2D Phase

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Fabrication of 0D/2D Bismuth Molybdates Phase Heterojunction for Enhancing the Visible-Light-Driven Photocatalytic Activity Yanhua Peng, Qinghua Liu, Yan Zhang, Mengjie Geng, Jinqiu Zhang, and Jianqiang Yu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11567 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Enhanced Visible-Light-Driven Photocatalytic Activity by 0D/2D Phase Heterojunction of Quantum Dots/Nanosheets on Bismuth Molybdates Yanhua Peng,a* Qinghua Liu,b Jinqiu Zhang,a Yan Zhang,a Mengjie Geng,a and Jianqiang Yu,a,c* a

School of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266071, Shandong, P. R. China.

b

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, Anhui, P. R. China c

Qingdao Center of Resource Chemistry & New Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 36 Jinshui Road, Qingdao, 266000, Shandong, P. R. China. *Corresponding Authors: Email: [email protected] and [email protected] Abstract The development of semiconductor heterojunctions with strong light-response and high charge separation efficiency is highly desired for addressing the global environmental and energy-related concerns. Herein, we present a new type of 0D/2D phase junctions of bismuth molybadates Bi4MoO9 quantum dots (QDs)/Bi2MoO6 nanosheets (NSs) to boost the interfacial charge transfer and electron conductivity for excellent visible-light-driven photocatalytic activity. Photocurrent and Mott-Schottky spectroscopy demonstrate that the strong coupling between 0D Bi4MoO9 QDs and 2D Bi2MoO6 NSs via phase heterojunctions could effectively tune the semiconductor work function for reducing the interfacial Schottky barrier by 0.05 eV, which greatly shortens the effective charge-transfer length and increases the number of photo-excited electrons in conduction band of Bi4MoO9 QDs. Hence, the as-obtained 0D/2D phase junctions of bismuth molybadates achieves excellent photocatalytic degradation of RhB under visible light irradiation (λ > 420 nm), and increases the catalytic efficiency by 13–23 times compared to that of pristine Bi4MoO9 and Bi2MoO6. This work offers a new concept to construct multifunctional phase junctions for the design of an efficient photocatalyst in the application of solar energy. Keywords: 0D/2D; Phase Heterojunction; Bismuth Molybdates; Charge Separation; VisibleLight-Driven Photocatalysis

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1. Introduction Semiconductors, such as TiO2, ZnO and bismuth molybdates-based materials, have emerged as attractive photocatalysts because of their excellent light harvesting, tunable band gaps and low cost.1-3 However, most of semiconductor-based photocatalysts show very low efficiency in conversion of solar energy, the simultaneous splitting of water and reduction of CO2. One of the key issues is the limited charge separation efficiency upon photo-excitation, which largely depends on the intrinsic electronic and structural properties of semiconductors.4-8 To solve the problem, it is highly urgent to develop an rational design that can efficiently promote charge mobility and separation. Fabrication of heterojucntion among different semiconductors (n-type and p-type) has been proved to be an efficient strategy for separating the photo-generate charges, while several drawbacks largely restrict their practical applications.9-11 Thus how to design efficient junctions for the overall efficiency still remains a challenge. Recently, a novel design, phase junction on the same semiconductor, has attracted great attention due to their unique advantages of the same containing elements, strong coupling, suitable band alignment and high electron affinity.12-16 For example, Li and co-workers fabricated TiO2 (anatase-rutile junctions) and Ga2O3 (α-β junctions) semiconductors with different phase structures by phase transformation, which displayed the enhanced photocatalytic performances due to efficient charge separation and transfer across the phase junction.12,13 Preethi et al. further studied the band alignment and charge transfer pathway in tri-phase junctions (anatase-rutile-brookite), which revealed the photo-excited electrons transferred from rutile to anatase to brookite in this system.14 Bismuth molybdates also have many crystalline phases, dependent on the formula Bi2O3·nMoO3 (n = 1, 2, 3) or mBi2O3·MoO3 (m = 1~2), corresponding to γ-Bi2MoO6, Bi3.64Mo0.36O6.55 and Bi4MoO9, respectively, which are commonly used in photocatalytic reactions.17-19 However, the largely investigated bismuth molybdates phase junctions do not exhibit obviously enhanced photocatalytic activity due to their bulk counterparts. With the rapid development of nano-technology, the fabrication of materials with different dimensions has been paid much more attention. Recently, 0D/2D heterojunction photocalysts have been greatly developed, in which the coupling of quantum dots and nanosheets is the most successful combinations.20,21 Due to the large surface area and short effective charge-transfer


