Synthesis and Optical Properties of Size-Quantized BiVO4 Semiconductor Particles with Tetragonal Zircon-Type Structure

1. Introduction

In recent decades, an immense research attention has been focused on the development of novel methods of fabrication of multifunctional nanostructured materials with unusual physical, electrical and optical properties originated from the strong quantum confinement effect or surface defects. The successful development of the multifunctional nanostructured materials opens a broad spectrum of new opportunities for their applications in sensor -basedchip devices, electromagnetic interference shielding, energy generation and storage, chemical- bio- and electro-sensing, fuel cells, multimodal imaging and photothermal therapy, solar and photovoltaic cells and photocatalytic water splitting [1,2,3,4,5,6,7,8,9,10].

The nanostructured materials with high surface area show improved chemical reactivity, magnetic moment and polarizability and enhanced photoactivities under UV/Vis light irradiation, in comparison to larger particles or bulk materials, due to increasing number of the photo-generated electron–hole pairs on the surface [11,12].

Among all nanostructured materials, the bismuth-based compounds have been extensively studied in the last decade due to their broad spectrum of potential applications. Among all bismuth-based compounds, publications based on bismuth orthovanadate (BiVO₄) have seen an exponential increase in number over the last years. These compounds have many interesting and unique properties originating from the electronic and/or steric influences of the 6s² lone pair of Bi³⁺ that has a strong role in determining the site occupancy of the Bi³⁺ ions. The bismuth-vanadate (BiVO₄) is widely applied as a yellow pigment [13], photoelectroanalytical sensor [14], ferroelectric material [15], antibacterial agent [16], luminescent material and a host for rare-earth ions [17,18], as well as a photocatalytic material [19].

BiVO₄ exists in nature in three crystalline forms: orthorhombic pucherite, tetragonal dreyerite (tz-BiVO₄, zircon-type structure, space group I41/amd), and monoclinic clinobisvanite (ms-BiVO₄, distorted scheelite-type structure, space group I2/b) [20]. Up to date, large number of methods has been adopted for the preparation of ms-BiVO₄, such as: hydrothermal method with and without using surfactant or template [21,22,23], microwave-assisted hydrothermal [24,25], solvothermal [26], and co-precipitation method [27]. Several methods have also been utilized for the synthesis of tz-BiVO₄: co-precipitation method [28,29,30,31], hydrothermal method [32,33,34,35,36], rapid microwave assisted method [37] and epitaxial growth on FTO substrate [38]. An enhanced photocatalytic activity of un-doped or no hybrid tetragonal zircon-type nanostructured BiVO₄ semiconductor is reported in dozen published papers [39]. Recently, MOF- derived tetragonal BiVO₄ and rare-earth doped BiVO₄ systems [40,41,42,43] with enhanced photocatalytic properties for water splitting were studied, where the presence of RE³⁺ could induce the progressive stabilization of the tetragonal phase [44].

This work was motivated by recent evidence that tetragonal zircon-type BiVO₄, tz-BiVO₄, one of three commonly found polymorphs of BiVO₄, is moderately photocatalytically active [45]. Only 30 or so relevant papers can be found in the literature: thus, tz-BiVO₄ has not been extensively studied and, in particular, its photocatalytic properties are insufficiently examined, new synthetic approaches to tz-BiVO₄ are most needed [46,47]. Interestingly, synthesis of tz-BiVO₄ in a non-aqueous medium is an obvious way to avoid hydrolysis and precipitation of side products.

Herein, aiming at producing nanocrystalline tz-BiVO₄ a new and non-conventional way of synthesis was attempted through a novel straightforward room-temperature non-aqueous preparation method. The as-prepared colloids and the obtained nanostructured particles were examined with a view of quantum size effects on their optical properties and their suitability in photocatalytic applications. The ultra-small tetragonal zircon-type BiVO₄ nanostructures, with size range from 2 to 8 nm, were prepared by ethylene glycol-assisted colloidal route and characterized using optical, structural and microscopic techniques. These size-quantized nanoparticles present a remarkable synthetic achievement. Adsorption behaviors and mechanisms of methyl orange on the size-quantized tetragonal BiVO₄ nanoparticles were studied in details, as well as photocatalytic activities. Good optical performances and enhanced photocatalytic activity in comparison to titania photocatalyst Degussa P25, give to the size-quantized tetragonal BiVO₄ nanoparticles the potential for different applications such as degradation of methyl orange (MO) and other organic dye-pollutants.

