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Facile Synthesis of CeO Nanosheets Decorated Upon BiOI Microplate: A Surface Oxygen Vacancy Promoted Z- Scheme Based 2D-2D Nanocomposite Photocatalyst With Enhanced Photocatalytic Activit Sabiha Sultana, Sriram Mansingh, and K.M. Parida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08534 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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The Journal of Physical Chemistry

Facile Synthesis of CeO2 Nanosheets Decorated upon BiOI Microplate: A Surface Oxygen Vacancy Promoted Z- scheme Based 2D-2D Nanocomposite Photocatalyst with Enhanced Photocatalytic Activity S. Sultanaa, S. Mansingha and K. M. Parida a*

a

Centre for Nano Science and Nano Technology SOA University, Bhubaneswar—751 030,

Odisha (India)

*

Corresponding author

E-mail: [email protected] & [email protected] Tel. No. +91-674-2379425, Fax. +91-6 74-2581637

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 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

Abstract Construction of visible light driven Z-scheme based photocatalytic system is a hot topic of research because of their potential to alleviate both energy and environmental issues. In the present study a series of surface vacancy mediated BiOI-CeO2 nanocomposites were prepared and their crystallographic, morphological, optical, electrochemical behaviour were characterised through XRD, TEM, FESEM, UV-vis DRS, PL, ESI and Mott-Schottky techniques. From the XPS analysis it could be said that a high amount of oxygen defects and metallic bismuth were present on the composite than neat CeO2 which was the major cause of enhancement in the photocatalytic activity. Further the photocatalytic efficiency of the as prepared samples was tested towards RhB decolourization/ Phenol oxidation as well as for O2 gas evolution. It was observed that 40%wt BiOI-CeO2 nanocomposite exhibited highest photocatalytic activity among neat and other composites i.e. 89% decolourization of 100ppm RhB and produces 323µmol/2h of O2 under visible light illumination. To justify the enhanced photocatalytic activity of the material a Z-scheme based charge transfer mechanism was proposed. Where surface oxygen vacancy on CeO2, I3-/I- reversibility pair through Bi metal on BiOI, scavenger experiment, PL spectra and Nyquist plot provide solid evidence towards Z-scheme charge transfer pathway. This work will provide some useful information in course of developing of Z-scheme based photocatalyst without an external mediator. 1. Introduction Topics related to water splitting, organic/inorganic pollutant degradation and reduction of heavy metals over semiconductor surface via solar light irradiation were found on the cover page many scientific journals. 1, 2 Photocatalysis have emerged as one of the cost effective, clean and ecofriendly technology in order to subside problem related to environmental pollution and energy crisis.

3, 4

Therefore, the scientific community invest all their skill and

experience towards the development of highly efficient photocatalytic systems that are active


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in the visible zone of solar spectrum. 4, 5, 6 Materials with comprehensive properties like wide visible light absorption range, high charge separation efficiency, large active surface area and long term photostability are proved to be the best in the field of photocatalysis. 7, 8 But it is quite tough to achieve all the above characteristics in a single material component. However, a sign of relieve was felt when the research world came forward with a solution popularly known as heterogeneous Z-scheme type photocatalysis, which is a prototype of natural photosynthesis. In this type of system two different photocatalysts with specify properties were coupled.

9, 10

Compared to conventional heterostructured photocatalysis,

more interest was given towards fabrication of visible light driven Z-scheme based photocatalysis, which serves as a improved photocatalytic system by promoting the spatial separation of excitons and keeping oxidation and reduction centre at two different photocatalyst by an electron shuttling mechanism.

9, 10, 11

Majority of Z-scheme oriented

catalysis reports are on water splitting and CO2 reduction reactions such as CdS/WO3, 10 gC3N4/BiOI, 16

12

g-C3N4/ZnO,

13

TiO2/rGO/CuGaS2,

CoOx/rGO/BiVO4,17 MoO3-C3N4,

18

14

Pt-SrTiO3: Rh/BiVO4,

SnO2-x-g-C3N4

19

15

g-C3N4/TiO2,

and Ag3PO4-g-C3N4

20

etc.,

however very few works are on pollutants degradation. 3, 8 So it is very essential to construct nanohybrid materials which will efficiently produce H2 and O2 from water and purify water system under visible light irradiation. In the present era of research, metal oxides based photocatalyst have covered the lime light of all time photocatalytic fields. Among the various oxides, TiO2, 14, 16 WO3, 10, 21 ZnO, 13, 22

CoO, 17 CeO2 6 etc. have been widely studied as one of the major component in z-scheme

mediated photocatalysis. In this work we have taken CeO2 as one of the reaction centre (oxidation) because of its specific features such as cost effectiveness, high photostablity, fast shuttling of oxidation state and ease of forming oxygen vacancies etc.

23, 24

As like ceria,

other surface oxygen vacancy rich metal oxides are more efficient in conserving their


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intrinsic geometric and electronic structure which alters the catalytic performance. Due to its 24, 25

numerous specialities it is considered as a catalyst in many applications

and it is often,

used to support transition or noble metals and provide distinctive catalytic functionality so it plays a dual role both as catalyst and catalyst supporter. 24, 26 Generally tailoring its shape into rod, cube and sheet results in a drastic change in its catalytic activity, so currently more emphasis was given on morphology controlled ceria photocatalyst.

27

In spite of all these

amazing characteristics property, major drawback lies with its large band gap which restricts the absorption of visible photon and decreases its efficiency under solar light illumination. Therefore fabricating visible active materials with an optimum balance between efficiency and stability remains a demanding challenge in this field. 25 So as to meet the above problem numerous works has been done on CeO2 by making its composite with other material or by doping appropriate foreign element on it. 23, 24, 27, 28 In the present investigation, we have chosen BiOI as the partner (reduction site) of CeO2 in the Z-scheme mechanism. It is another attractive material not only due to its visible light absorption ability but also due to its photocatalytic performance. 26 29 As like CeO2, numerous studies have been carried out by constructing its nanohybrid with other materials with remarkable catalytic performance.

