Synthesis and Characterization of Highly Crystalline Bi-Functional Mn-Doped Zn₂SiO₄ Nanostructures by Low-Cost Sol–Gel Process

Abstract

Herein, we demonstrate a process for the synthesis of a highly crystalline bi-functional manganese (Mn)-doped zinc silicate (Zn₂SiO₄) nanostructures using a low-cost sol–gel route followed by solid state reaction method. Structural and morphological characterizations of Mn-doped Zn₂SiO₄ with variable doping concentration of 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 wt% were investigated by using X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) techniques. HR-TEM-assisted elemental mapping of the as-grown sample was conducted to confirm the presence of Mn in Zn₂SiO_4. Photoluminescence (PL) spectra indicated that the Mn-doped Zn₂SiO₄ nanostructures exhibited strong green emission at 521 nm under 259 nm excitation wavelengths. It was observed that PL intensity increased with the increase of Mn-doping concentration in Zn₂SiO₄ nanostructures, with no change in emission peak position. Furthermore, magnetism in doped Zn₂SiO₄ nanostructures was probed by static DC magnetization measurement. The observed photoluminescence and magnetic properties in Mn-doped Zn₂SiO₄ nanostructures are discussed in terms of structural defect/lattice strain caused by Mn doping and the Jahn–Teller effect. These bi-functional properties of as-synthesized Zn₂SiO₄ nanostructures provide a new platform for their potential applications towards magneto-optical and spintronic and devices areas.

1. Introduction

Multifunctional nanomaterials play an important role in different fields of applied sciences including semiconductor electronics, solar energy, memory devices, and optoelectronics devices for development of efficient nanosensors and nanosystems [1,2,3,4,5,6]. Therefore, tailoring of these nanomaterials for their desired properties is extremely important, especially for applications in advanced portable devices [7,8,9,10,11,12]. There are several nanomaterials which show tunability with size, shape, and properties with variation of doping concentration [9,13,14,15,16]. Zinc silicate especially (Zn₂SiO₄) is a promising candidate for applications in flat panel optical displays, UV detectors, gas sensors, adsorption of toxic ions from water, and thin film electro-luminescence devices applications [17,18,19]. Furthermore, Zn₂SiO₄ structures have received significant attention due to their unique wide range of multiple color luminescent properties, wide band gap (5.5 eV), and excellent chemical stability [20]. The willemite Zn₂SiO₄ host material is heavily used for applications in the field of phosphors for light emitting devices [21].The Zn₂SiO₄ is a natural orthosilicate with a phenacite-like structure in which Zn–O tetrahedral and Si–O tetrahedral share corners to form hollow ‘tubes’ parallel to [0001] planes; and it is a well-known host material for potential applications in the area of phosphors for optoelectronics and blue, green, and red color light emitting devices [1,3,20]. The incorporation of transition metal centers and rare earth ions (luminescence centers) in a Zn₂SiO₄ host lattice results in a various interesting luminescent property. Various methods such as vapor phase synthesis, high temperature solid-state reaction, and ball milling have been previously reported for synthesis of the Zn₂SiO₄ nanostructures. However, these reported methods require complicated procedure, huge thermal energy, high reaction temperature and time, and the synthesized materials exhibit irregular micro structure morphologies which is unsuitable for applications in devices where precise control over shapes and properties is required.

Recently, several new methods have been reported for the synthesis of Zn₂SiO₄ including sol–gel, polymer precursor, spray pyrolysis, and hydrothermal to get different luminescence emissions for various potential application [21,22,23,24]. R. Krsmanovic et al. prepared Sm- and Tb-doped Zn₂SiO₄ particle by sol–gel method to get reddish orange and pseudo white emissions [25]. H. Wang et al. and R. Pozas et al. prepared the Zn₂SiO₄ with Co dopant using hydrothermal method to obtain the blue pigments [26,27]. K Omri et al. reported the yellow phosphor of Mn-doped Zn₂SiO₄ using sol–gel method [28]. G. Gao et al. reported the Eu-doped Zn₂SiO₄ nanoparticles prepared by using solid state method to get red color emission [29]. Moreover, K. Omri et al., M. Hafeez et al., and W. Zheng et al. recently reported the diamagnetic and ferromagnetic properties from Mn-doped Zn₂SiO₄ nanostructures [30,31,32].

