Shell Nanostructures for

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NaNbO3/MoS2 and NaNbO3/BiVO4 Core/Shell Nanostructures for Photoelectrochemical Hydrogen Generation Sandeep Kumar, TAMANNA MALIK, Deepanshu Sharma, and Ashok Kumar Ganguli ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00098 • Publication Date (Web): 18 Apr 2019 Downloaded from http://pubs.acs.org on April 18, 2019

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ACS Applied Nano Materials

NaNbO3/MoS2 and NaNbO3/BiVO4 Core/Shell Nanostructures for Photoelectrochemical Hydrogen Generation Sandeep Kumar,† Tamanna Malik,† Deepanshu Sharma,‡ and Ashok K. Ganguli†* †Department of Chemistry, Indian Institute of Technology, HauzKhas, New Delhi 110016, India ‡Department of Physics, Indian Institute of Technology, HauzKhas, New Delhi 110016, India

* Author for correspondence e-mail: [email protected] Tel No. 91-11-26591511 Fax 91-11-26854715

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Abstract: NaNbO3/MoS2 and NaNbO3/BiVO4 core -shell heterostructures show absorption range extending to visible region and high charge transfer rate at interface, lower charge recombination which result in overall enhanced photocatalytic activity in the visible region. NaNbO3/MoS2 core-shell heterostructures show higher solar-to-hydrogen conversion efficiency due to better alignment between core and shell interface. The Rct and RIFCT values of NaNbO3/MoS2 core-shell (from EIS studies) are significantly smaller than in NaNbO3/BiVO4 core-shell heterostructures suggesting the charge separation in NaNbO3/MoS2 is more suitable and hence shows higher photocatalytic activity towards photoelectrochemical water splitting and dye degradation. The experimental results were well supported by photoluminescence as well as time-resolved spectroscopy. Enhancement of cathodic current in NaNbO3/MoS2 core-shell heterostructure and from MottSchottky plots also indicates appearance of p-n junction formation between core and shell materials. The p-n junction assists in separation of photogenerated charge carriers at the core-shell interface. Increasing negative shift of the flat band potential for the NaNbO3/MoS2 photoelectrode suggested higher charge carrier concentration with reduced charge recombination in comparison with pristine MoS2, BiVO4 and NaNbO3/BiVO4 core-shell heterostructures. The enhanced performance makes these heterostructures ideal candidates for photoelectrochemical hydrogen evolution via water splitting. KEY WORDS: Photocatalysis, photoelectrochemical, water splitting, NaNbO3, MoS2, BiVO4, core-shell.


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INTRODUCTION Waste water from textile, paints, cosmetics materials, printing and many other industries have become one of the most challenging environmental problems influence people's lives, environment and our ecosystem by directly or indirectly.1,2 In the textile dyeing processes a large numbers of synthetic dyes (azo-based, polymeric, anthraquinone, triphenyl-methane, and heterocyclic-based) are employed and almost 12-15% of synthetic dyes (unutilized) are responsible for water pollution.3,4 Conventional methods like biodegradation, adsorption, combined coagulation and flocculation are not able to remove these synthetic dyes from wastewater because of stability, high salinity and solubility in water.5 Additionally, above mentioned methods convert these dyes from liquid phase to other media, resulting in secondary pollutants. Consequently, scientific communities need to develop environmental friendly technologies to prevent dye contamination by removal of these pollutants which are resistant to conventional methods. We can use photocatalytic reactions system to eliminate dissolved organic pollutants from wastewater.6,7 Simultaneously, for clean environment we need clean energy, molecular hydrogen production is envisaged with photoelectrochemical (PEC) water splitting powered by solar energy, a clean and renewable fuel is a possible alternate for the fossil fuels in future.8,9 The production of renewable energy with the help of solar irradiation is the most pursued solution to the energy and environmental concerns.10 The need of cheap and efficient process for water splitting and sustainable hydrogen production has gained increasing attention. Significant number of researchers are involved in efforts devoted to developing low-cost and high-efficiency photocatalysts in recent years.11 Currently, most of the hydrogen is produced from fossil fuels or natural gas with significant carbon emission.12 Photoelectrochemical (PEC) water splitting is considered to be one of the most capable techniques for hydrogen generation and directly 3 ACS Paragon Plus Environment


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converting water and sunlight to hydrogen and oxygen among these different methodologies.13,14 In a PEC method, some critical points are to be considered, like selection and design of the photoelectrode, the light absorption property and charge carrier transportation of the photoelectrode materials all of which determine the efficiency of the PEC cell towards the water splitting. The vital aim is to utilize the solar energy that is freely and easily accessible from sunlight and turn it into most demanding and less polluted energy (electricity) or use it to produce safe fuels such as hydrogen (H2) in a cost effective manner. Water electrolysis is one of the best technique and commonly used with other renewable power sources for example solar or wind provides a competent path for sustainable hydrogen generation for fuel cell electric vehicles15 and variable power sources performed via power-to-gas storage.16,17 For PEC applications, to reduces the overpotential we need additional driving force apart from the minimum thermodynamic requirement and minimize interfacial charge resistance due to the electron transfer processes at semiconductor/semiconductor/liquid interface.18-21 Many factors are considered for water splitting reactions by semiconductor photoelectrode, followings are important: (i) For visible region activity we need appropriate band gap (ii) higher charge carriers conductivity to assist photogenerated charge carrier transportation (iii) suitable band positions for efficient insertion of charge carriers (iv) stability of materials and (v) requirement of abundant and eco-friendly elements.22 Due to various positive aspects with TiO2, (photocatalytic activity, stability, and nontoxicity) we have considered as one of the most capable photocatalytic materials.23 But unfortunately due to its large band gap the light response window in solar spectrum and quantum efficiency confined in limited applications.24 Therefore, visible light driven efficient photocatalysts have become the centre point of research in this field. Among the strategies employed to solve the above problem is to modify the wide band gap semiconductor materials by appropriate narrow band gap sensitizers for


