Introduction

Research on the binary Bi2O3-V2O5 allowed us to highlight, at the end of the 1980s, the compound Bi4V2O11 [1]. This oxide has a remarkably high O2− ion conductivity and can be used as the electrolyte in oxygen sensors, oxygen separation systems, and solid oxide fuel cells [2,3,4,5].

Bi4V2O11 is a member of the Aurivillius 2(Bi2O2)2+(An-1BnO3n+1)2− family compounds, whose structure consists of alternating fluorite-like (Bi2O2)2+ layers and perovskite-like (An-1BnO3n+1)2− blocks. In the case of Bi4V2O11 (n = 1), the perovskite-like layers are reduced to a layer of oxygen-deficient octahedra (VO3.5□0.5)2− [6].

The presence of oxygen vacancies and the non-bonding electronic pair of bismuth (III) allowed Bi4V2O11 to acquire a high electrical conductivity explained by the high mobility of O2− ions. Furthermore, the compound Bi4V2O11 exhibits, as a function of temperature, three crystal phases α-monoclinic, β-orthorhombic, and γ-tetragonal. The description of the crystal lattices of these polymorphs can be performed by means of an orthorhombic mean lattice of parameters: am ≈ 5.53, bm ≈ 5.61, cm ≈ 15.28 Å [7], such that aα = 3am, bα = bm, cα = cm; aβ = 2 am, bβ = bm, cβ = cm, and aγ = bγ ≈ am/√2, cγ ≈ cm. The α to β transition occurs around 430 °C upon heating. The transition β → γ occurs by the disappearance of the anionic vacancy order around 550 °C. Thus, the modulation lines related to the β-orthorhombic form disappear reversibly, and the cell becomes of tetragonal symmetry, with the space group I4/mmm. The γ phase, obtained at high temperature, is the most disordered and therefore, the most conductive [8, 9]. The tetragonal phase is characterized by a total disorder of the oxygen vacancies, which corresponds to the statistical distribution of the vacancies around cations in the perovskite layers. This facilitates the migration of O2− ions in these layers. However, the order of the anion vacancies leads to a superstructure with a doubling of the a parameter (β-phase) or a tripling of the a parameter (α-phase). This order affects the mobility of O2− ions in these phases. The partial substitution of vanadium (V) by a cation (ME) allows the stabilization of the latter phase at low temperature. Thus, the obtained Bi4V2-xMexO11±δ solid solutions present, around 600 °C, electrical conductivities around 10−1–10−2 S.cm−1 [3,4,5,6, 10]. These new materials are known by the acronym BIMEVOX; ME is the substituent cation. Depending on the degree of substitution of the vanadium, the nature of the doping element ME (ionic radii and covalence of the bond ME-O), and the rate of oxygen vacancies, the resulting phases can take the form of one of the three polymorphs α, β, or γ. BIMEVOX materials are tested as solid electrolytes [11], oxygen separation membranes [2], and potentiometric and amperometric gas sensors [12], and they are used as base materials for SOFC fuel cells (as electrolytes or anodes).

Moreover, as we know, the band structure of semiconductors is a fundamental element to develop the applications of photo-electrochemistry. The compound Bi4V2O11 is a semiconductor that reacts to visible light. This oxide can absorb nearly 11% of the sunlight due to its band gap higher than 2 eV. The band gap energy varies according to the authors: 2.86 eV [13], 2.08 eV [14], and 2.19 eV [15, 16]. Thus, doping this compound would produce intermediate energy levels in the band gap and improve the optical performance of this semiconductor. Bi4V2O11 and some BIMEVOX materials were used as photoanodes for the first time by K. Trzciński et al. [17], and it has been shown that Bi4V2O11 compound is a semiconductor with relatively narrow energy band gap Eg and that the substitution of vanadium by Cu, Zn, and Mn causes a decrease in the Eg values. This property makes BIMEVOXes potentially attractive for application in photoelectrocatalysis.

