Copper Selenide (CuSe) Monolith Fabricated by Facile Copper Foam Selenization for Efficient Photocatalytic Degradation of Methylene Blue

. A critical challenge that impedes the application of photocatalytic techniques for organic dye degradation from polluted industrial efuents is that traditional powdery photocatalysts exposed limited photo-absorption sites and exhibited inefcient recyclability. To overcome these challenges, this study designed a one-step process to synthesize a monolithic copper selenide (CuSe)-based photocatalyst. Te characterization results fully supported that the maintenance of the copper foam during the selenization process was the prerequisite for the monolithic photocatalyst to keep its structural integrity in photocatalytic reactions. Te surface of the monolithic photocatalyst fully covered by active CuSe is crucial for the exposure of photocatalytically active sites and the efcient degradation of methylene blue (MB). It was found that the CuSe-based monolithic photocatalyst exhibited excellent MB degradation performances under harsh pH conditions and high MB concentrations. From these perspectives, it is reasonable to conclude that the CuSe-based monolithic photocatalyst as prepared is a promising alternative to traditional powdery photocatalysts for organic dye degradation and industrial efuent cleaning.


Introduction
Severe water pollution caused by the release of synthetic dyes from industrial activities is one of the major environmental concerns in the 21 st century [1].According to documented estimations, the annual generation of synthetic dyes worldwide reaches approximately 7 × 10 5 tons with over 1,000 types [2].Among the synthetic dyes generated, 15% of them are lost during the dyeing process and released into the ecosystem with textile efuents [3].Te aqueous reservoir colored and polluted by these organic dyes thus sufers harmful and uncontrollable eutrophication that signifcantly reduces its oxygenation capacity [4].Besides, the bioaccumulation of these organic pollutants also damages aquatic life and eventually becomes a serious health threat to human beings.Upon ingestion, the adverse physical efects of synthetic dyes include but are not limited to nausea, diarrhea, vomiting, breathing difculties, and gastritis infections [5].From these perspectives, it is imperative to develop efective and efcient methods to degrade and remove these synthetic dyes from textile efuents and aquatic environment.Methylene blue (MB), as an extremely carcinogenic thiazine pollutant that has been widely used in various industries for diferent purposes (e.g., dyeing of cotton, wool, and fabrics; coloring of paper, as a hair coloring agent and redox reaction indicator in outer space, etc.), has received special attention during the technical development process [6,7].
Although diferent techniques including adsorption [8], membrane fltration [9], coagulation/focculation [10], ionexchange removal [10], catalytic ozonation [11], and photocatalytic degradation [12] have been proposed as efective pathways for MB degradation, considering its chemical stability and the inherent complexity of MB-polluted sources, there is still not an omnipotent technique to achieve the removal of MB from all industrial processes.Among the abovementioned techniques, photocatalytic degradation is known as one that possesses unique advantages such as the direct utilization of solar energy, the facile cyclic functionalization of photocatalysts, and the total mineralization of dyes.In addition, by purposefully manipulating the band gap of semiconductors, it is theoretically possible to achieve the rational degradation of target pollutants from aquatic environment, while keeping the residual properties of the aquatic environment generally unchanged.Such rationality can hardly be realized when other degradation techniques are adopted for organic dye removal.With these advantages, developing MB removal techniques based on photocatalytic degradation has been a vibrant research area in the past twenty years with the related international environmental standards becoming more stringent [13].
To further extend the applicability of photocatalytic techniques for organic dye degradation from polluted sources, the rational design and successful synthesis of effcient photocatalysts are highly desirable.Critical drawbacks sufered by traditional photocatalysts in powdery forms include (i) low absorption rate of light due to the shielding efects among powders, (ii) the inefective exposure of active sites because of the agglomeration of nanoparticles, and (iii) the possible introduction of new pollutants into the aquatic system and the tedious process for photocatalyst recycling [14].From these perspectives, developing a monolithic photocatalyst may be a feasible solution for the abovementioned challenges considering that (1) anchoring photocatalysts on monolithic substrates well-disperses active components, thus increasing the light absorption efciency and optimizing the contact between pollutants and photocatalysts [15] and (2) the tedious recovery process of photocatalysts is skipped as they can be easily recycled together with the monolithic substrates [16].Tese undeniable merits indicate that monolithic catalysts may be more practical and efective options for photocatalytic removal of organic dyes from the aquatic system.However, although monolithic photocatalysts have long been recognized as potential alternatives to powdery ones for organic pollutant degradation from industrial sources, previous studies mainly developed monolithic photocatalysts based on traditional photocatalysts such as titanium dioxide (TiO 2 ) and zinc oxide (ZnO) with wide band gaps.Because they can only absorb light within the ultraviolet range that only accounts for 4-5% of solar irradiation, these photocatalysts exhibit relatively limited photocatalytic degradation performances under practical scenarios [17][18][19][20][21]. Besides, to fabricate monolithic TiO 2 and ZnO, a posttreatment strategy is generally adopted to load them on another monolithic supporter such as honeycomb or silicon dioxide via physical interactions [22,23].Te unstable loading pattern makes it possible for the photocatalysts to be released into the aquatic system and cause secondary pollution.Finally, to enhance the photocatalytic performances of catalysts with wide band gaps, co-catalysts or heterocomponents are required to narrow their band gaps and enhance the absorption of light irradiation [24,25], which inevitably increases the operation costs of the photocatalytic techniques.To overcome these critical challenges in the fabrication of monolithic photocatalysts, it is of great importance to design new strategies to synthesize efective monoliths for photocatalytic organic dye degradation from industrial sources.
Accordingly, in this work, a one-step procedure was developed to load copper selenide (CuSe) on the surface of copper foam to achieve the efcient photocatalytic degradation of MB.CuSe with an ideal band gap of 2.1-2.5 eV exhibits excellent absorption abilities for light in both ultraviolet and visible ranges [26,27].Te in-situ growth fabrication process of CuSe ensures that the photocatalyst stably anchored on the copper foam substrate because of the formation of chemical interactions.By properly controlling the selenization time, the copper framework could be wellmaintained during the selenization process, thus ensuring that the monolithic photocatalyst keeps its structural integrity in photocatalytic degradation.With these advantages, the CuSe-based monolithic photocatalyst prepared exhibited excellent MB degradation performances when its concentration was extremely high or the pH condition was harsh.From these perspectives, it is reasonable to conclude that the CuSe-based monolithic photocatalyst is a promising alternative to traditional powdery photocatalysts, which might further extend the applicability of the photocatalytic techniques for organic dye degradation and industrial efuent cleaning.

