**1. International Religious Freedom the Background**

IRF is contentious and contested within the academy, and more importantly within diplomatic communities and the countries they interact with or represent. Contestation involves debate about the extent of religious persecution in countries and, if such persecution exists, what, if anything, should other governments do about it? Further, should this 'anything' include protecting and promoting religious freedom as a universal right? IRF is contested on realist grounds of non-interference in the domestic affairs of sovereign states, and whether what might loosely be regarded as US, or at best Western values, being universalised and imposed on other countries without consideration of the national context, history and cultural background is desirable. Such considerations, though sincerely held, go against a long tradition in the international community of supporting religious freedom as one among many human rights.

The United Nations General Assembly met in Paris in December 1948 to seek to build a new international order, where human rights were to be upheld. Article 18 of The Universal Declaration of Human Rights (UDHR) adopted at the assembly stated:

Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance.2

The International Covenant on Civil and Political Rights (ICCPR) introduced in 1966 and enacted since 1976 has been ratified by 173 countries and expands on the UDHR:


**Glycine–Nitrate Combustion Synthesis of Cu-Based Nanoparticles for NP9EO Degradation Applications**

**Hsu-Hui Cheng 1, \* , Shiao-Shing Chen 2, \* , Hui-Ming Liu 3 , Liang-Wei Jang 2 and Shu-Yuan Chang 4**

Received: 15 July 2020; Accepted: 1 September 2020; Published: 15 September 2020

**Abstract:** Copper-based nanoparticles were synthesized using the glycine–nitrate process (GNP) by using copper nitrate trihydrate [Cu(NO3)2·3H2O] as the main starting material, and glycine [C2H5NO2] as the complexing and incendiary agent. The as-prepared powders were characterized through X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy analysis. Using Cu(NO3)2·3H2O as the oxidizer (N) and glycine as fuel (G), we obtained CuO, mixed-valence copper oxides (CuO + Cu2O, G/N = 0.3–0.5), and metallic Cu (G/N = 0.7). The XRD and BET results indicated that increasing the glycine concentration (G/N = 0.7) and reducing the particle surface area increased the yield of metallic Cu. The effects of varying reaction parameters, such as catalyst activity, catalyst dosage, and H2O2 concentration on nonylphenol-9-polyethoxylate (NP9EO) degradation, were assessed. With a copper-based catalyst in a heterogeneous system, the NP9EO and total organic carbon removal efficiencies were 83.1% and 70.6%, respectively, under optimum operating conditions (pH, 6.0; catalyst dosage, 0.3 g/L; H2O2 concentration, 0.05 mM). The results suggest that the removal efficiency increased with an increase in H2O2 concentration but decreased when the H2O2 concentration exceeded 0.05 mM. Furthermore, the trend of photocatalytic activity was as follows: G/N = 0.5 > G/N = 0.7 > G/N = 0.3. The G/N = 0.5 catalysts showed the highest photocatalytic activity and resulted in 94.6% NP9EO degradation in 600 min.

**Keywords:** glycine–nitrate process; copper-based nanoparticles; photocatalysis activity; NP9EO

#### **1. Introduction**

In recent years, many studies have attempted to develop nanomaterials through green synthesis to solve environmental issues. To synthesize nanoparticles with a high, specific surface area, it is also very important to control the size distribution of ultrafine particles [1]. The glycine–nitrate process (GNP) is effective, simple, and environmentally compatible for the efficient synthesis of various nanostructured metal oxides [2–5]. The GNP is highly suitable for synthesizing fine crystalline powders and nanometer-scale particles with desirable characteristics, such as high chemical homogeneity, high purity, and a narrow size distribution range of nanoparticles [6]. The GNP is a type of solution combustion synthesis, which involves a self-sustained reaction between an oxidizer, such as a metal nitrate solution, and a fuel, such as glycine [5,7,8]. Furthermore, glycine supplies the energy required

for combustion and functions as a complexing agent. An advantage of the GNP is that the rapid evolution of a large volume of gaseous products during combustion dissipates heat from the process and limits temperature increases, which reduces the probability of local sintering among particles while facilitating fine powder formation [6].

Nanosized transition metal oxide (TMO) particles have recently attracted considerable attention because they have unique applications with various types of matter, such as catalysts [2,5], photocatalysts [8], solar cells [5], and gas sensors [9]. Preparation of high-quality nanostructures with defined and controllable size and morphology is the main determinant of catalytic performance [10]. Among nanocatalysts, TMO-based catalysts offer active sites with low activation potential for hydrogen evolution systems [11]; in particular, noble metals (e.g., gold, silver, and platinum) are widely used to optimize catalytic efficiency. However, the high cost of noble metals remains a major obstacle in the development of stable and highly efficient catalysts. Consequently, reducing the amount of noble metals without compromising their catalytic performance is an urgent requirement for catalysis.

