Optical properties of heat-treated hierarchical structure of Eu³⁺ modified C-CaIn₂O₄ of small core–shell crystallites

Abstract

A natural aloe-vera gel is promptly bridging the Eu³⁺, Ca²⁺ and In³⁺ species in a bio-complex of a polymer network, so as it yields a sample Eu³⁺:CaIn₂O₄ of small crystallites bonding over a grafted C-sp² surface layer when it is burnt in a self-propagating combustion in air. The Eu³⁺ doped C-CaIn₂O₄ sample appears in a core–shell structure in part of the carbon forming a conjoint surface layer (shell) thereon of the individual crystallites in a hybrid nanostructure. The results are analyzed in terms of XRD patterns, phonon bands (in IR and Raman spectra), XPS bands, and hierarchical microstructure in the samples prepared with different Eu³⁺ dosages in finely tuning the microstructure and optical properties as useful for an efficient phosphor, optical display system, visible lasers, energy-converters, photocatalysts, optical imaging, medical tools, and several others. A partial Eu³⁺ → In³⁺ doping in a crystal lattice CaIn₂O₄ clearly reflects in a marked expansion of the lattice, as much as 2.2% found in the Eu³⁺ content progressively raised to 2.0 mol% in a tailored hybrid composite structure. An inbuilt C-sp² shell structure of the crystallites is characterized in terms of its characteristic phonon bands in a conjoint polymer network, which are accounted well in modified XPS bands in the surface species.

Graphic abstract

1 Introduction

The rare-earth ions R³⁺ (for example, R = Nd, Sm, Eu, Tb, Dy, Ho, Er, etc.) are extensively used as an efficient optical dopant to incorporate tunable electronic energy-levels in many engineering oxides like ZnO, ZrO₂, TiO₂, Y₂O₃, Al₂O₃, ZnAl₂O₄, CaAl₂O₄, CaIn₂O₄ etc., so that the later promptly absorb and emit radiation of selective bands in UV, visible, as well as near infrared (NIR) regions [1,2,3,4,5,6,7,8,9,10]. Tailored optical and other engineering properties in such R³⁺-doped oxides have wide spread applications in photonics, lasers, optical witches, optical sensors, and photocatalysis. A unique feature of rare-earths is a specific shell structure of the valence shell of 4fⁿ–5s²p⁶ electrons in which the valence 4fⁿ electrons (n varies from 1 to 14 over the rare-earth series) are firmly shielded by a completely filled shell of the 5s²p⁶ electrons, which firmly control them from local microscopic interactions with neighboring ions in a crystal lattice. As a result, they promisingly retain in the characteristically sharp microscopic energy-levels and those do not change much either by changing the mother compound or a foreign host in a case of a doped material of overall an insulator or a semiconductor. Further, a spin–orbit coupling and local crystal fields result in a series of ^2S+1L_J multiplet bands. Each ^2S+1L_J electronic state in a multiplet can spilt into its ‘J’ levels of a maximum of ‘2J + 1’ sub-energy levels accounting in the local crystal field symmetry of the color-center, as described with different examples elsewhere [1,2,3].

CaIn₂O₄ is one of wide-bandgap semiconductor metal oxides with a energy gap of around 3.9 eV is used conventionally as an optical host in doping various transition or rare-earth elements [11]. An increasing market demand for such optical devices like LEDs, CRTs, PDPs and FEDs is giving new route for better light emitting materials [12]. Traditionally, it has already been synthesized using the solid-state reaction route. Certain demerits like, elevated temperatures of annealing and long reaction time for this material has made a trend for new type of materials and synthesis route. To improve the preparation process, some other approaches have been suggested [10]. To prevent these issues researchers shifted towards the new trend of biosynthesis of materials that incorporates plants as a source of reactant. The advantage to this route is being a simple method that requires aloe vera gel (green gel containing polysaccharides) instead of any chemical, additionally, time and temperature is less compared to traditional routes and lastly this route gives light quenching at higher concentration ~ 1 mol% which is larger than other chemical routes. In this research paper, we have incorporated green gel from aloe vera plant, which yields crystallites of controlled size and shape [13, 14]. Hence, it is helpful in synthesizing of different nanomaterials.

2 Selection of material and its compositions

2.1 Optimizing Eu³⁺ doping in a core–shell nanostructure

Selective dosage of optical Eu³⁺ ions of x = 0.1, 0.5, 1.0, 1.5 and 2.0 mol% have been chosen tentatively in doping a sample C-CaIn₂O₄ made of small crystallites in view of finely tuning the light absorption and emission properties in a sample xEu³⁺:C-CaIn₂O₄ of a hybrid composite structure. A larger Eu³⁺ content is not required over here as it leads to adversely quench the light emission in the UV–visible–NIR regions [15,16,17]. A technical advantage with a core–shell structure is that it tailors multiple surfaces in small crystallites so as it is promptly promoting light absorption and photocatalytic properties useful for various applications of a functionalized hybrid composite nanostructure.

2.2 Synthesis procedure

A hybrid composite xEu³⁺:C-CaIn₂O₄ was prepared of small core–shell crystallites using the raw materials (1) europium acetate, (2) calcium acetate, (3) indium acetate, and (4) aloe-vera gel in an aqueous medium. A stock indium acetate solution was made of a moderate 0.25 M concentration in a 100 mL of distilled water and admixed in a freshly obtained aloe-vera gel in a typical 1:1 ratio in synthesizing a basic C-CaIn₂O₄ phase. Then, a 25 mL of an aqueous solution of calcium acetate (1.0 M concentration) was mixed into the above solution by slowly stirring on a magnetic stirrer to disperse it in a homogenous nanocolloid sample. Finally, a europium acetate solution in water was added in a required volume by continuing the stirring. A sample so obtained was then heated at 80–90 °C in air, forming a viscous hydrogel gel in a hydrothermal reaction of the Eu³⁺, Ca²⁺, and In³⁺ species in small tissues of the aloe-vera gel of a biogenic complex. It was observed that an initial greenish color of the sample had changed progressively into a dark brownish color in account of a hydrothermal reaction of the cations with bio-polymers present in the aloe-vera gel over 80–90 min of a reaction time. A mixed hydrogel so obtained stands pretty stable in its visible color for a month or so in open air. A final product xEu³⁺:C-CaIn₂O₄ was recovered as on a dried hydrogel (at room temperature) was burnt in a self-propagating combustion with camphor in open air. A recovered powder in this way was pulverized by grinding in a mortar by a pestle for 10–20 min, and then was washed in a hot water in eliminating any free carbon present in the sample. Different batches of the samples thus were prepared in the Eu³⁺ doping varied as 0, 0.1, 0.5, 1.0, 1.5 and 2.0 mol% in a hybrid composite structure. These samples were annealed at 400–600 °C in a muffle furnace for 1–2 h in air in finely refining a core–shell structure.