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length of QDs, these make them highly promising in using accelerated charges in the photoelectrochemical and photocatalytic fields.22,23 However, 0-demensional QDs were limited in practical application due to their disadvantages of the vulnerable self-aggregation and the high photoluminescence.24 To solve these problems, QDs is loaded onto ultrathin 2-dimensional nanosheets to form a 0D/2D nanocomposites, which make QDs more dispersive and stable. Meanwhile the accelerated charge transfer facilitated by 2D NSs can effectively quench the photoluminescence of QDs, thereby suppressing the recombination of photoexcited charges.25,26 For instance, Ma and co-works fabricated a series of heterojunctions of vanadate QDs/g-C3N4 NSs, exhibiting excellent visible-light-driven photocatalytic activity.20 Liu et al. loaded CdS QDs onto graphene NSs, which brought enhanced photocatalytic and PEC performance.27 However, the largely used graphene NSs are usually photoinactive, which would decrease the effective light absorption of QDs due to the light shielding effect.28 Although the g-C3N4 NSs would be photoexcited, their band alignment and electron affinity hardly blended well in comparison with heterojunctions between same semiconductors with different phases. Therefore, exploring new styles of QDs/NSs composites, which could overcome these drawbacks, is a long and arduous task. Herein, we report a new style of 0D/2D heterojunctions of Bi4MoO9 QDs/Bi2MoO6 NSs on bismuth molybdates for the first time and demonstrate their outstanding visible-light-driven photocatalytic activity. It is found that the activity is significantly better than single Bi4MoO9 and Bi2MoO6 bulk counterparts. Unlike the general graphene or g-C3N4-based 0D/2D composites, the new formation strongly coupled the advantages of phase junctions with semiconductors and 0D/2D structures with especial charge mobility, which offers a new concept to construct multifunctional and multidimensional nanocomposites in photocatalysis. 2. Experimental Sections 2.1 Fabrication of 0D/2D heterojunctions of Bi4MoO9 QDs/Bi2MoO6 NSs The as-prepared 0D/2D heterojunctions of Bi4MoO9 QDs/Bi2MoO6 NSs were synthesized through in situ hydrothermal process using commercial chemicals of analytical grade without any further purification. In a typical procedure, (NH4)6Mo7O24·4H2O (0.36 mmol) was ultrasonically dissolved into 40 mL H2O and Bi(NO3)3·5H2O (5 mmol) was completely dissolved


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in 20 mL HNO3 solution (2.0 mol·L-1) under magnetic stirring. Then the bismuth nitrate solution was dropwisely added into the ammonium molybdate solution under vigorous stirring. Separately, the pH value of the mixtures was adjusted from 2 to 10 by an aqueous NaOH solution. After further stirring for 30 min, the mixtures were transferred into a Teflon-lined stainless autoclave and treated at 160 oC for 24 h. The reactor was cooled to room temperature. The products were filtered and washed several times with deionized water, dried in air at 60 oC for 8 h and got the final products with light yellow colour. 2.2 Characterization of the products X-ray diffraction (XRD) patterns were recorded on a Bruker D8-advance x-ray diffractometer using Cu Kα radiation (λ = 0.15478 nm). The scanning electron microscope (SEM) characterizations were performed on JSM-6700F field emission scanning electron microscope. The transmission electron microscope (TEM) analyses were performed by a JEOL JEM-2100 high-resolution transmission electron microscope. The optical diffuse reflectance spectrum was conducted on a UV-Vis diffuse reflectance spectrophotometer (U-41000, Hitachi) over the wavelength range of 200 nm to 700 nm. Photoluminescence spectra (PL) were detected with a Shimadzu RF-5301PC fluorescence spectrophotometer. 2.3 Photocatalytic activity and photoelectrochemical measurements The investigation of photocatalytic activity was carried out by irradiation RhB-photocatalyst suspensions under a 500 W Xenon lamp (cutoff filter λ ≥ 420 nm). In the degradation experiment, 0.10 g photocatalysts were dispersed into 100 mL RhB dye solution (7.0 mg/L) and stirred for 180 min in the darkness in order to achieve the desorption-adsorption equilibrium. After that, the light source was used to irradiate the suspension above with a distance of 10 cm. During the photocatalytic reaction, 4 mL liquid was taken from the reactor at a time interval of 30 min followed by the separation of photocatalyst through centrifugation. The concentration of RhB solution without catalyst was determined at 554 nm using a spectrophotometer. The photocurrent spectra of as-prepared materials were measured by an electrochemical workstation (CHI760E, Chenhua Instruments) with a standard three-electrode configuration. The catalysts loaded on Fluorine-doped tin oxide (FTO), Pt slice (2.0 × 2.0 cm2) and saturated Ag/AgCl electrode were employed as working electrode, counter electrode and reference