Compared with noble metal catalysts, copper is relatively inexpensive and moderately abundant [12]. Copper oxides (Cu*x*O*y*, where x:y = 1:1 or 1:2) and metallic copper are widely used as TMO catalysts. Copper oxides are heterogeneous catalysts with high recyclability, high catalytic efficiency, and low band-gap energies [10]. Cu oxides are used as TMO catalysts; Cu*x*O*y* catalysts are p-type semiconductors with a monoclinic structure, a bandgap of 1.7–2.17 eV [13], and a high theoretical photocurrent density of −14.7 mA/cm2 [14,15]; therefore, they are suitable for application in the photocatalysis field. Although these superior properties of copper oxide have been of great interest to researchers, the transformations of cubic copper (I) oxide (Cu2O) to copper (II) oxide (CuO) and Cu are still unavoidable due to the thermodynamically feasible parasitic reactions of self-oxidation and self-reduction [16]. Nevertheless, the presence of both Cu+ and Cu2+ in oxides has been shown to be beneficial in some cases [17]. In all applications of Cu*x*O*y* morphology, structure, size, and surface area are the main determinants of its catalytic performance [15,18]. Compared with other TMOs, such as titanium dioxide (TiO2), zinc oxide (ZnO), iron oxide (Fe2O3), and cadmium sulfide (CdS) [18], only a few studies have described synthesis strategies for different Cu*x*O*y* nanoparticles along with the introduction of their related applications.

Nonylphenol ethoxylates (NPEOs) are nonionic surfactants that belong to the alkylphenol ethoxylates (APEOs) family, and are used primarily in emulsifiers, detergents, wetting agents, and dispersing agents, etc. Of these, NPEOs with an average of nine ethylene oxide units (NP9EO) are well known for their commercial application [19]. The biodegradation of NPEOs in the environment leads to the formation of nonylphenol, which is a toxic xenobiotic compound, classified as an endocrine disrupter capable of interfering with living organisms. Directive 2003/53/EC of the European Union (EU) prohibited the use of NPEOs and nonylphenols (NP) and recommended replacing them by alcohol ethoxylates, which are expensive [20]. However, due to their low cost and higher efficiency, NPnEOs are still widely used in China. They are discharged continuously into the environment, and the occurrence of NPnEOs in environmental samples has been well documented [21].

In this study, we presented a low-GNP with varying glycine/nitrogen (G/N) ratios for synthesizing Cu*x*O*y* and metallic copper catalysts. Furthermore, the degradation of NP9EO under ultraviolet (UV) irradiation was investigated using a heterogeneous Cu*x*O*y* photocatalyst (and metallic copper) in a custom-made photoreactor. Moreover, the effects of operational parameters, such as catalyst activity, catalyst dosage, and H2O2 concentration, were examined. This study will provide useful evidence on the potential of this method as a treatment to remove toxic materials from the water environment.

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

#### *2.1. Characterization of Cu-Based Nanoparticles*

Figure 1 depicts the X-ray powder diffraction (XRD) pattern of as-synthesized copper-based nanoparticles synthesized through the GNP with different G/N ratios. The XRD pattern of the sample (a) obtained at a stoichiometric G/N ratio of 0.3 displays peaks at 32.5◦ , 35.7◦ , 38.7◦ , 48.5◦ , 53.6◦ , 58.3◦ , 61.6◦ , 66.3◦ , and 68.1◦ for (110), (−111), (111), (−202), (020), (113), (022) (−311), and (220) planes, respectively, confirming the formation of the CuO monoclinic phase (JCPDS File No. 01-089-5897). At a stoichiometric G/N ratio of 0.5 (see sample (b) in Figure 1), intense reflections were observed at 2θ values of 29.5◦ , 36.8◦ , 38.7◦ , 42.3◦ , 43.4◦ , 50.5◦ , 61.4◦ , and 74.2◦ . The peaks at 38.7◦ corresponded to the (111) plane of divalent CuO, whereas the reflections at 29.5◦ , 36.8◦ , 42.3◦ , and 61.4◦ corresponded to the (110), (111), (200), and (220) planes of cuprite, respectively (JCPDS File No. 01-078-0428), which indicated Cu2O nanocrystal formation (JCPDS File No. 01-077-0199) [22]. In addition, the peaks at 50.5◦ and 74.2◦ , which corresponded to the (200) and (220) peaks, respectively, of zero-valent copper (Cu0 ) were observed. These results clearly indicated that the Cu0 nanoparticles, formed during chemical reduction, underwent oxidation because of the limited stability of Cu [23,24], and Cu2O might have been formed through oxidation [25]. When the G/N ratio increased to 0.7, broad reflections from a highly dispersed Cu0 phase were observed, as shown in sample (c) in Figure 1. The XRD patterns (111), (200), and (220) of Cu0 planes at 43.4◦ , 50.5◦ , and 74.2◦ , respectively, were consistent with bulk copper crystallographic data (JCPDS File No. 01-085-1326). The results clearly revealed that Cu0 formation was favored with an increase in glycine (fuel) content during combustion synthesis. When the G/N ratio was reduced to 0.3, sample (a) exhibited numerous primary CuO phases and a few Cu2O phases (approximately 9.5%). These findings indicated that nanocrystalline CuO reduced to a stable Cu2O phase rather than forming metallic copper directly [26]. The average crystallite size was calculated from the XRD pattern according to the line width of the (200) plane refraction peak by using the Debye–Scherrer equation [27]:

