# A Simple Approach to Prepare a C3N4/MoO3 Heterojunction with Improved Photocatalytic Performance for the Degradation of Methylparaben

Abdelaziz Imgharn, Tingwei Sun, Jimmy Nicolle, Yassine Naciri, Abdelghani Hsini, Abdallah Albourine, Conchi Maria Concepcion Ovin Ania

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Abdelaziz Imgharn, Tingwei Sun, Jimmy Nicolle, Yassine Naciri, Abdelghani Hsini, et al., A Simple Approach to Prepare a C3N4/MoO3 Heterojunction with Improved Photocatalytic Performance for the Degradation of Methylparaben. *Catalysts*, 2024, 14 (3), pp.170. 10.3390/catal14030170. hal-04799661

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# A Simple Approach to Prepare a C₃N₄/MoO₃ Heterojunction with Improved Photocatalytic Performance for the Degradation of Methylparaben

Abdelaziz Imgharn <fnref n="1" />,<fnref n="2" />, Tingwei Sun <fnref n="2" />, Jimmy Nicolle <fnref n="3" />, Yassine Naciri <fnref n="1" />,†, Abdelghani Hsini <fnref n="1" />,‡,
Abdallah Albourine <fnref n="1" />,<fnref n="4" /> and Conchi Ania <fnref n="2" />,*

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**Citation:** Imgharn, A.; Sun, T.; Nicolle, J.; Naciri, Y.; Hsini, A.; Albourine, A.; Ania, C. *A Simple Approach to Prepare a C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub> Heterojunction with Improved Photocatalytic Performance for the Degradation of Methylparaben.* *Catalysts* **2024**, *14*, 170.
https://doi.org/10.3390/catal14030170

Academic Editor: Maria Victoria Lopez-Ramon

Received: 18 January 2024
Revised: 11 February 2024
Accepted: 19 February 2024
Published: 27 February 2024

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**Copyright:** © 2024 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/).

**Abstract:** The adequate optical properties, low cost, and thermal stability of graphitic carbon nitride and molybdenum oxide make them both promising materials for photocatalytic applications. However, they both suffer from strong recombination of their photogenerated charge carriers. Therefore, searching for strategies that enable an efficient charge carrier separation is desirable for improving the photocatalytic performance of both semiconductors. In this work, we have synthesized a g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub> heterojunction by a facile solid dispersion approach to the pristine semiconductors that allows a uniform dispersion of the two phases in the heterojunction. The resulting hybrid photocatalyst exhibits light absorption features similar to pristine g-C<sub>3</sub>N<sub>4</sub> and presents an improved separation of the photogenerated charge carriers, likely through a Z-scheme between both semiconductor phases, as inferred by photoelectrochemical measurements. As a result, the g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub> heterojunction showed better photocatalytic activity than the individual semiconductors and good cycling stability for the degradation of methylparaben and its reaction intermediates. We drew these conclusions based on total organic carbon (TOC) measurements.

**Keywords:** methylparaben degradation; water emergent contaminants; solid state heterojunction; photocatalysis; carbon nitride; molybdenum oxide

## 1. Introduction

Recent years have seen an increased awareness of the risks associated with contaminants of emerging concern to the aquatic ecosystems and to human health [1,2]. Among different families of substances, parabens—derivatives of para-hydroxybenzoic acid used as antimicrobial and antibacterial preservatives—are perceived as recalcitrant pollutants to water bodies. Their impacts on human health include their ability to disrupt the endocrine system or to produce pregnancy complications and adverse effects on fetal and childhood growth [3–5]. Parabens occur naturally in many fruits and vegetables, but they are also widely used in the formulation of common consumer products such as cosmetics, pharmaceutical compounds, personal care products, beverages, and in the food industry [5]. Owing to such high utilization, parabens have been detected in human tissues and various types of water (including drinking water, rivers, and seawater). The presence of parabens in water bodies is mostly attributed to discharges from wastewater treatment plants and industrial activities [6]. Since conventional sludge treatment methods have proven to be

<footnote n="1">¹ Laboratory of Materials and Environment, Faculty of Sciences, Ibn Zohr University, Agadir 80060, Morocco; abdelaziz.imgharn@edu.uiz.ac.ma (A.I.); yassine.naciri@edu.uiz.ac.ma (Y.N.); abdelghani.hsini@edu.uiz.ac.ma (A.H.); a.albourine@uiz.ac.ma (A.A.)</footnote>

<footnote n="2">² CEMHTI, CNRS (UPR 3079), Université d'Orléans, 45071 Orléans, France; tingwei.sun@cnrs-orleans.fr</footnote>

<footnote n="3">³ ICMN, CNRS (UMR 7374), Université d'Orléans, 45071 Orléans, France; jimmy.nicolle@cnrs-orleans.fr</footnote>

<footnote n="4">⁴ Laboratory of Industrial Engineering, Energy and Environment (LI3E), SupMTI Rabat, Rabat 10000, Morocco</footnote>

<footnote>* Correspondence: conchi.ania@cnrs-orleans.fr; Tel.: +33-238-25-55-13</footnote>

<footnote>† Current address: Institut de Chimie Physique, UMR 8000 CNRS, Université Paris-Saclay, Orsay 91405, France.</footnote>

<footnote>‡ Current address: Research Institute for Solar Energy and New Energies (IRESEN), Ibn Tofail University, Kenitra 14000, Morocco.</footnote>
inefficient at removing these recalcitrant pollutants, research on more efficient degradation processes has become a topic of interest in water remediation.

