Band gap engineering in ZnO based nanocomposites

Band gap engineering in ZnO based nanocomposites

Journal Pre-proof Band gap engineering in ZnO based nanocomposites M. Chitra, G. Mangamma, K. Uthayarani, N. Neelakandeswari, E.K. Girija PII: S1386...

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Journal Pre-proof Band gap engineering in ZnO based nanocomposites

M. Chitra, G. Mangamma, K. Uthayarani, N. Neelakandeswari, E.K. Girija PII:

S1386-9477(19)31016-1

DOI:

https://doi.org/10.1016/j.physe.2020.113969

Reference:

PHYSE 113969

To appear in:

Physica E: Low-dimensional Systems and Nanostructures

Received Date:

09 July 2019

Accepted Date:

17 January 2020

Please cite this article as: M. Chitra, G. Mangamma, K. Uthayarani, N. Neelakandeswari, E.K. Girija, Band gap engineering in ZnO based nanocomposites, Physica E: Low-dimensional Systems and Nanostructures (2020), https://doi.org/10.1016/j.physe.2020.113969

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GRAPHICAL ABSTRACT

Journal Pre-proof Band gap engineering in ZnO based nanocomposites M. Chitra,*a G. Mangamma,*b K. Uthayarani,a N. Neelakandeswaric and E. K. Girijad aDepartment bSurface

of Physics, Sri Ramakrishna Engineering College, Coimbatore 641022, India

and Nanoscience Division, Material Science Group, Indira Gandhi Centre for

Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamilnadu, 603102, India cDepartment

of Chemistry, N.G.M College, Pollachi, 642001, India

dDepartment

of Physics, Periyar University, Salem 636011, India

* Corresponding Author: [email protected][email protected] Abstract In the present work, individual zinc oxide (ZnO), tin oxide (SnO2) and vanadium oxide (V2O5), their binary and ternary combinations were prepared via hydrothermal route. The structure and the morphology of all the samples were characterised using X-ray Diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM) and Raman spectroscopic tools. UV-Visible spectroscopic analysis was recorded for all the samples. The samples were also tested for ethanol sensing (0 ppm – 300 ppm) at room temperature. It has been focussed on to investigate the Raman and UV spectroscopic studies on the ethanol sensing properties of these oxides. It is observed that the Raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite. The peculiar hierarchical nanostructures of zinc – tin – vanadium oxide (ZTV) nanocomposite with larger surface area (167.3 m2/g) provided the required active surface sites for the adsorption of ethanol molecules. The band gap of ZTV is calculated as 1.97 eV. This band gap narrowing observed in ZTV might be due to the competing effects of high free carrier concentration and BursteinMoss shift. Hence ZTV shows pronounced sensitivity of 98 % at a response time of 32 s and recovery time of 6 s. Moreover the synergistic effect of ZTV nanocomposite enhanced the

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Journal Pre-proof sensitivity at a faster rate by overcoming the problems associated with the energy consumption, reversibility, adsorptive capacity and fabrication cost. Keywords : zinc-tin-vanadium oxide; nanocomposites; ethanol; sensor; spectroscopy 1. Introduction The demand of gas sensing devices in industries, emission control, environmental monitoring, military, agriculture production and medical diagnosis is constantly increasing [1-2]. This has attracted the researchers across the globe towards efficient gas sensing materials. Nanotechnology is the prime solution, with which it is possible to produce efficient sensing materials by mastering the shape and size [3, 4].In the past few decades, scientific community focusses on to develop gas sensing devices with superior performance [5 8] using nanostructured metal oxide semiconductors (MOS). It has been reported by various researchers that compared to individual MOS, mixed MOS show pronounced behaviour in sensing of gases owing to their synergistic effect [9-13]. The sensing behaviour of such nanocomposites are strongly influenced by grain size, morphology, chemical composition, surface modification, heterostructure formation and band gap [14-18]. Among them, band gap narrowing is one of the crucial effects in nanocomposites and these nanocomposites are employed as gas sensing devices. Band gap of the composites mainly depends on the change in electronic configuration of different metal oxides. This in turn depends on the charge carrier concentration, work function and electronic affinity. Ultraviolet Visible (UV-Vis) spectroscopic tool is used to investigate the effect of modulation in the band gap of the composite materials [19, 20]. The generated defect levels in these nanocomposites induces variation in band energies and results in band bending. In mixed oxides, lattice distortion owing to lattice strain is mainly attributed to lattice mismatch among zinc oxide, tin oxide and vanadium oxide (oxides

