Dual Mode Photoluminescence Studies of Polyol-Mediated Ho3+ Doped CePO4

Introduction

With the rapid development of rare earth minerals, they are currently referred to as inorganic vitamins.  These rare earth-doped nano-phosphors are a distinct group of light-emitting materials whose spectrum properties are determined by the 4f level spacing of a certain lanthanide ion. This shows a range of narrow emissions from visible to near-infrared, and there are numerous uses for rare earth-doped phosphors. They are high-resolution display systems, electronic devices, security, and medical diagnostics, among other things.^1–6 new research on these phosphors. Bio-imaging and energy are the two main areas that it covers. The up-conversion (UC) and down-conversion (DC) properties of lanthanide nano-particles are used in collectors-imaging research because they are projected to improve bio-imaging from multiple aspects, including penetration depth, insignificant auto-fluorescence, and lesser scattering. 3 In this present work (CePO₄:Ho³⁺) here cerium phosphate is doped with holmium which gives tremendous applications and the emission peaks of Ho³⁺ give upconversion (UC) and down-conversion (DC).  Because of various restrictions imposed by its nature, such as the laser’s small spot size (1-2 mm diameter) and the necessity for suitable filters and collimators ^7-8, literature research using NIR light excitation is few. The NIR light’s deep penetration, which enables it to detect materials with a few cm thicknesses, therefore, has significant benefits over UV light excitation if the abovementioned limits are resolved. Due to its superior physical and chemical characteristics, cerium (Ce) would be a rare earth element that is frequently employed. Due to their appealing catalytic activities and electrical conductivity characteristics, Ce-based nanomaterials have grown to be very important for electrochemical biosensors.^9–11 The preparation of Ce-based nanomaterials has a substantial impact on their electrochemical application-related properties. In this method of doping, we can observe the phase transfer of hexagonal to monoclinic can be observed from most of the literature and the given data.

Figure 1(a): A Graphical representation of  properties studies of CePO4:Ho3+ at 980nm and 300nm excitations. Click here to View Figure

Materials and Methods                                                          

Chemicals required and process of synthesis

 Chemicals required

Reagents of the highest purity Sigma-Aldrich were utilized as reactants without additional purification. Cerium(III) acetate Hydrated(Ce(ac)₃).XH₂O, Ammonium dihydrogen phosphate (NH₄)₂HPO₄, Holmium (III) acetate hydrate (HO(ac)₃). XH₂O, dil HCl, Ethylene glycol, dil NaOH, and deionized water were used as precursors.

Synthesis of CePO₄:Ho³⁺ Nanoparticles:

We prepared the sample via the polyol technique.

Samples prepared were

  The polyol method was used to create CePO₄:Ho³⁺ luminescent nanoparticles with varying Ho³⁺ concentrations (1%, 3%, 5%, and 7% atomic percent). The procedure is described as follows:

Step I

Starting Materials:991.3 mg of (CH₃CO₂)₃Ce.xH₂O (cerium acetate hydrate),8.7 mg of (CH₃CO₂)₃Ho.xH₂O (holmium acetate hydrate),5 ml of concentrated HCl (hydrochloric acid),10 ml of deionized water,298.2 mg of (NH₄)₂HPO₄ (ammonium phosphate),2.64 g of NaOH (sodium hydroxide),20 ml of EG (ethylene glycol).

Step II

Holmium and Cerium Acetate Dissolution: In 5 cc of concentrated HCl, the cerium and holmium acetate hydrates are heated until they are completely dissolved. As a result, a crystal-clear solution containing the metal ions is created.

Step III

Elimination of Extra HCl: By alternative addition of 10 ml of deionized water to the solution by repeating this process 5 times excess of  HCl is eliminated. Throughout this procedure, the solution is heated to 80 °C.

Step-IV

The (NH₄)₂HPO₄ and NaOH solution are prepared:  In 10 ml of deionized water, 298.2 mg of (NH₄)₂HPO₄ and 2.64 g of NaOH are dissolved. This results in a translucent (NH₄)₂HPO₄ and NaOH solution.

