Bi2MoO6 by TiO2 Nanofibers for enhanced visible-photocatalysis

Journal Pre-proofs Separating Type I heterojunction of NaBi(MoO4)2/Bi2MoO6 by TiO2 Nanofibers for enhanced visible-photocatalysis Yuejun Li, Tieping Cao, Zemin Mei, Xiaoping Li, Dawei Sun PII: DOI: Reference:

S0301-0104(19)31187-5 https://doi.org/10.1016/j.chemphys.2020.110696 CHEMPH 110696

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Chemical Physics

Received Date: Revised Date: Accepted Date:

3 October 2019 23 January 2020 23 January 2020

Please cite this article as: Y. Li, T. Cao, Z. Mei, X. Li, D. Sun, Separating Type I heterojunction of NaBi(MoO4)2/ Bi2MoO6 by TiO2 Nanofibers for enhanced visible-photocatalysis, Chemical Physics (2020), doi: https://doi.org/ 10.1016/j.chemphys.2020.110696

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Separating Type I heterojunction of NaBi(MoO4)2/Bi2MoO6 by TiO2 Nanofibers for enhanced visible-photocatalysis Yuejun Li, Tieping Cao,* Zemin Mei*, Xiaoping Li, Dawei Sun Baicheng Normal University, Research Centre of nano-photocatalyst, Baicheng, Jilin, 137000 *Corresponding authors: [email protected]; [email protected] Abstract: Among tremendous efforts for enhanced photocatalysis, construction of heterojunction has been deemed as an effective means. In comparison with type II heterojunction, type I heterojunction receives less attention due to accumulation and increased recombination of charge carriers in one side of heterojunction. Herein, we take NaBi(MoO4)2/Bi2MoO6 as an example of type I heterojunction and explore means to improve the photocatalytic performance of NaBi(MoO4)2/Bi2MoO6 heterojunction. We employ TiO2 electronspun nanofibers as substrates and separately grow NaBi(MoO4)2 and Bi2MoO6 nanostructures on the skeleton of TiO2. The characterization results show that TiO2 changes type I heterojunction into multiheterojunction integrating two type II heterojunctions. Photocatalytic tests demonstrate that as-constructed multi-heterojunction is of higher activity than that of single type I or II heterojunction, respectively, where photogenerated electrons are accumulated on the surface of TiO2 and photogenerated holes are accumulated on NaBi(MoO4)2 and Bi2MoO6, respectively. This study highlights the potential application of TiO2 electronspun nanofibers in the construction of multiheterojunction for enhanced visible light photocatalysis. Key words ： TiO2 nanofibers; type I, type II, multi-heterojunction, photocatalytic, visible light 1. Introduction: Heterojunction photocatalysis has been attracting increasing attention due to the promoted charge separation, extended light absorption and boosted surface reaction.[1-4] Therein, multiheterojunction is a more promising candidate of photocatalyst than single heterojunction towards application in environment purification and energy conversion[5,6]. It provides more choices for extended optical absorption and promoted charge separation and hence enhanced photocatalytic 1

activity. In traditional photocatalyst, Staggered heterojunction[7,

8]

has displayed its

advantage in inhibition of charge recombination, and whereas straddling heterojunction is somehow inefficient for charge separation due to the growing charge recombination in one side of heterojunction[9-11]. However, in the construction of multiheterojunction, the type of heterojunction should be reconsidered because of the increased contact interface between different components. With this regard, not only components across type II but also type I heterojunction may be reactivated when they meet another component in multi-component composites. Therefore, it deserves particular attention on the re-assembly of multiheterojunction on the basis of type I heterojunction. Recent studies have witnessed that TiO2 electrospun nanofibers can serve as a good substrate to build heterojunction due to their high surface area, large porosity and interwoven architectures.[12] Many heterojunctions, such as TiO2/SnO2[13], TiO2/SrTiO3[14], TiO2/CeO2[15], TiO2/In2S3[16], TiO2/ZnO[17], TiO2/WO3/g-C3N4[18], BiVO4/TiO2[19], BiOBr/TiO2[20]and etc, have been built based on TiO2 electrospun nanofibers and display higher photocatalytic activity due to promoted charge separation by type II heterojunction as well as better ability of charge transfer along one-dimensional nanofibers. Moreover, the potential of conduction band of TiO2 facilitates reduction of oxygen and the potential of valance band of TiO2 is more positive than other visible light induced photocatalyst. Therefore, TiO2 is more promising in formation of type II heterojunction with other visible light induced photocatalyst and help transfer of electrons from other visible light induced photocatalyst. Besides, bismuth based compounds emerge as promising visible light induced photocatalyst because of significant light absorption in the visible region and naturally good charge separation by interlayer-built electric field[21-24]. In this study, we aim at fabricating multiheterojunction from NaBi(MoO4)2, Bi2MoO6 and TiO2, respectively. According to the position of energy band, NaBi(MoO4)2 and Bi2MoO6 form type I heterojunction. However, when they grow on the surface of TiO2 nanofibers, NaBi(MoO4)2/TiO2/Bi2MoO6