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Journal Pre-proof of different crystal structures). In addition, synergic effects between physicochemical and catalytic activity become apparent in mixed oxides [21, 22].This in turn results in the reduction of band gap. Also, the extent of band bending decreases the potential barrier of the contacts between the heterostructures and therefore causes an increase in the conductivity. Based on this detailed survey, in this present work, zinc oxide (ZnO), tin oxide (SnO2) and vanadium oxide (V2O5), their binary combinations such as zinc oxide – tin oxide combinations (ZT), zinc oxide – vanadium oxide combinations (ZV) and tin oxide – vanadium oxide combinations (TV) and ternary combination such as zinc – tin – vanadium oxide (ZTV) were prepared via hydrothermal route. The mechanism behind the enhanced ethanol sensing property of these materials based on crystallite size, morphology, surface area and synergistic effect has been investigated in detail and reported earlier. Here, it has been mainly focussed on to investigate the electronic effects based on band bending due to Fermi level equilibration for all the prepared samples. Herein, the effect of composite formation on the spectral shift, band gap modulation, band bending have been elucidated towards the enhanced ethanol sensing behaviour. 2. Experimental Details 2.1

Sample Preparation The chemicals zinc chloride (ZnCl2), stannous chloride dihydrate (SnCl2.2H2O),

vanadium chloride (VCl3), glyoxalic acid monohydrate (C2H2O3. H2O) and ammonia solution (NH4OH) were purchased from Rankem and were used as such without further purification. Double distilled water was employed as the solvent. Hydrothermal method was followed for the preparation of individual, binary and ternary oxides and is shown in Fig. 1

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Fig. 1 Composites in the formation of nanostructures of hierarchical morphology 2.1

Characterization Techniques PANalyticalX’Pert PRO diffractometer with CuKα radiation ( λ = 1.54 Ǻ)

ranging from 20 °- 80 ° has been used for recording the diffraction pattern of the samples. The surface morphology was recorded using ZEISS ultra-Field emission scanning electron microscopic (FESEM) analysis. The vibrational characterization was performed using Renishaw Raman spectroscopic instrument with 514.5 nm excitation of an Ar+ laser. A 50 Х objective lens was used to focus on the laser beam (about 1 mm spot size) and to collect the Raman spectral signal in the back-scattering configuration. Instrument calibration was carried out by checking the Si band at 520.7 cm−1. A grating with 2400 lines/mm was used for monochromatization of the scattered waves. The optical property of the samples was studied using the absorbance spectra taken by JASCO – Ultraviolet - visible spectrophotometer. The band gap of the samples was also calculated from the respective absorption edge. The specific surface area of the samples was analysed using Brunauer–Emmett–Teller (BET) analysis based on the nitrogen adsorption – desorption isotherm using a micromeritics 4

Journal Pre-proof apparatus. All the samples were tested for ethanol sensing ability at room temperature. The sensing characteristics of all the samples based on the recorded Raman spectra, surface area and band gap were discussed in detail. 3 Results and Discussion 3.1 Structural Characterisation 3.1.1 XRD analysis The X-ray Diffractometer (XRD) measurements taken for all the samples have been reported and correlated with the morphology and the influence of strain [23]. From the data, it has been observed that the peaks obtained for ZnO were indexed (ICDD File : 36-1451) and the sample exhibited wurtzite structured hexagonal phase [24]. The sharp and intense peak revealed the crystalline nature of ZnO. The indexed XRD pattern of SnO2 matches with the ICDD file: 41-1445 and it showed the existence of tetragonal rutile structure [25]. Similarly, the diffractions of vanadium oxide matched well with the standard diffraction file (41-1426) and it exhibited orthorhombic [26] structured V2O5. Furthermore, the XRD patterns recorded for all the binary systems have been reported earlier which reveals that the ZT system possess the individual phases ZnO (22%) and SnO2 (45%) along with the secondary phase Zn2SnO4 (33%). It is also evident that the ZT system experiences a strain due to the formation of the spinel phase. Broadening in the diffraction peaks of ZT when compared with the peaks of ZnO and SnO2was observed. A negligible amount of strain was also experienced by the ZT nanocomposite. The formation of Zn2SnO4 further supports for the reduction in the size of nanorods [27] and it is evidenced from the FESEM image (Fig. 2). Similarly, ZV system possess 31% of ZnO, 30% of V2O5, 23% of ZnV2O6 and 16% of ZnV3O8. The substitution of V5+ ions into Zn2+ lattice in ZV composite experienced a slight shift towards higher angle and suffer feeble variation in the lattice parameters. Similar reports

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Journal Pre-proof are also available for such shift due to V5+ ions into Zn2+ lattice [28]. In the case of TV nanocomposite, the pattern revealed only the individual phases SnO2 and V2O5 as 40% and 60% respectively [29, 30]. It is discussed earlier that the strain produced in these binary systems has a significant role in the structural formation [23]. Similar strain is also observed in the ZTV system which has the phases SnO2 (28%), Zn2SnO4 (19%), ZnV2O6 (36%) and ZnV3O8 (17%). 3.1.2 FE-SEM analysis FE-SEM image of ZnO in Fig.2 (a) clearly portrayed uniform nanorods with hexagonal facets. FE-SEM image of SnO2 in Fig. 2 (b) showed uniform distribution of nanoparticles whereas V2O5 shown in Fig. 2 (c) revealed nanoparticles of grain size around 5 nm distributed over the microflakes with well-defined grain boundaries. FE-SEM image of the ZT composite in Fig. 2 (d) depicted both the smaller spherical and larger hexagonal structured nanoparticles clouded among the irregular nanorods. FE-SEM image of the composite ZV shown in Fig.2 (e) exhibited randomly distributed aggregated nanorods and the spherical shaped V2O5 nanoparticles were observed to be dispersed over the nanorods. In ZV composite, V2O5 nanoparticles got attached over the nanorods resulting in surface irregularity. FE-SEM image of TV composite shown in Fig. 2(f) depicted fine nanoparticles.