2. Materials and Methods

2.1. Materials and Chemicals Used

All chemicals were of high purity and were used without further purification. These included: bismuth(III) nitrate pentahydrate (Bi(NO₃)₃x5H₂O, Sigma-Aldrich, 97%), ammonium metavanadate (NH₄VO₃, Alfa Aesar, 99.999%), trisodium citrate dihydrate (Na₃C₆H₅O₇x2H₂O, ≥99%, Sigma–Aldrich), ethylene glycol (C₂H₆O₂, Sigma-Aldrich, 97%), polyethylene glycol 200 (PEG-200, Alfa Aesar), nitric acid, HNO₃ (J.T. Baker, 65%) distilled water, methyl orange (C₁₄H₁₄N₃NaO₃S, Merck) and titanium(IV) oxide nanopowder, Degussa P25 (Sigma-Aldrich, > 99.0%).

2.2. Synthesis of Colloidal Tetragonal BiVO₄

Colloidal BiVO₄ samples were synthesized by modified ethylene glycol-assisted colloidal route at room temperature [48]. Here, NH₄VO₃ and Bi(NO₃)₃x5H₂O were used as precursors and ethylene glycol was utilized as a solvent for precursors, in order to avoid the hydrolysis of Bi(NO₃)₃ and precipitation of side products like bismuth(III)-hydroxonitrate, a reaction medium for a precipitation and a capping agent (to limit a particle growth and prohibit agglomeration). Ethylene glycol is a dihydroxy alcohol (HO–CH₂–CH₂–OH) that is liquid at room temperature; it is more viscous than water, biodegradable and boils at 197 °C. There are also additional advantages of an ethylene glycol-mediated synthesis; this is one of the most general and powerful methods for preparation of high-quality nanomaterials, is used for conventional glassware and the synthesis is simple, easily scalable, “green”, versatile and low-cost [49]. PEG-200 has role as a structure-directing agent to synthesize vanadate nanoparticles, as an organic additive and as a surface modifier.

The appropriate amounts of NH₄VO₃, Bi(NO₃)₃x5H₂O were separately dissolved in ethylene glycol to prepare different concentrations of bismuth and vanadium solutions precursors (0.075M, 0.050M and 0.025M). The same concentration of solution of trisodium citrate together with HNO₃ was added dropwise to a solution at a stoichiometric ratio of Bi³⁺ ions at room temperature. A white precipitate consisting of a Bi³⁺-Cit^3- complex was formed. The PEG-200 in ethylene glycol solution (with the same concetrations as precursors) was slowly, drop wise, added into the NH₄VO₃ containing solution and the resulting mixture was left under vigorous stirring for 1 hour. Afterwards, the ethylene glycol mixture solution of precursor of Bi³⁺ was slowly added into the mixture solution of NH₄VO₃ and PEG-200 under vigorous stirring for 2 hours and orange-yellow transparent colloids of BiVO₄ were obtained. It is important to emphasize that this preparation procedure proved to be fully reproducible over multiple trials, and, in all the as-prepared colloids, no evidence of precipitation has been noticed over a period of more than one year, thus indicating superior colloid stability of BiVO₄ nanoparticles in ethylene glycol solution. These colloids, corresponding to concentrations of precursor 0.075M, 0.050M and 0.025M, are hereafter referred to as samples A, B, C, respectively. In order to obtain the powders of BiVO₄, following the synthesis of colloidal BiVO₄, as-prepared colloids were additionally treated, centrifugated, washed with water several times and dried in an oven at 110°C for 24h. In the following, powders prepared using the colloids (A, B and C) are denoted as A-tz, B-tz and C-tz.