5, 12, 29, 30, 31

It is a established layered compound having

crystal structure of [Bi2O2][Xm], possess alternating layers of [Bi2O2]2+

by two slabs

comprising of iodine atom. BiOI exhibits unique optical, structural and electrical properties due to its weak interlayer Vander Waal’s interaction and strong intralayer bonding. Among the other bismuth oxyhalide compounds, BiOI (1.8 eV) has the strongest absorption in the visible region and exhibits excellent visible-light photocatalytic activity.

32, 33, 34, 35

Although

it satisfies all the features of an ideal semiconductor photocatalyst but its photocorrosive nature makes it a lame horse. So BiOI was heterostructured with other semicondutors such as


4

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g-C3N4,

12

TiO2,

30, 31, 36, 37

ZnO,

5, 38

WO3,

39, 40

BiOBr,

41

Bi12O17Cl2,

42

BiOCl0.5I0.5

43

etc

that shows better photocatalytic performance. Herein, we have reported a composite of 2D-CeO2 with 2D-BiOI, where CeO2 was combined with different wt % of BiOI by simple wet chemical process and a series of heterostructure were prepared to achieve visible light driven solid state Z-scheme for RhB dye decolourization, phenol degradation and water oxidation reaction. The main objective of this work compared to the other reported article is that the composite CeO2-BiOI exhibits a typical Z-scheme mechanism of charge transfer without an external mediator pair and was successfully proved by different analytical techniques (PL-TA, PL, Nyquist etc). It is suggested that the enhancement of the photocatalytic activity may be attributed to the efficient charge transfer through Z-scheme between CeO2 and BiOI, which could result in the photoexcited electrons of BiOI with a high reducibility and photoexcited holes of CeO2 with a high oxidizability participating in the photoredox reaction. 2. Experimental section 2.1. Reagents used Ce(NO3)3.6H2O, Bi(NO3)3.5H2O, Sodium dodecyl sulphate, HMT, Formamide, Ethylene glycol, KI, Rhodamine B, Phenol, EDTA, Benzoquinone, Isopropanol, t-BuOH, DMSO, Terepthalic Acid, TEOA, NaOH and Methanol. All the reagents used in this work were of analytical grade (Merck) and used as such without further purification. Deionized water was used for all the experiments. 2.2. Catalyst Preparation 2.2.1. Synthesis of nanostructure staked CeO2 sheets


5


Pure Nanostructure CeO2 was prepared by simple refluxed method and was carried out as follows: At first the aqueous solution containing the mixture of SDS and Hexamethylene tetramine (HMT) was taken in a conical flask. Then Ce(NO3)3.6H2O dissolved in 100mL of DW was added slowly into the above solution resulting in a white precipitate. The obtained colloidal solution was refluxed for 8h at 900C. The formed products were collected through centrifugation followed by water and ethanol washing and was dried in a vacuum oven under room temperature. 44 2.2.2. Synthesis of BiOI microplates Neat BiOI was prepared by simple precipitation method. A calculated amount of Bi(NO3)3.5H2O (0.0028mol) was taken in a beaker with 10mL of ethylene glycol and was stirred to obtain clear solution. Then this solution was added drop wise from a burette into another beaker containing aqueous solution of 0.280M KI and was stirred for 6h. The resulting suspension was filtered, washed and dried for 24h at 600C. 2.2.3. Synthesis of BiOI-CeO2 nanocomposites A specified amount of as prepared CeO2 and different weight percentage of BiOI (10%, 30%, 40% and 50%) was individually added to the 20mL of formamide and then subjected to ultrasonication for 30min each. Then the above solutions were mixed together and again sonicated for 2h at 800C in a water bath. After that the resulting solutions were centrifuged and were dried at 800C in a vacuum oven for 24h. The resulting products are labelled as x% BiOI-CeO2, where x stands for the different wt% of BiOI. 2.3. Analytical Charecterization The crystallinity of the materials were analyzed on a Bruker advance X-ray powder diffractometer using Cu Kα set at 40kV and 40mA voltage and current respectively.


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Diffraction patterns were recorded over a range of 10 to 70 2θ angle. UV-vis diffuse reflectance spectroscopy (DRS) spectra were carried out by JASCO 750 UV-visible spectrophotometer in the region of 200-800nm. By using Seki STR500 Raman spectrometer, a Raman spectrum was recorded having excitation at a wavelength of 532nm. The PL spectrum was recorded using a JASCO spectroflourometer FP8300 at room temperature and the excitation wavelength was 340 nm. The BET surface area, pore volume and Nitrogen adsorption-desorption isotherms were done on a Micromeritics ASAP-2020 surface area and porosity analyzer at 77 K after the sample had been degassed in the flow of N2 at 120°C for 8h.The chemical composition and surface morphology of the samples were measured by TEM and HRTEM images on a Philips TECNAI G2 instrument at an accelerating voltage of 200 kV. The XPS measurements were carried out on a VG Microtech Multi lab ESCA3000 spectrometer with a non-monochromatised Mg-Kα X-ray source. The binding energy correction was performed using the 1s peak of carbon at 284.6 eV as a reference. 2.4. Photocatlytic activity test The photocatalytic activity of the materials was evaluated by monitoring the decolourization of Rhodamine B under the irradiation of direct sunlight in the month of April 2015, where the average sun light intensity is about 10,4100 Lx as per Lux meter. Initially 100mg/L concentration of RhB stock solution was prepared and for each experiment 20mg of solid catalysts were dispersed in the 20 mL of prepared stock solution. Then the dispersed solutions were stirred in the dark for 30mins in order to achieve the sorption equilibrium. After that the above solutions were placed under direct solar light with slow stirring. The photodecolourization process was analyzed by monitoring the decrease concentration of RhB through JASCO 750 UV-visible spectrophotometer. Blank test were also performed in the absence of photocatalysts. Simultaneously the scavenger like IPA, t-BuOH, EDTA, BQ,