Photocatalyst Synthesis and Regeneration.
Te CuSebased monolithic photocatalysts used in this work were prepared through a one-step selenization process (as shown in Figure 1).Typically, the commercial copper foam (>99.99%) was frst cut into cubic pieces with the side length of 10 mm × 10 mm × 10 mm.Ten, hydrochloric acid (HCl, Aladdin) solution with the concentration of 3 mol•L −1 was adopted to wash out any copper oxides on the surface of copper foam that had been stored in air.In a separate beaker, 0.002 mol of selenium powder (Se, Aladdin) was dissolved in 0.01 mol•L −1 sodium borohydride (NaBH 4 , Aladdin) solution at 90 °C until it totally turned transparent.Ten, commercial copper foams with diferent densities (5 ppi, 10 ppi, and 20 ppi) were added into the Se-NaBH 4 solution with continuous stirring for 20 min.After the surface of the foams transformed into black, they were separated out from the solution, washed with deionized water/ethanol for several times, and dried at 100 °C for 3 h to obtain the fnal monolithic photocatalysts.To regenerate the deactivated photocatalyst, the CuSe-based monolith was impregnated in hydrogen chloride (HCl, Aladdin) at 40 °C for 2 min (the weight ratio between CuSe-based monolith and HCl equaled to 1 : 10).After the CuSe on the surface of copper foam was removed, the copper foam underwent a reselenization process following the abovementioned procedure.Besides, to prepared powder CuSe photocatalysts, the same synthesis procedure was adopted without adding copper foam.Commercial P25 TiO 2 and ZnO powders were 2 International Journal of Chemical Engineering purchased as reference photocatalysts to claim the efciency of the CuSe-based monolith.