Among other alternatives, solar-driven photocatalytic processes seem particularly attractive for the degradation of non-biologically degradable pollutants such as parabens. TiO₂ is considered the benchmark photocatalyst for many applications; however, its poor activity under natural sunlight and the recently raised issues related to its nanoparticulated nature (e.g., the European Union banned the use of TiO₂ nanoparticle suspensions in food products in 2020 [7]) prevent a widespread application to water remediation [8–10]. This has prompted investigations into new materials with enhanced photoactivity under natural sunlight [10–12]. In this context, graphitic carbon nitride (g-C₃N₄) has received much attention in photocatalytic applications. Its interesting electronic band structure, visible-light response, and relatively low cost have made it a potential alternative to transition metal-based semiconductors [13–16]. As for many other semiconductors, the main cornerstone of g-C₃N₄ for further advances in the field of photocatalysis is the severe recombination of the photogenerated charge carriers.

On the other hand, the use of solid-state heterojunctions (e.g., catalysts composed of distinct phases—typically semiconductors—with different energy levels) has become a promising approach in photocatalysis, with many examples showing improved photocatalytic conversion rates owing to the increased lifetime of the charge carriers. (The close contact between the phases in the heterojunction shortens the path of the charge carriers [13,14,17].) In particular, g-C₃N₄-based direct Z-scheme photocatalysis has been extensively applied in water remediation, with remarkable results for several pollutants, namely dyes, tetracyclines, pesticides, and pharmaceuticals [13–17]. Despite the abundant literature on heterojunctions based on g-C₃N₄, most studies focus on the use of wide bandgap semiconductors (e.g., ZnO, TiO₂) and complex synthetic approaches to obtain controlled optimized interfaces between the semiconductors of the heterojunction, as the interfacial properties define the flow of charge carriers upon illumination [18–24].

In this study we have selected MoO₃, an n-type gap semiconductor for the preparation of a g-C₃N₄/MoO₃ heterojunction as both materials have a staggered energy level structure that should allow the formation of a direct Z-scheme. Furthermore, the favorable optical properties of MoO₃ (e.g., it can be activated under solar light), its low cost, and its stability in aqueous environments make it a viable contender in solar photocatalysis for water remediation [10]. Indeed, the advantages of combining MoO₃ with other photocatalysts have been widely reported in the literature. Some examples include ZnO [25], CuO [26], BiVO₄ [27], or g-C₃N₄ [19].

In this study, we used a fast and simple solid dispersion method to prepare a g-C₃N₄/MoO₃ heterojunction showing enhanced photocatalytic performance for the degradation of a recalcitrant pollutant (methylparaben, MPB). Methylparaben is a pollutant of primary concern in France, owing to its toxicity and frequent detection in aqueous environments after sewage treatment [28,29]. Obtained data has shown that the heterojunction displays better photodegradation efficiency under simulated solar light than the bare semiconductors in terms of conversion of both MPB and its degradation intermediates. Our preliminary results monitoring the by-products formed indicate the presence of short alkyl chain organic acids, with only traces of aromatic derivatives. The enhanced separation of the photogenerated charge carriers (evidenced by the photoelectrochemical measurements) demonstrated the adequate interface contact between both semiconductors in the heterojunction. This resulted in an improved redox ability of the photoelectrons and photoholes retained in the heterojunction, hence accounting for the improved catalytic activity for the photodegradation of methylparaben, with a high mineralization extent (as evidenced by TOC measurements).
## 2. Results and Discussion

### 2.1. Characterization of the Catalysts

Figure 1 shows the XRD patterns of the g-C₃N₄/MoO₃ heterojunction and the pristine semiconductors g-C₃N₄ and MoO₃⁻. The latter shows the diffraction peaks corresponding to the well-reported structures of both materials in the literature [19–26]. The pattern of g-C₃N₄ exhibits two diffraction peaks, located at 13.11° and 27.52°, which are consistent with the reference diffractogram for this material (JCPDS, 87-1526). The peak at 13.11° corresponds to the in-planar repeated triazine units (100). The peak at 27.52° (002) is associated with the interlayer stacking of aromatic structures and confirms the graphitic-like structure of this material. A crystallite size value of 5.2 nm was calculated using the Scherrer equation (Table S1).