7


DMSO were taken along with the photocatalysts in order to study the different active species produced during the photodecolourization process. The photocatalytic O2 gas evolution experiment was performed in a batch reactor at vacuum pressure and ambient temperature. In a typical experiment calculated amount (0.02g) of powdered catalyst was suspended in 0.05 M aqueous AgNO3 (20mL) solution. Subsequently the solution was stirred constantly with the help of a magnetic stirrer, preventing settling of particles at the bottom of the reactor. Before the illumination of light, the reaction mixture was bubbled with nitrogen gas for 30mins to remove the dissolved gases. For visible light source a 125 W medium pressure Hg lamp (λ ≥ 420 nm) and for UV filter 1 M NaNO2 was used. The evolved gas was collected by downward displacement of water and was analyzed on a GC-17A (Shimadzu) using a 5 Å molecular sieves column and a thermal conductivity detector (TCD).

2.5. Electrochemical measurements Photoelectrochemical analyses of the prepared samples were performed on IVIUM n STAT multichannel workstation equipped with a standard three-electrode cell. Photocatalysts deposited over Fluorine doped Tin oxide(FTO) by electrophoresis method were used as working electrode, while Pt plate and Ag/AgCl served as counter and reference electrode respectively. Electrophorectic deposition technique was followed to prepare working electrode where 2mg of sample and 4mg of iodine were dispersed 30mL of acetone solution. Then two parallel FTO dipped in the above solution were subjected to a 10V bias for 5min under potentiostat control. The sample was deposited as thin film in 1cm2 area over the FTO. The developed FTO were kept in Air oven at 1000C for 2h. The working, Pt plate and reference electrodes were dipped in 50mL of 0.1M Na2SO4 solution and electrochemical


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measurements were carried out. The impedance study was made at 105Hz to 102Hz Where as Mott-Schottky measurements were carried out at 100Hz under dark condition. 3. Results and discussion 3.1. Characterization of BiOI/CeO2 Powder XRD is an analytic technique used for detecting the phase, purity and crystallinity of synthesized materials. Fig.1 represents the diffraction patterns of CeO2, BiOI and BiOICeO2 nanocomposites with different wt% of BiOI. In fig.1 sharp and intense diffraction peaks attributed to 111, 200, 220 and 311 lattice plane of face centred cubic phase of CeO2 (JCPDS #34−0394) which belongs to the Fm3m space group having lattice constant 5.41A0.

44

Whereas the obtained smooth peaks indexed to 001, 102, 110, 103, 004, 200, 114 and 212 planes of pure BiOI, are identical with the reported results of a tetragonal phase (JCPDS # 10-0445).

12

The tower peak of CeO2 (111) at 2θ=28.50 was used to calculate the average

nanocrystallite size (D) of neat CeO2 and BiOI-CeO2 by Scherer formula and the computed D values were in the range of 11-15 nm. From the diffraction patterns of nanocomposite, we can clearly see two sets of diffraction peaks which confirm polycrystalline structure and the coexistence of both CeO2 and BiOI phases. Interestingly, it was observed that diffraction peak intensity for CeO2 in the nanocomposite gradually decreases addition to peak broadening with increase in BiOI content. However, intensity of BiOI peaks became sharp and narrow in the composites. This increased or decreased in the peaks parameters were observed due to the synergistic effect between the two components, which reveals a strong interaction between CeO2 and BiOI. 12


9

(212)

(114)

(200)

(103) (004)

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

(102) (110)


BiOI

50% BiOI-CeO2 40% BiOI-CeO2 30% BiOI-CeO2 (111) (200)

20

30

(220)

40 50 2 theta (degree)

(311)

CeO2

60

70

Fig. 1 XRD patterns of CeO2, 30%BiOI-CeO2, 40%BiOI-CeO2, 50%BiOI-CeO2 and BiOI . The optical response of the designed photocatalysts were characterised through UV-vis DRS technique. Fig.2 displays the absorbance and optical band gap potential of CeO2, BiOI and CeO2-BiOI nanocomposites. Generally, CeO2 shows strong absorption in the UV region; however the band gap energy may change accordingly with respect to confinement effect and oxygen vacancies of the nanomaterials. 5 The spectra of neat CeO2 was observed in the lower visible region around 430-450 nm, which is attributed to ligand-to-metal charge transfer (LMCT) from O2-(2p) to Ce4+(4f) orbital, 6 whereas the absorbance of neat BiOI extends up to 660 nm. Interestingly, all the CeO2-BiOI nanocomposites were red shifted and shows absorption from 620-680nm in the colour zone of the spectrum as shown in fig. 2(a). These intriguing results in the absorbance by the composites not only happen due to the incorporation of strong visible light absorbing agents (BiOI) but also because of the presence of Ce3+ states in the grain boundaries of ceria which leads to localized states in the energy band.

23

As a crystalline semiconductor the optical band gap energy of all synthesised

photocatalysts were calculated by the following equation. ℎν = (ℎν − ) / where α, ν, Eg, and A are the absorption coefficient, light frequency, band-gap energy, and proportionality constant, respectively. In addition, n is a constant that depends on the characteristics of transition in the semiconductor, namely n=1 for direct type and n=4 for


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indirect transition. 5 In our case all the prepared samples suffers indirect type transition. The intercept of the tangent to the X-axis would give a good approximation of the band gap energy of the products .The estimated band gap energies of CeO2 and BiOI were found to be 2.85 and 1.75eV respectively as shown in fig. 2(b) and 2(c) respectively.

6

In addition, the

Urbach energy for CeO2, BiOI and 40%BiOI-CeO2 were calculated as shown in fig. S1 (detail procedure of calculation has mentioned in Supporting information). It was observed that nanocomposite (778meV) exhibits much higher Urbach energy than both CeO2 (417meV) and BiOI (194meV). The enhanced Urbach energy arises mainly due to the structural disorder related to the presence of oxygen defects and high Ce+3 concentrations. 21 So in the nanocomposites, more number of oxygen vacancies is present which was further confirmed from XPS and Raman analysis.