multiheterojunction

is

formed

instead

of

NaBi(MoO4)2/Bi2MoO6 heterojunction. Different from previous type I heterojunction, 2

multi-heterojunction is composed of two type II heterojunctions. Moreover, NaBi(MoO4)2 and Bi2MoO6 can be evenly distributed on TiO2, which is beneficial for charge

transfer.

Photocatalytic

tests

show

that

as-obtained

dispersed

multiheterojunction is superior to single heterojunction, and the reason for the enhanced activity will be discussed in detail. 2. Experimental Section 2.1 Reagents Polyvinylpyrrolidone (PVP, Ms = 1300000) was purchased from Aladdin Reagent Co., LTD. Butyl titanate, bismuth nitrate and ammonium molybdate are purchased from National Medicine Group Chemical Reagent Co., LTD. Ethylene glycol (EG), anhydrous ethanol and glacial acetic acid are purchased from Beijing Chemical Factory; Ethylenediamine tetraacetic acid disodium salt (EDTA) was purchased from Shanghai Research Industrial co., LTD.; All reagents are analytically pure. Double distilled water was used in all experiments. 2.2 Preparation of multiheterojunction TiO2

nanofibers

were

fabricated

as

reported

previously[25].

NaBi(MoO4)2/TiO2/Bi2MoO6 multiheterojunction was fabricated as following: 0.1769 g of (NH4)6Mo7O24·4H2O was dissolved in a 10 mL mixture of H2O and ethanol (VH2O/Vethanol=1:1). 0.485g Bi(NO3)3·5H2O was dissolved in 20 mL of ethylene glycol. Then the solution of (NH4)6Mo7O24·4H2O and Bi(NO3)3·5H2O was mixed and stirred. During stirring, 5 mL of 0.2 M EDTA was added to the mixture. The pH of the solution was adjusted to 10 by 1 M NaOH solution. After stirring for 30 mins, the mixture was transferred to 50 mL autoclave. Meanwhile, 0.01 g TiO2 nanofibers were immersed in the mixture. After sealing the autoclave, the autoclave was heated at 180 oC

for 24 h. After cooling to room temperature, the product obtained in the autoclave

was rinsed with ethanol and water for several times, dried at 60 oC for 12 h and finally obtained the colored nanofibers mat. For comparison, single heterojunction such as NaBi(MoO4)2/TiO2 and TiO2/Bi2MoO6 was fabricated by simply changing the ethanol/water mixed solvent to pure water and ethanol, respectively. Last, NaBi(MoO4)2/Bi2MoO6

heterojunction

was

fabricated

as

that

for

NaBi(MoO4)2/TiO2/Bi2MoO6 multiheterojunction, the only change is no addition of TiO2 nanofibers. 3

2.3 Characterization X-ray diffraction (XRD) pattern was acquired at X'Pert3 powder instrument, Cu Kα target（λ=0.154056nm）was equipped, pipe current and voltage was set at 40 mA and 40 kV, respectively. Morphologies of samples were observed on Hitachi SU8010 scanning electron microscope (SEM). Growth and distribution of bismuth molydbate on TiO2 nanofibers was observed on JEOL-JSM 2010 transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) was collected on PHI-5000 VersaProbe. UV-Vis-DRS spectra was collected on LAMBDA35. Fluoresce spectra was obtained on Hitachi F-4500. 2.4 Photocatalytic test The photocatalytic activity was evaluated by degradation of gaseous acetaldehyde under visible light (λ ≥ 420 nm) with a 150 W Xe lamp. The light intensity in the region of 420 nm to 800 nm was adjusted to 100 mW/cm2. The light was irradiated from the top of the reactor. 0.1 g of photocatalyst was put in a 500 mL reactor, then the reactor was fully filled with mixed gas of 20% O2/80% N2. Then, the reactor was injected into gaseous acetaldehyde and the concentration of acetaldehyde in the reactor was adjusted to 200 ppm. After irradiation for every 10 mins, the concentration of acetaldehyde and CO2 was monitored by GC. 3. Results and discussion

