Structural, Optical, Electrical and Photocatalytic Investigation of n-Type Zn2+-Doped α-Bi2O3 Nanoparticles for Optoelectronics Applications

Herein, n-type pure and Zn2+-doped monoclinic bismuth oxide nanoparticles were synthesized by the citrate sol–gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) analysis, ultraviolet–visible (UV–vis) spectroscopy, and Hall effect measurements were used to study the effect of Zn2+ on the structural, optical, and electrical properties of nanoparticles. XRD revealed the monoclinic stable phase (α-Bi2O3) of all synthesized samples and the crystallite size of nanoparticles increased with increasing concentration of dopant. Optical analysis illustrated the red shift of absorption edge and blue shift of band gap with increasing concentration of dopant. Hall Effect measurements showed improved values (2.79 × 10–5 S cm–1 and 6.89 cm2/V·s) of conductivity and mobility, respectively, for Zn2+-doped α-Bi2O3 nanoparticles. The tuned optical band gap and improved electrical properties make Zn2+-doped α-Bi2O3 nanostructures promising candidates for optoelectronic devices. The degradation of methylene blue (MB, organic dye) in pure and zinc-doped α-Bi2O3 was investigated under solar irradiation. The optimum doping level of zinc (4.5% Zn2+-doped α-Bi2O3) reveals the attractive photocatalytic activity of α-Bi2O3 nanostructures due to electron trapping and detrapping for solar cells.


INTRODUCTION
Since a few decades, scientific researchers have shown significant interest in metal oxides due to their environmentally friendly behavior, thermal stability, ferroelectric properties, mechanical strength, and biocompatibility.−10 Bismuth chalcogenides (Bi 2 E 3 , E = Te, O, Se) are compounds that have noteworthy and attractive features based on electrical and optical properties.Bi 2 E 3 compounds are semiconductors with novel properties, which are used in different industrial applications.Among the Bi 2 E 3 family, bismuth oxide (Bi 2 O 3 ) is of great interest because it has six crystallographic polymorphs: α-Bi 2 O 3 (monoclinic phase), δ-Bi 2 O 3 (face-centered cubic phase), γ-Bi 2 O 3 (body-centered cubic phase), ω-Bi 2 O 3 (triclinic phase), ε-Bi 2 O 3 (orthorhombic phase), and β-Bi 2 O 3 (tetragonal phase). 11,12Owing to its excellent environmental stability, high carrier mobility, and appropriate band gap, Bi 2 O 3 is a promising candidate for optoelectronic and electronic devices. 9Bi 2 O 3 is a semiconductor material with a wide band gap in the range between 2 and 3.96 eV. 13 Optical band gap and electrical conductivity are the most important features, while the optoelectronics applications are under investigation.Bi 2 O 3 is also known as an amphoteric semiconductor because it can show both p-type and n-type conductivity, which depends upon the method of preparation.The synthesis methods of Bi 2 O 3 and noble metal oxide nanostructures are similar, in which decomposition of precursors occurs to yield the nuclei. 14Bi 2 O 3 nanostructures can be prepared by coprecipitation, 15 solution combustion, 16 hydrothermal, probe sonication, 17 and sol−gel techniques. 18mong all mentioned techniques, the sol−gel method is known to be an inexpensive, ingenious, robust, and momentous method, by which size-controlled nanostructures can be prepared within a short time.−21 Doping of HO 3+ , Tb 3+ , and Sm 3+ in Bi 2 O 3 was investigated by Vishwakarma, et al., 22 Dixit, et al., 23 and Ashwini, et al., 16 respectively.
A number of research groups have investigated the effect of transition metal doping in Bi 2 O 3 nanostructures, but very few have explained the effect of metal doping in Bi 2 O 3 .In the present research work, n-type α-Bi 2 O 3 nanoparticles doped with different volume percentages (0, 1.5, 3.5, and 5.5) of zinc (Zn 2+ ions) were synthesized via the citrate sol−gel method.The structural, optical, and electrical properties of the synthesized zinc-doped α-Bi 2 O 3 nanoparticles were investigated for optoelectronics applications.The photocatalytic analysis of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles was carried out by degradation of the pollutant (MB) in solution under solar illumination.1 shows the schematic representation of the synthetic methodology for Zn-doped Bi 2 O 3 nanoparticles.