(a)

(b)

1.2

CeO2

1/2 (eV)1/2

1.0 0.8 30% BiOI-CeO2

BiOI

0.6

10% BiOI-CeO2 50% BiOI-CeO2

0.4

(αhυ)

Absorbance (a. u.)

40% BiOI-CeO2

CeO2

Eg= 2.85eV

0.2 200

300

400

500 600 700 Wavelength (nm.)

(c)

800

900

2.0

2.5

3.0 3.5 hυ (eV)

4.0

1/2 (eV)1/2

BiOI

(αhυ)

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


Eg= 1.75eV

1.4

1.6

1.8

2.0 hυ (eV)

2.2

2.4

2.6

Fig. 2 (a) UV-vis diffuse reflectance spectra of CeO2, BiOI and BiOI-CeO2 composites, (b) and (c) band gaps of CeO2 and BiOI.


11


Photoluminescence characterization is carried out in order to understand the charge separation efficiency and recombination rate of photogenerated excitons. The intense PL spectra indicate the faster recombination of photoexcited electron-hole and a lower PL intensity expresses a lower recombination rate.

45

Here PL spectra of CeO2, BiOI and BiOI-

CeO2 nanocomposites were analyzed with an excitation wavelength of 340nm, as shown in the fig.3. It was found that PL emission peaks for neat CeO2 occur at 428, 467, 481 and 491nm shown in fig. 3(a). Here a strong blue emission peak and week one centred at 415 and 439 nm respectively, are mainly associated with the defect levels localized between the O2p and Ce 4f band levels. The other weak blue emissions centred at 467, 481 and 495 are possibly due to the surface defects and Ce3+ states. 6, 23, 46 Pure BiOI shows an emission peak nearly at 420nm consistent with the emission characteristic of bismuth oxyiodide. 47 From the fig.3(a) it was found that 40%BiOI-CeO2 exhibits the stronger PL intensity peak than the neat CeO2 as well as BiOI suggesting faster recombination of charge carriers in the composite. However experimental outcome postulates that, composite material show much higher photocatalytic activity than neat CeO2 which contradicts the general PL concept. The observed PL intensity was found to increase with increasing amount of BiOI and followed this pattern 40%BiOI-CeO2> 30%BiOI-CeO2> 50%BiOI-CeO2 > 10%BiOI-CeO2 as shown in fig. 3(b). Interestingly, same pattern was also observed for the photocatalytic activity of the nanocomposites. In this case it can be summarized as higher the intensity of PL spectra higher is the photocatalytic activity. Islam et al. and Zhang et al. have observed similar types of PL behaviour and activity relation for CeO2-AgI and Bi2O3-gC3N4 nanocomposites respectively. 6, 8 This type of surprising observation of the composites may be attributed to the surface oxygen vacancies of CeO2 or to the Z-scheme type of charge transfer which will be discussed in the later part of this article.


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(a)

(b) (d)

PL Intensity(a.u.)

40% BiOI-CeO2

PL 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


CeO2

(c) (b) (a)

BiOI

400

450

500

550

400

Wavelength(nm)

450 500 Wavelength(nm.)

550

Fig. 3 (a) Photoluminiscence spectra of BiOI, CeO2 and 40% BiOI-CeO2 (b) photoluminescence spectra of BiOI-CeO2 composites (a), (b), (c), (d). X-ray photoelectron spectroscopy (XPS) was used to elucidate the chemical state of the elements and surface composition of the material.

48

Fig. S2 represents the survey XPS scan

of CeO2, BiOI and 40% BiOI-CeO2nanocomposite. The survey spectrum of the nanocomposite indicates the presence of only Ce, O, Bi and I in the nanocomposite without any additional peak of other elements, which are well consistent with elemental composition of EDX pattern. In order to explore the details regarding bonding nature of atoms in the composite with respect to its neat counterpart, the obtained data were plotted and are deconvoluted in CASA XPS software. Fig. 4(a) indicates the core level spectrum of Ce 3d in the neat CeO2 and in the composite, which ascertains the presence of Ce4+ and Ce3+ states owing to its non-stoichiometric nature and splitted d-orbital represented by spin state 3d5/2 (880-900eV) and 3d3/2(900-920eV)20 respectively. From the deconvoluted spectra of CeO2, the main characteristic peaks for Ce3+ 3d3/2 and Ce3+ 3d5/2 were labelled as v/ and u/ respectively, whereas a set of peaks denoted as v, v//, v/// and u, u//, u/// were assigned to Ce4+ 3d3/2 and Ce4+ 3d5/2 , which are well consistent with the literature value.

23, 49

The ratio

between Ce3+ to Ce4++ Ce3+ concentration gives useful information regarding the presence of


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surface defects, that play a crucial role in determining the catalytic activity of Ceria based systems. Here we have calculated the ratio both for CeO2 and the 2D-2D nanocomposite and found as 22% and 31% respectively. The surface chemical state of Ce3d and the high percentage of Ce3+ imply the existence of non stoichiometric oxygen vacancy.

49, 50

As

compared to neat CeO2, the composite shows a blue shift of around 0.1eV-0.2eV in the binding energy of Ce, this indicates the reduction of electron density around Ce atom in the composite. This decreased in electron density concludes that the generated electrons are transferred and recombine with the holes of BiOI which ascertain the possibility of Z-scheme mechanism.

51

In order to further confirm the oxygen vacancies in the nanocomposite, the

O1s spectra of neat CeO2, BiOI and the nanocomposite are shown in fig. 4(b). From the deconvoluted spectra three types of O1s (OI, OII, OIII) peaks are observed for CeO2 and the nanocomposite however 2 types of oxygen (OI, OIII) peaks are observed for neat BiOI. Generally, the lower binding energy peak observed at 529.27eV is attributed to the lattice oxygen (OI) (Ce-O) of neat CeO2, which at 530.4eV for neat BiOI (Bi-O) and peak positioned around 529.92eV for the composite. The other two higher binding energy peaks (OII and OIII) are assigned to the oxygen vacancy and chemiadsorbed oxygen.

