore, the total organic carbon (TOC) values of MB solution using TiO2/NiO-2 hybrids photocatalysts under different visible light irradiation times were also detected (Table S1). The apparent decrease in TOC values of MB solution indicates that most MB molecules were

-1

Amount of H2 (mol g h

600

-1

(a)

554.9

2000

500

(b)

1600

400

1200

310.7

300 200

117.4

100 0

2400

Amount of H2 (mol g )

700

-1

)

degraded into CO2 during the photocatalytic process.

Trace

52.5

800 400 0

-1 /N i O

Ti O 2 Ti O 2

-3 -2 -(PM /Ni O i O 2/Ni OO 2/Ni O T Ti O 2 Ti

)

0

2

4

6

8

10

Time (h)

12

14

16

Fig. 7 (a) Photocatalytic hydrogen evolution rates of TiO2 and TiO2/NiO samples under visible light (λ> 400 nm). (b) Photocatalytic stability of TiO2/NiO-2 towards visible-light hydrogen production.

The photocatalytic hydrogen evolution of TiO2 nanosheets and TiO2/NiO hybrids was also investigated under visible-light irradiation. As shown in Fig. 7a, the hydrogen generation rate using TiO2 nanosheets is obviously improved after coupling with NiO. The hydrogen generation rate increases first and then decreases with the increasing concentration of NiO, and the TiO2/NiO-2 hybrids present the highest hydrogen generation rate of 554.9 μmol g-1 h -1. The stability of TiO2/NiO-2 sample was also investigated, as shown in Fig. 7b, it can be seen that the catalyst shows the good stability of in H2 production. The photocatalytic performance for TiO2/NiO-2 heterojunction hybrids also surpasses the most reported TiO2/NiO photocatalysts (Tab. S1) [29-32, 4344].

240

TiO2

(a)

TiO2/NiO-2 TiO2/NiO-3 TiO2/NiO-(PM)

200

Counts

Intensity (a.u.)

TiO2/NiO-1

(b)

TiO2 TiO2/NiO-1 TiO2/NiO-2

160

TiO2/NiO-3 TiO2/NiO-(PM)

120 80 40 0

450

500

Wavelength (nm)

0

550

10

20

30

Time (ns)

40

50

TiO2

(c)

2

Current (mA/cm )

400

TiO2/NiO-1

TiO2/NiO-3

TiO2/NiO-2

TiO2/NiO-(PM)

20 40 60 80 100 120 140 160 180 200

Time (s)

Fig. 8 (a) Photoluminescence emission spectra, (b) time-resolved fluorescence decay and (c) photocurrent analysis of TiO2, TiO2/NiO heterojunction hybrids. Tab. 2 The fluorescence lifetime and their relative percentage of photoexcited charge carries in TiO2 nanosheet and TiO2/NiO hybrids. Catalysts

τ1 (ns)

τ2 (ns)

TiO2

1.32

14.0

TiO2/NiO-1

1.78

16.2

TiO2/NiO-2

2.06

18.6

TiO2/NiO-3

1.41

15.4

TiO2/NiO-(PM)

1.37

14.9

Charge carrier separation and transfer are important factors that have crucial part in the photocatalytic process. Photoluminescence (PL) is an effective way to analyze the separation process of photoexcited electron-hole in the photocatalysts. Thus, PL

emission spectra of different photocatalysts were obtained. Fig. 8a shows the PL decay curve of TiO2 and TiO2/NiO. As compared with pure TiO2, the main peak intensity of TiO2/NiO at about 460 nm is weakened remarkably, suggesting efficient charge separation and transfer in the TiO2/NiO heterostructures during photoexcitation process. As for TiO2/NiO-2, the emission intensity is the lowest, suggesting the best separation efficiency of photogenerated electrons and holes [34]. In addition, time-resolved transient PL spectra were obtained to further characterize the life of photogenerted electrons during the photocatalytic process. Fig. 8b shows a double exponential decay fitting, which is used to calculate the τ1 (short lifetime) and τ2 (longer lifetime) values as listed in Tab. 2. The τ2 value for the TiO2/NiO samples, corresponding to the recombination process of photoexcited electron-hole, are increased firstly and then decreased with increasing NiO concentration, and all longer than that of TiO2, which implies the formation of heterojunction realizes the effective separation of photogenerated charge carries to enhance photocatalytic activity [45, 46]. As seen from Fig. 8c, there is no current response for all samples in absence of visible light irradiations. Under the visible light irradiation, photocurrents produce for all samples, and the pure TiO2 shows lower photocurrent than that of TiO2/NiO. It is also noted that the TiO2/NiO-2 reveal a notably increase of photocurrent response and longer life of charge carriers, suggesting superior efficiency in charge transfer and separation [32, 47-52].

Scheme 1. Schematic illustration of photocatalytic mechanism of TiO2/NiO hybrids under light irradiation.

According to the above results, the mechanism of photocatalytic process for degradation of organic pollutions and hydrogen production using TiO2/NiO

heterojunction hybrids was proposed (Scheme 1). As soon as NiO nanorods are loaded on surface of TiO2 nanosheets, abundant p-n heterojunctions are formed at the interfaces. The photo-induced electrons and holes are created once TiO2/NiO heterojunction hybrids are excited by visible light. Attributed to the internal electric field formed by p-n heterojunctions, the photo-generated electrons on the CB of NiO will transfer to the CB of anatase TiO2 owing to its more negative CB potential, while photo-generated holes on the VB of TiO2 will transfer to the VB of NiO, which resulting in effective separation of photo-generated electron-hole pairs. In the visible light catalytic reactions, on the one hand, the electrons on the anatase TiO2 are captured by oxygen, generating oxygen radicals (·O2-). On the other hand, the electron can convert H+ into H2. At the same time, the holes on the NiO can generate hydroxyl radicals to react with organic pollutes to produce CO2 and H2O. The produced hydroxyl radicals (·OH) can be confirmed by fluorescence method using terephthalic acid as a probe molecule as shown in Fig. S6.

4. Conclusions In summary, TiO2 nanosheet/NiO nanorod heterojunction hybrids photocatalysts were synthesized through a hydrothermal route. The introduction of NiO nanorods forms the p-n heterojunctions leading to an enhancement in interfacial charge transfer as well as an increase in light absorption. The superior photo-generated electrons-holes separation ability via the synergistic interactions of heterojunctions and outstanding structure feature of the hybrids are in favor of enhancement for the photocatalytic performance.

Therefore,

the

as-prepared

photocatalysts

exhibited

excellent

photocatalytic performance for the degradation of MB, phenol and hydrogen evolution. We believe that the design and synthesis of well-connected multicomponent heterostructures will offer new perspectives in the construction of novel nanomaterials for advanced energy, environmental and other related applications. Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21673202 and 21922202), Qing Lan Project and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We would also like to acknowledge the technical support received at the Testing Center of Yangzhou University.

Supplementary dataon Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/

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Graphic Abstract

Author Contribution Statement

Jie Chen: Investigation, Writing-Original Draft. Minggui Wang: Methodology, Validation Jie Han: Supervision, Writing - Review & Editing Rong Guo: Investigation, Writing - Review & Editing