Stimulating the Visible-Light Catalytic Activity of Bi2MoO6 Nanoplates

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Stimulating the Visible Light Catalytic Activity of Bi2MoO6 Nanoplates by Embedding Carbon Dots for the Efficient Oxidation, Cascade Reaction, and Photoelectrochemical O2 Evolution Subhajyoti Samanta, Santimoy Khilari, and Rajendra Srivastava ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00282 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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ACS Applied Nano Materials

Stimulating the Visible Light Catalytic Activity of Bi2MoO6 Nanoplates by Embedding Carbon Dots for the Efficient Oxidation, Cascade Reaction, and Photoelectrochemical O2 Evolution Subhajyoti Samanta†, Santimoy Khilari†, and Rajendra Srivastava†* †

Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, Punjab, India

E-mail: [email protected] (Dr. R. Srivastava). Phone: +91-1881-242175; Fax: +91-1881-223395

________________________________________________________

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ABSTRACT ________________________________________________________ The present work demonstrates the facile synthesis and applications of carbon dots (CD) embedded Bi2MoO6 nanoplates photocatalyst in the oxidative coupling of amines and oxidation of toluene and ethylbenzene. The synthetic protocol is applied to afford good yields of benzimidazole/benzothiazole via the cascade reaction between benzylamine and oaminothiophenol/o-phenylenediamine. These photocatalytic reactions are carried out under very mild conditions using the household LED bulb as a light source and O2 (1 atmospheric pressure). The CD embedded, 2.4 wt% CD/Bi2MoO6 exhibits the best photocatalytic activity. Impressive visible light absorbance coefficient, quantum confinement, photoluminescence up-conversion, and stable photoelectrochemical properties of CD are contemplating the excellent photocatalytic activity of CD/Bi2MoO6 than the pristine Bi2MoO6. Generation and influence of various reactive species in these catalytic reactions are investigated by radical scavenging, fluorescence spectroscopy, and cyclic voltammetric analysis. Both qualitative and quantitative estimation of the in-situ generated H2O2 in the photocatalytic oxidative coupling of amines was ascertained using CV and redox titration, respectively. Further, the influence of substitution in the benzylamine and involvement of the carbocations are confirmed using Hammett plot. The developed catalysts are also used as photoanode for O2 evolution from water oxidation in a photoelectrochemical (PEC) cell. Several PEC techniques evaluate the PEC activity of the photoanodes. The reactivity order for various substituted benzylamine and the involvement of reactive oxygen species (O2-.) in the oxidation reaction was obtained and confirmed from the band edge potentials of the best photocatalyst using Mott-Schottky analysis. Efficient catalytic recyclability and photostability are additional important features of the present investigation. This study provides a feasible alternative to the development of non-noble metal (carbon dots) based nanocomposite photocatalysts that can manifest important photocatalytic and photoelectrocatalytic applications in chemical synthesis and solar fuel production. KEYWORDS: Photocatalysts, Photoelectrochemical activity, Amine oxidation, Cascade reaction, O2 gas evolution, Carbon dot, Bi2MoO6. ___________________________________________________________________________


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INTRODUCTION The formidable energy demand across the globe created an appetite among the researchers to design and develop new technologies for energy production1 and storage.2 Among the various technologies, artificial photosynthesis involving semiconductor materials is one of the promising methods that uses sunlight as natural energy source and make the process unique. 36

Semiconductor mediated water splitting (for the production of H 2 and O2 gas) is one of the

thrust research areas in the energy sector after the breakthrough discovery led by Honda and Fujhisima.7 The photo-excited electrons reduce water molecules to produce H2 while the separated holes oxidize the water molecules to produce O2.8,9 The main difficulty associated with this process is the sluggish kinetics of water oxidation by the holes (4 times slower than that of the electron).10 Thus, O2 production is kinetically less favourable in the total water splitting because it requires 4 electron abstraction which is difficult to endure as compared to H2 concerning to its thermodynamics.11,12 Therefore, photocatalytic/photoelectrochemical O2 evolution is comparatively challenging task than H2 evolution.13-17 This limitation impulse upon the researchers to develop sustainable photocatalyst for O2 evolution to make an efficient total water splitting system.18,19 Several co-catalysts including noble metals have been employed to facilitate the hole transportation, but the economy remains an issue for its practical implementation.20-24 Therefore, the development of economical co-catalyst is highly desirable. Heterogeneous photocatalytic organic transformations for the fine chemical synthesis is an emerging and rapidly growing areas due to its positive environmental impact. 25-27 Visible light driven semiconductor mediated organic reactions have several advantages over the conventional thermal catalysis such as the use of sun light as abundantly available and sustainable energy source, mild reaction condition, high product selectivity etc. One-pot oxidation and reduction can be accomplished by using light induced holes/electrons, and high selectivity can be achieved for the desired products. The selective oxidation of amine to imine is one of the most important organic transformations. Imines are important synthetic intermediate that are used as a building block for other valuable chemicals like pharmaceuticals, agrochemicals and other important fine chemicals synthesis.28-30 In conventional practice, imines are synthesized by the condensation of a carbonyl group with an amine by using a dehydrating agent. 31 Semiconductor mediated oxidation of amine in the presence of molecular O2 under the visible light exposure is the most environmentally benign process. Further, H2O2 is an interesting molecule due to its high commercial importance in 3 ACS Paragon Plus Environment