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fabrication of new type photocatalytic materials.25,26 Herein, we have chosen sodium niobate (NaNbO3) for designing efficient photocatalyst due to its characteristic perovskite structure and other useful properties including high absorption coefficient, good chemical stability, highly crystalline nature, ease of availability and low environmental impact.27-29 However, band gap of NaNbO3 lies in ultraviolet (UV) region and photocatalytic applications have been limited up to UV spectra. So, bare NaNbO3 is inefficient for photocatalytic applications in broad-range solar spectrum. The photocatalytic properties of NaNbO3 can be tailored by designing heterostructures with a narrow band-gap, chemically stable and efficient semiconductor material. Herein, we have selected MoS2 and BiVO4, two narrow band-gap (Eg=1.3 eV and 2.4 eV) semiconductors respectively for combining with NaNbO3 and form heterostructures. NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures, have been successfully synthesized via a two-step hydrothermal method. The photocatalytic performance of these heterostructures in the photoelectrochemical water splitting and degradation of Rhodamine B (RhB) organic dyes under visible light irradiation have been investigated. The sensitization effect due to MoS2 and BiVO4 is expected mainly via suppression of charge recombination, prompt charge transfer kinetics, and excellent electrochemical stability. Our studies revealed that, the core-shell heterostructures exhibited enhanced photocatalytic activity compared to that of the bare NaNbO3, MoS2 and BiVO4 counterparts and also with the commercially available TiO2, Degussa P25. This work visibly demonstrates high potential for the development of environmental friendly, non-noble metal based heterostructures for clean energy production and photocatalytic dye degradation applications based on the new NaNbO3/MoS2 and NaNbO3/BiVO4 heterostructures. EXPERIMENTAL SECTION: Preparation of NaNbO3: 5 ACS Paragon Plus Environment


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Hydrothermal route was used for synthesis of NaNbO3 nanocubes and the procedure is display in Scheme 1a. In synthesis procedure, niobium pentoxide (Nb2O5) was dispersed in de-ionized (DI) water. The dispersed amount of solution was then mixed with 2 molar NaOH solution and 0.5g of the surfactants (CTAB) and stirred at room temperature for 30 minutes. Finally, the reaction mixture was transferred to a 100 mL Teflon- hydrothermal vessel and heated at 130 °C for 60 h. At last synthesized materials were collected by centrifugation at room temperature.30 Preparation of MoS2: MoS2 nanoparticles were synthesized by the hydrothermal method. 0.88g of (NH4)6Mo7O24•4H2O and 0.7g of hydroxylamine hydrochloride were dissolved in 10 mL of DI water and the pH was adjusted to 6 with 2 M HCl. The solution was continuously stirred at 90º C for 30 minutes and then 2.64g of Na2S•9H2O dissolved in 10 mL of double distilled water was added and stirred for another 10 minutes. Teflon-lined stainless steel hydrothermal vessel was used for reaction at 130 °C for 12 h. The vessel was then cooled to room temperature naturally. Finally the products were centrifuged and washed four to five times with de-ionized water and ethyl alcohol. Preparation of BiVO4: For the preparation of BiVO4 nanomaterials, stoichiometric amount of Bi(NO3)3•5H2O and NH4VO3, 0.5 mM each were dissolved in 40 mL diethylene glycol-water solvent (3:1). The pH was adjusted to be neutral by adding HNO3 and NH4OH. The reaction mixture was then transferred to a 100 mL Teflon-lined stainless steel hydrothermal vessel and heated at 130 °C for 12 h. Centrifugation was used for sample collection, and final product washed four to five times with de-ionized water and ethyl alcohol and dried at 50 °C for 4 h in air.


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Synthesis of NaNbO3/MoS2 Core-Shell Heterostructures: For the Fabrication of NaNbO3/MoS2, core-shell heterostructures, we used surface functionalization route31 by using 3-mercaptopropionic acid (MPA) as the functionalization materials. 20 μL of MPA was used for functionalize the NaNbO3 nanocubes in deionized water and the resulting solution was continuously stirred for 1 h. Therefore, a solution of (NH4)2MoO4•2H2O was added gradually in reaction mixture at constant stirring. After 30 minutes, Na2S·9H2O was added to the above reaction mixture and continue stirred the solution for 30 minutes. Finally, the reaction mixture was shifted to a 100 mL Teflon-lined hydrothermal vessel and heated at 180 °C for 30 h. The final reaction products were collected by centrifugation, washed four to five times with de-ionized water and ethyl alcohol and dried at 60 °C for 4 h (Scheme 1b).

(a)

Centrifugation

Nb2O5 + NaOH + SDS/PA/CTAB on stirrer

Hydrothermal treatment

Cooling , washing the reaction mixture with H2O and ethanol

Nanocrystals of NaNbO3 in vial



(b)

3mercaptopr opionic acid

(c) Ammonium molybdate

3mercapto propionic acid

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Bismuth nitrate

Bi Bi

Bi

Bi Bi

Bi

Sodium sulfide

Shell (BiVO4)

Shell (MoS2)

B i3+ Bi V 3+ B Bi i

Core (NaNbO3)

Core (NaNbO3)

Ammonium metavanad ate

V

B i3+

V

B i3+ Bi B i3+ V

B i3+ Bi

Bi

V

B i3+

B i3+

Scheme 1: Schematic diagram showing the synthesis mechanism of (a) bare NaNbO3 nanocubes, (b) NaNbO3/MoS2 and (c) NaNbO3/BiVO4 core-shell heterostructures. Synthesis of NaNbO3/BiVO4 Core-Shell Heterostructures: The initially prepared NaNbO3 nanocubes were used as a raw materials (dispersed in double distilled water) and 3-mercaptopropionic acid (MPA) was used for surface functionalization materials. The final solution was stirred for 1 h and in next step, diethylene glycol solution of Bi(NO3)3•5H2O and NH4VO3 aqueous solution stirred separately for 30 minutes, and then mixed slowly and the reaction mixture was continuously stirred for another 30 minutes. The reaction mixture was then transferred to a 100 mL Teflon-lined hydrothermal vessel and heated at 180 °C for 40 h. The resulting products were collected after centrifugation, washed by de-ionized water and absolute ethanol several times and finally dried at 60 °C for 4 h (Scheme 1c). Results and Discussion: The formation of NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures and bare counterparts were analyzed by different instrumentation techniques. Powder X-Ray Diffraction (PXRD)


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Powder X-ray diffraction (PXRD) studies were conducted for investigation of phase composition, crystallinity and the structure of synthesized materials as shown in Figure 1. 1800

(a) Intensity (a.u.)

228

134

131

200

1200 NaNbO3/BiVO4 1000 103

800 600

NaNbO3/MoS2

400 200

20

600 400 200

NaNbO3/MoS2 0

NaNbO3 10

30

40 2(deg)

50

60

NaNbO3/BiVO4

(b)

800 220 008

1400

013

110

1600 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


70

NaNbO3 48

50

52

54 56 2 (deg)

58

60

62

Figure 1. (a) PXRD pattern of NaNbO3, NaNbO3/MoS2 and NaNbO3/BiVO4 core/shell heterostructures and (b) close look at the reflections from 48 to 62 degrees reflection range clearly shows shift towards higher angles in composite heterostructures.