We have shown in recent studies [10, 18, 19] that the electrical conductivity and the activation energy depend on the nature of the doping ion. We also found that the double substitution of vanadium in Bi4V2O11 improves these properties in most of the studied phases. The doping ions were chosen from the d and p blocks of the periodic table. Thus, it seemed interesting to us to explore the effect, on energy band gap, of these double substitutions with elements of a different nature, which, allowed the stabilization of the different polymorphs of Bi4V2O11 α, β, and γ. Our choice fell on doubly substituted phases by silicon and another element. The latter is chosen from the d (Cu and Co) and p (Si and P) blocks.

The present work is devoted to the structural analysis of the compound Bi4V1.7Si0.3O10.85 (the chemical composition of this compound Bi4V1.7Si0.3O10.85 has been calculated from the general formula Bi4V2-xSixO11-x/2 and is a theoretical composition) and the doped series Bi4V1.7Si0.15Me0.15O11-δ (Me = Si, P, Cu, and Co) obtained by substituting 15% vanadium (denoted BISIMEVOX.15 or Bi4V1.7(Si.Me)0.3O11-δ). The main objective is to study the effect of the nature of the dopant on the crystalline structure and on the optical properties.

Experimental

Polycrystalline samples of the formula Bi4V1.7(Si.Me)0.3O11-δ (Me = Si, P, Cu, and Co) were prepared by conventional solid-state reaction. Stoichiometric amounts of Bi2O3 (99.5%), V2O5 (99%), SiO2 (99.9%), (NH4)2HPO4 (99%), CuO (99%), and CoO (99%) were ground for 20 min and heated in air at 800 °C for 24 h, then cooled to room temperature (two anneals were necessary to obtain pure products, and 3 g of powder was prepared). For the phosphorus-based compound, a pretreatment allowing the release of NH3 and H2O was performed at 400 °C for 12 h. X-ray diffraction (XRD) analysis was conducted on the powder samples using a Rigaku SmartLab SE instrument, utilizing CuKα radiation with a wavelength of 1.54059 Å. The measurements spanned a 2θ range from 5 to 80, with a scanning rate of 2/min and a step width of 0.02. The unit cell parameters were determined using the Rietveld refinement method. The parameter values obtained with the DICVOL06 software are used as a starting model in the Full Prof software. The average crystallite size of the prepared powders was estimated using the Scherrer formula [20]:

$$D={~}^{0.9 \lambda }\!\left/ \!{~}_{\beta \mathrm{cos}\theta }\right.$$
(1)

where D is the average crystallite size in nm, β is the full width at half maximum (FWHM) of the X-ray diffraction peaks, θ is the Bragg angle, and λ is the wavelength of Cu-Kα radiation.

Raman scattering spectra were recorded for samples in the form of powders, using a Confotec MR520 spectrometer, with a laser emitting in the green at wavelength λ = 532 nm.

The IR spectra were recorded using the KBr pellet technique, in the 4000 to 400 cm−1 wavenumber region at room temperature, using a Bruker VERTEX 70 FTIR spectrometer equipped with a DTGS detector and OPUS 6.5 software; 128 scans with 2 cm−1 resolution, in transmittance mode.

Analysis of phase transitions with temperature for all samples was performed by differential scanning calorimetry (DSC) measurements in the air using a DSC 131 EVO at a heating rate of 10 °C min−1.

The surface morphology of the sintered ceramics and powders was studied using a scanning electron microscope (VEGA3), equipped with energy-dispersive X-ray spectroscopy (EDS). The measurements were carried out on pellets that had been sintered at a temperature of 830 °C for a duration of 5 h.

The optical properties of the powdered samples were studied by diffuse reflectance spectroscopy (DRS) at room temperature in the UV–vis regions in the wavelength range of 200–800 nm using a Shimadzu spectrophotometer (UV-3101).

Results and discussion

Structural characterization

Figure 1 shows the room temperature X-ray diffraction patterns of the prepared samples Bi4Si0.15Me0.15V1.7O11-δ (Me = Si, P, Cu, and Co) in the 2θ range 10–65. Depending on the nature of the substituent ion, the obtained phases are isotypical to either the monoclinic, orthorhombic, or tetragonal polymorph of Bi4V2O11.