fuel cell and also as a safe oxidizing agent in petrochemical/fine chemical industry. 32,33 Conventional synthesis of H2O2 involves the hydrogenation of anthraquinone on a Pd catalyst followed by the oxidation with O2. This process is not safe since it involves explosive O2/H2 gas mixture.34,35 Therefore, photocatalytic H2O2 production using O2 and water under the visible light illumination would be the best sustainable and safe method. Among the various photocatalysts, Bi-based oxides BiVO4, Bi2WO6, Bi2MoO6, BiOX (X=Cl, Br, I), and BiPO4 have been extensively studied in the last few years.36-42 Amongst the Bi-based photocatalysts, γ-Bi2MoO6 is a promising photocatalyst with high visible light absorbance coefficient.38,

43,44

Bi2MoO6 has a suitable band gap of 2.6 eV along with a

suitable band edge position that enables it to exhibit photocatalytic redox properties. 43,44 Most of the studies based on Bi2MoO6 are focused on the degradation of organic dyes as water pollutants.43-47 Extensive investigation with various Bi2MoO6 nanostructures has been reported for a large number of organic dyes.48,49 Very recently, Bi2MoO6 has been reported for visible light driven oxidation of alkenes and alcohols.50,51 Literature reports also suggest that Bi2MoO6 can oxidize water molecules to produce O2 gas.52 Some recent studies highlight that Bi2MoO6 has capability to reduce CO2 to CH4 under visible light condition.53,54 However, the photocatalytic activity of Bi2MoO6 is limited due to the rapid recombination of photogenerated charge carriers. Several strategies have been adopted to promote the photocatalytic activity which includes, heterojunction formation, Z-scheme system design, doping of non-noble metals, and the incorporation of noble metal nanoparticles. 55-59 Further, a recent study suggests that the photocatalytic oxidation for benzyl alcohol is enhanced by loading Pt nanoparticles as a co-catalyst. 60 Di et al. reported that upon incorporation of carbon dots over Bi2MoO6, it exhibit better photocatalytic activity for tetracycline oxidation than pure Bi2MoO6.45 Sun et al. reported achievement of better photocatalytic activity for Rh B and methylene blue degradation upon carbon dots loading on Bi2MoO6 nanosheet.46 Zhang et al. claimed that Bi2MoO6 became able to produce hydrogen gas from photocatalytic water reduction only after carbon quantum dots loading.47 To make the overall system economically viable without compromising the efficiency, carbonaceous materials are incorporated as photosensitizer or promoter into the phtocatalytic materials.61 In this context many nanocarbon comes to the lime light for hybrid photocatalyst design. Among different nanocarbons, carbon dots (CD) are considered to be a potential candidate for the design of an efficient nanocomposite photocatalyst. CD belong to the carbonaceous family and exhibited impressive visible light absorbance coefficient, quantum confinement, stable optoelectrochemical properties, mechanical and thermal properties. These properties make it a 4 ACS Paragon Plus Environment