The XRD patterns (Figure 1a) clearly indicate that all the reflections of NaNbO3 are associated narrowly with the reported orthorhombic phase of NaNbO3 (JCPDS card no. 895173). Further, reflections at 2θ values of 26.86 and indexed with (013) planes indicates for monoclinic phase of BiVO4 (JCPDS card no. 831698). Simultaneously, reflection at 2θ values of 39.73 were indexed with (103) planes revealed the hexagonal phase of MoS2 (JCPDS card no. 371492). Energydispersive X-ray spectroscopy (EDS) elemental mapping also clearly revealed the uniform distribution of MoS2 and BiVO4 over the NaNbO3 nanocubes in core-shell heterostructure. As shown in the PXRD pattern no other diffraction peaks (except (103) of MoS2 are observed in the NaNbO3/MoS2 heterostructures, because of low content of MoS2. The diffraction peaks of composites materials are well matched with bare counterparts and representing that new any other secondary phase is not formed in the presence of shell materials.32 Weak reflection at (101) 9 ACS Paragon Plus Environment


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characteristics planes in NaNbO3/MoS2 implies the low crystallinity and low proportion of MoS2 in the core-shell heterostructures (Figure S2). For further confirmation of shell materials we have performed XPS measurement and shown in supporting information (Figure S3). Clear evidence of core-shell formation is indicated by the observed shift of the XRD diffraction peaks to higher angles in the core-shell structures which is due to lattice shrinkage of the NaNbO3 core with the growth of MoS2 and BiVO4 shell (Figure 1b). We have also mentioned the d-spacing values of NaNbO3 and core-shell heterostructures after composite formation in supporting information (Table S1). Here, we can rule out the any possibility of substitution/doping of Mo (higher ionic radius, 0.68Å) in Nb (0.64Å) site since that would lead to increase in size (lattice parameter) and the X-ray reflection would have shifted to lower angles. Here the shift is observed towards higher angles. Williamson-Hall (W−H) plot was used for strain calculation in fabricated core-shell nanostructures and is determined by following equation33 βcosθ/λ = 1/D + η sin θ/λ

(1)

Here θ is the diffraction angle, β represent the full width at half-maximum (FWHM) of θ − 2θ peak, λ, η and D is represent X-ray wavelength, effective strain and crystallite size of nanomaterials. Slope of plotted data gives effective strain and crystallite size (Dsch) is calculated from the intercept of the plot of βcos θ/λ against sinθ/λ. Materials strain is shown in Figure 2 and slope of the fitted line in W−H plots revealed that the pristine NaNbO3 have a net tensile strain (positive strain, +0.5%), while after shell formation, NaNbO3 experiences compressive strain (negative), −1.22 and −1.55%

in case of NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell

heterostructures respectively (Figure 2). 10 ACS Paragon Plus Environment

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0.010

NaNbO3 NaNbO3/BiVO4

0.008

 cos/

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


NaNbO3/MoS2

0.006

0.004

Tensile strain +0.5%

compressive strain -1.55%

0.002 0.10

compressive strain -1.22%

0.15

0.20

0.25 sin/

0.30

0.35

0.40

Figure 2. WilliamsonHall plot of β cos θ/λ against sin θ/λ for bare NaNbO3, NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures. The tensile strain in NaNbO3 nanocubes is developed during the preparation of nanomaterials (due to nano size effect). The negative strain in NaNbO3 core-shell heterostructures is due to the outcome of the MoS2 and BiVO4 shell which compresses the lattice planes of NaNbO3 and simultaneously the PXRD diffraction peaks shift in the direction of higher angle34 in case of NaNbO3/BiVO4 core-shell heterostructure. From the strain calculation we can conclude that MoS2 (hexagonal) is much more compatible to NaNbO3 (orthorhombic) as compared to BiVO4 (monoclinic) system and this lattice compatibility is supportive for band bending between core and shell materials and is expected to be beneficial for better separation of the photogenerated charge carriers.35 Table 1: Lattice strain, crystallite size calculated by Scherrer equation Dsch (nm) and Williamson Hall method DWH (nm). 11 ACS Paragon Plus Environment


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Composition

DWH (nm)

Dsch (nm)

Lattice Strain (%)

NaNbO3

229.22

234.13

+ 0.5

NaNbO3/MoS2

125.95

129.21

─ 1.22

NaNbO3/BiVO4

107.36

105.31

─ 1.55

The calculated value of crystallite sizes and lattice strain obtained by using Williamson-Hall plot method and corroborate well with the crystallite sizes (DSch, in nm) calculated by the X-ray method using Scherrer’s formula and given in Table 1). Field Emission Scanning Electron Microscopy (FESEM) studies: Morphologies and chemical composition of prepared materials were analyzed by FESEM studies. The bare NaNbO3 nanoparticles have cube like (uniform) morphology with smooth surfaces (Figure 3a). The morphological alignment of the cube is not changed after the accumulation of MoS2 and BiVO4 shell precursors over core materials and it is visibly distinguished from the Figure 3b and c. However, compared to NaNbO3 cubes, the core-shell heterostructures display a rough surface and clearly representing the fine accumulation of MoS2 and BiVO4 particles as the outer coating on NaNbO3 nanocubes surfaces. The incorporation of MoS2 and BiVO4 shell on the NaNbO3 core does not significantly modify the morphology of bare NaNbO3. The presence of Mo and S in MoS2/NaNbO3 and Bi, V and O elements in NaNbO3/BiVO4 core-shell heterostructures is clearly revealed by EDS elemental studies (Figure 3d and e).


