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Abstract
This article reports the synthesis and purification of nanosilica (SiO2) derived from the husk of some rice varieties; Faro-44, Faro-46, Faro-52, Nerica-I, and Nerica-II. The husks were pretreated with H2O and HCl. From the measurement results, Nerica-I rice husks produced the highest silica yield and best photophysical properties. Thus, the husks of Nerica-I rice were further pretreated with H3PO4 to additionally remove more mineral impurities and the derived nanosilica labeled nSiO2 was selected to serve as a host matrix for doping with varying concentrations of Sm3+ions. The doped nanosilica (nSiO2:xSm3+; x = 0.01, 0.05, and 0.1 molar ratios) maintained the amorphous structure of the undoped nanosilica, while the surface morphology as revealed by SEM and TEM indicates agglomerates of spherical nano-sized particles with average particle size measuring 21 nm. The Sm3+-doped nanosilica shows a large surface area measuring 198.0 m2/g. The photoluminescence excitation spectra show that near UV and blue LEDs can effectively be used as excitation sources to produce yellow and yellow-green emissions from Sm3+ ion-doped nSiO2 suitable for display applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silica is an important material that is extensively used for several commercial applications such as molecular sieves, catalyst, biomedical, semiconductor, and ceramics applications [1] and as a source for the synthesis of very pure silicon, silicon nitride, silicon carbide, magnesium silicide, and other applications [2]. Silica is usually synthesized from different sources and among these sources, rice husk (RH) has proven to be the most vital and cheaper source nowadays. RH also contains approximately 15 to 28 wt % of the silica in the hydrated amorphous form [3–5]. The amount and quality of silica in RH are dependent on rice variety, location, soil composition, climate, and synthesis methods [6–8]. Though the rice variety factor is key to the preparation and inherent properties of silica from RH, much research is focused on the purity of the silica. Compared with other silica sources like sand, bentonite, and diatomaceous earth, rice husk has a very small amount of contaminants. In this regard, solar grade silicon with a purity of approximately 99.9999% was synthesized from rice husk as a biogenic silica source [1].
Globally, nanotechnology has aroused considerable interest because materials at the nanoscale show excellent and unique physical, chemical, mechanical, biological, and optical properties compared to their bulk counterpart [9–11]. Ceramic nanoparticles such as silica, zirconia, alumina, silicon nitride, titania, silicon carbide, and so on with enhanced properties have been the subject of extensive studies due to their wide range of applications [9]. The production of nanosilica from RH is a simple process compared to other conventional production technologies such as vapour phase reaction, sol-gel process among others. According to Jung et al (in [12]), the simplicity in the production of silica from RH is because rice husk has exclusive nanoporous silica layers.
Despite the great advances provided by silicon science to the microelectronics industry, the application in the area of photonics (the so-called Si photonics), is at the infancy stage [13]. Si – rare-earth technology is expected to provide superior attributes in photonics technology by combining the superior electronic characteristics of Si-based materials with the unique optical properties of RE-ions. Incorporation of RE ions into silica and silicate host matrices is of interest for a wide variety of applications especially in the areas of optics, catalysis, and biomedicine [14]. Trivalent Sm3+ ions with 4f6 electronic configuration is an important optical activator that exhibits unique and bright reddish-orange emission due to 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, 11/2) transitions. However, there are few reports on Sm3+-doped SiO2; Sendova-Vassileval et al. [15] studied the photoluminescence in Sm3+ co-sputtered SiO2 thin films, and Tan et al. [16] synthesized and characterized SiO2: Sm3+ nanotube arrays with 1.06 µm laser antireflective property. Both reports did not use RH as a silica source.
Motivated by the need to (i) find a rice variety that can provide high SiO2 yield and excellent photophysical properties such as stable structure and enhance optical properties, (ii) improve SiO2 purity from RH, and (iii) demonstrate strong luminescence of Sm3+ in silica derived from the husk of rice varieties that can provide the best characteristics mentioned in (i) above. Herein, we report extraction of nanosilica from RH of some rice varieties found in Nigeria and studied the photophysical properties of the prepared nanosilica. From the analysis of the measurements carried out on silica obtained from the rice varieties, we selected husk from the rice variety that provides the best photophysical attributes. Besides, the nanosilica from this variety is subjected to different chemical treatments to further remove mineral impurities and the final product doped with Sm3+ ions to produce yellow and yellow-green emitting nanophosphors.
