1. Introduction
Nanoparticles have attracted extensive research interest due to their great potential for many applications such as photoelectrochemical cells, heterogeneous catalyst, optical switching and single electron transistors [
1,
2,
3,
4]. The morphology of the particles has an effect on the properties and also influences the self-assembly process [
5,
6,
7].
Nickel and its oxide nanoparticles display superior magnetic, electrical, thermal, optical, catalytic and mechanical performance [
8,
9,
10,
11]. Nanostructured NiO particles are of great interest, since in addition to having important applications as thermistors, sensors and additives, for gas and ceramic [
12,
13,
14,
15], they are also of great interest as precursors of Ni-based catalysts [
16,
17,
18,
19,
20,
21]. NiO nanoparticles have also a wide range of applications as a p-type semiconductor due to its stable wide band gap (3.6–4.0 eV) [
22,
23], although bulk NiO is an antiferromagnetic insulator [
24,
25].
NiO crystalline nanoparticles have been synthesized by several physical, chemical and biological methods [
14]. Some of the synthesis approaches are chemical reactive processes, electrodeposition, solution growth, pulsed-laser deposition, high-temperature nickel oxidation, spray pyrolysis, sputtering, sol–gel technique, the reverse-micellar route, pulsed laser ablation and by microemulsion [
10,
11,
26,
27,
28,
29]. Metal oxides particles in a size between 1 and 100 nm with high surface area are desirable adsorbents, carriers and catalysts. NiO nanoparticles have catalytic activity in pyrolyzing biomass components, a property which is attributed to the different effects in terms of volume, quantum size, surface and macroscopic quantum tunnel. NiO nanoparticles have many important properties over those of bulk and micro-NiO particles [
30]. El Kemary et al. [
29] synthesized NiO nanoparticles using the reaction of nickel chloride with hydrazine at room temperature and thermal decomposition of the Ni(OH)
2 thus formed. It was reported that upon calcination of Ni(OH)
2 at 400 °C nanoparticles with size of 45 nm and the energy band gap of 3.54 eV are obtained [
29]. Salavati-Niasari et al. [
3] studied the effect of calcination temperature and the effect of the metal-to-ligand ratio on the particle size of NiO nanoparticles obtained from nickel phthalate complexes by a solid-state thermal decomposition route. Thus, by increasing the ligand-to-metal ratio and decreasing the calcination temperature, the particle size is reduced without agglomeration, obtaining nanoparticles in a range of 15 to 36 nm. Also, Hosny [
22] studied the effect of changing the metal anion and metal-to-ligand ratio on the crystal lattice and the particle size of NiO nanoparticles obtained by solid-state decomposition of nickel anthranilic acid semi-solid complexes, obtaining NiO nanoparticles with a size of 8 nm.
The combination of different compounds which have excellent electronic properties can improve the electronic properties of the resulting composite material [
31,
32]. In recent years, a great number of examples with heterojunctions to modify the photocatalyst and improve its photocatalytic activity has been reported. This type of constructions has been considered as one of the solutions to the serious recombination of photo-generated holes and electrons. Yu’s team proposed the concept of S-scheme heterojunction by an electrostatic interaction, the built-in electric field and the band bending to ensure strong redox photocatalyst [
33,
34].
Among the rare earth oxide series, lanthanum oxide (La
2O
3) is a p-type semiconductor that has been extensively researched due to its unique chemical and physical properties, which make it suitable for certain electronic applications. It is the only lanthanide oxide with an empty Ln-4f shell [
35]. La
2O
3 has a wide band gap of 5.5 eV, is thermally stable and non-toxic, and has a significant relative dielectric constant (K > 20) [
36,
37]. Because of these properties, La
2O
3 is used in potential applications like biosensors, catalysts, dielectric layers, fuel cells, gas sensors, rechargeable batteries, photoelectric conversion, and optical devices for measuring various body temperatures, and biomedical [
37]. Quan et al. [
38] designed and constructed a novel S-scheme La
2O
3/AgCl heterojunction catalyst with La-Cl bond by ball milling method. The catalytic activity and stability of La
2O
3/AgCl photocatalyst for BPA degradation were significantly improved; where the formation of heterojunction structure and interfacial La-Cl bond effectively promoted the transfer and quenching of relatively useless holes and electrons in La
2O
3 and AgCl.
Cerium oxide (CeO
2) is another lanthanide oxide with great interest. This oxide is a n-type semiconductor with a wide band gap which has high oxygen storage capacity, strong redox capability and it has been considered as a promising material for photocatalytic applications [
39,
40,
41]. By the combination of CeO
2 with other semiconductors it is possible to improve the photocatalytic efficiency [
42,
43,
44,
45]. This improvement is possible by the transference of photoexcited electrons from the conduction band of CeO
2 to the conduction band of the other semiconductor rather than recombining with the holes in the valence band. Meanwhile, the photoexcited holes will flow from the valence band of the semiconductor to the valence band of CeO
2. In this way, Sherly et al. [
46] have studied the effect of CeO
2 on the structural, optical and photocatalytic properties of ZnO, observing an improvement on the photocatalytic efficiency of ZnO by the inter particle electron and hole transfer between both semiconductors.