Photocatalyst Characterization. Te crystallinity of the
CuSe-based photocatalyst was measured by X-ray difraction (XRD, D8 Bruker AXS, Germany) with two thetas from 10 °to 80 °in Cu α (λ � 0.154 nm) radiation.Te morphologies of the CuSe-based photocatalyst were recorded by a scanning electronic microscope (SEM, FEI F50, USA), during which the energy-dispersion X-ray (EDX) scanning and mapping were conducted to determine the composition content and distribution.A transmission electron microscope (TEM, JEOL 2100F, Japan) was used to determine the morphologies of the as-prepared CuSe-based photocatalyst.Te CuSe as formed on the surface of monolithic copper foam was separated out before conducting the TEM and highresolution TEM (HRTEM) tests.Te primary goal was to identify the morphology and crystal phase of the CuSe as formed.Te total organic carbon (TOC) change during the degradation process was tested based on a TOC analyzer (Germany, Analytik Jena AG).A 10-ppi copper foam, 5 pieces of 1 × 1 × 1 cm of monolithic CuSe, 10 mg•L −1 of methylene blue, and a pH value of 7 were adopted to conduct the TOC analysis experiment.

Photocatalytic Performance
Test.MB as a typical dye pollutant was adopted to test the photocatalytic performance of the CuSe-based monolithic photocatalysts.When performing the degradation tests, the monolithic photocatalysts were suspended in MB solution under continuous stirring.A 300 W Xe lamp was used to supply irradiation in both ultraviolet and visible range with the peak wavelength of 320 nm.Te distance between the photoreactor and Xe lamp was fxed as 10 cm.During the testing process, the MB solution was continuously sampled with an interval of 5 min to draw the concentration change curves.Te MB concentrations in each test were determined by an ultravioletvisible (UV-vis) spectrophotometer at the wavelength of 525 nm.Before conducting the UV-vis test, the light absorption characteristics of the standard MB solutions with concentrations of 5, 10, 15, 20, and 25 mg•L −1 were frst determined to draw the standard curve.As shown in Figure 2, when the concentrations of MB equaled to 5, 10, 15, 20, and 25 mgL −1 , the absorbance intensity of 525-nm light of the solutions were 0.58, 1.22, 1.81, 2.35, and 2.96, respectively.Te correlation coefcient (R 2 ) of this standard curve reaching 0.9995 suggested its reliability to determine the specifc MB concentrations under practical scenarios.
Based on the abovementioned testing procedures, to fully test the photocatalytic performances of CuSe-based monolithic photocatalysts, eight sets of experiments were conducted.Te detailed experimental conditions are shown in Table 1 for reference.In particular, Set I experiment was designed to rule out the adsorption of MB on CuSe-based monolith under dark conditions.Te MB degradation performances of diferent photocatalysts were compared in Set II experiments.Set III experiments tested the infuence of the density of copper foams on the MB degradation performances of CuSe-based monolithic photocatalysts.Set IV experiment was dedicated to investigating the efects of photocatalyst dosage on the photocatalytic performance.Te impacts of MB concentrations on the photocatalytic kinetics were determined in Set V experiment, and the infuences of the pH value of the solutions were explored in Set VI experiment.Finally, Set VII and Set VIII experiments were conducted to test the recyclability and reusability of the CuSe-based monolithic photocatalysts.Besides, it should be noted that, after selenization, there was a new peak that did not belong to elemental copper appear at around 20 °.Trough scrutinizing possible phases, it was found that this peak was ascribed to the (004) planes of klockmannite (JPCD #34-0171, the hexagonal phase of CuSe).Te exposure of (004) planes is in line with the growth pattern of hexagonal CuSe (the structure is shown in the inserted fgure of Figure 3), i.e., according to the classic BFDH (a model developed by Bravais, Freidel, Donnay, and Harker) model, the growth rate of the surfaces of a growing polygon was generally in inverse proportion to their lattice spacing.In CuSe hexagonal case, the lattice space between the (001) planes equaling to 1.74 nm is much larger than that between the (100) planes equaling to 0.34 nm, thus the planes along the (001) direction will grow much more slowly than the six energetically equivalent planes along the (100) directions.Hence, the CuSe tended to self-assemble into hexagonal phases with the exposure of the (004) planes [28,29].

