V5+-stabilized δ-Bi2O3 for visible light-driven photocatalyst

Materials and Design 181 (2019) 108066

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Preparation and optical properties of Te4+/V5+-stabilized δ-Bi2O3 for visible light-driven photocatalyst Guitao Zhou a, Yanlin Huang a,⁎, Donglei Wei b, Shala Bi b, Hyo Jin Seo b,⁎ a b

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China Department of Physics, Interdisciplinary Program of Biomedical Engineering, Pukyong National University, Busan 608-737, Republic of Korea

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Te4+/V5+-codoping in Bi2O3 stabilizes its cubic δ-phase at room temperature. • Band gap of Te4+/V5+-codoped δ-Bi2O3 was narrowed in comparison with αphase. • Bi2−xTexVxO3 (x = 0.1, 0.2) showed the improved photocatalysis performances. • Existence of multivalent V ions plays an important role on photocatalysis.

a r t i c l e

i n f o

Article history: Received 23 March 2019 Received in revised form 19 July 2019 Accepted 20 July 2019 Available online 23 July 2019 Keywords: Semiconductor Photocatalysis Band energy Solar materials Optical properties

a b s t r a c t Te4+ and V5+-codoped Bi2O3 nanoparticles were synthesized via the sol-gel method. The sample showed a loose aggregation with the ball-like nanoparticles (of 200–400 nm). X-ray powder diffraction (XRD) refinements were used to investigate the crystallinity formations. As-prepared Bi2−xTexVxO3 (x = 0.1, 0.2) samples present δ-Bi2O3 phase with space group of Fm-3m at room temperature. The results indicate that the cubic structure could be stabilized via Te4+ and V5+-codoping in Bi2O3 lattices. UV–vis absorption measurements concluded that Te4+/V5+codoping could modify the band gap of δ-Bi2O3. Te/V-doped samples could harvest more visible light in longerwavelength region. The improved photocatalysis of Bi2−xTexVxO3 (x = 0.1, 0.2) on RhB photodegradation was in presence with the irradiation of visible-light (λ N 420 nm). To elucidate the photocatalytic mechanisms, XPS measurements, luminescence and lifetimes and impedance spectra were measured and analyzed. The improved photocatalysis was related with the microstructure changes and the multivalent V ions. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

1. Introduction Semiconductor photocatalysts have been paid great attention in the potential applications in new energy generation from watering splitting and photo-degradation organic pollutions [1–6]. In recent years, Bi2O3⁎ Corresponding authors. E-mail addresses: [email protected] (Y. Huang), [email protected] (H.J. Seo).

related materials have been intensively reported as one of the promising photocatalyst candidates due to its excellent properties [7–10]. Bi2O3 has a lone pair of 6s2 electrons, which can exert a strong repulsive force to other bonds. The hybridizations of O-2p and Bi-6s electronic orbitals in a Bi-semiconductor make a lower energy position in valence band (VB). This effect induces great distortion and polar electric field in the lattices, which can accelerate the charge separation resulting in an efficient photocatalytic activity [11,12].

https://doi.org/10.1016/j.matdes.2019.108066 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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Bi2O3 has five phase formations: monoclinic type (α), tetragonal (β), body-centered cubic (γ), face-centered cubic (δ), and triclinic (ε) structures [13,14]. Among them, δ-type with cubic structure processes the best oxygen ion conductivity in oxide materials [15], which can accelerate the mobility of oxygen vacancies and induces high polarizability of 6s2 lone electron pairs and weak Bi\\O bond. In recent years, many reports have been published on the stabilization of the cubic δ-phased Bi2O3 at room temperature via impurity doping [16–19]. δ-Bi2O3 has a typical defect fluorite-type structure with 25% of the oxygen sites are vacant in the unit cell [20]. Its best conductivity is favorable for the effective mobility and migration of photo-created charges to the semiconductor surface, where effective photocatalysis could take place. Moreover, δ-Bi2O3 has a low VB edge top (+2.63 V vs NHE), which permits photo-created holes have strong oxidation power in the VB. This feature has been reported to be greatly favorable for the oxidation reactions [21]. δ-Bi2O3 structure has been reported to be one of the potential candidates for photocatalytic applications [22–27]. Although δ-Bi2O3 has been used as a photocatalyst, its efficiency is not satisfied due to the easy combination of electron–hole pairs. In this work, we developed a new method to prepare δ-Bi2O3, that is Te4+/V5+-codoping in the lattices. The cubic phase of Te4+/V5+stabilized δ-Bi2O3 nanoparticles were obtained. We investigated the photocatalytic abilities on the following motivations. Firstly, in bismuth oxide, Bi3+ ion could be partially substituted by Te4+, which has the same stereoactive lone electron pairs. This substitution could exert an influence on the electrical properties such as great reduction of the impedance [28–30]. Under this situation, it can be expected that the improved photocatalysis could be realized because the mobility/ migration of the charges can be improved. Secondly, δ-Bi2O3 has a typical defect fluorite-type structure with 25% of the oxygen sites in the unit cell are vacant. Thus with substitution of Te4+/V5+-doping, the microstructure in the lattices could be distorted. The structural defects of oxygen vacancies can accelerate the ion conductivity via vacancy hopping. The oxygen vacancies could further delay hole-electron recombination via trapping electrons resulting in improving the photocatalysis effects. Bi2−xTexVxO3 (x = 0.1, 0.2) was fabricated through the conventional sol-gel method. The phases and structure properties were investigated via XRD measurements. The work is to elucidate the influence of the cation substitution on the band energies and electronic structures. The photocatalytic activities for RhB photodegradation were tested. The mechanism was discussed via the measurements of luminescence spectra, charge lifetimes, and XPS results. The results in this work suggested that Te4+/V5+-codoping is an effective method to stabilize δ-Bi2O3 at room temperature. Photocatalytic effects could be improved via the Te4+/V5+-codoping in a Bi-compound. 2. Experimental Bi2−xTexVxO3 (x = 0.1, 0.2) solid-solutions were synthesized via the facile sol-gel method. The raw chemicals are NH4VO3, Bi(NO3)3·5H2O and H2TeO4. Firstly, the stoichiometric amounts of each raw material weighted on the basis of formula Bi2−xTexVxO3 (x = 0.1, 0.2) were dissolved in dilute nitric acid. The acetic acid and methanol were added in the solutions. Secondly, the citric acids with the amount of double molar amount of the cation ions were added. pH = 7 was adjusted by adding some NH3·H2O (30%wt). Then, PVA solutions (polyvinyl alcohol) were added in order to adjust its viscoelasticity. The colloidal sols were prepared after the stirring, which were carefully coated on the glassy substrates. In this way, some precursor films were prepared, which contained Bi3+, Te4+, V5+ ions and some organic components. The precursor films were taken off when it became dried, which were sintered at 760 °C for 3 h to get the final nano-powders. XRD measurements were taken on a diffractometer (Rigaku D/Max) with an incident radiation from a Cu-Kα (30 mA, 40 kV). SEM and TEM were used to investigate the morphological characteristics and particle

