English

• Demand for catalyst progressive increase year by year, in order to mitigate global energy shortage crisis and environmental deterioration problems. As a significant category, photocatalyst exhibit fascinating superiority and potency in the field of energy and environment, which can be used to accomplish water splitting to produce hydrogen and oxygen, carbon dioxide reducing, and organic pollutants degrading [1-5]. In allusion to photodegradation of organic dyes, abundant semiconductor based photocatalysis have been exploited and investigated [6-8]. However, the narrow light absorption band will impede the holistic photocatalytic efficiency, which stimulates researchers to exploit novel photocatalyst and innovate picturesque configuration. Among the numerous classes of emerging photocatalysts, rare earth compounds with 4f configurations have revealed remarkable luminescent properties and improved photocatalytic activity due to the strong interaction with organic compounds [9-12]. Remarkably, cerium vanadate (CeVO₄) has been developed as a newfangled photocatalyst based on cerium with plentiful energy level and the VO₄³⁻ activated centers with valid light absorption in the crystalline structure [13-17].

  In previous studies, the photocatalysts with nanoscale emerge superior catalytic efficiency by virtue of exposure of abundant active sites and sufficient contact area with the organic pollutants [18-20]. Nevertheless, collection of the nanoparticles for recycling is intrinsic imperfection, which severely restrict practical applications of the photocatalysts for water decontamination. Therefore, the exploitation of individual photocatalysts with nanocrystal assembled and hollow configuration is extremely significative [21, 22]. Despite exposure of abundant active sites, the mono-component semiconductor materials with hollow configuration have intrinsic restrict in practical application on account of the rapid recombination of photogenerated electrons and holes. In an effort to overcome these issues, the semiconductor is incorporated with noble metals, which is a meaningful and efficient strategy [23-25]. The noble metals as electron sinks will effectively improve the integral photocatalytic performance through transferring the photogenerated electrons and efficiently reducing the recombination of the photogenerated electron-hole pairs [26-28].

  It is interesting to note that noble metals with one dimensional nanostructure reveal effortless electrons transference, which will play an important role in the heterostructure photocatalysts for improving photocatalytic activity [29-31]. Under optical radiation, the noble metals provide rapid electronic channels and reservoirs for accelerating the transfer of photo-generated electrons. As an excellent electron mediator, Ag with nanostructure is successfully used for impeding rapid recombination of electron and the hole and elevating photocatalytic disinfection activity of individual photocatalyst [32-34]. Therefore, exploiting original photocatalyst and designing particular morphology of heterostructures have always been of great interest and critical importance on behalf of investigation and optimization their catalytic activity [35-38].

  In this work, we demonstrate a facile and scalable strategy to prepare CeVO₄ photocatalyst with hollow submicrospheres. In addition, the combination of Ag nanowires (AgNWs) and CeVO₄ is rationally designed to construct core shell architecture, which is expected to show advantages over the single component of catalyst. Benefiting from the electron transfer and boosted optical absorption in the presence of the AgNWs, the resultant AgNW@ CeVO₄ reveals prominent photocatalytic activity towards contaminants of rhodamine B (RhB), methylene blue (MB) and 4-nitrophenol (4-NP). Under full spectrum light irradiation. Significantly, the good structural stability and recyclability of AgNW@CeVO₄ are observed, which ensure long-term usability of photocatalyst and great promise in water purification.

  The fabrication strategies of the CeVO₄ hollow spheres and AgNW@CeVO₄ nanowires are schematically depicted in Fig. 1. This strategy as for the synthesis of CeVO₄ hollow spheres involves in the construction of highly uniform spherical CeOHCO₃ precursor and transformation of CeVO₄ hollow spheres through ions exchange process based on Kirkendall diffusion mechanism. Firstly, introduction of PVP as surfactant provides new interfaces action, which plays a crucial role in controlling of size and shape and favors the formation of highly uniform spherical CeOHCO₃ precursor during the hydrothermal process. For such strategies without PVP the crystalline octahedron emerged in the initial reaction stage (Fig. S1 in Supporting information), as a result, the hollow structure and coating layer are strictly limited in generalization. Subsequently, the hollow construction of CeVO₄ can be accomplished due to the difference of diffusion effect for cationic and anionic species during chemical transformation. Based on the above conceptual innovation, the uniform CeOHCO₃ precursors are deposited on the surface of a robust AgNWs substrate to form AgNW@CeOHCO₃, and then AgNW@CeVO₄ were successfully synthesized by way of ion exchange process.

