Characterization of Waste Material Derived Willemite-Based Glass-Ceramics Doped with Erbium

1Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia 2Department of Physics, Faculty of Science, Universiti Putra Malaysia (UPM), 43400 Serdang, Selangor, Malaysia 3Materials Processing and Technology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia 4Institute of Microengineering and Nanoelectronics (IMEN), Level 4, Research Complex, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia


Introduction
In recent years, glass-ceramics doped with rare-earth ions have attracted significant attention because of their wide application in the field of laser technology and optical communications [1].Thus far, various glass-ceramics (silicates and phosphates) have been utilized as the proper hosts for the rare-earths.However, poor chemical stability and low transition temperature of phosphates have restricted the use of phosphates as the host for these ions [2].In contrast, silicates demonstrate much superior chemical stability when compared with the phosphates and therefore were advantageous for ion-exchange technique to produce optical waveguides [2].In addition, silicate materials are more economic due to the traditional technology used for telecommunication application.Recently, different rare-earth ions have been studied as dopants in silicate-based glass-ceramics [3][4][5].Among the doped materials, erbium oxide (Er 2 O 3 ) is one of the promising materials for the use as a dopant in the silicatebased glass ceramics.Besides being a rare-earth oxide, it was anticipated that Er 2 O 3 is unique due to the existence of trivalent charge (Er 3+ ) on the cation, which may lead to the formation of active site for the charge transfer, eventually causing a transition of the absorption bands from 4f 4 I 13/2 to 4 I 15/2 [1].The anticipation provoked, having known that the unique feature found in the rare-earth cerium oxide (CeO 2 ) was attributed to the existence of Ce 3+ in the oxide as a result of phase transition from Ce 4+ to Ce 3+ [6,7].This property has brought wide applications of the CeO 2 not only as an optical material but also as a high dielectric constant gate oxide used in metal-oxide-semiconductor based devices [8,9].To date, the Er 3+ -doped silicate has been widely used in developing elements and sources for telecommunication systems since the wavelength region for telecommunications, which is around 1500 nm, coincides with the inter-4f 4 I 13/2 → 4 I 15/2 transition of Er 3+ ion at 1535 nm [1].
Amongst the silicate-based glass ceramics, willemite has attained considerable attention over the last 180 years as the zinc silicate material.Following its discovery, interest in the zinc silicate was focused toward its occurrence, crystallography, and application as an industrial material [1].In addition, glass-ceramics are typically obtained by heat treatment of glassy precursors or by sintering green bodies consisting of glassy and crystalline materials [10,11].A number of literatures have been reporting on the preparation of Er 3+ :SLS (Soda Lime Silicate) glass and zinc silicate composites.Kaewwiset et al. [12] reported the preparation of Er 2 O 3 doped SLS glass using a solid-state reaction at 1200 ∘ C. In another study, Cho and Chang [13] used similar solidstate reaction at 1400 ∘ C to produce manganese-doped zinc silicate green phosphor with appropriate oxides.To the best of our knowledge, despite extensive researches on producing willemite glass-ceramics, doped with rare-earths and metals using pure materials [14,15], there is no report regarding the doping of Er 3+ into willemite, which is produced from waste material.Therefore, it is of interest to investigate in present study for the production of willemite-based glass ceramics from waste material before being doped with 3 wt% of Er 3+ by systematically studying the effects of various sintering temperatures on structural, chemical, and morphological properties of prepared samples.

Preparation.
First, SLS glass bottles were cleaned and crushed properly to <63 m using a mortar and pestle.Then, the SLS glass and ZnO powders were carefully mixed at a weight ratio of 1 : 1 during a ball milling process for 24 h.The mixture powder was afterward put in an alumina crucible and melted in an electric furnace at 1400 ∘ C for 4 h in air environment.Subsequently, the samples were quenched immediately into water in order to gain glass frit.The frit was again ground to <63 m particles followed by a heat treatment at 1000 ∘ C in order to produce the willemite glassceramics.Unlike previous techniques of doping rare-earths [16], in this work 3 wt% of Er 2 O 3 was doped in the last stage whereby the willemite had fully crystallized in the glassceramics.The mixing happened in a mortar and pestle for 1 h, followed by ball milling for the next 24 h.The obtained powder was pressed to pellets with 12 mm diameter and subsequently sintered at different temperatures (500-1100 ∘ C) for 4 h at a heating and cooling rate of 10 ∘ C/min.Willemite (glass frits)