size. The high-resolution transmission electron microscopy (HRTEM) was measured on JEOL JEM-2010F microscope. EDS was applied to measure the elements on the nanoparticle surfaces. The optical absorption spectra were taken on a UV–Vis spectrophotometer (Cary 5000). The photocatalysis effects were conducted by the degradation of RhB (10 mg L−1, 300 mL). The photodegradation experiment was processed in the reactors with volume of 500 ml. The light irradiations were generated from a xenon lamp (500 W) with a 420 nm cut-filter. A pump was applied to supply the air-flow in the reactor. To get the desorption/adsorption equilibrium before the photocatalysis experiment, the reactor was standing in a dark room for 30 min. 5 mL RhB solution was extracted from the reactor after a designed time-interval, which was conducted on the optical absorption measurements. Photocatalysis was calculated via [1 − (A / A0)] × 100%, where A0 and Ai are the intensities before and after irradiation. TOC (total organic carbon) analyses were carried out with a Rosemount Analytical Instrument, Dohrmann DC-190.

3. Results 3.1. The structural and chemical formations The experimental diffraction patterns of the samples have been conducted on Rietveld structural refinements via the GSAS (General Structure Analysis System) software. Fig. 1(a, b) shows the refined XRD patterns on the basis of the initial model of fluorite type cubic δ-Bi2O3 structure (space group Fm-3m) [31]. Two members were observed in the same pure cubic structure. No other diffraction signals were found. The XRD has some sharp peaks indicating good crystallinity of the samples. Fig. 1(c) displays the evolution of cation substitution-dependent unit cell volume of the samples compared with the reported value of by Harwig [31]. The cell size shows a continuous decrease of as increasing of the substitution concentration. This is due to the fact that the ionic radius of Te4+ (0.66 Å) and V5+ (0.355 Å) are smaller than that of Bi3+ (0.96 Å). It has been reported that the VO4 tetrahedron is smaller than [BiO4], thus the introduction of Te4+ and V5+ ions in the structure effectively decreases the interplanar spacing. Meanwhile, the linear relationship between the cell size and substitution level is in good agreement with the so-called Vegard's law, which shows that the unit cell size of a host has a linear relation with the continuous substitution in a solid solution system. Similar behavior was also observed for some other trivalent cations such as Ln3+-stabilized δ-Bi2O3-type phases. The refined crystallographic parameters are shown in Table 1. Table 2 lists the refined atomic coordinate parameters. The residual errors Rp (7.36%) and Rwp (8.54%) suggest the good refinement for the experimental patterns. Fig. 1(d) simply displays the schematic structure presenting the framework modeled on the atomic coordinates in the refinements (Table 2). XRD Rietveld refined results show that Bi, Nb and Te stand in (4a) positions, which O occupied the 8c and 32f sites. Each cation (Bi, Te, V) has six coordinations with O2− ions occupying corners of a cube cell in the framework; this result suggests the impurity ions V5+ and Te4+ were well doped in the lattices. Two O atoms at diagonally opposite corners of the cube are missing [21]. In addition, chemical compositions of the samples were checked by the EDS in face-scanning on the selected nanoparticles (Fig. 2(a, b)). All the spectra have the characteristic lines indicating the chemical elements of Bi, Te, V and O on the particle surfaces. The quantitative analyses well agree with the stoichiometric ratio in the formula under investigation. For example, the measured results of Bi/V and Bi/Te in Bi2−xTexVxO3 (x = 0.2) (Fig. 2(b)) are 8.1 and 8.3, which are consistent with the stoichiometric value Bi/Te [8] and Bi/V [8] in the formula. This indicates the experimental results are consistent with the designed formula of the cation substitution.

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Fig. 1. XRD Rietveld refinements (a, b), the dependence of lattice cell volume of Bi2−xTexVxO3 (x = 0.1, 0.2) (c) and the schematic cell structure presenting the framework modeled on the atomic coordinates in the refinement (d).

3.2. The surface properties The morphological characteristics of the nanoparticles were investigated via SEM measurements. Fig. 3 shows the typical SEMs with the different magnification scales of Bi2−xTexVxO3 (x = 0.2). The SEM results of Bi2−xTexVxO3 (x = 0.1) nanoparticles were shown in supplementary materials Fig. S1. The results indicated that the Te4+/V5+doping levels in the lattices had not obvious influences on the morphological profiles. It can be seen that the nanoparticles with ball-like shape closely packed together. The size of the particles is about 200–400 nm. Although there is an aggregation, the dispersion is facilitated under a weak stirring in water solutions.