Synthesis of
2.2.Characterization Tools.Structural analysis of fabricated nanoparticles was done using Bruker d8 (Cu−Kα, 1.54 Å) X-ray diffraction (XRD).The morphology and elemental composition of nanoparticles were collected by a scanning electron microscope (SEM), Hitachi SU-70.Optical investigation was carried out using a ultraviolet−visible (UV− vis) dual beam spectrophotometer, Lambda 25, PerkinElmer.Fourier transform infrared (FTIR) spectroscopy was carried out using PerkinElmer spectrum 2. A Horiba RAM-HR800 microscope fitted with a HE-Cd UV laser (29 mW power, 400 nm) was used to perform photoluminescence (PL) measurements.Electrical analysis was carried out using the NANO− CHIP Reliabilty grade Hall effect system; the nanoparticles were coated on glass substrates of 1 cm 2 area and contact was made by indium metal at four corners.

Photocatalytic Analysis.
To investigate the photocatalytic activity of bismuth oxides, 0.2 g of two selected samples ZBO-A (pure α-Bi 2 O 3 ) and ZBO−D (4.5% doped Zn 2+ , α-Bi 2 O 3 ) were mixed in 100 mL of 1 mg/1 MB and added in a quartz photoreactor.The prepared aqueous solution was mixed for 2 h in darkness and then irradiated under sunlight with 950 ± 25 Wm −2 fluctuations.To check the level of MB, samples were collected after every 20 min and centrifuged to separate the unmixed nanoparticles.The remaining solution in the centrifuge of MB used to measure the optical absorbance at 464 nm.The degradation (%) of the dye (MB) in the absence and presence of the catalysts was calculated by the following equation.The diffraction peaks related to Zn and its oxides are not observed in the XRD spectrum, which illustrates that the monoclinic structure of α-Bi 2 O 3 was not altered by the doping of Zn. 18 The crystallite size "D" of the fabricated samples was calculated using Scherer's formula 25 eq 2 for the diffraction peak (−121).The lattice parameters "a", "b", and "c" were also calculated using the following eq 2. 26 where "λ" is the wavelength (Cu−Kα radiation 1.54 Å), "θ" is the angle of diffraction, and "β" represents the full width at half-maximum (radians).The crystallite size of pure α-Bi 2 O 3 nanoparticles increases from 35.14 to 41.72 nm with increasing volume concentration of Zn, as shown in Figure 3.The increase in crystallite size due to Zn doping because of the Zn 2+ ion of 0.075 nm ionic radius caused the foreign contaminants' distortion in the host α-Bi 2 O 3 lattice at the place of the Bi 3+ ion of 0.1034 nm ionic radius. 27oreover, lattice parameters "a", "b", and "c", unit cell volume, and β angle were also calculated and the calculated values are shown in Table 1.Unit cell volume and lattice parameters did not exhibit the monotonic variation upon increasing the concentration of Zn.The lattice parameters are also shown in Figure 3. Substitutional doping of Zn 2+ ions is dominant in the α-Bi 2 O 3 crystal because the unit cell volume decreases with increasing concentration of Zn.If the interstitial doping of Zn 2+ ions is dominant, then expansion of unit cell volume occurs, 28 but in the present study, the substitutional doping is dominant.
3.2.FTIR Analysis.FTIR spectroscopy is a nondestructive technique used to investigate the functional groups in the subjected samples.Figure 4 exhibits the FTIR spectra of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles.All of the synthesized nanoparticles exhibited the absorption bands of OH (hydrogen-bonded) stretching at 3430 cm −1 . 29,30The absorption bands at 3430 cm −1 were attributed to the OH group in the synthesized samples due to deoxygenation.The vibration modes appearing at 12,000−1700 cm −1 represented the interlayer nitrate (NO 3 group).The stretching vibration of normal OH was also observed at 1080 cm −1 , which is due to the presence of water in the Bi−O lattice. 31,32Absorption bands due to interatomic vibrations are produced mostly below 1000 cm −1 in metal oxides.Stretching vibrations of Bi−O   bonds were observed at 840 cm −1 in pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles.Meanwhile, the peaks originated in 400− 700 cm −1 are associated with metal oxygen vibrations (Bi−O− Bi).In Figure 4, it is shown that the intensity of Bi−O bond stretching vibrations significantly increased with increasing concentration of Zn 2+ .This progressive enhancement is owing to the variation of the defect state density around the Bi ions when Zn 2+ ions are doped in the Bi−O lattice.The peaks of Bi−O stretching vibrations illustrated that the desired nanostructures were developed successfully and these spectral peaks are attributed to specific bonding modes or molecular vibrations in nanostructures, which are essential for investigating their potential applications. 33

