# Zinc Oxide Nanoparticles for White Light-Emitting Diodes and Solar Cells

Alexandra Apostoluk, Dorian Priano, Maciej Sibinski, Katarzyna Znajdek, Stéphane Danièle, Bruno Masenelli

► To cite this version:

Alexandra Apostoluk, Dorian Priano, Maciej Sibinski, Katarzyna Znajdek, Stéphane Danièle, et al. "Zinc Oxide Nanoparticles for White Light-Emitting Diodes and Solar Cells. Mastering Optoelectronics - Fundamentals, Applications, and Innovations [Working Title], IntechOpen, 2025, 10.5772/intechopen.1011840". hal-05381902

HAL Id: hal-05381902

https://hal.science/hal-05381902v1

Submitted on 25 Nov 2025

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# Zinc Oxide Nanoparticles for White Light-Emitting Diodes and Solar Cells

Alexandra Apostoluk, Dorian Priano, Maciej Sibinski,
Katarzyna Znajdek, Stéphane Danièle and Bruno Masenelli

## Abstract

ZnO nanoparticles have been synthesized by an easy, cheap, and environmentally friendly one-pot synthesis method at room temperature. Non-rare-earth (non-RE) elements are doped during the synthesis process to adjust the emission color of ZnO nanoparticles (ZNO NPs). The emission color of the ZnO NPs can be tuned from orange to yellow or green. These ZnO nanoparticles served as phosphors in the fabrication of white light-emitting diodes (WLEDs) or as down-shifters in solar cells. The internal luminescent quantum yield was 64% and stable over time. These results indicate that these ZnO nanoparticles have great potential applications in down-shifters for solar cells, lighting, and displays.

**Keywords:** zinc oxide, nanoparticle, sol-gel, structural characterization, photoluminescence, white light-emitting diode, solar cell

## 1. Introduction

The development of devices using solar energy as a source of electrical energy, complementary to other non-fossil energies, requires continuous research and innovation efforts towards achieving higher photovoltaic cell efficiencies. The goal is, of course, to improve electrical conversion efficiency and to reduce production costs. Silicon-based cells, still the majority on the market, have seen their conversion performance reach 23.3% for multicrystalline cells and 26.1% for monocrystalline cells [1] and 22.5% in industrial production (panels with IBC cells—Interdigitated Back Contact cells), knowing that the theoretical maximum efficiency of a Si-based junction is approximately 31% [2] of the total incident solar radiation. One of the main limits to the efficiency of these cells comes from the spectral response of the p-n junction, which is not totally adapted to the solar emission spectrum. Thus, there are losses due to reflection, another part of the photogenerated charge carriers is lost by thermalization or recombination. For hot carriers, an excess of their energy is converted into heat, creating additional losses, because this heating reduces the efficiency of the cell. Different strategies have already been proposed to address this efficiency reduction problem, such as sunlight concentration, multijunction cells, multiple exciton
generation, and spectral modification of the incident light by up-conversion or down-
conversion mechanisms (or down-shifting, see **Figure 1**) [3]. The latter two
approaches necessitate the addition of a concentrator on the top layer of the solar cell.
Consequently, in recent years, significant efforts have been made to obtain a better
material for down-conversion, which involves splitting a high-energy photon into two
lower-energy photons. Typically, a thin, electrically insulated layer containing the up-
or down-converting material is placed on top of the solar cell, but in some cases this
layer can be located on the back surface of the cell. The down-conversion mechanism
can be achieved using lattice defects or doping ions, sometimes achieving efficiencies
greater than unity. In this case, the excited ion can transit from a higher- to a lower-
energy state by transferring its energy to a second ion. Following the transfer, both
ions may emit photons as they return to their ground states.

A general approach consists of placing homogeneous films containing rare-earth
ions on a crystalline silicon solar cell, by a low-cost screen printing for example. Thin
films doped with a europium(III) ion (Eu³⁺) complex in a polymer matrix were
deposited as an antireflection layer on the solar cells. For these cells, an improvement
in photovoltaic conversion efficiency of 0.1% (from 15.2 to 15.3%) is then obtained
[4]. Even better efficiency was obtained for silicon nitride (SiNₓ) layers doped with
terbium ion (Tb³⁺) or co-doped with Tb³⁺-Yb³⁺ (ytterbium ion). In both cases, the
films are used to replace the antireflection layer of the underlying silicon solar cell,
and a significant gain in photovoltaic conversion efficiency is obtained. It was 17.13%
for the SiNₓ layer alone and increased to 17.31% for Tb³⁺ doping and to 17.36% for Tb³
⁺-Yb³⁺ doping [5]. In addition to lanthanide ions embedded in various host matrices
such as oxides, emerging materials are those operating at the nanoscale: inorganic
quantum dots including CdS, CdSe, CdTe, CuInS₂, CuS, PbS, PbSe, InP, InAs, Ag₂S,
Bi₂S₃, Sb₂S₃ [6] and core-shell systems such as CuInS₂/ZnS [7], graphene quantum
dots [8], silicon nanoparticles (Si NPs) [9], which, among others, have proven their
effectiveness in improving the final efficiency in different types of solar cells [10].
Photovoltaic systems are required to maintain their initial efficiency for at least