$$d = \frac{\kappa\_1 \times \lambda}{\beta \times \cos \theta} \tag{1}$$

where *d* is the crystallite size (nm), *k*1 is the constant with a value of approximately 0.9, λ is the wavelength of the X-ray radiation (λ = 0.1542 Å CuKα), β is the full width at half maximum intensity, and the value of 2θ is the characteristic peak of the XRD pattern. The crystalline sizes of copper-based nanoparticles were 76.3 ± 2.2 nm (G/N = 0.3), 35.4 ± 1.0 nm (G/N = 0.5), and 99.4 ± 2.9 nm (G/N = 0.7), respectively, as presented in Table 1.

Figure 2 shows the SEM with EDS images of as-prepared Cu-based nanoparticles at various G/N ratios. The typical EDS spectra of the particles (Figure 2) indicated that the particles were composed of copper (Cu) and oxygen (O). The SEM photographs showed that the catalysts formed sponge-like aggregates containing some nanoparticles; the size distribution range of the synthesized particles was broad (5–100 nm), and some irregular granules with a high degree of shape anisotropy were observed. The prepared Cu-based nanoparticles were not single-domain particles. The lack of homogeneity in particle shape and size, as well as the presence of sponges or flake-like structures, was typical of combustion synthesis. The results indicated that the large amounts of gas that evolved during combustion affected the porosity of the solid products [28,29]. Moreover, high combustion temperatures promoted nanoparticle agglomeration and crystal grain growth [30]. This finding was consistent with the surface area (BET) analysis results in the present study (Table 1). The BET results indicated that increasing the glycine concentration and reducing the particle surface area promoted the formation of metallic Cu. Consequently, large-scale agglomerates were observed when the ratio G/N = 0.7 was used.

**Table 1.** Specific surface area, crystallite size, and determination of copper oxide nanoparticles synthesized at different glycine/nitrogen (G/N) ratios.

**Figure 1.** The X-ray diffraction (XRD) pattern of combustion synthesized products of copper-based materials with various glycine/nitrate ratios (G/N) = 0.3 (**a**), 0.5 (**b**), and 0.7 (**c**).

X-ray photoelectron spectroscopy (XPS) spectra of the selected catalysts were obtained to confirm the surface composition and oxidation state of copper from information derived from binding energy values. Copper oxidation states occurred during two semiconducting phases: cupric oxide (CuO) and cuprous oxide (Cu2O) [31]. The binding energies in the XPS analyses were calibrated using the C 1s peak (BE = 284.6 eV), where the C 1s peak was attributed to residual carbon from the sample and adventitious hydrocarbons from the XPS instrument or adsorbed carbon. Figure 3 presents the three samples obtained from the XPS results. As displayed in Figure 3a, the main O 1s peak present at 532.8 eV was attributed to lattice oxygen in a metal oxide, such as CuO or Cu2O. The deconvolution of the Cu 2p3/2 XPS core level spectra of Cu*x*O*y* is illustrated in Figure 3b–d. Through spectral deconvolution, we observed that the binding energies of Cu2O and CuO were in the ranges 933.1–933.7 and 934.3–934.5 eV for different samples, respectively [32,33]. In addition, two peaks were observed, and the feature at 932.4 ± 0.1 eV was attributed to metallic Cu (2p3/2) [31,32], as presented in Figure 3c,d, respectively.

**Figure 2.** SEM with EDS images of prepared Cu-based nanoparticles at various G/N ratios: G/N = 0.3 (**a**), G/N = 0.5 (**b**), and G/N = 0.7 (**c**).

**Figure 3.** X-ray photoelectron spectroscopy (XPS) wide-scan spectra (**a**) of all samples; deconvolution of XPS spectra of (**b**) (G/N) = 0.3, (**c**) (G/N) = 0.5, and (**d**) (G/N) = 0.7 samples.

#### *2.2. Investigation of Photocatalytic Performance*

NP9EO degradation was used in this study to evaluate the photocatalytic activity of the as-prepared samples; its chemical structure is demonstrated in Figure 4. Economically, catalyst dosage is a main parameter in degradation studies. As illustrated in Figure 5, the custom-made photoreactor (Panchum Scientific Co., PR-2000) was operated in the atmosphere and 14 UV light tubes functioned as germicidal lamps with a wavelength of 254 nm (Sankyo Denki Co., Ltd., Kanagawa, Japan), surrounding the cylindrical quartz reactor in sequence. Of these, a light with a power of approximately 10 mW/cm2 was placed in the center of the reactor and measured by a power meter (Model 840-C Handheld). To optimize the dosage of nano-Cu for photocatalyst activity, experiments were performed by varying the nano-Cu (G/N = 0.3) amount from 0.1 to 1 g/L. Figure 6a illustrates that the degradation efficiencies for 0.1, 0.3, 0.5, 0.8, and 1.0 g/L nano-Cu (G/N = 0.3) were 80.1%, 83.1%, 78.2%, 73.6%, and 70.3%, respectively. Besides, the TOC value represents the total organic carbons concentration in the solution. It is an important indicator of mineralization [34]. Figure 6a also indicated that TOC removal efficiency and NP9EO degradation efficiency showed the same trend. TOC removal efficiency increased from 58.2% for a nano-Cu catalyst amount of 0.1 g to more than 70.5% for a nano-Cu catalyst amount of 0.3 g. However, the concentration of nano-Cu catalysts (G/N = 0.3) for 0.5 g/L was increased above the limiting value, and TOC removal efficiency decreased (the mineralization efficiency dropped from 60.7% to 54.9%) because of increasing suspension turbidity and decreasing light penetration from an increased scattering effect [35], as shown in Figure 6a. Therefore, the optimal dosage of nano-Cu catalysts (G/N = 0.3) in this custom-made photoreactor was 0.3 g/L in the present study.

