**1. Introduction**

In the past decades, photocatalytic technology through semiconductor oxides for the purification and treatment of polluted water and air has been extensively studied. Recent research activity in the field of heterogeneous photocatalysis is focused on exploiting novel and more efficient photocatalysts capable of using visible light for the degradation of organic contaminants. Many Bi-based semiconductors, such as BiVO4 [1], Bi2O3 [2], Bi2WO6 [3], Bi2O2CO3 [4], Bi2MoO6 [5], and BiPO4 [6] have been developed as visiblelight-driven photocatalysts. Among them, Bi2O3 has received significant attention in recent years. It is well known that Bi2O3 is a p-type semiconductor with five crystallographic polymorphs denoted as monoclinic α-Bi2O3, tetragonal β-Bi2O3, cubic (BCC) γ-Bi2O3, cubic (FCC) δ-Bi2O3, and triclinic ω-Bi2O3 [2]. Monoclinic α-Bi2O3, which is nontoxic and chemically stable in aqueous solution under irradiation, has been proved to be a visible-light-driven photocatalyst, owing to its narrow band-gap energy (band gap around 2.6–2.8 eV). However, as a photocatalyst, α-Bi2O3 suffered severe problems in practical

**Citation:** Li, H.; Luo, X.; Long, Z.; Huang, G.; Zhu, L. Plasmonic Ag Nanoparticle-Loaded n-p Bi2O2CO3/α-Bi2O3 Heterojunction Microtubes with Enhanced Visible-Light-Driven Photocatalytic Activity. *Nanomaterials* **2022**, *12*, 1608. https://doi.org/10.3390/ nano12091608

 Academic Editor: Diego Cazorla-Amorós

Received: 13 April 2022 Accepted: 7 May 2022 Published: 9 May 2022

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applications due to its low quantum yield, which is normally caused by the rapid recombination of its charge carriers [2]. Thus, novel photocatalysts based on α-Bi2O3 are required to be further explored in order to achieve increases in quantum efficiency and successes in practical applications.

Coupling a p-type α-Bi2O3 with another n-type semiconductor with matching band potentials to form a p-n heterojunction has been demonstrated to be an effective strategy to enhance the quantum yield. Driven by the internal static electric field built at the heterojunction interface, the photogenerated charges can transport from one semiconductor to another, thus improving the electron–hole pairs separation and interfacial charge transfer efficiency [7]. Bi2O2CO3 is an n-type semiconductor with a band gap of 3.55 eV. Growing attention has been paid to it, since Zhang et al. reported for the first time the application of Bi2O2CO3 as a photocatalyst in the degradation of methyl orange in aqueous solution under UV light irradiation [8]. Since α-Bi2O3 and Bi2O2CO3 are intrinsic p-type and n-type semiconductors, respectively; thus theoretically, an n-p Bi2O2CO3/α-Bi2O3 heterojunction is formed when the two dissimilar crystalline semiconductors combine. The reason for this is that the conduction band edge for α-Bi2O3 is much higher than that for Bi2O2CO3. As a well-defined interface is the key to improving the catalytic activities of heterojunction photocatalysts by facilitating charge transfer and separation, it is of grea<sup>t</sup> significance to develop a facile route to fabricate Bi2O2CO3/α-Bi2O3 heterostructures with effective contacts between Bi2O2CO3 and α-Bi2O3.

Noble metal nanoparticles (NPs), such as Au NPs [9,10], Pt NPs [11,12], Ru NPs [13,14], Ag NPs [15,16], and so on, have been used as co-catalysts to work with photocatalysts for enhanced photocatalytic performance, not only because they play the crucial roles of being photoinduced electron trappers due to their superior electron conductivities, but also because of the surface plasmon resonance (SPR) effect caused by the mutual oscillation between incident light and the electrons on the surface of noble metal NPs. Ag nanoparticles are a good choice for constructing noble metal NPs/semiconductor heterostructures, due to their facile preparation and relatively low cost. So far, several Ag NP-hybridized heterostructures have been reported, including Ag-Cu2O/PANI [17], Ag/ZnO@CF [18], Ag/AgCl/Ag2MoO4 [19], Ag/ZnO/3Dgraphene [20], Ag/GO/TiO2 [21], Bi2WO6/Ag3PO4- Ag [22], and g-C3N4/Ag/TiO2 [23], with enhanced photocatalytic activity. To the best of our knowledge, no study has been performed on synthesis and photocatalytic application of Ag NP-loaded Bi2O2CO3/α-Bi2O3 heterostructure composite systems.

In the present study, novel n-p Bi2O2CO3/α-Bi2O3 heterojunction microtubes with hexagonal cross sections were prepared via a facile one-step template- and surfactant-free solvothermal method for the first time. As Bi2O2CO3 was formed via in situ carbonatization of α-Bi2O3 microtubes on the surface, this method is more conducive to generate welldefined Bi2O2CO3/α-Bi2O3 heterojunction interfaces than two-step strategies. Co-catalyst Ag nanoparticles were evenly loaded on the surface of Bi2O2CO3/α-Bi2O3 heterojunction microtubes, using a photo-deposition process to construct a novel Ag/Bi2O2CO3/α-Bi2O3 microtube ternary system to further enhance the photocatalytic activity. The photocatalytic performances of the as-prepared samples were evaluated by examining the degradation of methyl orange (MO) under visible light (λ > 420 nm) irradiation.

## **2. Materials and Methods**

#### *2.1. Synthesis of Bi2O2CO3/α-Bi2O3 Heterostructure Microtubes*

Bismuth nitrate pentahydrate and ethylenediamine were purchased from Xilong Scientific Co., Ltd (Shantou, China) and Taicang Hushi Reagent Co., Ltd (Taicang, China), respectively. All reagents were of AR grade, and used without further purification. Distilled water was used in all experiments. As illustrated in Figure 1, in a typical synthesis, 0.00175 mol of Bi(NO3)3·5H2O was added into the ethylenediamine (en)–water mixture (80 mL), with a certain volume ratio of ethylenediamine and water (Ven:Vwater). After being stirred for 30 min, the resulting faint yellow suspension (donated as precursor) was transferred into a 100-milliliter Teflon-lined stainless steel autoclave. The autoclave was

sealed and maintained at 140 ◦C for 10 h and then cooled down to room temperature. The resulting precipitate was centrifuged, rinsed repeatedly with distilled water and ethanol, then dried at 80 ◦C in air to obtain the Bi2O2CO3/α-Bi2O3 heterostructure microtubes.

**Figure 1.** Schematic illustration for the synthesis of Ag NP-loaded Bi2O2CO3/α-Bi2O3 heterostructure microtubes.

#### *2.2. Synthesis of Ag NP-Loaded Bi2O2CO3/α-Bi2O3 Heterostructure Microtubes*

The fabrication of Ag NP-loaded Bi2O2CO3/α-Bi2O3 heterostructure microtubes was conducted as follows. First, 0.5 g of Bi2O2CO3/α-Bi2O3 heterostructure microtubes was dispersed into the AgNO3 ((AR grade, Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) aqueous solution under stirring. The theoretical loading amount of silver was set at 3 wt% in the Ag/Bi2O2CO3/α-Bi2O3 sample. After being ultrasonically treated for 10 min, the suspension was further magnetically stirred for 10 h in the dark, followed by UV illumination for 2 h under stirring. The black powder was centrifuged, rinsed with distilled water repeatedly to purify the product, and finally dried at 80 ◦C in air.