International Journal of Chemical Engineering
To further identify the morphology of the CuSe-based monolithic photocatalyst as prepared, its SEM images are shown in Figure 4.As shown in Figure 4(a), the CuSe-based monolithic photocatalyst well-kept its structural integrity during the photocatalytic reaction process, which further supported the critical role of the copper skeleton to maintain the monolithic architecture.Figures 4(b) and 4(c) demonstrate that, after selenization, the surface of the copper foam was covered by nanoparticles with diameters less than 1 µm, which was probably attributed to the formation of CuSe on copper foam surface.Increasing the resolution of the SEM image proved that the nanoparticles as formed on the surface of copper foam shaped in nanorod-like morphology.Compared to the previous studies, it was found that the coverage by these rod-like nanoparticles was not an inherent feature of copper foam [30][31][32].Contrarily, pure copper foam was characterized by a relatively smooth surface with no rod-like nanoparticle accumulation.Tus, it is reasonable to conclude that the rod-like nanoparticles appeared on the surface of selenized copper foam formed during the selenization process.
To further explore the phase of the rod-like nanoparticles as formed on the surface of copper foam, the energy dispersive spectrometry (EDS) mapping patterns of the CuSebased monolithic photocatalyst is shown in Figure 5.As shown in Figures 5(a) and 5(b), copper and selenium were homogenously dispersed on the surface of copper foam.Te EDS mapping results that well-agree with the XRD pattern of the CuSe-based monolithic photocatalyst further suggested that the rod-like nanoparticles were CuSe.Besides, the compositional ratios of copper and selenium in the CuSe-based monolithic photocatalyst equaled to approximately 15% and 85% (as shown in Figure 5(c)), respectively.Tese compositional ratios indicate that most of the copper component in copper foam was kept unchanged during the selenization process, which is also in accordance with the XRD result.Terefore, an important conclusion can be drawn from such consistency, i.e., the selenization process this work adopted is reasonable enough to fully cover the surface of selenized copper foam with abundant heterocomponents, while its skeleton structure ensuring that the monolithic photocatalyst should not collapse during the photocatalytic reaction process, which was well-maintained due to the residual copper phase.
Finally, to confrm the phase of the rod-like heterocomponents as formed on the surface of copper foam, TEM tests were conducted in this work.As shown in Figure 6(a), after separating the nanoparticles from the surface of copper International Journal of Chemical Engineering   International Journal of Chemical Engineering foam and well-dispersing them in ethanol, it was found that the rod-like nanoparticles were comprised of structured nanosheets.Tis observation agrees with the initial assumption that CuSe might grow along six energetically equivalent planes along the (100) directions, thus selfassembling into hexagonal phases that exclusively explored the (001) planes.Such a growth pattern would structure the CuSe into a nanosheet or nanoplanet that favors the exclusive exposure of a specifc surface.Furthermore, based on the HRTEM image of the CuSe-based monolithic photocatalyst as prepared (as shown in Figure 6(b)), a lattice fringe of 0.34 nm was obtained for the nanosheet-structured material, which is attributed to the characteristic interlayer space between the (100) planes of CuSe [33].Tus, based on the XRD, EDS, and HRTEM results, it is plausible enough to declare that the heterocomponents as formed on the surface of copper foam during the selenization process was CuSe in its hexagonal phase.Its growth preference made it shape as a nanosheet, which selfassembled into rod-like nanoparticles and fully covered the copper foam surface eventually.Te monolithic structure and the adequate exposure of active sites might make the CuSe-based photocatalyst as synthesized an efcient and efective solution to organic dye pollution.

