Photoluminescence, photocatalysis and Judd–Ofelt analysis of Eu³⁺-activated layered BiOCl phosphors

Abstract

Eu³⁺-activated layered BiOCl phosphors were synthesized by the conventional solid-state method at relatively low temperature and shorter duration (400 °C for 1 h). All the samples were crystallized in the tetragonal structure with the space group P4/nmm (no. 129). Field emission scanning electron microscopy (FE-SEM) studies confirmed the plate-like morphology. Photoluminescence spectra exhibit characteristic luminescent ⁵D₀ → ⁷F_J (J = 0–4) intra-4f shell Eu³⁺ ion transitions. The electric dipole transition located at 620 nm (⁵D₀ → ⁷F₂) was stronger than the magnetic dipole transition located at 594 nm (⁵D₀ → ⁷F₁). The evaluated Commission International de l'Eclairage (CIE) color coordinates of Eu³⁺-activated BiOCl phosphors were close to the commercial Y₂O₃:Eu³⁺ and Y₂O₂S:Eu³⁺ red phosphors. Intensity parameters (Ω₂, Ω₄) and various radiative properties such as transition probability (A_tot), radiative lifetime (τ_rad), stimulated emission cross-section (σ_e), gain bandwidth (σ_e × Δλ_eff) and optical gain (σ_e × τ_rad) were calculated using the Judd–Ofelt theory. The experimental decay curves of the ⁵D₀ level in Eu³⁺-activated BiOCl have a single exponential profile. In comparison with other Eu³⁺ doped materials, Eu³⁺-activated BiOCl phosphors have a long lifetime (τ_exp), low non-radiative relaxation rate (W_NR), high quantum efficiency (η) and better optical gain (σ_e × τ_rad). The determined radiative properties revealed the usefulness of Eu³⁺-activated BiOCl in developing red lasers as well as optical display devices. Further, these samples showed efficient photocatalytic activity for the degradation of rhodamine B (RhB) dye under visible light irradiation. These photocatalysts are useful for the removal of toxic and non-biodegradable organic pollutants in water.

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1. Introduction

The unique spectroscopic properties of rare-earth ions in different host lattices prompted the development of rare-earth luminescent materials for lamps, cathode ray tubes, radiation monitoring systems, lasers, scintillators, biosensors and white light-emitting diodes (WLEDs).^1,2 White light-emitting diodes are potential materials for significantly improving lighting efficiency, resulting in reduction of the excitation energy and also reduction in pollution from fossil fuel power plants.³ The demand for developing efficient luminescent materials such as rare earth activated inorganic compounds has attracted researchers. Due to their possible photonic applications, good luminescent characteristics, stability in high vacuum, biosensors and absence of corrosive gas emission under electron bombardment when compared to currently used sulfide based phosphors. Among all the rare-earth ions, the red emitting trivalent Eu³⁺-activated phosphors have significant importance because of their potential application in color display and lighting technologies. Trivalent europium ions (Eu³⁺) activated phosphors were considered to obtain a red-emitting phosphor with proper CIE chromaticity coordinates. This was because the lowest excited level (⁵D₀) of the 4f⁶ configuration in Eu³⁺ was situated below the 4f⁵5d configuration. Eu³⁺ provides a particularly favorable situation for substitution in Bi³⁺ sites with suitable isostructural replacement. Trivalent europium ions exhibited narrow band emissions, long lifetimes and large Stokes shifts (emission of lower energy radiation upon excitation by higher energy radiation). The intensities and splitting of the spectral lines provide useful information concerning the local site symmetry, sizes of cations and properties of the chemical bonding.⁴ Eu³⁺ ions exhibit intense ⁵D₀ → ⁷F₂ emission in the red spectral region at 615 nm, when they occupy the lattice sites without centrosymmetry.

Bismuth oxychloride (BiOCl) belongs to V-VI-VII family, an important ternary phosphor among various inorganic phosphors due to their unique layered structures and high chemical stability.⁴ Oxyhalides-based phosphors would be advantageous when compared to oxidic hosts due to their low phonon energy, which improved the quantum efficiency of the luminescence. BiOCl compound crystallize in the tetragonal matlockite (PbFCl) structure with [Cl–Bi–O–Bi–Cl] layers stacked together by nonbonding van der Waals interaction through the halogen atoms along c-axis.⁵ Due to the strong intralayer bonding and weak interlayer van der Waals interaction, BiOCl show promising applications in photocatalysis, pharmaceuticals, battery cathodes, and photoelectrochemical devices.^6,7 The internal static electric fields between these layers enable the effective separation of photogenerated electron–hole pairs. Moreover, the electron–hole recombination was prohibited due to the indirect band gap nature of BiOCl where the excited electron in conduction band has to gain the crystal momentum in k-space with the emission or absorption of phonon before annihilating a hole in valence bond radiatively.⁸ Several methods were employed for the synthesis of BiOCl, namely hydrothermal,⁹ hydrolysis,¹⁰ sonochemical,¹¹ reverse microemulsion,¹² microwave⁴ and chemical vapor deposition⁷ methods. All these synthesis methods involve several problems, such as high pressure, less yield, lack of composition control, requirement of solvents and long reaction time.