Fig. 1
figure 1

X-ray diffraction patterns of Bi4(Si.Me)0.3V1.7O11-δ (Me = Si, P, Cu, and Co)

Full size image

For the single substitution by silicon, the XRD analysis shows that Bi4Si0.3V1.7O11-δ is a β-orthorhombic phase with the Amam space group. In the case of the double substitution (Si, P), the diffractogram of the obtained compound Bi4Si0.15P0.15V1.70O11-δ, reveals the presence of doublets at 2θ = 32, 34, 40, 48, and 55, as well as a superstructure peak at 2θ ≈ 24.2°, which are characteristic of the α-monoclinic polymorph with the C2/m space group. On the other hand, in the case of double substitution by (Si, Cu) and (Si, Co), the doublet diffraction peaks at 2θ ≈ 32, 34, and 55 become singlets which reveal the formation of a γ-tetragonal phase with the space group I4/mmm for the compounds Bi4Si0.15Cu0.15V1.70O11-δ and Bi4Si0.15Co0.15V1.70O11-δ. For all the compounds studied, no diffraction line indicating the presence of a secondary phase was detected.

It should be noted that in the case of the BISIVOX (Bi4V2-xSixO11-δ) solid solution (0.0 ≤ x ≤ 0.4), which we have studied in previous work [21], only the composition x = 0.35 allowed the γ-Bi4V2O11 form to be obtained. However, in the case of the BISIPVOX (Bi4V2-xSix/2Px/2O11-δ) solid solution (0.0 ≤ x ≤ 0.5), we did not obtain any γ-Bi4V2O11 type phase (only the α and β polymorphs were obtained) [18]. As it has been shown for the compounds BIPVOX (Bi4V2-xPxO11, 0.0 ≤ x ≤ 0.2), the strongly covalent character of the bonds is not favorable to the disorder which characterizes the γ phase [22]. The two compounds Bi4V1.7Si0.15Cu0.15O11-δ and Bi4V1.7Si0.15Co0.15O11-δ compounds crystallize in the γ-form as was the case with Bi4V1.7(Al.Cu)0.3O11-δ synthesized by Essalim et al. [10].

In summary, it can be seen that phosphorus doping of the compound with theoretical formula Bi4Si0.3V1.7O10.85, accompanied by a decrease in the rate of oxygen vacancies on the one hand and an increase in the covalence of the M–O bond on the other hand, leads to the transition to a less symmetrical structure (from β-Bi4Si0.3V1.7O10.85□1.15 to α-Bi4Si0.15P0.15V1.70O10,925□1.075) (decreased disorder). It can be noted that the double substitution by P, Si nevertheless leads to obtaining a solid solution existing over a wider composition range with x > xmax of BIPVOX solid solution (xmax = 0.2). However, Cu and Co, being weakly charged and of medium sizes, the M–O bonds are not very covalent; we also see that the doping of Bi4Si0.3V1.7O10.85 by Cu or by Co leads to an increase in the rate of oxygen vacancies (vacant oxygen quantities are theoretical). These two parameters are favorable to the creation of disorder in the network. Thus, in both cases, we observe the transition to a more symmetrical structure (from β-Bi4Si0.3V1.7O10.85□1.15 to γ-Bi4Si0.15Cu0.15V1.70 O10,70□1.30 and γ-Bi4Si0.15Co0.15V1.70 O10,70□1.30).

The variation of the lattice parameters of BISIMEVOX.15 (Me = Si, P, Cu, and Co) is presented in Table 1. To allow comparison, all lattice parameters were related to the dimensions of the average cell. Thus, without taking into account the nature of the polymorph, we compare the values of the cell parameters of the four compounds. We find that the a parameter varies weakly (Δa/a ≈ 0.3%) compared to the c parameter (Δc/c ≈ 1.3%), showing that the crystal structure has a pronounced two-dimensional character. Furthermore, we found, when studying several BIMEVOX [18, 19, 23, 24] that the parameter a depends little on the size of the dopant ion and is rather sensitive to the covalent character of the Me–O bond and the rate of anionic vacancies in the perovskite layers (the vacancies are preferentially located in equatorial positions). Considering the difference in the ionic radii and the charge of the cations, we can make a qualitative comparison of their polarizing powers and therefore we can qualitatively compare the covalence of the bonds. Thus, the compound BISIPVOX presents the lowest value of the parameter a because of the strong covalent character of the ME–O bond. While the other three BISIMEVOX.15 compounds, Bi4V1.7Si0.3O10.85, Bi4V1.7(Si.Cu)0.3O10.7 and Bi4V1.7(Si.Co)0.3O10.7), which have almost the same ratio of anionic vacancies, have almost the same value of parameter a despite the difference in the size of the doping element ME. However, the c parameter varies in the same direction as the ionic radius of ME (ionic radii: ri(P5+) = 0.38 Å ri(Si4+) = 0.40 Å, ri(Cu2+) = 0.73 Å, and ri(Co2+) = 0.74 Å [25]).