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potential candidate as a photosensitize/promoter for the efficient harnessing of solar energy.62-65 The above mentioned fascinating properties of carbon dots have encouraged us to develop carbon dots-Bi2MoO6 (here after designated as CD/BiMO) based nanocomposite materials for unique photocatalysis. The novelty of this study is that CD/BiMO nanocomposites are demonstrated here for the selective oxidation of amine to imine under visible light (10 W LED lamp) illumination using O2 (at 1 atm). One-step tandem reaction for the synthesis of benzimidazole and benzothiazole via photo assisted cascade protocol is demonstrated here for the first time using this photocatalyst. The efficient solar to fuel conversion (photo-electrochemical O2 evolution) activity is achieved using this material for the first time. In-depth mechanistic investigation is the main attraction of this manuscript. In this context carbon dots (designated as CD) were embedded on the Bi2MoO6 (BiMO) nanoplates by an in-situ hydrothermal method. Different amount of CD was incorporated on BiMO nanoplate to develop CD/BiMO based nanocomposites. Photocatalysts were investigated in the selective oxidation of amine to imine under visible light (10 W LED lamp) illumination using O2 as an oxidant. Moreover, the investigation of photocatalytic oxidation of amines revealed that carbon dots facilitated the charge separation by withdrawing electrons towards its surface from the conduction band of Bi2MoO6. These accumulated electrons reduced the O2 to superoxide radical ions which further oxidize the amine molecules. The favourable redox potential of reactant and photocatalyst prefers the one electron reduction of O2 which generates in-situ H2O2 that subsequently photo-decomposed to hydroxyl radicals and complete the oxidation reactions. Additionally, the catalytic activity was also demonstrated in the oxidation of toluene/ethylbenzene and cascade reaction between benzylamine and o-phenylenediamine/o-aminothiophenol. Furthermore, photocatalysts were employed in the photoelectrochemical (PEC) O2 evolution under visible light illumination. A significant improvement of photocatalytic and photoelectrochemical activity was observed upon CD incorporation onto the Bi2MoO6 matrix. A detailed mechanistic investigation was carried out to find out the reasons behind this significant improvement of the photocatalytic and photo-electrochemical activities. Efforts were made to correlate the catalytic activity with various photo-physical properties of the CD/BiMO material using several state of the art analytical measurements. PEC and photoluminescence measurements confirmed that carbon dots on the Bi2MoO6 nanoplates prolonged the lifetime of photogenerated charge carriers and facilitated the separation and migration of the charge carriers efficiently which are responsible for the high photocatalytic activity of carbon dot-Bi2MoO6 nanocomposites in the 5 ACS Paragon Plus Environment


present investigations. The catalytic activity of the presently investigated material is also compared with commercially available TiO2 (Degussa P25) to demonstrate its superior efficiency than TiO2. This study will surely encourage developing new carbon dots based photocatalytic materials for several plausible applications. Such a comprehensive and systematic study for the photocatalytic fine chemical synthesis and solar to fuel production is reported here for the first time.

EXPERIMENTAL SECTION Details of materials, synthesis of Bi2MoO6 (BiMO) nanoplates,52 carbon dots,66 and carbon dots-Bi2MoO6 (CD/BiMO) nanocomposites; procedure for photocatalytic oxidation, cascade reaction, and photoelectrochemical (PEC) O2 evolution; Hammett plot, effect of substitution on benzylamine oxidation, determination of H2O2 concentration; hydroxyl radicals (ȮH) trapping experiments using fluorescence and CVs are provided in Supporting Information.

RESULT AND DISCUSSIONS Physico-chemical Characterizations Figure 1a shows the powder XRD patterns of BiMO and CD/BiMO nanocomposites. Reflections observed in the powdered XRD pattern of BiMO is well matched with the standard diffraction pattern of Bi2MoO6 (JCPDS file no. 7623-88). BiMO is formed in the periodic packing of [MoO4]2- and [Bi2O2]2+ in which [MoO4]2- occupied the corner octahedron sites leading to the alternate sandwich like staking in between them.52 This type of packing results in the formation of nanoplate like structure for BiMO. The synthesis scheme and crystal structure obtained from XRD of BiMO is presented in Scheme S1. Further, the CD/BiMO nanocomposites exhibit reflections corresponding to BiMO only. No significant reflection corresponding to the carbon dots is observed in the XRD patterns of CD/BiMO nanocomposites. This is attributed to the presence of small content of carbon dots in the resulting nanocomposites. However, one very less intense peak around 26 is observed in 6CD/BiMO, which is the characteristic reflection for standard graphite carbon67 suggesting the successful incorporation of CD over BiMO (Figure S1, SI). N2 adsorption-desorption analysis was carried out to determine the textural properties. Pristine BiMO exhibits a type II isotherm with an H3 hysteresis loop whereas 4CD/BiMO nanocomposite exhibits a type III isotherm with an H3 hysteresis loop (Figure S1b, SI). 6 ACS Paragon Plus Environment

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Textural properties of various materials prepared in this study are summarized in Table S1, SI. Textural properties depict that the surface area increases with the increase in CD contents up to 4 wt % in the nanocomposites (4CD/BiMO). However, further increase in the CD content (6CD/BiMO) leads to the lowering in the surface area.