Generally, solid state method offer several advantages over other conventional methods such as mass scale production, higher purity of products, better compositional control and solvent/surfactant free process. As per our knowledge, the spectroscopic properties of Eu³⁺-activated BiOCl have not been extensively reported so far and need careful examination. In the present study, we employed solid state method to prepare Eu³⁺-activated BiOCl compounds and reported their structural and optical parameters. The effect of Eu³⁺ concentration on photoluminescence (PL) properties was investigated. Judd–Ofelt theory was adopted to calculate the intensity parameters and various other radiative properties such as transition probability, radiative life time, quantum efficiency, stimulated emission cross-section, gain bandwidth and optical gain. Further, these compounds were used as photocatalysts for the degradation of rhodamine B (RhB) dye under visible light irradiation.

2. Experimental

2.1. Synthesis

Bi_1−xEu_xOCl (x = 0, 0.01, 0.03 and 0.05) compounds were synthesized by conventional solid-state method. For the synthesis of host BiOCl, the required amount of Bi₂O₃ (1.1649 g) and NH₄Cl (0.3209 g, 20% excess) were used as the starting materials and were ground finely in an agate mortar with pestle. This mixed powder was transferred into crucible and calcined at 400 °C for 1 h. Similarly, Eu³⁺-activated BiOCl compounds were synthesized using Bi₂O₃, Eu₂O₃ and NH₄Cl as the raw materials. The equation for the formation of BiOCl compound was represented by the following reaction:

Bi₂O₃ + 2NH₄Cl → 2BiOCl + 2NH₃ + H₂O (1)

2.2. Characterization

The crystallinity and phase purity of the samples were examined by powder X-ray diffraction (PANalytical X'Pert Pro Powder diffractometer) using Cu Kα radiation (λ = 1.5418 Å) with a nickel filter. Rietveld refinement data were collected at a scan rate of 1° min⁻¹ with a 0.02° step size for 2θ from 10° to 80°. The structural parameters were estimated by Rietveld refinement method using FullProf Suite-2000 programme. The surface morphology and elemental mapping of these phosphors was investigated using FE-SEM (FEI Sirion). Fourier transform infrared (FTIR) spectra were recorded using Perkin Elmer Spectrometer, Frontier using KBr as a reference. UV-Visible absorption spectra have been recorded for powders on Perkin Elmer Lambda 750 spectrophotometer. Thermogravimetric analysis (TGA) was performed using Mettler-Toledo system in the presence of N₂ as a carrier gas upto 900 °C. The PL studies have been carried out using a JobinYvon spectrofluorometer (Fluorolog-3, Horiba) equipped with a 450 W xenon lamp as the excitation source. The photoluminescence (PL) decay lifetime measurements were performed at low temperature (77 K) with a Perkin Elmer spectrofluorimeter. The photocatalytic activity of rhodamine B (RhB) dye solution was analyzed by Perkin Elmer UV-Visible spectrophotometer (Lambda 35) in the range from 200 to 800 nm periodically. All the measurements were performed at room temperature.

2.3. Photocatalytic activity measurement

The photocatalytic activities of Bi_1−xEu_xOCl compounds were measured by examining the degradation of RhB dye under visible-light irradiation (λ > 420 nm). A 500 W metal halide lamp held at 15 cm from the sample was used as the visible-light source. In each experiment, about 50 mg of photocatalysts was added into 50 mL of RhB solution with a concentration of 5 × 10⁻⁶ M. Prior to Visible illumination, the suspensions were magnetically stirred in the dark for 30 minutes to ensure an adsorption–desorption equilibrium. Then, the stable aqueous dye solution was exposed to visible light for 45 minutes. During the photoreaction, about 3 mL of suspension was collected at different time intervals and centrifuged to remove the catalyst. The transparent solution was analyzed using a UV-Vis spectrophotometer, and the absorbance was measured at a wavelength of 554 nm, which corresponds to the maximum absorption wavelength of RhB. The percentage dye degradation rate was calculated by the following equation:where C₀ is the initial concentration at 0 min and C is the concentration at time t.

3. Results and discussion

3.1. Powder X-ray diffraction (PXRD)

Fig. 1 shows the indexed powder X-ray diffraction patterns for Bi_1−xEu_xOCl compounds. The sharp intense peaks revealed the crystalline nature of the samples. All diffracted peaks were agreed well with the reported JCPDS Card no. 85-0861. On substitution of smaller Eu³⁺ ion (r_Eu³⁺ = 1.066 Å) to larger Bi³⁺ ion (r_Bi³⁺ = 1.17 Å) upto 5 mol%, we did not see any impurity line of Eu₂O₃ in the powder XRD patterns. This implies that the Eu³⁺ substituted to 8-coordinated Bi³⁺ ion in BiOCl lattice. On substitution of Eu³⁺ ion, we did not see any appreciable change in the intensity of diffracted lines. The structural parameters were refined from the Rietveld method using powder XRD data. The refinement results confirmed that all the samples were crystallized in the tetragonal phase with space group P4/nmm (no. 129) and the structural parameters were summarized in Table 1. It was noted that there was no appreciable change in the lattice parameters or cell volume upto 5 mol% substitution of Eu³⁺ ion. Observed, calculated and the difference XRD pattern of Bi_0.97Eu_0.03OCl compound is shown in Fig. 2a. The crystal structure of BiOCl was modeled using Rietveld refined structural parameters by VESTA program (Fig. 2b). In the BiOCl structure, the Bi³⁺ atom was coordinated to a square antiprism with four O atoms in one base and four Cl atoms in another base. The BiOCl layers were stacked over one another by the nonbonding van der Waals interaction through chloride atoms along the c-axis. The interplanar lattice spacing between two bismuth ions in BiOCl layers was found to be 4.85 Å.