2.3 Measurements of C-CaIn₂O₄ structure and properties

Using an X-ray diffractometer, X’pertPRO PANalytical having an X-ray beam of CuKα of λ = 0.15410 nm wavelength, to scan the XRD patterns over 20°–80° of diffraction angle 2θ to delineate a single-crystalline Eu³⁺:C-CaIn₂O₄ of as-burnt as well as the annealed powders. The data were collected slowly at small 2θ-intervals under 0.01° in resolving the weak intensity peaks. Surface topology, morphology and size in the Eu³⁺:C-CaIn₂O₄ samples were studied with field emission scanning electron microscopy (FESEM) using a ZEISS EVO 60 FESEM at 5–20 kV acceleration voltages. HRTEM images, SAED patterns and lattice images were studied of the samples mounted on a carbon coated copper grid, using an analytical TEM of FEI—TECNAI G2 20S-TWIN operating at 200 kV. The C-sp², as breeds a 2D-network (amorphous) on coherent Eu³⁺:CaIn₂O₄ facets, exhibits multiple D and G-bands in Raman spectra over 1200–1800 cm⁻¹ as studied by exciting the samples at 514.5 nm by an Ar⁺ ion laser. The XPS bands were calibrated with C1s at 284.6 eV to scan binding energies of elements. The light absorption and emission properties were measured using UV–visible spectrophotometer of Perkin –Elmer.

3 Results and discussions

3.1 X-ray diffraction of Eu³⁺ doped C-CaIn₂O₄ crystallites

XRD pattern change in a sample Eu³⁺:C-CaIn₂O₄ when thermal annealing it at a moderate 400–600 °C temperature for 2 h in air. For example, Fig. 1a compares XRD patterns measured for a typical 1.0 mol% Eu³⁺:C-CaIn₂O₄ sample of small core–shell crystallites (a) before and after thermal annealing at (b) 400 °C and (c) 600 °C for 2 h in air, wherein part of a residual C-sp² surface layer (shell) desorbs off and the bare crystallites start growing in reactive crystallographic facets. The sample is growing preferentially in the (200) planes so as it obtains a larger recurring (200) peak intensity over the master sample at the expense of a residual o-CaIn₂O₄ polymorph. Both the peak positions and intensities in the characteristic XRD peaks are tailored in account of a preferential t-CaIn₂O₄ growth in the (200) facets in a markedly tailored core–shell structure, which is it is eventually reinforcing a uniaxial high pressure effect propagating on the core with promptly tailored structural parameters as follows. A closer view of the XRD peaks, as can be seen from a magnified (200) peak in Fig. 1b, clearly reveals that the XRD peaks are duly shifting over larger 2θ-values (or smaller d_hkl values) in the lattice contacting markedly in growing crystallites in a tailored core–shell structure. In Fig. 1b, the d₂₀₀ value in (200) peak of t-CaIn₂O₄ thus has progressively decreased from 0.3053 nm in the as-prepared sample (D = 35 nm) to that at 0.3035 nm in the 400 °C annealed sample (D = 30 nm), while at 0.3033 nm in the 600 °C annealed sample (D = 40 nm), in a markedly tailored surface effect on growth of the small crystallites. A bulk t-CaIn₂O₄, as on prepared in a solid state reaction [18], exhibits only a weak (200) peak (I_p = 3.4) at a much larger d₂₀₀ = 0.3107 nm, possibly involves significant O²⁻ vacancies. In this case, the most intense peak occurs in (211) peak at 0.2674 nm with the second most intense peaks in the diffraction from (202) and (103) planes in markedly different morphology of growth in a bulk polycrystalline t-CaIn₂O₄. Other details of the lattice parameters and derived properties from the XRD patterns in a 1.0 mol% Eu³⁺:C-CaIn₂O₄ sample studied before and after the optimal thermal annealing are given in Table 2. When optimally annealed at 400–600 °C in air for 2 h, a sample 1.0 mol% Eu³⁺:C-CaIn₂O₄ has a considerably reduced o-CaIn₂O₄ content of 5–10% in its weaker XRD peak intensities. Eventually, a thermal annealing promotes a polymorphic o → t-CaIn₂O₄ transformation in a rather easily reordering phase at the expense of thermally mobile O²⁻ vacancies via different oxide polygons in a crystal lattice. The o-CaIn₂O₄ shares a rather higher crystal density ρ_o = 6.144 g-cm³ (V_o = 0.3606 nm³) than the t-CaIn₂O₄, but a bit lower value than a well-known high-density o-CaIn₂O₄ polymorph of ρ_o = 6.33 g-cm³ (V_o = 0.3500 nm³) [18]. A polymorphic o → t change incurs in a bared Eu³⁺:C-CaIn₂O₄ in a critically hot C-sp² surface layer destabilizes and releases on heating the sample at 400–600 °C in air. Uniquely, an inbuilt C-sp² shell as thinning down in a conjoint network by annealing, the V_t reorders gradually in due shifts (also tailored intensities) of the XRD peaks in Fig. 1a in annealing an as-burnt Eu³⁺:C-CaIn₂O₄ sample at 400–600 °C in air.

Fig. 1

A XRD patterns for (a) as-prepared and annealed 1.0 mol% Eu³⁺:C-CaIn₂O₄ at (b) 400 °C and (c) 600 °C for 2 h in air, with B a magnified (200) peak showing how it shifts over larger 2θ-values on thermal annealing

Further, a pretty large intensity of (200) peak in the XRD patterns in Fig. 1a reveals the Eu³⁺:C-CaIn₂O₄ crystallites are grown preferentially in the (200) facets bound in a conjoint (220) surface [19] so as it sharing a second most intense XRD peak at 0.2147 nm in the sample grown in thin plates of the crystallites. Thin sample plates are clearly shown in the HRTEM images as will be discussed later in this chapter. These thin microscopic Eu³⁺:C-CaIn₂O₄ plates are grown in support of a model planar biogenic template binding Eu³⁺, Ca²⁺, and In³⁺ species in small bio-polymer chains (from an aloe-vera gel) used in a hydrothermal reaction in this work. Average lattice parameters given in Table 1 fairly produce d_hkl values in the observed XRD peaks within a small deviation ± 0.0005 nm of an error in the measured values in the different samples. Eventually, an interstitial Eu³⁺ → In³⁺ doping causes a Eu³⁺:CaIn₂O₄ lattice to be expanding in its average volume, i.e., as much as V_t = 0.4302 nm³ (density ρ_t = 5.153 g-cm³) found in a 600 °C annealed sample over a bulk CaIn₂O₄ value of V_t = 0.3800 nm³ (ρ_t = 5.833 g-cm³) as given in Table 1.