(a)

100

Weight (%)

6CD/BiMO

4CD/BiMO

(b)

99

(0.5%)

98 97

(2.4%)

96

2CD/BiMO 4CD/BiMO BiMO 6CD/BiMO

95 94 93

200

10

20

30

40

50

2 (degree)

60

70

400

(3.7%)

600

Temperature/C

800

2CD/BiMO

Transmittance (%)

(202) (260) (331) (133) (262) (191)

BiMO

(062)

(002) (060) (220) (042) (240) (151)

(111)

(131)

(142) (080)

2CD/BiMO

(020) (120)

Intensity (arbitrary unit)

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


BiMO

(c)

4CD/BiMO

6CD/BiMO 1636 cm-1 1385 cm-1 1438 cm-1

3443 cm-1

500 1000 1500 2000 2500 3000-1 3500 4000

Wavenumber (cm )

Figure 1. (a) XRD patterns, (b) thermogramms, and (c) FTIR spectra of various materials synthesized in present study. The quantity of CD contents in various nanocomposites was investigated with the help of thermogravemetric analysis (Figure 1b). Pure BiMO exhibits high thermal stability and shows two decomposition temperature regions (50-300 C and 350-600 C) with negligible amount of weight loss in its TGA regime. The first weight loss is due to the removal of water molecules from the sample and the second weight loss is due to the condensation and removal of surface bound –OH in the form of water molecules. All the CD/BiMO nanocomposites exhibit TGA profiles with decomposition temperature around 500 C (Figure 1b). This decomposition is due to the removal of CD present in the sample in the form of CO2/CO gas. The CD contents incorporated in 2CD/BiMO, 4CD/BiMO, and 7 ACS Paragon Plus Environment


6CD/BiMO are found to be 0.6 wt %, 2.4 wt %, and 3.7 wt %, respectively. Amount of CD contents was also confirmed from CHN analysis (Table S1, SI). The input CD contents were more than the CD contents measured using TGA and CHN analysis. Therefore, these results demonstrate that all CD nanoparticles were not incorporated in the resulting nanocomposites and some of them were still present in the mother liquor of reaction mixture. The formation of CD/BiMO nanocomposites was further confirmed using FTIR analysis which gives useful information about the presence of various functional groups in the nanocomposite materials. Figure 1c presents FTIR spectra of pristine BiMO and all the CD/BiMO nanocomposites. FTIR absorptions in the range of 500-600 cm -1 and 700-950 cm-1 are attributed to Mo-O stretching vibrations and stretching/deformation modes of Bi-O bonds, respectively in all the samples. One characteristic peak at 730 cm-1 is due to the asymmetrical stretching vibration of MoO6 units while the two other peaks at 570 cm-1 and 600 cm-1 are attributed to the bending vibrations of MoO6 units.43 Moreover, one sharp peak at 1385 cm-1 is observed in all the nanocomposite materials which is due to the incorporation of NO2 group in the sample since all the materials were synthesized in acidic (HNO3) medium (pH=2).48 CD/BiMO samples also exhibited two additional peaks at 1636 cm -1 and 1438 cm-1 which are originated from the -COO- and –C=O groups of carbon dots present in the nanocomposites (Figure 1c). A broad band at 3443 cm -1 is assigned to the presence of surface hydroxyl groups in these samples. Thus, the FTIR analysis provides evidence for the incorporation of CD in the resulting CD/BiMO nanocomposites. The morphological information of the materials was obtained from FESEM analysis. Figure 2 displays the FESEM images of pure BiMO and CD/BiMO nanocomposites. Pristine BiMO exhibits assembly of nanoplate like morphology. Furthermore, FESEM images (Figure 2 (a) and (b)) indicate that the orientation of these nanoplates is not uniform and the average width of these nanoplates falls within 100 nm. The presence of individual elements in BiMO was confirmed from the EDAX and elemental mapping during FESEM analysis (Figure S2, SI). Addition of carbon dot in the synthesis composition of BiMO has only marginal influence on the final morphology of the nanocomposites. The 4CD/BiMO exhibits highly aggregated morphology in which BiMO nanoplates are orientated in different directions (Figure 2 (c)). Similarly, the 6CD/BiMO exhibits aggregated non-uniform morphology (Figure 2 (d)). Carbon dots are not visible in the FESEM images but their presence was confirmed with EDAX and elemental mapping recorded with the same sample (Figure 2(ek)). 8 ACS Paragon Plus Environment

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(a)

(b)

(c)

(d)

(e)

(f)

800 nm

(h)

(g)

(j)

300 nm

(i)

5 4 3

Mo

(k) C

O

Mo Bi

2 1

300 nm 0

Bi

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Figure 2. FESEM images of (a-b) BiMO, (c) 4CD/BiMO, (d) 6CD/BiMO. Total elemental mapping (f) from the region shown in (e) for 4CD/BiMO, and individual mapping of (g) C, (h) O, (i) Mo, and (j) Bi atom, and (k) EDAX spectrum for 4CD/BiMO. 9 ACS Paragon Plus Environment


