3.1. Structural analysis
To investigate the effect of La addition on the crystal structure of YVO4, XRD analyses of the undoped YVO4 and La3+-doped YVO4 powders have been employed. Figure 1(a) and (b) show XRD diffractograms of samples prepared with solid state reaction method. It was found that all the strong diffraction lines correspond to expected (200), (211), (400), (422), (433) and (600) diffraction reflections of crystalline tetragonal type crystal structure of YVO4 (JCPDS card no: 17–0341) .The XRD pattern of undoped and the La3+-doped YVO4 contain same YVO4 diffraction lines with similar relative intensities and no other diffraction lines related with any lanthanum based compounds like La2O3 and La(OH)3, or other impurity phases were detected within the detection limit of the XRD. In the previous studies, thermal equilibrium solubility limit of La in YVO4 was found as 6 at.% with coprecipitation method [19]. In this study, the dopant amount of YVO4 is chosen much lower than the solubility of La in YVO4. Therefore, according to XRD results, substitutional incorporation of the La3+ ions into YVO4 crystal is expected for the studied dopant concentration.
In order to obtain detailed structural information, the crystallite size (D, in nm) of the powders were calculated by Scherrer’s Eq. (1) using the XRD line broadening method:
D = 0.9λ/βcosθ (1)
where λ is the X-ray wavelength for CuKα, β is the full width in radians half maximum of the diffraction line and θ is the Bragg angle of (211) peak. Β is determined by β= (βobs2 - βins2), βobs is the measured broadening and βins is the instrumental broadening caused by the diffractometer. First, a decrease is observed on crystallite size, D, with La dopant. D value of 45.5 nm and 30.9 nm were obtained at undoped and La3+-doped YVO4, respectively, which suggest that there can be hindering of the crystal growth by the substitutionally incorporation of La3+ ions.
The effect of the La incorporation on the YVO4 crystal structure can be also investigated by examining the peak intensities and positions of YVO4 patterns as shown Fig. 1(c). It is found that the relative (211) peak intensities of undoped YVO4 drastically decreased by La doping which implies that dopant atoms inhibit the crystal growth of YVO4 particles at preferential growth direction. Furthermore, (211) diffraction line positions of undoped and La doped samples were determined as 2θ=~24.93° and ~ 24.84°, respectively. According to XRD spectra, insignificantly peak shift is observed to relatively lower 2θ values. The lattice parameters (a) and (c) variation is tabulated to observe doping effect. For undoped YVO4 particles, the lattice constants (a) and (c) are calculated as 7.135 and 6.27 Å, respectively. However, YVO4: La3+ powder demonstrates a slight lattice expansion with calculated lattice parameters (a) = 7.14 Å and (c) = 6.31 Å. The lattice expansion observed in the La3+-doped sample can be attributed to the substitution of La3+ ions into the Y3+ sites due to larger ionic radius of La3+ (1.172 Å) than that of Y3+ (1.140 Å) for the same coordination number [22]. It is normal that this expansion (hence the peak shift in the XRD pattern) has been sensitively affected as the doping rates are low. This perturbation on lattice parameters depending on La addition is preliminarily evident that incorporated La ions was selectively substitute into Y3+ sites of YVO4 crystal.
3.2. Morphological and elemental analysis
The SEM photographs (Figs. 2a and 3a) of the undoped and La3+-doped YVO4 phosphor powders both exhibits partially agglomerated grains and irregular particles both in shape and size. The EDS profile of both phosphor powders (Figs. 2b and 3b) indicate the presence of Yttrium, Vanadium, and Oxygen elements what in each sample should include. Furthermore, in the EDS profile of La3+-doped YVO4 powder, addition peaks related with Lanthanum was observed.