Table 1 A microscopic effect of thermal annealing on structural parameters in preferentially grown 1.0 mol% Eu³⁺:CaIn₂O₄ of small core–shell crystallites

The present microscopic results imply that a thermal annealing causes (1) a C-sp² surface layer (shell) thin down gradually in its network structure, (2) a hot C-sp² network readily reorder on conjoined Eu³⁺:CaIn₂O₄ facets, (3) a bare CaIn₂O₄ to be growing-up slowly, (4) mobile interstitial O²⁻ vacancies reorder in the oxide polygons, and (5) part of interstitial Eu³⁺ species segregate and precipitate onto reactive CaIn₂O₄ facets in a modified core–shell structure at reaction temperature. As a fact, a shortened d₂₀₀ = 0.3053 nm appears in a uniaxial compression along a-axis in the as-prepared sample (ρ_t = 5.762 g-cm³), which is dropped down further to 0.3035 nm in a 400 °C annealed sample (ρ_t = 5.153 g-cm³), or 0.3033 nm in a 600 °C annealed sample (ρ_t = 5.155 g-cm³) in comparison to a bulk 0.3107 nm value (ρ_t = 5.833 g-cm³). In compensating an average V_t value, the c-axis is expanding in a way the aspect ratio c/a increasing in a reduced tetragonality, showing a minimal c/a = 1.6898 in the as-prepared sample, which is raised to 1.9255 in the 400 °C annealed sample, or 1.9258 in the 600 °C annealed sample in a finely refined core–shell structure of small crystallites. A far lower c/a = 1.5838 persists in a bulk CaIn₂O₄ in a 3.7% larger a-value. A uniaxial compressed Eu³⁺:CaIn₂O₄ lattice yields a duly large γ = 0.65% in the as-prepared sample, which is grown further to 0.88% in the 400 °C annealed sample, or 0.72% in the 600 °C annealed sample. The present γ-values are estimated from asymmetric broadening (β) in the XRD peaks in the Williamson-Hall plot [20], γ = βcosθ − λ(Dsinθ)⁻¹. So obtained D-values in the different samples are included in Table 1.

Here, an open question remains why an average crystal lattice volume is decreased largely in the Eu³⁺:CaIn₂O₄ samples when annealed at 400–600 °C in air. It sounds that the as-prepared sample Eu³⁺:CaIn₂O₄ contains a significant number of O²⁻ vacancies, as marked in a model core–shell in a shape of a plate in Fig. 2a, according to its extended volume. These vacancies were created in a redox reaction with a hot carbon (which captures oxygen from the lattice and escapes as its oxide, C + O → CO↑) while burning the sample precursor in camphor to remove the byproduct species. Thermally mobile O²⁻ vacancies in the lattice captures Eu³⁺ species from the nearby interstitial sites and ultimately segregate and precipitate onto the reactive surfaces in the individual crystallites. This is shown schematically in Fig. 2b, c. In a due course of an optimal thermal annealing the sample at 400 °C in air, it replaces part of an inbuilt C-sp² surface layer of a rigid polymer network thereby in the individual crystallites in the form of a modified Eu³⁺-rich surface layer in a refined core–shell Eu³⁺:CaIn₂O₄ structure. Here, a migration of both the O²⁻ vacancies and Eu³⁺ ions (of characteristically large ionic sizes) from the lattice core to its surfaces well accounts in a progressively decreased lattice volume in a locally refined microscopic structure in this way as that is what it is observed in XRD in Fig. 1 in the annealed samples. Further, thermodynamically, so small crystallites do expand in the average lattice volume on predominating surfaces in a high Gibbs-free-energy above the equilibrium bulk value [21, 22], but here in this example of a hybrid core–shell structure at the same time owes a competing surface-effect of a rigid core–shell of small crystallites adversely more effectively compressing the lattice in a reasonably lower volume. Such core–shell crystallites cross-link one another via the surface layers in small self-assemblies of a “second level hierarchical structure” as shown schematically in a microscopic model in Fig. 2d.

Fig. 2

A model structure of reordering of a an inbuilt C-sp² surface layer (shell) followed by growth of b, c a Eu³⁺-rich layer on preferentially growing Eu³⁺ doped CaIn₂O₄ crystallites in (200) plates in d a core–shell hierarchical structure

3.2 Phonon bands of Eu³⁺ doped C-CaIn₂O₄ crystallites

The Eu³⁺ doped C-CaIn₂O₄ samples synthesized of in-built core–shells of small crystallites of a hybrid structure exhibit multiple phonon bands of the Raman spectrum over 1150–1750 cm⁻¹ region. The spectrum is fitted by the Gaussian components in two distinct spectroscopic regions of traditionally so called D-band and G-band of collective oscillations (C=C stretching) of the C-sp² rings in a 2D-planar network presumably bonding over the Eu³⁺ doped CaIn₂O₄ crystallites. As marked in Fig. 3a in Raman spectrum measured from an as-prepared 1.0 mol% Eu³⁺:C-CaIn₂O₄, the D-band contains two bands at 1185 cm⁻¹ and 1310 cm⁻¹ in a doublet as shown in a deconvolution (in place of a single symmetric D-band of 1329 cm⁻¹ known in a pure grapheme) [23, 24], which reveal two major kinds of a joint C-sp² network form and exist on the perpetual Eu³⁺:CaIn₂O₄ facets. As expected, also the G-band is split-up into two overlapping components lying at 1530 and 1650 cm⁻¹ (in place of a single G-band of 1591 cm⁻¹ known in a pure graphene [23, 24]) in two major kinds of the C-sp² network formed in an as-prepared sample. A relative peak intensity I_D/I_G ~ 0.82 persists in the D-band over the D-band in this sample. The I_D/I_G ratio describes a figure of merit of an ideal 2D-planar C-sp² network. Both the D and G bands reassume 15–45 cm⁻¹ of larger frequencies in a reasonably modified spectrum in a self-confined C-sp² network on the Eu³⁺:CaIn₂O₄ facets on annealing the sample at 400 °C in air for 2 h in Fig. 3b. During the annealing part of the hot C-sp² releases as a gas and the residual reorders in a more rigid and regular network on the facets (δ = 1–2 nm).

Fig. 3

Deconvoluted Raman bands in a network of a C-sp² surface layer in a an as-prepared and b annealed 1.0 mol% Eu³⁺:C-CaIn₂O₄ sample at (b) 400 °C in air for 2 h

Further, the I_D/I_G ration has grown further to 0.96 in account of an induced local microstrain in the conjoint structure, as evident from a large γ = 0.88% value analyzed for the Eu³⁺:CaIn₂O₄ crystallites in terms of inhomogeneous broadening in the XRD peaks in Table 1. Now, let us examine how the D and G bands reorder in frequencies and intensities on annealing the above sample Eu³⁺:CaIn₂O₄ at an effectively higher temperature of 600 °C in air for 2 h, so as an inbuilt C-sp² surface layer is thinned down successively to a larger extend of a single molecular level, δ ≤ 0.5 nm in Fig. 4a. Despite a reasonably reduced γ = 0.72% value (Table 1), both the D and G phonon bands are promptly converged in further larger frequencies of 1335 cm⁻¹ and 1595 cm⁻¹ in rather more symmetric and sharp shapes intrinsic of an effectively single molecular GO-network co-bonding in a highly rigid polymer structure on the Eu³⁺:CaIn₂O₄ crystallites via C-O bonds in the oxide polygons. Also, the I_D/I_G ratio is increased to 0.87, conferring a reasonably better C-sp² network reordered via C-O moieties on the oxide surfaces. A doublet of two overlapping phonon bands recurring at 460 cm⁻¹ and 815 cm⁻¹ in Fig. 4b is resolved only when an inbuilt C-sp² surface layer had been sufficiently thin down in a 600 °C annealed Eu³⁺:CaIn₂O₄ sample in air. Tentatively, the first phonon band of 460 cm⁻¹ represents an In-O stretching vibration in a distorted InO₆⁹⁻ octagon (O_h point group), while the other one of 815 cm⁻¹ represents an analogous Ca–O stretching vibration in a distorted CaO₄⁶⁻ tetragon (T_d point group) in a joint network in a spinel structure [25,26,27,28]. These phonon bands are heavily masked in characteristically strong phonon bands recurring in a sufficiently thicker inbuilt C-sp² surface layer (shell) on individual Eu³⁺:CaIn₂O₄ crystallites in the other samples processed at lower temperatures. An inbuilt shell exhibit intense phonon bands in account of a surface enhanced Raman scattering in multiple surfaces in a hybrid core–shell structure. All these results of distribution of phonons unambiguously confirm the fact that the Eu³⁺:C-CaIn₂O₄ samples prepared in a biogenic precursor in this work contain substantially stable inbuilt core–shells of small crystallites.