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The nanoscale morphology was evaluated by high resolution transmission electron microscope (HRTEM). The bright field TEM images depict that BMO exhibited nanoplate like morphology which is composed of several small nanoslabs (Figure 3a, Figure S3a). The average diameters of these nanoslabs are around 50 nm (Figure S3a, SI). The diameter of the nanoplates is found to be 900 nm (Figure S3c, SI). Further, the TEM images of the composite confirm the presence of carbon dots in the 4CD/BiMO sample (Figure 3c, Figure S3b (SI)). Several beautiful images were captured for 4CD/BiMO showing the presence of CD (Figure 3c-d). Carbon dots adhered to the surface of BiMO is shown in the representative HRTEM images (Figure 3c, d). Furthermore the decoration of carbon dots on the surface of BiMO can be clearly visualized that have an interface between CD and BiMO (Figure 3d,e). The inter planar distance between two adjacent planes is found to be 0.275 nm for BiMO and 0.321 nm for CD which are identical to the 200 lattice plane of standard BiMO and 002 lattice plane of standard graphitic carbon, respectively (Figure 3d).47 The selected area electron diffraction (SAED) pattern of BiMO exhibit cubical bright diffraction spot array indicating the highly crystalline nature of BiMO nanoplates which can be indexed as 202, 002, and 200 planes of standard Bi2MoO6 (Figure 3f).

(a)

(b)

(e)

(d)

Carbon dot Figure 3. TEM images of (a-b) BiMO and (c) 4CD/BiMO; (d, e) HRTEM of 4CD/BiMO and (f) SAED pattern of BiMO. 10 ACS Paragon Plus Environment

(c)

(f)

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In this context a more detailed insight on the topographical information can be obtained from the atomic force microscopy (AFM). The AFM images of BiMO and 4CD/BiMO and their corresponding height profiles are presented in Figure S4, SI. The height profiles indicate that the average distance/successive gaps between two adjacent nanoplates are 433 nm in BiMO (Figure S4a) whereas it is 424 nm in 4CD/BiMO (Figure S4b). The average surface sizes of these nanoplates in BiMO and 4CD/BiMO are 5.6 μm and 3.2 μm, respectively (Figure 4b). The AFM analysis infers that incorporation of CD in BiMO resulting in the lowering in the surface size and successive distance between two adjacent planes. The chemical constituents and their respective oxidation state in the 4CD/BiMO nanocomposite were confirmed by X-ray photoelectron spectroscopy (Figure 4). The presence of all the elements was verified from the full survey XPS scan profile (Figure 4a). Further, the high resolution XPS spectrum of Bi 4f level consists of two peaks at 159.3 eV and 162.7 eV which can be indexed to Bi 4f5/2 and Bi 4f7/2, respectively (Figure 4b). The measured binding energies are matched to the (+3) oxidation of Bi. 68 Moreover, the Mo 3d level shows one set of peak centred at 235.1 eV and 231.8 eV which can be assigned to Mo 3d5/2 and Mo 3d3/2, respectively (Figure 4c). Further, the binding energies of Mo 3d doublets refers to (+6) oxidation state of the Mo.68 The O 1s spectrum composed of two peaks centred at 528.4 eV and 530.1 eV, respectively (Figure 4d). The peak at 528.4 eV is ascribed to the framework Mo-O/Bi-O oxygen whereas the peak at 530.1 eV is corresponding to the surface hydroxyl oxygen atoms. The high resolution XPS spectrum of C1s consists of two peaks, one at 284.7 eV assigned to (‘C=C’) SP2 hybridized carbon bond and another peak at 287.5 eV can be indexed to ‘C=O’ carbon atom in the 4CD/BiMO nanocomposite (Figure 4e). The XPS analysis also provides authentication for the successful incorporation of carbon dots on the surface of BiMO resulting in the formation of 4CD/BiMO nanocomposite photocatalyst. Light absorption followed by the charge carrier generation and their efficient separation were investigated by diffuse reflectance ultra violet visible spectroscopy (DRUVvis) and steady state photoluminescence spectroscopy (PL), respectively. Figure S5a, SI shows the UV-visible spectra of all the samples investigated in this study. BiMO has an absorbance edge around 475 nm whereas all the nanocomposites have absorbance edge at 490 nm. This shift suggests CD incorporation boosted the visible light absorption capacity.