2. Experimental
2.1. Materials
Rice husks of five varieties of rice which are common in the middle belt region of Nigeria namely; Sipi (Faro-44), Turn 2 (Faro-46), zemuje (Faro-52), Osi (Nerica-I), and Mass (Nerica-II), all of Oryza sativa species were collected from Wurukum rice milling station, Makurdi, Benue State, Nigeria. The varieties were chosen because they are the common species cultivated in the state. Indeed, the amount and quality of silica derived from rice husk are dependent on factors such as location, soil composition, climate, rice variety, and synthesis methods [17]. The rice husk used in this research work were cultivated in the same location in Makurdi. Therefore, the climatic factor and to a greater extent the soil composition are the same. Thus, only the variety factor, in this case, can influence the amount and quality of silica in the husks of rice of the different varieties under consideration in this research work. Sm (NO3)3.6H2O, HCl (37 wt %), and H3PO4 (87 wt %) were purchased from Sigma Aldrich while sodium hydroxide and ammonium hydroxide were purchased from Meck. The products were used as received.
2.2 Preparation of nano-SiO2 (SiO2) from varieties of rice
Nanosilica from rice husk was synthesized using the same procedure reported in our previous research report [18,19]. Typically, the husks of rice varieties were collected, sieved, washed with deionized water, and dried in air for 24 hours under a shield to drain the water content. A 50 g of dried RH from Nerica-I was mixed with 200 mL of 10-wt % HCl and boiled for 2 hours under constant stirring. The boiled rice husks were rinsed with deionized water to a neutral pH of 7 and then dried in an oven at 110 °C for 24 hours. The dried HCl pretreated rice husk sample was calcined in a box furnace at a temperature of 550 °C for 6 hours with a ramping rate of 10 °C/min. The sample was allowed to cool down to room temperature, stored in airtight bottles, and labeled SiO2. The procedure was repeated to produce silica (SiO2) from the husk of other rice varieties selected in this research work.
2.3 Purification and synthesis of nanosilica doped with Sm3+ (nSiO2: Sm3+)
Rice husk of Nerica-I rice was selected for use in this research work after the characterization of the five silica samples derived from selected rice varieties. The reason for using the Nerica-I variety will become more apparent from the results. The Nerica-I rice husk gave the highest yield of silica and was more thermally stable in the amorphous state. Further purification was done as follows; the HCl pretreated sample of the Nerica-I rice husk (in section 2.1) was subjected to phosphoric acid pretreatment by soaking into 0.8 M phosphoric acid (H3PO4) for 72 hours and then dried in an oven at 110 °C for 24 hours. The sample was calcined in a box furnace at the temperature of 550 °C (ramping rate was 10 °C/min) for 6 hours to obtain rice husk nanosilica (nSiO2) and it was allowed to cool naturally overnight. The rest of the procedure followed that described in section 2.1. A portion of the H3PO4 acid purified nanosilica was stored in an airtight container for characterization and another portion was used for the synthesis of nSiO2: Sm3+ phosphors.
The nSiO2: Sm3+ nanophosphors were synthesized using the sol-gel process. A portion of the H3PO4 acid purified nanosilica was dissolved in 50 ml of deionized water contained in a 250 ml three-neck-round-bottom flask arranged in an oil bath and fixed to a reflux condenser attached to a thermostat. 2 ml of NH4OH was then added to the solution slowly with vigorous stirring. Then 10 ml of 0.01M (1.7784g) Sm(NO3)3.6H2O solution was added to the mixture slowly. The pH of the mixture was controlled at 10 and constant stirring continues. The solution was kept at 40 °C for 21/2 hours for the reaction to complete. The mixture was washed and centrifuged with abundant deionized water first and later with abundant ethanol for 5 times at 4500 rpm. The precipitate was dried in an oven at 120 °C for 3 hours. The process was repeated to produce nSiO2 samples doped with 0.05 and 0.1 molar ratios of Sm3+ions. All three samples were annealed at 900 °C for 8 hours at a heating rate of 5 °C /min and the final products were stored in airtight bottles for characterization.
2.4 Characterization techniques
All prepared samples were characterized by X-ray diffraction (XRD) using PANalytical X’Pert PRO with Cu Kα radiation (λ=0.15406 nm) to determine their structures. The morphology of samples was characterized using Zeiss Auriga Field Emission Scanning Electron Microscope (FESEM) and the composition was obtained by using Energy Dispersive X-ray (EDX) attached to the FESEM and X-ray fluorescence spectroscopy (XRF) model minipal4 pw425/45B © 2005 with Rh tube. The Jeol JEM-2100 Transmission Electron Microscope (TEM) was used to determine the microstructure as well as the particle sizes of the samples. The optical properties of the samples were obtained using the Perkin-Elmer Lambda 1050 UV/Vis/NIR spectrophotometer equipped with an integrated sphere at room temperature. Fourier Transform Infrared (FTIR) spectra of samples were obtained using the Perkin-Elmer Ate-FTIR 100 spectrometer. The surface area was measured using Micromeritics TRISTAR II 3020 BET Surface area analyzer. Photoluminescence (PL) measurements were done at room temperature using an F-7000 spectrophotometer.