Raman Analysis.
Raman analysis provides information about the crystal structure, defects, and composition of nanomaterials, semiconductors, polymers, etc. 34 The Raman spectra of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles in the range 200−1000 cm −1 are shown in Figure 5a−d.For the α-Bi 2 O 3 group, theory states that the optical modes with good agreement of 15A g + 15B g are given by the following equation.Force constants improved with Zn doping; due to this reason, the observed peaks of α-Bi 2 O 3 nanoparticles shifted toward a higher frequency.The peak observed at 530 cm −1 is associated with oxygen vacancies.Raman spectra revealed the Raman active phonon at 622 cm −1 .A deep investigation of the Raman spectra for ZBO-C and ZBO−D revealed that small intense bands occurred at 666, 724, and 826 cm −1 , as shown in Figure 5c,d, and these bands represent the rearrangement of the anionic sublattice. 36,37The obtained overtones and energy position (cm −1 ) from Raman analysis for pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles are shown in Table 2.
3.4.PL Analysis.PL emission analysis was recorded at room temperature with 400 nm excitation wavelength for pure  and Zn 2+ -doped α-Bi 2 O 3 nanoparticles.As shown in Figure 6, the major visible emission peak for pure α-Bi 2 O 3 nanoparticles is observed at 456 nm at room temperature.The valence band of Bi 2 O 3 is associated with the 6s and 2p orbitals of Bi and O 2 , respectively.Under the irradiation excitation, charge transfers from O 2 2p and Bi 6s orbitals to the conduction band of Bi, which is a 6p orbital, and the peak at 465 nm forms due to the recombination of free excitons after the de-excitation. 30,38The band edge emission peak at 456 nm for pure α-Bi 2 O 3 nanoparticles (denoted as * in Figure 6) shifted toward shorter wavelengths with increasing concentration of Zn 2+ ion in α-Bi 2 O 3 nanoparticles; e.g., for ZBO−D it occurs at 441 nm.The shifting of band edge emission peak toward the lower wavelength with increasing concentration Zn 2+ is due to the improvement in energy of the band-to-band recombination according to the Burstein−Moss effect. 39In Zn 2+ -doped α-Bi 2 O 3 nanoparticles, the other three peaks were also recorded.The first one was recorded at 470 nm, which is produced due to the recombination of Zn interstitial or vacancies with valence band. 40,41The second extra peak occurs at 495 nm, which represents the deep-level emission due to surface defects. 42The peak located at 549 nm corresponds to the defect emission due to oxygen interstitials or ion vacancies in the structure.This peak is also associated with Zn vacancy in case of zinc oxides. 43,44Usually the peak appearing at 549 nm is broad and intense due to oxygen chemisorptions, but in our work, the intensity of the peak is low, which indicates the moderate percenatge.nanoparticles shifted toward shorter wavelengths with increasing concentration of Zn 2+ dopant.Absorption analysis indicates that the doping of Zn 2+ ions introduced the new absorption energy levels because the width of the energy gap progressively improved with the increasing concentration of Zn 2+.45,46 Tauc's relationship was used for the measurement of optical band gap "E g " by extrapolating the linear portion of the (αhν) 2 versus hν (eV) curve. 47

h
A h E ( ) where the value of "n" is 2 for the direct allowed band gap, hν (eV) is the energy of photons, "A" is related to the slope of the Tauc line and is the band tailing parameter, and "α" is the absorption coefficient.An increase in optical band gap of the synthesized samples was observed with increasing concentration of Zn 2+ ions; it increased from 2.79 eV for ZBO-A (pure α-Bi 2 O 3 ) to 2.94 eV for ZBO−D.The increase in optical band gap with Zn 2+ doping is due to Burstein−Moss effect and this shift is already confirmed in the PL analysis section.8a and 8c,d, respectively.The SEM images show that the fabricated nanoparticles have plate-like morphology.Plate-like morphology provides significantly improved interactions due to a greater surface area.Measurement of particle size is difficult from the obtained SEM images because there is hazy morphology with different particle sizes.SEM also provided the energy dispersive spectroscopy (EDS) spectra of 3% (volume percentage) Zn 2+ -doped α-Bi 2 O 3 nanoparticles, which is shown in Figure 8d.EDS spectra showed the elemental peaks of Bi and O with addition of Zn peaks. 48The EDS results illustrated that the nanoparticles are produced of only the desired atoms without any impurity.
For further investigations of the crystalline size and morphology of nanoparticles, TEM analysis was done.Figure 9 shows the TEM images of ZBO-A and ZBO-C samples.Figure 9a illustrates that the pure α-Bi 2 O 3 nanoparticles are spherical and well separated with sizes of 10−20 nm, while if we compare doped nanoparticles (ZBO-C) with pure ones, the shape of the particles are found to remain spherical but doped nanoparticles are aggregated with sizes of 40−51 nm (Figure 9b).