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Fig.2 FE-SEM image of (a) ZnO, (b) SnO2, (c) V2O5, (d) ZT, (e) ZV and (f) TV From the table 1, it is observed that ZnO reveals nanorods, SnO2 reveals nanoparticles and V2O5 reveals nanoparticles distributed over the micro flakes with well-defined grain boundaries. Among the individual oxides, it has been noted that the elongated nanostructures of ZnO possess the larger surface area. In the case of binary systems, the effect of the secondary phase and composite formation on the morphology has been studied and it has been emphasised that the modulation in the morphology imparts large surface area which in turn provides more number of surface active sites for the adsorption of gas molecules to take place. Apart from these, ZTV which possess both hierarchical flower like microstructures and spherical nanoparticles as shown in Fig.3 exhibits the largest surface area. The linear relationship between the surface area and catalytic activity would have an impact on the sensing characteristics of ZTV.

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Journal Pre-proof Table. 1 Morphology and surface area values of all the samples Sample ZnO SnO2 V2O5 ZT ZV TV ZTV

Morphology Nanorods Nanoparticles Nanoparticles Nanorods Nanorods Nanoparticles Flower like microstructures and spherical nanoparticles

Specific surface area (m2/g) 123.9 118.8 116.5 156.7 151.6 126.5 167.3

Fig. 3 FE-SEM image of ZTV Table 2 EDAX data of ZTV Element (Atomic %)

OK

VK

Zn K

Sn L

Total

On the surface of the Flakes

65.00

11.72

13.86

9.41

100

76.33

7.13

7.46

9.09

100

On the surface of the spherical particles 3.2 Raman Spectra analysis 3.2.1 ZnO, SnO2, ZT

Generally, the crystal structure of ZnO is hexagonal with space group C46V. As per Group theory, Raman-active modes for ZnO crystals are A1 + 2E2 + E1. The polar modes A1 and E1 can split into TO and LO modes, where TO is the transverse optical mode and LO is

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Journal Pre-proof the longitudinal optical mode [31, 32]. The E2 mode is nonpolar and it is composed of two modes with a low and high frequency. In the present investigations, from Fig. 4(a), peaks of Raman spectrum of pure ZnO appear at 438 cm-1 and 588 cm-1. The maximum intensity occurs at 438 cm-1 which corresponds to E2 (high) modes. This sharp and E2 mode peak confirms the formation of wurtzite structured ZnO nanorods which is also evidenced from the XRD pattern [23]. The peak at 588 cm-1 corresponds to multiphonon process which occurs due to oxygen imperfections present in the sample and have been assigned to the second order Raman spectrum arising from the zone boundary phonons - [E2 (high) +E2 (low)] [33].

Intensity (arbitrary units)

Experimental -1 441 cm -1 635 cm Fitted Curve

400

450

500

550

(c)

600

650

(b)

Experimental SnO2 -1

633 cm Fitted curve

500

550

600

650

Experimental -1 438 cm -1 582 cm Fitted curve

400

700

700

(a)

600

800 -1

wavenumber (cm )

Fig. 4 Raman spectrum of (a) ZnO, (b) SnO2 and (c) ZT Tin oxide occurs in rutile form and it belongs to the symmetry space group D4h. The 6 unit cell atoms give rise to 18 vibrational modes. Two modes are infrared active (the single A2u and the triply degenerate Eu), four are Raman active (the three non-degenerate A1g, B1g, B2g modes and the doubly degenerate Eg one) and two others are silent (the A2g and B1u modes) [33, 34]. In the present work, Raman spectrum of SnO2 in Fig. 4(b), shows strong peaks at 633 cm-1 which correspond to A1g modes which could be attributed to the presence of amorphous phase [34, 35]. 9

