Synthesis and characterization of CuO/Co0304 and CuO/Fe:O; composites and their potential application in the photocatalytic CO2 reduction process

Cupric oxide is a prominent material used as a photocatalyst due to its narrow bandgap; coupling it with other metal oxide semiconductors improves its efficiency due to the favored charge transference. This work reports the synthesis of the composites CuO/CosO4 and CuO/Fex0s, prepared in a sol-gel and hydrothermal two-step methodology to disperse the cocatalyst particles over CuO. The effect of the cocatalyst's concentration over CuO in its structural, optical, and photocatalytic properties was analyzed. A better distribution of the Fe203 particles over CuO was observed, which resulted in the largest cfficiency in the photocatalytic CO2 reduction to formic acid. Despite this, increasing the cocatalyst concentration reduces the photocatalytic activity due to the surface saturation, probably causing the formation of recombination centers. The presented methodology represents a low-cost way to obtain highly efficient composites in photocatalytic reductive processes. Palabras clave: Copper oxide, composites, characterization, CO> reduction. been used to prepare many metal oxide particles, resulting in an easy and low-cost methodology due to the simple Simple oxide semiconductor materials have attracted equipment used for the particle’s preparation. attention recently due to their multiple applications thanks to their valuable properties. For that reason, optimizing their synthesis methods and trying to obtain them in lees steps and under low-cost methodologies has been the current challenge for materials scientists.


Introduction
On the other hand, it is well-known that using composites is favored in different applications compared to using single materials.This is mainly associated with the improved properties achieved with the union of two or more materials with different independent characteristics Cupric oxide is a transition metal oxide with that add value to the formed material, even more, if both semiconductor properties (p-type) due to its narrow have exhibited good results in a specific application.bandgap [1].Among its properties are superthermal conductivity, -stability, antimicrobial -activity, and photovoltaic properties [2]; for that reason, it has been used in multiple applications, such as photocatalysis [3], supercapacitors [4], antibacterial surfaces [S], gas sensors [6], chemical absorbers [7], batteries [8], among In photocatalytic applications, the formation of composites is one of the most used strategies to reduce the recombination of photo-generated charges.Photocatalysis is a process that uses irradiation to activate a semiconductor material, forming electron (e-) -hole pairs ().In this context, both charged species are responsible thers.others for most redox reactions performed on the catalyst's In this context, different methods have been used to -surface.Unfortunately, those charges tend to recombine prepare CuO particles, including hydrothermal [9], co--themselves, reducing the photocatalytic activity.precipitation [10], sol-gel [11], microwave irradiation [12], etcetera, resulting in particles with different structures, allowing it to modify its properties.Sol-gel is a method used to produce solid materials from molecular precursors with the formation of colloidal particles.It has Cos0; and Fe:O; have been used as cocatalysts forming composites in different photocatalytic processes such as degradation of recalcitrant organic compounds [13,14], hydrogen production [15,16], and CO> reduction QUIMICA HOY Jorge A. Quilantán-Serrano, L u i s F .Garay-Rodríguez, Lorena L. Garza-Tovar', M. Torres-Martínez, 1 .Juárez-Ramírez [17,18].In this context, decorating the surface of a base material with both metallic oxides has been responsible for an enhancement in light harvesting, the formation of heterostructures that favors the charge transfer, and in most cases, guide the selectivity of the reaction [19].
Considering the above points, the present work explores the formation of composites of CuO decorated with Co3O4 and FesO; particles in different concentrations in a two-step sol-gel -hydrothermal synthesis for the CuO and composites preparation, respectively.All the prepared composites were structural and optical characterized, and their -potential -application was evaluated in -the photocatalytic CO2 reduction reaction to value-added chemicals.