Fig. 4

Raman bands in a network of a a conjoint C-sp² surface layer and b oxide polygons in an annealed 1.0 mol% Eu³⁺:C-CaIn₂O₄ at 600 °C in air for 2 h

3.3 XPS bands of Eu³⁺ doped C-CaIn₂O₄ crystallites

To find out the chemical state and distribution of the different atoms in a sample Eu³⁺:C-CaIn₂O₄ prepared of small core–shell crystallites, we studied XPS bands in the different atoms in the sample as follows. For example, a Eu³⁺:C-CaIn₂O₄ sample as annealed at 600 °C in air for 2 h exhibits two broad C1s peaks at 288.1 eV and 284.1 eV in Fig. 5a arising in carbon present in C=O (or C–O) and C–C [29, 30] bonds respectively in a presumed GO-like surface layer in the sample of the small crystallites. The C1s band arising from the C=O species has much smaller intensity according to their small numbers present only on the surfaces in a core–shell structure. Figure 5b presents a characteristic doublet of two Ca2p_{1/2, 3/2} XPS bands of 345.0 eV and 348.8 eV, with a doublet separation Δ = 3.8 eV, as marked thereby on the respective bands in the Eu³⁺:C-CaIn₂O₄ sample annealed at 600 °C in air for 2 h.

Fig. 5

a C1s, b Ca2p_{1/2, 3/2} and c In3d_3/2,5/2XPS bands from a sample of 1.0 mol% Eu³⁺:C-CaIn₂O₄ of small core–shell crystallites (annealed at 600 °C in air for 2 h)

There is no much change in these bands in comparison to those observed before doping Eu³⁺ → In³⁺ in a crystal C-CaIn₂O₄ lattice. A similar characteristic doublet of XPS bands is observed in In3d_5/2 and In3d_3/2 bands at 442.1 eV and 450.1 eV (with Δ = 3.8 eV) in the In³⁺ specifies presumably in the forms of InO₆⁹⁻ polygons in a Eu³⁺:C-CaIn₂O₄ core–shell structure. Consistently, more or less similar E_b-values have been observed earlier in these Ca2p_{1/2, 3/2} and In3d_{3/2, 5/2} bands within a small deviation of ± 0.2 eV in solid solutions of In₂O₃-CaIn₂O₄ [11] and Eu³⁺:CaIn₂O₄ [31] which were prepared using a conventional high-temperature solid state reaction in ambient air. A marked variation appears in the XPS bands in these species owing to a due change in the local structure especially when mobile O²⁻ vacancies are reordering in a core–shell network structure. The Eu³⁺ species doped in a C-CaIn₂O₄ lattice of small crystallites of the above sample share a characteristic doublet of Eu3d_5/2 and Eu3d_3/2 bands at 1159.1 eV and 1165.0 eV (with Δ = 5.9 eV) as marked in a deconvolution in Fig. 6a. In a pure Eu₂O₃, these bands appear of bit larger values of 1154 eV and 1163 eV (with Δ = 9.0 eV) in a rather more insulator behavior of 4f⁶-valence electrons [17, 32]. As seen in Fig. 6, XPS bands for europium ion are marked with two bands at 1165 and 1159 eV for Eu³⁺ and Eu²⁺ ions but the band at 1165 eV is 10 times higher in intensity compared to 1159 eV band. So, possibly effect of Eu³⁺ ions are more than Eu²⁺ ions.

Fig. 6

a Eu3d_3/2,5/2 and b O1s XPS bands from a sample of 1.0% Eu³⁺:C-CaIn₂O₄ of small core–shell crystallites (annealed at 600 °C in air for 2 h)

Uniquely, the sample reveals three kinds of O²⁻ species, which appear in three overlapping O1s XPS bands at 528.9, 529.6 and 530.3 eV in a triplet in a deconvolution of the observed spectrum in Fig. 6b. An undoped sample C-CaIn₂O₄ has a strong O1s band at 530.3 eV with only a weak shoulder at 531.8 eV, illustrating that a Eu³⁺ → In³⁺ doping significantly modifies a distribution of density of states of O²⁻ electrons in the different sites in a Eu³⁺ doped C-CaIn₂O₄ of small crystallites. Qualitatively, the first two bands observed at 528.9 eV and 529.6 eV in strong intensities attribute to O²⁻ species occupying the two kinds of the oxide polygons of CaO₄⁶⁻ and InO₆⁹⁻ respectively as per their ratio 1:2 in a spinel CaIn₂O₄ structure. Thus, a weak satellite band observed at 530.3 eV (or 531.8 eV in the undoped sample) can be assigned to the surface O²⁻ species present as functionalized groups, like C=O, C–O, or CHO [33,34,35], in a conjoint C-sp² surface layer of a stable network on the individual crystallites in a hybrid core–shell structure. It is a hybrid core–shell structure that finely tunes the E_b values and intensities in the three distinct overlapping O1s bands in a wide O1s distribution of electrons in the different sites in the lattice in this example.