3. Result and discussion
3.1 Photophysical characteristics of SiO2 derived from husks of rice varieties
3.1.1 Structure and chemical composition
The XRD patterns of nanosilica (SiO2) derived from the husk of rice varieties; Faro-44, Faro-46, Faro-52, Nerica-I, and Nerica-II, shown in Fig. 1 exhibit a single broad peak from 15 to 30° with a maximum at 2θ ∼22.19°, which indicates amorphous silica [20,21]. There is also a crystalline peak common to all the patterns at about 2θ ∼26.48° which is assigned to the (101) plane belonging to the quartz phase of silica. However, the XRD pattern of the SiO2-Faro-52 sample shows several low-intensity crystalline peaks, which are assigned to the quartz phase of silica according to the JCPDS file no.79-1910. This sample (SiO2-Faro-52) is predominantly crystalline. From the XRD result, it could be observed that SiO2-Nerica-I, SiO2-Nerica-II, and SiO2-Faro-46 have shown better stability in the amorphous phase at the prevailing synthesis temperature.
3.1.2 Microstructure, morphology, and chemical composition
The SEM images of SiO2 derived from the husk of rice varieties presented in Fig. 2(a)-(e), respectively show the formation of agglomerates of irregular shape particles. Larger particles agglomerate can be seen, particularly in Fig. 2(e). The silica particles in Fig. 2(a) appear to have smaller size distributions. The EDX results of SiO2 samples of the rice varieties under investigation in this research are displayed in Fig. 2(f) and Table 1. The result shows that all the samples are composed mainly of Carbon (C), Oxygen (O), and silicon (Si). Besides, elements such as Os, Ca, Hg, Ir, and Na are impurities that were not removed during acid pretreatment were also recorded. The weight percent of each of the elements present in the samples is obtained from Fig. 2 and shown in Table 1. The results show that among all the samples, SiO2-Nerica-I has the highest yield of Si and O and consequently SiO2.
Fig. 2. (a)-(e) SEM images of SiO2 derived from husk of Faro-44, Faro-46, Faro-52, Nerica-1, and Nerica-II rice varieties respectively, and (f) EDX of SiO2 derived from the husk of Nerica-1 rice.
Table 1. EDX result for SiO2 derived from husk of different varieties of rice.
Figure 3(a) - (e) display the TEM images of SiO2 from the husk of the rice varieties. The TEM micrographs show the formation of spherical nanosized particles. The average particle sizes obtained from TEM images are in the nanometer range (39 nm) i.e., 27 nm (Faro-44), 66 nm (Faro-46), 34 nm (Faro-52), 39 nm (Nerica-I), and 28 nm (Nerica-II). The SiO2-Faro-44 sample has the smallest average size, while SiO2-Faro-46 presented the largest average size distribution. The size distribution of the particles of silica derived from the husk of Nerica-1 rice is shown in Fig. 3(f). From Fig. 3(f), it could be observed that the histogram shows an average particle size of approximately 39 nm (Nerica-I).
Fig. 3. (a)-(e) TEM images of SiO2 derived from the husk of Faro-44, Faro-46, Faro-52, Nerica-I, and Nerica- II respectively, and (f) Histogram of SiO2 derived from husk of Nerica-I rice.
3.1.3 Fourier transforms infrared (FT-IR) analysis SiO2-Nerica-I sample
The FT-IR spectrum of SiO2-Nerica-I displayed in Fig. 4(a) contains two absorption bands; a weak absorption band at ${\sim} $802 cm-1 which is assigned to Si-O-Si symmetrical stretching vibrations [22–25] and a broad absorption band which is much stronger with a valley at 1062 cm-1, assigned to the asymmetrical stretching mode of Si-O-Si [21,22,25]. Pure and relaxed silica having a bond angle of 144° has a narrow absorption band and valley at approximately 1078 cm-1. The absence of other strong peaks at 3000 cm-1 indicates that the O-H stretching vibration of the adsorbed H2O molecules has been successfully removed during heat treatment.
3.1.4 Surface area measurement
The result of Brunauer-Emmett-Teller (BET) and Barret–Joyner–Halenda (BJH) measurement carried out on SiO2-Nerica-I sample (see Fig. 4(b)), show that this sample has 69.68, 5.48 × 10−9 m, 161.38 m2/g, 2.2 × 10−7 m3/g for BET constant (C), pore size, surface area, and Barret–Joyner–Halenda (BJH) pore volume, respectively. This result is similar to the result obtained by Liu et al. [26] and it indicates mesoporous silica.