Table 1 Unit cell parameters of BISIMEVOX.15 (ME = Si, P, Cu, and Co) and the average crystallite size (D)
Full size table

The values of the average crystallite size (D) were calculated from the Scherrer formula and are given in Table 1. The average size increases from the monoclinic α-phase to the orthorhombic β-phase and then to the tetragonal γ-phase, as the number of vacancies increases which facilitates the diffusion of ions in the lattice and promotes crystal growth [26].

FTIR and Raman analysis

Figure 2 shows the FT-IR spectra of the BISIMEVOX.15 compounds. The band around 510–525 cm−1 is mainly attributed to the stretching vibration of the Bi–O bond in the octahedral units of BiO6 [10, 27]. The absorption bands in the 710–730 and 800–890 cm−1 regions are attributed to the symmetric νs(V–O) and asymmetric νas(V–O) stretching vibration modes in the vanadate layers, respectively. It should be noted that the position of the latter band varies according to the nature of the dopant element ME, reflecting the variation of the V–O bond length under the effect of doping. Indeed, P5+ doping seems to create deformations leading to a decrease in the length of the V–O bond, which results in the displacement of the band from 815 cm−1 for BISIVOX.15 (Bi4V1.7Si0.3O10.85) to 824 cm−1 for BISIPVOX.15 (Bi4V1.7(Si.P)0.3O10.7). Furthermore, we note the band located at 620 cm−1 for the α-monoclinic phase BISIPVOX.15 disappears completely from the IR spectrum of the β-orthorhombic phase BISIVOX and the γ-tetragonal phases BISICUVOX.15 (Bi4V1.7(Si.Cu)0.3O10.7) and BISICOVOX.15 (Bi4V1.7(Si.Co)0.3O10.7); this seems to indicate the banning of some vibrational modes following a change in local symmetry. For the compounds BISICUVOX.15 and BISICOVOX.15, we note the appearance of two bands around 452 and 424 cm−1 which can be attributed to the metal dopant-oxygen bond (Cu–O) and (Co–O) [16, 27]. Thus, in agreement with the results of the XRD analysis, the FT-IR spectroscopy study highlights modifications of the local symmetry and bond strength according to the nature of the doping ion.

Fig. 2
figure 2

FT-IR spectra of the compound Bi4V1.7(Si.Me)0.3O11-δ (Me = Si, P, Cu, and Co)

Full size image

Room temperature Raman spectra for the compounds BISICUVOX.15, BISIVOX.15, and BISIPVOX are presented in Fig. 3.

Fig. 3
figure 3

Raman spectra at room temperature for BISIMEVOX.15 compounds

Full size image

Similar to our previous studies on BIMEVOX [18, 19], there are two strong signal regions in the high frequency (600–940 cm−1) and low frequency (130–450 cm−1) regions. The high-frequency region is mainly associated with the stretching vibrations of the V–O bond in vanadium polyhedra. The signal at 930 and 918 cm−1, as observed in BISIPVOX.15 and BISIVOX.15 spectra, respectively, gradually shifts to lower frequencies and merges with the main peak at the α → β transition. This peak is attributed to the stretching of the V–O bond in a V–O–V chain structure. The existence of such chains was observed in the α and β phases during a structural study as a function of the temperature of Bi4V2O11 carried out by Patwe et al. [28]. These chains disappear during the transition to the γ form. The low-frequency signals (130–260 cm−1) are mainly associated with external modes corresponding to chain bending. The remaining signals between 360–380 cm−1 are associated with V–O bending vibration modes. The band located at about 530 cm−1 corresponds to the symmetric stretching mode of the V–O bond, while the signals located at 600–860 cm−1 are assigned to the asymmetric stretching mode of the V–O bond [29, 30]. The strong mode observed in the region of 845–860 cm−1 is due to the symmetric stretching modes of the V–O band of VO4 groups [31]. Note that the band corresponding to this mode, located for BISIVOX.15 at 847 cm−1, gradually shifted to 855 cm−1 and 852 cm−1 for the compounds BISIPVOX.15 and BISICUVOX.15, respectively. From the positions of these bands, we calculated the corresponding V–O bond length using the empirical formula proposed by Hardcastle et al. [32] (Eq. 2):