Table 1 Refined crystallographic parameters of Bi2−xTexVxO3 (x = 0.2). Formula Radiation 2θ range (°) Symmetry Space group# a/Å α/° Z Rp Rwp X2 V/Å3

Cu Ka 10–80 cubic Pm-3m 90 2 0.0736 0.0854 3.7231

The typical microstructures of the nanoparticles were investigated via TEM and HRTEM micrographs as displayed in Fig. 4(a, b, c). TEM images of the sample further confirmed the packed ball-like nanoparticles with rough surfaces. These results well agree with the SEM measurements. The HRTEM micrographs were examined (Fig. 4(d)). The typical lattice spacing was detected to be about 0.32 nm. This value was ascribed to the (111) reflection. Inset in Fig. 4(c) shows the SAED measurement on a selected single particle. The measurements confirmed a cubic lattice and verified the single-crystallinity characteristic of the nanoparticles. The typical BET (Brunauer–Emmett–Teller) measurements of the nanoparticles are displayed in Fig. 5. The isotherm has a typical hysteresis loop, which agrees with the standard IV pattern in BET measurements. The adsorption region for P/P0 comes near 1.0. It could be suggested that the nanoparticles have both mesopores and macropores. On the calculations, the specific surface of the samples Bi2−xTexVxO3 were 65 m2 g−1 (x = 0.1) and 75.6 m2 g−1 (x = 0.2). Inset Fig. 5 shows the pore size, which have an average size about 45 nm. The pore volume of the particles is about 0.6 cm3/g, which was decided via the Barrett, Joyner and Halenda (BJH) methods.

Table 2 Refined atomic coordinate parameters of Bi2−xTexVxO3 (x = 0.2). Atom Bi1 Te V O1 O2

Wyck. 4a 4a 4a 8c 32f

x/a

y/b

z/c

Occ.

0.000 0.000 0.000 0.2500 0.3521

0.000 0.000 0.000 0.2500 0.3521

0.000 0.000 0.000 0.2500 0.3521

0.9000 0.0040 0.0040 0.9000 0.00720

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Fig. 2. The typical EDS measurements of Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles.

3.3. Optical absorption and bend energy The optical absorption properties of the nanoparticles were compared with α-Bi2O3 and commercial P25 (Fig. 6(a)). Typically, each sample has a cut-off absorption edge in the different wavelength. The results indicate that the cation substitution in the lattices could change the absorption abilities, i.e., the band gap energies could be modified by the Te4+/V5+-doping for δ-Bi2O3. Compared with α-Bi2O3 and commercial P25, Bi2−xTexVxO3 (x = 0.1, 0.2) presents the improved optical absorption in the visible wavelength region. Usually the band energy (Eg), i.e., the transition energy for an electron between a conduction band (CB) and a valance band (VB), could be calculated via Wood-Tauc theory and Eg is determined from the equation:
k αhυ∝ hυ−Eg

ð1Þ

where ν is the optical frequency incident on the sample, α is absorbance coefficient. h is the Planck constant. The absorption characteristics are quantified by constant k, which could be direct allowed, indirect

allowed, direct forbidden, and indirect forbidden nature when k can be best fitted to be 1/2, 2, 3/2 or 3, respectively. In this work, Eq. (1) could be well fitted in k = 2 for each sample (Fig. 6(b)). The transition of the series of semiconductors presents an indirect allowed type. The estimated values of Bi2−xTexVxO3 (x = 0.1, 0.2) were listed in Fig. 6 (b). The Egs of Bi2−xTexVxO3 were determined to be 2.53 eV (x = 0.1) and 2.408 eV (x = 0.2). Compared to α-Bi2O3 (2.83 eV) and commercial P25 (3.17 eV), the Eg value of Bi2−xTexVxO3 (x = 0.1, 0.2) was lower. This permits the sample to absorb light of wavelengths down to 520 nm in the visible region. Meanwhile, Bi2−xTexVxO3 (x = 0.1, 0.2) presents a smaller band energy than the reported Bi3+-semiconductors, for example, δ-Bi 2O 3 is 3.01 eV [32], α-Bi2 O 3 (2.80–3.4 eV) [33,34], monoclinic BiVO4 (2.4 eV) [34], tetragonal BiVO4 (2.90 eV) [34], Bi2 MoO 6 (2.44 eV) [35], Bi2 Mo3 O 12 (2.73 eV) [36], αBi 2Mo3 O12 (2.92–2.94 eV) [37]. The narrowed band gap of Bi 2 −xTexVxO3 (x = 0.1, 0.2) can be unambiguously ascribed to the doping ions of Te and V in the δ-Bi2O3 cubic lattices. On the present results, it is reasonable to suggest that cation substitution with Te/V codoping is a very effective method to enhance the optical absorption of bismuth oxides.

Fig. 3. Typical SEMs with the different magnification scales in Bi2−xTexVxO3 (x = 0.2).

G. Zhou et al. / Materials and Design 181 (2019) 108066

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Fig. 4. TEM images (a, b, c) and HRTEM image (d) of Bi2−xTexVxO3 (x = 0.2). Inset in (c) was the SAED pattern (selected area electron diffraction).

3.4. Band structure model Theoretically, the energy positions for CB and VB in a semiconductor could be calculated in the equations of ECB = X − Ee − 0.5 Eg and EVB = X − Ee + 0.5, respectively. Ee is energy for free electrons on the hydrogen scale (∼4.5 eV vs SHE) and X is the absolute electronegativity. On this calculations, CB bottom, VB top together with the band energy of Bi2−xTexVxO3 (x = 0.1, 0.2) were listed in Fig. 7. The results suggested that Te/V codoping for Bi3+ ions could cut down the CB bottom, and lift VB top in the semiconductors. The CB in Bi2−xTexVxO3 (x = 0.1, 0.2) could be wider and broader with the increase of substitution level (x). It is widely accepted that Bi2O3 compounds have the VB components from the hybridization between O-2p and Bi-6s, and CB from Bi6p electronic components [38]. The optical absorption of Bi2−xTexVxO3 (x = 0.1, 0.2) suggested that band energies could be modified by the Te/V doping for Bi3+ ions. This effect should have the complicated causes because there several optically active centers in the lattices. Te/V doping in cubic δ-Bi2O3 lattices could modify the electronic structure.