Fig. 1. XRD patterns of different samples 4

Phase composition of single heterojunction as well as multiheterojunction were detected by XRD. As shown in Fig. 1, TiO2 nanofibers are composed of mixed anatase and rutile phases, which agrees our previous works. For sample BMT, the pattern shows other 12 peaks in addition to TiO2 can be detected. These additional peaks are in coincidence with orthogonal Bi2MoO6 (JCPDS card No. 72-1524). This indicates that Bi2MoO6 can be obtained in the presence of ethanol as cosolvent in the solvothermal reaction. For sample NBMT, the pattern shows other 9 peaks in addition to TiO2 can be detected. These additional peaks are in coincidence with tetragonal NaBi(MoO4)2 (JCPDS card No. 51-1508). This indicates that NaBi(MoO4)2 can be obtained in the presence of water as cosolvent in the solvothermal reaction. For sample NBMTBM, we can observe three sets of peaks corresponding to NBM, BM and TiO2, respectively. In comparison with above three samples, we can conclude that the as-adopted synthesis route in the presence of water/ethanol as cosolvent can successfully produce mixture of NBM, BM and TiO2. a

b

500nm

c

500nm

d

500nm

Fig. 2

500nm

SEM images of different samples: a, TiO2; b, NBMT; c, BMT; d, NBMTBM

The morphology of different heterojunctions was observed by SEM. As shown in Fig.2a, the pure TiO2 sample exhibits morphology of nanofibers, demonstrating electrospinning technique is a powerful for producing nanofibers mat with either organic or inorganic component. For sample NBMT, nanorods are evenly grown on the surface of TiO2 nanofibers, indicating the formation of nanorods-nanofibers heterojunction. The nanorods are of ca. 10 nm in diameter and ca. 30 nm in length. In 5

combination with above XRD results, it is deduced that the nanorods are composed of NaBi(MoO4)2 phase. Similarly, for sample BMT, nanosheets are evenly distributed on TiO2 nanofibers. The nanosheets are of ca.10 nm in thickness and ca. 30 nm in width. The nanosheets are deduced to be composed of Bi2MoO6 phase. For sample NBMTBM, morphologies including nanofibers, nanorods and nanosheets can be simultaneously observed. Moreover, nanorods and nanosheets are uniformly grown and distributed along the nanofibers. In combination with its XRD pattern and morphology

observation

on

single

heterojunction,

it

is

concluded

that

multiheterojunction integrating TiO2, NaBi(MoO4)2 and Bi2MoO6 is fabricated. This implies that NaBi(MoO4)2 and Bi2MoO6 can be separately or simultaneously grown on

TiO2

nanofibers,

which

provides

a

very

feasible

means

to

study

multiheterojunction and refer to that of single heterojunction. More importantly, the control over the growth component can be realized by simple tuning the cosolvent (ethanol or water or ethanol/water) in the precursor solution, which is easy for experimental replication and drawing reliable conclusions. a

b (111) 0.31nm

0.35nm (101)

NaBi(MoO4)2

TiO2 500 nm

c

5 nm

d (101)

TiO2

0.35nm

0.32nm (131) Bi2MoO6

500 nm

5 nm

Fig. 3. TEM images of (a) NBMT and (c) BMT; HRTEM images of (b) NBMT and (d) BMT To have a further confirmation of formation of heterojunction, TEM and HRTEM were performed and the images are shown in Fig. 3. As shown by TEM in Fig. 3a, nanorods are deeply rooted in the skeleton of nanofibers. HRTEM in Fig. 3b 6

clearly shows the interface between nanorods and nanofibers. Lattice distances of 0.35 nm in the region of nanofibers stem can be ascribed to (101) facet of anatase TiO2. Meanwhile, lattice distances of 0.31 nm can be ascribed to (111) facet of NaBi(MoO4)2. These observations can safely prove the formation of NBMT heterojunction. Similarly, for BMT shown in Fig. 3c, nanosheets can be vertically rooted into the skeleton of nanofibers. HRTEM image in Fig. 3d show a lattice distances of 0.32 nm corresponding to (131) facet of BM, proving the formation of BMT heterojunction. The distribution of NBM and BM can also be evaluated by EDS. As can be seen in Fig. S1, elements of Na, Bi, Mo and O are evenly distributed without aggregation and in well accordance with the morphology of nanorods. Accordingly, it can be again concluded that the as-adopted synthesis route can selectively grow secondary NBM or BM on the surface and hence form heterojunction.