Photocatalytic Performance.
After elucidating the chemical composition of the CuSe-based monolithic photocatalyst, MB was selected as a typical organic dye pollutant to test its photocatalytic performance.As shown in Figure 7, the adsorption of MB on CuSe-based monolith under dark condition was frst ruled out.It was found that in the absence of light irradiation, the CuSe-based photocatalyst negligibly degraded MB, while, after the lamp was switched on, the MB concentration decreased rapidly.Te results indicate that MB did not adsorb on the surface of CuSe-based monolith, and its degradation was primarily attributed to photooxidation.Besides, this work also compared the MB degradation performances of CuSe-based monolithic photocatalyst with powder CuSe, commercial P25, and commercial ZnO.As shown in Figure 8, the CuSe-based monolith obviously outperformed powder CuSe, commercial P25, and commercial ZnO in MB degradation.Compared to powder CuSe, the monolithic CuSe exposes more active sites due to its homogeneous distribution on the surface of copper foam.Compared to commercial TiO 2 and ZnO, CuSe possessed more favorable band gap that can efectively absorb both visible and ultraviolet light.Ten, this work tested the infuence of copper foam density on the MB degradation performance of CuSebased monolithic photocatalyst (as shown in Figure 9).When the density of copper foam equaled to 5 ppi, the CuSe-based monolithic photocatalyst as synthesized possessed a ∼97% MB degradation efciency within 120 min with the concentration of MB decreasing from 10 mg•L −1 to approximately 0.3 mg•L −1 .By adjusting the density of the copper foam supporter to 10 and 20 ppi, it was found that the MB degradation performance of the CuSe-based monolithic photocatalyst was only negligibly afected.As shown, the CuSe-based monolithic photocatalysts supported on 10-and 20-ppi copper foam exhibited comparable MB degradation performances with that supported on 5-ppi copper foam, i.e., by decreasing the MB concentration from 10 mg•L −1 to around 0.3 mg•L −1 in a 120-min experiment.It is thus concluded that the density of copper foam is not a primary factor infuencing the MB degradation performance of the CuSe-based monolithic photocatalyst.In the following experiments, the copper foam with a density of 10 ppi was used to test the efects of other infuential factors.
Te efects of photocatalyst dosage on the MB degradation performance of the CuSe-based monolithic photocatalyst are shown in Figure 10.As shown, when the dosage of the CuSe-based monolithic photocatalyst was two pieces, International Journal of Chemical Engineering it achieved a ∼96% MB degradation performance within 170 min, slightly inferior to the MB degradation performance of fve-piece photocatalyst.Such inferiority may be attributed to the fact that extremely low dosage of the CuSebased monolithic photocatalyst provided insufcient active sites for the degradation of MB.However, if the dosage of the CuSe-based monolithic photocatalyst increased to more than fve pieces, i.e., eight and ten pieces, their MB degradation performances did not signifcantly improve compared to the fve-piece scenario.Tis observation indicates that redundant active sites negligibly contributed to the improvement of MB degradation performance of the CuSe-based monolithic photocatalyst.In the case where the MB concentration equaled to 10 mg•L −1 , fve-piece dosage of 1 × 1 × 1 cm CuSe-based monolithic photocatalyst was enough to achieve a relatively high MB degradation performance.
After identifying the optimal density and the photocatalyst dosage, the infuence of MB concentration on the photocatalytic performance was that the CuSe-based monolithic photocatalyst was explored in the presence of 5, 10, 15, and 20 mg•L −1 of MB.As shown in Figure 11, after 150-min degradation, the CuSe-based monolithic photocatalyst could achieve a >98% MB degradation efciency when the MB concentration equaled to 10 mg•L −1 .Slightly increasing or decreasing the pH value to 9 or 5 had an unobvious inhibitive efect on the MB degradation, i.e., the time used to achieve a >90% MB degradation efciency was extended to more than 180 min.Te adverse efect under the pH value of 5 might be attributed to acidic condition that made the photocatalyst positively charged, which compromised the adsorption of MB as a cationic component [34], while the detrimental infuence under the pH value of 9 might be ascribed to the relatively high pH value that decreased the H + concentration in the MB solution, hence inhibiting the formation of HO 2 − (H + + O 2 + 2e − ⟶ HO 2 − ), a benefcial radical that facilitates MB photocatalytic degradation [35].However, it was intriguingly found that, when the pH value further decreased from 5 to 3, the MB degradation performance of the CuSe-based monolithic photocatalyst was contrarily improved.Tis was because abundant H + ions in the MB solution favored the formation of HO 2− through a photocatalytic process [36].Besides, a similar observation was also identifed when further increasing the pH value from 9 to 11, which indicates that, under this circumstance, the adsorption of MB on the surface of the CuSe-based monolithic photocatalyst was signifcantly enhanced due to its negatively-charged nature [37].Considering the adsorption of MB is the preliminary step for its photodegradation; the enhanced adsorption capacity towards MB is benefcial for its further conversion under light irradiation.Based on the abovementioned discussions, it is reasonable to declare that the CuSe-based monolithic photocatalyst as synthesized exhibited excellent compatibility and adaptability to harsh pH conditions, which did extend its application opportunities under practical scenarios.Te TOC analysis shows that, within a 2hour experiment, the TOC removal efciency reached >70%, which demonstrates that both decolorization and mineralization played functional roles during the degradation of MB, while the mineralization played the dominant role (as shown in Figure 13).
Finally, to justify the cost-efectiveness of the CuSe-based monolithic photocatalyst, a recycling experiment was conducted to determine the MB degradation performances in diferent cycles with the same batch of photocatalyst.As International Journal of Chemical Engineering shown in Figure 14, in a cyclic experiment, the CuSe-based monolithic photocatalyst exhibited comparable MB degradation performances for ten continuous cycles, which indicates that the photocatalyst is relatively stable in degrading MB.As shown in Figure 15, in the regeneration experiments, it could be found that the MB degradation performances of regenerated CuSe-based monolithic photocatalyst negligibly declined compared to the original one.Te regenerated CuSe-based monolithic photocatalyst also exhibited >95% MB degradation efciency within a 3 hour experiment, which suggests that the HCl-impregnation followed by reselenization process can efectively regenerate the deactivated photocatalyst.From these perspectives, the CuSebased monolithic photocatalyst possessed a relatively decent recyclability and reusability in the photocatalytic degradation of MB.Such favorability will further save the operational cost if the CuSe-based monolithic photocatalyst was used as alternatives to traditional powdery photocatalysts for MB removal from dye-polluted industrial efuents.Te performance tests involving the infuence of copper foam density, photocatalyst dosage, MB concentration, pH value, and cyclic number fully demonstrate the possibility to progressively extend the research and discussion on the CuSe-based monolithic photocatalyst in the future (as briefy illustrated in Figure 16).