3.1.5 Optical properties
Diffuse reflectance spectroscopy (DRS)
The result of diffuse reflectance spectra of SiO2 derived from the husk of the five rice varieties (Faro-44, Faro-46, Faro-52, Nerica-1, and Nerica-II) presented in Fig. 5(a) shows absorption bands located at about 206, 218, 251, 320, 475 and 519 nm for all varieties except SiO2-Faro-52 and SiO2-Faro-44, which show the absence of the absorption band at 218 nm. The absorption bands at 206 and 218 nm are assigned to near band absorption of silica. The bands at 251, 320, 475, and 519 nm are attributed to impurities/defect states in silica or centers such as oxygen vacancies [27,28].
Fig. 5. (a) Diffuse reflectance spectra (b) Bandgap of SiO2 derived from the husk of different rice varieties.
The energy band gaps of all the silica samples were calculated from the relationship between Kubella-Munk and Tauc relations [29] in Eq. (1).
where A is a proportional constant, n is the nature of transition, hν is photon energy, Eg is the energy bandgap, and F(R) is the Kubelka–Munk coefficient calculated from reflectance data, R using Eq. (2). The energy band gap is obtained at the point on the energy axis where the extrapolated linear portion of the [F(R)hν]2 vs hν plot is such that [F(R)hν]2 = 0. From Fig. 5(b), it could be observed that the energy bandgap of the samples SiO2-Faro-44, SiO2-Faro-46, and SiO2-Faro-52, are 5.52, 5.29, and 5.39 eV respectively. Meanwhile, both SiO2-Nerica-1 and SiO2-Nerica-II have the same energy bandgap located at 5.18 eV. The bandgap of SiO2 samples is in the range of 5.18-5.52 eV higher than the 3.54 eV bandgap of TEOS derived silica [30].Photoluminescence studies
The photoluminescence excitation (PLE) spectra of SiO2-Faro-44, SiO2-Faro-46, SiO2-Faro-52, SiO2-Nerica-I, and SiO2-Nerica-II samples obtained by monitoring emission at 435 nm (Fig. 6(a)) present bands with peaks at about 275, 335, and 365 nm. The 365 nm excitation band has been previously reported for SiO2 to be due to defects in the silica [27,28,31]. It could be observed from Fig. 6(a) that the Faro-46 sample (red line) has a single peak in the range 320-370 nm, by marked contrast; other samples displayed two peaks in this range. The observed single peak from the Faro-46 sample could be due to the large particle size distribution as can be seen in Fig. 3(a)-(e). Silica derived from husk of Faro-46 has average particle size of 66 nm, which is larger than the average particle sizes obtained from the other respective varieties. The PL spectra (Fig. 6(a)) obtained at 365 nm excitation are continuous, broad, and cover the range 380-650 nm. SiO2 is known for broadband emission and the emission is in the blue-green region due to the presence of defect sites in the large bandgap [19,27]. The deconvoluted PL spectrum of SiO2 derived from the husk of Nerica-I rice variety (Fig. 6(b)) gave 6 Gaussian peaks with a maximum at about 402 nm (i), 412 nm (ii), 427 nm (iii), 447 nm (iv), 475 nm (v) and 517 nm (vi). The emission bands may be due to the presence of some defect states such as oxygen-deficiency centers (ODCs), and non-bridging oxygen hole centers (NBOHC), etc. in the energy bandgap of the silica [27,28,31]. The peaks at 475, and 517 nm, may be due to carbon impurities [5,23,26,28,32] or surface defects such as silanone and dioxasilane [33–35]. By looking at the PL spectra in Fig.7a, it is clear that the bandwidth ($\Delta \lambda$) of the emission spectra change with the variety it was derived from. The bandwidth ($\Delta \lambda$) of SiO2 samples derived from the husk of rice varieties are 80, 90,104, 111 nm, and 111 nm for SiO2-Faro-44, SiO2-Faro-46, SiO2-Faro-52, Nerica-I, and Nerica-II, respectively. The variation in the bandwidth may be related to the nature of defects and the kind of impurities that are present in the husk of the rice variety. According to Fourier's analysis, the bandwidth when considered in terms of frequency ($\Delta v$) is governed by the relation in Eq. (3).
where $\Delta t$ is the average lifetime of the light-emitting particle in its excited state. Thus, the bandwidth is related to the average lifetime by Eq. (4). where c is the speed of light. From Eq. (4), it is observed that an increase in bandwidth (decrease in energy) increases the average lifetime of the emitting particle. This means that the longer a particle stays excited, it dissipates the larger portions of its energy and then finally emit smaller energy (PL energy) [36,37].Fig. 6. (a) PL excitation and PL of nanosilica (SiO2) derived from the husk of five different rice varieties where x = SiO2 and (b) Deconvoluted Gaussian peaks of SiO2 derived from the husk of Nerica-I variety.