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FESEM NaNbO3/MoS2

FESEM NaNbO3/BiVO4

(a)

50 nm

50 nm

NaNbO3 100 nm

(b) cps/eV

Nb Mo S

Nb Mo Nb Mo C S O

4

O Na

S

NaNbO3/BiVO4

6

NaNbO3/MoS2

5

Nb Mo S

Na

Object 2718

(e)

7

6

5

cps/eV

Object 2719

(d)

7

100 nm

(c)

100 nm

Nb Nb V C O

4

V Na Nb Nb

Na

Bi

O

3

3

2

2

1

1

Bi

V

Bi

Nb

V Mo

Bi

0

0 0

2

4

6 keV

8

0

10

2

4

6 keV

8

10

Figure 3. FESEM Images of (a) bare NaNbO3 (b) NaNbO3/MoS2 and (c) NaNbO3/BiVO4 composite nanostructures with EDS elemental examination data of (d) NaNbO3/MoS2 and (e) NaNbO3/BiVO4 core-shell heterostructures. Simultaneously, for revealed the uniform shell formation over core we have performed EDS elemental mapping was conducted which clearly show the formation of cube-shaped core-shell heterostructures. FESEM images of core-shell heterostructures and EDS mapping clearly indicates a uniform distribution of shell material (MoS2 or BiVO4) over core and gives better understanding of the morphology of the nanocubes after core-shell fabrication (Figure 4). NaNbO3/MoS2

(a)

(c)

(b)

100 nm

(g)

100 nm

(Nb)

(Na)

NaNbO3/BiVO4 (h)

(d)

(i)

(Na)

(e)

(O) (j)

(Nb)

(f)

(Mo) (l)

(k)

(O)

(S)

(Bi)

(V)



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Figure 4. (a) FESEM picture of NaNbO3/MoS2 core-shell heterostructures and EDS elemental detection of (b) Na, (c) Nb, (d) O, (e) Mo, and (f) S. In (g) display the NaNbO3/BiVO4 core-shell nanomaterials with elemental mapping results of (h) Na, (i) Nb, (j) O, (k) Bi, and (l) V. The FESEM image of NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell nanomaterials depicted the coexistence of Na, Nb, O, Mo, S, Bi, and V elements in the as synthesized core-shell heterostructures which shows clear evidence of uniform core-shell formation. Thus, MoS2 and BiVO4 shell materials successfully deposited on the surface of NaNbO3 nanocubes, resulting in the formation of core-shell heterostructures having cubic morphology. Transmission Electron Microscopy (TEM) Studies TEM analysis in Figure S3a and b (supporting information) depicts core-shell formation of NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures respectively. HRTEM images as shown in Figure S3c and d show that different planes of core and shell materials are observed at the

interface.

Sodium

niobate

nanocomposites

heterostructures

(NaNbO3/MoS2 and

NaNbO3/BiVO4) are well crystallized and carefully match lattice fringes with intense reflection observed in PXRD. The TEM results also supported the fabrication of core-shell heterostructures of NaNbO3 with MoS2 and BiVO4 narrow band gap sensitizers. UV-vis Diffuse Reflectance Spectra: UV−vis diffuse reflectance spectroscopy (UV−vis DRS) study was used for analysis of absorption characteristic of the as-synthesized core-shell heterostructures. Figure 5a shows the reflectance spectra plot of NaNbO3, NaNbO3/BiVO4, and NaNbO3/MoS2 composite nanostructures. Unsurprisingly, NaNbO3 nanocubes reveal a distinct absorption band in the UV region at 380 nm but after formation of core-shell heterostructures with BiVO4 and MoS2 a significant bathochromic


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shift is observed in the absorption band of NaNbO3 which implies the contact at the interface between NaNbO3 with BiVO4 and MoS2 in the core-shell heterostructures.

12

120

(a)

NaNbO3 NaNbO3/MoS2

10

100 -2

(h) (eV .cm )

(b)

NaNbO3 NaNbO3/MoS2

NaNbO3/BiVO4

NaNbO3/BiVO4

80

2

8 6 4 2

40 20

0

200

60

2

F(R)∞

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


300

400

Wavelength  nm

500

600

0

2.5

3.0

3.5

4.0

4.5

5.0

h (eV)

Figure 5. (a) UV−vis absorption spectra of NaNbO3 and its composite nanomaterials (b) Tauc’s plot for optical band-gap calculations. Assuming the core-shell photocatalyst acts as direct band gap semiconductor, like NaNbO3 (linear behavior is recorded for the designed core-shell materials with 1/2 exponent equivalent to direct band gap transition). From the Tauc’s plot (Figure 5b) (modified Kubelka−Munk function versus the energy) calculated band gap values of NaNbO3, NaNbO3/BiVO4, and NaNbO3/MoS2 core-shell were 3.27, 2.90, and 2.60 eV respectively. The DRS result clearly revealed visible-light motivated photocatalytic activity was performed after the composite heterostructure formation. Photoelectrochemical Water-Splitting The linear scan voltammograms (LSVs) of the core-shell and bare materials are shown in Figure 6a. All the four samples displayed low dark currents and remarkable photoresponse upon illumination. For photo electrochemical water-splitting applications, we have fabricated photoelectrodes of bare NaNbO3, MoS2, BiVO4 nanomaterials, NaNbO3/MoS2 and 15 ACS Paragon Plus Environment


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NaNbO3/BiVO4 core-shell heterostructures. I-V (current-voltage) properties of PEC cells of all synthesized pristine materials and heterostructures used as the photoelectrode in 0.5M Na2SO4 electrolyte were analyzed without light (dark) as well as under visible light irradiation with a 150 W Xe arc lamp (Figure 6). The photo-response analyses determined via current–voltage plots are extremely encouraging. The NaNbO3 nanomaterials produced lower photo-oxidation current (~38 μAcm-2 at 0.9V vs. Ag/AgCl) and at onset potential about 0.12 V on anodic side.36 The core-shell heterostructure performed as a photocathode and exhibits higher photo reduction current density compared to bare materials and produced hydrogen at materials surface. The opencircuit voltage (OCV) was –0.24 V for NaNbO3/BiVO4 and –0.22 V for NaNbO3/MoS2 core-shell heterostructures versus Ag/AgCl reference electrode and short-circuit current was produced approximately 0.5 mAcm-2. The observed photocathodic current density is about 4.54 and 3.56 mAcm-2 at –1.0 V for NaNbO3/MoS2 and NaNbO3/BiVO4 respectively, higher than bare materials (1.08, 0.74 mA cm-2 for MoS2 and BiVO4 respectively) (Figure 6a). The onset potentials observed for core-shell heterostructures under visible light illumination was lower compared to bare counterparts (MoS2 and BiVO4) calculated at 0.6 mA current (Figure 6a inset). The photocurrent density enhancement is achieved with core-shell formation, which is consistently higher in NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures. The performance enhancement may be due to the greater width of light absorption window, increased separation of the photo induced charge carriers and also the enhanced electrochemically electrode/electrolyte active area due to core-shell surface. Transient photocurrent responses for NaNbO3/MoS2 core-shell heterostructures were also observed in 0.5 M Na2SO4 aqueous solution (Figure S4) and this study revealed the variation in the dynamics of photogenerated charge carriers.


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Additionally, electrochemical impedance spectroscopy (EIS) analysis was performed for further clarify the co-sensitization effect of MoS2 and BiVO4 in the core-shell heterostructure of NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell photoelectrode.