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electrode, respectively. Typically, 10 mg of catalyst was dispersed into 1 mL of Triton X-100 and 2 mL acetylacetone under grinding for 10 min to obtain slurry, which was further spread onto FTO with conductive glue to obtain film with an area of 1.0 × 1.0 cm2. The as-prepared working electrodes were dried at 200 oC for 2 h. 0.1 M Na2SO4 and 0.1 M Na2SO3 mixture solutions were used as the electrolyte solution. The 500 W Xenon lamp with a 420 nm cut-off filter was employed modelling a visible light source. 3. Results and discussion

Figure 1. (a) Schematic structure of the 0D/2D heterojunctions of Bi4MoO9 QDs/Bi2MoO6 NSs, in which the purple ball presents Bi atom, the green one presents Mo and the red one presents O, (b)-(d) TEM images of Bi2MoO6 sheets, Bi4MoO9 spheres and 0D/2D heterojunctions of Bi4MoO9 QDs/Bi2MoO6 NSs (e) and (f) SAED patterns taken on the area marked point 1 and point 2 in (d), of, (g) HRTEM images of Bi4MoO9 QDs/Bi2MoO6 NSs, and (h) the top view of the schematic structure.


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Proper junctions formed in semiconductors could lead to enhanced activity in application of solar energy conversion. Currently, the major approach to construct phase junction is phase transformation by annealing.12,13,29 Unfortunately, particles usually sinter at the same time, and even the morphology and dimension of photocatalysts could hardly be controllable. The 0D/2D heterojunctions of bismuth molybdates were firstly synthesized through in situ hydrothermal process. As shown in Figure 1a, the new style presents several superiorities for enhancing performance. Firstly, Bi4MoO9 QDs and Bi2MoO6 NSs themselves are efficient visible-lightdriven photocatalytsts.30,31 Secondly, Bi2MoO6 NSs have unique electrical conductivity property to increase the effective charge transfer of NSs and QDs.32 And importantly, the same Bi-O tetrahedron structure of Bi2MoO6 NSs surface with Bi4MoO9 QDs make phase junction stable than graphene and g-C3N4 as an ideal host for QDs robustly accommodating.12,13 Firstly, the morphology and structure of the as-prepared bismuth molybdates were investigated by transmission electron microscopy (TEM). A platelet shape of Bi2MoO6 was sheeted with the length between 800 nm and 3.0 µm as shown in Figure 1b. Pure Bi4MoO9 presenting the irregular spherical particles with the diameters about 100 nm (Figure 1c), could be considered as the quasi-quantum dots from the generalized definition. It could be clearly seen from Figure 1d that the heterojunctions were composed of sheets and nanoparticles embedded in the sheets. To further confirm the composition, the heterojunctions were characterized by the selected-area electron diffraction (SAED) analysis. The SAED pattern (Figure 1e) taken along the [010] direction on the area marked point 1 in Figure 1d indicated that the sheet was Bi2MoO6 single crystal. The angle between the (002) and (200) planes was 90.0°, which was consistent with the structure of Bi2MoO6.17 The SAED pattern (Figure 1f) was taken along the [110] direction on the point 2 marked in Figure 1d, indicating that the particles were also a single crystal. Moreover, the angle between the (1-11) and (1-1-1) planes was 69.5o, which indicated the cubic structure of Bi4MoO9 similar to the structure of Bi3.64Mo0.36O6.55.17,18 Interestingly, the HRTEM image of Bi4MoO9 QDs/ Bi2MoO6 NSs heterojunctions on the area marked in Figure 1d, shows highly dispersed QDs loaded on the 2D NSs and the size is 3-5 nm (Figure 1g), which are greatly smaller than the pure Bi4MoO9 spheres. Two typical lattice distance of 0.308 nm and 0.325 nm are clearly identified, which can be assigned to the (131) facet of orthorhombic Bi2MoO6 and (111) facet of cubic Bi4MoO9, respectively.19,33 From top view of the 0D/2D


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heterojunctions as shown in Figure 1h, the fringes of QDs are very similar with the results of HRTEM.