<figure><img src="image_5.png" /><figcaption>**Figure 1.** XRD patterns and SEM images of the pristine semiconductors and the heterojunction: (A,B) g-C₃N₄/MoO₃; (C,D) MoO₃; (E,F) g-C₃N₄.</figcaption></figure>

For MoO₃, all the diffraction peaks indexed with the orthorhombic structure (JCPDS, 05-0508) of the semiconductor, with a crystallite size of ca. 61 nm. The XRD patterns of the g-C₃N₄/MoO₃ heterojunction revealed the coexistence of the crystalline phases of both semiconductors, with negligible differences in the position and/or widths of the peaks compared to the patterns of the bare semiconductors. The peaks corresponding to the (020), (110), (040), and (060) of MoO₃ are clearly seen in the pattern of the heterojunction, along with the wide (002) peak of g-C₃N₄. The absence of additional diffraction peaks in the heterojunction confirms the lack of structural modifications to the semiconductors during the synthesis (i.e., solid state dispersion of the individual phases at 100 °C). Similarly, the morphology (Figure 1) and porosity (Figure S1) of the heterojunction were similar to that of the pristine semiconductors. More specifically, the heterojunction showed particles of irregular shapes ranging from a few hundred nanometers to several microns that correspond to the morphology of pristine g-C₃N₄, along with scattered micrometric particles of regular and smooth shape (see arrows in Figure 1) corresponding to the MoO₃ semiconductor. The average particle sizes ranged from a few hundreds of nanometers to several microns. EDS analysis of the heterojunction (Figure S2) revealed the presence of C, N, O, and Mo atoms with atomic ratios C:N and Mo:O of 1:2 (ca. 1.1 wt.%) and 0.0.1 (ca. 0.6 wt.%), respectively. These atomic ratios are lower than the expected stoichiometric values considering the composition of the heterojunction (0.8 and 0.33 atomic ratio for C:N and Mo:O, respectively). This mismatch could be attributed to either the lack of sensitivity
of EDS analysis preventing a precise estimation of the atomic ratios, or to a homogeneous distribution of MoO₃ particles in the bulk of the heterojunction and not on the surface (EDS being a surface technique). A similar observation was inferred from the XPS analysis, as will be discussed below.

Regarding porosity (Figure S1), all the samples displayed type II N₂ adsorption isotherms characteristic of non-porous solids, and a hysteresis loop relative pressure above 0.8, indicating the presence of large cavities (mesopores and macropores). Furthermore, the main textural parameters of sample g-C₃N₄/MoO₃ (pore volumes and surface areas) are similar to the theoretical values predicted by the mixing rules. This was rather expected since the synthetic route used in the preparation of the heterojunction consisted of a solid dispersion of the pristine semiconductors at moderate temperatures.

The optical features of the photocatalysts were evaluated by diffuse reflectance spectroscopy. The corresponding spectra of the heterojunction and the pristine semiconductors, along with Tauc representations for the evaluation of the optical band gap, are shown in Figure 2. As seen, all three materials presented an abrupt decrease in reflectance values associated with the optical absorption edge of the materials. The absorption edge of g-C₃N₄ and MoO₃ is observed at around 400 nm and 350 nm, respectively, indicating the higher absorption of g-C₃N₄ in the visible light region. The corresponding bandgaps were calculated assuming direct electronic transitions for MoO₃ and indirect transitions for g-C₃N₄ and g-C₃N₄/MoO₃, as commonly reported in the literature [19,21,25–27]. The obtained values were 2.83 eV for g-C₃N₄, 3.03 eV for MoO₃ and 2.85 eV for g-C₃N₄/MoO₃ (+ 0.01 eV in all cases).

<figure><img src="image_6.png" /><figcaption>**Figure 2.** Experimental diffuse reflectance spectra (a) and Tauc graphical representations of the studied catalysts (b–d). In Tauc representations: experimental data (black lines); fitted range (orange line); baseline fitting (green line); dotted lines represent the extrapolation of the double linear fitting ranges.</figcaption></figure>
The calculated optical band gap of the heterojunction is slightly higher than that calculated for g-C₃N₄ (although the differences are subtle). This points out that, despite the differences in the optical band gap of the pristine semiconductors, the incorporation of a small amount of MoO₃ (ca. 10wt.%) in a solid-state dispersion of carbon nitride matrix does not have a strong impact on the optical features of the latter.

The surface composition of the catalysts was analyzed by XPS (Figure 3). For sample g-C₃N₄, the C1s spectrum was deconvoluted into several contributions: a peak at 284.6 eV was assigned to C–C bonds of graphitic carbon; a peak at 285.8eV attributed to the C–N=C backbone, and the peak 288.2 eV characteristic of sp²-hybridized carbon in an N-C=N configuration. The N1s core level spectrum was deconvoluted into three peaks at 398.7, 400.0, and 401.1 eV, which represent triazine rings of C-N=C, the tertiary nitrogen in N-(C)₃ groups, and the N(N-H) groups, respectively. The O1s core level spectrum revealed small contributions (ca. 3 at.%) for oxygen species, mainly attributed to adsorbed oxygen/water. All these assignments are consistent with previously reported results for carbon nitride [30,31]. The analysis of pristine MoO₃ showed the characteristic peaks corresponding to Mo 3d3/2 and Mo 3d5/2 at 236.6 and 233.4 eV assigned to Mo⁶⁺, along with two small peaks at 232.10 eV and 235.2 eV that can be assigned to Mo⁵⁺ (oxygen vacancies) [32]. The calculated atomic ratio Mo 3d:O 1s of 1.6 confirms the existence of oxygen vacancies in MoO₃.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** (a) High resolution C1s, O1s, and N1s core spectrum of the studied catalysts before and after three consecutive photocatalytic assays; (b) atomic concentration of the different species.</figcaption></figure>

For the heterojunction, a distribution of species similar to that in g-C₃N₄ was found for all the elements, confirming the minor modification in the surface composition of the heterojunction. On the other hand, the Mo 3d signal was not detected on the heterojunction (the sample was measured three times with similar outcomes). The presence of MoO₃ in the heterojunction was confirmed by ICP-OES (upon acid digestion of the sample) and by the XRD patterns (Figure 1). The lack of a Mo 3d signal indicates that the MoO₃ phase is not
accumulated in the external surface of the heterojunction (note that XPS is a surface analysis technique probing ca. 1–10 nm in the external surface), but rather dispersed in the bulk of the material. (Hence, the surface composition determined by XPS does not represent the bulk material.) A similar conclusion was inferred from the mismatch in the element's atomic ratios detected by EDS analysis (another surface technique), as mentioned above.