<figure><img src="image_2.png" /><figcaption>**Figure 1.**
Down-shifting diagram. λexcitation—wavelength of the incident light, which will excite an electron from the valence band to the conduction band. From there, the electron will deexcite non-radiatively to a localized state in the band gap (a defect in the crystal lattice, a dopant atom, etc.) and then recombine with a hole in the valence band, this time radiatively. Light with a longer wavelength (λemission) than the excitation wavelength is emitted.</figcaption></figure>
20 years, which necessitates both mechanical stability and resistance to degradation from atmospheric exposure. To successfully implement down-conversion-optimized solar cells in commercial systems, much work remains to be done to improve their stability and robustness under environmental conditions. Promising nanomaterials used for down-conversion or down-shifting, such as quantum dots or nanoparticles, can be synthesized in solution and deposited from them, which would facilitate their large-scale production. While the use of nanomaterials is a very attractive approach, there are serious challenges to overcome, such as controlling the interaction between the environment and the solar cell, ensuring large-scale deposited layer uniformity, and among others, the fact that surfaces in nanostructures increase the probability of recombination of photogenerated charge carriers. Methods for synthesizing various materials can be divided into two groups: physical and chemical. Selecting the appropriate material and the most practical synthesis method depends on the optical correspondence between the solar cell's band gap and the optical spectral response of the converter layer.

Recently, nanomaterials have attracted significant research attention due to the ease with which their optical properties can be tuned by adjusting their size and shape. ZnO (and especially its nanostructures) turns out to be promising as a down-shifter material for photovoltaic cells, thanks to its ability to convert the ultraviolet (UV) part of the solar spectrum into wavelengths more suitable for efficient absorption. Its high luminescent efficiency and stability make it an interesting candidate for increasing the energy conversion of solar cells. In order to assure a cost-effective and scalable method, suitable for large-scale production of ZnO nanoparticles for industries such as electronics, medicine, and environmental protection, we used chemical synthesis. It also allows precise control over their size, shape, and surface properties. We performed the sol-gel-based synthesis permitting the synthesis and simultaneous encapsulation of the phosphor, the originality lying in performing a hydrolysis-condensation reaction in a basic medium on an acidic polymer.

## 2. Material synthesis and characterization

In the case of sol-gel synthesis method, it is relatively cheap, the luminescence quantum yield of the synthesized materials is high, and process parameters can be easily controlled. Moreover, in sol-gel synthesis, various precursors, solvents can be used and various additives (like surfactants) added to the synthesis milieu *during* or *after* the synthesis. In our case, diethylzinc (ZnEt₂) was used to perform the hydrolysis in water, with the addition of PAAH (polyacrylic acid, (CH₂-CHCO₂H)ₙ) and PAANa (sodium polyacrylate) as stabilizing surfactants, according to the exothermic reaction:

$$ \mathrm{ZnEt_2 + H_2O \rightarrow ZnO + 2 EtH} \quad (1) $$

PAAH with a molecular weight of M_w = 2000 g·mol⁻¹ and with the different ratios (0, 0.063, 0.63, 6.3 wt%) was added into water during the synthesis to control the surface states of ZnO NPs. It can be noticed from the X-ray diffraction (XRD) diffractograms presented in **Figure 2** that as the PAAH concentration increases, the diffraction peaks associated with the wurtzite ZnO structure decrease and finally disappear at the highest PAAH concentration. It confirms that the crystalline quality degrades as the PAAH content increases. It can therefore be concluded that more defects are gradually introduced into the structure of ZnO nanoparticles and finally an
<figure><img src="image_3.png" /><figcaption>**Figure 2.** X-ray diffractograms of ZnO nanoparticles synthesized via hydrolysis of ZnEt₂ with varying PAAH concentrations (0.063, 0.63, and 6.3 wt% from top to bottom) are shown. The curves are vertically offset for clarity. Peaks marked with an asterisk (*) correspond to the wulfingite phase Zn(OH)₂. Reprinted with permission from Zhu et al. [11].</figcaption></figure>

amorphous material is obtained. At the same time, the particle size, estimated using the Scherrer equation, varies from 17 nm (no PAAH), passing through 15 nm (0.063 wt%) to 8 nm (0.63 wt%). Note that at the concentrations of 0.063 and 0.63 wt %, some traces of ε-Zn(OH)₂ (wulfingite) are present in parallel with the wurtzite phase of ZnO, as can be observed in **Figure 2**.

Photoluminescence (PL) spectroscopy was performed using a setup with continuous laser excitation at 266 nm (1.2 mW, Crylas FQCW 266-10). The emitted light was dispersed by a spectrometer (iHR Triax 320, Jobin-Yvon) and detected using a liquid-nitrogen-cooled Si charge-coupled device (CCD) detector. The photoluminescence quantum yield (PL QY) was determined with an integrating sphere, employing the same excitation and detection configuration as used for the PL measurements.