Now, let us comment on how a thermal annealing leads to tailor characteristic (A) In3d_3/2,5/2, (B) O1s, and (C) C1s XPS bands at the expense of an inbuilt C-sp² surface layer (shell) in a typical 1.0 mol% Eu³⁺ doped sample C-CaIn₂O₄ of small crystallites of a tailored core–shell structure. As a result, Fig. 7 compares the three band groups of A, B, and C observed in (a) the as-prepared and annealed samples at (b) 400 °C and (c) 600 °C for 2 h in air in a way the shell thins down in successive layers {from an average δ = 2–3 nm thickness in the beginning sample (a) to a single molecular level δ → 0.5 nm or smaller in sample (c)} in progressively tuned core–shell crystallites. Peculiarly, the In3d_3/2,5/2 bands are sharpened (over bit smaller E_b values) in account of a reasonably manifested intensity in an optimally 400 °C annealed sample and then they lose intensity in diffuse bands of significantly promoted E_b—value by nearly 0.5 eV in a critically 600 °C annealed sample. Also the O1s band assumes a minimum 528.9 eV value in this sample relative to that of 529.2 eV in the as-prepared, or 529.5 eV in a 600 °C annealed sample in a marked reordering of O²⁻ species in the shell and surface-interfaces in a hybrid core–shell structure. Also, the C1s band component (band-1) lying of a lower E_b-value at 283.7 eV readapts a markedly reduced intensity compared to its higher E_b-value component (band-2) at 288.1 eV in an optimally 400 °C annealed sample. The result accords with a reasonably increased number density of C–C and C=C bonds (band-1) reformed in a top surface layer in a C-CaIn₂O₄ core shell at the expense of the functional C–O, C=O, CHO groups in a 600 °C annealed sample. Further, diffuse In3d_3/2,5/2 XPS bands observed in Fig. 7A in a 600 °C annealed Eu³⁺:C-CaIn₂O₄ sample illustrate a disordering of InO₆⁹⁻ polygons on the In³⁺ cations on the surfaces in the small crystallites. This is a very similar observation as that we described in ACS SC&E research paper [19] in critically hot InO₆⁹⁻ polygons refine into tripods ‘InO₃’, which co-bond one another in a more stable 2D-network involving C-sp², C–O and C=O species in different ways at the surfaces in a disordered structure. At an elevated temperature, a regular network of the hot C-sp² species dissociates to release highly reactive C-sp², part of which reacts with the ‘InO₃’ tripods, creating an ‘alloyed In₂O₃–C surface’ on the nascent CaIn₂O₄ facets. That is what it appears in diffuse characteristic In3d_3/2,5/2 XPS bands as observed over here in a critically annealed sample of Eu³⁺ doped C-CaIn₂O₄ at 600 °C for 2 h in air. It is useful to fruitfully tailor the local core–shell structure and in turn the useful functional properties as will be described further.

Fig. 7

A In3d_3/2,5/2, B O1s, and C C1s XPS bands measured from (a) as-prepared and annealed 1.0 mol% Eu³⁺:C-CaIn₂O₄ at (b) 400 °C and (c) 600 °C for 2 h in air

3.4 Microstructures of Eu³⁺ doped C-CaIn₂O₄ crystallites

The size, shape and surface morphology in as-prepared and annealed samples Eu³⁺:C-CaIn₂O₄ of small crystallites, prepared with Eu³⁺ contents varied in small dosages of 0.5, 1.0, 1.5 and 2.0 mol%, can be analyzed in terms of their FESEM and HRTEM images studied at selective magnifications. The results help to find out the mechanism in a sample of small core–shell crystallites refines on the optimal annealing in a hierarchical structure via a residual carbon of a thin C-sp² microscopic layer in a hybrid structure. The FESEM images present mostly thin plates of the Eu³⁺:C-CaIn₂O₄ sample of tiny crystallites binding one another in small groups in a hierarchical structure, with 100–150 nm widths and 20–40 nm thickness (which represents the effective crystallite size). This is a characteristic feature of the small crystallites are cross-linked via the C-sp² surface in this specific structure. An aloe-vera gel used as a precursor medium in synthesizing the sample is found to be highly efficient in controlling the final size, shape and morphology in the small Eu³⁺:C-CaIn₂O₄ crystallites. Distinct plates of the Eu³⁺:C-CaIn₂O₄ crystallites grown preferentially in the (200) planes as illustrated with XRD patterns has been published in MRS Advances in 2017 can be observed in the HRTEM images at a reasonably higher magnification [36]. A SAED pattern taken from a thin plate grown in (200) planes of HRTEM images revealed arrays of the lattice points. The (200) arrays are marked at d₂₀₀ = 0.3050 nm in agreement with the XRD value. An inbuilt C-sp² surface layer shares a broad diffraction halo of a ring (near the center) in its characteristic (002) arrays in a graphene like network structure of hexagonal rings with d₀₀₂ = 0.3460 nm [19]. The result confers that the plate in the observation does contain an inbuilt surface layer of a core–shell crystallite. These images are clearly bound by a conjoined peripheral surface layer (of distinct darkish contrast), showing the plate, which is grown in (200) facets, contains a bonded C-sp² surface over layer, what is it is marked over its left top corner. The results characterize the sample Eu³⁺:C-CaIn₂O₄ forms a hybrid composite structure of small core–shell crystallites. A core shell Eu³⁺:C-CaIn₂O₄ structure and its hierarchical composite structure bonding through a conjoint C-sp² surface layer distinctly vary in an annealed sample at 400–600 °C for 1–2 h in air. This is clearly observed in FESEM and HRTEM images studied in the annealed samples. For example, Fig. 8a, b presents FESEM and HRTEM images of a sample 1.0 mol% Eu³⁺:C-CaIn₂O₄ annealed at 400 °C for 2 h in air. Thin plates of core–shell crystallites as observed in an as-prepared sample are grown into a hierarchical structure in a shape laminates, with 500–1000 nm widths and 30–50 nm thickness (represents the average crystallite size), in the FESEM images in Fig. 8a.

Fig. 8

a FESEM, b HRTEM, c lattice image, and d SAED pattern from a 1.0 mol% Eu³⁺:C-CaIn₂O₄ sample of small crystallites annealed at 400 °C for 2 h in air

Small plates Eu³⁺:C-CaIn₂O₄ embedding in a carbon film are also observed in HRTEM images in Fig. 8b as measured from the same sample. Figure 8c displays lattice images of (004) planes of a thin plate of HRTEM images in Fig. 8b, with a d₀₀₄-spacing of 0.2568 nm). A thin crystal plate grown in (200) planes, as observed in the HRTEM images in Fig. 8b, displays a SAED pattern of regular arrays of the lattice points in two different series of (004) and (112) planes as shown in Fig. 8d. The lattice separations of d₀₀₄ = 0.2568 nm and d₁₁₂ = 0.3295 nm observed in these arrays fairy reproduce the corresponding values observed in the XRD peaks of 0.2568 nm and 0.3310 nm in this sample. As observed in the as-prepared sample, a presumed inbuilt C-sp² surface layer of a hexagonal network is sharing a characteristic broad diffraction halo of a circular ring (near the centre) in its characteristic (002) arrays, with a modified d₀₀₂ = 0.3455 nm,⁷ as on it presumably confining in a thinner conjoin shell in the crystallites. Figure 9 displays a very complicated hierarchical structure of pretty long thin flakes of FESEM (1–2 μm widths and 25–35 nm thickness), which were grown in support of presumably long surfaces of the biogenic templates as used in the synthesis, in a 2 mol% Eu³⁺:C-CaIn₂O₄ sample annealed at 400 °C for 2 h in air.