600

400

Experimental peaks Peak 1 Peak 2 Sum of fitted peaks

Mo 3d5/2 (c)

Mo 3d3/2

238

200

0

Binding energy (eV)

236

234

232

230

Binding energy (eV)

228

Experimental peak Fitted peak 1 Fitted peak 2 Sum of fitted peaks

534

532

530

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Experimental peaks Bi 4f 7/2 Peak 1 Peak 2 Sum of fitted peaks

(b)

Bi 4f5/2

166 O 1s

528

164

162

160

158

Binding energy (eV)

Binding energy (eV)

(d)

526


800

C 1s Mo 3d

O 1s Bi 4d3 Mo 3p1 Bi 4d5 Mo 3p3

Bi 4p3

O KKL

1000


Bi 4f

4CD/BiMO

Mo 4p O 2s

(a)



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



Experimental peaks Fitted peak 1 Fitted peak 2 Sum of fitted peaks

290

288

286

156 (e)

C 1s

284

Binding energy (eV)

282

Figure 4. (a) Surface survey scan spectrum; high resolution XPS spectra of (b) Bi 4f, (c) Mo 3d, (d) O 1s, and (e) C 1s of 4CD/BiMO nanocomposite.

Only 4CD/BiMO has an absorbance coefficient higher than pure BiMO while the other two nanocomposites have lower absorbance coefficient than BiMO. So the obtained result indicates that the incorporation of carbon dots in BiMO influence the extent of light absorption in the nanocomposites. Further, the Tau plot is derived to measure the band gap of all the materials investigated in this study. The band gap energy of all the photocatalysts can be calculated by the following (eq 1).69 αh = k (hv - Eg)n

(1)

where, Eg represents the band gap of the corresponding material, h is the photon energy, k is the constant, α is the Kubelka–Munk function and n is dependent on the type of transition involved. Commonly, if n = 1 then it is contemplated as a direct transition and if n = 4 then the transition become indirect.69 In the present study, n = 4 found to be the best fit for (αh)1/n versus photon energy (h) plots proposing indirect allowed transition in these materials. The band gap of BiMO, 2CD/BiMO, 4CD/BiMO and 6CD/BiMO are found to be 2.65 eV, 2.67 eV, 2.69 eV, and 2.71 eV respectively (Figure S6).


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The migration and efficient separation of photo-induced charge carriers in the photocatalysts was investigated by photoluminescence spectroscopy. Materials were excited with a wavelength of 405 nm in the steady state photoluminescence analysis. All the material absorbed light intensity around 445 nm in the PL spectra (Figure S5b). Among all the semiconductors, BiMO exhibits the highest intense reflection in the PL spectrum which indicates that the extent of recombination of the charge carriers is the highest in this material. Among the nanocomposites, 4CD/BiMO exhibits the lowest intense peak implying the extent of separations and migration of photo induced charge carriers is the highest for this material. This analysis confirms that the incorporation of an optimum quantity of CD on BiMO can efficiently separate and minimize the charge carrier’s recombination process by extending the short carrier diffusion length70 as well as the lifetime, which will be beneficial for the efficient photoelectrocatalysis. For most of the condensation reactions, total acidity with medium strength play important role.69 Therefore, acidity was measured using NH3-temperature programme desorption technique (Figure S7). The BiMO shows the NH3 desorption in the temperature range of 250-375 C which demonstrate its medium strength acidity. Furthermore, a shift in the NH3 desorption temperature is observed for the all the CD/BiMO nanocomposites. With increase in the CD content (up to 4 wt %), the amount of NH3 desorption increases. However, with further increase in the CD content, the NH3 desorption amount is reduced. But, in the case of 6CD/BiMO, the desorption temperature is shifted to higher temperature, proposing the material is having the highest acid strength. The amount of NH3 desorbed follows the trend with the values of 3.3×10-3 mmol/g, 2.7×10-3 mmol/g, 2.4×10-3 mmol/g, and 2.1×10-3 mmol/g for 4CD/BiMO, 2CD/BiMO, 6CD/BiMO, and BiMO, respectively (Figure S7). Photocatalytic Investigation The aim of present study was to develop efficient, non-precious metal oxide based ecofriendly photocatalyst which can be operated under very mild condition using simple LED bulb in the presence of molecular oxygen at atmospheric pressure for the oxidative coupling of amines. Reaction parameters were optimized using benzylamine as a reactant. Benzylamine was converted into the corresponding product, N-benzylidenebenzylamine, with moderate yield under exceptionally mild reaction condition over BiMO (Table 1, entry 1). In addition to the desired product, benzaldehyde was obtained as a minor product. In the absence of light, no reaction took place (Table 1, entry 2). Reaction did not proceed when it