50

Interestingly, the oxygen vacancy (OII) peak is absent in parent BiOI where as in compared to neat CeO2, OII/OI ratio is more in the nanocomposite i.e. more oxygen defect. A remarkable shifting of OI peak and the increment of OII/OI ratio confirms the nanocomposite formation between CeO2 and BiOI. 52 As demonstrated in fig.4 (c), two major peaks visualised at 159.01 and 164.32eV in neat BiOI and 158.92 and 164.23eV for the composite, represents Bi 4f7/2 and Bi 4f5/2 states of Bi3+ in BiOI respectively. There is a negative shifting of major peaks as well as two satellite peaks are observed for the BiOI-CeO2 composite. 12 Additionally, two peaks metallic bismuth (for Bi-Bi bond) are observed in both BiOI and the composite. However the plotted XRD


14

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result doesn’t support the above claim, due to low concentration of metallic Bi. Zhao et al. and Luo et al. have observed similar type of Bi 4f XPS in Bi2MoO6 and BiOI. They have concluded that metallic Bi is the main reason for the enhanced photocatalytic activity.

53, 54

The high resolution XPS spectra of I 3d displays two humps at 618.74 and 630.26eV for BiOI and 618.23 and 629.74eV in the 2D-2D nanocomposite, depicting I 3d5/2 and I 3d3/2 level as pictured in fig.4 (d). These values are reliable with the previously published papers. 12, 35, 55

A remarkable negative shifting was observed for both Bi 4f as well as I 3d in the

composite compared to neat BiOI, which indicates that there is a significant rearrangement occurs between the each atoms in the composite. The presence of surface oxygen vacancy, Ce3+ concentration and metallic bismuth in the composite traps the photo induced excitons at the surface and enhances the photocatalytic degradation efficiency. All of above results further confirm the coexistence of both BiOI and CeO2 in the nanocomposite. The physiochemical properties of CeO2, BiOI, and 2D-2D CeO2-BiOI nanocomposite is further analyzed through BET analysis. From the N2 adsorption and desorption isotherm as shown in fig. S3, all the samples display type-IV isotherm. The BET surface area of CeO2 nanosheets is calculated to be 123m2g-1 with a total pore volume of about 0.102cm3g-1. The mesoporous structure of CeO2 is due to rearrangement of CeO2 nanocrystal in 2D array. 37 However pure BiOI do not posses any porous network and has a very low surface area of 2.64m2g-1(Vm=0.00345cm3g-1). Further the specific surface area of 2D-2D CeO2-BiOI nanocomposite is calculated to be 67.8m2g-1, which is much higher than neat BiOI but comparably low than CeO2 nanosheets. Such a high surface area than some of the previously reported metal oxide-BiOI system, 5, 31 could be the major essential contribution for enhanced photocatalytic application. 23 This observation justifies that, the surface area of the composite is not essentially to be greater than both its components to increase the catalytic activity.

30

This is seen in our case, additionally the adsorption oriented catalytic activity (RhB) of the


15


composite takes place on BiOI surface whose area of exposure is greater compare to neat BiOI.

(a) C e 3 d v

v

///

/ CeO2 v u

Intensity (a.u.)

v

u

///

// u

//

/

u

4 0 % B iO I-C e O 2

920

(b)

910 900 890 B in d in g E n e rg y (e V )

O 1s

880

O I

C eO 2 O II

Intensity (a.u.)

O III

B iO I

4 0 % B iO I- C e O 2

536

(c)

534 532 530 528 B in d in g E n e r g y ( e V )

Bi 4f

526

4 f 7 /2 4 f5 /2

B iO I

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

4 0 % B iO I - C e O 2

168

166

164 162 160 158 B in d in g E n e r g y ( e V )

156

154


16

Page 16 of 39

Page 17 of 39

(d) I 3 d

3 d 7 /2 3 d 5 /2 B iO I

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


4 0 % B iO I-C e O 2

633

630

627 624 621 618 B in d in g E n e rg y (e V )

615

Fig. 4 Typical high resolution deconvulated XPS spectra of CeO2, BiOI and 40%BiOI-CeO2 (a) Ce 3d (b) O 1s (c) Bi 4f and (d) I 3d. Generally, Raman spectroscopy is applied to investigate the vibrational and structural properties of crystal as well as the defects present on the materials.

27

Fig. 5 displays the

Raman spectra of CeO2, 40%BiOI-CeO2 nanocomposite and BiOI. From the figure it was seen that a distinct peak at ̴ 464 cm-1 has risen due to the F2g mode vibration of CeO8 unit in CeO2 nanosheets.

26

In addition a very weak phonon D mode at 580cm-1 corresponds to the

presence of structural point defects i.e surface oxygen vacancy in CeO2.

27

However, the

intensity of the observed defect band (D) intensifies in the nanocomposite indicating more oxygen voids as depicted in the black framed image. For more clear view, the zoom plot of the framed portion was highlighted just above it. Further a week band at 253cm-1 is noticed which was attributed to the displacement of oxygen atoms from their ideal point.

26

In the

neat BiOI a strong band E1g at 148.8cm-1 was observed due to the internal Bi-I stretching vibration. 33 All characteristic peak of CeO2 in addition to peak corresponding to Bi-I internal stretching mode (E1g) was observed in the nanocomposite. The ratio between the D band to F2g band was increased as compared to neat CeO2 and there was a shifting of E1g and F2g modes were observed, which indicates a stronger interaction between BIOI and CeO2 and as well as demonstarate the higher surface oxygen vacancy in the composite.


17


Intensity (a.u.)