Furthermore, the synthesis of NiO nanoparticles using layered double hydroxides (LDH) as precursors has been described in the literature. Zheng et al. [
47] generated
in situ NiO and Co
1.29Ni
1.71O
4 by oxidizing and calcining Ni-Co layered double hydroxides (LDHs) to construct an S-scheme heterojunction heterogeneous catalyst in order to improve the electron transfer efficiency and promote the catalyst performance. The resulting catalyst has higher light absorption intensity and photocurrent response than Ni-Co LDH, and also a smaller electronic impedance and good separation efficiency of electrons and holes.
Layered Double Hydroxides, also known as hydrotalcite-like compounds, are a family of solids with at least two different metal cations in their layers and anions occupying the interlayer space [
48,
49,
50]. The layered structure of these materials could be explained from the brucite structure, where, during the coprecipitation process, an isomorphic substitution of divalent cations by trivalent cations takes place in the octahedral environment formed by OH
- ions [M(OH)
6]. The isomorphic substitution gives rise to positively charged layers. The electroneutrality of the compound is achieved by the incorporation of anions in the interlayered space, so that LDH are also known as anionic clays. LDHs could incorporate a great diversity of anions in their interlayer, both organic and inorganic ones, with a variety of size and charge [
51]. In addition, water molecules may occupy the remaining free space in the interlayer space. The wide range of anions and divalent and trivalent cations that can be used to prepare LDH, provide them a diversity of composition, based on the general formula
,
and
being the divalent and trivalent cations in the octahedral positions, and
the interlayer anion, with
defined as the
molar ratio [
48,
52].
LDHs are easily prepared, are cheap and their composition can be easily tuned. Therefore, LDH have emerged as promising materials due to their properties and applications in numerous fields, such as water decontamination [
53,
54], catalysis [
55,
56,
57,
58], drug delivery [
59,
60], electroactivity [
61], biomedicine [
62], and others. Moreover, LDHs prove to be excellent precursors of high specific surface area mixed metal oxides (MMOs) with applications in various industrial sectors such as semiconductors, photocatalyst, catalysts, electrodes, adsorbents, etc. [
63,
64,
65,
66].
Among the methods reported in the literature to prepare LDH, coprecipitation is the most commonly used. This method is based on the slow addition of a mixed solution of the desired
and
cations in a fixed ratio over an alkaline solution, generally an aqueous NaOH solution. The synthesis is carried out at constant pH by the addition of a second alkaline solution, allowing the coprecipitation of both metallic salts [
52]. The nature and concentration of the cations and the anions, the precipitation medium, the pH and temperature are important parameters to control the process. Kloprogge et al. [
67] reported the influence of the pH in the synthesis of Zn/Al LDH, the range 11-12 being that in which the samples exhibit the best crystallinity.
The alkaline compound used to provide the precipitation medium could influence in the precipitation rate and agglomerate formation of the final material. Studies on the effect of the precipitating agent used during the synthesis process of the LDHs precursors of the Ni-Fe-Al
2O
3 catalysts obtained by calcination and applied in the methanation reaction can be found in the literature. Hwang et al. [
68,
69] used NaOH, NH
3aq, Na
2CO
3 and (NH
4)
2CO
3 as precipitating agents, showing that the precipitation rate is different depending on the compound used, with Na
+-based precipitation media leading to a higher precipitation rate than NH
4+-based ones. The crystal size varies in the sense NiFeAl-NaOH > NiFeAl-NH
3aq> NiFeAl-Na
2CO
3 > NiFeAl-(NH
4)
2CO
3. As for the catalytic performance, it increases in the sense: NiFeAl-NaOH < NiFeAl-NH
3aq < NiFeAl-Na
2CO
3 < NiFeAl-(NH
4)
2CO
3.
In a previous work, the synthesis of ZnAl LDHs using different amines [methylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA)] in the coprecipitation medium as modifying agents of the morphology and crystallinity of LDH have been reported [
70]. The use of organic compounds of a basic nature, such as amines, allows to obtain well crystallized compounds, overall, when DMA or TMA are used.
In this work the results obtained using dimethylamine (DMA) as precipitating agent to prepare NiO-Al2O3, NiO-La2O3-Al2O3 and NiO-Ce2O3-Al2O3 semiconductors are discussed. The aim is to study the effect of the precipitating agent on the properties of the nanostructured NiO particles using LDH as precursors. It is aimed to achieve a higher dispersion of Al, La and Ce oxides and the formation of highly dispersed NiO nanoparticles.