Fig. 1 XRD patterns of Bi_1−xEu_xOCl compounds.

Table 1 Rietveld refined structural parameters for Bi_1−xEu_xOCl compounds

Compounds	BiOCl	Bi0.99Eu0.01OCl	Bi0.97Eu0.03OCl	Bi0.95Eu0.05OCl
Crystal system	Tetragonal	Tetragonal	Tetragonal	Tetragonal
Space group	P4/nmm (no. 129)	P4/nmm (no. 129)	P4/nmm (no. 129)	P4/nmm (no. 129)
Lattice parameters
a (Å)	3.893(2)	3.894(2)	3.895(3)	3.895(2)
c (Å)	7.366(3)	7.374(3)	7.372(7)	7.371(5)
Cell volume (Å^3)	111.63(8)	111.81(3)	111.85(2)	111.82(1)
Atomic positions
Bi/Eu (2c)
x	0.2500	0.2500	0.2500	0.2500
y	0.2500	0.2500	0.2500	0.2500
z	0.1707(2)	0.1703(3)	0.1704(7)	0.1703(5)
O (2a)
x	0.2500	0.2500	0.2500	0.2500
y	0.7500	0.7500	0.7500	0.7500
z	0.0000	0.0000	0.0000	0.0000
Cl (2c)
X	0.2500	0.2500	0.2500	0.2500
Y	0.2500	0.2500	0.2500	0.2500
z	0.6447(9)	0.6560(8)	0.6479(5)	0.6535(5)
Rfactors (%)
Rp	0.103	0.127	0.124	0.118
Rwp	0.141	0.168	0.171	0.160
Rexp	0.119	0.135	0.156	0.148
χ^2	0.013	0.016	0.012	0.012
RBragg	0.056	0.087	0.065	0.064
RF	0.035	0.065	0.050	0.047

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Fig. 2 (a) Observed, calculated and the difference XRD patterns of Bi_0.97Eu_0.03OCl and (b) crystal structure of BiOCl compound.

3.2. Field emission scanning electron microscopy (FE-SEM) analysis

Fig. 3 shows FE-SEM images of BiOCl and Bi_0.95Eu_0.05OCl compounds. The micrographs revealed the plate-like morphology. Similar morphology was obtained by Wang et al.⁶ using hydrolysis method and Li et al.⁹ via hydrothermal route. The thickness of the plates in both compounds was found in the range of 0.116–0.140 μm. To confirm the homogeneity of elements in the compounds, we have carried out the elemental mapping using energy dispersive X-ray (EDX) analysis. Fig. 4 shows the elemental mapping of (a) BiOCl and (b) Bi_0.95Eu_0.05OCl compounds. The elemental mapping of BiOCl confirmed that all the elements were homogeneously distributed over the entire area. For 5 mol% Eu³⁺doping, although the concentration of Eu was low and therefore faintly visible but was evenly distributed.

Fig. 3 FESEM micrographs of (a and b) BiOCl and (c and d) Bi_0.95Eu_0.05OCl compounds.

Fig. 4 Elemental mapping of (a) BiOCl and (b) Bi_0.95Eu_0.05OCl compounds.

3.3. Fourier transform infrared (FTIR) studies

The purity and functional groups of all the compounds were confirmed by the FTIR spectra (Fig. 5). All the samples show absorption peak at 524 cm⁻¹ which was attributed to the characteristic symmetrical stretching vibration of the Bi–O bond. Further, we did not observe any additional bands which confirm that all the compounds were pure phase.

Fig. 5 FTIR spectra of Bi_1−xEu_xOCl compounds.