## 2.2. Photocatalytic Performance

Before irradiation, the photocatalysts were put in contact with a solution of methylparaben and stirred in dark conditions for 30 min (see further details in the experimental section below). This takes into account the removal of the pollutant due to adsorption on the catalysts (before irradiation). For clarity purposes, this has been marked as -30 min in Figure 4 (dark contact region). Preliminary kinetic adsorption studies in dark conditions on all of the catalysts confirmed that under our experimental conditions, the uptake of MPB reaches equilibrium conditions (i.e., stable concentration in solution) in 20–30 min. The uptake in dark conditions was relatively low and similar to all three studied catalysts, which is consistent with their low porosity (Figure S2). This assured us that the initial concentration of the pollutant in the solution, when the catalysts are illuminated, is the same for all of them. This is important to the comparison of their photocatalytic performance, disregarding the effects of adsorption rates since it is well-known that photodegradation kinetics is affected by the initial concentration of the pollutant in the solution. Also, this experimental approach allows us to discriminate between pollutant removal due to (non-degradative) adsorption and the photocatalytic degradation reaction, which may occur simultaneously in porous catalysts.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Concentration decay curves (a) and conversion (b) of methylparaben upon illumination of the studied materials; (c) methylparaben degradation rate constants; (d) TOC reduction.</figcaption></figure>
Figure 4 shows the concentration decay curves of MPB for all three studied photocatalysts, compared to the photolytic reaction (in the absence of a catalyst). For comparison purposes, the data are also plotted as the conversion of methylparaben. Note that under our experimental conditions the decay in MPB contribution can be directly related to its photooxidative degradation (disregarding adsorption effects as discussed above).

With the exception of sample MoO₃, the photocatalytic degradation of MPB was more efficient than the photolytic reaction (in the absence of a catalyst). Even though the decay in MPB concentration during the photolytic degradation was relatively high (ca. 40% and 90% decay after 1 and 4 h respectively), the photolytic reaction was quite inefficient for the complete mineralization of MPB. The evolution of the Total Organic Carbon (Figure 5) showed that only ca. 0.4% of complete degradation of MPB was achieved after 5 h of irradiation in the absence of a catalyst. This evidences the low conversion of the degradation intermediates of MPB in the photolytic reaction. The relatively large decay of methylparaben concentration on the photolytic reaction under our experimental conditions may be due to the low concentration (ca. 5 ppm) of the pollutant in the solution. Indeed, most studies reported in the literature on the photocatalytic degradation of methylparaben typically use higher initial concentrations of the pollutant (Table 1).

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Photocatalytic performance of the g-C₃N₄/MoO₃ heterojunction upon several consecutive cycles (1st, 2nd, 3rd): (a) concentration decay curves of methylparaben and (b) TOC reduction.</figcaption></figure>

<table><caption>**Table 1.** Comparison of the photocatalytic degradation studies of parabens with g-C₃N₄ semiconductor and heterojunctions with other semiconductors.</caption><thead><tr><th>Catalyst</th><th>Synthesis</th><th>Paraben Concentration, Catalyst Loading</th><th>Irradiation Source</th><th>Photodegradation Efficiency (%)</th><th>Ref.</th></tr></thead><tbody><tr><td>g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub></td><td>Solid state dispersion</td><td>Methylparaben 5 ppm, 0.5 g/L catalyst</td><td>125 W lamp (UV-vis)</td><td>100% after 120 min; TOC reduction of 93%</td><td>This work</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub> and WO<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub></td><td>hydrothermal treatment</td><td>Methylparaben 10 ppm, 1 g/L catalyst</td><td>Xe lamp (300 W)</td><td>36% after 60 min (g-C<sub>3</sub>N<sub>4</sub>)<br>44-98% (WO<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub>)</td><td>[31]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub></td><td>calcination</td><td>Methylparaben<br>0.1-1 ppm, 0.05-0.5 g/L catalyst</td><td>100 W Xe lamp</td><td>100% after 90 min of irradiation</td><td>[33]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>/BiVO<sub>4</sub></td><td>hydrothermal treatment</td><td>benzyl paraben 10 ppm, 1 g/L catalyst</td><td>300 W Xe lamp</td><td>86% after 150 min</td><td>[34]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub> (nanosheets, bulk)</td><td>Thermal exfoliation</td><td>methyl-, ethyl- and propyl paraben<br>10 ppm, 0.2 g/L catalyst</td><td>300 W Xe lamp</td><td>Nanosheets > 95%<br>Bulk < 30%</td><td>[35]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub>/BiVO<sub>4</sub></td><td>hydrothermal treatment</td><td>benzyl-paraben<br>20 ppm, 1 g/L catalyst</td><td>300 W Xe lamp</td><td>62% after 150 min<br>5% composite</td><td>[36]</td></tr></tbody></table>
<table><caption>**Table 1.** *Cont.*</caption><thead><tr><th>Catalyst</th><th>Synthesis</th><th>Paraben Concentration, Catalyst Loading</th><th>Irradiation Source</th><th>Photodegradation Efficiency (%)</th><th>Ref.</th></tr></thead><tbody><tr><td>N-vacancies on g-C<sub>3</sub>N<sub>4</sub></td><td>N<sub>2</sub> etching method</td><td>Propylparaben (no details)</td><td>70 W metal halide lamp</td><td>94%</td><td>[37]</td></tr><tr><td>g-C<sub>3</sub>N<sub>4</sub> (various precursors)</td><td>Thermal polymerization method</td><td>Mixture of methyl-, ethyl-propyl-paraben, 1 ppm each, 0.2g/L catalyst</td><td>ultraviolet-A, visible LED, natural sunlight</td><td>28–32% under UV light</td><td>[38]</td></tr><tr><td>P25 (TiO<sub>2</sub>)</td><td>Commercial</td><td>methylparaben 10 ppm, 2 g/L catalyst</td><td>125 W UV lamp (365 nm)</td><td>99% after 120 min</td><td>[39]</td></tr><tr><td>P25 (TiO<sub>2</sub>)</td><td>Commercial</td><td>methylparaben 30 ppm, 2.5 g/L catalyst</td><td>15 W UV lamp (350–410 nm)</td><td>20% after 100 min</td><td>[40]</td></tr></tbody></table>