CuO synthesis
CuO synthesis was performed by the sol-gel methodology.For this purpose, copper acetate (Fermont) was dissolved in isopropyl alcohol and continuously stirred at 70 "C for 1 hour.Subsequently, solvent evaporation was carried out at 90 °C.The recovered powder was thermally treated at 400 "C for one hour in order to achieve phase formation.an appropriate amount to achieve 1, 5, and 10 wt.% composites.After the complete dissolution, the pH was adjusted to 9 using NHOH (DEQ), being added drop to drop.Finally, the CuO prepared in the previous step was suspended in the solutions, and the resulting suspension was transferred to a stainless-steel autoclave to be heat treated at 120 *C for 12 hours.The recovered powders were washed three times with deionized water, dried for 12 hours at 80° C, and stored for later use.For easier reading, the composites will be labeled as follows: Co 1, Co 2, and Co 3 for the CuO/Co304 composites with 1, 5, and 10 wt.% of Co0s; and F3 1, Fe 2, and Fe 3, for the CuO/Fe:03 composites with 1, 5, and 10 wt.% of Fe20s, respectively. Characterization.
The structural properties of the prepared composites Ka Bcoso (1) Where d is the crystallite size in nm, k is the shape factor constant, which is 0.89, B is the full width at half maximum in radian, 4 is the wavelength of the X-ray which is 1.540598 nm, and 8 is the Bragg diffraction angle in radian [20].The morphological properties were visualized with a JEOL 6490LV SEM.Some HR-TEM analyses were performed in a JEOL 2010 microscope.The diffuse reflectance spectra were collected in a UV-Vis NIR spectrophotometer Cary 5000 coupled with an integration sphere.
As a potential application, the prepared composites were evaluated in the photocatalytic CO: reduction reaction in aqueous media.0.1 g of the material was dispersed in 100 mL of deionized water, and the suspension was bubbled with CO: for 15 minutes to promote an anoxic media and saturate the system.After that, the reactor was pressurized with 2 psi of CO2 as the initial concentration and irradiated from the outside with two LED lamps with a maximum emission of 420 nm.At the end of the reaction (3 hours), liquid products were analyzed with gas chromatography (methanol) and liquid chromatography (formic acid and formaldehyde).and Fe 3 samples is notorious the presence of peaks at 24, 33, and 57 degrees, belonging to the (0 12), (1 0 4), and (0 1 8) planes of 01-089-0598 Fe:O3 with rhombohedral structure and R-3c spatial group, being those peaks, in o both cases increasing in intensity due to the increase in g were analyzed with a Bruker D8 advance XRD, using that their concentration.The Crystallite size was also al information to calculate the crystallite size using the calculated using this data (Table 1).In this context, bare  Some micrographs were taken to bare CuO and the composites with larger cocatalyst concentrations (Co3 and Fe3), and those are presented in Figure 2. As seen, bare CuO exhibits an undefined morphology with an average particle size of 900 nm; however, most particles have created agglomerates.As other authors report, this is a common feature of CuO particles synthesized by sol-gel (i On the other hand, some differences are observed in the SEM images of the Co 3 and Fe 3 samples.As seen in Figure 2, in both cases, small particles are notorious for covering the surface of others with larger sizes.Despite this, in the case of the Co 3 sample, these smaller particles clump together to form agglomerates, contrary to the Fe 3 sample, where the small particles are well dispersed over the bigger CuO particles.This feature is better appreciated in the elemental mapping obtained for both composites.Despite having the same concentration of the metal oxide cocatalysts, Fe seems to be more dispersed, making the CuO on the surface less visible.In contrast, as Co3Os forms agglomerates of large particles, it does not cover all the surface, creating only some zones rich in that element.
From these results, it is possible to assume that in the case of Fe203, the hydrothermal method favored the formation of nanoparticles, contrary to the Co3Os.Figure 4 shows the HR-TEM images of the Co 3 and Fe 3 composites.In both images, it is possible to observe the grain boundaries between CuO and the deposited Co0304 and FezO; particles, confirming their presence in the composite.
As seen, CuO particles grew in the (1 1 1) face, in concordance with the information obtained in the XRD patterns.A similar behavior is observed in the Co3Os and Fex03 particles where the (4 0 0) and (1 2 1) planes are observed, confirming the growth of that metallic oxides as cocatalysts.
Synthesis and characterization of Cu0/Co,0,and CuO/Fe, composites and their potential application in the photocataly reduction process The absorption spectra of the prepared composites and the bare sample are presented in Figure 5.As can be observed, all the samples exhibit photo-absorption in the visible light zone, suggesting their photo-activation under solar light.This is an expected result due to the coloration of the obtained powders.
On the other hand, comparing the spectra of the composites with the one of CuO, it is possible to observe the apparition of characteristic bands related to the presence of CosOs and Fe:O:.For instance, in the case of the Co composites, a band was remarked between 500 -800 nm, related to 0? -Co** transition [21], while in the Fe composites, the band in = 540 is associated with 0? -Fe**transition [22].These data were used to calculate their bandgap energy, summarized in Table 1.As seen, the calculated bandgap value for the CuO sample is 2.8 eV, similar to other reports in the literature [23].This parameter significantly decreases with the incorporation of both metallic oxide nanoparticles.This change in the bandgap energy can be associated with the good incorporation of both metal oxides forming the composite and the increase in the cocatalyst concentration.As seen, all the samples exhibited selectivity to the formic acid generation, evolving this compound in larger concentration (> 10 pmol; right scale) compared to formaldehyde or methanol (< 1 pmol; left scale).Those results can be associated with the fewer quantity of electrons (e) and protons (H*) required for its formation compared to the other products, according to equations 2 Despite this, there was a significant increase in the evolution of all the products with the incorporation of the cocatalysts, compared to bare CuO, which is commonly associated with the well synergistic effect between both materials forming the composite and the efficient electronic transfer achieved.On the other hand, it can be noticed in the case of all the products that despite the increase in the photocatalytic activity, it reaches a maximum and after it decreases.This behavior can be related to the dispersion of the formed Co304 or Fex0s3 particles over CuO.In this context, according to some reports [25,26], the saturation of the main photocatalysts surface minimizes the photocatalytic efficiency due to radiation is no longer captured efficiently or particles could a c t as recombination centers.espite this, Which presented the largest efficiency: mainly associated with the good electronic transference between both metal oxides and CuO.Those results confirm that the formation of composites is a good strategy for obtaining highefficiency photocatalysts.