Firstly, the charge transfer transitions (CT) between O2− and V5+ from VO3− can give more and more contributions with increasing V5 4 + -substitution content in the lattices [39]. This can provide electronic components in the bottom of the conduction band. Secondly, it is reasonable that Te4+ electronic components could have contributions to the band structures in semiconductors. It has been confirmed by calculation and experiments that the presence of a heavy element Te4+ in a semiconductor could contribute its p-orbitals to the valence band [40]. Usually in a telluride, the electronic states near the Fermi level could be dominated by Te-5p orbitals, meanwhile, Te-5s and Te-5p electronic orbitals have a contribution to the CBs and VBs of a telluride semiconductor, respectively [40]. For example, a significant red-shift of the absorption edges (i.e., narrowed band) of WO3 [41] and ZnO [42] and have been reported with the increase of the Te4+ concentration in each lattices. Besides, the band-gap of BiF3 shows a great decrease with increasing Te4+ doping levels. Te-5p states can easily give a contribution to VB components in a semiconductor [43]. On the basis of the discussions above, the band structure models of Bi2−xTexVxO3 (x = 0.1, 0.2) were suggested in Fig. 7. It is reasonable to suggest that Te5p

Fig. 5. The isotherm curves of N2- desorption–adsorption in Bi2−xTexVxO3 (x = 0.1, 0.2). Inset is the corresponding pore-size distributions.

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Fig. 6. Optical absorption in UV–vis region (a) and band energy calculations (b) in Bi2−xTexVxO3 (x = 0.1, 0.2) together with α-Bi2O3 and commercial P25.

and Te5s components have a big contribution to the VB and CB of Bi2 −xTexVxO3, respectively. 3.5. Photocatalytic activities The photocatalysis of Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles was evaluated on RhB degradation. The detailed photo-degradation effects were compared with α-Bi2O3 and commercial P25 as shown in Fig. 8 (a). A blank experiment without the photocatalyst was conducted because RhB solutions have self-bleached effects under visible light excitation. The experiments suggested that the self-bleached effects could be ignored. Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles have much more efficient effects than that of α-Bi2O3 and commercial P25. The improved photocatalytic degradation was realized in the Te4+ and V5+-codoped samples. The best photocatalysis was observed in Bi2−xTexVxO3 (x = 0.2), which had a degradation rate 95% in 85 min. The component analysis of TOC removal was performed to detect the mineralization degree of the organic species. The typical TOC removal effects of Bi2−xTexVxO3 (x = 0.1) nanoparticles were shown in supplementary materials Fig. S2. The mineralization rate of RhB reached N85% after light irradiation. To quantitatively discuss the experimental data, the photocatalysis was evaluated following a first order kinetic equation: ln(Ct/C0) = kt, where C0 is the initial concentration, and Ct is the concentration at

time t. The rate constant (k) was decided through the slope of the fitted curves. It can be clearly seen that all the Eu3+-doped samples presented the deteriorated effects in a comparison with pure BiVO4. This could be reflected via the dye color photos with irradiation time 0 and 6 h for all the samples shown in Fig. 8(b). The dye solutions with the presence of Bi2−xTexVxO3 (x = 0.1, 0.2) showed a better photocatalytic effects. + ∙ − Usually, several possible reactive species such as •O− 2 , h , and OH have been reported to have dominant role for photocatalysis process [44]. In this work, the quenching effects on photocatalysis in presence of several quenchers were conducted. The tert-butanol, EDTA, and benzoquinone with a concentration of 1.0 mmol/L were added in the photocatalysis systems as scavengers for hydroxyl radicals (•OH−), hole (h+) and superoxide anions (∙O− 2 ), respectively. Fig. 9 is the photo-degradation effect by Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles with three quenchers. Compared with the degradation effects of EDTA and benzoquinone, tert-butanol shows the most delayed efficiency. The experiments suggested that •OH− and •O− 2 should be the dominant reactive species responsible for the degradation. The electrons and holes were firstly created in VB and CB, respectively, after the excitation (Eq. (2)). þ

Bi2−x Tex Vx O3 →h ðCBÞ þ e− ðVBÞþ

ð2Þ

The holes (h+) could react with OH− creating •OH (Eq. (3)). þ

h þ OH− →˙OH

ð3Þ

The electrons can react with the O2 resulting in O2•− (Eq. (4)). HO2• could be subsequently produced via the reaction between H+ and O2•− (Eq. (5)). e− þ O2 →O2 ˙−

ð4Þ

Hþ þ O2 ˙− →HO2 ˙

ð5Þ

HO− 2 could be formed if an electron reacts with HO2• (Eq. (6)), which could further react with H+ inducing a H2O2 (Eq. (7)). HO2 ˙ þ e− →HO2 −

ð6Þ

HO2 − þ Hþ →H2 O2

ð7Þ

•OH− could be created by reaction between H2O2 and e− (Eq. (8)), which could degrade dye molecules resulting in H2O and CO2 (Eq. (9)).

Fig. 7. The suggested CB, VB and possible local energy levels of Bi2−xTexVxO3 (x = 0.1, 0.2) compared with α-Bi2O3.

H2 O2 þ e− →OH− þ ˙OH−

ð8Þ

˙OH− þ RhB→CO2 þ H2 O

ð9Þ

G. Zhou et al. / Materials and Design 181 (2019) 108066

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Fig. 8. The RhB degradation (a) and the plots of ln(Ct/C0) on with time (b) by Bi2−xTexVxO3 (x = 0.1, 0.2) under irradiation wavelength N420 nm. The data was compared with α-Bi2O3 and commercial P25.