-2

-3 -2

(b)

NaNbO3/MoS2-L NaNbO3/BiVO4-D

-30

0.1

0.0 0.5 0.0 Applied Potential (V vs RHE)

-1 NaNbO /MoS -L, MoS -L 3 2 2 NaNbO3/BiVO4-L, BiVO4 -L 0

NaNbO3/MoS2-D

-0.12

NaNbO3-D

-0.09

-20

Dark current

1.0 0.5 0.0 Applied Potential (V vs RHE)

-0.5

NaNbO3/BiVO4-L NaNbO3/MoS2-L NaNbO3/BiVO4-D NaNbO3/MoS2-D

-0.06 -0.03 0.00 0.00

-10

1 1.5

NaNbO3/BiVO4-L

-40

(a)

Z''(KOhm)

-4

-0.2

Z''(KOhm)

-2

Current Density (mAcm )

-5

Current Density (mAcm )

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


0.02

0.04 0.06 Z(KOhm)

0.08

0 0

2

4 6 Z(KOhm)

8

10

Figure 6. (a) Measurement of photocurrent density under dark and visible light irradiation at a scan rate of 20 mVs-1 as a function of applied potential (V vs. Ag/AgCl) (b) EIS Nyquist plots for bare core materials, NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures in the presence of light and in dark. EIS characterization was performed on core-shell nanomaterials without light as well as under visible-light irradiation. The EIS plot (semicircles), revealed the participation of charge transportation at the electrode-electrolyte interface (Figure 6b). When moving from higher to lower frequency region the semicircle radius of the EIS spectrum revealed the charge transfer resistance at the interface. The magnitude of the EIS spectrum obtained for the NaNbO3/MoS2 and NaNbO3/BiVO4 heterostructures represent their elevated conductivity compared with bare NaNbO3. Figure 6b shows the results taken at –0.22 V and the observed slope in the high frequency range (Figure 6b inset) represents the Warburg behavior characteristic of synthesized materials.37 It is obvious, the EIS plots radii of the NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures in presence of irradiation are smaller in diameter than in dark condition. 17 ACS Paragon Plus Environment


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Therefore, the photo sensitization effect of MoS2 and BiVO4 shell over NaNbO3 (core) changes the generation and transfer rate of the photogenerated charge carriers at the interface. The EIS Nyquist plots were fit to circuit as mentioned in supporting information (Figure S5), where Rs represents series resistance, Rct, the surface resistance value and RIFCT, the charge transfer resistance at the interface (i.e. interfacial resistance). All EIS parameters of the analyzed data are given in Table S2 in supporting data. The Rct and RIFCT values of NaNbO3/MoS2 coreshell are 8.86 Ω and 3.4 Ω respectively, significantly smaller than NaNbO3/BiVO4 core-shell heterostructures (12.1 Ω and 8.36 Ω) which means that the charge separation in NaNbO3/MoS2 is more suitable and hence shows higher photocatalytic activity towards photoelectrochemical water splitting and dye degradation. EIS result is a clear appearance of easier electron-hole separation through the heterostructure which is also supported by photoluminescence spectroscopy and time resolved spectroscopy. The MoS2 core-shell heterostructures are more effective than BiVO4 towards enhancing interfacial charge transfer, lower charge recombination and delivered an ideal route for charge carrier’s transport to the current collector. Mott-Schottky Studies In order to confirm the consequence of core-shell formation on charge transfer at the coreshell interface and to study electronic properties like flat band potential, nature of semiconductor materials and donor density, we employ the Mott-Schottky plots as shown in Figure 7. The data was recorded at 1 kHz in 0.5 M Na2SO4 solution. Resulting flat band potential and donor density of the synthesized core-shell materials were calculated by using Mott-Schottky equation:36 1/C2 = (2/qε0εNd)(V − VFB − kBT/q)

(2)


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Where C is the specific capacitance (F/cm2), ε is the dielectric constant ε0 the permittivity of vacuum, Nd is the donor density, V the applied electrode potential, Vfb the flat band potential, and kBT/q has usual significance.

BiVO4

4.0

NaNbO3/MoS2 NaNbO3/BiVO4

3.5

MoS2

4

2.5

10

3.0 2.0

2

-2

1/C x10 (cm F )

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


1.5

NaNbO3

1.0 0.5 0.0 -1.0

-0.5

0.0 E/V vs Ag/AgCl

0.5

1.0

Figure 7. Mott Schottky plots of bare NaNbO3, BiVO4, MoS2 and NaNbO3/MoS2 and NaNbO3/BiVO4 core–shell nanostructures. In inset the apparent p-n junction formation between NaNbO3 and MoS2 in magnified views of NaNbO3/MoS2 core-shell heterostructures. For flat band potential calculation we extrapolated the X-axis intercepts of the linear region in Mott-Schottky plots (1/C2 vs. V) and calculated the Vfb values of NaNbO3/MoS2 and NaNbO3/BiVO4 which were found to be ─0.34 and ─0.097 V vs NHE respectively. Huge enhancement of photocathodic current in NaNbO3/MoS2 core-shell heterostructure is also the clear manifestation of p-n junction formation between core and shell interface. The coexistence of both positive and negative slopes in NaNbO3/MoS2 core-shell (Figure 7 inset) (inverted bell shape) clearly revealed the p-type behavior of MoS2 and n-type behavior of NaNbO3. This indicates that the p-n junction assists in photogenerated charge carriers (electrons and holes) separation at core19 ACS Paragon Plus Environment


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shell interfaces. Bare BiVO4 shows n-type behavior as shown by Mott-Schottky plot in Figure 7 but during core-shell synthesis, n-type property of BiVO4 become transform into p-type38,39 as visibly indicated in Figure 7 because equilibration of oxygen and bismuth was expected to pin the Fermi level. Therefore, composite materials perform like photocathode.40 The increasing negative shift of the flat band potential for the NaNbO3/MoS2 electrode suggests an elevated charge carrier concentration and lower recombination in the NaNbO3/MoS2 electrode in comparison with the pristine MoS2, BiVO4 and NaNbO3/BiVO4 core-shell heterostructures.41 Efficiencies of the most active photocathodic system The generation rate of H2 gas is a function of the applied bias voltage at the electrode and the photocurrent.42 During the analysis, the H2 production is visible at the electrode surface of the three electrode assembly. The transient H2 evolution measurement has been carried out for NaNbO3/MoS2 core/shell heterostructures (highly efficient materials) in 0.5 M Na2SO4 aqueous solution under the applied bias voltage of -0.35 V vs Ag/AgCl reference electrode. The measurement was performed for 90 min and shown in Figure 8. It is clearly apparent from the figure that the H2 generation rate is increased with time and average generation rate was 61 μmol per hr. Further, from the Faraday law of electrolysis, the hydrogen evolved is calculated as,43 t  Idt 10 No. of moles of Hydrogen: 2 F

(3)


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In this equation, I is the measured photocurrent, F is the Faraday constant (C/mol) and t is the time for which measurement has been done. The experimental results matched well with the calculated observations (solid green line) in figure 8 (a).