2.3. Characterization Methods and Instrumentation

Absorption measurements were performed by the UV-vis spectrophotometer (LLG-uniSPEC 2 UV/VIS-Spectrometer 190-1100 nm) in a range of wavelengths from 300 to 500 nm with 1 nm step. X-ray photoelectron spectroscopy (XPS) data were collected using a PHI Versa Probe III-XPS-spectrometer equipped with a monochromatic Al-Kα X-ray source and a hemispherical analyzer. Phase and purity of the powder samples were examined by powder X-ray diffraction (XRD) measurements on a Rigaku SmartLab diffractometer using Cu-Kα radiation (λ = 0.15405 nm). Diffraction data were collected with a step size of 0.02^◦ and a counting time of 0.7^◦/min over the angular range 2θ from 15^◦ to 70^◦. The photoluminescence (PL) measurements were recorded at room temperature on a Fluorolog-3 spectro-fluorimeter in the range of wavelengths from 450 nm to 600 nm. HRTEM measurements were performed using a FEI Tecnai F20 at 200 kV electron acceleration voltages after drop-casting of sample material on lacey carbon TEM grids.

2.4. Photocatalytic Experiment

Methyl orange (MO), anionic and water soluble azo dye, was widely used material in the dye industry and is commonly selected as a model organic pollutant to evaluate the behavior of a material for the removal of organic pollutants from its aqueous solutions. The removal of MO in aqueous solutions was carried out in double-walls cylindrical photochemical reactor. Temperature was maintained at 18° C by continuous flow of water through the reactor walls during the adsorption and photocatalytic experiments. Adsorption experiments were carried out in the dark before subsequent (photodegradation) experiments with visible light illumination. The suspension of BiVO₄ powder and solution of MO was than exposed to visible light from a LED lamp (Xled E27-15W, 220 V, 3000 K, 1350Lm) placed 15 cm above the reactor.

In a typical experiment, MO was dissolved in 200 ml of deionized water to obtain a 5 mgL^-1 solution while optimal photocatalyst concentration was found to be 1 mgmL^-1. Prior to irradiation, the adsorption−desorption equilibrium of the dye on the photocatalysts surface was achieved through the vigorous stirring for 90 minutes in the dark to ensure adsorption/desorption equilibrium before lighting. During the irradiation procedure, the reaction sample was collected and centrifuged to remove photocatalyst particles. The change in dye concentration over time was monitored by measuring the absorbance of MO at wavelength of 464 nm using the UV-Vis spectrophotometer. At given time intervals, samples were collected from the reaction mixture during 240 minutes of irradiation.

3. Results and Discussion

3.1. Optical Properties of Colloidal BiVO₄ Nanoparticles

3.1.1. UV-Vis Absorption and Photoluminescent Spectra

It is a well-known fact that the optical properties of semiconducting nanoparticles are essentially determined by their energy band gap (Eg). BiVO₄ is a semiconductor with a direct optical band gap with energy of 2.9 eV for tetragonal structure, measured at room temperature [50]. The optical UV-Vis absorption spectra of dispersions of colloidal tz-BiVO₄ nanoparticles in ethylene-glycol are given in Figure 1a. It is noteworthy that there are very few reports in the literature on absorption spectra of solutions of any polymorph of BiVO₄ [51,52,53]; UV-Vis diffuse reflection spectra, however, are more common [28,54,55]. It was observed that all the prepared samples exhibited absorption bands in the near UV and in the blue-violet regions. The absorption, centered at around 362 nm can be readily explained by ligand-to-metal charge transfer transitions localized within tetrahedral vanadate VO₄^3-groups; electrons from filled oxygen 2p levels are excited into vacant vanadium 3d levels [56,57].

The extended absorption tail in UV-Vis spectrum is assigned to subgap absorption due to defect states or intra-band absorption [57].

In general, the band gap energies of semiconducting materials can be extracted from their absorption spectra by using Tauc’s plots, i.e., by plotting (αhν)^1/n versus incident photon energy hν. The determination of optical band gap is obtained by Tauc’s Equation (1):

(αhν)^1/n = A(hν − E_g),

(1)

where A is a proportionality constant, α is the measured optical absorption coefficient and Eg is the band gap energy of the material. The exponent n depends on the type of the transition and the values of n = 1/2, 2, and 3/2, are for direct allowed transitions, indirect allowed and direct forbidden transitions, respectively. To determine the type of transitions, (αhν)^1/n versus hν was plotted; the band tail constant A is obtained through the slope of the linear region of the graphs, while the corresponding optical band gaps were estimated by extrapolating to zero absorption in the linear part of the graph at (αhν)^1/n = 0 Tauc’s plots (with n = 1/2) of the three as-prepared samples of colloidal BiVO₄ are given in Figure 1b. The estimated band-gap values of 3.07, 3.09 and 3.12 eV are higher than the value of 2.9 eV reported for tz-BiVO₄ [31]. The calculated values of band gaps are shifted to larger values than those for bulk materials due to quantum confinement. Obtained Eg values are in agreement with reported band gap values by other authors for tz-BiVO₄ nanoparticles. [57,58]