To better compare the performance of the studied materials, the rate constant values of the photocatalytic assays were determined from the concentration decay curves using a first-order reaction model. The data are shown in Figure 4c. The rate constant of methylparaben conversion was twice as large for sample g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub> than that of bare g-C<sub>3</sub>N<sub>4</sub> and almost three times larger than that of bare MoO<sub>3</sub>. This demonstrates the faster degradation rate of the heterojunction.

The extent of MPB mineralization (i.e., complete conversion to CO<sub>2</sub> and water) was monitored by observing the evolution of the total organic carbon of the solutions (Figure 4d) and the analysis of the degradation subproducts in the reaction by HPLC (Figure S4). For g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub>, a TOC abatement of ca. 93% was achieved after 300 min of illumination, demonstrating the capacity of this photocatalyst to degrade both MPB and its reaction intermediates. The extent of mineralization was higher for the heterojunction than that observed for the bare semiconductors, with TOC abatements ranging between 55–75% for g-C<sub>3</sub>N<sub>4</sub> and MoO<sub>3</sub>, respectively. This is in agreement with the low amounts of intermediates detected in the solution by HPLC (Figure S4). A preliminary screening of the degradation subproducts pointed out the presence of small amounts of polyhydroxylated intermediates (e.g., trihydroxybenzoic acid isomers and tri- and di-hydroxybenzene isomers) in the first 60 min of irradiation for the heterojunction. These intermediates were not detected after longer periods of the reaction in the heterojunction, but were still seen when the bare semiconductors were used as photocatalysts.

The good photocatalytic performance of g-C<sub>3</sub>N<sub>4</sub>/MoO<sub>3</sub> in terms of methylparaben conversion, degradation of intermediates, and TOC abatement was maintained after three consecutive illumination assays (ca. 15 h) (Figures 5 and S4). The stability of the heterojunction during the cycles was further confirmed by the post-mortem analysis of the composition by XPS (Figure 3) after the consecutive photocatalytic assays. The C1s and N1s core level spectra of the heterojunction after the consecutive runs were similar to those of the initial material. Small differences were observed in the O1s signal; after the photocatalytic cycles, the heterojunction displayed a slightly higher oxygen content (ca. from 3 at.% to 6 at.%). While the O1s core level spectrum in the as-prepared material was deconvoluted in one peak at 532.2 eV (adsorbed oxygen/water), an additional contribution to the O1s signal appeared after the cycles. A small peak featuring at 533.21 eV was observed, assigned to surface hydroxyl radicals [41]. The relative abundance of the O1s peaks was still relatively low after the cycles (ca. 6.7 at.%), confirming the stability of the catalysts upon three consecutive cycles. The peaks attributed to Mo-O bonds (either O1s or Mo 3d signals) were still not present in the heterojunction, as discussed above for the initial sample before the subsequent photocatalytic runs.