It turns out that with the hydrolysis method carried out in water permits to easily modify the surface of the NPs by changing the synthesis conditions and it is possible to synthesize visible-emitting ZnO NPs (see **Figure 3**) and that these NPs are organized into nanospheres of about 200 nm in diameter, as can be seen in transmission electron microscopy (TEM) images in **Figure 4**. It turns out that the presence of these spheres is mandatory for a high quantum yield of the PL (see **Figure 5**).

The studies presented in **Figure 5** underline the importance of the existence of nanospheres for the photoluminescence quantum yield. It was also observed that the destruction of nanospheres leads to the decrease of the quantum yield of the PL, as can be seen in **Figure 6**. Studies done for ZnO NPs synthesized with PAAH having a short molecular chain ($M_w$ = 2000 g mol⁻¹), heating the ZnO NPs up to 400°C leads to the decomposition of PAAH (as can be seen in TEM images in **Figure 5**), which results in the loss of the nanosphere structure and the decrease of PL QY. The reason that it is higher at 400°C than at 350°C is due to the fact that at 400°C ZnO has probably improved its crystalline quality (hence some emission in the UV appeared) and stabilized its surface defects after the decomposition of the polymer chains.
<figure><img src="image_4.png" /><figcaption>**Figure 3.** PL spectra of ZnO nanoparticles synthesized via hydrolysis of ZnEt₂ with varying PAAH concentrations (from bottom to top: 0, 0.063, 0.63, and 6.3 wt%). The spectra are vertically offset for clarity. Reprinted with permission from Zhu et al. [11].</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** TEM images of ZnO nanoparticles synthesized via hydrolysis of ZnEt₂ with (a) 0.063 wt% PAAH and (b) 0.63 wt% PAAH. The inset in (b) shows a magnified view of the surface of a nanoparticle. Reprinted with permission from Zhu et al. [11].</figcaption></figure>

We proposed a model of the energy levels of the ZnO/PAAH NPs nanocomposite, with the conduction and valence band levels of ZnO and the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of PAAH, presented in **Figure 6**. The two materials form a type II heterojunction. When a UV photon is absorbed by either ZnO or PAAH, the electron readily moves to the bottom of the ZnO conduction band, while the hole becomes trapped in the HOMO of PAAH. This process reduces exciton formation in ZnO when the materials are mixed on a scale smaller than the exciton diffusion length. Subsequently, the electron relaxes to a defect level within the ZnO band gap, where it is trapped. Finally, the recombination of the trapped electron and hole produces visible emission. The emission efficiency is
<figure><img src="image_6.png" /><figcaption>**Figure 5.** TEM images of nanospheres containing ZnO NPs synthesized by hydrolysis of ZnEt₂ with 0.63 wt% PAAH and the corresponding PL spectra (excited at 266 nm) with the PL quantum yield value. The temperature at which the sample was heated before PL and quantum yield measurement is indicated in each case.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Energy level diagram for ZnO/PAAH NPs nanocomposites and the supposed mechanism of the observed photoluminescence [11].</figcaption></figure>

enhanced due to the strong spatial overlap of the trapped electron and hole wave functions.

The disadvantage of all solutions in which such a luminescent layer is used is that the emission of photons generated via down-shifting is bidirectional. For this reason,
in the case of the luminescent layer placed on the surface of the solar cell, as well as the luminescent layer placed between two cells, approximately 50% of the light energy converted via down-shifting will be lost for conversion into electricity. We have proposed a converter system that significantly increases the photoconversion energy efficiency of the solar cell. The subject matter of the patent is shown in **Figure 7** [12]. It consists of a solar cell equipped with a p-n semiconductor junction, covered with a luminescent layer 1, based on ZnO nanoparticles in a transparent matrix of polymer PAAH or its derivative PAAN (CH₂-CHCO₂N)ₙ, used to convert photons in UV into photons in the green and red via down-shifting. The system is equipped with a directional optical transmission layer 2 (one-way mirror layer), with optical transmission in the direction of the p-n junction, made by low-temperature sintering of silver pastes, or by application of optically selective polymer layers. It reflects the light emitted via down-shifting entirely towards the semiconductor junction (the cell itself) by this layer based on zinc oxide nanoparticles in a polymer matrix and deposited on the surface of the solar cell by screen printing or centrifugation.

The broad visible emission in ZnO nanostructures originates from various native point defects, such as oxygen interstitials (Oᵢ), oxygen vacancies (V_O), oxygen antisites (O_Zn), zinc interstitials (Znᵢ), zinc vacancies (V_Zn), and zinc antisites (Zn_O) [13]. Because these defects greatly affect the photoluminescence of ZnO nanoparticles, it is crucial to control their formation and understand how they influence the emission properties of ZnO NPs for the advancement of targeted optoelectronic devices.