The Commission Internationale de I’Eclairage (CIE) chromaticity diagram in Fig. 7. The Commission Internationale de I’Eclairage (CIE) chromaticity coordinates were obtained by inputting PL data in OSRAM color calculator software, version 7.77. From Fig. 7, there are no significant variations in the emission color from the SiO2 derived from the husk of all the varieties in this research. The results of all the nanosilica samples show emission in the blue with that of SiO2- Faro-52, SiO2-Nerica-I, and SiO2-Nerica-II showing a slight shift from the deep blue position to sky-blue light. Table 2 presents the CIE coordinates of the blue emissions from SiO2 obtained from the husk of varieties of rice.
Fig. 7. CIE chromaticity coordinate diagram of nanosilica (SiO2) derived from the husk of different rice varieties.
Table 2. CIE coordinates of nSiO2 derived from the husk of different rice varieties.
It is noted that going by the observed results that SiO2 derived from the husk of Nerica-I rice does not only produce the highest yield of silica but also shows better stability in the amorphous phase and remarkable photophysical properties, explains our choice of this variety for the remaining part of this work.
3.2 Photophysical characteristics of Sm3+-doped nSiO2 derived from the husk of purified Nerica-I rice variety
On account of producing the highest SiO2 yield, superior optical properties, and microstructural stability as demonstrated in DRS, PL, XRD, SEM, and TEM analysis above, the sample (RH Nerica-I) was selected for further studies. Rice husk ash from the untreated RH was used as the control and the XRF result presented in Table 3 shows the presence of SiO2 and 18 impurities. By pretreating RH Nerica-I with water, impurities such as MgO, Al2O3, BaO, and Cr2O3 were removed and the yield of silica increased from 77% of the untreated to 86.2%. More impurities including P2O5, MgO, V2O5, Rb2O, ZrO2, Y2O2, and Cr2O3 were removed when RH Nerica-I was pretreated with HCl and the yield increased to 99.30% indicating the effectiveness of HCl to remove mineral impurities and increase the purity of the produced nanosilica. Further treatment with phosphoric acid did not increase the silica yield (99.30%) but remove more impurities such as BaO and Eu2O3. Table 3 presents the composition of the sample (RH Nerica-I) from different pretreatment chemicals. From now onward, the silica derived from RH Nerica-I rice pretreated with HCl and soaked in Phosphoric acid (nSiO2) will be called undoped nSiO2 or host.
Table 3. XRF of SiO2 derived from different pretreatment chemicals
3.2.1 Structural analysis of undoped and Sm3+-doped nSiO2 phosphors
Figure 8 shows XRD patterns of nSiO2 (undoped) and nSiO2: xSm3+ (x = 0.01, 0.05 and 0.1 molar ratios). All samples exhibit an amorphous structure with a broadband peak in the range of 15 to 40°. However, the samples doped with higher concentrations of Sm3+ ions (x = 0.05 and 0.1 molar ratios) show peak shifts to larger 2θ values with a center maximum at about 2θ = 29.31° indicating an increase in disorderliness in the structure of the samples because of higher doping concentrations. The stability in the amorphous phase is associated with the removal of metal (alkali) impurities such as K+ that promotes crystallinity [17,32]. The treatment of rice husk is known to significantly reduce crystallinity and promote the amorphous phase [17,38].
Fig. 8. XRD result of RHS doped with different concentrations of samarium ions (nSiO2: xSm3+; x=0.01, 0.05. and 0.1 molar ratios).
3.2.2 Morphology and chemical composition
Figure 9(a) and 9(b) display the SEM micrographs of the host and nSiO2: 0.05 Sm3+ respectively, which show the presence of agglomerate of spherical nano-sized particles. Figure 9(c) & 9(d) show EDX spectra of nSiO2 (host) and nSiO2: 0.05 Sm3+ nanophosphors to be composed of Si, O, C, Hg, and Si, O, C, Sm, Al, respectively. The carbon impurities are from either the host (nSiO2) or carbon tape used to hold the samples. The elements such as Hg and Al are impurities from the rice husk used. The elemental maps displayed in Fig. 9(e-i) show homogeneous distribution of the major elements (Si, O, C and Sm) in nSiO2: 0.05 Sm3+ nanophosphor.
Fig. 9. (a) SEM image of nSiO2 (host), and (b) SEM image of nSiO2: xSm3+ (x = 0.05 mol %), (c) EDX spectrum of nSiO2 (host), (d) EDX spectrum of nSiO2: xSm3+ (x = 0.05 mol %), and (e–i) Elemental mapping of nSiO2: xSm3+ (x = 0.05 mol %).
Figure 10(a) and (b) show the TEM images of nSiO2 (host), and nSiO2: xSm3+ (x = 0.05 mol %), respectively. The TEM image of the host (Fig. 10(a)) shows dispersed spherical nanosized particles with an average particle size of 30 nm. Meanwhile, the TEM image of nSiO2: 0.05 Sm3+ shows clusters of spherical nanosized particles with an average size of 21 nm.