E 1g

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

F 2g

520

5 40

560

580 600 620 640 R a m a n S hift ( c m -1 )

660

680

BiOI

2TA D

40% BiOI-CeO 2 CeO 2

200

300

400

500

600

700

800

900 1000

Raman Shift (cm -1 )

Fig. 5 Raman plot of CeO2, 40%BiOI- CeO2, BiOI and the zoomed version of D band in the inset. Electron-microscopic techniques like SEM and TEM were carried out of the prepared materials in order to confirm structural and morphological details. 56 From the SEM images it was seen that as prepared BiOI are of irregular rectangular plate like morphology (fig. S4(a)) of width between 0.5µm- 1.5µm. Whereas CeO2 (fig.S4(a)) appears to be stacked nanosheets. When both the parent materials dissolved in formamide and were sonicated, the stacked nanosheets of CeO2 (yellow framed) were exfoliated and deposited upon the microplates of BiOI as seen in fig. 6(a) and (b). The yellow rectangle portion in fig.6(c) representing ceria nanosheets were zoomed further to demonstrate that ceria nanosheets were composed of ceria nanocrystals connected two dimensionally as shown in fig.6(d). 44 The HRTEM image in fig. 6(e) displays well defined lattice fringes with an interplanner distance of 0.31nm and 0.27nm corresponds to the 111 and 200 planes of CeO2 particles. Similarly, the lattice fringe of BiOI as shown in the inset of fig.6(c) indicates the well crystalline 110 plane of BiOI with an interlayer distance of 0.28nm. The EDX pattern of the composite in fig. S5 confirms the presence of elements like Ce, Bi, I and O which was further supported by XPS analysis.


18

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Fig. 6 (a), (b) and (c) TEM images of 40%BiOI-CeO2 (d) High resolution TEM image of selected area of CeO2 nanosheet in composite and (e) lattice fringes of CeO2 The electrochemical impedance measurments (Nyquist plot) is carried out in order to illustrate the migration and transfer process of photoexicted electrons and holes. The plot mainly consists of a high frequency conductive loop region, describing features like interfacial charge transportation resistance, surface trap charging by excitons and charge diffusion in the space charge layer. The second one is a straight line called inductive loop at low frequency region associated with the Warburg resistance.

48, 57, 58

The Nyquist plots for

BiOI, CeO2 and 40%BiOI-CeO2 nanocomposite electrodes at an applied bais of 0.0V in 0.1M Na2SO4 solution over the frequency range 105 to 102Hz with amplitude of 10mV were represented in fig.7. Apparently, the smaller diameter arc for nanocomposite electrode compared to their parent materials signifies lower interfacial charge transfer resistance and high electrical conductivity.

57

A significant change in Rct occurs by introducing BiOI to

ceria, which proves the presence of synergetic interaction between the two, resulting in


19


enhanced catalytic activity of 40%BiOI-CeO2 hybrid material. Meanwhile the observed straight line at lower frequency region of BiOI-CeO2 composite exhibited a much smaller diffusion path length of ions in the electrolyte as compared to neat BiOI and nearly same as CeO2. The above evidences from Nyquist plot proves that the composite posses higher charge separation and lower recombination rate of excitons. CeO2

60

BiOI

40%BiOI-CeO2

50 -Z"(ohm)

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 30 20 10 0 10

20

30

40

50

60

Z'(ohm)

Fig. 7 EIS Nyquist plots of CeO2, BiOI and 40%BiOI-CeO2 (105 Hz to 102Hz). In an another electrochemical analysis, the flat band potential measurement through MottSchottky methods was carried out in order to know the band structure of BiOI and CeO2 and its composite which were shown in fig. S6. Generally flat band potential (Efb) values are calculated by the following Mott-Schottky equation. 34, 58

1 2 = − − ∈∈ Where C is the space charge capacitance, ∈, ∈ are the dielectric constants of material electrodes and free space respectively, Nd is the donor density, kB is the Boltz’s man constant, q is the electronic charge and T is the absolute temp. The Efb value can be determined from the extrapolation to 1/C2=0 and the type of semiconductor material can be determined from the nature of slope viz. positive slope for n-type and negative for p-type


20

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materials.

48

The Mott-Schottky plot of CeO2 and BiOI reveals n-type feature. As it is well

known that the CB potential of n-type semiconductor lies 0-0.1V above to the flat band potential thus the Efb positions for CeO2, BiOI obtained by the tangent intersection on potential axis were found to be

-0.67eV and -1.05eV(vs. Ag/AgCl) respectively.

58

Accordingly the Conduction band positions were estimated to be -0.58eV and -0.96eV (vs. NHE).

59

Combined it with Eg band gap data the relative valence band edge potential (VB)

were calculated to be 2.27eV and 0.79eV (vs. NHE) respectively. 3.2. Photocatlytic activity The photocatalytic performance of the as prepared samples was examined towards RhB decolourization under direct solar light. The rates of the decolourization were calculated by plotting C/C0 versus time t

60

and were represented in fig.8. A separate experiment was

performed to demonstrate the photolysis of RhB without catalyst, the result shows only negligible decolourization of RhB. So the reason behind this high rate of decolourization is mainly due to the presence of photocatalyst.

34

Initially, RhB decolourization of parent CeO2

and BiOI was carried out and found to be 23% and 73% respectively. As observed from fig. 8(a), 40%BiOI-CeO2 nanocomposite decolourises about 89% of RhB within 60mins which is 3.8 and 1.2 fold higher than neat CeO2 and BiOI respectively. The photocatalytic activity of BiOI/CeO2 nanocomposite multiplies with the increase in BiOI content up to 40wt%, and then drops for 50wt%. Thus it is obvious that the amount of BiOI plays an important role on the photocatalytic activity of CeO2 nanocomposite.

8

Gu et al. and Islam et al. reported

decolourization efficiency of RhB dye as 96.8% (40ppm) and 98.99% (10ppm) respectively over CeO2-CdS and CeO2-AgI nanocomposites under stimulated sunlight.