We studied the emission of pure and doped ZnO NPs synthesized by hydrolysis of diethylzinc (see Eq. (1)). To avoid the use of rare-earth (RE) materials as dopants, which can pose environmental risks due to the toxicity of some of these elements and could lead to increased production costs of down-shifting layers, white light-emitting diodes, and waste treatment associated with these materials, we opted for non-RE elements. The dopant ions we used typically have smaller ionic radii than lanthanides, resulting in higher solubility in ZnO and facilitating their incorporation. This enables easier tuning of the properties of the resulting ZnO-based materials. Among the most studied non-rare-earth dopants are transition metals (TMs), with elements such as Co, Fe, and Mn being particularly promising for inducing ferromagnetism in otherwise non-ferromagnetic ZnO. Regarding optical properties, Cu and Cr are among the most studied non-RE elements in ZnO. Doping rates of 0.1, 1, and 5% were tested for the following atoms: Ag, Bi, Ce, Cu(I), Cu(II), Ni, Mn, Co, Cr, Fe(II), Fe(III), and Al,

<figure><img src="image_8.png" /><figcaption>**Figure 7.**
Schematic of a solar cell with a frequency converter layer (1) covered by a one-way mirror (2) to ensure that all light generated via down-shifting is directed towards the cell at the bottom [12].</figcaption></figure>
ranked according to their decreasing ionic radii. The priority was always given to preserve the mesosphere structure, so important for the high quantum yield of luminescence. The first effect is the enhancement of light-matter interaction through mesosphere-induced scattering. Mesosphere presence can also help passivate the surface of ZnO nanocrystals and reduce non-radiative surface states. Together, these effects are expected to improve the luminescence quantum yield of the final material. However, this effect is not dominant in this case, as the mesospheric (Cu-doped) samples exhibit the same PL quantum yield as samples without mesospheres (Bi-doped, see **Figure 8b**).

The influence of dopant atom and its concentration on the morphology of the ZnO NPs and mesospheres was studied. The different atoms do not behave in the same way. The various dopant atoms exhibit different behaviors. As illustrated in **Figure 9**, a low Mn ion concentration (0.1%) maintains the ZnO mesospheres, which have an

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Morphologies of ZnO nanoparticles doped with different elements at 0.1% concentration: (a) Cu(II)-doped and (b) Bi-doped. Reprinted with permission from Zhang et al. [14].</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 9.** Morphologies of ZnO nanoparticles doped with Mn at (a) 0.1% and (b) 5% concentrations, respectively. Reprinted with permission from Zhang et al. [14].</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 10.** PL spectra of Co, Fe(III), Mn, and Cu(II)-doped ZnO NPs with 0.1% concentration compared to the PL spectrum of undoped ZnO NPs. Reprinted with permission from Zhang et al. [14].</figcaption></figure>

average diameter of about 50 nm. These spheres consist of nanoparticles embedded within an organic PAAH matrix, as shown in the inset of **Figure 9a**. At 5% Mn doping, the mesospheres remain present but increase in size, reaching an average diameter of approximately 100 nm (see **Figure 9b**). But this is not always the case. **Figure 8** shows that doping at the same level, but with two different ions, can destroy the structure of mesospheres already at 0.1%, which is especially clearly visible for bismuth (**Figure 8b**).

We observed a tendency that larger dopant ions are unfavorable for mesosphere formation [14]. Dopant ions and their doping levels will also modify the luminescent emission spectra of ZnO NPs. **Figure 10** shows the PL spectra for the 0.1% doping level for Co, Fe(III), Mn, and Cu(II), where a clear redshift is observed compared to the spectra of undoped NPs. For both 0.1% doped and undoped ZnO NPs, the PL emission peak (max emission intensity) can be tuned by doping from 2.46 eV (undoped ZnO) down to 2.16 eV, corresponding to a wavelength range of 504 to 574 nm.

Valence also matters—if we look at the case of copper, for example, the similarity in the visible part of the emission spectra suggests that the two valence states of Cu affect deep defects in a similar way. At 1% doping, the spectra are very similar [15]. A notable difference between the two valence states of Cu doping is that Cu(II) manifests itself at 5% doping, where Cu(I) doping causes a more drastic redshift than Cu(II). The emission peaks are in a small range between 2.30 and 2.25 eV for Cu(II) doping at three doping concentrations, while 5% Cu(I) ranges from 2.30 to 2.18 eV (see **Figure 11**). The largest redshift can be observed for 5% doping for Cu(I) and Fe(III) (see **Figure 12**). Considering all doping cases at all doping concentrations, the emission range extends from 2.46 (undoped ZnO) to 2.08 eV, or from 504 to 596 nm [14].