Conclusions
In conclusion, this work primarily developed a method to prepare a CuSe-based monolithic photocatalyst as an efcient alternative to traditional powdery ones.Te CuSebased photocatalyst as synthesized well-kept its monolithic structure during the photocatalytic reaction process due to the maintenance of the copper skeleton after selenization.Besides, the surface of the monolithic photocatalyst fully covered by CuSe ensured that it exhibited decent performance towards MB degradation.In particular, activity test results showed that the CuSe-based monolithic photocatalyst as prepared could successfully achieve a ∼97% MB degradation efciency.Notably, this efcient photocatalyst also exhibited resistance to harsh pH conditions, i.e., under the pH values of both 3 and 11, the MB degradation performance of the CuSe-based monolithic photocatalyst was further improved compared to the performance under a pH value of 7. Tese outstanding performances support that the CuSe-based monolithic photocatalyst may be a promising alternative to traditional powdery photocatalysts to ensure the recyclability and extend the applicability of the photocatalytic techniques for organic dye removal and industrial efuent cleaning.
Chemical Engineering of the peaks in the XRD pattern centering at approximately 43 °, 50 °, and 74 °were attributed to elemental copper (JPCD #04-0836), which indicates that the structural integrity of the copper skeleton was generally maintained during the selenization process.Such excellent maintenance is indispensable for the CuSe-based photocatalyst to keep its monolithic structure instead of collapsing during the photocatalytic reaction process.

Figure 3 :
Figure 3: XRD pattern of the CuSe-based monolithic photocatalyst (inserted with the crystal structure of klockmannite).

Figure 7 :Figure 8 :
Figure 7: MB degradation performance of CuSe-based monolith under dark and light-irradiated conditions.

Figure 16 :
Figure 16: Diagrammatical illustration of the MB degradation on the CuSe-based monolithic photocatalyst.