(200)

(111)

◊ Bi4MoO9

pH 10

pH 8

pH 8

pH 6

pH 6

pH 4

pH 4

pH 2

pH 2 (131)

pH 10

60

70

26

28

∆ JCPDS 21-0102 (200) (002) (060)

(133) (262)

30 40 50 2 Theta (degree)

◊ JCPDS 12-0149

∆ Bi2MoO6

∆ JCPDS 21-0102

(202) (062)

(131)

20

(200) (002) (060) (151)

(020)

10

(b)

(400)

(311) (111)

(220)

◊ JCPDS 12-0149

Intensity (a.u.)

◊ Bi4MoO9

(111)

∆ Bi2MoO6

(200)

(a)

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


30 32 2 Theta (degree)

34

36

Figure 2. (a) XRD patterns and (b) local amplified XRD patterns of different samples prepared by tuning

pH values.

The phase transformation was followed by XRD spectroscopy. As shown in Figure 2a, the XRD patterns suggest that the bismuth molybdates undergo gradual orthorhombic γ-Bi2MoO6 to cubic Bi4MoO9 phase transformation upon increasing pH value from 2 to 10, and bismuth molybdates with different phase structures could be obtained during the in situ hydrothermal process. All the diffraction peaks of the obtained samples, pH value from 2 to 6, can be well indexed to γ-Bi2MoO6 (JCPDS No. 21-0102). For Bi4MoO9 spheres (pH 10), all the diffraction peaks can match the standard card of cubic Bi4MoO9 (JCPDS No. 12-0149). While for Bi4MoO9/Bi2MoO6 heterojunction, it exhibits all the featured peaks of orthorhombic γ-Bi2MoO6 and cubic Bi4MoO9 and no additional impurity phase is found in the diffraction pattern, which indicates formation of phase junction between different bismuth molybdates. Additionally, Figure 2b depicts that the intensity of Bi2MoO6 (060) increases from pH 2 to 8, meanwhile the intensity of low index facet (020) also increases at 2θ of 10.9° (Figure 2a), implying that Bi2MoO6 crystal was special anisotropic growth along the (010) plane, which was also reported


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in the previous articles.30,34 The average size of the Bi4MoO9 particles was calculated by the Scherrer Formula as follows,

where K is constant, λ is the source wavelength, and β is the FWHM of peak. According to the equation, the average size of the Bi4MoO9 spheres was about 96.7 nm, which was close to the results of TEM. The morphology of the bismuth molybdates was shown in Figure 3a-c and corresponding crystal structure of them is also presented (Figure 3a'-c'). SEM images of Bi4MoO9 spheres illustrate the typical nanostructure. The crystal structure (Figure 3a') exhibited only a distorted [BiO]4 and [MoO]4 tetrahedral structure with four oxygen coordinate. For Bi2MoO6 sheets (Figure 3c), the SEM image showed that the surface of sheets was very flat and smooth, which facilitated the excited-charge rapidly transfer. Only [BiO]4 tetrahedral structure and [MoO]6 octahedral structure existed in Bi2MoO6 (Figure 3c'). After coupling with Bi4MoO9 spheres, the surface of Bi2MoO6 became rough, indicating a substantial loading of guest species (Figure 3b). In order to confirm the surface structure, the UV Raman spectra were used to detect the discrepancies of different bismuth molybdates (Figure 3d). Three characteristic bands at 850, 803 and 723 cm-1 were observed in the spectrum of Bi2MoO6 sheets (the black line), which corresponded to the stretching motions of distorted [MoO6] in Bi2MoO6.18,35-36 The different peaks at 880, 813 and 321 cm-1 in the Raman spectrum of Bi4MoO9 (the blue line) were assigned to Mo-O stretching motions and O-Mo-O bending motion of [MoO4] tetrahedral species, which had a little shift comparing with the same structure of Bi3.64Mo0.36O6.55 in previous reports.18 Like the results of XRD patterns, it was remarkable both [MoO4] and [MoO6] species were observed in the Raman spectrum of 0D/2D heterojunction, which indicated both Bi2MoO6 and Bi4MoO9 phases coexisted on the surface of heterojunction. Since the change in the chemical environment of 0D/2D heterojunction and interfacial contact of the two components, a slight peak shift of stretching motions of [MoO4] octahedron and [MoO6] tetrahedron was found in the Raman spectrum. Overall, by finely tuning the pH value in the range of 2-10, a series of bismuth molybdates with different phase structure had been prepared.


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Figure 3. SEM images of (a) Bi4MoO9 spheres, (b) 0D/2D heterojunction and (c) Bi2MoO6 sheets, the

crystal structures of (a') Bi4MoO9 spheres, (b') 0D/2D heterojunction and (c') Bi2MoO6 sheets and (d) the Raman spectroscopy.