Contrary to several reports in the literature, some specific dopants appear to preserve or even enhance the PL quantum yield (PL QY). For example, Ce⁴⁺, Ag⁺, Bi³⁺, and Ni²⁺
<figure><img src="image_12.png" /><figcaption>**Figure 11.** PL spectra of Cu-doped ZnO NPs with two different Cu valencies (+1 and +2) at 5% doping concentrations compared to undoped ZnO NPs.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 12.** PL spectra of undoped ZnO NPs and of 5% doping concentration of Fe(II), Fe(III), Cu(I), and Mn. Reprinted with permission from Zhang et al. [14].</figcaption></figure>

maintain PL QY values nearly identical to undoped ZnO nanoparticles, while Cr³⁺, Cu²⁺, Al³⁺, and Fe³⁺ show improved PL QY compared to undoped ZnO NPs. In **Figure 13**, the normalized PL QY of the undoped ZnO nanohybrids and the ionic radius of Zn²⁺ are indicated by dotted lines. Notably, the most effective dopants cluster in the upper-right
<figure><img src="image_14.png" /><figcaption>**Figure 13.** PL QY of doped ZnO NPs normalized to PL QY of undoped ZnO NPs. Reprinted with permission from Zhang et al. [14].</figcaption></figure>

region of the plot, indicating that a small ionic radius combined with a high precursor valence tends to enhance PL QY. However, this alone is not sufficient, as some dopants with a 2+ valence and relatively small ionic radius, such as Mn and Co, do not improve the PL QY of ZnO nanoparticles.

**Figure 14** presents the emission color of three doped samples and the undoped ZnO NPs on a gamut chart in Judd's Uniform Chromaticity Scale (UCS) diagram. The

<figure><img src="image_15.png" /><figcaption>**Figure 14.** Locations of the spectra from selected doped samples within the gamut chart on Judd's UCS diagram. Reprinted with permission from Zhang et al. [14].</figcaption></figure>
sample's emission projection onto the Planckian locus reveals a wide range of achievable color tuning solely through doping.

The next step in this work is the fabrication of ZnO-PAAH nanomaterials surrounded by a tetraethyl orthosilicate (TEOS) shell synthesized by the sol-gel method. A silica-coated structure may have some advantages, such as improved material stability. Silica is widely used as a coating material because of its high stability, particularly in aqueous environments, ease of application, compatibility with other materials, and optical transparency.

The silica coating is applied to a ZnO/PAAH suspension using tetraethyl orthosilicate (Si(OC₂H₅)₄, TEOS). This process results in ZnO nanocrystals embedded within PAAH mesospheres, which are unevenly coated with silica. Two key factors influencing the coating process will be examined: the TEOS-to-ZnO ratio (x) and the amount of NH₄OH in the alkaline solution (solution A, composed of EtOH, H₂O, and NH₄OH in mass ratios of 1:0.93:0.027y, y being different amounts of ammonia in solution A, varied from 1 to 4). These parameters control the formation of the ZnO/PAAH core-shell structure and ultimately affect the photoluminescence quantum yield (PL QY).

We observed that the greater the NH₄OH content used during the synthesis, the lower the presence of the PAAH crystalline phase [14]. This behavior can be attributed to the acid-base reaction between PAAH and NH₄OH, which gradually converts PAAH into its salt form, PAANH₄. Unincorporated ZnO nanocrystals are also observed within the mesospheres, indicating that high ammonia concentrations disrupt the PAAH mesospheres. Thus, ammonia has a dual effect: increasing its concentration promotes silica deposition but simultaneously leads to the progressive degradation of mesospheric structures that are essential for high photoluminescence efficiency.

Since the ZnO/PAAH spectrum was not modified by the silica coating [14], the PL QY becomes a key parameter for determining the attractiveness of silica-coated ZnO

<figure><img src="image_16.png" /><figcaption>**Figure 15.** PL QY improvement of xSi-1A samples as a function of TEOS to ZnO ratio in the synthesis reaction [15].</figcaption></figure>
nanohybrids. The PL QYs of the xSi-1A samples are plotted in **Figure 15**. An optimal amount of silica coating is crucial for enhancing the PL quantum yield (PL QY). Insufficient silica coverage—such as in the 1.1Si-1A sample shown in **Figure 15**—leads to more surface states on the ZnO/PAAH nanoparticles, which reduce PL QY by decreasing charge trapping efficiency at deep defects, causing charges to be captured by non-radiative surface states. Conversely, excessive silica coating—as seen in the 10Si-1A sample—can partially absorb the UV excitation light before it reaches the ZnO, diminishing excitation efficiency. Therefore, a TEOS ratio of 3.8 was considered as an appropriate amount of silica coating to observe a strong increase in PL QY (see **Figure 15**).

For this amount of NH₄OH, an increase in the luminescence quantum yield of the final material was observed. The exact mechanism behind the improvement in PL quantum yield (PL QY) remains unclear, but two complementary hypotheses can be proposed. The first suggests that silica fills the pores within the PAAH, making the mesospheres impermeable to atmospheric contaminants or impurities from the synthesis solution that might otherwise fill defects—mainly oxygen vacancies—critical for ZnO's visible emission.

The second hypothesis involves the electronic charge state of certain defects. Visible emission in ZnO nanoparticles primarily arises from ionized oxygen vacancies (V_O). Although the introduction of TEOS does not directly affect the defects responsible for visible emission, it may increase either the overall concentration of oxygen vacancies or, more likely, the concentration of singly ionized V_O through silica formation. It is unlikely that TEOS induces new oxygen vacancies in ZnO nanoparticles embedded in PAAH mesospheres. However, since PAAH mesospheres are not completely impermeable to charge migration, a change in the charge state of oxygen vacancies in some ZnO nanoparticles could occur, effectively increasing the concentration of ionized vacancies.