The experiments confirmed that Te4+/V5+ codoping in the lattices is a very effective method to enhance the photocatalytic activities in the bismuth oxides. The band gap energies could be modified by Te4+/V5 + codoping in the lattices directly enhancing the optical response in visible wavelength region. Meanwhile, Te4+/V5+ certainly induces strong structural distortion in the lattices. This can give dispersions in both CB and VB, which are greatly favoring mobility of photo-induced charges. 4. Discussions In a Bi-compound, the electrical investigations have confirmed that the Bi3+ ions could be partially substituted by Te4+ ions, which also have stereo-active lone electron pairs. This substitution could exert a great influence on the electrical properties such as great reduction of the impedance [28–30]. The differences of stereo-chemical activities between the lone electron pairs of Te-5s2 and Bi-6s2 enhance the repulsions between O2− and the lone pairs. Consequently, the polarization will increase, which is favorable for enhancement of the electrical conduction and photocatalysis for the RhB decomposition. To further elucidate the photocatalysis, the XPS measurements were conducted to investigate the microstructure and electronic dynamic properties. Fig. 10(a) is XPS total survey measurement for Bi2−xTexVxO3 (x = 0, 0.1, 0.2), which indicates the elements of Bi, and O in Bi2O3 and Bi, Te, V and O in Bi2−xTexVxO3 (x = 0.1, 0.2). Fig. 10(b) is the XPS measurements of Te-3d confirming the successful incorporation into Bi2

−xTexVxO3 (x = 0.1, 0.2) lattices. There are two dominant Te-3d XPS sig-

nals on the spectra. Fig. 10(c) shows the XPS for Bi4f with the typical spin-orbit split in Bi2−xTexVxO3 (x = 0, 0.1, 0.2). The binding energies at 159.6 eV and 165.2 eV were shown for Bi-4f7/2 and Bi-4f5/2, respectively, with a splitting energy about 5.6 eV. This is in good agreement with the reported spin-orbit splitting in bismuth oxides such as 5.3 eV [45]. There are no big differences for the XPS spectra of Bi4f and Te-3d. It is obvious that the Te/V-codoped samples have much more deteriorative asymmetry characteristics in the XPS results of O-1s and V-2p as shown in Fig. 10(d). The asymmetric shape of O-1s profile indicates more than two kinds of oxygen in the nearby region, which was given in three Gaussian decompositions. The dominated peak (I1) can be indexed to the lattices O. whereas the weaker shoulder peaks (I2 + I3) are widely assigned to the oxygen species from the defect oxygen and adsorbed oxygen surface hydroxyl corresponding to O\\H bonds [46]. The same manner was conducted for XPS of V-2p as shown in Fig. 10 (d), which could be divided into two sub-peaks at 516.9 eV and 515.1 eV assigned to V5+ [47] and V4+ [48], respectively. The same results of multivalent V species have been reported in stabilized δ-Bi2O3 [26]. It have been confirmed that the coexistence of V4+/V5+ is one of the dominant reasons to improve the photocatalysis of a vanadate [49]. On the observation, V4+/V5+-icodoped samples have the dominated contents of intrinsic O and V in compared with the parent compound δ-Bi2O3. This is important to improve the photodegradation in V4+/V5+ codoped samples. To verify the multivalent V ions in this Te/V-coded δ-Bi2O3, the bond valence method was applied to calculate the chemical valence of V-ion. The method is derived from the bond valence model, which is a development of Pauling's rules. It is popular method to estimate the oxidation state of an atom in coordination chemistry [50]. The bond valence sum Vi of an atom (i) coordinated with j oxygen ions can be calculated via the following empirical formula [51]: Vi ¼

X j

Fig. 9. The photocatalytic effects with three selected quenchers of the EDTA, benzoquinone and tert-butanol in Bi2−xTexVxO3 (x = 0.2).

Sij ¼

X j

  l0 −lij ; exp 0:37

ð10Þ

where Sij is the bond valence, l0 is a constant for the bond valence parameter, which is reported to be 1.803 for V5+ [52], lij is the interatomic distance. This average value of V\\O is 2.3132 Å obtained from the structure refinement in Te/V-codoped δ-Bi2O3. In this calculation, V ion has a bond valence sum of 4.38. Usually, this method could have an error of 10% because the variations of the inter-atomic distances and feature of the empirical formula [53]. Therefore, it is reasonable that the V ions in this host have a lower valence than 5. This agrees with the assignment in XPS measurement. The EIS (Electrochemical Impedance Spectroscopy) spectra of Bi2 −xTexVxO3 (x = 0.1, 0.2) photocatalysts were shown in Fig. 11. The

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Fig. 10. XPS survey result (a), Te-3d (b), Bi-4f (c), V-2p and O2p (d) of Bi2−xTexVxO3 (x = 0, 0.1, 0.2).

results clearly indicate that the Te and V codoping has an obvious influence on its impedance properties. The arc radius of the spectra shows a decrease with the codoping levels from 0.1 to 0.2. This indicates that the charge-separation efficiencies for the samples are improved. It is reasonable that the existence of oxide vacancies could capture the photocreated electrons and holes. This is certainly critical for the separation of induced charges (holes and electrons) prolonging its decay lifetimes.

Fig. 11. EIS spectra of Bi2−xTexVxO3 (x = 0.1, 0.2) photocatalysts at room temperature.

This insures the charges have more chances to realize the photocatalysis reactions. The luminescence spectra and decay curves were investigated to get a clear concept for the electronic transitions in Bi2−xTexVxO3 (x = 0.1, 0.2). The result usually can determine the separation efficiency between light-excited exciton, i.e., holes and electrons, in a semiconductor. In this work, all the samples were not detected any luminescence signals at room temperature. The emission of Bi2−xTexVxO3 could be only measured at a low temperature below 100 K and 150 K for samples with x = 0.1 and 0.2, respectively. Fig. 12(a) presents the luminescence of the samples at low temperatures. The emission spectra present a broad emission band. The publications have been widely reported that the emission in Bi2O3 has two possible emission centers, i.e., intraionic transitions (3P0,1 → 1S0) of Bi3+ and some complex vacancy defects [54]. Usually the intrinsic intraionic transitions were reported to have near UV or blue emission such as photoluminescence (PL) at 441 nm (2.81 eV) [55] and 397 nm (3.12 eV) [56], cathodoluminescence (CL) at 381 nm (3.25 eV) [57]. The different wavelength at 540 nm (2.3 eV) and 476 nm (2.6 eV) were reported in PL and CL emission [58]. Usually the longer wavelength emission was ascribed to oxygen defect complexes [54]. The emission in Bi2−xTexVxO3 (x = 0.1, 0.2) (Fig. 12(a)) could be originated from the oxygen vacancies in the lattices. The photocatalysis and luminescence in a semiconductor have a close relationship. Usually, a stronger luminescence suggests an active recombination between the light-created electrons and holes, accordingly, the photocatalysis should have a smaller opportunity. Meanwhile, a quick luminescence lifetime in a semiconductor can provide ample opportunities for photocatalysis [59]. Fig. 12(b) shows the

G. Zhou et al. / Materials and Design 181 (2019) 108066

9

Fig. 12. The emission spectra (a), the decay curves (b) in Bi2−xTexVxO3 (x = 0.1, 0.2). The samples were excited with 355 nm via a YAG:Nd pulsed laser.

luminescence decay curves of two samples by monitoring the maximum emission wavelength. The non-exponential curves were fitted in the following equation to calculate the average lifetime (τavg).