Fig. 4

(a): XPS Survey spectra of different samples; High resolution XPS spectra of (b): Bi4f ;

(c): Mo3d ;

(d): Ti2p ; (e): O1s ;

(f): Na1s

The surface element composition and valence state of samples NBMT and BMT were analyzed by XPS, and the results are shown in Fig. 4. As can seen in Fig. 4a, NBMT and BMT are composed of Na, Bi, Mo, C, Ti, O and Bi, Mo, C, Ti, O elements, respectively, where the binding energy is 283.18 eV. The C1s peak is derived from the standard measurement method of the instrument and is corrected. 7

The other elemental compositions are in good agreement with the XRD and EDS analysis results. Figures 4b-4f are the XPS high resolution spectra of Bi4f, Mo3d, Ti2p, O1s and Na1s in the samples tested. Fig. 4b shows that Bi4f of sample NBMT exhibits two asymmetric peaks at binding energies of 159.0 eV and 164.4 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively, indicating that Bi is predominantly present in Bi3+ form in the NBMT sample. The Bi4f photoelectron peak of sample BMT is more complicated. After the peak deconvolution, in addition to the two asymmetric peaks at 159.0 eV and 164.4 eV, two photoelectrons belonging to Bi5+ appear at 159.7 eV and 165.0 eV. The peak indicates that the Bi species of the BMT sample exist in both Bi3+ and Bi5+ forms. According to the principle of electrical neutrality, it can be speculated that oxygen vacancies exist in the BMT sample[26], which will be beneficial to the improvement of photocatalytic activity of the sample. As shown in Fig. 4c, the Mo3d of the two samples showed the same symmetric photoelectron peak at 232.5 eV and 235.6 eV, corresponding to Mo3d5/2 and Mo3d3/2, respectively, meaning that the Mo species in both samples were in the form of Mo6+[27]. Fig. 4d shows the XPS spectrum of Ti2p. Two photoelectron peaks of Ti2p1/2 and Ti2p3/2 appear at the binding energy of 464.4 eV and 458.6 eV in both samples, and the peak positions are basically the same[28]. Fig. 4e is an XPS high resolution spectrum of O1s in both samples. The photoelectron peak at a higher binding energy of 531.7 eV is attributed to surface chemisorbed oxygen (Oα), while the photoelectron peak at a lower binding energy of 529.7 eV is attributed to the material lattice oxygen (Oβ) [29]. As shown in Fig. 4f, the sample NBMT showed a Na1s photoelectron peak at 1072.3 eV[30]. Fig. 5a shows the UV-Vis-DRS spectra of different samples. The growth of NBM can extend the absorption of TiO2 from UV to visible region as long as 500 nm. As reported previously, NaBi(MoO4)2 is of yellow color and its band gap falls with 2.9 eV[31]. Herein, the red shift of absorption for NBMT in comparison with that of pure TiO2 results from the narrower band gap of NaBi(MoO4)2. For BMT heterojunction, the absorption can be extended to 600 nm, which is believed to be induced by the compositing TiO2 with BM[32,33]. The more significant red shift over BMT than NBMT is due to a smaller band gap of BM than that of NBM. As for the NBMTBM multiheterojunction, the absorption is extended to 550 nm, which is a synergistic effect of NBM and BM. In general, the fabrication of heterojunction can 8

effectively bring visible light absorption to TiO2, which is essential for endowing photocatalytic activity under visible light.

Fig. 5. (a) UV-vis spectra of different samples; (b) PL spectra of different samples. Fig. 5b shows the PL of different samples. The PL intensity follows an order: NBMTBM < BMT < NBMT. It can be deduced that formation of BMT and NBMT heterojunction can inhibit the charge recombination in TiO2. Moreover, BMT gives a stronger ability for promoting charge separation than that of NBMT, which may be related to the difference of matched degree of energy band position. Furthermore, in comparison with single heterojunction, NBMTBM multiheterojunction shows the lowest PL intensity, indicating the strongest ability for promoting charge separation among all heterojunctions. This also implies that NBMTBM multiheterojunction is a more promising candidate for photocatalysis under visible light.

Fig. 6. Times curves of (a) photocatalytic degradation of gaseous acetaldehyde and (b) CO2 evolution over different samples.

9

Fig. 6 shows the photocatalytic test results of acetaldehyde degradation over different samples. As can be observed, no activity can be observed in dark (curves magnified in Fig. 6, inset) over all samples. Meanwhile, the decrease of acetaldehyde and CO2 evolution can be hardly detected over pure TiO2, suggesting TiO2 is inactive for degradation of acetaldehyde under visible light irradiation. Other three heterojunctions display activity under visible light, the activity follows an order: NBMTBM > BMT > NBMT.

Notably, sample NBMTBM gives the highest activity.