The radius (r) of the nanoparticles were estimated using Brus equation [59]:

$$E_{g\left(nano\right)}+E_{0\left(bulk\right)}\left({\displaystyle \frac{1}{m_e^{*}}}+{\displaystyle \frac{1}{m_h^{*}}} \right){\displaystyle \frac{h^2}{8m_0{r}^2}}-{\displaystyle \frac{1.8e^2}{4\pi \epsilon {\epsilon }_{0 }r}},$$

(2)

where E_g(nano) is the values of the energy gap determined as the x-intercept of the linear portion of the absorbance as a function of wavelength for nanoparticles with unknown radius (r); E_0(bulk) is the energy gap for bulk material (E_0(bulk) = 2.90 eV, for tetragonal BiVO₄) [60], me* and mh* are the effective masses of electrons and holes (m_e* = 17.322×m_e, m_h* = 1.210 × m_e) for tetragonal BiVO₄, respectively [61], m₀ is the mass of electron (m₀ = 9.110x10^-31 kg), ε₀ is the permittivity of vacuum (ε₀ = 8.854187817×10⁻¹² F×m⁻¹), ε is the dielectric constant for the tetragonal-BiVO₄ (ε = 68) [62], h is the Planck constant (h = 6.62607004×10-34 m²kgs^-1) and e is the charge of an electron (e = 1.60217662×10^-19 C). The second term in equation (2), which dominates when r is small, corresponds to the confinement energies for an electron–hole pair in a spherical nanoparticles, while the third term accounts for the Coulomb interaction between an electron and hole modified by the screening of charges by the crystal. After multiplying by r², rearranging, and using the quadratic formula:

$$\mathrm{r}={\displaystyle \frac{-\left(\frac{1.8e^2}{4\mathrm{\pi }\mathrm{\epsilon }{\epsilon }_{0 }}\right)+\sqrt{{\left(\frac{1.8e^2}{4\mathrm{\pi }\mathrm{\epsilon }{\epsilon }_{0 }}\right)}^2+\left(E_{g\left(nano\right)}-E_{g\left(bulk\right)}\right)\frac{h^2}{2m_0} \left(\frac{1}{m_e^{*}}+\frac{1}{m_h^{*}} \right) }}{2\left(E_{g\left(nano\right)}-E_{g\left(bulk\right)}\right)}},$$

(3)

one obtains the radii (r) of the nanoparticles equal to 3.24, 3.20 and 3.12 nm, for A, B, and C samples, respectively. These values are consistent with average nanoparticles size determined from TEM images.

Photoluminescent spectra, PL, with excitation wavelength at 428 nm of dispersions of tz-BiVO₄ nanoparticles are shown in Figure 1c. A vanadate group, VO₄^3-, where the central vanadium ion is coordinated by four oxygen ions in a tetrahedral (Td) symmetry, is known to be an efficient luminescent center. Hence, strong emission band centered at 495 nm could be attributed to the charge-transfer transitions (generated upon photo-excitation) between vanadium 3d and oxygen 2p orbitals in VO₄^3-. The results presented in this paper are similar to those of Bajaj et al. [28]. Photographs of colloidal solutions under day light and UV-lamp (253 nm) are presented in the Figure 1d.

3.1.2. XPS Spectra

To explore the chemical states of surface Bi and V sites in tetragonal nanostructured BiVO₄ semiconductor, the X-ray photoelectron spectroscopy (XPS) technique was used. Figure 2a presents the wide energy range 0-1200 eV XPS survey spectrum of BiVO₄ nanoparticles with tetragonal structure.