56, 6

However in

our case the nanosheet oriented system shows an exceptional decolourization activity i.e. 89% of 100ppm RhB in just 60min in open sunlight. This photocatalytic decolourization of the dye was found to follow the first order kinetics and the equation be ln (C0/C) = k t where


21


C is the final concentration at a particular time t, C0 is the initial concentration and k is the apparent rate constant as shown in fig.8(c). The apparent rate constant k is obtained from the slope of the linear plot of ln (C0/C) with time

23, 34

and was shown in fig.8(c) Among all

prepared composites 40% BiOI-CeO2 exhibited much higher photocatalytic activity which can be attributed to stronger visible light absorption ability, a higher charge transfer and separation efficiency of photogenerated charge carriers and possesses best synergistic interaction between the parent materials than all other composites.60 Further the absorption spectra of RhB with 40%BiOI-CeO2 nanocomposite photocatalyst under solar light irradiation clearly showed that the characteristic absorption peaks corresponding to RhB decreases rapidly as the irradiation time increases, indicating rapid RhB decomposition as shown in fig. 8(d). Additionally, the shift in absorption peak to shorter wavelengths of RhB in the presence of the BiOI-CeO2 nanocomposite suggests that the RhB degradation process occurs by the stepwise removal of the four N-ethyl groups in the dye (successively yielding N, N, N/-triethylated rhodamine, N, N/-diethylated rhodamine, Nethylated rhodamine, and rhodamine). These changes in the chromophore structure during the degradation process are reflected in the decrease in intensities of characteristic peaks and shifting of the peaks.

61

The coloured organic dyes have a sensitization effect that greatly

affects the photocatalytic performance of the catalyst. Therefore the non-coloured phenol was chosen as a model compound to determine the intrinsic photocatalytic activity of the photocatalyst, where sensitization effect was negligible. From the fig. S7, it was seen that 40%BiOI-CeO2 exhibits much higher degradation rate of phenol compared to parent CeO2. The degradation efficiency of nanocomposite and neat CeO2 was 77% and 21% respectively in 90min. So this study provides solid evidence that the catalytic performance is mainly due to the photocatalysis though some sensitization effect is there as in case of RhB but very negligible.


22

Page 22 of 39

Page 23 of 39

The stability of the 40%BiOI-CeO2 nanocomposite was further tested by recycling the photocatlyst in order to know the efficiency of the sample. From the fig. 8 (e) it was seen that there is a slight loss in the photocatalytic activity was observed which illustrated the high stability and efficiency of nanocomposite in dye solution. (c)

0.4

BiOI 10% BiOI-CeO2 30% BiOI-CeO2 40% BiOI-CeO2

0.5

0.2

50% BiOI-CeO2

0.0

0.0 0

10

20

30 40 Time(min)

-ln(C0/C)

Blank CeO2

50

1.0

60

(d)

0

10

20

30 40 Time(min)

(e)1.0 0 min 15 min 30 min 45 min 60 min

1.5

C/C0

1.0

4

CeO2

0

0.8 0.6

0.4

w/o Quencher EDTA BQ t-BuOH

0.0

0

10

20

19

(f)

0.6

0.2

600

17

10

0.8

0.0 500 550 Wavelength(nm.)

22

20

1.0

0.5

450

60

C/C0

2.0

50

34 29

30

BiOI

1.5

0.6

40

40% BiOI-CeO2

40% BiOI-CeO2

50% BiOI-CeO2

0.8

C/C0

10% BiOI-CeO2

30% BiOI-CeO2

2.0

BiOI

30% BiOI-CeO2

50% BiOI-CeO2

CeO2

10% BiOICeO2

(b)2.5

1.0

Apparent rate constant Ka (min-1 10-3)

(a)

Absorbance(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


1

st

2nd Run

Run

3rd Run

0.4 0.2 0.0

30 40 Time(min)

50

60

0

20

40

60

80

100 120 140 160 180

Time(min)

Fig. 8 (a) Photocatalytic decolourization plots of RhB under visible light (b) First order kinetic study, (c) apparent rate constant of the materials, (d) Concentration change of RhB as a function time under sunlight, (e) Reusability study of 40%BiOI-CeO2 and (f) Photodecolourization of RhB over 40%BiOI-CeO2 composite in the presence of different scavengers, the suspensions containing 0.2g catalysts in 20ml solution, initial concentration = 100 mg/L, direct sunlight. To defend strongly the proposed photocatalytic mechanism, radical trapping experiments were performed to determine the main active species in the photocatalysis process as highlighted in fig. 8(f). The decolourization efficiency of RhB was decreased considerably upon the addition of EDTA (1mM, hole scavenger)

43

suggesting that h+ are the main active

species in the PCO process. Similarly in addition of BQ (1mM, superoxide scavenger),

40

decolourization efficiency was decreased significantly which confirmed that super oxide ions


23


were also an important active species in decolourization process. However, when t-BuOH (1mM)

60

was added for scavenging the hydroxyl radicals no significant change in

decolourization efficiency observed, indicating that hydroxyl radicals didn’t play a vital role in PCO process, this result was further confirmed by the TA photoluminescence probe analysis as shown in fig. S8. The fluorescent intensity of TAOH for CeO2 was very negligible because of the fact that the standard redox potential of Ce4+/Ce3+ (2.03V) and OH./OH(1.99V) are very close,

13

as a result the holes can’t oxidized to OH. and for BiOI, the VB

potential is more negative than E0 (OH./OH-). This indicates that OH. generation couldn’t be possible both from neat materials and nanocomposite. Thus from the above quencher and TAOH photoluminescence probe test it was confirmed that after coupling of the bands in composite, the fluorescent intensity of TAOH wasn’t changed which validates that OH. wouldn’t play a key role in the PCO process. In order to strongly support the assumed mechanism, photo-induced O2 evolution reaction was performed for CeO2 and all the composites under visible light (less than 400nm) irradiation shown in fig.9. In this experiment, required amount of powdered photocatalyst was added to an aqueous solution of AgNO3 (0.05M, 20ml) and nitrogen (150ml min-1) was degassed for 30mins. It was observed that, the rate of oxygen evolution increases upto 40 wt% of BiOI, and suddenly decreases for the higher wt% content (50wt% BiOI), which is quite similar to the trend observed in case of RhB dye decolourization. It was found that 40%BiOI-CeO2 displayed the best activity with an average range of 323 µmole/2h oxygen, much higher than neat CeO2. Similarly Lavarato et al. also reported oxygen vacancy rich CeO2-GO composites which showed the higher oxygen evolution rate but quite less than the present system.