  Figure 1

  

  Figure 1.  Conceptual scheme of synthesis route for (Ⅰ) CeVO₄ hollow spheres and (Ⅱ) AgNW@CeVO₄.

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  The X-ray diffraction (XRD) patterns of CeOHCO₃ precursors and CeVO₄ hollow spheres obtained in the presence of PVP are revealed in Fig. S2a (Supporting information). It can be seen that no obvious diffraction peaks are observed in the XRD pattern of CeOHCO₃, signifying the sample exhibits an amorphous character. In the XRD pattern of CeVO₄ hollow spheres, the typical diffraction peaks can be readily indexed with the tetragonal structured CeVO₄ (JCPDS card No. 84-1457), which manifests that the single phase CeVO₄ is synthesized. Subsequently, the morphology and microstructure of the products in different stages were indagated by Field-emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM) analyses. Fig. S2b (Supporting information) exhibits that the CeOHCO₃ precursors possess a monodisperse characteristic and highly homogeneous spherical morphology with a diameter of approximate 200 nm and a relatively smooth surface. After undergoing ion exchange process, the spherical morphology of samples (with an average diameter of 200 nm) is maintained. Nevertheless, high-magnification FE-SEM image (the inset in Fig. S2c in Supporting information) reveals that the as-prepared CeVO₄ presents a hollow feature and a rough surface texture, which is composed of small nanoparticles with an average diameter of 25 nm. To further confirm the hollow structure of CeVO₄, TEM analyses are performed, as shown in Fig. S2d (Supporting information). It can be seen clearly that the samples present hollow and porous architecture, which will be conducive to improve the degradation property of samples for organic contaminants by virtue of sufficient contact between catalysts and contaminants. Additionally, from the high-resolution TEM image of CeVO₄ hollow spheres (Fig. S2e in Supporting information), clear lattice fringes are observed and the average lattice space of 0.369 nm is obtained, corresponding to the (202) planes of tetragonal structured CeVO₄. The distinct dot array (Fig. S2f in Supporting information) in the fast Fourier transfer (FFT) diffraction pattern collected from the high−resolution TEM image presents the single crystallinity for CeVO₄. These results confirm

  In the cases of AgNW@CeVO₄, the XRD pattern (Fig. 2a) exhibits that the main diffraction peaks can be assigned to the tetragonal structured CeVO₄ (JCPDS card No. 84-1457) and the extra peaks located at 38.2°, 44.4° and 64.6° match well with correspond to (111), (200) and (220) of Ag (JCPDS card No. 87-0720), respectively, confirming the coexistence of Ag and CeVO₄. FE-SEM and TEM analyses are applied to verify the morphology and microstructures of AgNWs and AgNW@CeVO₄ samples. Fig. 2b reveals the microstructure of the AgNWs, which presents homogeneous one-dimensional nanowires structure with an average diameter of around 90 nm and smooth surface [39]. By contrast, the AgNW@CeVO₄ samples maintain the one-dimensional structure and exhibit a binary hierarchical architecture, including the AgNWs core layer and in situ grown CeVO₄ shell layer with a rough surface, as shown in Fig. 2c. Additionally, the low-magnification TEM image (Fig. 2d) of single AgNW@CeVO₄ confirms the ore-shell structure with an obvious shell thickness of approximately 40 nm and the corresponding high-magnification TEM image displays legible crystalline phase with a d-spacing of 0.369 nm, which is assigned to the (202) interplane spacing of CeVO₄. The EDS elemental mapping analysis is employed to examine the element distribution information of samples, as revealed in Figs. 2e-h. It is clear that Ag element is uniformly distributed in the core region, while homogenous Ce and V elements are distributed in the shell region, further confirming the formation of AgNW@CeVO₄ with core-shell structure.

  Figure 2

  

  Figure 2.  (a) XRD patterns of AgNW@CeOHCO₃ and AgNW@CeVO₄, SEM images of (b) AgNWs and (c) AgNW@CeVO₄ samples, (d) TEM images of the AgNW@CeVO₄, (e-h) STEM image and the corresponding EDS elemental mapping of the AgNW@CeVO₄ pure phase and high crystallinity of the resulting CeVO₄ hollow spheres.