100

0.70

(a)

(b)

0.65

H2 evolution ( mol)

80

0.60

60

0.55

STH  

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


40 20

0.50 0.45 0.40

0

0.35 0

20

40 60 Time (min)

80

100

0

20

40 60 Time (min)

80

100

Figure 8. (a) H2 evolution as a function of time measured for NaNbO3/MoS2 sample and (b) solar to hydrogen (STH) conversion efficiency with respect to time. Another important factor is solar to hydrogen (STH) conversion efficiency which is used as a standard for water splitting application and generally expresses the efficiency of a PEC water splitting device with respect to solar irradiance. STH efficiency is presented by the following expression

J STH (η) =

ph

(V

V ) redox bias P light

(4)

Where, Jph is the current density at a given bias voltage, Vbias is the applied bias voltage, Vredox is the Gibbs free energy per electron and Plight is the power of the irradiance.44 21 ACS Paragon Plus Environment


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The STH conversion efficiency is measured with respect to time and shown in Figure 8 (b). The efficiency increases and then saturates after a certain time. Photocatalytic dye degradation The creation of charge carriers and their enhanced lifetime is efficiently used in photocatalytic dye degradation. These active species are used to oxidize or reduce organic dyes via waste water treatment to get rid of organic pollutant. Lot of methods are being used to secure the aquatic environment from toxic dyes, such as adsorption, electro coagulation, ultrasonic decomposition, nanofiltration, chemical coagulation and is followed by sedimentation technique to remove toxic dyes from wastewater.45 In this context, to explore the efficiency of the NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures, we have carried out photocatalytic experiments via an aqueous solution of Rhodamine B dye (RhB) (Figure 9). RhB dye has fluorescence property and can thus be detected easily and inexpensively with instruments called fluorometers and UV absorbance method. Standard Degussa P25 (TiO2) also used for comparative study with synthesized photocatalytic materials. The results shows P25 have relatively lower catalytic efficiency than core-shell heterostructures which may be due to its limited range of photoabsorption and also faster charge recombination at interface. But higher surface area and high crystalline nature of the P25 materials is also to be a factor in the degradation (43 %) of RhB in 60 min (Figure 9(b)). Photocatalytic activity compared with bare materials disclose the catalytic performances of NaNbO3, BiVO4 and MoS2 nanoparticles which result in∼1.1%, 25% and 46% degradation respectively of RhB under visible light illumination. However, introduction of the NaNbO3/MoS2 and NaNbO3/BiVO4 heterostructures significantly enhances the degradation efficiency and successfully proving the fact that fabricating appropriate semiconductor heterostructures boost the photocatalytic 22 ACS Paragon Plus Environment

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efficiency. The NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures reveal enhanced photocatalytic efficiency than bare MoS2 and BiVO4 (Figure S6), affording ∼95.2% and ∼74.6% photo degradation of the RhB solution after 60 minutes respectively. Many irradiation sources are used to study the kinetics of dye degradation process. The rate constant of the reaction was calculated using the following equations and represent in Table S3. ln(C/C0) = −kt t1/2= (ln2)/ k

(5) (6)

Where ‘t’ represent the degradation time, and k is the rate constant of the photodegradation reaction mechanism. Calculated values of rate constant of the degradation reaction was relatively higher in case of NaNbO3/MoS2 as compared to NaNbO3/BiVO4 core-shell which leads to the more rapidly degradation of organic pollutant (dye) under sunlight. The degradation of dye in polluted water usually follows the Langmuir−Hinshelwood mechanism.46 r = kKC / (1+KC)

(7)

Where r and k represent the photocatalytic reaction rate and rate constant (mol L−1 min−1), K is the equilibrium adsorption coefficient (L mol−1), and C represent the reactant concentration (mol L−1) at time t.



100 NaNbO /MoS 3 2

0.4

NaNbO3/BiVO4

0.2

NaNbO3/MoS2

(a)

0.0

95.2

60

P25 40 20 0

0

10

20 30 40 50 Irradiation Time (minutes)

NaNbO3/BiVO4

43.1

P25

0.6

80

(b)

74.6

0.8

Degradation Efficiency (%)

NaNbO3

1.0

C/Co

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

60

NaNbO3 1.1 At 60 minutes

Figure 9. (a) Photocatalytic activity representation and (b) Photodegradation efficiency of bare NaNbO3 nanocubes, Degussa P25, NaNbO3/BiVO4 and NaNbO3/MoS2 core-shell heterostructures. Since substrate concentration is very low, we can neglect KC (equation 7) and the calculated reaction kinetics display first-order performance, i.e., r = kKC. Integral form of the rate equations is given below: ln(C0/C) = kKt = kappt

(8)

Where C0 and kapp represent the initial reactant concentration and apparent photocatalytic rate constant respectively. Pseudo-first-order model was used for comparative study of reaction kinetics of composite materials and its individual counterparts. The plots of Rh B concentration versus time show approximately linear behavior and revealed the first order reaction kinetics performed (Figure S7). The lower photocatalytic activity of NaNbO3/BiVO4 may be attributed to the compatibility between core and shell materials. It may be noted that higher strain is developed (lesser compatibility) for orthorhombic NaNbO3 with BiVO4 as compared to that of MoS2 coreshell heterostructure. The band gap and photocatalytic activity of different materials such as,


Page 25 of 40

NaNbO3, P25-TiO2, NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructure for degradation of Rh Bare shown in supporting information (Table S3). Photoluminescence (PL) spectroscopy Photoluminescence and time resolved spectroscopy were sued for calculation of charge recombination rate of photogenerated charge carriers and lifetime decay in NaNbO3/BiVO4 and NaNbO3/MoS2 core/shell heterostructures and bare counterparts (Figure 10).

(a)

NaNbO3

(b)

BiVO4 MoS2

 NaNbO3/MoS2 C/S  NaNbO3/MoS2 C/S (3-exp fit)

1000

NaNbO3/BiVO4

PL Intensity (a.u.)