Journal Pre-proof In the case of ZT as in Fig. 4 (c), ZnO peak at 441 cm-1 is assigned to vibration modes E2 for wurtzite ZnO. The peak shifts to higher wavenumber and decrease in intensity of the peaks is observed which might be due to the shorter bond lengths. The peaks of SnO2 in ZT nanocomposite appear at 635 cm-1 which corresponds to A1g mode for casseriterite SnO2 show a shift towards higher wavenumber when compared to individual SnO2 and results in decrease in the intensity [36]. It reflects that the compressive stress is due to the structural defects generated during calcination. The compressed lattice would have provided a wider band gap because of the increased repulsion between the oxygen 2p and the zinc 4s bands of ZT nanocomposite. The band at 514 cm-1 corresponding to E1 (LO) mode observed for pure ZnO has been collapsed after reaction with tin chloride and on further calcination. A very small band corresponding to Zn2SnO4 is also observed at 303 cm-1 which has been shifted to lower wavenumber (Fig. 3(c)) compared to bulk zinc stannate and it is attributed to the tensile stress experienced by the nanocomposite [36 - 38]. Also, this stress could have been the reason for the shrinkage of nanorods as observed from FESEM image (Fig. 2(d)). 3.2.2 ZnO, V2O5, ZV In the case of pure V2O5 as in Fig. 4 (b), the strong peaks appear at 406 cm-1, 482 cm1,

528 cm-1 and 701cm-1. Peak at 406 cm-1 corresponds to the bending vibration of the V=O

bonds [24]. Peak at 482 cm-1corresponds to the bending vibrations of the bridging V–O–V (doubly coordinated oxygen). Peak at 528 cm-1 triply coordinated oxygen (V3 –O) stretching mode which results from edged-shared oxygen in common to three pyramids [39]. Peak at 701 cm-1 correspond to the doubly coordinated oxygen (V2–O) stretching mode which results from corner-shared oxygen common to two pyramids [30]. In the case of ZV nanocomposite, a very small peak corresponding to ZnO (Fig. 5(c)) is observed at 324 cm-1 and it gets shifted to lower wavenumber when compared to pure ZnO reported. Similarly, V2O5 peak at 515 cm-1 and 912 cm-1 shifts towards lower wavenumber

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Journal Pre-proof (higher wavelength). The shift in wavenumber is of the order of 13 cm-1. The observed shift towards lower wavenumber is due to the tensile stress exerted by the intercalated ZnO nanorods sandwitched between the layers. Especially, in ZV nanocomposite, 374 cm-1 peak relates to E1 mode (TO) corresponding to ZnV2O6 which occurs due to lattice vibrations in crystal lattice [40 - 44]. The peak at 807 cm-1 and 862 cm-1 is mainly due to influence of V into the ZnO lattice and also due to the formation of ZnV3O8 . The formation of these phases

Intensity (arbitary units)

is also evidenced from the XRD pattern which has been reported earlier.

(c)

Experimental -1 289 cm -1 373 cm -1 514 cm -1 710 cm -1 807 cm -1 862 cm -1 912 cm Fitted

200

400

600

800

(b)

Experimental -1 285 cm -1 304 cm -1 406 cm -1 482 cm -1 528cm -1 702 cm -1 994 cm Fitted

200

400

600

800

1000

1200

(a)

Experimental -1 438 cm -1 582 cm Fitted curve

400

1000

600

800 -1

wavenumber (cm )

Fig. 5 Raman spectrum of (a) ZnO, (b) V2O5 and (c) ZV 3.2.3 SnO2, V2O5, TV From Fig. 6 (a) for pure SnO2, the strong peaks appear at 473 cm-1 and 630 cm-1 . For pure V2O5 (Fig. 6 (b)), the strong peaks appear at 406 cm-1, 482 cm-1, 528 cm-1 and 701cm-1. In the case of TV nanocomposite, peak of SnO2 at 478 cm-1 shifts to longer wavenumber and intensity decreases; but the peak at 630 cm-1get vanished. In the case of TV nanocomposite

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Journal Pre-proof (Fig. 6(c)), all the peaks shift towards shorter wavenumber and decrease in intensity is observed [45, 46].

Intensity (arbitrary units)

Experimental -1 284 cm -1 304 cm -1 405 cm -1 482 cm -1 526 cm -1 526 cm Fitted Curve

200

300

(c)

400

500

600

700

Experimental -1 285 cm -1 304 cm -1 406 cm -1 482 cm -1 528cm -1 702 cm -1 994 cm Fitted

200

400

600

800

800

(b)

1000

1200

(a)

Experimental SnO2 -1

633 cm Fitted curve

500

550

600

650

700

-1

wavenumber (cm )

Fig. 6 Raman spectrum of (a) SnO2 (b) V2O5 and (c) TV 3.2.4 ZTV In the Raman spectrum of ZTV nanocomposite (Fig. 6), the peaks at 375 cm-1 and 513 cm-1, 715 cm-1, 806 cm-1, 859 cm-1 and 912 cm-1 correspond to the presence of ZnV2O6 phase and 432 cm-1 correspond to ZT phase. In ZTV, the peaks are broad (62 cm-1) which indicates the nanocrystalline nature of the material. Such broad nature of the peak observed in the Raman spectrum of ZTV might be attributed to its larger surface area (167.3 m2/g). Further, peaks are shifted towards lower wavenumber which might be due to the tensile stress exerted by the ZT phase which is sand witched between the matrix of SnO2 nanoparticles.