Step-V

Addition of (NH₄)₂HPO₄ Solution: Until the (NH₄)₂HPO₄ solution turns translucent, the NaOH solution is gradually added drop by drop. The solution acquires a little yellow hue as a result of this reaction.

Step-VI

Development of Precipitate: In a 100 ml round-bottom flask, the evaporated metal ion solution from step 3 is transferred. The flask is filled with 20 ml of EG. The mixture is refluxed at 80 °C for 10 minutes. The (NH₄)₂HPO₄ solution is then poured into the flask drop by drop. Over two hours of heating at 120 °C, the solution progressively becomes white. During this procedure, a white precipitate is created in the solution.

Step-VII

Cleaning and Gathering: Ten ml of acetone is used to rinse the precipitate twice. It is then collected as a dry powder after being dried using an infrared (IR) light.To separate and gather the nanoparticles, centrifugation is carried out at 5000 rpm for 5 minutes.

Step-VIII

Annealing: The same method is used to create CePO₄ nanoparticles doped with different amounts of Ho³⁺ (3%, 5%, and 7% atomic percent). All of the synthesized samples are subjected to a 4-hour annealing process at 900 °C to improve their crystallinity and optical characteristics.

Figure 1(b): Schematic representation of Synthesis of   CePO4:Ho3+ Nanoparticles. Click here to View Figure

Results and Discussion

Instrumentation

For all the prepared samples these are the below characterization techniques and the instruments are used commonly. The crystalline and regular crystallite sizes of the given samples were checked using angle dispersive X-ray diffraction (source: Rigaku MiniFlex 600 X-ray diffractometer). Using a scanning electron microscope, microstructural characterizations of changes in shape and particle size are carried out. (SEM: QUANTA 200). To examine the vibrational structure of the generated materials, FTIR spectroscopy ( Bomem MB 102 spectrophotometer) was used. UC emission was measured using a monochromator (iHR320, Horiba Jobin Yvon) fitted with a photomultiplier tube. Using a diode laser, 980 nm radiation was used to stimulate the samples (2 W, power continuous mode, adjustable). In up-conversion studies, photoluminescence excitation (PLE) is used to examine the emission spectra of different samples are examined CePO_4:Ho³⁺

X-ray diffraction Studies

The XRD pattern of CePO₄:1%Ho³⁺ nano phosphor is described in Fig. 2. Here’s a summary of the key points: The XRD pattern of CePO₄:Ho³⁺ exhibits strong and distinct peaks, indicating a well-defined crystalline structure. The presence of strong peaks and the absence of impurity peaks suggest that the dopants (Ho³⁺ ions) are uniformly dispersed within the CePO₄ host lattice. The diffraction pattern matches the characteristic peaks and peak positions expected for the monoclinic phase of CePO₄. This indicates that the material has a monoclinic crystal structure. The coordination number (CN) of Ce³⁺ ions in the CePO₄ lattice is 8, meaning each Ce³⁺ ion is surrounded by eight oxygen anions. Ho³⁺ ions are substituted at the Ce³⁺ sites of the CePO₄ lattice due to their similar ionic radii, resulting in the presence of Ho³⁺ dopants within the material. The coordination of Ce³⁺ to eight oxygen anions results in a hexagonal to monoclinic distortion in the nano phosphor material, consistent with the JCPDS NO.01-083-0652 reference. The reference citation provided ³⁴ likely corresponds to the source where the XRD pattern and its interpretation can be found in more detail.

Figure 2: XRD pattern of CePO4:Ho3+ samples respectively. Click here to View Figure

SEM Studies

The SEM picture of CePO₄:1%Ho³⁺ nano phosphor material is annealed by applying 900 °C which is shown in Fig.3. It depicts nanoparticles with irregular shapes (a big number of Sponge shapes and a small number of other shapes) (cuboid, cone, rectangle shapes). The normal size of spherical particles is 50 nanometers.