Figure 1 .
Figure 1.XRD patterns of the prepared composites

Figure 2 .
Figure 2 .SEM images of the bare CuO and the Co3 and Fe3 sites with the respective particle size distribution of the CuO particles

Figure 3 .
Figure 3. (a) EDS elemental mapping of the Co 3 and Fe 3 composites.

Figure 4 .
Figure 4. HR-TEM images of the Co 3 and Fe 3 composites.

Figure 5 .
Figure 5. Absorption spectra of the prepared composites.

LoteFigure 6 .
Figure 6.Accumulation of formic acid, formaldehyde, and methanol in the reaction media after three hours of visible-light irradiation.
an adequate cocatalyst concentration favors the surface properties, and light absorption results in an improved efficiency evolving CO2 reduction products as the Co 1 and Fe 1 samples.Synthesis and characterization of CuO/Co,0,and CuO/Fe.0),composites and their potential application in the photocatalytic Co, reduction process 4. Conclusions The CuO photocatalytic activity in the CO2 reduction reaction was improved by forming composites with Co3O4 and Fe:03 in a two-step sol-gel and hydrothermal methodology.Their structural characterization confirmed the formation of the expected phases forming the composites in the different cocatalysts concentrations.It was observed that the presence of Co30s and FexO4 over the CuO surface decreased its bandgap energy favoring the absorption of visible-light irradiation.Formic acid was evolved as the main CO reduction product, being the samples loaded with the lowest cocatalyst concentration

Table 1 .
Summary of the erystallite size and bandgap of the repared composites.