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Intensity (arbitrary units)

Experimental -1 165 cm -1 221 cm -1 375 cm -1 432 cm -1 513 cm -1 715 cm -1 758 cm -1 806 cm -1 859 cm -1 911 cm Fitted

200

400

ZTV

600

800

1000

-1

wavenumber (cm )

Fig. 7Raman spectrum of ZTV(pl indicate peak position in 3.3 UV spectroscopic studies The optical band gap properties of the individual (ZnO, SnO2 and V2O5), binary (ZT, ZV and TV) and ternary (ZTV) systems were studied by UV-visible spectroscopy (Fig. 8). From them, Tauc plots were drawn by using the Tauc equation [47] where α is the absorption coefficient of the material, λ is the wavelength, h is the Planck’s constant, C′ is the proportionality constant, υ is the frequency of light and Eg is the band gap energy. The graph is plotted between (αhυ)2 vs hυ and it is observed that the band gap of ZnO nanorod is 3.3 eV which is smaller than the band gap of bulk ZnO (3.37 eV). This decrease in the band gap in turn alter the electronic structure by producing localised resonant states [48 - 50]. But the band gap of SnO2 and V2O5are 4.35 eV and 2.35 eV respectively which is larger than the corresponding bulk values (3.67 eV and 2.24 eV respectively) [51 - 54]. It is evident that the absorption edges gets shifted to higher energy region as the particle size decreases [55, 56]. The observed blue shift (Fig. 8) in these systems is also in accordance with the smaller grain size of SnO2 (20 nm) and V2O5 (5 nm). The increase in the band gap value of these MOS 13

Journal Pre-proof nanostructures is ascribed to the quantum confinement effect [52].Such a slight shift towards shorter wavelength is attributed to the Moss-Burstein effect. Owing to the defects, the absorption edge in the conduction band is pushed to the higher energies. As a result, all the states near the conduction band edge will be populated and the value of the optical band gap increases.

527 nm

Absorbance (arb.units)

c

285 nm

b

376 nm

a 200

400

600

800

Wavelength (nm)

Fig. 8 UV-Vis spectra of (a) ZnO, (b) SnO2 and (c) V2O5 Fig 9. (a) shows the UV-Vis absorption spectrum of the ZT composite material measured in the wavelength range of 200 nm – 700 nm. The absorption band for the composite material is found to be around 375 nm which is little larger than the characteristic absorption peak of ZnO. Also, a weak absorption is experienced around 250 nm and it may be attributed to the presence of spherical SnO2 and hexagonal Zn2SnO4 nanoparticles with grain size in the range of 20 – 50 nm. Investigations done on such equimolar mixture of

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Journal Pre-proof Zn/Sn composites reveal similar kind of absorption for the nanoparticles ranging from 40 – 70 nm. The optical band gap of ZT (Fig. 9 (a)) is calculated as 4.44 eV which is apparently larger than that bulk zinc stannate (3.6 eV). This blue shift in the optical band gap is due tothe Burstein–Moss effect arising from the smaller particle size [57 - 59]. Similar trend (Fig. 9(b and c)) has been observed in ZV and TV systems in which the quantum confinement effect due to smaller crystallite size is observed [60, 61].

Absorbance (arb.units)

257 nm 250 nm

305 nm 358 nm

c

286 nm 223 nm

376 nm

b a

200

400 600 Wavelength (nm)

800

Fig. 9 UV-Vis spectra of (a) ZT, (b) ZV and (c) TV

Samples

Reported Band gap(eV) Bulk

Reported Band gap (eV) 15 Nano

Band gap (eV) Observed

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ZnO SnO2 V2O5 ZT ZV TV ZTV

3.37 3.64 2.24 3.60 -

Rods – 3.53, Rods – 3.3 Particles – 3.78 Particles - 3.10 Particles – 3.94, Rods - 3.75 Nanoparticles - 3.23 Particles – 2.21 Present study

3.30 4.35 2.35 4.44 3.47 4.96 1.97

3:

Comparison of observed band gap values and the corresponding reported values

Kubelka-Munk absorption curve obtained using the diffuse reflectance spectrum (DRS) in Fig. 10 (a) of ZTV sample revealed no characteristic absorption for individual oxides. A broad absorption edge noticed around 600 nm may be due to the synergistic effect of the composite material. Band gap for ZTV is found to be 1.97 eV from the plot between (F(R)hυ)n and hυ where n = ½ [36] (Fig. 10 (c)). This reduction in band gap could be due the formation of defect states [62] and it was further confirmed by the Urbach energy value as 3030 meV calculated from Fig. 10 (d).