3.4. UV-vis absorption spectroscopy

Fig. 6a illustrates the UV-Vis absorption spectra of Bi_1−xEu_xOCl compounds. A red shift was observed in the absorption edge with increasing Eu³⁺ concentration in BiOCl. The optical energy band gap (E_g) was estimated from these absorption spectra using Wood and Tauc relation.¹³ The energy band gap was calculated with absorbance and photon energy by the following equation:

αhν = A(hν − E_g)ⁿ (3)

where α is the absorption coefficient, hν is the photon energy, A is a proportional constant, E_g is the energy band gap and n depends on the characteristics of the transition in a semiconductor. (n = 1/2, 2, 3/2, or 3 for allowed direct, allowed indirect, forbidden direct, and forbidden indirect electronic transitions, respectively). BiOCl exhibits an optical absorption spectrum governed by indirect electronic transitions (n = 2).¹⁴ The intercept of the tangent to the x-axis gives good approximation of the band gap energies of Bi_1−xEu_xOCl compounds. The energy band gaps for BiOCl, Bi_0.99Eu_0.01OCl, Bi_0.97Eu_0.03OCl and Bi_0.95Eu_0.05OCl were found to be 2.92 (±0.0031), 2.90 (±0.0026), 2.88 (±0.0024) and 2.86 (±0.0017) eV, respectively (Fig. 6b). The obtained band gap energy values were comparable with the reported literature.¹⁵ On substitution of smaller Eu³⁺ ion (r_Eu³⁺ = 1.066 Å) to larger Bi³⁺ ion (r_Bi³⁺ = 1.17 Å), the band gap values decrease. The influence of Eu³⁺ on the electronic structure of the host material was investigated by calculating the positions of the conduction band (CB) and valence band (VB) edges using the equation:

E_CB = X − E^C − 1/2E_g (4)

where E_CB is the CB edge of the compound at the point of zero charge, X is the absolute electronegativity of the compound (6.34 eV), E^C is the energy of free electrons on the hydrogen scale (∼4.5 eV) and E_g is the band gap energy of the compound. The calculated bottom of the CBs and top of the VBs of Bi_1−xEu_xOCl compounds are listed in Table 2. The band gap structures for Bi_1−xEu_xOCl compounds are shown in Fig. 6c. As can be seen, the VB edge potential of Bi_1−xEu_xOCl decreases from 3.30 to 3.27 eV due to the smaller ionic radii of Eu³⁺ ion in the BiOCl lattice.

Fig. 6 (a) UV-Vis absorption spectra, (b) plots of (αhν)^1/2 vs. hν and (c) band gap structures for Bi_1−xEu_xOCl compounds.

Table 2 Absolute electronegativity, calculated CB edge, calculated VB position and band gap energy for Bi_1−xEu_xOCl compounds

Compounds	Absolute electronegativity X (eV)	Calculated CB position (eV)	Calculated VB position (eV)	Band gap energy, Eg (eV)
BiOCl	6.34	0.38	3.30	2.92
Bi0.99Eu0.01OCl	6.34	0.39	3.29	2.90
Bi0.97Eu0.03OCl	6.34	0.40	3.28	2.88
Bi0.95Eu0.05OCl	6.34	0.41	3.27	2.86

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3.5. Thermogravimetric analysis (TGA)

Thermogravimetric analysis was used to evaluate the stability of BiOCl compound. Fig. S1† shows that no significant weight loss was observed upto 590 °C. This confirms that the BiOCl compound was stable. Above 590 °C, a drastic weight loss around 12% was observed in the range of 590 to 780 °C due to the decomposition of BiOCl into bismuth oxide and chlorine gas. Further, the decomposition of bismuth oxide was started above 780 °C.

3.6. Photoluminescence (PL) properties

In Fig. 7a, an excitation spectrum was presented for Bi_0.99Eu_0.01OCl phosphor recorded at λ_em = 620 nm. A broad excitation peak at 350 nm (⁷F₀ → ⁵D₄) was observed and this band was due to the Eu³⁺–O²⁻ charge transfer (CT) which arises from the transition of 2p electrons of O²⁻ to the empty 4f orbitals of Eu³⁺ ions. The peak at 394 was observed which correspond to ⁷F₀ → ⁵D₃ transition of Eu³⁺. The excitation spectrum shows a peak at 466 nm (⁷F₀ → ⁵D₂) due to the intra-4f transitions of Eu³⁺ ions in the host lattice.⁴ The emission spectra obtained for Eu³⁺-activated BiOCl phosphors under the visible excitation (466 nm) displayed the typical emissions in the 570–720 nm region (Fig. 7b). The sharp peaks in the emission spectra were correspond to the electronic transitions as ⁵D₀ → ⁷F₀ (580 nm), ⁵D₀ → ⁷F₁ (594 nm), ⁵D₀ → ⁷F₂ (612, 620 nm), ⁵D₀ → ⁷F₃ (651 nm) and ⁵D₀ → ⁷F₄ (698 nm). Fig. S2† depicts the energy level diagram for Eu³⁺-activated BiOCl phosphors. The presence of unique emission line at 580 nm was due to the J–J mixing by the crystal field effects.¹⁶ The ⁵D₀ → ⁷F₁ transition peak originates from magnetic dipole transition. In contrast, the radiative transitions from ⁵D₀ to ⁷F₂ and ⁷F₄ levels were electric dipole in character.^16,17 The ⁵D₀ → ⁷F₃ transition was forbidden from both electric and magnetic dipole considerations. It can also be observed that the relative emission intensity of ⁵D₀ → ⁷F₂ transition was higher than ⁵D₀ → ⁷F₁ transition. This was normally associated with Eu³⁺ ions occupying crystallographic sites without an inversion center.¹⁶ The relative intensities of the emissions fall in the order ⁵D₀ → ⁷F₂ > ⁷F₄ > ⁷F₁ > ⁷F₀ > ⁷F₃ for all Eu³⁺-activated BiOCl phosphors (Fig. S3†). Additionally, it was worth noting that the emission intensities increase with the increase in Eu³⁺ doping concentration from 1 to 5 mol%. The splitting of ⁵D₀ → ⁷F₂ peak was due to the composition, coordination environment and crystal field symmetry.¹⁸

Fig. 7 (a) Excitation spectra of Bi_0.99Eu_0.01OCl phosphor (inset shows the magnified view of excitation spectra) and (b) emission spectra of Bi_1−xEu_x OCl phosphors.