There is no change of morphology and crystal phase after photocatalytic test (shown in Fig. S1). Since the activity order is consistent with above PL analysis, it is believed that the superior performance of NBMTBM originates from its best performance for charge separation. In other words, the multi-heterojunction assembled by double type II heterojunction promotes the electron transfer to O2 and hole transfer to H2O, as shown in Scheme 2. Moreover, the advantages of charge separation by double heterojunction outperforms sing heterojunction, resulting higher photocatalytic activity towards degradation of acetaldehyde of NBMTBM than BMT and NBMT. The performance of NBMTBM has also been compared with related works on acetaldehyde degradation. As shown in Table S1, the photocatalytic activity is comparable to other works, demonstrating the potential of NBMTBM in the application of visible light induced photocatalysis. To have a deep understanding on the enhanced activity over NBMTBM heterojunction, Motto-Schottky curves are measured to compare the energy band positions of the three components. As shown in Fig. 7a, the Mott-Schottky plots of all samples have a positive slope, which agrees with their n-type semiconductor. The Fermi levels of NaBi(MoO4)2, Bi2MoO6 and TiO2 are -1.13 V (vs. SCE), -1.04 V (vs. SCE) and -0.94 V (vs. SCE), respectively. Because the flat-band potential of n-type semiconductor lies very close to the bottom of the conduction band, it is inferred that the conduction band position of NaBi(MoO4)2 is the highest, followed by Bi2MoO6, and then TiO2. As is well reported, the bandgap of NaBi(MoO4)2, Bi2MoO6 and TiO2 is 2.9, 2.75 and 3.2 eV, respectively. It can be deduced that the valance band position of TiO2 is the lowest, followed by NaBi(MoO4)2, and then Bi2MoO6[34]. Accordingly, type I heterojunction can be formed between NaBi(MoO4)2 and Bi2MoO6, which is worse for charge separation. When they are dispersed on TiO2 nanofibers, type II 10

heterojunction can be formed at the interface of NaBi(MoO4)2/TiO2 and Bi2MoO6/TiO2, respectively, which is beneficial for charge separation. The schematic illustration on the photocatalytic process over double heterojunction is shown in Fig. 7b.

9

4x10

9

3x10

9

2x10

9

1x10

9

b NaBi( MoO4) 2 Bi2MoO6 TiO2

-2

-2

C (F )

a 5x10

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

E (V vs SCE)

Fig. 7: (a) Mott-Schottky curve of different samples, (b) schematic illustration of photocatalytic process over NBMTBM heterojunction. Last, the growth mechanism of NBM and BM on TiO2 is illustrated in Scheme 1. On the basis of literature results and possible reaction between reactants[35,36], the steps of chemical reaction are listed as following: Bi(NO3)3 = Bi3+ + 3NO3-

(1)

(NH4)6Mo7O24 + 14OH- = 7MoO42- + 6NH3↑ + 10H2O

(2)

Bi3+ + Na+ + 2MoO42- = NaBi(MoO4)2↓

(3)

2Bi3+ +4OH-+ MoO42- = Bi2MoO6↓ + 2H2O

(4)

At the initial stage, surface hydroxyl groups on TiO2 nanofibers react with carboxyl group in EDTA molecular (step 1). Such reaction is similar to the esterification between alcohol and organic acid[37,38]. This helps binding of EDTA onto the surface of TiO2. Meanwhile, EDTA is a good chelating agent, which can coordinate with Bi3+[39,40]. Accordingly, bismuth ion can be aggregated around the surface of TiO2, which is beneficial for nucleation of bismuth related crystals on TiO2. When water and ethanol are used as cosolvent, NaOH can react with (NH4)6Mo7O24 and generate MoO4

2-

ion (step 2). However, different cosolvent may induce a

different reaction pathway for the formation of heterojunction. When water is presented as cosolvent, the solution media allows high concentration of both Na+ and 11

Bi3+ ions (step 3). Therefore, MoO42- ion tends to react with Na+ and Bi3+ ions to generate NaBi (MoO4)2 embryos on the surface of TiO2[41]. With the reaction proceeds, NaBi(MoO4)2 nanorods gradually grow on the embryos and resulting in NaBi(MoO4)2/TiO2 heterojunction. Differently, when ethanol is presented as cosolvent, MoO42- ion tends to react with Bi3+ in a solvothermal reaction due to hard ionization and slow mobility of Na+ in organic solvent (step 4). Note that this reaction is very common in most works Bi2MoO6 is obtained by similar solvothermal route. In addition, it should be noted that EDTA plays a key role for the growth of bismuth compounds on the surface of TiO2. We tried to synthesize NBMT and BMT composites without the addition of EDTA, while other conditions are the same as that for NBMT and BMT heterojunction. Fig. S2 shows the SEM images of samples synthesized without EDTA. As can be seen, both NBM and BM form aggregates by themselves rather evenly distribute on TiO2. Therefore, no heterojunctions are obtained in the absence of EDTA. It is deduced that EDTA acts as a “molecular glue” that binds TiO2 by esterification and capture Bi3+ and Na+ by coordination. This EDTA-mediated synthesis route may work well in fabrication of other heterojunctions on TiO2 or other semiconductor oxide supports.