The binding energy levels of Bi, V and O, along with the Auger peaks, identified from the XPS survey spectrum, indicate that no other impurity elements or secondary phases were found in obtained BiVO₄ nanoparticles. The C1s peak arises from the reference. The chemical binding energy of C 1s at 284.54 eV was used for calibration to adjust the binding energies of the other elements. As it can be seen from Figure 2b, the Bi 4f orbital of tz-BiVO₄ can be well reproduced by two peaks with binding energies of 159.60 eV and 164.91 eV, which can be assigned to the Bi 4f_7/2 and Bi 4f_5/2 orbitals of Bi³⁺ indicating the absence of the metallic state of Bi⁰.The Bi4f photoelectron core level spectra point out a spin orbit splitting of about 5.31 eV between Bi 4f_7/2 and Bi 4f_5/2 peaks and corresponds to the binding energy of the Bi³⁺ state. For the V2p region (Figure 2c), the peaks located at binding energies of 515.91 eV and 523.37 eV are assigned to V 2p_3/2 and V 2p_1/2 of V⁵⁺, respectively. Figure 2d presents the high-resolution O1s spectra of the BiVO₄ photocatalyst, which were fitted into two components at binding energy values of 529.28 and 531.71 eV, which can be assigned to the lattice oxygen and oxygen of surface hydration of the nanostructured tz-BiVO₄ [63]. The obtained values for binding energies of Bi4f, V2p and O1s are in accordance with literature [64,65,66].

3.1.3. Photocatalytic Performance

The photocatalytic activities of the tz-BiVO₄ samples were evaluated for the photodegradation of MO, which was used as a model of organic pollutants. The powders A-tz, B-tz and C-tz were evaluated for photocatalytic performance, and compared with performance of powdered Degussa P25, a standard commercially available photocatalyst, by photocatalytic degradation of MO, under simulated solar light.

Upon reaching the equilibrium (under no illumination) the initial time was chosen and the dye solution with photocatalyst was exposed to simulated solar light for 240 minutes. As is customary, a relative concentration (C/C₀) of MO versus a contact time t describes kinetics of the dye removal, C₀ and C being respectively the concentration of MO before illumination (t = 0) and the concentration of MO after illumination for t min, while the decolorization efficiency is defined as percentage of decreasing absorbance intensity according to the following equation:

% decolorization = (A₀-A)/A₀*100%,

(4)

where A₀ is the absorbance at the maximum absorption wavelength (464 nm) of initial methyl orange solution, and A is the maximum absorption wavelength at 464 nm of the same solution mixed with ultra-small tz-BiVO₄ nanoparticles after the treatment. The efficiency of commercial TiO₂ P25 in removing the MO dye was found to be about 10%, while efficiencies of about 26%, 32% and 29% were observed for the samples A-tz, B-tz and C-tz, respectively, as presented in Figure 3a. The proposed mechanisms of the photocatalytic oxidation of MO in presence of tz-BiVO₄ photocatalyst are drawn in Figure 3b. Although still preliminary, this is a remarkably good performance of the system for MO removal and it would be interesting to examine it further for use in environmental purification [67]. This result suggests that ultrasmall tetragonal zircon-type BiVO₄ nanoparticles have a good photocatalytic performance and could be used in water and wastewater treatments, due to their large surface area and good efficiency under UV/Vis and solar light. Before doing any of the experiments, stability of methyl orange was tested under the proposed photocatalytic conditions. MO showed great stability under the conditions (lamp, temperature, etc.) used for all photocatalytic experiments confirming that the decreasing absorbance intensity should be attributed exclusively to the presence of BiVO₄ nanoparticles. The absorbance band at 464 nm corresponds to the conjugated structure that was associated with the azo bond (–N=N–) under the strong influence of the electron-donating dimethylamino group. Decreasing of absorbance intensity at 464 nm for A-tz, B-tz and C-tz Degussa P25 is presented in Table 1.

The possible mechanism of the photocatalytic oxidation of MO over tz-BiVO₄ can be represented by the following equations:

tz-BiVO₄ + hv → BiVO₄ (e^-) + BiVO₄ (h⁺)

MO + hv → MO*

tz-BiVO₄ + MO* → BiVO₄ (e^-) + ^.MO⁺

tz- BiVO₄ (e-) + O₂ → ^.O^2-

^.MO⁺ + O₂/^.O^2- → intermediate product,

(5)

Under irradiation with visible-light tz-BiVO₄ nanoparticles can be excited to electron–hole pairs and the electron would transform the valence band to the conduction band of tz-BiVO₄. At the same time, the photo-generated electrons are transferred to the excited state (MO*) of the MO owing to the intramolecular π–π* transition. The photogenerated electrons of MO* are immediately injected into the CB of BiVO₄, leaving ^.MO+ radicals [68,69].