For practical lighting applications, color quality was specified in terms of 1931 Commission International de l'Eclairage (CIE) chromaticity color coordinates. These color coordinates recognize that the human visual system uses three primary colors: red, green and blue.¹⁹ In general, the color of any light source can be represented on the (x, y) coordinates in this color space. To characterize the luminescence of Bi_1−xEu_xOCl materials under visible excitation (466 nm), the CIE coordinates were calculated using the color calculator program radiant imaging.²⁰ Fig. 8 shows the CIE 1931 chromaticity diagram of Bi_1−xEu_xOCl phosphors. The calculated CIE color coordinates of Bi_1−xEu_xOCl (x = 0.01, 0.03 and 0.05) phosphors were found to be (0.6193, 0.3802), (0.6322, 0.3653) and (0.6371, 0.3615), respectively. The evaluated color coordinates for Bi_0.95Eu_0.05OCl phosphor was close to the commercial Y₂O₃:Eu³⁺ (0.645, 0.347) and Y₂O₂S:Eu³⁺ (0.647, 0.343) red phosphors.²¹ The location of the color coordinates of Eu³⁺-activated BiOCl on the CIE chromaticity diagram presented in Fig. 8 revealed that the color properties of the phosphor materials can be useful for the production of red component in white LEDs.

Fig. 8 CIE 1931 chromaticity diagram of Bi_1−xEu_xOCl phosphors.

3.7. Judd–Ofelt intensity parameters and radiative properties

The PL emission spectrum was further used for the calculation of Judd–Ofelt parameters. The J–O intensity parameters, Ω_J (J = 2, 4) provide an insight into the local structure and bonding in the vicinity of rare earth ions. So the Judd–Ofelt analysis of the PL emission can be used for calculating the parity-forbidden electric-dipole radiative transition rates between the various levels of the rare earth ions.^17,22 Through these analyses, the local environment around the metal ion and the bond covalency of metal–ligand bonds can be interpreted. The integrated emission intensities of the spontaneous (radiative) emission of the transition between two manifolds ⁵D₀ and ⁷F_J (J = 2, 4) were associated with the radiative emission rates and can be written as:where I_0–J and I_0–1 are integral intensities for ⁵D₀ → ⁷F_J and ⁵D₀ → ⁷F₁ transitions and hν_0–J and hν_0–1 are their energies, respectively. The magnetic dipole radiative emission rate A_0–1 has a value of 50 s⁻¹.²³ The radiative emission rates A_0–J were related to forced electric dipole transitions and they may be written as a function of the J–O intensity parameters:where χ is the Lorentz local field correction factor given as function of the index of refraction n of the host . The 〈⁵D₀|U^(J)|⁷F₂〉² are the square reduced matrix elements whose values are independent of the chemical environment of the Eu³⁺ ion. Their values were known and are 〈⁵D₀|U⁽²⁾|⁷F₂〉² = 0.0032 and 〈⁵D₀|U⁽⁴⁾|⁷F₄〉² = 0.0023. Thus, using eqn (5) and (6) the values of Ω_2,4 were obtained. The calculated Judd–Ofelt parameters have been used to predict some important radiative properties such as radiative transition probability (A_rad) and lifetime (τ_rad).

The radiative transition probability (A_rad) can be calculated using the equation below:

The radiative lifetime τ_rad of an excited state in terms of A_rad, is given by:

The Judd–Ofelt intensity parameters, radiative transition probability and calculated lifetime of Bi_1−xEu_xOCl phosphors are displayed in Table 3. The relative intensity of the hypersensitive electric dipole ⁵D₀ → ⁷F₂ transition depends on the local symmetry of the Eu³⁺ ions. This transition was mainly responsible for the Ω₂ value, and it depends on short distance effects. The change of Ω₂ parameter was due to distortions around occupied Eu³⁺ ion sites. The intensity of this transition increases with decreasing local symmetry of the Eu³⁺ ion. It was observed that the value of Ω₂ parameter increases with increase in the Eu³⁺ concentration. This result reveals the high covalence of the metal–ligand bonds and more distortion of the symmetry of Eu³⁺ sites in BiOCl matrix.²⁴ The parameter Ω₄ was not directly related to the symmetry of the Eu³⁺ ion but to the electron density on the surrounding ligands. Its value increases with increase in the concentration of Eu³⁺, indicating the decrease in the electron density on the ligands.