In the present investigations, we synthesized Mn-doped Zn₂SiO₄ nanostructures phosphors with various doping concentration using the cost effective and facile sol–gel method. Crystal structure, morphology, and photo luminescence properties of Mn-doped Zn₂SiO₄ nanostructures were studied. Effect of doping concentration of Mn in Zn₂SiO₄ on its structural, morphological, luminescence, and magnetic properties was also studied. Based on this, a correlation of lattice strain with observed magnetic properties and luminescence behavior is presented.

2. Materials and Methods

2.1. Materials

Manganese acetate (Mn(CH₃COO)₂; 99.999%), zinc chloride (ZnCl₂; 99.999%), tetraethyl orthosilicate (Si(OC₂H₅)₄; 99.999%), and ethanol (C₂H₅OH; 99.999%) were procured from Sigma Aldrich Chemicals Pvt Ltd., Delhi, India. Deionized water (DI) was obtained from a Direct-Q 3UV Millipore water purification system. Copper wires and silver paste were used to make the connection and contact, respectively.

2.2. Synthesis of Zn₂SiO₄ Nanoparticles

A flow chart for the preparation of Mn-doped Zn₂SiO₄ nanoparticles is shown in Figure 1. Mn-doped Zn₂SiO₄ nanoparticles were synthesized by sol–gel method under ambient atmosphere followed by thermal annealing. In the first step, 2.0433 mg of ZnCl₂ aqueous solution was prepared using 16 mL H₂O and 64 mL EtOH. Then, “x” amount (x = 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, 2.0 wt%) of Mn(CH₃COO)₂ was dissolved into main solution. The prepared solution was mixed in a magnetic stirrer at 400 rpm at room temperature for 1 hr. Further, 2 mL of TEOS was slowly added drop by drop using micropipette while stirring after 1 h. Then, the solution was heated at 70 ⁰C resulting, initially in the formation of a white gel and then into a powder when the gel was left overnight. This white powder was then calcined at 1000 °C in box furnace under natural atmospheric pressure for 6 h. Finally, the resultant white Zn₂SiO₄ nanoparticles was washed with distilled water and collected for characterization.

2.3. Measurement and Characterizations

The structural properties of the prepared samples were measured by X-ray diffraction (XRD) technique using a Rigaku (ultima IV model, Japan) benchtop X-ray diffractometer equipped with a monochromatic Cu-Kα radiation (λ = 1.541 Å) X-ray source. The morphology of the samples was analyzed by high resolution transmission electron microscopy (HR-TEM) JEM 2100F operating at 200 keV and the attached GATAN (Version: GMS 2.32) digital-micrograph software. Elemental mapping was performed using Oxford attachment in HRTEM. The room temperature photoluminescence (PL) investigation was performed using a Perkin Elmer LS-55 fluorescence spectrophotometer with a Xenon (Xe) lamp as the source of excitation. The magnetic properties at room temperature were examined by a SQUID magnetometer (MPMS7 Tesla, Quantum Design, San Diego, CA, USA) in the field range ± 2T with a step size of 400 Oe and a field scan rate of 100 Oe per minute.

3. Results and Discussion

3.1. Formation of α-Phase Zn₂SiO₄

The chemical reaction process and crystal growth mechanism for formation of α-phase Zn₂SiO₄ nanostructures can be explained by the following chemical reaction steps:

$$ZnC{l}_2+H_{2}O \stackrel{70 °\mathrm{C}}{\to }Zn\left(OH\right)Cl+HCl$$

(1)

$$Zn\left(OH\right)Cl+C_2{H}_{5}OH \stackrel{70 °\mathrm{C}}{\to }{H}_5{C}_{2}O-Zn-OH+C{l}_2 \uparrow$$

(2)

$$H_5{C}_{2}O-Zn-OH+Si{C}_8{H}_{20}{O}_4\stackrel{70 °\mathrm{C}}{\to }Z{n}_4{\left(OH\right)}_{2}S{i}_2{O}_7.H_{2}O+C{O}_2\uparrow$$