According to the literature, such enhanced photocatalytic activity of the heterojunction may be attributed to an efficient charge separation via the charge migration between the two semiconductors [13,42]. To corroborate this hypothesis, a photoelectrochemical characterization of the materials was carried out. The cyclic voltammograms of ITO/semiconductor electrodes under on/off illumination obtained in 0.1 M Na<sub>2</sub>SO<sub>4</sub> are presented in Figure 6.
<figure><img src="image_10.png" /><figcaption>**Figure 6.** Cyclic voltammograms recorded at 20 mV/s of ITO/semiconductor electrodes in 0.5 M Na₂SO₄ (pH 6.10) under dark conditions and illumination using a 340 nm LED. Before the measurements, the electrodes were stabilized for 20 cycles in the dark (last cycle shown).</figcaption></figure>

In dark conditions, the profiles of bare g-C₃N₄ and MoO₃ showed the characteristic shape of n-type semiconductors with two distinctive regions in the voltammograms. A first region between +200 and +1200 mV is characterized by a flat small current at anodic potentials and corresponds to the depletion region where electrons cannot move (the material behaves like a diode). The second region (accumulation region) between -400 and +200 mV is characterized by a cathodic current indicating a flux of electrons. (Note that the average cathodic current in this region is slightly higher than that in the depletion region.) The dark currents were slightly more intense in the g-C₃N₄/MoO₃ heterojunction, compared to the pristine semiconductors, which could be attributed to the small differences in capacitive contribution (in agreement with the porous features, Figure S2). Upon illumination, cathodic and anodic photocurrents were observed at potentials below -100 mV and above +200 mV, respectively, due to the migration of the majority charge carriers, indicating the formation of hole-electron pairs (recombination is not dominant at these bias potentials). In an aqueous inert supporting electrolyte, the anodic photocurrent is related to water oxidation by the photoholes (through the formation of ●OH radicals). In the anodic sweep, the photocurrent gradually increased with the potential bias; this is characteristic of systems governed by a photohole capture regime [43].

Figure 7 shows the transient chronoamperometric curves upon on-off illumination at +600 mV of the three studied materials. All the catalysts showed a similar qualitative response, with an initial sharp current spike on switching on the light followed by a gradual decrease in the photocurrent (the pulses were very fast). The current returned to the original values almost instantaneously upon switching off the light. The photocurrent response was reproducible during repeated on/off illumination cycles, reaching a steady state regime after the second cycle (this cycle was used for the semi-quantitative analysis of the photoelectrochemical response). This confirms that the initial current decay is due to fast recombination of the charge carriers, rather than to photocorrosion of the electrode. For the second cycles and so forth, the photocurrent decay would be most likely associated with the trapping of electrons in deep states, resulting in the redistribution of interfacial potential [43].

The photocurrent intensity of the g-C₃N₄/MoO₃ electrode was much larger (ca. 3 times) than that of pristine g-C₃N₄, demonstrating the reduced recombination of the charge carriers in the heterojunction. The estimated photocurrent rate constants using a simple first-order kinetic approach also demonstrated the slower photocurrent decay for g-C₃N₄/MoO₃, compared to the bare semiconductor (Figure S3). This is in line with the matched band alignment reported in the literature for both semiconductors [19,44,45]. It also confirms that the preparation of the heterojunction via a solid-state dispersion of both phases is adequate to provide a close interfacial contact between them and to facilitate the flow of the charge carriers in the solid state without the need of a mediator in solution.
<figure><img src="image_11.png" /><figcaption>**Figure 7.** Transient photocurrent response of ITO/semiconductor electrodes recorded at a bias potential of +600 mV vs. SCE in 0.5 M Na₂SO₄ (pH 6.10). Illumination source is an LED at 340 nm: on/off cycles of 2 min were recorded.</figcaption></figure>

## 2.3. *Photocatalytic Mechanism*

To explore the photoactivity mechanism of the g-C₃N₄/MoO₃ heterojunction system, we estimated the alignment of the conduction and valence band edges of the semiconductors used in the heterojunction using the Butler and Ginley method [46]. (The complete dataset and equations are compiled in Table S2.) The calculated energy levels for the valence band (VB) and conduction band (CB) of MoO₃ are 0.47 and 3.32 eV, respectively. Those calculated for g-C₃N₄ are 1.63 eV (VB) and -1.07 eV (CB). The proximity of the CB of MoO₃ to the VB of g-C₃N₄ makes possible a charge transfer upon a Z-scheme, as illustrated in Figure 8.

<figure><img src="image_12.png" /><figcaption>**Figure 8.** Schematic illustration of the photocatalytic activity on methylparaben following the direct Z-scheme in the g-C₃N₄/MoO₃ heterojunction.</figcaption></figure>

g-C₃N₄ is a metal-free n-type indirect semiconductor that can be synthesized from earth-abundant organic precursors. Its structure is based on triazine or heptazine units
polymerized in a layered structure. It has a high visible light absorption capacity, which makes it attractive in photocatalytic applications, with an optical bandgap of ~2.7 eV. The energy levels of the CB of g-C₃N₄ enable the reduction of water into hydrogen and the reduction of oxygen into superoxide radicals (via the photoelectrons' reactivity). Still, they cannot promote the oxidation of water (via photoholes reactivity) into hydroxyl radicals [13,14]. However, owing to its 2D layered structure, this semiconductor presents a rapid surface charge recombination. Similarly, orthorhombic MoO₃ is a n-type indirect semiconductor, with the orthorhombic polymorph presenting a lamellar structure consisting of edge-and corner-sharing distorted MoO₆ octahedra in an ABA arrangement. It has a band gap of 2.8–3.6 eV, depending on the oxygen vacancies. The relatively low VB edge level of MoO₃ has a strong oxidation ability (i.e., the high oxidation power of photoholes), but the electrons in the CB edge are unable to reduce water to hydrogen due to slow electron mobility [45–48]. This causes the surface accumulation of electrons in the CB upon illumination of MoO₃, increasing the recombination rate and ultimately reducing the photocatalytic performance.