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  The bandgaps of semiconductor photocatalysts possess crucial effect for their catalytic performance and result in the absorption range of light during the photocatalytic process. The UV-vis-NIR absorption spectra are employed to survey investigate the optical absorption range and determine the bandgaps of the CeVO₄ and AgNW@CeVO₄ samples. The absorption curves of as-prepared samples are presented in Fig. S3a (Supporting information). It is noted that the optical absorption range is broadened after introducing AgNWs to CeVO₄ photocatalyst, which will lead to shrink the bandgap of AgNW@CeVO₄ sample and aggrandize the utilization efficiency of visible light. The bandgaps of the CeVO₄ and AgNW@CeVO₄ samples are calculated based on the following equation (Kubelka−Munk formula) [40, 41]:

  (1)

  where α denotes absorption coefficient, ν corresponds to light frequency, h is Planck constant, and Eg represents the bandgap of sample. Fig. S3b, c (Supporting information) show the curves of (αhν)² versus hν, deriving from the corresponding absorption spectra. The bandgaps of the CeVO₄ and AgNW@CeVO₄ are determined to be 2.76 and 2.12 eV, respectively. The diminution of bandgaps for AgNW@CeVO₄ indicates that the incorporation of CeVO₄ with AgNWs is beneficial to aggrandize the utilization efficiency of visible light and improve the photocatalytic activity.

  In order to evaluate the separation efficiency of photogenerated charge carriers in the as-prepared photocatalysts, the photocurrent variations are monitored according to I-t curve tests with several on-off cycles of full spectrum light irradiation. Fig. S4a (Supporting information) reveals the photocurrent density of CeVO₄ and AgNW@CeVO₄ photocatalysts under intermittent light illumination. Obviously, both of the photocatalysts exhibit fast and reproducible light response, and AgNW@CeVO₄ photocatalyst possesses clearly stronger photocurrent intensity than pure CeVO₄, which indirectly reflects that the introduction of AgNWs promotes the generation of charge carriers and separation efficiency of the photogenerated electron-hole pairs [42, 43]. Such a highly sensitive photocurrent response and separation efficiency of carriers will make AgNW@CeVO₄ photocatalyst to achieve more excellent photocatalytic activity. As can be seen from the PL spectra (Fig. S4b in Supporting information), high PL intensity of CeVO₄ suggests fast recombination of photogenerated electron-hole pairs [44]. Interestingly, the PL spectrum of AgNW@CeVO₄ reveals weaker emission intensity. The results further confirm that the introduction of AgNWs is conducive to separation and transfer of electron-hole pairs.

  The catalytic activity of the as-prepared CeVO₄ and AgNW@CeVO₄ photocatalysts was evaluated via photodegradation of RhB under simulated sunlight irradiation. Fig. 3a reveals the degradation efficiency of photocatalysts for RhB at different irradiation time. The direct photolysis behavior of RhB as a blank experiment is compared. Before photodegradation, the photocatalysts are dispersed in RhB solution for accomplishing an adsorption-desorption equilibrium under a dark condition for 60 min. It can be seen that the CeVO₄ and AgNW@CeVO₄ photocatalysts exhibit a slight adsorption effect, which suggests the photolysis of RhB is chiefly attributed to catalytic function of photocatalysts. For the CeVO₄, the degradation of RhB gradually increases with the prolongation of irradiation time, and 64% of RhB is degraded within 120 min under simulated sunlight illumination. Accompany with the introduction of AgNWs, the obtained AgNW@CeVO₄ sample reveals a considerably higher photocatalytic activity. As can be noticed, the photodegradation rate of RhB reaches up to 94% within 120 min of full spectrum light irradiation. Furthermore, based on the Langmuir Hinshelwood (L-H) kinetic model, the photocatalysis degradation results conform to pseudo-first-order photocatalysis kinetics, and the corresponding reaction rate constants (k) of the different photocatalysts are calculated from the following equation [45, 46]:

  (2)

  Figure 3

  

  Figure 3.  (a) Photocatalytic degradation of RhB under full spectrum light irradiation as a function of the irradiation time without catalyst, and over the as-prepared CeVO₄ and AgNW@CeVO₄ samples. (b) Corresponding plots of ln(C₀/C) against irradiation time for the photocatalytic degradation of RhB under full spectrum light over different catalysts. (c) Photodegradation of MB and 4-NP under full spectrum light irradiation over the as-prepared AgNW@CeVO₄ catalysts. (d) Schematic diagram of possible photocatalytic mechanism for AgNW@CeVO₄ heterogeneous system.