Table 3 Judd–Ofelt intensity parameters (Ω₂, Ω₄), radiative transition probability (A_rad), non-radiative relaxation rate (W_NR), total transition probability (A_tot), calculated radiative (τ_rad) lifetime, experimental (τ_exp) lifetime, quantum efficiency (η) and asymmetry ratio of Bi_1−xEu_xOCl phosphors

Eu^3+ (mol%)	J–O intensity parameters (×10^−20 cm^2)		Arad (s^−1)	WNR (s^−1)	Atot (s^−1)	τrad (ms)	τexp^a (ms)	η (%)	Asymmetry ratio
	Ω2	Ω4
a Maximum experimental lifetime error: τexp = 0.0016 ms.
1	1.4167	0.6861	191.98	22.19	214.17	5.21	4.67	90	1.77
3	1.5156	0.7454	203.05	5.95	209.00	4.92	4.78	97	2.06
5	1.5567	0.7506	205.68	1.71	207.39	4.86	4.82	99	2.09

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Asymmetric ratio was used to understand the variation of symmetry and coordination environment around Eu³⁺ ion doped in BiOCl matrix. The asymmetric ratio is a relative ratio of integrated areas under the peaks of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions.²⁵ In a site with inversion symmetry, the ⁵D₀ → ⁷F₁ transition was dominant while in a site without inversion symmetry, the ⁵D₀ → ⁷F₂ transition dominates. The asymmetric ratios for Bi_1−xEu_xOCl phosphors increase with increase in Eu³⁺ concentration (Table 3). This was due to the distortion in symmetry around Eu³⁺ ion in BiOCl which caused an enhancement in the relative intensity of the hypersensitive electric dipole transition. This indicates strong electric fields of low symmetry at the Eu³⁺ ions.

Fig. 9 shows the PL decay curve for Bi_1−xEu_xOCl phosphors measured at low temperature (77 K). The decay curve was monitored for the emission band at 620 nm, corresponding to the ⁵D₀ → ⁷F₂ transition, under the excitation of 466 nm. These PL decay profiles exhibit single exponential behavior and the excited state lifetime was calculated from the slope of the fitting line using the following expression:

I_t = I₀e^−t/τ (9)

where I_t and I₀ are the luminescence intensities at time t and at t = 0 respectively, τ is the lifetime of the excited state energy level.

Fig. 9 Photoluminescence decay curve for the Eu³⁺-activated BiOCl phosphors measured at 77 K with λ_ex = 466 nm and λ_em = 620 nm. The dots represent the observed points and the solid line gives its exponential fitting.

The experimental lifetime (τ_exp) values of the ⁵D₀ level of Eu³⁺ in BiOCl are presented in Table 3. These lifetime values support the increase in luminescence intensity of ⁵D₀ → ⁷F₂ transition in Eu³⁺-activated BiOCl phosphors. It was obvious that the decay time increases with increase of Eu³⁺ ion concentration due to the decrease in symmetry around Eu³⁺ ion.²⁶ It was observed that the experimental lifetime (τ_exp) values were lower than the calculated radiative lifetime (τ_rad) values. This was due to the non-radiative relaxation rate (W_NR) of the ⁵D₀ energy level of the Eu³⁺ ions which was determined by the relation:

The total transition probability is the sum of radiative and non-radiative emission rates. Its value decreases with increase of Eu³⁺ concentration in BiOCl matrix due to the decrease in symmetry around Eu³⁺ ion (Table 3). Luminescence quantum efficiency (η) is defined as the ratio of number of photons emitted to the number of photons absorbed. But for the Ln³⁺ ion system, it is defined as the ratio of the experimental lifetime to the calculated radiative lifetime of the ⁵D₀ level and was measured from the following expression:

The non-radiative relaxation rate and quantum efficiency of Bi_1−xEu_xOCl phosphors are shown in Table 3. The calculated W_NR values of the present phosphors were less than lead fluorophosphate²⁶ and telluro fluoroborate²⁷ glasses. Table 4 illustrates a comparison of the experimental lifetime and quantum efficiency for ⁵D₀ level of Eu³⁺ ion in BiOCl along with reported Eu³⁺ doped different host matrices. As can be seen from Table 4, it was found that the experimental lifetime and quantum efficiency of Bi_1−xEu_xOCl phosphors were higher than the Eu³⁺ doped LaOF,²⁸ KLa(PO₃)₄,²⁹ KLu(WO₄)₂,³⁰ telluro fluoroborate,²⁷ lead fluorophosphate²⁶ and tellurite³¹ glass. The quantum efficiency of 99% of ⁵D₀ level suggests the efficiency of the Bi_0.95Eu_0.05OCl phosphors as a potential host material for red laser applications.