Electrical Analysis.
To investigate the effect of Zn 2+ ion on the electrical properties of α-Bi 2 O 3 nanoparticles, commonly used Hall measurements (Van der Pauw technique) were carried out to measure the carrier concentration, carrier type, resistivity, conductivity, and mobility of the fabricated nanoparticles. 3 All of the synthesized nanoparticles showed ntype conductivity, and it was also observed that the conductivity and carrier mobility increase from 5.91 × 10 −6 S cm −1 for ZBO-A to 2.19 × 10 −5 S cm −1 for ZBO−D and 0.611 cm 2 /V•s for ZBO-A to 6.89 cm 2 /V•s for ZBO−D, respectively.Maximum conductivity, minimum resistivity, and improved carrier mobility were obtained for ZBO−D.The carrier mobility improved with increase in the volume concentration of Zn 2+ ions because of the reduction in scattering probability of charge carriers. 49The details of carrier concentration, resistivity, conductivity, and mobility are shown in Figure 10 for all samples.Three recycling studies were carried out on the ZBO−D nanocatalyst to investigate its photostability.At the end of each photocatalytic round, the nanocatalyst was recovered and cleaned for the next round.Figure 12a illustrates the significant photostability of the ZBO−D nanocatalyst during the photocatalytic recycling investigations.At the end of the three recycling investigations, the examined nanocatalyst showed a 3.1% reduction in photocatalytic activity, which is a mere one.
Scavenger experiments were performed to investigate the role of active species in the degradation of MB dye under solar irradiation.Ethylenediaminetetraacetic acid (EDTA), benzoquinone (BQ), ZnNO 3 , and 2-propanol (IPA) were used to remove holes, superoxide radical ions, electrons, and hydroxyl radicals, respectively.Figure 12b illustrated that the ZBO−D nanocatalyst mineralizes approximately 95% of the MB dye after 140 min without any scavenger (mentioned as without a scavenger in Figure 10b).When EDTA was used as the scavenger, the mineralization of MB dye was effective up to 85%.When BQ and ZnNO 3 were used as scavengers, the mineralization of MB dye was effective up to 74 and 57%, in that order.The dye mineralization efficiency of the ZBO−D nanocatalyst reduced a lot when IPA was used as the scavenger (up to 18%), which illustrates that hydroxyl radicals are the active species in the process of photodegradation.
Photocatalytic activity was improved due to the phenomenon of electron trapping and detrapping, as well as the separation of photogenerated charge carriers.In semiconductor materials, a photoenergy is required to excite the electrons from the valence (E VB ) to conduction band (E CB ).E VB and E CB , the potentials for the ZBO−D nanocatalyst, were measured using the following equations. ) where χ is the absolute electronegativity and the calculated value of χ for the ZBO−D semiconductor is 6.2.In eq 6, 4.5 represents the free electron energy at the hydrogen scale.The conduction and valence bands for ZBO−D semiconductor are 0.23 and 3.17 eV, respectively.The possible reactions in our work are mentioned below.Figure 13 shows the possible mechanism of photocatalytic degradation of the MB dye when ZBO−D is used as a nanocatalyst.