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  where C₀ corresponds to the initial concentration of RhB solution, C is the concentration of RhB solution at t min, and the t represents the irradiation time. Significantly, as presented in Fig. 3b, AgNW@CeVO₄ photocatalyst possesses higher k value (0.02216 min⁻¹) than CeVO₄ hollow sphere, which demonstrates AgNWs contribute to optimizing photocatalytic activity of CeVO₄ for RhB. The phenomenon can be explained that the AgNWs with excellent electronic transmission capability are beneficial to transfer the photogenerated electrons, inducing reducing the combination of electron-hole pairs.

  The stability and recyclability of AgNW@CeVO₄ photocatalyst are also crucial parameters for cyclic utilization in practical applications. In order to evaluate the recycling performance of AgNW@CeVO₄, the cycling tests of photodegrading RhB are performed under full spectrum light irradiation, as shown in Fig. S6 (Supporting information). The photodegradation efficiency of photocatalysts for RhB nearly constant during five sequential cycles, manifesting AgNW@CeVO₄ presents great recycling stability during the photocatalytic reactions. In addition, we also evaluated the photodegradation property under solar irradiation by using MB and 4-NP as contaminants. Obviously, AgNW@CeVO₄ exhibited the high catalytic activity for degradation of MB and 4-NP due to the fact that the AgNW@CeVO₄ can provide active sites for adsorption and catalytic reduction of contaminants. Fig. S7 (Supporting information) presents the XRD pattern and SEM images of AgNW@CeVO₄ photocatalysts after photodegradation cycling tests, which reveal that the photocatalyst maintains the original crystal structure and morphology features. The results further determine a good stability of AgNW@CeVO₄ photocatalyst.

  To investigate surface catalysis of AgNW@CeVO₄ photocatalyst, the adsorption of the O₂ molecules on the surfaces of CeVO₄ (110) are studied on the basis of density functional theory (DFT). Fig. S8 (Supporting information) shows optimized structure of pristine CeVO₄ and Ag-CeVO₄. Based on the optimized structures of CeVO₄ and Ag-CeVO₄, the effect of introducing Ag on O₂ adsorption and separation of electron and hole is further explored. Adsorption modes and adsorption energy of O₂ on the CeVO₄ and Ag-CeVO₄ surfaces are presented in Fig. 4. The adsorption energy for of O₂ on the CeVO₄ and Ag-CeVO₄ are −0.53 and −0.66 eV, respectively. It is obvious that an adsorption energy gain of −0.13 eV can be obtained when the presence of Ag. The increase of adsorption energy insinuates that the presence of Ag is beneficial to the reduction of oxygen molecules on the surface of AgNW@CeVO₄. It can be speculated that the photocatalytic activity of CeVO₄ is promoted by introducing Ag.

  Figure 4

  

  Figure 4.  Optimized O₂ adsorption structure on (a) CeVO₄ and (b) Ag-CeVO₄.

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  The Muliken charge analysis was achieved based on the charge changes of Ag atom before and after adsorption on CeVO₄. The results show that the positive charge of Ag atom (0.485 e) is mainly contributed by the surrounding O atoms and the O atoms around the V atoms located directly under the Ag atom. Through the charge differential density diagram after O₂ adsorption as shown in Fig. S9 (Supporting information), we can see that the charge depletion region and the charge accumulation region mainly distribute in the O₂ molecular and V atom region, respectively. However, Muliken charge analysis shows that the positive charge of the V atom did not change significantly, and the negative charge of the surrounding O atoms is reduced. Therefore, we believe that electrons are mainly transferred from O atoms around V to O₂ molecules through V atoms after the adsorption of O₂ molecules [47, 48].