NaNbO3/MoS2

 NaNbO3/BiVO4 C/S  NaNbO3/BiVO4 C/S (3-exp fit)

Counts (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


100

τavg = 5.47 ns

10 τavg = 5.31 ns 400

500 600 700 Wavelength () nm

800

20

40 60 Time (ns)

80

100

Figure 10. (a) Photoluminescence spectra of NaNbO3, BiVO4, MoS2, NaNbO3/BiVO4, NaNbO3/MoS2 and (b) Time resolved PL decay curves of NaNbO3/BiVO4 and NaNbO3/MoS2 core-shell heterostructures, the measurement was performed at the energy analog to the PL peak taken from the Pl spectra as shown in (a). Here solid lines represent the result of multi-exponential fit to the exponential data. The optical properties of NaNbO3, BiVO4, MoS2, NaNbO3/BiVO4 and NaNbO3/MoS2 core-shell heterostructures were analyzed by using the photoluminescence spectra obtained after excitation at 350 nm and all PL experiments and time-resolved analysis were performed at room temperature under ambient conditions. Figure 10a shows a comparison among the PL spectra of all photocatalytic materials. After core-shell formation the new emission band is observed to have 25 ACS Paragon Plus Environment


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a red shift compared to the pure NaNbO3. Additionally, the PL intensity of NaNbO3/BiVO4 and NaNbO3/MoS2 core/shell heterostructures show a significant decrease of intensity compared to the pristine NaNbO3, BiVO4 and MoS2 nanomaterials. Lowest PL intensity observed for NaNbO3/MoS2 core-shell materials suggests the lowest charge recombination in this material. Time resolved-photoluminescence (TR-PL) decay spectra also recorded at the PL peak energy provides further details about the transfer of charge carriers in the core-shell (NaNbO3/BiVO4 and NaNbO3/MoS2) heterostructures (Figure 10b).47 A multi-exponential fitting is used to determine the relaxation time and other parameters for understanding the decay dynamics across the full decay time of core-shell materials at different time intervals. Exponential functions were used for quantitative assessment and the obtained decay profiles were fit successfully. 𝑛

I(t) = ∑𝑖 = 1Ai e

-𝜏

/τi

(9)

where I(t) represents intensity, τi and Ai are excited state lifetime and intensity related with ith component.48 NaNbO3/MoS2 heterostructures show higher decay time (τavg= 5.47 ns) compared to NaNbO3/BiVO4 (τavg = 5.31 ns) indicating that NaNbO3/MoS2 core-shell heterostructures are more compatible for charge transfer and have better band alignment between the core and shell semiconducting nanomaterials. The calculated parameters of the fluorescence lifetimes from the time-resolved PL decay plots are given in Table 2. The visual assessment of the residuals of the fit function to the data lies between −0.1 and 0.1 counts, which is within experimental error limits and the (χ2) criterion was used to accept the fitting (Figure S8). This residual fitted plot also revealed that there were no systematic trends in both core-shell materials. Time resolved decay 26 ACS Paragon Plus Environment

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spectra clearly indicate efficient charge separation of photogenerated charge carriers facilitated by the staggered (type-II) band alignment between NaNbO3, BiVO4 and MoS2 interfaces. On the basis of calculations, valence and conduction band of BiVO4 and MoS2 positioned above that of NaNbO3, resulting in the staggered band bending (Figure 11), which permit the transport of photogenerated electrons at interface from the conduction band of lower band gap materials (MoS2 and BiVO4) which acts as a sensitizer to the conduction band of the wide band gap semiconductor, NaNbO3 and at the same time holes are transferred in the opposite direction. The low PL intensity and higher decay time of NaNbO3/MoS2 compared to NaNbO3/BiVO4 core-shell heterostructures (Figure 10a and b) indicates the lower charge recombination, lower interfacial resistance and higher materials compatibility (for charge transfer) between NaNbO3 and MoS2. Compatibility of core-shell and interfacial resistance has been discussed using lattice strain and EIS measurement earlier. Consequently, the results obtained from the PL and time resolved study clearly show the core-shell heterostructures to significantly diminish the photogenerated charge recombination and extend the excited state lifetime, ensuring higher photocatalytic activity. Table 2. Fluorescence lifetimes parameters calculated from time-resolved PL decay plots. Sample

A1

NaNbO3/MoS2

0.67

0.18

0.25

1.8

NaNbO3/BiVO4

0.82

0.15

0.11

1.6

τ1(ns)

A2

τ2(ns)

A3

X2

τ3(ns)

τAV(ns)

0.089

8.5

5.47

1.21

0.034

9.4

5.31

1.33

Photocatalytic Reaction Mechanism: Here, it is essential to identify the reason behind the better photoelectrochemical water splitting and photocatalytic dye degradation ability of the NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell 27 ACS Paragon Plus Environment


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nanomaterials. The enhanced photocatalytic efficiency is a result of a combination of factors: First, the range of absorption by the core-shell materials was enhanced from approximately 380 to 440 nm from UV to Visible range (red shift) as shown in Figure 6. Secondly and most important factor is the efficient separation and reduced charge recombination rate of photo generated charge carriers at core-shell interface as revealed by photoluminescence studies.

Before contact formation

After contact formation Sun

e- e-

CB

CB

CB 2.6

Ef

Ef 3.27

VB

2.9

hv

Ef

MoS2

e-

CB Ef

Ef

VB h+ h+

VB NaNbO3

e- e- e- e-

VB h+

MoS2

BiVO4

NaNbO3

VB h+ h+

BiVO4

Figure 11. Schematic representation of the band alignmentbetween NaNbO3, MoS2 and BiVO4nanomaterialsafter core-shell heterostructure formation. Figure 11 represents the type-II band alignment exists between NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures. The alignment of the bands in the proper energy scale, were based on the calculated CB and VB positions for the concerned nanomaterials obtained using the following equation.49

ECB = X + E0 – 1/2Eg and EVB = ECB + Eg 28 ACS Paragon Plus Environment

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where Eg, ECB, EVB, represented band gap energy, conduction band and valence band potential, E0 indicated scale factor (E0 = −4.5 eV vs NHE) and X is the electronegativity of the semiconductor (expressed as the geometrical mean of the absolute electronegativity of the individual atoms). As seen in the energy bands scheme of NaNbO3, MoS2 and BiVO4 (Figure 11) the Fermi level of p-type MoS2 is close to their valence band maximum (VBM) while that of n-type NaNbO3 and BiVO4 is closer to there conduction band minimum (CBM). As the CBM of p-type MoS2 is located at elevated energy levels than NaNbO3 CBM, but after interface formation between n-type NaNbO3 and p-type MoS2 it forms a p-n heterojunction, leading to the alignment of the Fermi level and MoS2 energy band. Similarly in NaNbO3 and BiVO4 core-shell heterostructures, the Fermi levels are aligned after interfacial contact formation between core and shell materials. Table 3. Calculated parameters of band gap, CB and VB positions of bare NaNbO3, MoS2, BiVO4 and composite nanomaterials. Materials