3.3. Photoluminescence of undoped and La3+-doped YVO4
The PL spectra of undoped YVO4 and YVO4: La3+ powders in Fig. 4a show same excitation peaks and considerable number of emission peaks between 450 and 750 nm, representing the similar positions of peaks by varying intensities. The PL emission spectrum of undoped YVO4 has two clear peaks in the wavelength range of 600–800 nm (Fig. 4b) which the major peak is at 620 nm. The other peak at 710 nm can be related to the recombination of electrons and holes at oxygen vacancies in YVO4. The maximum intensity emission peak located at 571 nm for YVO4: La3+ powder may be assigned to singly ionized oxygen vacancies which became dominated after La3+ doping. With the La3+ doping, emission bands from 450 to 650 nm were greatly suppressed and emission around 571 nm became the dominated component on the emission spectra of YVO4: La3+. The red luminescence centered at 620 nm was suppressed with respect to the blue one (at 478 nm) and the yellowish green one (at 571 nm) which are also the low intensity bands of undoped YVO4. This indicates La3+ doping modifies intrinsic lattice defects and intensively changes luminescence characteristic with respect to luminescence of undoped YVO4 [23, 24].
PL excitation and emission spectra given in Fig. 4 depicts the similar luminescence characteristics of undoped and La3+ doped YVO4 powder samples. The reason for red-shift from 282 nm to 290 nm in PL excitation spectra of undoped and La3+-doped YVO4 powders, respectively can be assigned to the La3+ doping, hence O2−-La3+ charge transfer (CT) from oxygen 2p excited state to La3+ 4f state and O2−-V5+ CT from oxygen 2p states to the empty d states of central vanadium in the VO43− group, indicating that there is a strong energy migration from host to La3+-activator in tetragonal YVO4 [25]. Further, the detailed study of PL emission spectrum of undoped YVO4 powder is shown in Fig. 5. The intense broad emission peak centered at 620 was deconvoluted using Gaussian fitting in the wavelenumber (cm− 1) unit of energy to be able to interpret and demonstrate in detail the various defects/vacancies present in the system, as shown in Fig. 5. All these peaks combined with the major peak at 16130 cm− 1 (620 nm) obtained after deconvolution could be specified to emission as shallow and deep level trap states. Hence, it can be said that oxygen vacancies are the prime defects responsible for PL emission. The emission peak at 485 nm for undoped YVO4 could correspond to characteristic intrinsic emission of VO43− groups [26] defect related luminescence and/or surface impurities such as oxygen vacancies, intrinsic defects and surface state and intrinsic metal ions in YVO4 crystal formed during growth. Accordingly, the effect of La3+ dopant ion has emerged based on the luminescence properties of the undoped material since they have same host crystal. In previous studies with similar results demonstrates doping of impurities with different oxidation state can create or increase defect levels [27, 28]. The impurity ions could give raise to either radiative or non-radiative recombination. In this case and studied material, La ion plays a significant role in modifying the defect chemistry of YVO4 which can be clearly seen in the variation in PL intensities between undoped and La doped YVO4. The effect of La additive on lattice is as follows: Along with doping, La ion migrates inside the YVO4 lattice and occupies the vacancy site. There could not be charge imbalance and ionic radii mismatch affect because it was supposed to be that La3+ ions could substitute the Y3+-sites since ionic radii are close to each other (La3+= 1.172 Å vs Y3+= 1.140 Å). The defects in YVO4 may also vary with synthesis method and condition, particle size and morphology, hence, the emission peak at 485 nm, which is of relatively low intensity compared to the others is related to deep level defects of La3+-doped YVO4.
3.4. Absorption spectra of undoped and La3+-doped YVO4
Figure 6 shows the absorption spectra of undoped and La3+ doped YVO4 powders.
It can be clearly made inference that both the samples are nearly transparent in the visible region between 400–800 nm. La-doped sample being a little more evident and severe, also exhibit an intense band-to-band absorption originated from the contribution of 1A1→1T1 charge transition around 300 nm overlapped with 1A1→1T2 charge transition at between 200–350 nm of VO43− groups. Additionally, the powder colors formed in both undoped and La3+-activated YVO4 samples have a light-yellow color, which can be attributed to the reduction of V5+ to V4+, subsequently formed V4+ defects and oxygen vacancies. It can be concluded that the formation of V4+ defects is rather related with redox between Ln3+ (here, Ln means La) ions and different types of vanadium ions [26, 29].