Fig. 9

a, b FESEM images of a 2 mol% Eu³⁺ doped C-CaIn₂O₄ sample (annealed at 400 °C for 2 h in air) grown as thin platelets as magnified in b

A refined C-sp² surface layer on the crystallites (in a shape of thin plates) promptly cross-links the bare crystallites in this specific structure of a hybrid composite. A similar microstructure prevails on raising the Eu³⁺ → In³⁺ doping further in a sample 2.0 mol% Eu³⁺:C-CaIn₂O₄ studied in this series. Nevertheless, presumably small plates of small crystallites can be resolved in these samples only after etching out most of the C-sp² surface layers in critically annealed samples in air. For example, Fig. 10a, b presents a rather clear picture of thin flakes or plates of the crystallites in the Eu³⁺:C-CaIn₂O₄ samples (having 1–2 mol% Eu³⁺ → In³⁺ doping, so as it effectively promoting growth of the crystallites over larger exposed facets) optimally annealed at 600 °C for 2 h in air. Bare crystallites grown in a specific shape of rectangular prisms with well-developed facets (200) and (112) facets are observed in Fig. 10b, c in a sample 2.0 mol% Eu³⁺:C-CaIn₂O₄ annealed at 600 °C for 2 h in air. Well-separated prisms are 50–100 nm long, with W = 25–50 nm widths and δ = 25–40 nm thicknesses. The model shapes of these crystallites are given in Fig. 10d illustrating how the small crystallites are grown in the perpetual facets. It is interesting to observe that, also the single crystallites are shown to be cross-linking (via O²⁻ in the oxide polygons at reactive facets in a CaIn₂O₄ lattice) one over others in a ‘second level hierarchical structure’. Now, let us view HRTEM images in Fig. 11a from a typical 2.0 mol% Eu³⁺:C-CaIn₂O₄ sample annealed at 600 °C for 2 h in air., wherein the small crystallites are binding one another via the inbuilt C-sp² surface layers (δ ≤ 0.5 nm) in self-assemblies of a ‘hierarchical structure’. A typical lattice image measured from a selected plate A (as marked in the HRTEM images in Fig. 11a) in Fig. 11b reveals how it is preferentially grown in a single crystallite in (200) planes, with d₀₀₂ = 0.3060 nm (consistent to a corresponding value of 0.3062 nm observed in the XRD pattern), and how it contains a distinct peripheral boundary (of a whitish contrast) in part of a bonded surface layer. As magnified in Fig. 11c, it exhibits a large number of linear interference fringes of an internal separation of 0.8265 nm (which is much larger than expected d_hkl in this sample) in interference of the electron beams reflected from successive C-sp² layers on a perpetual crystal facet. Eventually, the interference fringes are intersecting the lattice arrays at a specific internal angel of 38°, which is nearly the same throughout the observed fringes.

Fig. 10

FESEM images of a 1.0 mol% and b, c 2.0 mol% Eu³⁺ doped C-CaIn₂O₄ grown in thin a plates and b, c rectangular prisms as annealed at 600 C for 2 h in air, with d the model crystal plates grown with distinct facets

Fig. 11

HRTEM images of a thin crystal plates bonding over an inbuilt C-sp² surface layer and b, c lattice patterns from a 2.0 mol% Eu³⁺ doped C-CaIn₂O₄ sample (annealed at 600 °C for 2 h in air). Interference fringes from an inbuilt surface layer on a plate are shown

Figure 12a presents another example of a lattice image taken from one of the single crystallites grown in a self-assembly as marked in region ‘A’ in Fig. 11a. As marked by numbers 1, 2, 3 and 4, this crystallite contains a distinct phase boundary of a conjoint surface layer (of a disordered phase) formed on its multifacets, which eventually appear in four distinct microscopic regimes, likely from four conjoint facets in this crystallite. Uniquely, all the four facets share a uniform thickness of an inbuilt conjoint surface layer, δ ~ 0.7 nm, in a regular molecular structure at an atomic scale, accounting in a continuous interconnected C-sp² network forms over a crystallite in minimizing the total surface-energy in a hybrid core–shell structure. The said crystallite in this example (a 2.0 mol% Eu³⁺ doped C-CaIn₂O₄ sample and annealed at 600 °C for 2 h in air in thinning down a surface layer in a critical core–shell structure) reveals regular (112) arrays of lattice images, with d₁₁₂ = 0.3325 nm (consistent to its value shown in the XRD peak at 0.3310 nm) as studied using an electron beam incident on its (112) facet. As magnified in Fig. 12b, it exhibits a wide series of linear interference fringes of an internal separation of 0.8065 nm and an average 0.51 nm fringe-width when the indent electron beam reflecting from successive C-sp² surface layers on the (112) facet in a way the later get a conjoint constructive interference in an ordered pattern.

Fig. 12

a Lattice images from a 2.0 mol% Eu³⁺:C-CaIn₂O₄ sample of a single crystal plate grown in (112) facets (annealed at 600 °C for 2 h in air), with b a magnified view containing interference fringes. A boundary phase extends over four segments 1 → 4

Eventually, these fringes are inclined uniformly over the (112) lattice arrays at a specific internal angel of 41°, which does not change noticeably over the entire range of the observed fringes. Further, these fringes have an average width of 0.51 nm, which is nearly the same as observed above in Fig. 12b in a similar pattern of the interference fringes formed on a (200) crystal facet in a similar preferentially grown single crystallite in the (200) planes. A fringe width observed in this experiment seems to be critical of a conjoint C-sp² network (in terms of its thickness and molecular structure) formed on the sample of its bare surfaces. These are general microscopic features in unambiguously characterizing a core–shell structure of metals, metal oxides, and derived structures. A spatial distribution of the atoms in a Eu³⁺:C-CaIn₂O₄ hybrid nanostructure was studied in terms of its characteristic elemental mapping of selected regions of the FESEM images with an in situ EDX analysis, which evaluates the Eu, C, Ca, In and O species in the chosen microscopic regions. As shown in the typical spectra in Fig. 13, all the four kinds of atoms are duly distributed in a due distribution in a 1.0 mol% Eu³⁺:C-CaIn₂O₄ sample, as-prepared in a self-combustion of a biogenic complex in air. For a comparison at a glance, the different elements are given in different colors; white for Eu, red for C, green for Ca, blue for In, and yellow for O. Similar results were obtained for all other samples studied in this work. As expected, an initial C-content has been progressively reduced in optimally annealed samples at 400–600 °C for 1–2 h in air.

Fig. 13

The elemental mapping of an as-prepared 1.0 mol% Eu³⁺:C-CaIn₂O₄ hybrid nanostructure; a all the different elements shown together and the individual ones b C, c Ca, d In, e O and f Eu marked in the different visible colors

4 Tailored optical properties of Eu³⁺ doped C-CaIn₂O₄ crystallites

Figure 14 shows absorption spectra of annealed Eu³⁺:C-CaIn₂O₄ samples at 400 °C for 2 h in air, containing (a) 0.1, (b) 0.5, (c) 1.0 and (d) 2.0 mol% Eu³⁺ doages. The primary band-1, which is observed at 205 nm (215 nm before the annealing) [35] in Fig. 14a in a sample doped by only 0.1 mol% Eu³⁺, is shifted progressively over lower λ-values below 200 nm and only its tail extends over longer λ-values above 200 nm in a distinct band edge absorption. Uniquely, a weak absorption band is developed in the visible region at 645 nm in a 7F₃ → ⁵D₀ transition from the Eu³⁺ ions in Fig. 14c after a critical 1.0 mol% Eu³⁺ doping in C-CaIn₂O₄ in a core–shell structure. Another weak band is developed in the visible region at 785 nm (in a 7F₆ → ⁵D₀ transition [15, 36, 37] of the Eu³⁺ ions) in Fig. 14a after a rather smaller doping of only 0.1 mol% Eu³⁺ in a core–shell C-CaIn₂O₄ structure. Both these bands were not so visible before thermal annealing the sample in a marked effect of the annealing in which a significant part of the Eu³⁺ ions presumably segregates from the lattice (core) and migrates to the interfaces (shell) in a refined hybrid core–shell structure. Eventually, both of the bands are the hot bands, which can be excited only in the high-energy carriers of occupied sites in the interfaces in a firmly stable core–shell structure [1, 2, 17].