Reflectance (%)

30

20

10 500

600

700

16

Wavelength (nm)

800

900

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Fig. 10(a) Diffuse reflectance spectrum of ZTV

10

8

4

2

0

500

600

700

800

900

Wavelength (nm)

Fig. 10(b) Kubelka-Munk absorption curve of ZTV

5.0 4.5

1/2

4.0

(F(R)h)

F(R)

6

3.5 3.0 2.5 2.0 1.5 1.0 1.4

1.6

1.8

2.0

2.2

2.4

2.6

h (eV)

Fig. 10(c) Band gap determination plot of ZTV

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2.5 2.0

ln[F(R)]

1.5 1.0 0.5 0.0 1.9

2.0

2.1

2.2

2.3

2.4

2.5

h

Fig. 10(d) Plot of ln[F(R)] versus hυ for ZTV 4.4 Mechanism for ZTV system towards better sensing characteristics In a single component system, ZnO, the sensitivity towards 250 ppm of ethanol at room temperature is 51.98 %. When ZnO is clubbed with SnO2, the response shoots up to 77.93% which might be due to the oxygen defects present. Similarly when ZnO is clubbed with V2O5, again the sensitivity is increased (72.97 %). The next individual system SnO2 responds as 42.92% and while it occurs as a binary component with ZnO, the higher response observed might be due to the large surface area (156.7 m2/g) and oxygen defects present in the sample. In a similar fashion, TV system also shows good response of 63.96%. The contributions from the binary systems enable ZTV to show a pronounced response of 98.96% at a faster adsorption – desorption rate of 32 s and 6 s respectively. The following reasons are elucidated for such a drastic increase in the sensing characteristics of ZTV.

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4.4.1 Based on the vibrational modes in ZTV system In the present investigation, peaks of Raman spectrum of pure ZnO (Fig. 3) appear at 438 cm-1 and 588 cm-1. The maximum intensity occurs at 438 cm-1 which corresponds to E2 (high) modes. This sharp and E2 mode peak confirms the formation of wurtzite structured ZnO nanorods which is also evidenced from the XRD pattern. The peak at 588 cm-1 corresponds to multiphonon process which occurs due to oxygen imperfections present in the sample and have been assigned to the second order Raman spectrum arising from the zone boundary phonons - [E2 (high) +E2 (low)]. In the present work, Raman spectrum of the heat treated SnO2 shows strong peaks at 633 cm-1 which correspond to A1g modes and it can be attributed to the presence of amorphous phase (Fig. 1). In the case of pure V2O5 (Fig. 2), the strong peaks appear at 406 cm-1 , 482 cm-1, 528 cm-1 and 701cm-1. Peak at 406 cm-1 corresponds to the bending vibration of the V=O bonds. Peak at 482 cm-1corresponds to the bending vibrations of the bridging V–O–V (doubly coordinated oxygen). Peak at 528 cm-1 triply coordinated oxygen (V3 –O) stretching mode which results from edged-shared oxygen in common to three pyramids. Peak at 701 cm-1 - the doubly coordinated oxygen (V2 –O) stretching mode which results from corner-shared oxygen common to two pyramids. It is observed that the Raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite (Fig. 3, Fig. 4 and Fig. 5) which affects the sensing characteristics of the samples [63]. In a single component system, ZnO, the sensitivity towards 250 ppm of ethanol at room temperature is 51.98%. When ZnO is clubbed with SnO2, the response shoots up to 77.93% which might be due to the oxygen defects present. Similarly when ZnO is clubbed with V2O5, again the sensitivity is more (72.97%). The other individual system SnO2 responds as 42.92% and while it occurs as a binary component with ZnO, the higher response

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Journal Pre-proof might be due to the large surface area (156.7 m2/g) and oxygen defects present in the sample. In a similar fashion, TV system also shows good response of 63.96%. Among all the systems, the contributions from the binary systems enable ZTV to show a pronounced response of 98.96% at a faster adsorption – desorption rate of 32 s and 6 s respectively.

4.4.2 Based on surface area of ZTV system From the Table 4, it is understood that, in the case of individual metal oxides, though the surface area values lie in the range from 115 m2/g to 125 m2/g, there occurs drastic increase in the sensitivity from 30.98% (V2O5) to 51.98% (ZnO). This might be due to more number of surface active sites available for the adsorption of ethanol molecules on the surface of ZnO nanorods. Table. 4 Band gap, sensitivity, response-recovery time, surface area values of all the samples Sample

Eg (eV)

Sensitivity (%)

Response time(s)

Recovery time (s)

Surface area (m2/g)

ZnO SnO2 V2O5 ZT ZV TV ZTV

3.30 4.35 2.35 4.44 3.47 4.96 1.97

51.98 42.92 30.98 77.93 72.97 63.99 98.96

105 162 156 68 98 87 32

130 118 128 79 84 93 6

123.9 118.8 116.5 156.7 151.6 126.5 167.3

In the case of binary system, there occurs drastic increase in surface area value from 125 m2/g to 157 m2/g when compared with individual oxides. This in turn abruptly enhances the sensitivity to 78%. Among the binary system, ZT possess larger sensitivity due to its large surface area. With respect to SnO2, the binary system TV shows drastic increase in sensitivity from 42.92% to 63.93%. Similarly, with respect to ZnO to ZV system, the large change in surface area (123.9 m2/g) to (151.6 m2/g) imparts relative increase in sensitivity from 51.98% to 72.97%. This may be ascribed to the catalytic activity of V2O5 which could 20

