**1. Introduction**

Deterioration of the environment and a shortage of sustainable energy supply have become major societal issues that are threating the development of human society and the preservation of our planet [1]. Whereas water shortage was for many decades associated to certain regions from Africa and central Asia, nowadays becomes a worrying problem worldwide, being entangled to the climate changes from the last decades [2]. Nowadays a large variety of organic pollutants have been found in effluents of sewage treatment plants, rivers, surface and ground waters [3]. Among them, man-made consumables containing

**Citation:** Ignat, E.C.; Lutic, D.; Ababei, G.; Carja, G. Novel Heterostructures of Noble Plasmonic Metals/Ga-Substituted Hydrotalcite for Solar Light Driven Photocatalysis toward Water Purification. *Catalysts* **2022**, *12*, 1351. https://doi.org/ 10.3390/catal12111351

Academic Editors: Ioan Balint and Monica Pavel

Received: 9 October 2022 Accepted: 27 October 2022 Published: 2 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

phenols, dyes, nitrobenzene or halobenzene compounds are the leading sources of water pollution [4]. The quest to provide clean water has led to a tremendous boost in the scientific efforts to develop novel performant technologies for environmental remediation [5]. Toward this, photocatalysis has received much attention because, among the traditional physical techniques, it provides a powerful tool for the removal of organic contaminants by completely degrading them [6]. Inside this, solar-light-driven photocatalysis is an effective and very promising way to meet both energy demands and water pollution issues [7]. It utilizes photogenerated carriers (electrons and holes) to initiate redox reactions and realize solar-to-chemical energy conversion.

The demonstrations of solar-light-driven chemical transformations on plasmonic nanostructures have led to the emergence of a new field in heterogeneous catalysis known as plasmonic catalysis [8]. Plasmonic metals are light-harvesting nanostructures that interact with visible light through the excitation of localized surface plasmon resonance (LSPR) [9]. A question that has emerged recently is whether it is possible to take advantage of the functionality of the plasmonic behavior in multicomponent catalysts. These are formed by close conjunctions of a plasmonic metal, which amplifies and concentrates the photons' energy within the material and, a non-plasmonic component that is able to play the role of support to stabilize the nanometal and, further, to extract the plasmon energy in the form of electronic excitations to perform a targeted catalytic function [10]. On such a plasmonic/non-plasmonic interface the light energy harvested by the plasmonic metal can modulate specific interactions with the support that are entangled to the rearrangement of electrons, transfer of photogenerated carriers and their prolonged lifetime and the extended light-response range within heterostructured components [11].

LDH are 2D layered matrices with a brucite-like structure that are conventionally described by the general formula [MII <sup>1</sup>−xMIIIx(OH)2] x+ · <sup>A</sup>n−x/n · mH2O, where the divalent MII and trivalent MIII cations might be defined as Mg2+, Zn2+, Cu2+, Al3+, Ga3+, etc. and, the An<sup>−</sup> can be almost any organic or inorganic anion [12]. The possibility to incorporate specific cations in the 2D layers of LDH delivers a generous palette of semiconductive materials useful for photocatalytic devices [13–15]. Further, LDH are relatively simple and cheap to prepare and own the ability to reconstruct its layered structure when the calcined LDH are introduced in the aqueous solutions containing anions [16,17]. By virtue of their unique 2D-layered structure, tuned optical absorption, their hydroxylated surfaces and ease of preparation, LDHs have emerged as very promising candidates for obtaining versatile and robust catalysts for many actual and potential applications and several reviews on this topic are available [18–25]. Importantly, the close conjunction of the LDH matrix and plasmonic nanoparticles affords to obtain multicomponent catalysts in which the LDH unit plays multifarious roles including immobilization and stabilization of nanoparticles [26], and further provides a unique interface-space confined in a 2D matrix for controlling nanoparticle spatial distribution [27]. Additionally, serving as a support, LDH will afford to stabilize low nuclearity nanospecies on its surface by minimizing atom diffusion, controlling the nanoparticle morphology and tuning active nanometal electronic structure [28].