Characterization. X-ray diffraction (XRD) analysis was
performed by X-ray diffractometer (PANAalytical (Philips) X'Pert Pro PW3050/60) with CuK radiation (Bragg angle 2 in the angular range of 20 to 80 ∘ ) equipped with a copper X-ray tube and scintillation detector.To identify crystalline phases in the analyzed samples, the powder diffraction file (PDF2) was used.Fourier transform infrared (FTIR) spectra of the samples were recorded over the range of 0-4000 cm −1 on a Perkin-Elmer 1752X spectrophotometer (Waltham, MA) using a KBr disc method.In order to obtain good quality FTIR spectra, the samples were crushed in an agate mortar to obtain fine powder.Microstructural observations were performed using a field-emission scanning electron microscopy (FESEM) modelled FEI NOVA NanoSEM 230 and equipped with energy dispersive X-ray (EDX) spectrometer.Finally, grain size of the samples was measured using mean linear intercept method.2. The bands detected in the FTIR spectra have been summarized in Table 1.The experimental data gained in present work were compared with those of some related vitreous and crystalline compounds [21][22][23][24][25].It was deduced that the FTIR spectra of glass-ceramic matrix (Zn 2 SiO 4 :Er 2 O 3 ) consisted of eight wide and strong transmission bands, positioned at 395 cm −1 , 485 cm −1 , 595 cm −1 , 702 cm −1 , and 902 cm −1 .Normally, the metal-oxide vibrations would occur below 1000 cm −1 .The transmission peak in the lower wave number of 395 cm −1 was assigned to the asymmetric deformation of SiO 4 group (] 4 SiO 4 ) [26].The band positioned at 485 cm −1 corresponded to SiO 4 asymmetric stretching vibration (] 3 SiO 4 ) [27,28].The peak at 595 cm −1 was marked as the asymmetric stretching vibration of ZnO 4 group (] 3 ZnO 4 ) [29][30][31][32][33]. The distinct band placed at 702 cm −1 could be assigned as the totally symmetric stretching vibration of SiO 4 group (] 1 SiO 4 ) [34][35][36].The broad band located at 902 cm −1 corresponded to asymmetric stretching of SiO 4 (] 3 SiO 4 ) [17].

Results and Discussion
4000 3500 3000 2500 2000 1500 1000 500 0 Transmission (a.u.) Willemite (1000 ∘ C) As a whole, the presence of vibrations associated with SiO 4 and ZnO 4 groups would clearly suggest the formation of Zn 2 SiO 4 phase [37].The increase of sintering temperature from 500 to 1100 ∘ C would cause an increase in intensities of the FTIR bands.On the other hand, at higher temperature of 1100 ∘ C, the band situated at 700 cm −1 disappeared.A compositional evaluation of the FTIR properties of the [(ZnO) 0.5 (SLS) 0.5 ] 1−x [Er 2 O 3 ] x system in present work proposed that the presence of Er 3+ ions would affect the surroundings of Si-O bond and the trivalent Er 3+ would occupy the position.The literature reported that FTIR spectrum of crystalline (cubic) Er 2 O 3 would show characteristic transmission bands located at 469 cm −1 , assigned to the Er-O bond vibrations [38].Hence, the band located at 469 cm −1 would belong to the vibrations of the Er-O group present in the studied glass-ceramic system.In addition, the most significant modification produced by the addition of Er 3+ and the increase of sintering temperature of the studied samples was related with a drop in intensity of the FTIR band located at 702 cm −1 .Results indicated that the addition of Er 2 O 3 into willemite as well as the increase of sintering temperature had led to a reduction in the content of SiO 4 group.As it can be observed in Figure 3(a), the crystallized particles aggregated and were irregular in shape.In addition, the willemite surface showed a homogenous distribution of rhombohedral-like particles.By observing Figure 3(b), it could be seen that when the willemite was doped with 3 wt% of Er 3+ and sintered at the temperatures of 700 ∘ C, the Er 3+ ions were dispersed on surface of the willemite and did not contribute to the ceramic's lattice.On the other hand, after increasing the sintering temperature up to 800 ∘ C and 900 ∘ C (Figures 3(c) and 3(d)), surface morphology of the impact powder became granular and appeared to have a homogenous distribution, which could be due to a reaction of the Er 3+ with the willemite.However, agglomeration of the willemite occurred while approaching the temperature of 1000 ∘ C (Figure 3(e)) whereby the clustered Er 3+ remained on the surface.Subsequently, at the temperature of 1100 ∘ C (Figure 3(f)), the willemite melted and had completely covered the dopant particles.As a conclusion, it could be said that the amorphous phase was reduced during sintering.In addition, as the temperature was increased to 1000 ∘ C, less grain boundaries were present due to grain growth.However, at the temperature of 1100  calculate grain size of the sample due to the melting process.A summary of the obtained grain sizes of the samples has been presented in Figure 4.The distribution of Er 3+ in the willemite matrix was confirmed by FESEM-EDX analysis, as shown in Figure 5.The EDX spectrum distinctly showed the presence of Zn, O, Si, Al, and Er, in addition to the Au peak, resulting from the sputtered gold on the surface during the characterization process.However, the detection of Al peak was due to the SLS glass from the waste material.The sintering temperature was at or above 1200 ∘ C [39,40] in most of the recently reported investigations; however, it had been decreased to a lower temperature (900 ∘ C) in the present work.