Fig. 14

A UV–visible absorption spectra of annealed Eu³⁺:C-CaIn₂O₄ at 400 °C, with (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 mol% Eu³⁺ dosages, B part of magnified spectra over 400–800 nm, and C Tauc plots of tail of band-1 over lower energies

As usual, the Tauc plots of the tail in absorption band-1 over lower energies are used in estimating the E_g-values from the spectra given in Fig. 14 from four annealed samples Eu³⁺:C-CaIn₂O₄ at 400 °C for 2 h in air. The values of λ_max, extinction coefficient (ε_max) and E_g obtained from absorption spectra in the different the as-prepared and annealed Eu³⁺:C-CaIn₂O₄ samples are given in Table 2. Further, as compiled in Fig. 15, the present light absorption spectra were further modified in the samples Eu³⁺:C-CaIn₂O₄ were annealed at an effectively higher temperature of 600 °C for 2 h in air. Here, band-1 is markedly broadened in samples (a, b), while the band-2 is grown in a more pronounced band with its average position lying at 265 nm, which does not exhibit a measurable in the as prepared and annealed sample at 400 °C for 2 h in air. Obviously, it is the result of a presumably refined grafted C-sp² surface layer reformed on the facets of small core–shell Eu³⁺:CaIn₂O₄ crystallites. An energy-transfer Eu³⁺ → C-sp² thus accounts in an enhanced intensity in the π → π* transition in excited C-sp² species in a rigid core-shell. These samples have bit lowered E_g-values (as given in Table 2) than those having more Eu³⁺ species, which help mixing in the closely lying energy levels involved between the two types of the species in a joint 2D-network in this example.

Table 2 Optical parameters derived from light absorption spectra in Eu³⁺:C-CaIn₂O₄ of small core shell crystallites dispersed in water

Fig. 15

A UV–visible absorption spectra of Eu³⁺:C-CaIn₂O₄ annealed at 600 °C for 2 h in air, with (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 mol% Eu³⁺ dosages, B part of magnified spectra over 400–800 nm, and C Tauc plots band-1 over lower energies

As usual, characteristically weak absorption bands expected in the spin-forbidden 4f → 4f transitions of Eu³⁺(4f⁶) ions [38] are masked in the strong background absorption from a due band edge absorption tail (of a strongly electric-diploe-allowed transition) in a result of the excitation of the charge carriers from the valence band to the condition band in a hybrid core-shell Eu³⁺:C-CaIn₂O₄ structure. Such microscopic structure has large multiple surfaces of core, interface, top network layer that all share strong recurring surface scattering throughout the spectral region over 200–800 nm studied in this example. Nevertheless, the Eu³⁺⁻energy levels present in this spectral region exhibit strong surface enhanced light-emission. Technically, it is very useful that the Eu³⁺ ions doped in a host C-CaIn₂O₄ of a core structure of the small crystallites exhibit strong light-emission of a broad spectrum over the UV–visible regions. For example, Fig. 16 compares (A) light emission and (B) excitation spectra for the samples (as-prepared) having (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 mol% Eu³⁺ dosages, as on measured using a pulse xenon lamp of a light source. The emission spectra were obtained by exciting the samples at λ_ex = 240 nm, while the excitation spectra were obtained by irradiating the samples in the emission band at λ_em = 512 nm in finding the optical origin of electronic transitions in these bands. In general, two broad groups of emission appear over (1) 300–580 nm and (2) 580–900 nm on exciting the samples at a common λ_ex = 240 nm, while those of the excitation spectra appear in a strong group in the far UV region over 210–310 nm with rather weak bands extending over longer wavelengths 310–430 nm on a common λ_em = 512 nm irradiation by the xenon lamp.

Fig. 16

A Light-emission (λ_ex = 240 nm) and B excitation spectra (λ_em = 512 nm) measured from Eu³⁺:C-CaIn₂O₄ core–shell crystallites (as-prepared); (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 mol% Eu³⁺-doping dosages, with associated energy-levels in the insets

On the basis of light-emission reported in Eu³⁺ ions doped in different optical hosts [31], the blue light-emission of average peak position lying around 435 nm ascribes to the ⁵D₃ → ⁷F₀ transitions, while the red emission of average peak position lying around 660 nm ascribes to the ⁵D₀ → ⁷F₄ transitions of the Eu³⁺ ions in the CaIn₂O₄ lattice. A strong excitation band prevails below 240 nm due a prominent metal-to-ligand Eu³⁺ → O²⁻ charge-transfer [1, 37] in the small Eu³⁺:C-CaIn₂O₄ crystallites. This charge transfer is between europium ions and oxygen ions present in indium oxide polygons in interstitial sites. As stated in this research that the reordering of indium oxide polygons is taking place on annealing, lastly it’s a mixed structure (CaIn₂O₄) with few oxygen vacancies. The 4f–4f electronic energy-levels of Eu³⁺(4f⁶) ions, which share the light-emission and excitation spectra in these samples are given in the insets in Fig. 16. Eventually, the Eu³⁺ ions exhibit a series of emission and excitation bands extending over the entire UV–visible regions what is it is a great demand nowadays in finely tuning the light absorption, emission and associated properties in a kind of a hybrid nanocomposite of small core–shell crystallites.

Both the excitation and emission spectra in Fig. 16 had acquired a maximum intensity in a critical dosage of 0.5 mol% Eu³⁺ ions, i.e., the concentration quenching (Q_c), in the as-prepared Eu³⁺:C-CaIn₂O₄ samples. A similar Q_c ~ 1.0 mol% Eu³⁺ value has been reported in Eu³⁺:Y₂O₃, while an order of lower value is reported in Eu³⁺:C-CaIn₂O₄, which were prepared in pretty bigger crystallites by heating the precursors at an effectively higher temperature 1000 °C for 3 h in air [31]. Now, let examine how the light emission and excitation spectra change on a core–shell structure is finely modified in the annealed Eu³⁺:C-CaIn₂O₄ samples. Figure 17 compares light-emission spectra obtained for the samples (annealed at 400 °C for 2 h in air) under identical to those reported in Fig. 1. An obviously effect of an annealing is that it induced an higher-energy emission ⁵D₄ → ⁷F₀ displaced at 355 nm and a long wavelength ⁵D₀ → ⁷F₄ red-emission at 695 nm as marked on the individual bands. A maximum light-emission is retained in the same sample as that it contains 0.5 mol% Eu³⁺ before the annealing. Involved energy-levels in these spectra are given in the insets in Fig. 17. As can be seen from the energy-level diagram, the ground state multiplet ⁷F_J (0, 1, 2, 3, 4, 5, 6) is more sensitized in propagating the emissions from different levels of the excited state ⁵D_J′ multiplet (J′ = 0, 1, 2, 3, 4). Further, the results of the light-emission observed in the samples annealed at 600 °C for 2 h in air are composed in Fig. 18. As marked in the individual bands, the ⁵D_J′ → ⁷F_J transitions are significantly modified over those in the above samples in a duly modified core–shell structure.