62

In addition n-type BiOI didn’t produce any O2 because of its insufficient

VB potential to oxidize water. Besides activity, the stability of the composite material was evaluated by performing recycling experiments (four cycles) under similar conditions as


24

Page 24 of 39

Page 25 of 39

shown in fig. 9(b). The data reveals that, the oxygen evolution rate remained constant upto two cycles and thereafter decreases because of the fact that Ag nanoparticles from electron scavenger AgNO3 deposited on the catalyst surface leading to the retardation of light absorbing ability of photocatalyst and hence decreased oxygen production activity. Though upto the 2nd cycle the oxygen evolution was almost constant, suggesting a better stability and productivity of the material. 63, 64

(b)

40% BiOI-CeO2

Amount of evolved O2 gas (µ mol)

(a)350 Amount of evolved O2 gas (µ mol/2h)

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


300 250

30% BiOI-CeO2

200 150

50% BiOI-CeO2 10% BiOI-CeO2

100 50 CeO2

0

BiOI

40% BiOI-CeO2

300 250 200 150 100 50 0 0

60

120

180

240

300

360

420

480

Time (min)

Fig. 9 (a) Histograms of O2 production rate under visible light irradiation and (c) stability study of 40%BiOI-CeO2.

3.3 Photocatalytic mechanism of BiOI-CeO2 nanocomposite On the basis of the above two application result, the photocatalytic mechanism for the composite was proposed and illustrated in scheme 1. According to the Mott-Schotky plot CB edge potentials of CeO2 and BiOI were calculated to be -0.58 and -0.96eV and corresponding VB positions were estimated to be 2.27 and 0.78eV vs NHE scale respectively. Taking the band edge potential under consideration there may be 2 possible ways of efficient charge separation mechanism: (1) double charge transfer and (2) the Z-scheme type. On the basis of the band positions, Zhang et al. and Wang et al. also supposed these two types of charge


25


Page 26 of 39

transfer mechanism over WO3-CdS and g-C3N4-BiOI respectively.

10, 12

By following them,

as per the conventional double charge transfer mechanism, BiOI would harvest maximum visible light and get excited to produce electrons and holes. The photogenerated electrons then migrate from the CB of BiOI with higher potential to the lower potential CB of CeO2, resulting in the apparent decrease of electron reducibility.

13

In order to maintain electro

neutrality holes from ceria moves towards the VB of BiOI. The accumulated electrons in the top position CB of CeO2 couldn’t reduce more of O2 to .O2- although the CB is more negative than standard redox potential E0 (O2/·O2- =-0.33 V vs. NHE).

56

It is because of the fact that

the accumulated electrons both from CeO2 and BiOI at the CB of CeO2 (4f0 5d0 6s0) immediately reacts with Ce4+ and forms Ce3+ (4f1 5d0 6s0), which is stabilized by creating an oxygen vacancy state. 6, 13 Due to this process an oxygen vacancy state slightly below the CB edge potential position would form which is more positive than the standard redox potential E0 (O2/·O2- = -0.33 V vs. NHE)). Thus a large number of ·O2- couldn’t be produced from the composite material which contradicts the quencher experiment, where ·O2- are the main active species for RhB decolourization.


26

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Scheme. 1 Photogenerated electron and hole pair separation and possible reaction mechanistic pathway. Similarly, double charge transfer mechanism couldn’t able to explain the cause of O2 evolution by the prepared photocatalyst. Additionally, as the VB edge potential of BiOI (0.78V) is less positive than water oxidation potential (0.82 V vs NHE at pH 7) hence possesses a very poor ability to oxidize H2O to O2.

65

So by conventional double charge

transfer mechanism it is not possible to produce such a large amount of O2 gas and couldn’t be able to interpret the unexpected behaviour of PL spectra. On the other hand if the composite catalytic path follows the Z-scheme of charge transfer, then upon visible light irradiation both BiOI and CeO2 simultaneously can get excited in the composite and produces electron-hole pairs. The excited electrons from the CB of CeO2 will combine with the photogenerated holes of BiOI (scheme 1), thus preserving the stronger reducible electrons in the CB of BiOI and stronger oxidizability holes in the VB of CeO2. As suggested by Wang et al. here also we suppose that I3- act as the electron carriers in the process of electron transfer across the junction through metallic Bi which we have confirmed from XPS analysis. Under the light irradiation I- of BiOI reacts with the hole to form I3-, simultaneously I3- reacts with photogenerated electrons of CeO2 to again produce I-. This reversibility in the I- / I3- pairs accelerates the electron and hole quenching at the metallic Bi resulting in the efficient charge separation of energetic electrons-holes through Z-scheme

12

which has also confirmed from Nyquist plot. This undesired fast recombination between the holes of BiOI and electrons of CeO2, is the main cause of high intense PL peaks for the composite than CeO2. Finally, the energetic electrons from the CB of BiOI react with the molecular oxygen to yield high amount of super oxide ion which is mainly responsible for RhB decolourization along with holes. Similarly the more oxidizable holes on the CeO2


27


surface can react with the water molecules to produce a large amount of O2 gas. The possible photocatalytic reactions were listed below. & * ) + ℎν → ($% … … . ℎ) & * ) $ + ℎν → $ ($% … … . ℎ) & ) & ) $ ($% → $ ($+) ,-. / ,.

* ) & ) (ℎ) 012 $ ($+) .

& ) ($% + → . + .

. + 3ℎ → 3ℎ 4$567689:;

Facile Synthesis of CeO 2 Nanosheets Decorated ... - ACS Publications

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