**Figure 4.** Chemical structure of NPnEO.

**Figure 5.** Schematic representation of the custom-made photoreactor.

–

–

**Figure 6.** (**a**) Effect of catalyst dosage on the degradation of NP9EO (G/N = 0.3, NP9EO = 100 mg/L, H2O2 = 0.05 mM (1000 mg/L), pH = 6), (**b**) Effect of increasing H2O2 concentration on NP9EO degradation. Data presented are the average of duplicate experiments with relative standard deviation. (G/N = 0.3, NP9EO = 100 mg/L, pH = 6, dosage = 0.3 g/L).

– In addition, the effects of hydrogen peroxide (H2O2) concentration (0–0.1 M) on photocatalytic oxidation were investigated. The experimental results indicated that, initially, on increasing the H2O2 concentration up to 0.05 mM for nano-Cu catalysts, the highest exhibited NP9EO and TOC removal efficiencies were 83.1% and 70.6%, respectively; the rate of degradation increased due to the availability of the hydroxyl radical (·OH) by the decomposition of more H2O2 molecules [36]. Subsequently, NP9EO and TOC removal efficiency gradually decreased (70.3% and 60.1%, respectively) with increasing H2O2 concentration above 0.08 mM (Figure 6b). The propagation step in the oxidative cycle is hindered by an excess of H2O2, which scavenges the ·OH radicals in the solution due to the reaction between excess H2O2 and ·OH radicals (Reactions (2) and (3)), thereby reducing the amount of ·OH available for reaction with NP9EO [37,38].

$$\rm H\_2O\_2 + OH \rightarrow HO\_2 + H\_2O \tag{2}$$

$$\rm HO\_2 + HO\_2 \to O\_2 + H\_2O \tag{3}$$

The effect of different G/N ratios of Cu-based nanoparticles on photocatalytic activity was also investigated in this study (Figure 7). For photocatalytic activity, each respective dose of Cu-based nanoparticles was dispersed in 1L NP9EO aqueous solution, and then the suspension was stirred in a dark condition for 30 min to establish the adsorption–desorption equilibrium. Subsequently, the suspension was subjected to UV light irradiation. In addition, the background experiments, such as NP9EO, were performed to ensure that the actual photocatalytic activity came from the photocatalysts without the influence of direct pollutant degradation by light irradiation. Photolysis experiment was conductedin the reactor without photocatalysts at 254 nm UV light irradiation. The results in Figure 7 clearly indicate that no significant NP9EO photolysis was detected under UV light irradiation. The order of photocatalytic activity was as follows: G/N = 0.5 > G/N = 0.7 > G/N = 0.3. It was observed that the percentage of NP9EO degradations with different G/N ratios were 83.1%, 94.6% and 87.7%, respectively, and the corresponding TOC removal percentages were 52.8%, 70.6%, and 57.6%, respectively. These results clearly suggested that at G/N = 0.5, the catalyst exhibited a high activity, which was represented as the degradation of NP9EO. This finding is consistent with the results of the XPS analysis of the oxidation state of copper (see Table 1). Cu2O (bandgap of approximately 2.2 eV) has a higher electric conductivity than CuO (bandgap of approximately 1.6 eV) [39]; therefore, in the experiment on photocatalytic performance, Cu2O exhibited a higher removal effect than CuO under the UV-light irradiation system in our study. Furthermore, Cu2O and CuO are more stable at a higher potential for photoelectrochemical/hydrogen-evolution reaction because a high potential restrains

the photoreduction of Cu2O (or CuO) to metallic copper [40]. Dasineh et al. [39] reported that the integration of Cu2O and CuO significantly increased charge collection and reduced the recombination rate inside the photocatalyst. Luna et al. [41] also reported that the Cu species modified catalysts presented a high photocatalytic activity which was attributed to an appropriate amount of copper phases acting as photogenerated electron traps, avoiding charge recombination. This indicates that Cu2O and CuO improved optical absorption and facilitated charge transfer at the interface between the photocatalyst and electrolyte. Consequently, with copper-based catalysts in a heterogeneous system, the highest photocatalytic performance was obtained with a G/N ratio of 0.5. In our results, the efficiency of TOC removal was consistently lower than that of NP9EO removal, indicating that, in addition to CO2 (formed by mineralization), some intermediates were generated (such as by-products) [42].