Data curation, Funding acquisition, Project administration, Supervision, Validation. Acknowledgements

R∞ τavg ¼ R0∞ 0

tI ðt Þdt Iðt Þdt

ð11Þ

Bi2−xTexVxO3 (x = 0.2) has a longer lifetime (1.32 μs) than that of Bi2 (x = 0.1) (0.71 μs). It is reasonable that the longer emission lifetime in Bi2−xTexVxO3 (x = 0.2) can provide more chances for the photo-induced charges to take part in photocatalysis [60]. From the discussions above, the improved photocatalysis in Bi2−xTexVxO3 (x = 0.2) could be related to some properties, such as the improved optical absorption in visible light region, easy creation of •OH− radicals, the longer luminescence lifetime.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03029432). Appendix A. Supplementary data

−xTexVxO3

5. Conclusions Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles (200–400 nm) were prepared by the sol-gel synthesis. As prepared samples have typical δBi2O3 formation with space group of Fm-3m. This concluded that cubic structure could be stabilized viaTe4+ and V5+-codoping in Bi2O3 lattices. The cation substitutions could greatly modify band energies of Bi2O3, which shows a decrease with the codoping. Bi2−xTexVxO3 (x = 0.1, 0.2) nanoparticles present an indirect allowed electronic transition with the band energies of 2.53 eV and 2.408 eV for x = 0.1 and x = 0.2, respectively. The valence and conduction band levels have a dispersion via mixing (V3d + Te5s) and Te-5d in conduction band and valence band, respectively. Compared with α-Bi2O3 and commercial P25 Bi2 −xTexVxO3 (x = 0.1, 0.2) nanoparticles present the improved degradation on the RhB dye solutions. The improved photocatalysis has a close relation with the multivalent V5+/4+ ions, the delayed lifetimes of the changes, and the reduced impedance. This work provides a new method to stabilize δ-Bi2O3 at room temperature via Te4+/V5+-codoping. Cation substitution with Te4+/V5+ codoping is a very effective method to enhance the optical absorption of bismuth oxides. This is a valuable reference in designs bismuth compounds with the improved photochemical abilities. CRediT authorship contribution statement Guitao Zhou: Investigation, Formal analysis, Methodology, Writing original draft. Yanlin Huang: Conceptualization, Data curation, Investigation, Validation, Writing - original draft. Donglei Wei: Investigation, Software, Writing - review & editing, Resources. Shala Bi: Investigation, Writing - review & editing, Resources. Hyo Jin Seo: Conceptualization,

Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2019.108066. References [1] Mitra Mousavi, Aziz Habibi-Yangjeh, Shima Rahim Pouran, Review on magnetically separable graphitic carbon nitride-based nanocomposites as promising visible-lightdriven photocatalysts, J. Mater. Sci. Mater. Electron. 29 (2018) 1719–1747. [2] Bhabhina Ninnora Meethal, Abdul Faisal Panichikkal, Jabeen Fatima Manamkeri Jafferali, Dharsana M. Vidyadharan, Sindhu Swaminathan, Shining black nanoscopic ternary zincospiroffite: a panchromatic light harvester for depollution, Mater. Des. 165 (2019), 107600. https://doi.org/10.1016/j.matdes.2019.107600. [3] Ying Ye, Zhigang Zang, Tingwei Zhou, Fan Dong, Yubo Zhang, Theoretical and experimental investigation of highly photocatalytic performance of CuInZnS nanoporous structure for removing the NO gas, J. Catal. 357 (2018) 100–107. [4] Puhong Wen, Fangyi Yao, Dengwei Hu, Jingjing Guo, Yuzhu Lan, Chuanchuan Wang, Xingang Kong, Qi Feng, Changes in cell parameters and improvement in photocatalytic activity of KNbO3 and NaNbO3 crystals via polarization, Mater. Des. 158 (2018) 5–18. [5] Wentao Zhang, Shuo Shi, Xinnan Liu, Xinyi Ren, Min Li, Tianli Yue, Shaohui Ouyang, Jianlong Wang, Tetrathiomolybdate@ZIFs nanocrystal clusters: a novel modular and controllable catalyst for photocatalytic application, Mater. Des., In press, accepted manuscript, Available online 16 July 2019, Article 108042. doi:https://doi.org/10. 1016/j.matdes.2019.108042. [6] Maryam Shekofteh-Gohari, Aziz Habibi-Yangjeh, Masoud Abitorabi, Afsar Rouhi, Magnetically separable nanocomposites based on ZnO and their applications in photocatalytic processes: a review, Crit. Rev. Environ. Sci. Technol. 48 (2018) 806–857. [7] Sonal Singh, Rishabh Sharma, Bi2O3/Ni-Bi2O3 system obtained via Ni-doping for enhanced PEC and photocatalytic activity supported by DFT and experimental study, Sol. Energy Mater. Sol. Cells 186 (2018) 208–216. [8] A. Sivakumar, B. Murugesan, A. Loganathan, P. Sivakumar, A review on decolourisation of dyes by photodegradation using various bismuth catalysts, J. Taiwan Inst. Chem. Eng. 45 (2014) 2300–2306. [9] J. Dai, J.B. Xu, F. Wang, Y. Tai, Y. Shen, R.Q. Shen, Facile formation of nitrocellulosecoated Al/Bi2O3 nanothermites with excellent energy output and improved electrostatic discharge safety, Mater. Des. 143 (2018) 93–103. [10] G.L. Zhu, W. Yang, W.Q. Lv, J.R. He, K.C. Wen, W.R. Huo, Facile electrophoretic deposition of functionalized Bi2O3 nanoparticles, Mater. Des. 116 (2017) 359–364. [11] Y. Bao, T.T. Lim, Z. Zhong, R. Wang, X. Hu, Acetic acid-assisted fabrication of hierarchical flower-like Bi2O3 for photocatalytic degradation of sulfamethoxazole and rhodamine B under solar irradiation, J. Colloid Interface Sci. 505 (2017) 489–499. [12] F. Wang, K. Cao, Q. Zhang, X. Gong, Y. Zhou, A computational study on the photoelectric properties of various Bi2O3 polymorphs as visible-light driven photocatalysts, J. Mol. Model. 20 (2014) 2506. [13] O. Madelung, U. Rossler, M. Schulz, Non-tetrahedrally Bonded Elements and Binary Compounds, Springer, Berlin, 1998 41.