was carried out in the absence of catalyst (Table 1, entry 3). Optimization shows that when the reaction was performed in Ar or N2 atmosphere, very low conversion of benzylamine was achieved (Table 1, entries 4, 5). However, conversion of benzylamine was less than half when the reaction was carried out in air when compared to the reaction performed in O2 (Table 1, compare entries 1 and 6). Based on these results, O2 was optimized as preferred atmosphere for this reaction. The influence of the solvent on the reaction was also investigated (Table 1, entries 1, 7-11). In all the solvent, the reaction proceeded and produced N-benzylidenebenzylamine selectively in different yield. Not only the solvent adsorption and oxidation but the polarity of solvent is also important because it influences the condensation reaction step which is required to produce imine. DMF is having the highest polarity among the various polar solvents investigated in this study but the lower product yield was obtained in DMF because of the competitive adsorption between the solvent and the substrate might result in the sluggish reaction rate. Similarly, reaction proceeded well in toluene but in this medium comparatively higher amount of benzaldehyde was obtained as a by-product. Based on the results obtained, CH3CN was chosen as the optimum solvent for this reaction. Moreover, at the optimum reaction condition, 250 W medium pressure Hg lamps exhibited only marginally higher activity and selectivity when compared to the reaction performed using 10 W LED bulb (Table 1, compare entries 1 and 12). Hence, considering the low cost and easy availability, further reaction was optimized using 10 W LED bulbs. It was found that benzylamine conversion and N-benzylidenebenzylamine selectivity were increased using CD/BiMO nanocomposite. With increase in the CD content (from 2CD/BiMO to 4CD/BiMO), both benzylamine conversion and N-benzylidenebenzylamine selectivity were increased. However, with further increase in CD content (6CD/BiMO), benzylamine conversion and N-benzylidenebenzylamine selectivity were reduced. Based on the screening experiments, it is concluded that 4CD/BiMO is the most effective photocatalyst investigated in this study. Further the catalytic activity of the presently investigated material (4CD/BiMO) is also compared with the commercially available photocatalyst TiO2 (Degussa P25) and the obtained results implies that CD/BiMO exhibits superior activity than TiO 2 for benzylamine oxidation (Table 1, compare entries 14 and 16). After the optimization of reaction parameters and catalysts, various benzylamines and N-alkyl benzylamines were subjected to oxidative coupling reactions (Table 2). Para-substituted benzylamines were successfully converted to corresponding N-benzylidenebenzylamine in high yields. Benzylamine substituted with electron-donating groups undergo oxidative coupling more efficiently than that of benzylamine substituted with electron-withdrawing groups. 14 ACS Paragon Plus Environment

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Table 1. Optimization of reaction parameters in the oxidative coupling of benzylamine. E.No. Catalyst

Atmosphere

hv source

Solvent

(1 atm)

Benzylamine Imine conv. (%)a

selec. (%)a

1

BiMO

O2

10 W LED

CH3CN

69

96

2.

BiMO

O2

Dark

CH3CN

benzylamine (Figure S11b, SI). The influence of different gaseous environment in the oxidation of benzylamine was also investigated (Figure S11c, SI). The results indicate that the oxidation leads to produce a significant amount of H2O2 in the presence of O2 while under inert atmosphere very less amount of H2O2 was obtained. Further in the presence of air atmosphere, higher amount of H2O2 was produced which well matched with the yield of the oxidized imine studied earlier. This is due to the presence of O2 in air atmosphere (Figure S11c, SI). Stability and recyclability of the photocatalyst


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The PEC O2 evolution was recycled for three successive runs which confirmed its high reproducible PEC activity (Figure 6d). The important feature of this catalytic process is that the catalyst was easily separated and recycled. Only a marginal decrease in the benzylamine conversion was obtained during the oxidative coupling of benzylamine in five successive runs over 4CD/BiMO (Figure S12, SI). The recycled catalyst exhibited no significant difference in terms of local structure [(as confirmed by X-ray diffraction analysis) (Figure S13, SI), morphological analysis (Figure S14, SI), textural properties (as confirmed by N2adsorption study) and carbon content (TGA analysis)] with reference to the fresh catalyst. Moreover, the 4CD/BiMO exhibited a stable current response over the period of 300 mins under illumination condition, which reflects its high stability (Figure S15, SI). Based on the findings it can be concluded that the developed photocatalyst has high stability and recyclability which is one of the key parameters to be an excellent heterogeneous photocatalyst.