$$\nu = 21349 \times exp (-1.9176 \times R)$$
(2)

(ν is the vibration band frequency and R is the length of the V–O bond). This equation was established with the diatomic approximation where each vanadium–oxygen bond vibrates independently of the crystal lattice. The values obtained are 1.6828 Å, 1.6797 Å, and 1.6779 Å for BISIVOX.15, BISICUVOX.15, and BISIPVOX.15, respectively. These values show that changes in the local environment of vanadium are induced by doping with elements of different natures. We also note that the shortest bond is obtained in the case of doping by phosphorus, which is in agreement with the lowest values of the lattice parameters obtained by the XRD analysis. We can conclude that the study by Raman spectroscopy has shown, in agreement with the results of FT-IR spectroscopy and those of XRD, that depending on the nature of the doping element, structural changes and modifications of the local symmetry have been well confirmed.

DSC and SEM

The DSC thermograms of the compound BISIMEVOX.15 from 50 °C to 550 °C are shown in Fig. 4. The DSC curve of the α-monoclinic phase BISIPVOX shows two endothermic peaks appeared upon heating at 453 °C and 489 °C, attributed to α → β and β → γ phase transitions, respectively (these transitions are not accompanied by a gain or loss of mass). The BISIVOX.15 compound shows a single endothermic peak at 425 °C, attributed to the β → γ transition. In the case of the BISICUVOX.15 sample, no thermal phenomena were recorded, indicating the stabilization of the γ-tetragonal polymorph at room temperature. Upon cooling, all phenomena recorded during the temperature rise are observed at almost the same temperatures, showing the reversibility of the local transformations. These results are in good agreement with the results of the XRD analysis.

Fig. 4
figure 4

DSC thermo-grams of BISIMEVOX.15 (Me = Si, P, and Cu): a heating and b cooling

Full size image

Figure 5 shows SEM micrographs of ceramics sintered at 830 °C for 5 h and synthesis powder (in the inset of the figure) for all BISIMEVOX.15 samples (Me = Si, P, Cu, and Co). Note first that the morphology of the synthesized powders BISIMEVOX.15 is almost independent of the nature of the doping ion and that the grain size varies from 10 to 30 µm. Nevertheless, the microstructure of sintered samples is strongly dependent on the doping element. The SiIV-PV doped ceramic shows a large grain size of about 25 µm, and some microcracks are also observed. This can be explained by the decrease in melting point in the presence of phosphorus [18, 22], and consequently, excessive grain growth leads to the appearance of microcracks. SiIV, SiIV-CuII, and SiIV-CoII doped ceramics show well-defined grains separated by visible grain boundaries. Moreover, intergranular pores are observed for the SiIV-CoII doped sample. One can also observe, for this sample, relatively darker spots which indicate the presence of a secondary phase less rich in bismuth or a phase of the V-Me system as seen in other works [33].

Fig. 5
figure 5

SEM micrographs of BISIMEVOX.15 ceramics and powders (sans in the inset)

Full size image

EDX analysis was performed for the ceramics and the results are presented in Table 2. The identified elements are the following: Bi, V, Si, P, Cu, Co, and O. Overall, the experimental atomic percentages (% exp) agree with the stoichiometry of the synthesized compounds (% theo).

Table 2 Elemental EDS analysis of BISIMEVOX.15 ceramics (atom%)
Full size table

UV–Vis

The optical absorption properties of semiconductors associated with the electronic structure play an essential role in photo-catalytic activity [34, 35].