Figure 3: SEM images of CePO4:Ho3+ respectively Click here to View Figure

FTIR Studies

The vibrational modes and chemical bonds that are present in a material may be learned a lot about FTIR spectroscopy. The CePO₄:Ho³⁺ acquired FTIR spectrum exhibits various distinctive peaks that aid in the comprehension of its vibrational structure. FTIR spectrum seen in Fig. 4. has the following full explanation: The CePO₄:Ho³⁺ nanomaterial’s peak is associated with a particular vibrational mode at a certain wavenumber (cm^-1) in the spectrum. Stretching or bending vibrations of the Ce-O, P- O, or Ho-O bonds are examples of potential vibrational modes. The FT-IR spectrum shows another peak at a different wavenumber. A discrete vibrational mode in the nanomaterial is represented by this peak. Each CePO₄:Ho³⁺ system’s chemical vibrations and bonds determine its exact identification. Phosphate (PO₄) group vibrations or interactions between the dopant ion (Ho³⁺) and the host lattice are examples of potential assignments. The FTIR spectrum could also show multiple peaks at different wavenumbers. The CePO₄:Ho³⁺ nanomaterial’s overall vibrational structure is influenced by the individual vibrational modes represented by each peak. we can observe the O-H peak frequently appears in the FTIR spectrum between 3200 and 3600 cm^-1, P-O peak at 900–1200 cm^-1 and Cerium-Oxygen (Ce-O) bands at 800-–1000 cm^-1from Fig.4.

Figure 4: FTIR of CePO4:Ho3+ respectively. Click here to View Figure

Photoluminescence Studies

Photoluminescence of CePO₄:Ho³⁺

Due to its energy levels and electron configurations, the rare-earth element holmium is known for having distinctive optical features. The unique energy level structure of Ho, which has several closely spaced energy levels, is one of the elements that contributes to its dual-mode behavior. The observed up-conversion and down-conversion processes result from interactions between these energy levels and incoming photons of various energies. A defence of the assertion based on Ho’s dual-mode behavior is provided here: Holmium has tightly spaced energy levels that make it possible for it to interact with a variety of photon intensities effectively. This implies that transitions between these energy levels can occur as a result of the holmium ions’ ability to absorb both lower- and higher-energy photons. Up-Conversion: Holmium ions can boost electrons to higher energy levels when they absorb lower-energy photons. The transitions to even higher energy levels that these excited electrons can make in the future can result in the emission of more powerful photons. This process of “up-conversion” combines many low-energy photons to create a single high-energy photon. Down-conversion: higher-energy photons can cause electrons to be promoted to higher energy levels when they are absorbed by holmium ions. In the process of returning to lower energy levels, these excited electrons can emit a number of lower-energy photons. A single high-energy photon is divided into many low-energy photons during the down-conversion process. Peaks are observed: Holmium’s closely spaced energy levels enable effective absorption and emission of photons with a range of energies. As a result, you can see peaks that correspond to both up-conversion and down-conversion processes when examining the emission or absorption spectra of holmium under various circumstances. The energy transitions involved in photon absorption and emission would be represented by these peaks. Due to this dual mode behavior   we can observe the similar type of peaks will observe yet Up-conversion & Down-conversion studies from dike book and also we can observe additional peak yet 450nm shows P-O weak charge transfer band. For the given content justification we observe from UC&DC study.

UC Study

Regarding the nano-phosphor substance from Fig.5. CePO₄:Ho³⁺‘s up-conversion luminescence: A multiphoton optical process called up-conversion luminescence produces high-energy photons while absorbing low-energy ones. It causes photons with energy greater than the excitation source to be emitted as a consequence. The nanophosphor CePO₄:Ho³⁺ displays up-conversion fluorescence. Emission of Ho³⁺ Depending on the concentrations of the host and co-dopant, Ho³⁺ ions in CePO₄:Ho³⁺ can produce either green or red emissions^12-16. CePO₄:Ho³⁺ emits in the ranges of 520–550 nm (green) and 630–670 nm (red), respectively. The Ho³⁺ ion undergoes electronic transitions at ⁵S₂ to ⁵I₈, ⁵F₅ to ⁵I₈, and ⁵F₄ to produce these emissions. Laser Excitation A laser is used to stimulate CePO₄:Ho³⁺ nano-phosphor material yet 980 nm. The P-O charge transfer band (CTB) and the CePO₄:Ho³⁺ absorption peaks at 450 nm are seen in the excitation spectra. The intensity of Up-converted Emission^17-18: The number of photons included in the up-converted emission bands (n) and the laser input power (P) both affect the up-converted emission intensity (I). The up-converted intensity rises with rising laser power and the number of photons taking part in the process, as shown by the relationship I  and Pn. Investigation and optimization. For studies involving up-conversion luminescence, CePO₄:1%Ho³⁺ nano-phosphor material, created from 1% Ho³⁺ doped CePO₄, is particularly intriguing. When stimulated beyond 980 nm, the optimized nano phosphor material displays up-conversion emission spectra with green and red bands^19-20.