EtOH+EG pH=10 (B aq) EDTA

Bi3+

Bi2MoO6/TiO2 H2O+EG pH=10 (A aq)

TiO2

NaBi(MoO4)2/TiO

Scheme 1. Formation mechanism of NBMT and BMT.2

4. Conclusions In this work, by employing electrospun TiO2 nanofibers as the substrate, bismuth nitrate as the bismuth source, and EDTA as the chelating agent, we successfully prepared single heterojunction of Bi2MoO6/TiO2 and NaBi(MoO4)2/TiO2 as well as double heterojunction NaBi(MoO4)2/TiO2/Bi2MoO6. Structural analysis demonstrates the double heterojunction is more efficient for charge separation than single 12

heterojunction. Photocatalytic test via acetaldehyde degradation shows the best performance of NaBi(MoO4)2/TiO2/Bi2MoO6 under visible light, benefiting from the ultradispersion of heterojunction along TiO2 nanofibers. This study indicates that electrospun inorganic nanofibers is promising candidate for fabrication of multi-heterojunction.

Acknowledgements: This work is supported by National Natural Science Foundation of China (21573003)

References: [1] H. Hou, X. Zeng, X. Zhang, Production of hydrogen peroxide through photocatalytic

processes:

a

critical

review

of

recent

advances,

doi:

10.1002/anie.201911609. [2] Y. Liu, Z. Zhang, Y. Fang, B. Liu, J. Huang, F. Miao, Y. Bao, B. Dong, IR-Driven strong plasmonic-coupling on Ag nanorices/W18O49 nanowires heterostructures for photo/thermal synergistic enhancement of H2 evolution from ammonia borane, Appl. Catal. B, Environ., 2019, 252, 164-173. [3] D. Dong, C. Yan, J. Huang, N. Lu, P. Wu, J. Wang, Z. Zhang, An electron-donating strategy to guide the construction of MOF photocatalysts toward co-catalyst-free highly efficient photocatalytic H2 evolution. J. Mate. Chem. A, 2019, 7, 24180-24185. [4] X. Li, J. Xiong, X. Gao, J. Huang, Z. Feng, Z. Chen, Y. Zhu, Recent advances in 3 D g-C3N4 composite photocatalysts for photocatalytic water splitting, degradation of p ollutants and CO2 reduction, J. Alloys Compounds, 2019, 802, 196-209. [5] A. Meng, L. Y. Zhang, B. Cheng, J. G. Yu, TiO2-MnOx-Pt Hybrid Multiheterojunction

Film

Photocatalyst

with

Enhanced

Photocatalytic

CO2-Reduction Activity, ACS Appl. Mater. Interfaces., 2019, 11(6): 5581-5589. [6] Z. Y. Zhang, K. C. Liu, Y. A. Bao, B. Dong, Photo-assisted self-optimizing of charge-carriers transport channel in the recrystallized multi-heterojunction nanofibers for highly efficient photocatalytic H2 generation, Appl. Catal. BEnviron., 2017, 203: 599-606.

13

[7] A. Samal, S. Swain, B. Satpati, D.P. Das, B. K. Mishra, 3D Co3(PO4)2-Reduced Graphene Oxide Flowers for Photocatalytic Water Splitting: A Type II Staggered Heterojunction System. ChemSusChem, 2016, 9, 1-13. [8] X. Li, J. Xiong, Y. Xu, Z. Feng, J. Huang. Defect-assisted surface modification en hances the visible light photocatalytic performance of g-C3N4@C-TiO2 direct Z-s cheme heterojunction. Chin. J. Catal., 2019, 40, 424-433. [9] K. X. Wang, Y. S. Zhang, L. N. Liu, N. Lu, Z. Y. Zhang, BiOBr nanosheets-decorated TiO2 nanofibers as hierarchical p–n heterojunctions photocatalysts

for

pollutant

degradation, J.

Mater.

Sci.,

2019, 54 (11):

8426-8435. [10] S. J. A. Moniz, S. A. Shevlin, D. J. Martin, Z. X. Guo, J. W. Tang, Visible-light driven heterojunction photocatalysts for water splitting – A critical review, Energy Environ. Sci. 2015, 8(3): 731-759. [11] X. Li, J. G. Yu, M. Jaroniec, X. B. Chen, Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels, Chem. Rev., 2019, 119 (6): 3962-4179. [12] H. Hou, L. Wang, F. Gao, G. Wei, B. Tang, W. Yang, T. Wu, General Strategy for fabricating thoroughly mesoporous nanofibers, J. Am. Chem. Soc., 2014, 136(48), 16716-16719. [13]

C.