Figure 6 shows the diffuse reflectance spectra (DRS) of BISIMEVOX.15 in the 250–800 wavelength range. The direct band gaps (Eg) of the prepared compounds were obtained by plotting F(R) hν)2 as a function of hν (1240/λ (nm)) and then extrapolating the linear portion of the graph to the x-axis, as shown in Fig. 7. The results of the band gap determination are given in Table 3. The band shape of the studied compounds is similar to that of Bi4V2O11 [15, 36], except for the compound BISICOVOX.15, where an additional fuzzy reflection minimum is observed. This minimum can be explained by the existence of electron transfers that correspond to the absorption of emission in cobalt polyhedra characteristic of the BICOVOX structure [37, 38]. On the other hand, the compounds BISIPVOX.15, BISIVOX.15, BISICUVOX.15, and BISICOVOX.15 show absorption in visible light with an absorption edge at 525.7, 551.5, 579.8, and 705 nm, respectively.

Fig. 6
figure 6

UV–vis spectra of BISIMEVOX.15

Full size image
Fig. 7
figure 7

Curves of (F(R)*hν).2 versus energy for BISIMEVOX.15

Full size image
Table 3 Direct band gap of BISIMEVOX.15 compounds
Full size table

As shown in Fig. 7, the band gap energies Eg of BISIPVOX.15, BISIVOX.15, BISICUVOX.15, and BISICOVOX.15 were estimated to be 2.36 eV, 2.25 eV, 2.14 eV, and 1.76 eV, respectively. It should be noted that the experimental values of Eg are different, due to the difference in phase symmetry and the doping element ME. The smaller optical band gap energy is obtained for BISICOVOX.15. This is undoubtedly attributed to the additional contribution of Co3d orbitals to the conduction band and the increased concentration of oxygen vacancies in the vanadate layers [39]. Globally, the values of band gap energy Eg are lower than that of Bi4V2O11(2.46 eV) [17] and BiVO4 (2.4 eV) [40]. Moreover, the estimated Eg values obtained by K. Trzciński et al. [17], for the orthorhombic α-Bi4V2O11 and for compounds with substitution rate x = 0.2 in BICUVOX (Bi4V2-xCuxO11-δ) (γ-form), BIZNVOX (Bi4V2-xZnxO11-δ) (α-form), and BIMNVOX (Bi4V2-xMnxO11-δ) (γ-form) systems, are 2.46, 2.34, 2.27, and 2.18 eV, respectively. It is clear that the Eg values obtained for our samples are not very different from those obtained for the monosubstituted phases studied by K. Trzciński. However, the bisubstituted γ-BISICOVOX.15 compound has a significantly lower Eg value, 1.76 eV. Thus, the double substitution (Si, ME) of vanadium in Bi4V2O11 made it possible to lower the energy of the gap, which may be favorable to the improvement of the photocatalytic activity in the visible light of the obtained compounds.

Conclusion

In this work, the series of BISIMEVOX.15 compounds, Bi4V1.7(Si.Me)0.3O11-δ (Me = Si, P, Cu, and Co), was obtained by coupled substitution of 15% vanadium with Si and Me in Bi4V2O11. XRD showed that the three polymorphs of BIMEVOX, α, β, and γ, are stabilized at room temperature for ME = P, Si, and Cu/Co, respectively. The obtaining of these polymorphs was confirmed, and local deformations were highlighted by Raman and FT-IR spectroscopies in relation to the nature of the doping element. DSC showed two reversible phase transitions α ↔ β and β ↔ γ for Bi4V1.7(Si.P)0.3O11-δ, a single reversible transition β ↔ γ for Bi4V1.7Si0.3O11-δ and no transition for the tetragonal phases Bi4V1.7(Si.Cu)0.3O11-δ and Bi4V1.7(Si.Co)0.3O11-δ. The DRS spectra reveal that the band gap energy values of the double-doped BISIMEVOX.15 compounds (1.76 ≤ Eg ≤ 2.36 eV) are lower than some values encountered in the literature for the parent compound Bi4V2O11. The band gap reduction can be explained by generated local defects that improve the optical properties of Bi4V2O11.