The enhancement factor shown in the graph was determined by comparing the PL QY of each sample to that of its ZnO/PAAH precursor and averaged over multiple samples. The highest PL QY improvement was observed in the 3.8Si-2A samples, where PL QY increased by a factor of 4, reaching an absolute value of up to 64% [16].

Further development requires that the ZnO/PAAH nanophosphor be dispersible in a silicon resin to fabricate a film to be used in WLEDs or solar cells. The highly hydrophobic property of silicon resin makes the dispersion complicated in the case of hydrophilic nanophosphors, such as ZnO/PAAH. Improving the dispersion of such nanophosphors in silicon resin is a real challenge. Li et al. show that bi-modal silica coating of ZrO₂ nanoparticles can improve properties of the obtained film, such as dispersion, real permittivity, and breakdown field [17]. This coating was also efficient with CdSe quantum dot [18]. To perform a bi-modal silica coating, two silica alkoxides were used, tetraethyl orthosilicate (TEOS), which provides hydroxide groups, and vinyl triethoxysilane (VTES), which provides vinyl groups on the nanoparticle's surface. After that, a hydrosilylation can be done with short or long polydimethylsiloxane chain. We performed coating of ZnO/PAAH NPs with silica by the Stöber process. ZnO/PAAH nanoparticles in a suspension were synthesized by hydrolyzing 7.95 mmol of diethylzinc in 100 mL of a water solution containing 0.63 wt% of polyacrylic acid under ambient temperature according to Eq. (1).

Before studying the encapsulation of ZnO/PAAH nanoparticles by the sol-gel process, it is important to determine the optimum reaction parameters between TEOS and VTES, which concern the reaction time, the amount of catalyst (NH₄OH), the hydrolysis ratio, and the ratio between TEOS and VTES. The aim is to obtain the
conditions for developing a silica shell with hydroxyl (OH) and vinyl groups on the surface, in order to achieve bi-functionalization of the surface with silicone oligomers via condensation and hydrosilylation reactions, respectively. The hydrolysis-condensation reaction used here is known as the Stöber method in a basic medium. The first part of the study focused on the ratio between the quantities of TEOS and VTES, following a protocol described in the literature [19]. Initial syntheses were carried out with TEOS mass fractions ranging from one to 0.8 and reaction times of 1 or 24 hours, as shown in **Table 1**. The amount of ammonia was set at 2.6 molar equivalents relative to the amount of silica based on work carried out on the optimum amount of ammonia in a sol-gel reaction using the Stöber method [19].

After 24 hours of reaction, yields were significantly higher than those obtained after 1 hour. Time therefore has a positive impact on the reaction. Nevertheless, yields still decrease as the amount of VTES increases, as VTES seems to have slower condensation hydrolysis kinetics than TEOS. The reason is that VTES has a non-hydrolyzable vinyl group, which will induce a difference in electronegativity on silicon. Nanoparticle size was determined by dynamic light scattering (DLS) and shows a decrease when VTES proportion increases.

¹H MAS NMR is a technique where high-resolution magic angle spinning (MAS) is used in combination with proton nuclear magnetic resonance (¹H NMR) spectroscopy. ¹H NMR spectra presented in **Figure 16** (left image) give no information about the presence of vinyl groups because only ethanol can be seen.

<table><caption>**Table 1.**

Mass yield and size of particles obtain at different mass fractions of TEOS and VTES.</caption><thead><tr><th rowspan="2">Mass fraction of TEOS:VTES</th><th colspan="2">Mass yield</th><th colspan="2">Size</th></tr><tr><th>After 1 h</th><th>After 24 h</th><th>After 1 h</th><th>After 24 h</th></tr></thead><tbody><tr><td>0.8:0.2</td><td>1%</td><td>65%</td><td>78.5 nm</td><td>110 nm</td></tr><tr><td>0.85:0.15</td><td>10%</td><td>97%</td><td>117 nm</td><td>126 nm</td></tr><tr><td>0.9:0.1</td><td>24%</td><td>100%</td><td>100 nm</td><td>138 nm</td></tr><tr><td>0.95:0.05</td><td>24%</td><td>100%</td><td>129 nm</td><td>152 nm</td></tr><tr><td>1:0</td><td>34%</td><td>100%</td><td>163 nm</td><td>178 nm</td></tr></tbody></table>

<figure><img src="image_17.png" /><figcaption>**Figure 16.**

MAS ¹H NMR spectra (left) and CP-MAS ²⁹Si NMR spectra (right) of 0.8:0.2 TEOS:VTES ratio after 24 h.</figcaption></figure>
$^{29}$Si cross polarization magic angle spinning (CP-MAS) NMR spectroscopy is a method used to analyze insoluble organosilicon compounds. On CP-MAS $^{29}$Si NMR spectra shown in **Figure 16** (right image), peaks at -105 and -110 ppm (parts per million) correspond, respectively, to Q3 and Q4 groups presented in **Figure 17** for the completely hydrolyzed and condensed TEOS [20]. Peaks at -80 and -68 ppm correspond, respectively, to T3 and T2 groups [21].