4-Nitrophenol (4-NPh) is one of the highly toxic organic pollutants found among the substances bearing nitro groups, which are common components of industrial effluents. It has been detected in urban and agricultural waste and is recognized as a priority hazardous pollutant by the Environmental Protection Agency (EPA) due to its poisonous and volatile nature. [29]. Furthermore, para-Dichlorobenzene (p-DCB) is the main component of moth balls, disinfection fumigants and toilet deodorization cakes. It is considered a low toxicity compound, but it causes skin, eyes and gastrointestinal tract irritation, causing nausea, vomiting and diarrhea. Neurotoxic effects (retardation, dysarthria, ataxia, cognitive decline, memory disorders) were reported in cases of ingestion [30].

Herein, by exploiting the "structural reconstruction" of the LDH in the aqueous solutions of Au(CH3COO)2 and Ag2SO4, respectively, we successfully constructed novel plasmonic heterostructures defined by the close conjunction of nanoparticles of Au or Ag with gallium partially substituted hydrotalcite-like 2D matrix, denoted as Au\_MgGaAl and Ag\_MgGaAl, respectively. Next, we present their structural, morphological and plasmonic characteristics and applications in plasmon-induced photocatalysis toward degradation of both 4-NPh and p-DCB from water. Results point out the enhanced catalytic performances of the synthesized plasmonic heterostructures in comparison to their calcined forms and the basic LDH, and further reveal the advantage of plasmonic metals in catalyst composition. A discussion of the kinetic models that govern the studied plasmonic catalysis is also included.

#### **2. Results and Discussion**

### *2.1. Synthesis Procedures and Structure Characterization*

In our method, MgGaAl was obtained by coprecipitation at constant pH and 65 ◦C, while Ag\_MgGaAl and Au\_MgGaAl were obtained, at room temperature, after the reconstruction of the calcined MgGaAl in the aqueous solutions of Au(CH3COO)3 and Ag2SO4, respectively [31]. In fact, the synthesis procedures exploited the LDH capability to manifest its structural memory in the aqueous solution containing CH3COO− and SO4 <sup>2</sup><sup>−</sup> [31]. The purities and crystalline phases of the as-prepared samples were analyzed by X-ray diffraction (XRD). Figure 1a shows the XRD diffractograms of MgGaAl as "as synthesized" and after the reconstruction processes. The recovery of the LDH structure by reconstruction is shown by the XRD analysis, revealing patterns that could be perfectly indexed to the LDHs phase (ICDD file No. 22-700), with a series of sharp and symmetric basal reflections of the (00-, - = 3, 6, 9) planes and broad, less intense, reflections for the nonbasal (01-, - = 2,5,8) planes [32]. Particularly, MgGaAl shows a well crystalized LDH structure though, other phases such as gallium oxyhydroxide GaOOH are easily identified, as indicated as their characteristic reflections at 2θ = 34.5, 39 and 46.88 (JCPDS file no. 36-1451) denoted in Figure 1a as (\*). Importantly, after the reconstruction, the structural features of Ag\_MgGaAl and Au\_MgGaAl are defined as a single crystalline LDH-like phase, pointing out that the reconstruction procedure, at ambient temperature, promoted the reconstruction of the LDH.

**Figure 1.** XRD patterns of (**a**). MgGaAl, Ag\_MgGaAl and Au\_MgGaAl; (**b**) after calcination at 870 ◦C. (**\***) GaOOH; Δ Au and Ag; ♦ MgGa2O4.

The most intense peak corresponds to (003) reflection and is associated with the distance between two consecutive brucite-like layers in the LDH structure. On the contrary, for Ag\_MgGaAl and Au\_MgGaAl the position of (003) reflection shifted to lower 2θ degrees. The (003) peak is related to the interlayer distance between the brucite-like layers that is established by the size of the anions of the interlayers and the 2θ values of (003), which are 11.52◦, 9.92◦ and 6.94◦ for MgGaAl, Ag\_MgGaAl and Au\_MgGaAl, respectively. This shows that the reconstruction process altered the LDH interlayer space. Therefore, the replacement of carbonate anions of MgGaAl with the acetate and sulfate anions, after the reconstruction, promoted the increase of the interlayer spaces from 7.675 nm for MgGaAl to 8.909 nm for Ag\_MgGaAl and 12.727 nm for Au\_MgGaAl, as shown in Table 1. The position of the