3.2.3 FTIR analysis
Figure 11(a) and (b) are the Fourier transform infrared (FT-IR) spectra of host and nSiO2: 0.05 Sm3+, respectively. Figure 12(a) shows the host contains two absorption bands; a weak absorption band at ${\sim} $802 cm-1 which is assign to Si-O-Si symmetrical stretching vibration [21–24,39] and a strong broad absorption band with a valley at 1072 cm-1 assigned to the asymmetrical stretching mode of Si-O-Si [20,21,23]. The FT-IR spectrum of nSiO2: 0.05 Sm3+ (Fig. 11(b)) has only one broad absorption band, which is strong with a valley at 1063 cm-1 indicating unrelaxed silica. Pure and relaxed silica should have a narrow band with its valley at approximately 1078 cm-1 [23]. The intense broadband observed in the FT-IR spectra of nSiO2: 0.05 Sm3+ is due to the doping of Sm3+ ions in the sample and it overwhelmed the 802 cm-1 band which is assigned to Si-O-Si symmetrical stretching vibrations. The 1063 cm-1 absorption band is assigned to the asymmetrical stretching mode of Si-O-Si [20,21,23]. The absence of a strong peak at 3000 cm-1 indicates that adsorbed H2O molecules have been successfully removed during heat treatment.
3.2.4 Surface area measurement
Figure 12(a) and (b) display nitrogen adsorption-desorption isotherm curve of nSiO2 (host) and nSiO2: 0.05 Sm3+, respectively. From Fig. 12, the surface areas of nSiO2 (host) and nSiO2:0.05 Sm3+ are 219 m2/g and 198 m2/g, respectively. These results show that doping reduces the surface area. The reduced surface area is due to the observed increased agglomeration of particles in the SEM and TEM images (Fig.9b & 10b) [40]. The surface areas of nSiO2 and nSiO2: 0.05 Sm3+ are larger than that of commercial silica which has a specific surface area of 53.80 m2/g due to reduced particle size [41,42].
3.2.5 Diffuse reflectance spectroscopy
Figure 13(a) shows diffuse reflectance spectra of the host (nSiO2) and nSiO2: xSm3+ (x = 0.05 molar ratios) samples. From Fig. 13(a), the spectrum of the host sample shows absorption bands located at about 206 nm (i), 250 nm (ii), 318 nm (iii), 472 nm (iv), and 516 nm (v), which can be assigned to near band absorption (206 nm) and defect-related absorption bands (250, 318, 472 and 516 nm) [27,28,31]. The Sm3+-doped samples show absorption bands at 304 nm, 320 nm, 345 nm, 362 nm, 376 nm, 402 nm, 438 nm, 474 nm, 528 nm, and 562 nm which can be exclusively assigned to the 6H5/2→3H9/2, 4F11/2, 3H7/2, 4D3/2, 4D5/2, 6P3/2, 4G9/2, 4I11/2, 4G7/2, and 4G5/2 transitions of Sm3+, respectively [43]. It can be observed from Fig. 13(a) that the hypersensitivity transition 6H5/2→6P3/2 has the strongest intensity among all the transitions. Figure 13(b) displays the Kubella-Munk and Tauc-Wood plots for energy bands of the host and the Sm3+- doped samples. The energy band gaps were evaluated from the plots ${[{F(R )h\nu } ]^2}vs\,h\nu$as shown in Fig. 14(b). From Fig. 13(b), it could be observed that the host (nSiO2) has three sets of energies 3.90 eV, 4.96 eV, and 5.43 eV. Meanwhile, the energy bandgap for nSiO2: 0.05Sm3+ nanophosphor is at 5.53 eV. It can be observed that the energy bandgap increases after doping. The expansion of the energy bandgap after doping may be attributed to the Burstein–Moss effect. This effect is observed when the bandgap of a material is increased as the absorption edge is shifted to higher energies as a result of some states close to the conduction band being populated by doping [44–47].
Fig. 13. (a) Diffuse reflectance spectra and (b) energy band gaps of nSiO2 (host) and nSiO2: 0.05 Sm3+.
Fig. 14. PLE of (a) nSiO2 (host) and (b) nSiO2: xSm3+ (x = 0.01, 0.05 & 0.1 molar ratios) nanophosphors (λem=568 nm).