The formation mechanism of different bismuth molybdates was illustrated in Figure 4. Most published articles showed that different phase of bismuth molybdates largely depend on the pH values of reaction solution.18,19,37 The orthorhombic structure of Bi2MoO6 favorably formed in the acidic condition, while the high Bi/Mo ratio composites (like cubic Bi4MoO9 or Bi3.64Mo0.36O6.55) were usually obtained in the alkaline medium.31,33 One of the key issues was the presence of Bi ions in the hydrothermal process, which were greatly influenced by the initial pH value of solutions. It was well known that Bi(NO3)3·5H2O, a strong acid weak-base salt, could ionize into Bi3+ and NO3- in strongly acidic solutions. For this reason, we firstly dissolved Bi(NO3)3·5H2O into 2M HNO3 solution. But when the pH value of solution was weakly acidic through gradually adding NaOH solution (pH 10.0), all Bi3+ ions directly generate Bi2O3 precipitate rather than Bi2O2(OH)+ ions due to the solubility product. Then, Bi2O3 nanospheres could be etched by amount of MoO42- ions in solution to form high Bi/Mo ratio composites.31,39 On the basis of the reaction time and etching strength, a family of bismuth molybdates with the formula mBi2O3·MoO3 were obtained, such as Bi3.64Mo0.36O6.55 with 9.0 % Bi sites substituted by Mo atoms in Bi2O3 with fluorite structure. We got a lower BiMo substituted composites (Bi4MoO9) because of the mild pH value and reaction time. For 0D/2D heterojunction of bismuth molybdates, the Bi precursors included both Bi2O2(OH)+ and Bi2O3 in the solution. With an Aurivillius structure, Bi2MoO6 was easily sheeted to decrease the surface energy, which could capture Bi2O3 nanoparticles as an island-like morphology on the surface originating from the Volmer-Weber mode due to their distinct crystalline structure (Figure 4).20,40 The growth of Bi4MoO9 was effectively confined by the binding effect of Bi2MoO6 during the reaction. After hydrothermal process, the generated uniform QDs presented similar size and clear lattice fringes, while the fast kinetics and confinement effect of NSs hindered the overgrowth of nanoparticles.

Figure 4. Schematic illustrating the fabrication of different bismuth molybdates composites.

The visible light photocatalytic activities of different bismuth molybdates on the degradation of RhB, under λ > 420 nm, were shown in Figure 5. Before illumination, the dark


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adsorption with and without photocatalysts (blank test) were performed. As shown in Figure 5a, the absorption equilibrium of RhB could be reached on all the samples within 30 min in dark, and 0D/2D heterojunction exhibited the highest absorption capability comparing to Bi2MoO6 sheets and Bi4MoO9 nanoparticles. Moreover, the 0D/2D heterojunction also displayed a much faster adsorption-desorption equilibrium rate than bulk Bi3.64Mo0.36O6.55/Bi2MoO6 heterostructure prepared in the previous reports.18 Figure 5b showed the variation of RhB concentration (C/C0) with irradiation time over different bismuth molybdate samples. For comparison, direct RhB photolysis and photodegration over as-prepared samples were also performed under identical conditions. Clearly, the direct photolysis of RhB under visible-light irradiation was negligible. Among the bismuth molybdate materials, 0D/2D heterojunction still displayed the highest photocatalytic activity, which could completely decompose RhB dye within 180 min. Moreover, the efficiency was higher than that of the mechanical mixture between Bi2MoO6 sheets and Bi4MoO9 nanoparticles, suggesting superiorities on photocatalytic activity of 0D/2D heterojunction than single Bi2MoO6 sheets and Bi4MoO9 nanoparticles counterparts. However, the degradation efficiencies of RhB over pristine Bi4MoO9 spheres and Bi2MoO6 sheets were only about 12% and 20% during the same time of visible light irradiation, respectively. The kinetics of RhB photodegradation fits an apparent pseudo-first order model, which exhibited a linear relationship between ln(C0’/Ct) and kt, where C0’ and Ct represent the concentration of RhB before and after irradiation, t is the irradiation time and k is the apparent rate constant. The calculated rate constants of all the samples at Bi2MoO6, 0D/2D heterojunction and Bi4MoO9 were 6.01×10-4 min-1, 1.36×10-2 min-1 and 9.97×10-4 min-1, respectively (in Table 1). Obviously, it was found that the k value of 0D/2D heterojunction was 23 times higher than pristine Bi2MoO6 and 14 times as high as that of pristine Bi4MoO9. In order to reveal the reason for activity differences of bismuth molybdates, the BET surface area of these three photocatalysts were measured (Table 1). Compared to pure Bi2MoO6 sheets (6.31 m2·g-1) and Bi4MoO9 nanoparticles (4.58 m2·g-1), 0D/2D heterojunction (2.87 m2·g-1) possess a relatively lower specific surface area. Generally, low specific surface area means a bad photocatalytic activity. However, photocatalytic experimental results indicated that 0D/2D heterojunction owed the highest photocatalytic activity, which could be explicated by some reasons in the previous articles.41 And the RhB degradation rate had be normalized with the BET specific areas. The degradation rate per area (DRpA) was 9.53×10-5, 2.18×10-4 and 4.74×10-3