Band gap (eV)

Conduction band (eV)

Valence band (eV)

NaNbO3

3.27

─0.23

3.04

MoS2

1.8

─0.29

1.8

BiVO4

2.7

0.2

2.9

NaNbO3/MoS2

2.6

─0.23 (NaNbO3) ─0.09 (MoS2)

3.04 (NaNbO3) 1.8 (MoS2)

NaNbO3/BiVO4

2.9

─0.23 (NaNbO3) 0.2 (BiVO4)

3.04 (NaNbO3) 2.9 (BiVO4)

From calculated values of band gap, CBM and VBM (Table 3) it is clear that after band bending establishment between core-shell nanomaterials the CB and VB position of BiVO4 and MoS2 lie at upper potential than that of NaNbO3 and successfully correlates with the type-II band alignment where both the CB and VB of shell materials (BiVO4 and MoS2) are may be above or below than



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those in the core materials (NaNbO3). Electrons are produced in the presence of light are transferred from the CB of shell materials towards the CB of NaNbO3 and hence the charge carriers generated photo chemically are frequently separated at the core-shell interfaces. These charge carriers migrate in opposite direction and minimize their charge recombination probability and hence can efficiently participate in photocatalytic oxidation and reduction reactions at different sites.50-52 The active species (superoxide radicals and hydroxyl radicals) involved in photodegradation reactions at the synthesized core-shell heterostructures surface. Earlier, published work clearly revealed the different reactive molecules participate in chemical reactions and take part for degrade the toxic organic molecule in simple nontoxic forms.53-55 Conclusions: Design of efficient materials for photoelectrochemical applications based on appropriate band – edge alignment and formation of p–n junctions were realized in NaNbO3/MoS2 and NaNbO3/BiVO4 core-shell heterostructures. These semiconductor heterostructures show higher efficiency towards photoelectrochemical water splitting and RhB dye degradation under solar visible light spectra compared to its individual counterparts (NaNbO3 or MoS2 or BiVO4). NaNbO3/MoS2 is more compatible for photocatalytic applications compared to NaNbO3/BiVO4 core-shell, primarily due to suitable lattice match with compatible band alignment (less strain). The higher H2 generation rate for NaNbO3/MoS2 (61 μmol per hr) compared with NaNbO3/BiVO4 and enhanced solar-to-hydrogen conversion efficiency also support the design of efficient alignment between core-shell heterostructures. Here, synergistic effect of the enhanced photo response range and low recombination of photogenerated charge carriers in the heterostructured materials is crucial for the higher efficiency. Such materials have a possibility to be employed for the production of clean renewable energy and for environmental remediation. 30 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Characterization instrumentation details, d-spacing table, PXRD bare MoS2 and BiVO4, XPS measurement of core-shell heterostructures, TEM and HRTEM data of core-shell heterostructures, photocurrent responses plot, EIS Nyquist equivalent circuit, Nyquist plot parameters table, Dye degradation data for bare MoS2 and BiVO4, Dye degradation kinetic plot and Residual plot for the fitted functions to the actual time resolved decay of (a) NaNbO3/BiVO4 and (b) NaNbO3/MoS2 core-shell heterostructures. AUTHOR INFORMATION Corresponding Authors Prof. Ashok K Ganguli E-mail: [email protected], Tel No. +91-11-26591511 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS S. Kumar and Ashok K. Ganguli thanks to Department of Science and Technology (DST) Govt. of India for financial support. T. Malik thanks to CSIR for fellowship and D. Sharma thanks to Indian Institute of Technology Delhi for financial support. References:



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1. Singer, h.; Muller, S.; Ceäline, T.; Laurent.; P. Triclosan: Occurrence and Fate of a Widely Used Biocide in the Aquatic Environment: Field Measurements in Wastewater Treatment Plants, Surface Waters, and Lake Sediments. Environ. Sci. Technol. 2002, 36, 4998-5004. 2. Kolpin, D. W.; Furlong, E. T.; Meyer, M. T.; Thurman, E. M.; Zaugg, S. D. Z.; Barber, L. B.; Buxton, H. T. Pharmaceuticals, Hormones, and Other Organic Wastewater Contaminants in U.S. Streams, 1999-2000: A National Reconnaissance. Environ. Sci. Technol. 2002, 36, 1202-1211. 3. Ehrampoush, M. H.; Moussavi, G. R.; Ghanian, M. T.; Rahimi, S.; Ahmadian, M. Removal of Methylene Blue Dye from Textile Simulated Sample using Tubular Reactor and TiO2 /UV-C Photocatalytic Process. J. Environ. Health Sci. Eng. 2011, 8, 35-40. 4. Natarajan, T. S.; Thomas, M.; Natarajan, K.; Bajaj, H. C.; Tayade, R. J. Study on UV LED/TiO2 Process for Degradation of Rhodamine B Dye. Chem. Eng. J. 2011, 169, 126134. 5. Kapdan, I. K.; Kargi, F. Simultaneous biodegradation and adsorption of textile dyestuff in an activated sludge unit. Pro. Biochem. 2002, 37, 973-981. 6. Kumar, S.; Khanchandani, S.; Thirumal, M.; Ganguli, A. K. Achieving Enhanced VisibleLight-Driven Photocatalysis Using Type-II NaNbO3/CdS Core/Shell Heterostructures. ACS Appl. Mater. Interfaces 2014, 6, 13221-13233. 7. Shanmugam, M.; Alsalme, Ali.; Alghamdi, A.; Jayavel, R. Enhanced Photocatalytic Performance of the Graphene‑V2O5 Nanocomposite in the Degradation of Methylene Blue Dye under Direct Sunlight. ACS Appl. Mater. Interfaces 2015, 7, 14905-14911. 8. Modestino, M. A.; Haussener, S. An integrated device view on photo-electrochemical solar-hydrogen generation. Annu. Rev. Chem. Biomol. Eng. 2015, 6, 13-34.


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Table of Contents

H H

Sun

e-

1000 Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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e- e- e-

e- e- e-

NaNbO3 CORE

100

10

h+ h+

h+

O2

SHELL

τavg = 5.47 ns τavg = 5.31 ns

20

40 60 Time (ns)

ACS Paragon Plus Environment

80

100

Shell Nanostructures for

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