**Figure 7.** NP9EO degradation curves for Cu-based catalysts synthesized at different G/N ratios. Data presented are the average of duplicate experiments with relative standard deviation. (Catalyst dosage = 0.3g/L, NP9EO = 100 mg/L, H2O2 = 0.05 mM (1000 mg/L), pH = 6.)

#### *2.3. Analysis of Variance (ANOVA) Study of Photocatalytic Activity*

– – – − − − We further evaluated the effects of nano-Cu*x*O*y* catalyst dosage (range 0.1–1 g/L), H2O2 concentration (range 0–0.1 mM), and different G/N ratios (range 0.3–0.7) on the degradation of NP9EO by using a one-way ANOVA analysis. The nano-Cu*x*O*y* photocatalytic activity assays were performed in triplicates. For all statistical analyses, values were presented as the mean ± standard deviation, and the significance between different groups was examined (*p* < 0.05). According to the results in Table 2, the nano-Cu*x*O*y* catalyst had significant effects as the *p* values were 4.99 × 10 −7 , 1.42 × 10 −13 , and 9.46 × 10 −6 for catalyst dosage, H2O2 concentration, and different G/N ratio percentage removals, respectively. This suggests that there were significant differences in the effect of the nano-Cu*x*O*y* catalyst on the percentage removal of dosage, H2O2 concentration, and different G/N ratios, hence this practice can be effectively used to explain the effect of nano-Cu*x*O*y* on photocatalytic activity efficiency.

−

−

−

**Table 2.** One-way ANOVA results of various Cu-based nanoparticles effects on the degradation of NP9EO.

#### **3. Materials and Methods**

#### *3.1. Catalyst Preparation*

Cu-based materials were synthesized through the GNP. Copper nitrate trihydrate [Cu(NO3)2·3H2O] (Merck, Kenilworth, NJ, USA, 99.5% purity) and glycine [C2H5NO2] (Merck, Kenilworth, NJ, USA, 99.7% purity) were dissolved in deionized water. Various amounts of glycine (fuel) were added to the metal nitrate (oxidizer) solution to obtain three different glycine–nitrate (G/N) ratios (G/N = 0.3, 0.5, and 0.7). As shown in Figure 8, the glycine–nitrate solutions were then mixed and heated overnight over a hot-plate stirrer (Corning, Glendale, AZ, USA) at 105 ◦C to form clear, homogeneous, and viscous gel-like solutions. Each gel was then poured into a ceramic bowl that was placed in an oven. The gels were heated to 200 ◦C until they underwent self-ignition and produced catalyst ashes. Finally, we obtained various shades, from russet brown to dark brown, of copper oxide nanoparticles displayed in the forms of fine powder. – –

**Figure 8.** Schematic illustration of combustion synthesis for the copper G/N reaction system.

The crystal structure of the samples was determined through XRD by using Cu Kα radiation (*λ*

− −

#### *3.2. Characterization*

The crystal structure of the samples was determined through XRD by using Cu Kα radiation (λ = 1.5418 Å) on a Rigaku DMAX 2200VK/PC diffractometer (Tokyo, Japan). The allocation/distribution of all measured peaks from XRD referred to the data from the Joint Committee on Powder Diffraction Standards (JCPDS) [43]. The XPS (VG Escalab 250 iXL ESCA, VG Scientific, West Sussex, UK) was applied to verify the chemical composition of the samples to obtain the chemical analysis. The specific surface area (m2 g −1 ) was calculated by the BET equation [44], and the total pore volume (Vt, m3 g −1 ) was examined by converting the adsorption amount at P/P0 = 0.95 to the volume of the liquid adsorbate. The morphologies of the synthesized samples were analyzed through SEM (S-4800, Hitachi, Tokyo, Japan), coupled with EDS measurements.

#### *3.3. Photocatalytic Experiments*

The stock solutions of NP9EO were prepared in distilled water to obtain a concentration of 100 mg/L, which was then ultrasonicated for 30 min to obtain a stable emulsion. The emulsion was stored at 4 ◦C for a maximum of 4 weeks. The photocatalytic activities of nano-Cu samples under UV irradiation were evaluated based on the degradation rate of NP9EO in a quartz reactor (120/115 mm outer/inner diameter, height: 240 mm) containing 0.3 g of nano-Cu and 1 L of a 100 mg/L aqueous solution of NP9EO. The system was operated in the atmosphere and 14 UV light tubes functioned as germicidal lamps with a wavelength of 254 nm (Sankyo Denki Co., Ltd., Kanagawa, Japan), surrounding the cylindrical quartz reactor in sequence.

#### *3.4. Analytical Procedures*

The photo-degradation rate of NP9EO solution was determined through HPLC (Agilent 1100 series; Agilent Technologies, Santa Clara, CA, USA) with a Wondasil-C18 (4.6 × 150 mm, 5 µm), and a 2487-UV detector at 277 nm. TOC was measured by a model 1010 TOC analyzer (O.I. Analytical, College Station, TX, USA) following the NIEA W532.52C standard methods (Taiwan's Environmental Protection Agency). Of these, the principle of TOC was calculated by the subtraction of the IC (inorganic carbon) value from the TC (total carbon) of the sample. The pH value was measured with a pH meter (Model TS-1, Suntex Co., New Taipei City, Taiwan).