(3)

$$Z{n}_4{\left(OH\right)}_{2}S{i}_2{O}_7.H_{2}O \stackrel{550 °\mathrm{C}}{\to }Z{n}_4{\left(OH\right)}_{2}S{i}_2{O}_7$$

(4)

$$Z{n}_4{\left(OH\right)}_{2}S{i}_2{O}_7\stackrel{750 °\mathrm{C}}{\to }\mathsf{\beta }-phase Z{n}_{2}Si{O}_4$$

(5)

$$\mathsf{\beta }-phase Z{n}_{2}Si{O}_4\stackrel{1000 °\mathrm{C}}{\to }\mathsf{\alpha }-phase Z{n}_{2}Si{O}_4$$

(6)

The reaction mechanism leading to the formation of zinc silicate can be summarized in the following steps. In the first step, ZnCl₂ reacts with H₂O and C₂H₅OH at 70 °C and is transformed into Zn (OH)Cl. Then, this Zn(OH)Cl reacts with TEOS, and hydrated hemimorphite leading to formation of Zn₄(OH)₂Si₂O₇.H₂O powder. This powder, when annealed at ~550 °C forms dehydrated hemimorphite Zn₄(OH)₂Si₂O₇. Dehydrated hemimorphite Zn₄(OH)₂Si₂O₇ is transformed into β-phase ZnSi₂O₇ by annealing at ~750 °C and into α-phase Zn₂SiO₄ by annealing at 1000 °C.

3.2. Structural Analysis

The X-ray diffraction (XRD) pattern of the annealed Zn₂SiO₄ nanostructures collected at room temperature is shown in Figure 2. All diffraction peaks in the pattern can be indexed to the crystalline willemite (α-Zn₂SiO₄, JCPDS card 37-1485) structure of rhombohedral system with lattice parameters a = b = 13.938 Å and c = 9.31 Å, α = β = 90°, γ = 120° revealing the crystallinity of the samples [33].

The average crystallite size of the as grown samples was calculated using the Williamson and Hall method which is a combination of the Scherrer’s equation for size broadening and the Stokes and Wilson expression for strain broadening [34,35,36,37].

$$\mathsf{\beta }\cos {\theta }_B=\frac{k\lambda }{D}+4\mathsf{\eta }\sin {\theta }_B$$

(7)

where β is the full width at half maximum of diffraction peak, η is the strain in the crystallites, and D is the crystallites size [38,39,40]. The constant k is typically close to unity and ranges from 0.8 to 1.39. To calculate the strain and crystallite size, a graph was plotted in between $\mathsf{\beta }\cos {\theta }_B$and $\sin {\theta }_B$. The strain (η) was then extracted from the slope and the crystallite size was determined from the y-intercept. The average crystallite size of the undoped Zn₂SiO₄ nanostructures was found to be 28 nm.

The XRD patterns of Zn₂SiO₄ samples doped with various doping concentration of Mn are shown in Figure 3a. The result shows that the diffraction peaks of nanostructures shifted towards lower angle with an increase in Mn concentration. This shift in the diffraction peak can be attributed to lattice expansion as a result of induced strain from Mn doping. The likely position of Mn substitution in Zn₂SiO₄ lattice is shown in the Figure 3b and it is proposed that Mn²⁺ occupies the Zn²⁺ position. Furthermore, a close view of unit cell can be easily seen from right inset (top and bottom) of Figure 3b.

The different doping concentration of Mn substitution in Zn₂SiO₄ lattice and lattice expansion can be easily understood by strain effect as shown in Figure S1. N. Tripathi et al., D.H. Hwang et al., and M.S. Kwon et al. observed the same trend where strain induced effect of Mn in the Zn₂SiO₄ nanocrystals enhances the green emission efficiency with short decay time [41,42,43]. The result clearly indicates that the different doping concentration significantly affects the packing of unit cell such as dislocation of host Zn site. Further, the dislocation is confirmed by a Williamson and Hall in the form of strain developed in the lattice which results in slight angle shifts in XRD spectra as shown in Figure 3a. Moreover, depending on different θ positions, the separation of size and strain broadening analysis is done using Williamson and Hall method. Crystal imperfections and distortion of strain-induced peak broadening can be related by the well-known Williamson and Hall method shown in Figure S1.