CONCLUSION Present work demonstrated the synthesis of carbon-dots embedded Bi2MoO6 nanoplates for photocatalytic amine oxidation and PEC water splitting. Microscopic and spectroscopic investigations confirmed the successful formation of 5-7 nm carbon dots embedded Bi 2MoO6 (with nanoplate dimension of  1 m). The developed materials exhibited excellent photocatalytic activity in the selective oxidation of amines, toluene, and ethylbenzene using a house hold LED lamp with O2 gas as an oxidant. Among various materials, 2.4 wt % of carbon dots on Bi2MoO6 (4CD/BiMO) fascinated the best photocatalytic activity which can be attributed to the highest charge carriers migration and separation leading to the minimal recombination, suitable band edge position, and high visible light absorbance coefficient of carbon dots. 4CD/BiMO nanocomposite offered excellent product yields in the oxidative coupling of amines (>90%) and oxidation reactions (benzaldehyde and acetophenone yields of 26 % and 38 %, respectively). Photochemical oxidation activity and acidity of the catalyst enabled us to demonstrate the application of this catalyst in the one-pot synthesis of benzimidazole/benzothiazole with very good yield ( 80%) by the reaction of benzylamine and o-phenylenediamine/o-aminothiophenol using 10 W LED bulb. Furthermore, 4CD/BiMO photoanode evolved 1.40 μM.cm-2.h-1 O2 gas and 1.05 mA.cm-2 photocurrent density at 1.23 V vs. RHE bias potential under visible light illumination, whereas pristine BiMO produces 0.77 μM.cm-2.h-1 O2 gas under the potential bias of 1.23 V vs. RHE and 394 μA.cm -2 27 ACS Paragon Plus Environment


photocurrent density. The magnitude of half cell solar to thermal conversion efficiency (HCSTH %) was found to best for 4CD/BiMO photoanode (0.075%) among all the photoanodes. The photocatalytic oxidation reaction of amine produces H2O2 as a reaction intermediate which was confirmed by CV, radical scavenging experiments, and quantified by KMnO4 redox titration. Radical scavenging experiments and fluorescence study accomplished that electrons, holes, superoxide radicals and hydroxyl radical ions are the reactive species involved in the oxidative coupling of amines. Moreover, Hammett plots with a BrownOkamoto constant of (σ+ = -0.320) implied that substituent’s on benzylamine influenced the kinetics of these oxidation reaction. Furthermore, extremely low product and H 2O2 yields in Ar, N2, and He confirmed the role of O2 in the oxidation of organic compounds investigated in this study. The Mott-Schottky analysis confirmed that 4CD/BiMO has conduction band potential (VCB) of -0.97 V and the valance band (VVB) potential of 1.72 V that are suitable for the oxidative coupling of amines using O2. The electrons relaying from the conduction band of BiMO to CD after photo-excitation and accumulation on the surface of CD facilitated the charge separation process and photocatalytic activity. We believe that this study will encourage materials scientists, heterogeneous catalyst researchers and energy/environmental researchers to develop several versatile catalysts based on carbon dots for their innumerable possible photoelectrocatalytic applications that have the ability to replace noble metal based photocatalytic materials.

ASSOCIATED CONTENTS The Supporting Information (experimental details, characterization techniques, Table S1-S3, Figure S1-S15, and Scheme S1-S4) is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Rajendra Srivastava *email for (RS): [email protected] ORCID ID (RS): 0000-0003-2271-5376 (SS): 0000-0002-4461-1319 Phone: +91-1881-242175; Fax: +91-1881-223395 Notes 28 ACS Paragon Plus Environment

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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is financially supported by Nano Mission, Department of Science and Technology, New Delhi with the award of a research project (SR/NM-NS-1054/2015). SS is grateful to MHRD, New Delhi for providing him a SRF. Authors acknowledge the DST-FIST funded XPS facility at Department of Physics, IIT Kharagpur. Authors appreciate the help of Prof. Nilmoni Sarkar, Department of Chemistry, IIT Kharagpur for the PL measurement. Authors thank SAIF, IIT Bombay for HRTEM analysis. Authors are also grateful to Director IIT Ropar for providing interdisciplinary project on energy.

DECLARATION Work plan was decided by RS and SS. All experimental work was executed by SS. SK contributed in the PEC measurements. The manuscript was written by SS and RS.



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For Table of Contents Only Stimulating the Visible Light Catalytic Activity of Bi2MoO6 Nanoplates by Embedding Carbon Dots for the Efficient Oxidation, Cascade Reaction, and Photoelectrochemical O2 Evolution Subhajyoti Samanta†, Santimoy Khilari†, and Rajendra Srivastava†*

e- O2

O2 e-

O2 nanoplate

h+


H2O

Stimulating the Visible-Light Catalytic Activity of Bi2MoO6 Nanoplates

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