Journal Pre-proof enhance the sensing behaviour [64]. But in the case of ZT system, the improved sensitivity of 77.93% is observed which exceeds ZV (72.97%) and TV (63.93%) systems which might be purely attributed to the more number of surface active sites provided due to large surface area (156.7 m2/g) of the material. In the case of ternary system ZTV, as we reported in the earlier paper, the highest sensitivity of 98.96% is due to the catalytic sites provided by ZV and large surface active sites contributed from ZT system. Therefore, from the detailed investigation towards ZTV, a probable account for the enhanced sensing properties at room temperature is attributed to the hierarchical architecture with axially oriented flowers. Due to the intermission between ZnO, SnO2 and V2O5, the hierarchical architectures evolves the BET surface area to 167.3 m2/g when compared to individual and binary systems (Fig. 11) [65]. This provides more active sites for absorption of ethanol and reaction of ethanol with surface-adsorbed oxygen ions, thus the decrease in resistance is much noticeable so that the gas sensing response is enhanced accordingly.

Fig. 11 Surface area contribution to ZTV 21

Journal Pre-proof 4.4.3 Based on the Band gap in ZTV system Among all the studied systems, the ternary system, ZTV possess the maximum sensitivity of 98.96% at a faster adsorption and desorption rate of 32 s and 6 s respectively. The band gap of ZTV calculated from the DRS spectrum is 1.97 eV (Fig. 12) which is much lower than the reported band gap value of individual oxides [48, 52, 55]. The red shift or decrease in energy with a decrease in particle size arises due to surface and interface effect, which is responsible for reduction in band gap. But present results can be featured to the lattice strain as well as the formation of localised energy states or defect states between the valence band (VB) and conduction band (CB) and interfaces/grain boundaries. These defect states [62] contribute a tailing effect (Urbach tail) in both CB and VB and lead to the formation of an additional energy gap lesser than the actual value. The localized energy states act as defects plays an important role in the electronic transitions. These defect states lead to decrease the optical energy gap and hence shifts the absorption edge towards the higher wavelength of the incident photons. It is also reported that the ability to attract the gas molecules is high for a material with smaller band gap due to its high carrier concentration [66].

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6

2

Surface area (m /g) 126.5

5

118.8

156.7

SnO2

ZT

Band gap (eV)

4

123.9

151.6

ZnO

ZV

3

116.5 2

167.3

1

0 ZTV

V2O5

TV

Samples

Fig. 12 Surface area and band gap values of all the samples The inherent catalytic activity, provision of active surface states and lowering of depletion layers of the n-type transition metal oxide promotes ZTV in the sensing ability. The reduction in the band gap (Fig. 12) of ZTV could possibly lead to more excited electrons compared to the other compositions [55]. The factors such as surface area, crystallite size, structural deficiencies and catalytic activity contributed from the secondary semiconductor to the structure of a primary semiconductor could be the reason for enhanced sensing behaviour of ZTV. Similarly, the addition of two or more phases in equimolar proportion increases the actual surface area of the nanocomposite because the phases in the composite act as the sintering inhibitor to each other.

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120

Band gap (eV) 100

1.97

Sensitivity (%)

80

4.4

3.47

4.96 60

3.3

40

4.35

2.35

20

0 ZTV

V2O5

ZnO

ZV

SnO2

ZT

TV

Samples

Fig. 13 Band gap and sensitivity values of all the samples

4.4.4 Mechanism for modified optical and electrical properties based on energy band diagram Based on the above results, the binary and ternary nanoheterostructure-based sensor shows enhanced sensing properties when compared with individual systems. The influence of nanocomposite formation is also mainly through the increasing adsorption capability by injecting more electrons into the active surface area. This also reduces the rate of charge recombination by separating active charge carriers and making more negative conduction band levels at the hetero-interfaces [67]. Herein, an analogous model for the binary and ternary systems has been proposed and is shown in Fig. 14 and Fig. 15. Initially, an n–n-type heterojunction can be formed at the interface between ZnO and SnO2, ZnO and V2O5 and SnO2 and V2O5 and a schematic illustration of electron transport at the heterojunction can be proposed as shown in Fig. 14 (a), 14 (b) and 14(c).