3.2. Structural and Microstructural Properties

Representative high-resolution TEM (HRTEM) images of BiVO₄ particles of the as-synthesized colloidal dispersions A, B and C are depicted in Figure 4 (see Figure 4a, 4d and 4g). All TEM specimens were prepared by evaporating a drop of a colloidal dispersion of BiVO₄ in ethylene glycol on a carbon-coated specimen grid. Well-defined, non-agglomerated highly crystalline nanoparticles together with the diffractogram patterns produced from the digitally-recorded HRTEM images of the samples by means of the two-dimensional Fast Fourier Transformation (FFT) (see Figure 4b, 4e and 4h) can be seen in the TEM micrographs. The nanoparticle with size of 6-8 nm, 4-6 nm and 2-4 nm were found in the samples A, B and C, respectively.

From these figures it could be concluded that the diameter of colloidal tetragonal zircon-type nanostructured BiVO₄ particles decreases as the concentration of precursor decreases. These findings from the HRTEM micrographs of the colloids A, B and C were fully supported by X-ray analysis of the powders A-tz, B-tz and C-tz obtained from the dispersions [70,71]. Moreover, the average crystallite sizes of 4–9.4 nm estimated from the XRD diffraction peaks by the Halder–Wagner method were consistent with the sizes evaluated from the TEM images. In particular, similar values of the crystalline domain size and microscopically estimated average particle size of the nanostructured BiVO₄ imply that each particle consists of a single crystallite. It is in accordance with theoretically estimated radii using Brus equation.

The obtained BiVO₄ particles crystallized in tetragonal zircon-type structure (space group I41/amd, a = b = 7.300 Å and c = 6.457 Å, JCPDS card no. 00-014-0133) and corresponding XRD patterns are given in Figure 4c, 4f and 4i. All patterns clearly show the presence of a single tetragonal zircon type phase of BiVO₄. The relatively intense reflection peaks suggest that the as-synthesized nanostructured BiVO₄ are highly crystalline. The effect of size-dependent lattice expansion in nanoparticles is observed. The size dependence can be explained naturally from the increasing surface-to-volume ratio and the sensitivity of the surface stress to environmental conditions. Also, point defects may cause lattice expansion in special cases, where the particles are not in thermodynamic equilibrium or where special effects modify the thermodynamic conditions [72].

4. Conclusions

In summary, the colloidal dispersions of highly nanocrystalline tetragonal zircon-structured BiVO₄ particles with 2-8 nm in size were successfully prepared through a facile inexpensive room-temperature precipitation method using ethylene glycol simultaneously as a solvent, a reaction medium and a capping agent. The preparation procedure has proved to be fully reproducible over multiple runs and the as-prepared colloids have been stable and homogeneously colored over longer periods of time. The optical, structural and microstructural properties were examined in detail. The obtained band-gap values, using Tauc’s plot, of 3.07, 3.09 and 3.12 eV are higher than the values for tz-BiVO₄ reported in literature before. X-ray photoelectron spectroscopy technique confirmed the occurrence of Bi, V, and O elements and also found that Bi and V exist in +3 and +5 oxidation states, respectively. The radius (r) of the nanoparticles was estimated theoretically using Brus equation. The findings from the HRTEM micrographs of colloidal dispersions were fully supported by X-ray analysis of the powders obtained from the dispersions. Similar values of the crystalline domain size and microscopically estimated average particle size of the nanostructured BiVO4 imply that each particle consists of a single crystallite. It is in accordance with theoretically estimated radii using Brus equation. Interesting experimental findings in photodegradation experiments with a suspension of tz-BiVO₄ catalyst powders and MO dye under simulated solar light were encouraging and deserve further, more elaborate investigation of poorly studied tz-BiVO₄. It was found that adsorption was the dominant mechanism by which the MO solution was purified, whereas photocatalysis played a minor role under indoor lighting conditions. The interactions between the p-electrons of the MO ring and the surface (-OH) groups dominated the mechanism according to which adsorption occurred. This study demonstrated that the ultrasmall BiVO₄ nanoparticles with tetragonal structure could be excellent candidate for wastewater treatment via their highly efficient adsorption and photocatalytic properties.