H.

Wang,

C.

L.

Shao,

X.

T.

Zhang,

Y.

C.

Liu,

SnO2 Nanostructures-TiO2 Nanofibers Heterostructures: Controlled Fabrication and High Photocatalytic Properties, Inorg. Chem., 2009, 48(15): 7261-7268. [14] T. P. Cao, Y. J. Li, C. H. Wang, C. L. Shao, Y. C. Liu, A Facile in Situ Hydrothermal Method to SrTiO3/TiO2Nanofiber Heterostructures with High Photocatalytic Activity, Langmuir, 2011, 27(6): 2946-2952. [15] J. Tian, Y. H. Sang, Z. H. Zhao, W. J. Zhou, D. Z. Wang, X. L. Kang, H. Liu, J. Y. Wang, S. W. Chen, H. Q. Cai, H. Huang, Enhanced Photocatalytic Performances of CeO2/TiO2 Nanobelt Heterostructures, Small, 2013, 9(22): 3864-3872. [16] X. Zhang, X. H. Li, C. L. Shao, J. H. Li, M. Y Zhang, P. Zhang, K. X. Wang, N. Lu, Y. C. Liu. One-dimensional hierarchical heterostructures of In2S3 nanosheets on electrospun TiO2 nanofibers with enhanced visible photocatalytic activity, J. Hazard. Mater., 2013, 260: 892-900. [17] J. Y. Park , S. W. Choi, J. W. Lee, C. Lee, S. S. Kim, Synthesis and Gas Sensing 14

Properties of TiO2–ZnO Core‐Shell Nanofibers, J. Am. Ceram. Soc., 2009, 92(11): 2551-2554. [18] H. Hou, F. Gao, L. Wang, M. Shang, Z. Yang, J. Zheng, W. Yang, Superior thoroughly mesoporous ternary hybrid photocatalysts of TiO2/WO3/g-C3N4 nanofibers for visible-light-driven hydrogen evolution, J. Mater. Chem. A, 2016, 4, 6276-6281. [19] H. Hou, L. Wang, F. Gao, X. Yang, W. Yang, BiVO4@TiO2 core–shell hybrid mesoporous nanofibers towards efficient visible-light-driven photocatalytic hydrogen production, J. Mater. Chem. C, 2019, 7, 7858-7864. [20] K. Wang, Y. Zhang, L. Liu, N. Lu, Z. Zhang, BiOBr nanosheets-decorated TiO2 nanofibers as hierarchical p-n heterojunctions photocatalysts for pollutant degradation, J. Mater. Sci., 2019, 54, 8426-8435. [21] J. L. Zhang, L. S Zhang, N. Yu, K. B. Xu, S. J. Li, H. L. Wang, J. S. Liu. Flower-like

Bi2S3/Bi2MoO6

heterojunction

superstructures

with

enhanced

visible-light-driven photocatalytic activity, RSC Adv., 2015, 5(92): 75081-75088. [22] J. S. Cai, J. Y. Huang, Y. K. Lai, BiMoO6 nanosheet/TiO2 nanotube array heterostructure with enhanced UV and visible-light photocatalytic activity, J. Mater. Chem. A., 2017, 5(31):16412-16421. [23] J. X. Liu, R. J. Wei, J. C. Hu, L. Z. Li, J. L. Li, Novel Bi2O3/NaBi(MoO4)2 heterojunction with enhanced photocatalytic activity under visible light irradiation, J. Alloy. Compd., 2013, 580: 475-480. [24] M. Zhang, Y. Qi, Z. Zhang, AgBr/BiOBr Nano-Heterostructure-Decorated Polyacrylonitrile Nanofibers: A Recyclable High-Performance Photocatalyst for Dye Degradation under Visible-Light Irradiation. Polymers, 2019, 11, 1718 [25] T. P. Cao, Y. J. Li, C. H. Wang, Z. Y. Zhang, M. Y. Zhang, C. L. Shao, Y. C. Liu, Bi4Ti3O12 nanosheets/TiO2 submicron fibers heterostructures: in situfabrication and high visible light photocatalytic activity, J. Mater. Chem., 2011, 21(19): 6922-6927. [26]

Q.

He,

Y.

H.

Ni,

S.

Y.

Ye.

Heterostructrued

Bi2O3/Bi2MoO6

nanocomposites:simple construction and enhanced visible-light photocatalytic performance, RSC Adv., 2017, 7: 27089-27099. [27] W. Z. Lv, D. Z. Huang, Y. M. Chen, Q. Qiu, Z. K. Luo, Synthesis and characterization

of

microwave-assisted

Mo-W

co-doped

hydrothermal

VO2

method, 15

(R)

nano-powders

Ceram.