The same observation was done on 100 $_{w}$% of TEOS after 24 h, as it can be seen in
the spectra presented in **Figure 18**.

TEM pictures shown in **Figure 19** show that the morphology of silica nanoparticles is perfectly spherical for syntheses ranging from 0 to 10 $_{w}$% of VTES. From 15 $_{w}$% of VTES upwards, nanoparticle morphology is more irregular, which can be explained by surface modification by vinyl groups creating irregularity. To conclude the compatibility of TEOS and VTES, a ratio of 0.9:0.1 of TEOS:VTES was kept in order to perform bi-modal coating of ZnO/PAAH, because increasing the amount of VTES tends to increase the reaction time and irregularity on nanoparticle's surface.

One of the problems of encapsulating ZnO/PAAH nanoparticles using the Stöber
process concerns the use of a base as a catalyst, which can be consumed by acid-base
reaction with PAAH and/or also destroy the ZnO/PAAH self-assembly. The interest-
ing optical properties of mesosphere are based on the entanglement of polyacrylic acid
with crystallized zinc oxide nanoparticles, which generates efficient light-matter
interactions. The solvent plays an important role in terms of mesosphere stability and
pH, so particle encapsulation was carried out mainly in ethanol, as ZnO/
PAAH suspensions were shown to be more stable when ammonia was added to an
ethanol suspension. Coating was performed with different ratios of ZnO/PAAH and
silicon alkoxide, as shown in **Table 2**.

<figure><img src="image_18.png" /><figcaption>**Figure 17.**
Structures of TEOS and VTES after hydrolysis condensation. X = H or R.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>**Figure 18.**
MAS $^{1}$H NMR spectra (left) and CP-MAS $^{29}$Si NMR spectra (right) of 1:0 TEOS:VTES ratio after 24 h.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure 19.** TEM pictures of silica nanoparticles at (a) 20, (b) 15, (c) 10, (d) 5, and (e) 0% of VTES after 24 h.</figcaption></figure>

<table><caption>**Table 2.** Ratio tested between ZnO/PAAH and SiO<sub>1.5</sub>(C<sub>2</sub>H<sub>3</sub>)<sub>0.5</sub> and yield obtained.</caption><thead><tr><td>Ratio of Zn:Si</td><td>Mass percentage of ZnO/PAA</td><td>Mass percentage of SiO<sub>1.5</sub>(C<sub>2</sub>H<sub>3</sub>)<sub>0.5</sub></td><td>Mass yield</td></tr></thead><tbody><tr><td>1:99</td><td>1</td><td>99</td><td>87%</td></tr><tr><td>1:49</td><td>2</td><td>98</td><td>84%</td></tr><tr><td>1:1</td><td>50</td><td>50</td><td>92%</td></tr><tr><td>2:1</td><td>67</td><td>33</td><td>93%</td></tr><tr><td>4:1</td><td>84</td><td>16</td><td>83%</td></tr></tbody></table>

**Table 2** shows good mass yields, despite the presence of ZnO/PAAH, and yields do not change with the increase of ZnO/PAAH ratio.

**Figure 20** (left image) shows the FTIR-ATR spectra showing the characteristic bands of n<sub>C=O</sub> at around 1590 cm<sup>-1</sup>, n<sub>C-H</sub> at around 2940 cm<sup>-1</sup>, and n<sub>O-H</sub> between 3200 and 3400 cm<sup>-1</sup> of polyacrylic acid. Note that the intensity of these peaks grows as the mass percentage of ZnO/PAAH increases. The characteristic Si-O-Si bond band is also found at around 1050 cm<sup>-1</sup>, the latter being weaker for the 4:1 ratio as the amount of silica is greatly reduced. TGA curves in the **Figure 20** (right image) confirm the increase in the amount of ZnO/PAAH within the core-shell systems, with very significant mass loss between 400 and 500°C from the 1:1 ratio. Regarding mass loss, the 1:1 ratio (brown curve in the right image of **Figure 20**) should have a mass loss of its organic part of 25% of the total mass. According to the TGA curve, the mass loss related to polyacrylic acid around 440°C represents 20% of the total mass. For the 2:1 ratio (purple curve), the theoretical mass loss is 34% and is around 25% according to the TGA curve. For the 4:1 ratio (green curve), the theoretical mass loss is 42% and is around 30% according to the green TGA curve. Overall, the observed mass losses
<figure><img src="image_21.png" /><figcaption>**Figure 20.** Fourier transform infrared-attenuated total reflection (FTIR-ATR) (left) and thermogravimetric analysis (TGA) (right) of different ratios of ZnO/PAAH coating by TEOS and VTES.</figcaption></figure>

are slightly lower than theoretical. The hypothesis is that ammonia reacts with polyacrylic acid.

In fact, as discussed above, the possibility of a reaction between polyacrylic acid and ammonia cannot be ruled out. As polyacrylic acid acts as a surfactant and stabilizer for ZnO/PAAH particles, the presence of ammonia can lead to an acid-base reaction between the two, destabilizing the ZnO/PAAH nanoparticle. If the amount of base is significantly greater than the amount of acid, this can lead to the dissolution of ZnO to the hydroxide $Zn(OH)_2$. From the results of thermogravimetric analysis, it would appear that ammonia slightly disturbs the formation of nanoparticles.