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Journal Pre-proof In ZT system

Fig. 14 (a) Band Energy Diagram of ZnO and SnO2 In ZV system

Fig. 14 (b) Band Energy Diagram of V2O5 and ZnO

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Fig. 14 (c) Band Energy Diagram of V2O5 and SnO2

In ZTV system, Fig. 14 shows the possible arrangement of nano hybrid gas sensor

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Fig. 15 Possible Band Energy Diagrams of ZTV system When an n-type semiconductor ZnO and n-type semiconductor SnO2 enter into an equilibrium with each other, electrons would transfer from ZnO to the lower-energy conduction band of SnO2 until their Fermi energy levels become equal. This leads to band bending and some of the electrons also takes the same path [68]. Secondly, in the presence of air oxygen species are adsorbed on the surface of the ZnO / SnO2 nanostructures and are then ionized into oxygen ions by capturing free electrons from the nanostructures which leads to the formation of a thick depletion layer at the interface and results in increase in the height of the energy barrier [68]. But when ZT nanocomposite is exposed to ethanol vapour, ethanol could react with the adsorbed oxygen species which leads to the release of the adsorbed electrons. This in turn reflects on the depletion layer so that it becomes thinner at the oxide surface and a decrease in the height of the energy barrier at the interface is observed Another possible explanation for the enhancement of ZT nanocomposite is the formation of n–n heterojunctions between ZnO and SnO2 [68, 69]. The resistance R of the heterojunction can be expressed by the following equation [70]:

𝑹 𝜶 𝑩 𝐞𝐱𝐩 (𝐪𝚽/𝐤𝐓)

27

……………….. (1)

Journal Pre-proof where B is a constant and qΦ is effective energy barrier at the heterojunction. In air, as discussed earlier, the free electrons are captured by oxygen species to ionize into oxygen ions, the effective energy barrier increases. But after exposure to ethanol gas, ethanol molecules could react with the adsorbed oxygen species and lead to the release of adsorbed electrons which leads to the decrease of the energy barrier. So the notable change in the energy barrier of the heterojunctions can induce great change in the conductivity and improvement of the gas sensing performance because Ra/Rg is in direct proportion to the value of exp (ΔqΦ). This could be purely due to the synergistic effect of nanocomposite and is applicable to studied binary systems such as ZV, TV and ZTV. In addition, the heterojunctions could provide more additional active sites resulting in the improved performance of gas sensing materials. Furthermore, ZT, ZV, TV and ZTV systems act asmore efficient catalyst (due to the reasons depicted in Fig. 16) than pure ZnOnanorods, SnO2 nanoparticles and V2O5 nanoflakes which could be capable of promoting the sensing reaction between the test gas and adsorbed oxygen species.

Fig. 16 Grain and Pore effects in ZTV From the above all detailed investigations made on the individual, binary and ternary systems, the better performance experienced by ZTV towards ethanol can be ascribed to the following two factors.

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Journal Pre-proof (i) larger surface area and complete electron depletion due to hierarchical architecture (ii) non-interface dependent complimentary behaviour (synergistic effect), more number of surface active sites and efficient catalytic capability induced by effective heterojunction formation. 5. Conclusion In the present work, ZnO, SnO2 V2O5, ZT, ZV, TV and ZTV were prepared via hydrothermal route followed by calcination. All the samples were characterised by various state-of-the-art techniques and subjected to ethanol sensing at room temperature. Especially, Raman spectroscopic and UV spectroscopic studies were correlated with the ethanol sensing property of the samples at room temperature. It is observed that the Raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite. In the case of ZT – ZnO peak at 441 cm-1 is assigned to vibration modes E2 for wurtzite ZnO. The peak shifts to higher wavenumber and the intensity of the peaks gets diminished which might be due to the shorter bond lengths. In the case of ZV nanocomposite, a very small peak corresponding to ZnO is observed at 324 cm-1 and it gets shifted to lower wavenumber when compared to pure ZnO reported. Among ZnO, SnO2 and V2O5, ZnO showed pronounced sensitivity of 51.98% owing to its large surface area and morphological characteristics. In the case of binary nanocomposite, ZT showed better response compared to the other two systems which might be attributed to the catalytic activity. Among all the studied systems, the ternary system, ZTV possess the maximum sensitivity of 98.96% at a faster adsorption and desorption rate of 32 s and 6 s respectively. The defect states proved from urbach energy value (3030 meV) of ZTV lead to decrease the optical energy gap as 1.97 eV and hence shifts the absorption edge towards the higher wavelength of the incident photons. Hence, the small band gap ZTV once again proves to be the best ethanol sensor at room temperature.

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Journal Pre-proof Acknowledgement Authors wish to thank Dr.M. Kamruddin, Dr. S. Dhara, Dr T.N.Sairam, Dr.A. Rajesh, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603012, India.

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Conflict of Interest ZTV material has been studied for the first time. Studies were in detail for device fabrication. We request you consider our paper for publication based on the following important points about our paper which are given below: 1. Nano composite is a multifunctional material which has enhanced activity toward gas sensing along with other improved properties like 2. The raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite 3. The defect states proved from urbach energy value (3030 meV) of ZTV lead to decrease the optical energy gap as 1.97 eV. 4. ZTV has a pronounced sensitivity of 98.96 % with a response and recovery time of 32 s and 6 s at room temperature. Such materials are very rare. There is no conflict of interest .

Journal Pre-proof Highlights   

Zinc-tin-vanadium oxide nanocomposite is prepared via hydrothermal route ZTV possess large surface area of 167.3 m2/g and smaller band gap of 1.97 eV Raman and UV-Visible spectroscopic studies have been carried out