The Z-scheme heterojunction g-C₃N₄/MoO₃ compensates for this limitation of both semiconductors, as observed in their enhanced photocatalytic performance (Figure 4). Upon light excitation of the heterojunction, the photoelectron injected in the CB of MoO₃ (a semiconductor with a low reduction potential) can be easily transferred to the VB of g-C₃N₄ due to the alignment of the energetic levels, then recombine with the low oxidation potential photoholes in the VB of g-C₃N₄ [19,44], forming a direct Z-scheme photocatalytic system without an electron mediator. As a result, isolated photoholes and photoelectrons formed in the heterojunction show a higher redox ability than those in the individual semiconductors. This would account for a higher capacity to generate radical reactive species (hydroxyl, superoxide) and ultimately promote the photodegradation of methylparaben and its intermediates to reach a high mineralization extent (Figure 6).

## 3. Materials and Methods

### 3.1. *Catalysts Preparation*

Bulk g-C₃N₄ was obtained upon calcination of melamine powders at 550 °C during three hours in a muffle. MoO₃ powders were prepared by solid-state decomposition of (NH₄)₆Mo₇O₂₄·4H₂O at 500 °C for 4 h under an air atmosphere. The resulting products were thoroughly washed in ultrapure water and dried at 70 °C for 12 h. The g-C₃N₄/MoO₃ heterojunction (composition fixed to 10:90 wt.%) was prepared by dispersion of the solids under mild sonication. Briefly, 90 g. of g-C₃N₄ were dispersed in 20 mL of a mixture of ethanol:water (3:1 *v*/*v*) and sonicated for 30 min. Likewise, 10 g. of MoO₃ were dispersed in 20 mL of ethanol:water and sonicated for 30 min. Then, the MoO₃ dispersion was gradually added to the g-C₃N₄ one under constant stirring. The mixture was stirred for 2 h at room temperature, and then placed in a hot water bath at 100 °C for 1h, followed by cooling to room temperature. The resulting catalyst powders were washed, filtered and dried for 6 h at 70 °C. They were finally calcined in a muffle furnace at 300 °C for 3h. For comparison purposes, the bare semiconductors were sonicated and calcined in the same conditions.

### 3.2. *Characterization*

The porosity of the catalysts was evaluated by N₂ adsorption at –196 °C in a volumetric analyzer (Tristar, Micromeritics, Norcross, GA, USA). The samples were previously outgassed at 120 °C overnight under primary vacuum. The gas adsorption isotherms were used to calculate the specific surface area, total pore volume, and micropore volume using the Dubinin-Radushkevich equation [49]. X-ray diffraction (XRD) patterns were recorded in a Bruker instrument (D8 Advance, Billerica, MA, USA) operating at 40 kV and 40 mA and using CuKα (0.15406 nm) radiation. The optical band gap of the photocatalysts was determined by UV-Vis diffuse reflectance spectroscopy in a spectrophotometer (Shimadzu UV-2501, Kyoto, Japan) equipped with an integrating sphere. Powders of the catalysts were pressed in holders of ca. 3 cm diameter and 5 mm depth. The measurements were recorded
between 220–700 nm in the diffuse reflectance mode, using BaSO₄ as reference. Spectra were collected at 1 nm intervals with a spectral bandwidth of 2 nm. Data were transformed to a magnitude proportional to the extinction coefficient through the Kubelka-Munk function, F(R∞). The graphical representation of the Tauc method was used to evaluate the optical band gap, using a double linear fitting approach [50]. The morphology of the catalysts was determined by scanning electron microscopy (SEM) using a microscope (JEOL JSM-IT200, JEOL Ltd., Akishima, Japan) coupled with energy dispersive X-ray spectrometry (EDS) analysis. X-ray photoelectronic spectroscopy (XPS) experiments were collected in a K-Alpha ThermoScientific spectrometer (ThermoScientific, Waltham, MA, USA) using monochromatic AlKα (1486.6 eV) radiation at 3 mA × 12 kV. The spectra of dried samples were recorded using a 400 µm diameter analysis area. Processing of the XPS spectra was performed in Avantage software (v5), with energy values referenced to the C1s peak of adventitious carbon located at 284.6 eV, and a Shirley-type background.

### 3.3. *Photoelectrochemical Measurements*

The photoelectrochemical properties of the prepared photocatalysts were evaluated at room temperature in a conventional three-electrode electrochemical using an electrochemical workstation (Biologic VMP3, Seyssinet-Pariset, France). The working electrodes were prepared by spin-coating ca. 200 microL of inks containing the active material (ca. 90:10 photocatalyst:nafion 5 wt.% dispersed in dimethylformamide) onto conductive ITO substrates. A saturated calomel (SCE) and a graphite rod were used as reference and counter electrodes, respectively. A monochromatic 340 nm LED (Thorlabs Inc., Newton, NJ, USA) was used as irradiation source. The electrodes were immersed in 0.5 M Na₂SO₄ (pH 6) inert electrolyte, placed in front of the irradiation source and allowed to equilibrate for at least 3 h before recording any measurements. The photoelectrochemical response of the prepared electrodes was recorded by cyclic voltammetry under on/off illumination in a potential sweep from –400 to +1200 mV vs. SCE (scan rate 20 mV/s). The transient photocurrent response of the electrodes upon on/off illumination at a fixed bias potential was recorded to evaluate the electron transfer mechanism upon calculation of the rate of photocurrent decay from the transient profiles using a first-order kinetic approach [51]. (See details in Supplementary Materials File).