10

G. Zhou et al. / Materials and Design 181 (2019) 108066

[14] M. Mehring, From molecules to bismuth oxide-based materials: potential homo-and heterometallic precursors and model compounds, Coord. Chem. Rev. 251 (2007) 974–1006. [15] S.J. Skinner, J.A. Kilner, Oxygen ion conductors, Mater. Today 6 (2003) 30–37. [16] C.D. Ling, R.L. Withers, S. Schmid, J.G. Thompson, A review of bismuth-rich binary oxides in the systems Bi2O3–Nb2O5, Bi2O3–Ta2O5, Bi2O3–MoO3, and Bi2O3–WO3, J. Solid State Chem. 137 (1998) 42–61. [17] L. Malavasi, C.A.J. Fisher, M.S. Islam, Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features, Chem. Soc. Rev. 39 (2010) 4370–4387. [18] A. Dapčević, D. Poleti, J. Rogan, A. Radojković, G. Branković, A new electrolyte based on Tm3+-doped δ-Bi2O3-type phase with enhanced conductivity, Solid State Ionics 280 (2015) 18–23. [19] T.N. Soitah, C.H. Yang, Effect of Fe3+ doping on structural, optical and electrical properties δ-Bi2O3 thin films, Curr. Appl. Phys. 10 (2010) 724–728. [20] X.Z. Wang, R. Jayathilake, D.D. Taylor, E.E. Rodriguez, M.R. Zachariah, Study of C/ doped δ-Bi2O3 redox reactions by in operando synchrotron X-ray diffraction: bond energy/oxygen vacancy and reaction kinetics relationships, J. Phys. Chem. C 122 (2018) 8796–8803. [21] H. Sudrajat, S. Hartuti, Boosting electron population in δ-Bi2O3 through iron doping for improved photocatalytic activity, Adv. Powder Technol. 30 (2019) 983–991. [22] X.M. Gao, Y.Y. Shang, L.B. Liu, F. Fu, Multilayer ultrathin Ag-δ-Bi2O3 with ultrafast charge transformation for enhanced photocatalytic nitrogen fixation, J. Colloid Interface Sci. 533 (2019) 649–657. [23] X.M. Gao, Y.Y. Shang, L.B. Liu, F. Fu, Chemisorption-enhanced photocatalytic nitrogen fixation via 2D ultrathin p–n heterojunction AgCl/δ-Bi2O3 nanosheets, J. Catal. 371 (2019) 71–80. [24] S. Bandyopadhyay, A. Dutta, Thermal, optical and dielectric properties of phase stabilized δ–Dy-Bi2O3 ionic conductors, J. Phys. Chem. Solids 102 (2017) 12–20. [25] J. Zhang, Q.F. Han, X. Wang, J.W. Zhu, G.R. Duan, Synthesis of δ-Bi2O3 microflowers and nanosheets using CH3COO(BiO) self-sacrifice precursor, Mater. Lett. 162 (2016) 218–221. [26] H. Sudrajat, P. Sujaridworakun, Low-temperature synthesis of δ-Bi2O3 hierarchical nanostructures composed of ultrathin nanosheets for efficient photocatalysis, Mater. Des. 130 (2017) 501–511. [27] J.C. Medina, M. Bizarro, P. Silva-Bermudez, M. Giorcelli, S.E. Rodil, Photocatalytic discoloration of methyl orange dye by δ-Bi2O3 thin films, Thin Solid Films 612 (2016) 72–81. [28] A. Castro, R. Enjalbert, P. Baules, J. Galy, Synthesis and structural evolution of the solid solution bi(Bi12−xTexO14)Mo4−xV1+xO20(0≤xb2.5), J. Solid State Chem. 139 (1998) 185–193. [29] R. Enjalbert, G. Hassselmann, J. Galy, [Bi12O14E12] n columns and lone pairs E in Bi13Mo4VO34E13: synthesis, crystal structure, and chemistry of the Bi2O3-MoO3V2O5 system, J. Solid State Chem. 131 (1997) 236–245. [30] A. Castro, P. Millan, R. Enjalbert, E. Snoeck, J. Galy, An original oxide of antimony and tungsten related to aurivillius phases, Mater. Res. Bull. 29 (1994) 871–879. [31] H.A. Harwig, On the structure of Bismuthsesquioxide: the α, β, γ, and δ-phase, Z. Anorg. Allg. Chem. 444 (1978) 151–166. [32] H.F. Cheng, B.B. Huang, J.B. Lu, Z.Y. Wang, B. Xu, X.Y. Qin, Synergistic effect of crystal and electronic structures on the visible-light-driven photocatalytic performances of Bi2O3 polymorphs, Phys. Chem. Chem. Phys. 12 (2010) 15468–15475. [33] J.L. Hu, H.M. Li, C.J. Huang, M. Liu, X.Q. Qiu, Enhanced photocatalytic activity of Bi2O3 under visible light irradiation by Cu (II) clusters modification, Appl. Catal. B Environ. 142 (2013) 598–603. [34] K. Brezesinski, R. Ostermann, P. Hartmann, J. Perlich, T. Brezesinski, Exceptional photocatalytic activity of ordered mesoporous β-Bi2O3 thin films and electrospun nanofiber mats, Chem. Mater. 22 (2010) 3079–3085. [35] M. Malys, G. Fafilek, C. Pirovano, R.N. Vannier, Redox stability of phases based on bismuth oxide studied by voltammetry on microsamples, Solid State Ionics 176 (2005) 1769–1773. [36] E. Luévano-Hipólito, A.M.L. Cruz, Q.L. Yu, H.J.H. Brouwers, Photocatalytic removal of nitric oxide by Bi2Mo3O12 prepared by co-precipitation method, Appl. Catal. A 468 (2013) 22–326.