3.2.6 Photoluminescence
Figure 14(a) and 14(b) present the Photoluminescence excitation (PLE) spectrum of the host (nSiO2) and nSiO2: xSm3+ (x = 0.01, 0.05, and 0.10 molar ratios) samples, respectively. The PLE spectrum of the host (Fig. 14(a)) obtained by monitoring emission at a wavelength of 435 nm shows two prominent peaks at 275 and 365 nm which are attributed to defects centers in the SiO2 (host) [48,49]. Besides the defect-related bands, there is also broadband in the range of 200 to 260 nm, which can be assigned to the O2- to Si charge transfer state (CTS) [43,50–52]. The PLE spectra of all the doped samples (Fig. 14(b)) obtained by monitoring the emission at 568 nm show the CTS band and three other excitation lines at 320, 365, and 400 nm assigned to 6H5/2→4F11/2, 4F9/2, and 4K11/2 f-f transitions of Sm3+ ions, respectively [50,51]. The position of this CTS band in Fig. 14(b) is similar to that in Fig. 15(a) except for the peak shift from 260 nm to 280 nm after doping with Sm3+ ion. These results indicate that near UV and blue LEDs can be effectively used as excitation sources to produce yellow and yellow-green emission from Sm3+ ions doped-nSiO2.
Fig. 15. (a) Photoluminescence (PL) emission spectrum and (b) Deconvoluted bands of host (nSiO2) (c)&(d) PL nSiO2: xSm3+ (x = 0.01, 0.05 & 0.1 molar ratios), (d) Overlap of excitation spectrum of nSiO2: Sm3+ and emission spectrum of host (nSiO2).
Figure 15(a) shows the emission (PL) spectrum for the nSiO2 (host), obtained under 365 nm excitation wavelength. The emission band covers the blue-green regions of the electromagnetic spectrum indicating the overlap of several emission bands due to the presence of different defect centers. The broad emission band was deconvoluted (Fig. 15(d)) into five bands using Gaussian fits; 411 nm (i), 431 nm (ii), 453 nm (iii), 480 nm (iv), and 528 nm (v). The emission bands at 411, 431, and 453 nm are due to defects in the silica [48,27,49] while the peaks at 480, and 528 nm may be due to organic impurities [23,26,32,48] or surface defects such as silanone and dioxasilane [33–35]. Fig. 15(b) and Fig. 15(c) show the emission spectra of nSiO2: xSm3+ (x = 0.01, 0.05, and 0.10 molar ratios) at the indirect and direct excitation wavelengths of 365 nm and 400 nm, respectively. The obtained PL spectra of Sm3+ doped silica at both direct and indirect excitations consist of three main lines at 568, 605, and 652 nm which are attributed to the intra-4f shell transitions from the excited level 4G5/2 to ground levels 6H5/2, 6H7/2, 6H9/2 respectively [50,51,53,54]. From the selection rules, 4G5/2$\to $6H5/2, and 4G5/2 $\to $6H7/2, transitions are magnetic and electric dipole transitions whereas the 4G5/2$\to $6H9/2 transition is purely an electric dipole transition [55,56].
The intensities of the transitions 4G5/2$\to $6H5/2, and 4G5/2 $\to $6H7/2, are stronger than the intensity of 4G5/2$\to $6H9/2 transition with that of 4G5/2$\to $6H5/2 transition being the strongest for all the concentrations under both 365 and 400 nm excitation wavelengths. In literature, it is shown that a low doping concentration of activator ions gives weak emission (luminescence) while excess doping results in luminescence quenching [51]. In this research, under the indirect excitation at 365 nm, the highest concentration (0.1molar ratio of Sm3+ ions) has the strongest emission lines indicating energy transfer and no luminescence quenching. However, under the direct excitation at 400 nm, the PL intensity is strongest at 0.05 molar ratios. The highest concentration 0.1 molar ratio has the least emission intensity indicating luminescence quenching.
Figure 15(d) shows the overlap of the emission band of the host (nSiO2) and excitation band of the samarium-doped sample between 390-500 nm. The overlap of these bands is an indication of the possibility of energy transfer from the donor (host nSiO2) to the acceptor Sm3+ [57,58]. The energy transfer mechanism for SiO2:Sm3+ nanophosphor is depicted in the energy diagram shown in Fig. 16. It can be noticed from Fig. 16 that when the host is excited, the electrons absorb the pumping energy and electrons are excited to higher energy states or localized energy levels created by defects or impurities. This lead to three important processes; first, there is a resonance energy transfer from the defects in the host to samarium ions [57,58] and secondly, the nonradiative relaxation via localized energy levels or states created by defects or impurities finally recombination with holes in the valence band thereby generating overlapping blue emission bands with a broad feature [58,59]. Thirdly, direct excitation of Sm3+ ions at 400 nm followed by relaxation to grounds states emitting the characteristic colours of the dopant ion. The low extinction coefficients of rare earth ions makes direct photoexcitation to be inefficient [60], so the common strategy to improve the absorption of rare earth ion is using energy transfer from a sensitizer (host or organic complex) to the rare earth ion; the weak narrow absorption band of the rare earth ions is supplemented using these compounds that present broad intense absorption band. From Fig. 15(c), direct excitation of Sm3+ at wavelength of 400 nm produces relatively poor emission (luminescence) with quenching at x = 0.1 molar ratio. Meanwhile, indirect excitation through the host (nSiO2) leads to energy transfer from the host to the Sm3+ producing stronger luminescence without quenching at x = 0.1 molar ratio. This result indicates that high emission intensity could be obtained by increasing Sm3+ doping concentration beyond 0.1 molar ratio via host photoexcitation.