Surface Morphological Analysis.
In addition, other impurities, such as ferum and ferum oxide were not detected for all of the investigated samples.

Conclusions
Structural changes induced by sintering temperature in the willemite-based glass-ceramic doped with trivalent erbium ions were well reflected in X-ray diffraction patterns, FTIR spectra, and FESEM images.Sintering of the samples produced homogeneity.The presence of willemite and Er 3+ was being evidenced by X-ray diffraction analysis.The FTIR data suggested the presence of SiO 4 , ZnO 4 , and Er-O structural units.The microstructural study proposed that, by increasing the sintering temperature, the grain size increased.One of the most dominant results of this work was regarding the sintering of the synthesized pellets at a relatively lower temperature (900 ∘ C), which resulted in the formation of polycrystalline erbium-doped willemite samples.The structural properties of synthesized samples were comparable with previously reported materials.

Figure 2 :
Figure 2: FTIR spectra of the willemite-based glass-ceramic samples doped with 3 wt% Er 2 O 3 sintered at the temperature of 500-1100 ∘ C for 4 h.

Table 1 :
[20][18][19][20]Zn 2 SiO 4 :Er 2 O FTIR transmission features and their assignments for oxide [(ZnO) 0.5 (SLS) 0.5 ] 1− [Er 2 O 3 ]  system.The results agreed well with the findings from previous researchers[17][18][19][20], wherein rhombohedral crystalline phase of willemite was formed after the mixing of ZnO and SLS glass followed by a heat treatment at 1000 ∘ C[20].By doping with 3 wt% of Er 2 O 3 into the willemite and sintering at 500 ∘ C, an additional peak, ascribed to (222) diffraction plane of cubic crystalline phase of Er 2 O 3 , appeared at 2 = 29.29 ∘ .The detection of single diffraction peak associated with Er 2 O 3 indicated that the Er 2 O 3 did not react with the crystalline Zn 2 SiO 4 and therefore remained as an unreacted phase at 500 3.1.StructuralStudies.basedglass-ceramics.It was observed that main diffraction peak of erbium oxide (Er 2 O 3 ) was located at diffraction angles (2) of 29.29 ∘ while the XRD pattern shown for glass frits indicated amorphous nature of the glass frits.In the willemite sample sintered at 1000 ∘ C shown in the third XRD pattern, diffraction peaks detected at 2 of 22.14 ∘ , 25.61 ∘ , ∘ C. As the sintering temperature was increased from 500 to 700 ∘ C, the intensity decreased gently with a dramatic fall when temperature reached 800 ∘ C. Beyond 900 ∘ C, it was seen that the diffraction peak of crystalline Er 2 O 3 started to diminish, leaving only the main diffraction peaks attributed to the willemite.The occurrence of this might be due to a complete entering of the Er 3+ cations into the Zn 2 SiO 4 lattice.Phase formation of the willemitebased glass-ceramics was further confirmed using FTIR.3.2.FTIR Analysis.FTIR spectroscopy was performed to obtain fundamental information concerning the functional groups of the studied glass-ceramics.FTIR spectra of the willemite-based glass-ceramics with the content of 3 wt% Er 2 O 3 sintered at different temperatures are presented in Figure