#### **4. Conclusions**

A nanocrystalline Cu-based heterogeneous catalyst was synthesized using the GNP. The nature and amount of fuel were crucial factors controlling combustion and final product composition. The formation of CuO, mixed-valence copper oxides (CuO + Cu2O), and metallic Cu was achieved by varying the G/N ratio, in which glycine was used as fuel. The effect of the G/N ratio on catalyst morphology was analyzed using SEM. With a G/N ratio of 0.7, agglomerate structure formation was observed. Furthermore, Cu-based catalysts exhibited high photocatalytic activity during NP9EO degradation in a custom-made photoreactor under UV light. The experimental results revealed that, under optimal reaction conditions, namely a catalyst dosage of 0.3 g/L and H2O2 concentration of 0.05 mM, NP9EO and TOC removal efficiency was 83.1% and 70.6%, respectively. However, the concentration of nano-Cu catalysts (G/N = 0.3) for 0.5 g/L was increased above the limiting value, and TOC removal efficiency decreased because of increasing suspension turbidity and decreasing light penetration from an increased scattering effect. The NP9EO removal efficiency increased with H2O2 concentration but decreased when H2O2 concentration exceeded 0.05 mM. With Cu-based catalysts in a heterogeneous system, the highest photocatalytic activity was observed with a G/N ratio of 0.5. Furthermore, TOC was consistently lower than the removal efficiency of NP9EO, indicating that, in addition to the mineralization of CO2, some intermediates were also generated.

**Author Contributions:** S.-S.C. and H.-H.C. conceived and designed the experiments that produced/generated this paper. S.-S.C. proposed methodology and project administration related to the research areas. H.-H.C. wrote original manuscript, drew figures, reviewed and edited the article. H.-M.L. analyzed quantitative data, statistically analyzed data, reviewed original manuscript, and provided critical comments. L.-W.J. performed the experiments, collected and analyzed the data. S.-Y.C. performed data processing of the experiments, statistically analyzed, reviewed original manuscript, revised grammar and provided critical comments. H.-H.C. and S.-Y.C. contributed to the final revision of the paper. All authors contributed to the manuscript and discussed the results. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by a grant from Hsuteng Consulting International Co., Ltd. (No. HCI 20190113-A).

**Acknowledgments:** The authors gratefully acknowledge the experimental apparatus support from Institute of Environment Engineering and Management, National Taipei University of Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 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 (http://creativecommons.org/licenses/by/4.0/).

*Article*


**4. Discussion**

A unique feature of *M.tb* is the ability to persist in the host for a long time, despite functioning mechanisms of acquired immunity. In turn, approximately 2.3 billion individuals have latent tuberculosis infection without evidence of the clinical manifestation of active TB. It is hypothesized that active TB is usually caused by the reactivation of endogenous infection and untreated LTB is a

major source of new active TB infections and transmission. The risk for severe active tuberculosis reactivation is increased several times among the immunocompromised individuals, diabetics, organ transplantation recipients, patients with hematologic malignancies, or HIV-infected subjects. Several treatment regimens of LTB are recommended, between isoniazid monotherapy for six months, rifampicin plus isoniazid for three months or rifapentine plus isoniazid for three months [27]. Currently, interferon-gamma release assays (IGRA) are used to test LTB based on the production of IFN-γ by Th1 cells responding to specific *M.tb* antigens, although the IGRA's accuracy in immunocompromised individuals is still limited. Moreover, the definition of active tuberculosis infection might not be accurate using IFN-γ responses to *M.tb* antigens even in combination with tuberculin skin test [28]. At the same time, neither the IGRA test nor the tuberculin skin test can distinguish between latent and active TB infection. Strategies for rapid di fferentiation of patients with active TB and people with LTB and prevention of tuberculosis reactivation in LTB individuals are urgently needed.

To meet such needs, we compared the expression of genes of the IL-18 pathway, functional receptor of this cytokine IL-18R, and IFN-γ, as well as the expression of IL-18BP and IL-37 genes in groups of patients with active TB, healthy individuals with LTB and healthy controls without *M.tb* infection.