Int.,

2014,

by

the

40(8):

12661-12668. [28] W. J. Ren, Z. N. Ai, F. L. Jia, L. Z. Zhang, X. X. Fan, Z. G. Zou, Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2, Appl. Catal. B-Environ., 2007, 69(3-4):138-144. [29] F. X. Xie ,X. M. Mao, C. M. Fan, Y. W. Wang, Facile preparation of Sn-doped BiOCl photocatalyst with enhanced photocatalytic activity for benzoic acid and rhodamine B degradation, Mater. Sci. Semicond. Process., 2014, 27: 380-389. [30] G. Iucci, C. Battocchio, M. Dettin, F. Ghezzo, G. Polzonetti, An XPS study on the covalent immobilization od adhesion peptides on a glass surface, Solid State Sci., 2010, 12: 1861-1865. [31] J. X. Liu, R. J. Wei, J. C. Hu, L. Z. Li, J. L. Li. Novel Bi2O3/NaBi(MoO4)2 heterojunction with enhanced photocatalytic activity under visible light irradiation, J. Alloy. Compd., 2013, 580: 475-480. [32] Y. Xu, M. A. A. Schoonen, The absolute energy positions of conduction and valence bands of selected semiconducting minerals, Am. Mineral., 2000, 85(3-4): 543-556. [33] Y. S. Xu, Z. J. Zhang, W. D. Zhang, Facile preparation of heterostructured Bi2O3/Bi2MoO6

hollow

microsph-erees

with

enhanced

visible-light

photocatalytic and antimicrobial activity, Mater. Res. Bull., 2013, 48: 1420-1427. [34] A. H. Nethercot, Prediction of Fermi Energies and Photoelectric Thresholds Based on Electronegativity Concepts, Phys. Rev. Lett., 1974, 33: 1008-1091. [35] G. W. Keulks, J. L. Hall ,C. Daniel, K. Suzuki. The catalytic oxidation of propylene: IV. Preparation and characterization of α-bismuth molybdate, J. Catal., 1974, 34(1): 79-97. [36] J. P. Zou, S. L. Luo, L. Z. Zhang, J. Ma, S. L. Lei, L. S. Zhang, X. B. Luo, Y. Luo, G. H. Zeng, C. T. Au, One-pot solvothermal syntheses of ternary heterostructured TiO2–Bi2MoO6/Bi3.64Mo0.36O6.55 controllable in terms of composition, morphology and structure: Materials of high visible-light driven photocatalytic activity, Appl. Catal. B-Environ. 2013, (140-141): 608-618. [37] L. Palmisano, M. Schiavello, A. Sclafani, C. Martin, I. Martin, V. Rives, Surface properties

of

iron-titania

photocatalysts

16

employed

for

4-nitrophenol

photodegradation in aqueous TiO2 dispersion, Catal. Lett., 1994, 24(3-4): 303-315. [38] M. Primet, P. Pichat, M. V. Mathieu, Infrared study of the surface of titanium dioxides. I. Hydroxyl groups, J. Phys. Chem.,1971, 75(9):1216-1220. [39] H. Wullens, N. Bodart, M. Devillers, New bismuth(III), lanthanum(III), praseodymium(III), and heterodinuclear Bi–La and Bi–Pr complexes with polyamino-carboxylate ligands, J. Solid State Chem., 2002,167(2): 494-507. [40] H. Kumagai, C. J. Kepert, M. Kurmoo, Construction of Hydrogen-Bonded and Coordination-Bonded Networks of Cobalt(II) with Pyromellitate:  Synthesis, Structures, and Magnetic Properties, Inorg. Chem., 2002, 41(13): 3410-3422. [41] A. Kabalnov, Ostwald Ripening and Related Phenomena, J. Disper. Sci. Technol., 2001, 22(1):1-12.

Author Statement

Yuejun Li: Performing the experiment, Writing- Original draft Tieping Cao: Propose the idea, Discussion on the raw data, Writing-Editing drfat Zemin Mei: Discussion on the raw data Xiaoping Li: Characterization on SEM, Photocatalytic Test Dawei Sun: Characterization on XPS, Mott-Schottky

Graphical Abstract

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TiO2 nanofibers helps disperse type I heterojunction to two type II heterojunctions. This leads to higher photocatalytic activity towards degradation of gaseous acetaldehyde.

Highlights: 

NaBi(MoO4)2 and Bi2MoO6 can be grown on TiO2 nanofibers



Type I heterojunction can be separated to two type II heterojunctions by TiO2



Two type II heterojunctions are more active than type I heterojunction

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