[37] H.H. Li, K.W. Li, H. Wang, Hydrothermal synthesis and photocatalytic properties of bismuth molybdate materials, Mater. Chem. Phys. 116 (2009) 134–142. [38] P. Malathy, K. Vignesh, M. Rajarajan, A. Suganthi, Enhanced photocatalytic performance of transition metal doped Bi2O3 nanoparticles under visible light irradiation, Ceram. Int. 40 (2014) 101–107. [39] T. Nakajima, M. Isobe, T. Tsuchiya, Y. Ueda, T. Kumagai, A revisit of photoluminescence property for vanadate oxides AVO3 (A:K, Rb and Cs) and M3V2O8 (M:Mg and Zn), J. Lumin. 129 (2009) 1598–1601. [40] S. Sabah Abed Al-Abbas, M. Kadhim Muhsin, H. Rahman Jappor, Tunable optical and electronic properties of gallium telluride monolayer for photovoltaic absorbers and ultraviolet detectors, Chem. Phys. Lett. 713 (2018) 46–51. [41] A.H.Y. Hendi, M.F. Al-Kuhaili, S.M.A. Durrani, M.M. Faiz, A. Ul-Hamid, A. Qurashi, Modulation of the band gap of tungsten oxide thin films through mixing with cadmium telluride towards photovoltaic applications, Mater. Res. Bull. 87 (2016) 148–154. [42] F. JamaliSheini, R. Yousefi, M.R. Mahmoudian, N.A. Bakr, A. Saaedi, N.M. Huang, Facile synthesis of different morphologies of Te-doped ZnO nanostructures, Ceram. Int. 40 (2014) 7737–7743. [43] Z. Yang, X. Wang, Y. Pei, L. Liu, X. Su, First principles study on the structural, magnetic and electronic properties of Te-doped BiF3, Comput. Theor. Polym. Sci. 60 (2012) 212–216. [44] S.J. Liang, S.Y. Zhu, Y. Chen, W.M. Wu, X.C. Wang, L. Wu, Rapid template-free synthesis and photocatalytic performance of visible light activated SnNb2O6 nanosheets, J. Mater. Chem. 22 (2012) 2670–2678. [45] W.E. Morgan, W.J. Stec, J.R.V. Wazer, Inner-orbital binding-energy shifts of antimony and bismuth compounds, Inorg. Chem. 12 (1973) 953–955. [46] W. Zhu, X. Feng, Z. Wu, Z. Man, On the annealing mechanism in PbWO4 crystals, Physica B 324 (2002) 53–58. [47] J. Mendialdua, R. Casanova, Y. Barbaux, XPS studies of V2O5, V6O13, VO2 and V2O3, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 249–261. [48] W.T. Draai, G. Blasse, Energy transfer from vanadate to rare-earth ions in calcium sulphate, Phys. Status Solidi A 21 (1974) 569–579. [49] Z.Y. Zhang, C.L. Shao, L.N. Zhang, X.H. Li, Y.C. Liu, Electrospun nanofibers of V-doped TiO2 with high photocatalytic activity, J. Colloid Interface Sci. 351 (2010) 57–62. [50] I.D. Brown, VALENCE: a program for calculating bond valences, J. Appl. Crystallogr. 29 (1996) 479–480. [51] I.D. Brown, D. Altermatt, Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database, Acta Crystallogr. 41 (1985) 244–247. [52] N.E. Brese, M. O'Keeffe, Bond-valence parameters for solids, Acta Crystallogr. B 47 (1991) 192–197. [53] J. Awaka, N. Kijima, J. Akimoto, S. Nagata, Synthesis and crystallographic studies of garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12, J. Phys. Chem. Solids 69 (2008) 1740–1746. [54] A.M. Srivastava, S.J. Camardello, H.A. Comanzo, Explanation for the variance of the Ce3+ emission energy in LnI3 [Ln=Lu3+, Y3+, Gd3+], Opt. Mater. 32 (2010) 936–940. [55] Y. Xiong, M. Wu, J. Ye, Q. Chen, Synthesis and luminescence properties of hand-like α-Bi2O3 microcrystals, Mater. Lett. 62 (2008) 1165–1168. [56] W. Dong, C. Zhu, Optical properties of surface-modified Bi2O3 nanoparticles, J. Phys. Chem. Solids 64 (2003) 265–271. [57] M. Vila, C. Díaz-Guerra, J. Piqueras, Luminescence and Raman study of α-Bi2O3 ceramics, Mater. Chem. Phys. 133 (2012) 559–564. [58] M. Vila, C. Díaz-Guerra, J. Piqueras, α-Bi2O3 microcrystals and microrods: thermal synthesis, structural and luminescence properties, J. Alloys Compd. 548 (2013) 188–193. [59] L. Ge, C.C. Han, J. Liu, Novel visible light-induced g-C3N4/Bi2WO6 composite photocatalysts for efficient degradation of methyl orange, Appl. Catal. B Environ. 108 (2011) 100–107. [60] D.C. Lee, I. Robel, J.M. Pietryga, V.I. Klimov, Infrared-active heterostructured nanocrystals with ultralong carrier lifetimes, J. Am. Chem. Soc. 132 (2010) 9960–9962.