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g·m-2·min-1 responding to Bi2MoO6 sheets, Bi4MoO9 spheres and 0D/2D heterojunction, respectively. The quantum efficiency (QE) was calculated from equation as follows,42

where Ip is the photocurrent, C = 6.25×108, I = 5.54 mW/cm2, A = 26.4 cm2, t = 1 s, λ= 420 nm, h = 6.62×10-34 J·s and c = 3.0×108 m/s. According to the equation, the apparent quantum efficiency of Bi2MoO6 sheets, Bi4MoO9 spheres and 0D/2D heterojunction was 1.21%, 1.41% and 2.82%, respectively. Consequently, the results revealed the highest photocatalytic activity of 0D/2D heterojunction. TABLE 1: Summarized surface area from BET measurement, degradation rate from photocatalytic activity, degradation rate per area and quantum efficiency of Bi2MoO6 sheets, Bi4MoO9 spheres and 0D/2D heterojunction.

Bi2MoO6 sheets 0D/2D heterojunction Bi4MoO9 spheres

SA (m2·g1 ) 6.309

DR (min-1)

DRpA (g·m-2·min-1)

QE (%)

6.01×10-4

9.53×10-5

1.21

2.869

1.36×10-2

4.74×10-3

2.82

4.578

9.97×10-4

2.18×10-4

1.41

Note: SA presents surface area, DR presents degradation rate, DRpA presents degradation rate per area, and QE presents quantum efficiency.

To speculate the active species of photocatalytic activity, species trapping experiments including ·O2- radicals, ·OH radicals and h+, the typical reactive species generated in the photocatalytic reaction, were carried out. From Figure 5c, there was no obviously changes of photocatalytic efficiency when Iso Propyle Alcohol (IPA) was added to trap ·OH radicals. In contrast, as the Ethylene Diamine Tetraacetic Acid (EDTA) and p-benzoquinone (BQ) were added to the solution to trap h+ and ·O2- radicals, the photocatalytic activity obviously decreased, which indicated that ·O2- radicals and h+ were the major reactive species in the degradation process. Based on the viewpoint of applications, the chemical and physical stability of


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photocatalysts is a very important factor. Thus, 0D/2D heterojunction of bismuth molybdates was also tested. As shown in Figure 5d, the heterojunction still remained superior photodegradation efficiency after five consecutive photodegradation cycles, which indicated 0D/2D heterojunction of bismuth molybdates possessed excellent photocatalytic activity and favourable stability.

(a)

In dark

0.8

Bi2MoO6 sheets

0.6

0D/2D heterojunctions Bi4MoO9 spheres

(b) 1.00

Visible light irradiation

0.75

C / C0

C / C0

1.0

1.0

0.50 Blank Bi2MoO6 sheets

0.25

0D/2D heterojunctions Bi4MoO9 spheres

without photocatalyst 0.00

0.9 -60

0

60

120

180

Time/min

100

82.5%

Mechanical mixture

0

(d)

60

120

180

Time/min 1.00

79.8%

80

0.75

C / C0

(c)

Degradation (100%)

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


60 40

0.50

33.1%

30.2%

0.25

20 0.00 0 No Scavenger EDTA

IPA

BQ

0

1

2

3

4

5

Cycles

Figure 5. (a) Dark absorption-desorption over as-prepared samples and direct photolysis in the absence of

photocatalyst, (b) photocatalytic RhB degradation, (c) the species trapping experiments for degradation of RhB and (d) recycle RhB photodegradation results over 0D/2D heterojunction on bismuth molybdates.