diffraction maximum is seldom varying with the nature of the cation, since the distance in the layer is depending on fitting the cations in the octahedral cage defined by six hydroxyl groups [19,33]. The local lack of order or deformations issued in the brucite-like sheet are due to discrepancies in the ordered arrangement of the octahedral units and promoted the overlap of (110) and (113) peaks for Ag\_MgGaAl and Au\_MgGaAl. As consequence, the characteristics of the diffraction patterns of Ag\_MgGaAl and Au\_MgGaAl demonstrate the reconstruction of the LDH structure, but further points out that this procedure may affect the ordering inside the layers leading to the formation of the structural defects. The intensity of the peaks due to (015) and (018) reflections further indicate the distortion of the layered structure. The "*a*" and "c" structural parameters [17,31] were calculated as: a=2 × d (110) and c = 3 × d (006), where d (110) and d (006) are given by the Bragg relation and presented in Table 1. The increases of parameter "c" indicate the different nature of anions in Ag\_MgGaAl and Au\_MgGaAl, and agrees well to the previously reported values for the LDH containing CH3COO− and SO4 <sup>2</sup><sup>−</sup> in the interlayers, as previously reported [33,34].


**Table 1.** The XRD structural characteristics of the MgGaAl based catalysts.

Characteristic reflections of gold or silver phases are not observed in the XRD patterns of Ag\_MgGaAl and Au\_MgGaAl catalysts. This can be due to the small sizes and/or the low content of Au and Ag nanoparticles. Hence, to promote the growing of the small sizes nanoparticles and to study them by XRD, we further calcinated the heterostructured samples at 870 ◦C. After the calcination, the XRD patterns of the heterostructures (see Figure 1b) show the formation of MgGa2O4 (denoted with ♦ in Figure 1b) though further reveal the specific reflections of the face-centered cubic (fcc) of Ag (denoted with () in Figure 1b), namely (111), (200) and (220) (JCPDS data no. 04-0783) and the diffraction lines of the (111) and (200) planes of (fcc) of Au (JCPDS Card No. 65-2870), denoted with (Δ) in Figure 1b.

Next, we used FT-IR analysis to get information about the nature of the anions of the LDH structure. The FT-IR spectra of the catalysts (see Figure 2) resemble those of the LDH phases [33,35]. Typical of all spectra are the strong broad absorbance band between 3600 and 3200 cm−<sup>1</sup> associated with the stretching mode of the hydroxyl groups, both from the brucite-like layers and the interlayer water molecules, as well as, the water molecules physisorbed on the external surface of the crystallites [36]. For MgGaAl, the ν3 mode of interlayer carbonate is responsible for the intense band at 1384 cm−1. This is a degenerated mode for the D3h symmetry of the original carbonate anion. However, it can be noticed that the band at 1384 cm−<sup>1</sup> shows a clearly identified shoulder at 1491 cm−1, which can be considered a result of the splitting of the ν3 mode [37]. The replacement of carbonate anions by acetate anions in Au\_MgGaAl and by sulphate anions in Ag\_MgGaAl is clearly disclosed by FT-IR analysis. For Au\_MgGaAl, the bands at 1575 and 1410 cm−<sup>1</sup> indicate the presence of acetate anions, after the reconstruction; for Ag\_MgGaAl, the peak around 1110 cm−<sup>1</sup> reveal the presence of the SO4 <sup>2</sup><sup>−</sup> in the interlayers. Weaker bands below 1000 cm−<sup>1</sup> correspond mainly to vibration of lattice bonds of Me–OH while the characteristic peak at 668 cm−<sup>1</sup> is due to the vibration of the Me−O−Me bonds associated with the cations of the LDH layers [36].

**Figure 2.** FT-IR spectra of MgGaAl, Ag\_MgGaAl and Au\_MgGaAl.