Figure 4 shows the calculated strain and crystalline size of Zn₂SiO₄ as a function of % of Mn doping. It is clearly observed that the strain increases with an increase in Mn doping in the host lattice. The obtained results also indicate that crystallite size increases due to differences in ionic radius between Mn²⁺ and Zn²⁺. (The ionic radii of Zn and Mn are 0.083 and 0.091 nm, respectively).

3.3. Morphology Analysis

The morphology and microstructure of the as-synthesized Zn₂SiO₄ nanostructure was analyzed by high-resolution transmission electron microscope (HR-TEM). Figure 5a shows the nanorod-like morphology of 0.2 wt% Mn-doped Zn₂SiO₄. The estimation of the dimension of individual nanorods is presented in the inset of Figure 5a. Figure 5b shows the magnified view of nanorod-like morphology of the individual rod with 620 nm length and 300 nm width. Its selected area electron diffraction (SAED) pattern recorded without sample tilting is shown in the inset of Figure 5b. The SAED result reveals that the nanorod is crystalline in nature. The HR-TEM image of the nanorod in Figure 5c shows regular lattice fringes with d-spacing of 0.26 nm, which is in agreement with the inter planar spacing of (300) plane in XRD pattern. Figure 5d shows the magnified view of nanoflake-like morphology of individual rod with length of 970 nm and width of 370 nm. The inset in Figure 5d shows the clear lattice fringe of nanorods which corresponds to 113 plane.

The elemental mapping of the as-synthesized 0.2 wt% Mn-doped Zn₂SiO₄ samples was done by energy dispersive spectroscopy (EDS) added to the HR-TEM instrument. The spectra obtained from the EDS mapping are shown in Figure 6 which confirms the Mn doping in Zn₂SiO₄ lattice. The elements present in two different regions of the sample (Figure 6a) are shown in the Figure 6b–e for first region and 6f–i for the second region highlighting the elemental homogeneity of the grown samples. The elemental maps micrographs of Mn, Zn, Si, and O show a uniform distribution over the entire morphology of Mn-doped Zn₂SiO₄ nanostructure in both the regions. A quantitative analysis carried out to estimate the weight and atomic percentage of samples is shown in the Figure 6j. The observed spectra show that peak detected at 1.740 eV, 0.525 eV, 8.639 eV corresponds to the Kα₁ of Si, Kα₁ of O, and Kα₁ of Zn, respectively. Further, the peaks detected at 9.572 eV and 1.012 eV correspond to the Kβ₁ of Zn, and Kα₁, kβ₂ of Zn, respectively. The peak observed at 5.899 eV correspond to the Kα₁ of Mn and peak, 6.491 eV, 0.636 eV corresponds to the Kβ₁ of Mn and Lα₁ and Lα₂ of Mn, respectively [30,44].

3.4. Photoluminescence Analysis

Figure 7a shows the photoluminescence (PL) spectra of Mn-doped Zn₂SiO₄ measured with an excitation wavelength of 259 nm at room temperature (Figure 7b). The emission spectra of the samples show broad green emission centered at 521 nm which arises due to Mn²⁺ ion transitions. The green emission from the sample can be understood in terms of various transition states of Mn element. The green PL emission results transition from the ⁴T₁(⁴G) excited state to the ⁶A₁ (⁶S) ground state in 3D orbital electrons of the Mn²⁺ ion [45]. The variation in the PL intensity is due to the size variation and defect level introduced from the doping. It is noted that the nanostructure has large surface area, which significantly impairs the PL intensity by introducing a correspondingly large number of defects. The transition of free electrons in the conduction band by the charge transfer transition of Mn is as described below

$${\mathrm{Mn}}^{2+}\left(3{\mathrm{d}}^5\right)\to {\mathrm{Mn}}^{3+}+\left(3{\mathrm{d}}^4\right)+{\mathrm{Eg}}_{\mathrm{CB}}$$

(8)