Fig. 17

Light-emission (λ_ex = 240 nm) in thermally annealed Eu³⁺:C-CaIn₂O₄ of small core–shell crystallites at 400 °C for 2 h in air; (a) 0.1, (b) 0.5, (c) 1.0, and (d) 2.0 mol% Eu³⁺-dosages, with associated energy-levels in the insets

Fig. 18

a Light-emission (λ_ex = 240 nm) in thermally annealed Eu³⁺:C-CaIn₂O₄ of small core–shell crystallites in as-prepared, 400 and 600 °C for 2 h in air for 1 mol% Eu³⁺-dosages, b Light absorption for 0.1, 0.5, 1, 2 mol% Eu³⁺-dosages for 400 and 600 °C annealed Eu³⁺:C-CaIn₂O₄

In Fig. 19a, we compared the light-emission in a critically Q_c ~ 0.5 mol% Eu³⁺ doped C-CaIn₂O₄ before and after optimally annealed at 400 °C and 600 °C for 2 h in air, showing a way in a refined core–shell structure promotes a maximum light-emission in this hybrid structure. Qualitatively, the first band group perceives an order of enhanced intensity over 300–580 nm regime, while the second band group obtains two order of enhanced intensity over 580–900 nm regime, in the annealed samples. Obviously, only a molecularly thin grafted C-sp² surface is required in harvesting an enhanced light-emission of a broad band over UV–visible regions in a hybrid Eu³⁺:C-CaIn₂O₄ core–shell structure. As shown in Fig. 19b, also the excitation spectrum reveals a maximum intensity in optimally annealed samples only, showing a maximum value in a Q_c ~ 0.5% Eu³⁺ doped C-CaIn₂O₄ sample as on it is annealed at 600 °C for 2 h in air. As a result, it is confessing an inherent correlation between the emission and the extraction spectra in this example. Here, characteristically sharp Eu³⁺ bands are merged in markedly broad bands of the multiple ⁵D_J′ → ⁷F_J transitions mediated through a presumably grafted C-sp² surface layer of a conjoint network. As a result, also a Eu³⁺-decorated graphene is reported of promptly combining the otherwise sharp ⁵D_J′ → ⁷F_J (Eu³⁺) bands in the form of a single broad blue-emission only [38]. As marked over the spectra, a metal to ligand “Eu³⁺ → O²⁻” charge-transfer [37] in a stable interface results in a prominent 240 nm excitation band irrespective of irradiating the samples at any λ_em-values of the emission bands. In fact, it is a core–shell Eu³⁺:C-CaIn₂O₄ structure that promptly channels a strong spontaneous “Eu³⁺ → O^2−” charge-transfer (also energy-transfer), resulting in a concomitantly surface enhanced light-emission in the UV–visible regions as long as irradiating the sample within its wide 200–300 nm regime.

Fig. 19

(a) Light-emission (λ_ex = 240 nm) in a critical Q_c ~ 0.5 mol% Eu³⁺ doped C-CaIn₂O₄ before and after annealed at 400 °C and 600 °C for 2 h in air, with (b) excitation spectra (λ_em = 512 nm) of different Eu³⁺:C-CaIn₂O₄ samples as described in the insets

Figure 20 displays a CIE chromaticity diagram estimated using the light-emission spectra observed over 300–580 nm region for a typical Eu³⁺:C-CaIn₂O₄ sample having a critical Q_c ~ 0.5 mol% Eu³⁺ doping, so as it exhibits a maximum emission intensity in the as-prepared sample as well as when it is optimally annealed at 400 °C and 600 °C for 2 h in air. The CIE co-ordinates so obtained for these three samples are given in Table 3 in the right in Fig. 20. In these three Eu³⁺:C-CaIn₂O₄ samples, the co-ordinates are varied very marginally as x = 0.15, y = 0.13 for the as-prepared sample, x = 0.18, y = 0.17 for the 400 °C annealed sample, while x = 0.22, y = 0.20 for the 600 °C annealed sample, which are all in a primarily blue light emission zone. A similar bulk sample Eu³⁺:CaIn₂O₄ is well-known to serve to be a white light-emitter [38]. Another similar oxide phosphor Eu³⁺:Y₂O₃ (0.5–1.5 mol% Eu³⁺ ions) is known to display a primarily red emission [32]. A comparison of all these results implies that an average visible color can be very finely tuned in such oxides depending on their local microscopic structure.

Fig. 20

CIE chromaticity of as-prepared and annealed 0.5 mol% Eu³⁺:C-CaIn₂O₄ at 400 °C and 600 °C for 2 h in air, with the (x, y) co-ordinates given in the right

Table 3 CIE coordinates in Eu^3+:C-CaIn₂O₄ core–shell crystallites

5 Conclusions

An in situ chemical Eu³⁺ → In³⁺ doping (0–2.0 mol% Eu³⁺ dosages) of a compound CaIn₂O₄ by using a biogenic synthesis in this work finely tailors its lattice parameters of a high-energy t-CaIn₂O₄ polymorph of small Eu³⁺:C-CaIn₂O₄ crystallites. It promotes a phase o → t-CaIn₂O₄ transformation in the annealed samples at moderate temperature 400–600 °C in air. As analyzed in terms of the XRD patterns and HRTEM studies, part of Eu³⁺ ions used here occupies the In³⁺ sites in a spinel structure and the other orders on the resulting crystallites in a hierarchical core–shell structure. An inbuilt surface C-sp² layer shares multiple D and G phonon bands of 1200–1800 cm⁻¹ in Raman spectra of the as-prepared and optimally annealed samples (synthesized of selective compositions) of finely tuned facets on a duly modified surface C-sp² structure. A core–shell distinctly exhibits Eu3d, O1s and C1s XPS bands in two microscopic parts of core and shell in a hybrid structure. An optical doping of Eu³⁺ ions in selective dosages (up to 1 mol%) in a parent C-CaIn₂O₄ lattice of an insulator, as prepared of tunable core–shell crystallites, promptly adds and tailors multiple electronic bands in UV–visible region of the solar energy. As on thinning down part of a thin inbuilt C-sp² surface layer (shell) binding over a doped CaIn₂O₄ lattice by thermal anneals at 400–600 °C in air, it finely tunes the characteristic light absorption and emission bands in three microscopic regimes of core, interface and the surface layer in a hierarchical core–shell structure. In general, the Eu³⁺ ions exhibit a broad light emission in two major groups (1) 300–600 nm and (2) 600–900 nm of multiple bands in the optimally annealed samples. As a result, a co-doping of Eu³⁺ ions thus can conduct an efficient energy-transfer between pertinent energy levels useful for energy conversion, photocatalysis and other applications.