Our results for the first time showed a significant increase in the relative expression of IL-18 and IL-18BP mRNA in the group of patients with active TB and LTB individuals when compared to healthy controls, according to available literature data. It might suggest a permanent activation of the immune cell signaling pathways in the course of *M.tb* infection either in the control or progression to active TB disease. Moreover, no significant di fferences in relative IL-18 and IL-18BP mRNA expression were observed between active TB patients and LTB individuals. Pechkovsky et al. demonstrated an increased expression of IL-18 mRNA in type II lung epithelial cells obtained from patients with pulmonary TB. Pneumocytes that were cultured in the presence of *M.tb* cell lysate showed an increased IL-18 mRNA expression in comparison with unstimulated cells and pneumocytes stimulated with PPD or LPS [29]. Higher levels of IL-18 mRNA expression were observed in monocytes responding to *M. leprae* antigens [30]. Corbaz et al. showed significantly higher levels of IL-18 and IL-18BP mRNA expression in the intestinal mucosa of patients with Crohn's disease as compared to healthy controls. We detected a similar increase in IL-18 and IL-18BP mRNA expression in the group of patients with active TB and healthy LTB individuals. These results are somewhat out of line with our earlier definition of a simultaneous increase in serum IL-18 and IL-18BP protein expression, which might be treated as a discriminatory biomarker of active tuberculosis and LTB. Yet, it is worth noting that only the complex co-expression of serum IL-18BP and IL-37, IP-10, and IFN-γ were identified as the most accurate discriminative biomarker set for diagnosis of active TB [21].

Our study allowed for the discovery of a novel relation—the significantly lower expression of functional IL-18R receptor mRNA in the LTBI group as compared to the active TB and healthy controls. This low expression of IL-18R mRNA in LTB individuals was accompanied by IFN-γ mRNA expression at the 'baseline' level, characterizing healthy individuals without *M.tb* infection. Similarly, Taha et al. pointed out that the expression of IFN-γ genes was significantly higher in the group of patients with active TB as compared to those who were infected but did not develop active TB [31].

Among many possible mediators of host response to *M.tb* is the activity of the indoleamine 2,3-dioxygenase-1 (IDO1), the enzyme of tryptophan metabolism, which leads to the formation of tryptophan metabolites, including quinolinic and picolinic acids [32]. In animal models, increased IDO-1 expression and the activation of the tryptophan-kynurenine pathway were indicated to play a crucial role in *M.tb* pathogenesis [29,30,33,34]. The depletion of tryptophan, which is required for microbial growth, as well as the accumulation of biologically active tryptophan metabolites, impaired e ffective anti-mycobacterial immune response and, thus, favoured survival and persistence of the pathogen [32]. In *M.tb*-infected mice, IFN-γ receptor-deficiency in nonhaematopoietic cells led to a lack of IDO-1 expression and it was associated with exuberant neutrophil recruitment and increased mortality [33]. In macaques, the suppression of IDO activity led to the reduction of the bacterial burden and clinical symptoms of active TB that was accompanied by increased lung T cell proliferation, the induction of inducible bronchus-associated lymphoid tissue, and the relocation of e ffector T cells to the center of the granuloma [34]. Mehra et al. demonstrated that IDO induction in the periphery of the granuloma correlated with active TB disease [30]. A few studies have investigated IDO-mediated tryptophan metabolism and its metabolites in humans in the context TB. Li et al. demonstrated increased IDO expression and activity in the pleural fluid from TB patients [35]. Almeida et al. found significantly higher expression levels of immune-suppressive mediators, including IDO-1, in patients with active pulmonary TB as compared to patients with other infectious lung diseases and healthy volunteers [36]. The authors suggested that the increased levels of immunosuppressive mediators may render the immune activation and counteract the development of Th1-type immune response against *M.tb*. The IDO levels were elevated at the time of TB diagnosis and declined after TB treatment, which serves as evidence that IDO expression might be both: a useful diagnostic marker of active TB as well as prognostic factor in TB treatment of HIV-negative patients [36]. Additionally, Adu-Gyamfi et al. showed that plasma IDO expression is a potential biomarker of active TB in HIV-positive patients [37], while Shi et al. confirmed that IDO activity might have an auxiliary diagnosis value for the early discrimination of multi-drug resistant TB patients [38]. The increase in IDO activity was noticed in both HIV-infected and uninfected active TB patients as compared with individuals with latent TB infection [36,37,39]. In relation to this statement, the simultaneous reduction of IL-18R mRNA expression together with significant overexpression of IL-18 mRNA observed by us in the LTB group is of particular interest.

The presented results entitle us to hypothesize that: the increase in IL-18 gene expression, the lack of increase in IFN-γ gene expression, and the remarkably reduced expression of IL-18R gene may be a novel set of conditions that partially describe the homeostasis between *M.tb* and host-immunity in latent tuberculosis infection. As an obligate intracellular pathogen, *M.tb* has numerous adaptive mechanisms of modifying cellular processes in the fight against the host immune response. In latent TB infection, *M.tb* bacilli benefit from epigenetic changes that occurred in the host immune system under mycobacterial infection [40]. These changes make the *M.tb* favorable environment in the host cells and promote mycobacterial survival, growth, and latency. In a study that was conducted among Chinese patients with pulmonary TB and healthy controls, single nucleotide polymorphisms in the IL-18R promoter were associated with genotype-specific methylation status and genotype-specific IL-18R expression [41]. In the author's opinion, the relationship between decreased mRNA expression of IL-18R that is caused by an SNP and increased DNA methylation can partially mediate the susceptibility to TB risk. No statistically significant di fferences were observed in the relative mRNA IL-37 expression among the groups in our study. IL-37 is a new member of the IL-1 family, which reduces systemic and local inflammation. IL-37 is expressed in various cells and tissues and it is regulated by numerous inflammatory stimuli and cytokines via di fferent signal transduction pathways [42]. Mannose-capped lipoarabinomannan purified from *M.tb* induces IL-37 production via enhancing TLR2 expression in human type II alveolar epithelial cells; this process might contribute to the persistence of *M.tb* infection [43]. Zhao et al. indicated that IL-37 is also a negative regulator of immune responses in *Listeria monocytogenes* infection due to reduced production of colony-stimulating factors and increased macrophage apoptosis [44]. In our earlier studies, the serum concentration of the IL-37 protein was similar in the group of patients with active pulmonary TB and healthy individuals with or without latent *M.tb* infection [21]. However, the complex co-expression between the two IL-18 inhibitors, IL-18BP and IL-37, was identified as the strongest discriminative biomarker of active TB disease.