To understand the reason for the enhanced photocatalytic activity of 0D/2D heterojunction, the photoeletrochemical performances were also investigated. As the photogenerated electronhole pairs are the dominant species in photocatalytic reaction, the separation of photogenerated carriers is of great importance. The photocurrent generation and photoluminescence (PL) spectra were performed to investigate the electron excitation and the separation efficiency of the photogenerated electron-hole pairs. As shown in Figure 6a, the photocurrent generated by the heterostructure was significantly higher than that of Bi2MoO6 sheets and Bi4MoO9 particles, suggesting more photogenerated electrons were produced in 0D/2D heterojunction under visible


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light irradiation. It was also found that the heterostructure exhibits the lowest photoluminescence intensity compared to Bi4MoO9 and Bi2MoO6 (Figure. 6b) which indicated that the QDs/NSs phase heterojunction could effectively suppress the recombination of the photogenerated charge carriers. In addition, the time-resolved PL measurement was also carried out to investigate the decay kinetics, the longest average lifetimes was observed for QDs/NSs phase heterojunction with 8.71 ps, and 5.91 ps for Bi2MoO6 sheets, 7.15 ps for Bi4MoO9 nanoparticles, respectively. The results are consistent with the steady-state PL spectra, thus resulting in enhanced photocurrent and photocatalytic performance.

Figure 6. (a) photocurrent vs time measurements in 0.10 mol·L-1 Na2SO4 and 0.10 mol·L-1 Na2SO3

solution at 0.20 V bias potential, 500 W xenon lamp with 420 nm cutoff filter, (b) Steady-state photoluminescence (PL) spectra, (c) band gap from UV-Vis diffuse reflectance spectra, (d) Mott-Schottky curves of Bi2MoO6 and Bi4MoO9 measured the above solution used and (e) illustration of charge transfer cross the 0D/2D heterojunction on bismuth molybdates.

The band structures of Bi2MoO6 and Bi4MoO9 were further examined by UV-Vis absorption spectra and Mott-Schottky measurements. From the absorption spectra, the band gap energy (Eg) of the bismuth molybdates can be determined by the following formula,43


14

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α hv = A(hv − Eg ) n 2

where α, h, v, Eg and A are absorption coefficients, Planck constant, light frequency, band gap and a constant, respectively. The values of Eg for pure Bi2MoO6 and Bi4MoO9 can be estimated as 2.66 eV and 2.89 eV (Figure 6c), respectively. The band gap of Bi4MoO9 is larger than that of Bi2MoO6, which are consistent with the previous reports.17,19 Mott-Schottky (MS) analysis is a standard technique, commonly used to determine the dopant density, the type of semiconductor (p or n type) and flat-band potential at semiconductor/liquid contacts.44 MS curve exhibits a positive slope, suggesting that the bismuth molybdate semiconductors were n-type semiconductors.45 The flat band potential of semiconductor in a liquid junction could be estimated from the Mott-Schottky equation,46

where

is the space charge capacitance in F cm-2,

dielectric constant of the semiconductor, density in cm-3,

is the electronic charge in C,

is the permittivity of free space,

is the applied potential in V,

is the

is the carrier

is the flat band potential in V,

is the

Boltzmann constant and T represents the temperature in K. As shown in Figure 6d, MS curves of Bi2MoO6 and Bi4MoO9 exhibited positive slopes, suggesting that both Bi2MoO6 and Bi4MoO9 are n/n heterojunctions rather than p/n junctions. It is generally known that the conduction band potentials (CB) of n-type semiconductor is very close to (about 0-0.2 V more negative) the flatband potentials.47,48 The flat band potential of Bi2MoO6 and Bi4MoO9 are estimated to be -0.16 V and -0.11 V vs. NHE by Mott-Schottky analysis. Therefore, the bottom of the conduction band of Bi2MoO6 and Bi4MoO9 are -0.36 V and -0.31 V, respectively. By further talking into account the band structure, a Bi4MoO9 QDs/Bi2MoO6 NSs phase junction is drawn to interpret the mechanism of the enhancement of photocatalytic activity, as shown in Figure 6e. Upon irradiation of bismuth molybdates with 0D/2D phase junctions, the photogenerated electrons tend to transfer from the Bi2MoO6 phase to the Bi4MoO9, while the photogenerated holes transfer from the Bi4MoO9 phase to the Bi2MoO6 phase, driven by the potential difference caused by the differing band levels of Bi2MoO6 and Bi4MoO9. Such transportation of the photogenerated carriers could extend their transfer path or stabilize the photogenerated holes in the valence band of Bi2MoO6, leading to the prolonged lifetime of the charge carriers.18,20 Although such potential