### 3.4. *Photocatalytic Performance Assays*

Methylparaben (98% purity) was purchased from Sigma Aldrich (St. Louis, MO, USA). Photodegradation experiments were performed at room temperature in a 500 mL batch reactor using an immersion photoreactor of cylindrical configuration (125W, Helios ItalQuarz (Cambiago, Italy), photon flux 900 W/m²). The lamp was provided with a double-walled cylindrical quartz jacket cooled by flowing water (to prevent overheating of the treated solution during the photocatalytic assays), and vertically immersed in the solution. Before the photocatalytic assays, the catalyst was stirred in the dark in contact with the pollutant solution for 30 min. An excess of oxygen was provided during the photocatalytic assays by supplying a continuous air flow. Aliquots of the solution were periodically removed and filtered (0.45 mm nylon filter), then analyzed by reverse-phase HPLC (C18 column 125 mm × 4 mm; 45:55 methanol: phosphate buffer at pH 2.5 as mobile phase; 40 °C; flow rate of 0.7 mL/min; injection volume 10 microL; photodiode array detector). The degradation of MPB in the absence of catalysts and with the bare semiconductors was also carried out. All photodegradation experiments were performed with a catalyst loading at 1 g/L at least in duplicate; average data is herein presented (accuracy of ca. 5%). For total organic carbon (TOC) analysis, samples were filtered with 0.45 µm filters (Whatman, Maidston, UK) and measured in a Shimadzu VCSN TOC analyzer.

## 4. Conclusions

This study focused on the synthesis of a g-C₃N₄/MoO₃ heterojunction via a fast solid-state dispersion of the individual semiconductors. This simple synthetic route allowed us
to obtain a good interfacial contact between both semiconductor phases, favoring a direct solid-based Z-scheme photocatalytic mechanism (i.e., an interfacial electric field across the semiconductors enabling the flow of the photogenerated charge carriers). The reduced recombination rate of the e⁻-h⁺ pairs upon illumination of the heterojunction was demonstrated by the increased photocurrent intensities and rates, as well as the increased oxidation performance for the photooxidation of methylparaben (a recalcitrant pollutant) and its derivatives. The reduction in overall total organic content increased by two times in the heterojunction compared to the isolated bare semiconductors. The heterojunction showed a stable performance through 3 consecutive cycles, with no modifications of the composition of the heterojunction. These results point out that matching the optical bandgap and the energy levels of the conduction and valence bands of semiconductors becomes more important for achieving efficient photocatalytic Z-schemes in solid state heterojunctions, than the synthetic route for the preparation of the heterojunction itself. A good interfacial contact can be achieved using a solid dispersion of powdered semiconductors, leading to efficient photocatalytic performance if the energy levels are adequately matched. Hence, efforts to synthesize new semiconductor formulations and make advances in the field of photocatalysis for environmental remediation should be focused in this direction.

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14030170/s1, Figure S1: N₂ adsorption/desorption isotherms at -196 °C of the studied catalysts. Inset: main textural parameters evaluated from the isotherms; Figure S2: SEM micrographs and EDS elemental spectrum of the g-C₃N₄/MoO₃ heterojunction; Figure S3: Photocurrent decay rate constant (s⁻¹) of the studied catalysts evaluated from the transient photocurrent responses of ITO/semiconductor electrodes at +600 mV vs. SCE in 0.5 M Na₂SO₄ (pH 6.10); Figure S4: Evolution of methylparaben degradation intermediates detected by HPLC for the studied catalysts, assigned to isomers of trihydroxybenzoic acid and/or, tri- and di-hydroxybenzene derivatives. Note that the photolytic reaction has been plotted in the second axis for clarity purposes (due to the high intensity of the peaks); Table S1: Crystallite size (D) calculated by Scherrer equation from the X-ray diffraction data; Table S2: Electronegativity (eV) of the semiconductors used for the estimation of the energy levels of the conduction and valence bands [52–55].

**Author Contributions:** Conceptualization, Y.N., A.H., A.A. and C.A.; methodology, A.I., T.S. and C.A.; investigation, A.I., T.S. and C.A.; writing—original draft preparation, A.I., T.S. and C.A.; Experimental, A.I., T.S., C.A. and J.N.; writing—review and editing, A.I., T.S., J.N., Y.N., A.H., A.A. and C.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by region Centre Val de Loire (France), through the APR-IA program (grant number 240602, MATHYFON).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors thank the Region Centre Val de Loire for the financial support. A.I. Thanks Association Française de l'Adsorption (AFA) for a mobility grant to Orléans. T.S. thanks China Scholarship Council for her grant (fellowship 202006460009) to perform a PhD in France. This project has benefited from the facilities of the Platform MACLE-CVL, co-funded by the European Union and Centre-Val de Loire Region (FEDER).

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

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