**1. Introduction**

Aluminium nitride (AlN) thin films, being a staple for commercial acoustic wave resonators, have been well investigated over the last two decades to improve understanding of the growth–performance relation [1–3]. The advent of the Al1−*x*Sc*x*N (AlScN) pseudobinary alloys [4] instigated further studies on the enhancement of the piezoelectric properties, yielding the interplay of the wurtzite and hexagonal lattice configurations, which led to the overall lattice softening [5] and the increased electromagnetic coupling [6]. The phase formation diagram is complicated by the fact that pure ScN crystallises in the rock-salt structure [7]. It was initially shown that only up to *x* = 0.22 could be introduced into the AlN lattice before it converts to the cubic system [8]. However, the maximum amount of Sc within the wurtzite lattice was later optimised: *x* = 0.41 [9]. The amount of Sc atoms dispersed in the AlN wurtzite lattice has a dominant role on the physical properties of the AlScN, which can, for instance, show even ferroelectric behaviour for *x* > 0.28 [10].

The phonon properties of these pseudobinary alloys are of great interest not only to enable insights into the structural integrity and thermal properties of their crystals, but also to provide reliable and non-destructive characterisation of the thin films in terms of their dielectric and optical properties. The first results on the infrared-active phonon modes were shown by Mayrhofer et al. for AlScN films of varied Sc composition in a narrow range up to 15% [11]. The redshift of the E1(TO) band of AlN was shown to be proportional to the Sc content, which was interpreted as the indication of the elongating M-N (M = Al, Sc) bonds. Similar behaviour was also observed for the E2(high) and A1(TO) Raman bands and compared to the evolution of the rest IR-active bands, thereby confirming a decrease in the bond length as a result of increased bond ionicity [12].

**Citation:** Solonenko, D.; Zukauskaite, ˘ A.; Pilz, J.; Moridi, M.; Risquez, S. Raman Spectroscopy and Spectral Signatures of AlScN/Al2O3. *Micromachines* **2022**, *13*, 1961. https://doi.org/10.3390/ mi13111961

Academic Editor: Alberto Tagliaferro

Received: 20 September 2022 Accepted: 8 November 2022 Published: 11 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

Despite the narrow range of the Sc concentration, the main trends in phonon properties of the AlScN lattice have been unravelled so far while largely omitting the obvious impact of the film's microstructure on the spectral data. While the phonon frequency shift is evident and expected in the framework of the lattice softening, the rest of the peak characteristics are intricately entangled with the quality of the AlScN films. Although the influence of the microstructure can be observed using both IR and Raman spectra, it can be approached more accurately in the latter case [13,14]. Indeed, the large peak broadening observed in the Raman spectra of AlScN alloys remains poorly understood, and so does their overall spectral signature, which includes a few Raman bands addressed earlier [12,15–19]. It is necessary for understanding the contributions of various mechanisms responsible for the dramatic broadening of the Raman-active spectral bands, such as the possibility of the rock-salt phase formation, alloy- or disorder-based phonon scattering, and size effect.

In this work, we investigated in detail the vibrational signatures of Al1−*x*Sc*x*N pseudobinary alloys in a broad range of the Sc concentrations (up to *x* = 0.41). Our results confirm the previous observations of the frequency shift and peak broadening for the Raman bands observed in the c–axis–oriented films [12,15]. We show that the Raman spectra of the alloys actually exhibit up to eight bands. Unreported bands are attributed to the second-order phonon modes of the wurtzite AlScN lattice. Moreover, we investigated the low–frequency spectral region, the investigation of which is usually limited by technical difficulties. We found the presence of Raman bands which are proposed to be related to the confined acoustic modes. In contrast to pure AlN, the second–order spectral features stemming from two-phonon modes and phonon density of states are greatly enhanced as a result of the bandgap shrinkage and a high density of the midgap states. The presence of these electronic states is attributed to the point defects, the density of which is two orders of magnitude higher than in pure wurtzite, as assessed via the phonon correlation length. This was corroborated by temperature–dependent Raman measurements showing that the phonon-defect scattering dominates over the phonon–phonon mechanism in AlScN regardless of the Sc concentration. The determined temperature coefficients can be used for the precise temperature determination in AlScN films via Raman spectroscopy.