The luminescence emission colour of nSiO2: xSm3+ (x = 0, 0.01, 0.05 & 0.1 molar ratios) nanophosphors prepared from rice husk was investigated using the 1931 commission international de l’e’clairage (CIE) system. Figures 17(a) and 17(b) present the CIE 1931 chromaticity coordinate (x = 0.1604, y = 0.1068) diagram of the host (nSiO2) showing blue light emission. The CIE chromaticity diagram of SiO2: xSm3+ nanophosphors are also presented in Fig.17a and 17b and the corresponding chromaticity coordinates are displayed in Table 3. The values are close to the coordinates of equal energy yellow light and yellow-green light respectively. The chromaticity coordinates are also close to that of the commercial yellow-emitting phosphor (Zn, Cd)S:Ag+ (x=0.49, y=0.48) [61,62]. This indicates that the nSiO2: xSm3+ (x = 0, 0.01, 0.05 & 0.1 molar ratios) nanophosphors have potentials to be used as an additional phosphor in the typical tricolor; Red, Green, and Blue (RGB) field emission displays (FED). The yellow colour will enhance the colour range and chromaticity saturation thereby improving the display quality of the full-colour FED. The display quality of full-color FEDs is enhanced because the four-color; Red, Green, Blue and Yellow (RGBY) phosphor system, displays more natural color than three primary (RGB) system [63]. Besides, the four-color system has a higher “information density” which is the pixels per unit area compared with three-color system [63]. The yellow colour can also serve as a complementary colour for white generation in lighting technology.
Fig. 17. CIE chromaticity coordinate diagram of nSiO2 and nSiO2: x Sm3+ (x = 0, 0.01, 0.05& 0.1 molar ratios) at excitation wavelengths (a) 365 nm (b) 400 nm.
Figure 17(a) and Fig.17b display the CIE chromaticity coordinate diagram of PL of nSiO2: xSm3+ (x = 0, 0.01, 0.05, and 0.10 molar ratios) at excitation wavelength of 365 nm and 400 nm, respectively. Table 4 contains the CIE chromaticity coordinates of the same samples. The results show that nSiO2: 0.01Sm3+ and nSiO2: 0.1Sm3+ which are of very low and high concentrations of samarium ions respectively give yellow light emission under indirect excitation at 365 nm while nSiO2: 0.05 Sm3+ which is a sample with moderate concentrations of samarium ions gives yellow-green light emission under the same excitation wavelength. The emission colour of light for the lowest concentration (0.01 molar ratios) is yellow due to weak emission resulting from deficiency of activator ions. An increase in the concentration to 0.05 molar ratio increases the population of the activator ions thereby resulting in a shift to yellow-green light. However, the color of the emitted light shifted back to yellow at higher concentrations (0.1 molar ratios) due to luminescence quenching. This concentration quenching is caused by the increase in the ion-ion interaction aggravated by the shorter distance between interacting activator ions as the concentration increases [53]. However, the reverse of this behavior is the case under direct excitation at 400 nm as the sample with the lowest concentration (0.01 molar ratios) and highest concentration (0.1 molar ratios) shift to yellow-green towards white while the moderately doped sample (0.05 molar ratios) shift to yellow.
Table 4. CIE chromaticity coordinates values and correlated color temperature (CCT) of nSiO2: xSm3+ (x = 0.01, 0.05, 0.1 molar ratio) nanophosphors.
4. Conclusion
The rice husks of all the varieties of rice under consideration in this research produced a high yield of nanosilica with the silica from the Nerica-I rice variety having the highest yield. The nanosilica derived from the husk of Nerica-I rice shows outstanding structural, morphological, and optical properties, which show it is a promising candidate for phosphors, and can serve as a host matrix for different kinds of rare-earth ions. The nSiO2: xSm3+ (x = 0.01, 0.05 & 0.1 molar ratios) phosphors have been successfully synthesized by the sol-gel method. The nanophosphor has a large surface area and exhibits an amorphous structure. It also shows strong absorption bands, which could be direct or indirect. The photoluminescence excitation spectra show that near UV and blue LEDs can effectively be used as excitation sources to produce yellow and yellow-green emissions from Sm3+ ion-doped SiO2 nanophosphors. The nanophosphors are suitable for display applications.
Funding
Tertiary Education Trust fund, Abuja, Nigeria.
Acknowledgment
This work was supported by the Tertiary Education Trust fund, Abuja, Nigeria.
Disclosures
The author declares no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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