These excited free electrons further relax from the conduction band to the ⁴T₁(⁴G) lowest excitation energy level of the Mn²⁺ ion via non-radiative transitions, followed by a radiative transition from the ⁴T¹(⁴G) level to the ⁶A₁(⁶S) ground state or valance state which results in the green PL emission peak centered at 521 nm. A schematic image of the corresponding transition is shown in the Figure 7b. Changing the doping % of Mn²⁺ resulted in only a very small shift in peak position. However, the PL intensity shows nonlinear trend with doping concentration. For better understanding, the PL peak intensity as a function of x at 521 nm was also plotted along with the FWHM as a function of x and is shown in Figure S2. It is clearly observed that the PL intensity first increases with doping concentration with a maxima at 0.2 wt% and then decreases with a further increase in doping. As Mn concentration increases from 0.03 wt % to 0.2 wt%, stress in the crystal lattice also increases as discussed in the XRD section and also shown in Table S1. The decrease of intensity with higher concentration of Mn doping is due to activator concentration quenching and the decrease in the inter-ionic distance between two adjacent Mn²⁺ ion pairs. This reduction of inter-ionic distance between the Mn²⁺ ion pairs leads to exchange of excited free electron migration from one Mn²⁺ ion to another, which causes non-radiative transitions and reduces the emission efficiency and the demonstration at 0.4 wt% Mn doped Zn₂SiO₄ is shown in Video V1 [46].

3.5. Magnetic Analysis

The M-H plot of Mn-doped Zn₂SiO₄ nanorods at room temperature is shown in Figure 8a. It is observed that the value of magnetization increased with increase in the Mn doping concentration, which may be due to the increased amount of impurity in the host lattice of Zn₂SiO₄ [47,48]. It was observed in the present study that pristine Zn₂SiO₄ exhibits only diamagnetic behavior at room temperature. Further, observed remanence and coercivity for higher Mn content indicates nucleation of ferromagnetic characteristic, whereas diamagnetic behavior was found to be still prevailing for lower Mn²⁺ content. The M-H loops of all the samples were plotted after substituting the diamagnetic contribution of each of the sample [36,37,49]. Magnetization of Zn₂SiO₄ with increased Mn doping can be explained in terms of the Jahn–Teller effect which relates the suppressing the spiral spin structure of Zn₂SiO₄ by the incorporation of Mn at Zinc sites [30,31]. It is also proposed that Mn cations presented in the Zn₂SiO₄ are in mixed valence states of 2+, 3+, and 4+ which could support ferromagnetic coupling through Zener double exchange interaction. In low concentration, the Jahn–Teller effect would be expected to cause a reduction in the tendency toward ferromagnetism since it leads to a splitting of the states. The splitting will reduce the interactions between Mn pairs since hopping requires an extra energy [9,14,34,35,50]. On the other hand, the interaction between Mn–Mn pairs at realistic distances is sufficiently large that it dominates over the Jahn–Teller effect. The interactions between Mn atoms is not greatly affected by lattice relaxations and there is always a clear tendency for ferromagnetic alignment of Mn pairs as evidenced in the present study for higher Mn concentrations. Figure 8b shows value of magnetization with increase in Mn doping wt% concentration into Zn₂SiO₄ host lattice.

4. Conclusions

We successfully synthesized highly crystalline bi-functional Mn-doped Zn₂SiO₄ nanostructures with various doping concentration using a low-cost sol–gel process. The X-ray diffraction results confirmed the rhombohedral phase of Mn-doped Zn₂SiO₄ nanostructure. The HR-TEM results confirm the nanorod-like morphology. The PL results show the strong green emission peak centered at 521 nm under 259 nm excitation wavelength. The optimization for highest photoluminescence intensity was achieved by varying different concentrations of Mn-doping in Zn₂SiO₄ lattice. We observed that 0.2 wt% Mn doping in Zn₂SiO₄ nanostructure showed the highest PL intensity. Magnetic studies confirmed the magnetic nature of Mn-doped Zn₂SiO₄ nanostructures and magnetization increased with increase of Mn concentration. Thus, the bi-functional behavior of Mn-doped Zn₂SiO₄ nanostructures offers a new avenue to further exploit its potential application in magneto-optical devices.