<figure><img src="image_22.png" /><figcaption>**Figure 21.** TEM pictures of nanoparticles obtained with (a) 1:99, (b) 1:49, (c) 1:1, (d) 2:1, and (e) 4:1 ratios of Zn:Si.</figcaption></figure>
<figure><img src="image_23.png" /><figcaption>**Figure 22.** EDX analysis of the ratio of Zn:Si of 2:1.</figcaption></figure>

In TEM images presented in **Figure 21**, the (a) 1:99 and (b) 1:49 Zn:Si ratios gave quite spherical nanoparticles with some irregularities on their surface. The nanoparticle sizes are larger than those of ZnO/PAAH and silica alone. Energy-dispersive X-ray spectroscopy (EDX) or scanning transmission electron microscopy (STEM) analyses were not carried out on these samples to confirm the presence of ZnO/PAAH due to the very low percentages.

The (c) 1:1 ratio of Zn:Si proved highly polymorphic and was not studied in detail. On the other hand, the (d) 2:1 ratio of Zn:Si showed a large number of spherical nanoparticles associated with filamentous structures. EDX analysis (**Figure 22**) confirmed that silicon and zinc were present throughout, but that the filaments were predominantly composed of silicon. This confirms that some ZnO/PAAH nanoparticles have been disassembled during encapsulation.

### 3. Conclusions

To summarize, a sol-gel synthesis method that enables both the formation and simultaneous encapsulation of the phosphor, with the novelty stemming from carrying out a hydrolysis-condensation reaction in a basic medium on an acidic polymer, was performed. The concentration of PAAH has a significant impact on the synthesis of ZnO nanoparticles. An optimal concentration is observed around 0.63 wt% PAAH, resulting in the formation of small ZnO nanocrystals (~8 nm) embedded within large polymer spheres (~200 nm), and achieving a maximum PL quantum yield of nearly 20%. At lower PAAH concentrations, the PL spectrum and quantum yield show little
to no difference compared to samples synthesized without PAAH. In contrast, at
higher PAAH concentrations (6.3 wt%), there is no evidence of crystalline ZnO
formation, and the PL emission is notably blueshifted with a significantly reduced
quantum yield. Interestingly, the degradation of PL QY is not an unavoidable outcome
in our nanocomposites. For certain dopants, low doping concentrations can actually
enhance or at least preserve the PL QY, enabling the tuning of visible emission spectra
between 2.17 and 2.46eV. This finding opens up promising opportunities for the
development of rare-earth-free phosphors for white LEDs. Such materials can be
fabricated and tailored to emit either warm or cool white light through an environ-
mentally friendly synthesis process. Notably, PL QY improvement is only achievable
with dopant ions whose ionic radii are smaller than or comparable to that of Zn²⁺ (see
**Figure 13**). The chemical nature of the dopants and their precursors does not have a
clear and obvious effect on the size of ZnO NPs. However, it is likely that the dopant's
chemical nature influences the synthesis of hybrid nanocomposites—particularly
during the complexation between PAAH and Zn²⁺, which ultimately affects the
mesostructure of the hybrid inorganic/organic system. In conclusion, it appears that
the conditions of the ratio 2:1 of Zn:Si are a very good compromise between meso-
sphere stability and the amount of base added to lead to the efficient encapsulation.
Optimization seems necessary to avoid, if possible, the presence of SiO₂ in the form of
filaments, before moving on to the second stage of encapsulation of ZnO/PAAH NPs
by hydrosilylation.

## Acknowledgements

This article is based upon the work from COST Action ReNewPV CA21148,
supported by COST (European Cooperation in Science and Technology). A. A. and B.
M. thank PHC Polonium programs and PHC Parrot 2025 program number 52291TG.

The authors would like to thank the Auvergne-Rhône Alpes region for funding the
LUXIL project through the AAP Pack Ambition Recherche 2021 and the doctoral
contract of M. Dorian PRIANO.
**Author details**

Alexandra Apostoluk¹*, Dorian Priano², Maciej Sibinski³, Katarzyna Znajdek⁴, Stéphane Danièle² and Bruno Masenelli¹

1 INSA Lyon, Ecole Centrale de Lyon, CNRS, Universite Claude Bernard Lyon 1, CPE Lyon, INL, UMR5270, Villeurbanne, France

2 Université Claude Bernard Lyon 1, CP2M, ESCPE Lyon - CNRS, UMR 5128, Villeurbanne, France

3 Department of Material and Environmental Technology, Tallinn University of Technology, Ehitajate tee 5, Tallinn, Estonia

4 Department of Semiconductor and Optoelectronic Devices, Lodz University of Technology, Lodz, Poland

*Address all correspondence to: aleksandra.apostoluk@insa-lyon.fr

**IntechOpen**

© 2025 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

<figure><img src="image_24.png" /></figure>

BY
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