CONCLUSIONS
The citrate sol gel method was used for the synthesis of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles with varying volume concentrations of zinc.XRD analysis show that all of the synthesized nanoparticles were formed in monoclinic stable phase (α-phase).Optical analysis revealed that the introduction of Zn 2+ ions into the bismuth oxide lattice generated the new energy levels and improved the optical band gap.Doping of Zn 2+ ions improved the conductivity and carrier charge mobility of the α-Bi 2 O 3 nanostructures.The tuned optical band gap, improved conductivity and carrier mobility, and significantly reduced resistivity make these Zn 2+ -doped α-Bi 2 O 3 nanoparticles a better candidate for optoelectronic Pure and Zn-Doped Bi 2 O 3 Nanoparticles.All of the chemical reagents were purchased from Sigma-Aldrich and used without any further purification.Zn x Bi 2−x O 3 , for pure Bi 2 O 3 (x = 0.00) and for zinc-doped Bi 2 O 3 with doping of Zn taken in different percentages such as x = 0.015, 0.03, and 0.045, nanoparticles were successfully developed by the citrate sol−gel method.High-purity bismuth nitrate pentahydrate (Bi(NO 3 ) 3 •5H 2 O, ≥98%) and zinc nitrate hexahydrate (Zn(NO 3 ) 2 •6H 2 O, ≥99%) were used as initial precursors for Bi and Zn, respectively.In the first step, pure Bi 2 O 3 nanoparticles were fabricated.A 0.16 M solution of Bi(NO 3 ) 3 •5H 2 O was prepared in nitric acid (HNO 3 ), and further, this solution was diluted in HNO 3 in the ratio 1:3.The obtained mixture was heated at 100 °C in a water bath.0.16 M (equimolar) citric acid (C 6 H 8 O 7 ) was added to the hot mixture, and the temperature of the water bath was kept at 100 °C until a gel of pale yellow color formed.A heating mantle was used to heat the formed gel and autocombustion took place to convert the gel into foam to obtain the final product.The final products were annealed at 600 °C for 2 h in a muffle furnace to obtain the improved phase ordering of metal oxides.In the second step, Zn-doped Bi 2 O 3 was fabricated with the same process.A 0.16 M solution of Zn(NO 3 ) 2 •6H 2 O in HNO 3 was mixed with a 0.16 M solution of Bi(NO 3 ) 3 •5H 2 O in HNO 3 in different volume percentages (1.5, 3, and 4.5) by keeping the volume constant, before diluting in HNO 3 in the ratio 1:3.The samples were denoted as ZBO-A, ZBO-B, ZBO-C, and ZBO−D for pure Bi 2 O 3 , 1.5, 3, and 4.5 volume percentage doping of Zn in Bi 2 O 3 nanoparticles, respectively. Figure

C
o and C t are the concentrations of the dye (MB) before and after the illumination under sunlight, respectively.

35
degenerated E g and nondegenerated A 1g , B 1g , and B 2g modes are first-order active Raman modes.B 1u and A 2g are the silent modes.Two E u and one A 2u modes are associated with longitudinal (LO), transverse optical (TO), and acoustic modes.The Bi−O and Bi−O−Bi vibrations are the origin of bands produced in the region 200 to 550 cm −1 .The peak around 210 cm −1 is due to the vibration of oxygen present in the Bi 2 O 3 structure.The Bi−O stretching vibrations are associated with A g and B g observed at 314 cm −1 .

3 . 5 .
UV−Vis Spectroscopy.The optical transmittance and absorbance of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles in the wavelength range 200 to 600 nm are shown in Figure 7a,b.
Figure 7a depicts the optical transmittance of pure and Zndoped α-Bi 2 O 3 nanoparticles.The synthesized nanoparticles showed less optical transmittance in the UV region, but in the visible region, nanoparticles exhibited significant optical transmittance.Pure α-Bi 2 O 3 nanoparticles showed the maximum optical transmittance in the visible region as compared to Zn 2+ -doped α-Bi 2 O 3 nanoparticles.From Figure 7b, it is clear that the synthesized nanoparticles absorb both ultraviolet and visible regions.The absorption edge of α-Bi 2 O 3

3. 6 .
SEM and TEM Analysis.The morphology and elemental composition of the synthesized samples were analyzed by SEM.The SEM images of pure α-Bi 2 O 3 nanoparticles (ZBO-A) and Zn 2+ -doped α-Bi 2 O 3 nanoparticles (ZBO-B and ZBO-C) are shown in Figure

3 . 8 .
Photocatalytic Analysis.The photocatalytic investigations of pure and Zn 2+ -doped α-Bi 2 O 3 nanoparticles were carried out on the basis of the percentage degradation of organic dye (MB) in solution under solar irradiation.The change in optical absorbance at 464 nm was measured to calculate the percentage degradation of MB as a function of irradiation interval (20 min).Figure 11a,b shows the variation in optical absorbance as a function of irradiation interval due to the change in concentration of MB in pure (ZBO-A) and Zn 2+doped α-Bi 2 O 3 (ZBO−D).The investigations revealed that pure α-Bi 2 O 3 exhibits partial degradation and Zn 2+ -doped α-Bi 2 O 3 shows complete degradation.The results of percentage degradation are shown in Figure 11c, and the doped sample shows a much higher (≈95%) degradation compared with that of the pure sample.A linear relation between the logarithm of the relative concentration (C o /C t ) of the MB solution and the irradiation time intervals is shown in Figure 11d to monitor the

Figure 10 .
Figure 10.(a) Carrier concentration and mobility.(b) Resistivity and conductivity of pure and doped samples.

Table 1 .
XRD Parameters of the Synthesized Nanoparticles