Table 4 Comparison of experimental lifetime (τ_exp), quantum efficiency (η) of Eu³⁺ doped different host matrices

Host matrix	Eu^3+ (mol%)	τexp (ms)	η (%)	Reference
BiOCl	1	4.67	90	Present work
	3	4.78	97	Present work
	5	4.82	99	Present work
LaOF	0.5	1.394	95	28
	20	0.427	24	28
KLa(PO3)4	2	3.45	66	29
	30	3.48	66	29
KLu(WO4)2	1	0.26	—	30
	1.5	0.68	—	30
	3	1.04	—	30
	5	1.29	72	30
Telluro fluoroborate glass	0.05	1.452	39	27
	1	1.601	63	27
Lead fluorophosphate glass	10	2.29	86	26
Tellurite glass	0.5	0.82	64	31
	1.5	0.88	74	31

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The stimulated emission cross-section (σ_e) is an important factor to predict the laser performance of a material and also the rate of the energy extraction from the lasing material, and was calculated as

where λ_p is the emission peak wavelength, c is the velocity of light and Δλ_eff is the effective bandwidth of the emission transition.

Some of the radiative properties such as the effective bandwidth of the emission transition (Δλ_eff), stimulated emission cross-section (σ_e), gain bandwidth (σ_e × Δλ_eff) and optical gain (σ_e × τ_exp) for Bi_1−xEu_xOCl phosphors are displayed in Table 5. The trend of σ_e for the ⁵D₀ emission transition were observed as ⁷F₂ > ⁷F₄ > ⁷F₁ for Eu³⁺-activated BiOCl phosphors. The stimulated emission cross-section values of the prepared phosphors for ⁵D₀ → ⁷F₂ transition were found to be better than those reported for Eu³⁺ doped KLaEu(PO₃)₄,²⁹ Eu³⁺ doped ZnAlBiB (ref. 32) and Eu³⁺ ions in lithium aluminium borophosphate³³ glasses. The higher stimulated emission cross section value was an attractive feature for low threshold and high gain laser applications which were used to claim good laser action.²⁹ The product of emission cross-section and the effective bandwidth of the emission transition is a significant parameter to predict the bandwidth of the optical amplifier. The higher the product values were, the better was the amplifiers performance. It was noteworthy that the maximum value of gain bandwidth was 6.20 × 10⁻²⁸ cm³ corresponding to ⁵D₀ → ⁷F₂ transition for Bi_0.95Eu_0.05OCl phosphor. Similarly, the product of experimental lifetime and stimulated emission cross section is a vital aspect for high optical amplifier gain. The obtained gain bandwidth and optical gain values corresponding to ⁵D₀ → ⁷F₂ transition were found to be larger than Eu³⁺ doped ZnAlBiB (ref. 32) glass. Therefore, we can conclude that the ⁵D₀ → ⁷F₂ transition (620 nm) provide favorable lasing action, high gain bandwidth and optical gain for amplifiers. The large values of lifetime, stimulated emission cross-section, gain bandwidth and optical gain suggest that the Eu³⁺-activated BiOCl phosphors can be useful for red laser applications and also for the development of color display devices.

Table 5 Effective bandwidth of the emission transition (Δλ_eff), stimulated emission cross-section (σ_e), gain bandwidth (σ_e × Δλ_eff) and optical gain (σ_e × τ_exp) for Bi_1−xEu_xOCl phosphors

Eu^3+ (mol%)	Transitions	Δλeff (nm)	σe (×10^−22 cm^2)	σe × Δλeff (×10^−28 cm^3)	σe × τexp (×10^−25 cm^2 s^−1)
1	^5D0 → ^7F1	8.88	2.63	2.33	41.80
	^5D0 → ^7F2	5.66	8.95	5.06
	^5D0 → ^7F4	5.40	7.43	4.01
3	^5D0 → ^7F1	7.83	2.98	2.34	45.31
	^5D0 → ^7F2	5.87	9.48	5.56
	^5D0 → ^7F4	5.20	8.26	4.29
5	^5D0 → ^7F1	7.96	3.12	2.48	49.40
	^5D0 → ^7F2	6.05	10.25	6.20
	^5D0 → ^7F4	5.37	9.27	4.98

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3.8. Photocatalytic activity

Fig. 10 shows the photocatalytic degradation of RhB dye over Bi_1−xEu_xOCl catalysts by monitoring the temporal changes of UV-Vis absorbance spectra. A significant reduction in maximum absorption spectra of RhB aqueous solution was verified during the photodegradation process. Before irradiation, the N,N,N′,N′-tetra-ethylated rhodamine molecule, RhB dye, has one band with a maximum absorption centered at λ_max = 554 nm. Visible light irradiation of the aqueous RhB/Bi_1−xEu_xOCl dispersion leads to a decrease in absorption with a concomitant wavelength shift of the band to shorter wavelengths. The blue shift depicted in Fig. 10 was caused by de-ethylation of RhB due to an attack by one of the active oxygen species on the N-ethyl group.³⁴ The shifting of the maximum absorption of RhB dye to other wavelength positions of its major absorption band which moved toward the N,N,N′-tri-ethylated rhodamine (λ_max = 539 nm); N,N′-di-ethylated rhodamine, (λ_max = 522 nm); N-ethylated rhodamine, (λ_max = 510 nm) and rhodamine (λ_max = 498 nm) species.³⁵ In the process of photodegradation, the intensity of the main peak reduced gradually to zero at 45 min (BiOCl), 40 min (Bi_0.99Eu_0.01OCl), 35 min (Bi_0.97Eu_0.03OCl) and 30 min (Bi_0.95Eu_0.05OCl). This revealed that the molecular structure of RhB was decomposed to yield environmental friendly species. Fig. 11a shows the percentage of dye degradation after irradiation for different times. When the reaction was performed under dark for 30 min, we observed 42%, 44%, 48% and 51% RhB dye degradation for Bi_1−xEu_xOCl (x = 0, 0.01, 0.03 and 0.05) catalysts, respectively. This shows that the Bi_0.95Eu_0.05OCl catalyst has good adsorption capacity. Under visible illumination, the 100% RhB dye was decomposed within 45, 40, 35 and 30 min for Bi_1−xEu_xOCl (x = 0, 0.01, 0.03 and 0.05) catalysts, respectively.