Energy band gap (\({E}_{g}\)) of the undoped and La doped YVO4 nanoparticles were obtained using the well-known Tauc plot which is depicted in Fig. 7. Tauc method relies on the following equation:
$${\left(\alpha h\upsilon \right)}^{1/n}=A\left({E}_{g}-h\upsilon \right)$$
2
Where \(\alpha\) is the absorption coefficient, \(h\upsilon\) is the energy of the incident photon, A is a constant and \(n\) depends on the optical transition of the material. YVO4 possess a direct transition (\(n=1/2\)) and a band gap of 3.8 eV [30]. As given in Fig. 7, \({E}_{g}\) of undoped YVO4 is obtained as 3.8 eV from fit of the red curve which is consistent with the literature. On the other hand, \({E}_{g}\) of the YVO4: La is determined as 3.5 eV from the fit of the blue curve. This decrease in energy band gap with doping is highly desirable for the photocatalytic performances. It is obvious that doping YVO4 with La will provide an increase in the photocatalytic activity due to formation of new localized energy levels between valence and conduction band.
3.5. Photocatalytic activity of YVO4 phosphors
To understand the influence of lanthanum dopant on the photoactivity of the YVO4, the photocatalytic activities of undoped and La3+-doped YVO4 powders have been evaluated by degradation of MB dye under UV light irradiation. Photocatalytic reaction was carried out with a UV light exposure time of 180 minutes. The wavelength of maximum absorbance of MB at 664 nm was used for the measurement of MB concentration. As seen in Fig. 8, a set of experiments were performed with the photocatalysts and MB under UV light irradiation. Both figures show decreases at the vicinity of 664 nm with a slightly shift. This little hypsochromic shift (blue shift) is attributed to N-demethylation of MB [31].
The normalized temporal concentration changes of photocatalytic degradation of MB aqueous solution in the presence of the photocatalysts were shown in Fig. 9a. As seen in Fig. 9a, La3+-doped YVO4 with 76.7% degradation efficiency led to more degradation under UV light illumination compared to the undoped one (38.8%). These results show that the photocatalytic activity of YVO4 changes and enhances with the substitutionally incorporation of La3+ ions into YVO4 crystal structure. The photocatalytic degradation rate of MB dye solutions fits a pseudo-first-order reaction model which is based on the Langmuir-Hinshelwood mechanism and can be calculated from the following equation:
$$-\text{ln}\frac{{C}_{t}}{{C}_{0}}={k}_{app}t$$
3
where \({C}_{0}\) is the initial concentration of the dye, \({C}_{t}\) is the concentration at irradiation time \(t\) and \({k}_{app}\) is the apparent pseudo-first-order reaction rate constant. The plots of \(-\text{ln}\left({C}_{t}/{C}_{0}\right)\) versus time \(t\) are given in Fig. 9b for UV irradiation. The \({k}_{app}\) values were calculated from the slope of these curves. According to the results obtained, the \({k}_{app}\) values are 0.00287 min− 1 and 0.00846 min− 1 for YVO4 and La3+-doped YVO4 particles, respectively. This result shows that La doping increases the \({k}_{app}\) value by 66%.
A possible mechanism can be proposed to explain the increasing in the photocatalytic activity as follows (Fig. 10): YVO4 can only be excited by UV light (λ < 367 nm) due to its wide band gap (3. 8 eV) [30]. La doping results in localized impurity levels between the conduction and valence bands. Thus, electrons can be excited from valence band to La doping energy level which provides the photo-generated electrons and holes with UV irradiation. While photo-generated electrons are captured by O2 molecules to form superoxide radicals, the photo-generated holes are held by H2O molecules to form hydroxyl radicals. Also, La3+ ions act as the electron traps and decrease the electron-hole recombination rate. These trapping prevent electron and hole recombination as they migrate to the catalyst surface [32]. The recombination of electrons and holes excites the 4f electrons of the La, converting the energy corresponding to the 4f→4f transitions into red emission [19].