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difference is small, it can still serve as the driving force for efficient charge separation and transfer.12,13,49 As a result, the photogenerated electrons and holes can be spatially separated into two different phases and thus charge recombination is drastically inhibited, which is of great benefit for enhancing activity in the photocatalytic reaction. Additionally, the energy of electrons at the conduction band of Bi4MoO9 is -0.11 eV vs. NHE, more negative than the redox potential of O2/·O2- (+0.28 eV vs NHE), which indicated that the electrons could react with the adsorbed O2 to form active oxygen species ·O2- radicals. However, the energy of photoinduced holes of Bi2MoO6 is 2.50 eV vs. NHE, lower than the redox potential of H2O/·OH (+2.68 eV vs NHE), which means that the photoinduced holes can not to directly oxidize adsorbed hydroxyl groups for generating hydroxyl radicals. Moreover, the redox potential of RhB is 1.43 eV vs. NHE, which is much lower than the energy of the photoinduced holes of Bi2MoO6. It is reasonable to consider that the degradation of RhB could be conducted by the direct oxidation of photoinduced holes from the Bi2MoO6. Thus, the important active species in the degradation of RhB over Bi4MoO9 QDs/Bi2MoO6 NSs heterojunction are ·O2- radicals and holes, which are agree with the trapping experiments. Summarizing all the above results, the strong interaction between Bi4MoO9 QDs and Bi2MoO6 NSs has been conclusively demonstrated to significantly improve the separation efficiency of photoexcited charges through better interfacial charge transfer and promote the photocatalytic activity. Thereby, the 0D/2D heterojunction of bismuth molybdates could degraded almost 100% 7.0 mg/L RhB within 3 hours under visible light irradiation (λ >420 nm) (see Figure 5b), 13-23 times increase relative to single bulk Bi4MoO9 and Bi2MoO6 counterparts and comparable to the other 0D/2D heterojunction of vanadate QDs/graphitic carbon nitride NSs reported recently. This enhanced photocatalytic activity is contributed to the novel 0D/2D structure and phase heterojunction of biumuth molybdates as shown in Figure 1a and Figure 6e. Firstly, a large number of carriers are excited due to the suitable band gap of bismuth molybdates with visible-light-driven photocatalytic activity, and then the electrons transfer near the 0D/2D heterojunction interface across the smooth surface of NSs, as proved by the UV-visible absorption and current-voltage curves under visible-light irradiation. Secondly, the Schottky barrier at the interface is greatly reduced by the formation of heterojunction between the Bi4MoO9 QDs and Bi2MoO6 NSs, and the conduction band of Bi2MoO6 NSs is more negative


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than that of Bi4MoO9 QDs, the electrons transferring from Bi2MoO6 NSs to Bi4MoO9 QDs and the holes from Bi4MoO9 QDs to Bi2MoO6 NSs, to suppress charge recombination and boost interfacial charge transfer, which is confirmed by the PL spectra, the flat band potentials measurement and the band gap results from the UV-visible absorption. Finally, the Bi4MoO9 QDs served as an electron acceptor to trap the photo-excited electron from Bi2MoO6 CB, and the electrons and holes play a major role as reductive and oxidative species, which is confirmed the scavengers experiments. In brief, as shown in Figure 6e, the phase herterojunction between Bi4MoO9 QDs and Bi2MoO6 NSs could effectively lower the Schottky barrier at the junction interface to lead to electron transfer from 2D to 0D and holes from 0D to 2D. Thus, the Bi4MoO9 QDs coated on the surface of Bi2MoO6 NSs could function as traps to capture the photoinduced electrons and the 0D/2D heterojunction could act as an active center for hindering the rapid recombination of electron and holes. 4. Conclusions In summary, a new style of 0D/2D phase heterojunction of quantum dots/nanosheets on bismuth molybdates are successfully synthesized by an in situ growth strategy. The enhanced photocatalytic performance has been shown to be due to strongly coupling the Bi2MoO6 nanosheets and the highly dispersed small Bi4MoO9 qutuam dots, which lead to efficient charge separation and transfer across the bismuth molybdates phase junction. As polymorphic semiconductors are quite common in nature, an atomically well-matched phase junction with novel multidimensional structure, can be conveniently fabricated by fine-tuning the synthesized conditions. The 0D/2D phase heterojunction will open new avenues for the development of efficient photocatalysts for solar energy utilization, as well as photoelectronic devices. Acknowledgements The work was supported by the Shandong Excellent Young Scientist Research Award Fund (No. BS2015CL002), the Basic Research Project of Qingdao Source Innovation Program Fund (17-1-1-82-jch), the Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), and Natural Science Foundation of Shandong Province (No. ZR2016BM08). The authors are grateful to many of the colleagues for constructive discussion.


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Density-Functional

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Graphical Abstract


23

2D Phase

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