Fig. 10 UV-Vis absorbance spectra of RhB dye as a function of time over (a) BiOCl, (b) Bi_0.99Eu_0.01OCl, (c) Bi_0.97Eu_0.03OCl and (d) Bi_0.95Eu_0.05OCl catalysts under visible illumination.

Fig. 11 (a) Percentage of RhB dye degradation and (b) first order kinetics of dye degradation over Bi_1−xEu_xOCl catalysts as a function of irradiation time.

To prove the efficiency of Bi_1−xEu_xOCl catalysts with the possibility for future industrial applications, the reaction kinetics was evaluated. The kinetics of RhB degradation reaction under visible light is presented in Fig. 11b, which follows apparent first order kinetics in agreement with a general Langmuir–Hinshelwood mechanism³⁶

where R is the degradation rate of reactant (mg L⁻¹ min⁻¹), C is the concentration of reactant (mg L⁻¹), t is the illumination time, K is the adsorption coefficient of reactant (L mg⁻¹) and k is the reaction rate constant. If C is very small then the above equation could be simplified towhere C₀ is the initial concentration (0 min) of the RhB aqueous solution and C is the concentration of the RhB aqueous solution for different times of visible illumination.

According to eqn (14), if ln(C₀/C) was plotted as a function of t, a straight line was observed, whose slope is equal to the apparent first-order rate constant k_app which were calculated to be 0.0844, 0.0898, 0.0957 and 0.1003 min⁻¹ for Bi_1−xEu_xOCl (x = 0, 0.01, 0.03 and 0.05) catalysts, respectively. These values substantiate the photocatalytic reaction rate being highest for Bi_0.95Eu_0.05OCl catalyst. Further, the rate constants increase with increase in Eu³⁺ concentration for Bi_1−xEu_xOCl catalysts. This was due to the decrease in band gap energy values with Eu³⁺ doping concentration. The indirect optical transition and layered structure of BiOCl play an important role in its excellent photocatalytic activity. In indirect transition, an excited electron has to travel certain k-space distance to reach valence band. This reduces the recombination probability of the excited electron and the hole. The layered structure of BiOCl can polarize the related atoms and orbitals by supporting large enough space,¹⁰ resulting in an increase in the electron–hole separation efficiency through the appeared dipole, and thus the enhanced photocatalytic activity was observed.

4. Conclusions

Layered Bi_1−xEu_xOCl compounds were prepared by conventional solid-state method at relatively low temperature. Rietveld refinement results confirmed that these compounds have a tetragonal structure. Based on the CIE chromaticity diagram, the Eu³⁺-activated BiOCl phosphors exhibit red luminescence under visible excitation. In order to investigate the nature of the luminescence behavior of Eu³⁺ in BiOCl, the Judd–Ofelt intensity parameters were calculated from the emission data. The J–O intensity parameters increase with the increase in Eu³⁺ concentration which reveals the high covalence of the metal–ligand bonds and more distortion of the symmetry of Eu³⁺ sites in BiOCl matrix. The high asymmetric ratio confirmed that the Eu³⁺ ions locate at sites with low symmetry and without an inversion center. The present Eu³⁺-activated BiOCl phosphors show higher lifetime, quantum efficiency, stimulated emission cross-section, gain bandwidth and optical gain parameters for ⁵D₀ → ⁷F₂ transition in comparison with different host matrix such as oxides and glasses. We achieved 100% RhB dye degradation using Bi_1−xEu_xOCl (x = 0, 0.01, 0.03 and 0.05) catalysts within 45, 40, 35 and 30 min under visible illumination. The apparent first-order rate constant was found to be in the decreasing order of Bi_0.95Eu_0.05OCl > Bi_0.97Eu_0.03OCl > Bi_0.99Eu_0.01OCl > BiOCl which followed the 1st order kinetic mechanism. The high efficiency was ascribed to their indirect band gap, layered structure and ability to utilize broad bands in the solar spectrum. The present work provides an effective and economic material ideal for developing red component in white LEDs, red lasers and environment remedy applications.

Acknowledgements

Authors are thankful to DST-IRHPA for FESEM and EDX facility. One of the author R. Saraf would like to thank Indian Academy of Sciences for awarding summer research fellowship 2014.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10163d

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