  Subsequently, a possible photocatalytic mechanism of AgNW@CeVO₄ for the degradation of pollutants is expounded, as illustrated in Fig. 3d. Initially, the RhB and MB dye molecules are excited and corresponding electrons migrate to photocatalysts, resulting in forming the reactive excited-state RhB⁺ and MB⁺ under simulated sunlight radiation [49]. For 4-NP adsorbed on the surface of AgNW@CeVO₄, the nitro group can be transformed into nitrophenolate ions and further reduced to amino group due to the electrons displacement and active hydrogen species derived from borohydride [50, 51]. Simultaneously, the electrons are excited from valence band (VB) the conduction band (CB) of Ag@CeVO₄ photocatalysts with the increase of absorbing photon energies, and then transfer to the Ag and V atoms on the interface between CeVO₄ and AgNWs for contributing to the degradation [52, 53]. The photoexcited electrons can reduce the dissolved oxygen molecules to produce the superoxide radicals O₂^•−, and the holes react with H₂O molecules to form hydroxyl radicals (^•OH) [54-56]. The associated superoxide radicals (O₂^•−), hydroxyl radicals (^•OH), and holes (h⁺) can efficiently degrade contaminants into nontoxic compounds.

  In summary, we develop a convenient strategy to fabricate CeVO₄ based photocatalyst with hollow submicrospheres, and combine AgNWs and CeVO₄ to rationally construct AgNW@CeVO₄ photocatalyst with core shell architecture. The introduction of Ag NWs not only endow the photocatalyst with one dimensional continuous structure, but also expand the optical absorption range of CeVO₄ photocatalyst. Moreover, AgNWs afford rapid charge transport channels and reservoir for the electrons in the AgNW@CeVO₄ heterostructure, which promote the separation efficiency of photogenerated electrons and holes. The theory analysis further validates that introducing Ag can strengthen O₂ adsorption on CeVO₄ surface, which facilitates the separation of electron and hole and the advance of photocatalytic activity of CeVO₄. Benefiting from the electron transfer and boosted optical absorption in the presence of the AgNWs, the resultant AgNW@CeVO₄ reveals prominent photocatalytic activity towards the decomposition of organic pollutants under simulated sunlight irradiation. Thereinto, AgNWs can further improve the photocatalytic efficiency of the AgNW@CeVO₄ based on the plasmonic effect under solar light irradiation. Significantly, the good structural stability and recyclability of AgNW@CeVO₄ are observed due to the strong synergistic effect, which ensure long-term usability of photocatalyst and great promise in water purification. From these findings, this is a valuable reference in designing Ce based compounds with improved photochemical abilities. From these findings, this study is a valuable reference into developing efficient Ce-based recyclable photocatalysts with excellent photochemical abilities for application in environmental purification.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This study is financially supported by the National Natural Science Foundation of China (Nos. 21701166, 51472236, 21603109), the National Basic Research Program of China (973 Program, No. 2014CB643803), the Fund for Creative Research Groups (No. 21521092), and Key Program of the Frontier Science of the Chinese Academy of Sciences (No. YZDY-SSW-JSC018), the Henan Joint Fund of the National Natural Science Foundation of China (No. U1404216).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.cclet.2021.07.060.

  
  
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  Figure 1  Conceptual scheme of synthesis route for (Ⅰ) CeVO₄ hollow spheres and (Ⅱ) AgNW@CeVO₄.

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  Figure 2  (a) XRD patterns of AgNW@CeOHCO₃ and AgNW@CeVO₄, SEM images of (b) AgNWs and (c) AgNW@CeVO₄ samples, (d) TEM images of the AgNW@CeVO₄, (e-h) STEM image and the corresponding EDS elemental mapping of the AgNW@CeVO₄ pure phase and high crystallinity of the resulting CeVO₄ hollow spheres.

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  Figure 3  (a) Photocatalytic degradation of RhB under full spectrum light irradiation as a function of the irradiation time without catalyst, and over the as-prepared CeVO₄ and AgNW@CeVO₄ samples. (b) Corresponding plots of ln(C₀/C) against irradiation time for the photocatalytic degradation of RhB under full spectrum light over different catalysts. (c) Photodegradation of MB and 4-NP under full spectrum light irradiation over the as-prepared AgNW@CeVO₄ catalysts. (d) Schematic diagram of possible photocatalytic mechanism for AgNW@CeVO₄ heterogeneous system.

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  Figure 4  Optimized O₂ adsorption structure on (a) CeVO₄ and (b) Ag-CeVO₄.

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