Figure 5: Emission spectrrum of CePO4: Ho3+ (1 at.%Ho3+) excited at 980 nm. Click here to View Figure

DC Study

 Down-conversion: The main points are summarized as follows:

The Stokes Shift

Following light absorption (Eexc), the procedure involves the emission of lower-energy radiative light (Eem). The emission of light at lower energy compared to the absorbed light is caused by the Stokes shift.^21-22 Emission from Down-Conversion (DC): Fig.6 displays the CePO₄:Ho³⁺ (1% atomic percent Ho) DC emission spectra after UV irradiation at 300 nm Excitation, Green (450, 520, and 550 nm), red (630-670 nm), and other emission bands are seen.^23-24 These bands are connected to electronic transitions of the Ho³⁺ ion (⁵F₄, ⁵S₂ to ⁵I₈, ⁵F₅ ⁵I₈). The emission spectra are measured at various excitation possible wavelengths (260 – 300 nm). Due to the low absorption cross-section for Ho³⁺ f-f transitions, direct stimulation of Ho³⁺ at 450 results in less intense emission so there is no change observed. The emission peaks linked to Ho³⁺ and the wide emission band 450 nm linked to PO₄^3- may be seen when stimulated at 300 nm.²⁵ Due to the P-O charge transfer band’s (CTB) permitted transition, the absorption cross-section at 300 nm is considerable. Ho³⁺‘s radiative rate is increased through energy transfer (ET), Excitation-Dependent Emission.²⁶Which occurs via resonance, from PO₄^3- to Ho³⁺. CePO₄:Ho³⁺ monitoring is shown in Fig. 6 at 520-550 nm emission, with a broad peak seen at 300 nm that is due to the P-O CTB transition. Due to Ho³⁺ emissions, sharp peaks with modest intensities are seen at 450 nm, 520 -550 nm, and 630-670 nm. Ho³⁺ emission peaks are seen when the CePO₄ host is stimulated at 300 nm with   amount of 1%Ho³⁺ doped in. As the Ho³⁺ ion concentration rises above 1%, the luminescence intensity drops.²⁷ET Process: The “conn” quenching effect serves as the primary energy source for the energy transfer (ET) process. The luminescence concentration quenching that is seen at greater dopant concentrations is known as the “conn” quenching effect.^28-34     

Figure 6: Excitation spectrum of CePO4: Ho3+(1 at.% Ho) excited at 300 nm. Click here to View Figure

Conclusion

In conclusion, a polyol-mediated process is successful in producing CePO₄:1% Ho³⁺nanophosphor. After the preparation process, the sample is annealed for 4 hours at 900°C to increase crystallinity, eliminate organic components, and reduce water content.  XRD revealed the monoclinic structural phase with space group I41/amd. Yet 980 nm excitation, CePO₄:0.01Ho³⁺ produces significant up-converted green and red color bands at ∼520-550 nm (⁵ F₄, ⁵ S₂→ ⁵ I₈), ∼630-670 nm ⁽⁵ F₅ →⁵ I₈) of Ho³⁺. The broad emission band results primarily from ligand to metal CT (P-O CTB). Peaks at 450nm, ∼520-550 nm (⁵ F₄, ⁵ S₂ → ⁵ I₈), ∼630-670nm (⁵ F₅→ 5 I₈), at 300nm excitations of DC   be grasped.

Acknowledgment

My heartfelt appreciation goes to GITAM Deemed to be University(Hyderabad) for providing a facility for synthesis.

Conflicts of interest

We have no competing interests that could Influence our research or findings.

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