Production of Methane and Ethane with Photoreduction of CO₂ Using Nanomaterials of TiO₂ (Anatase–Brookite) Modifications with Cobalt

Abstract

In this study, we present the synthesis of TiO₂ nanomaterials doped with different mol% cobalt, prepared by the sol–gel method for CO₂ reduction using UV light. The nanomaterials were calcined at 400 °C for 4 h. Characterization of the nanomaterials was performed using XRD-Rietveld refinement, XPS, Raman spectroscopy, diffuse reflectance spectroscopy (DRS), SEM-EDS, TEM-HRTEM, BET area, photoluminescence, and electrochemical techniques. The results show that the incorporation of cobalt into TiO₂ modifies the structural properties, binding energies, and oxygen vacancy generation, it undergoes a shift towards the visible region, the recombination of charge carriers decreases, and the BET area is slightly modified. The photoreduction of CO₂ with the highest production of methane and ethane is with 1% mol% of cobalt in TiO₂, exhibiting values 3 and 14 times higher with respect to TiO₂, which is attributed to the efficiency in the separation of photogenerated species (e⁻/h⁺) as a consequence of the generation of energetic states that function as an electron trap and thus improve the photocatalytic activity for the photoreduction of CO₂.

1. Introduction

One of the major concerns today is the increase in atmospheric CO₂ concentrations caused by the burning of fossil fuels, as it negatively impacts human health and the environment. The consequence of the excessive use of this type of fuel is the generation of CO₂; this gas is primarily responsible for the greenhouse effect, thus contributing to global warming worldwide [1,2,3]. To mitigate this environmental problem, it is necessary to develop new alternatives and implement clean, renewable technologies that are effective in achieving a sustainable world. Currently, there are some alternative renewable energy sources, such as biomass, wind, and geothermal, but they are insufficient and do not meet the current and future energy demand [4,5,6].

Another alternative is through the capture and transformation of CO₂ into safe chemicals. However, this conversion process is very complex and is not cost-effective, as it requires a large amount of energy and a catalyst with certain characteristics because CO₂ is a chemically stable molecule. Another natural alternative is photosynthesis by plants, but CO₂ fixation in plants is impossible. Over the past two decades, researchers have focused on imitating artificial photosynthesis (CO₂ reduction), as it is an alternative with the potential to convert CO₂ into high-value products at constant pressure and temperature, which can then be used as fuel or raw material for industry [6,7,8].

The photocatalytic reduction in CO₂ is a process that contains several stages, and, to date, different nanomaterials have been used for this process and a wide range of products such as CO, CH₄, HCHO, HCOOH, and CH₃OH have been obtained [9,10,11]. In the photoreduction reaction of CO₂, several researchers have used different semiconductors, such as g-C₃N₄ [5], CeO₂ [7] TiO₂ [9,10,11], and ZnO [12], among others. However, the most widely used is TiO₂ (anatase) as it has low toxicity, is environmentally friendly, and is low cost [7,8,13]. However, it has some disadvantages that limit its efficiency, such as a performance limitation due to the rapid recombination of charge carriers and its band gap that only adsorbs light in the UV region. TiO₂ has low efficiency in CO₂ reduction, due to the complex reaction routes, poor adsorption, and subsequent activation on the TiO₂ surface, which delay the conversion of CO₂. To overcome these types of deficiencies, we have tried to find strategies using different methods to modify the properties of TiO₂, such as doping with a metal or non-metal [9,14], impregnation with metals or non-metals [1,10,11,15], and synthesis of heterostructures [13], with the purpose of improving photocatalytic efficiency for CO₂ photoreduction.

One of the most important strategies is doping, since it allows for higher photocatalytic performance due to the modification of the electronic structure of the photocatalyst, as it increases the lifetime of the charge carriers and thus efficiently improves the transfer of charge carriers to the adsorbed species, such as CO₂ [10,16,17]. Additionally, cobalt-modified nanomaterials have advantages over other metals, such as high electrical conductivity, thermal stability, unique electronic properties, chemical properties, and high catalytic performance, making these types of materials promising for the CO₂ reduction reaction [18,19]. Furthermore, using cobalt as an impurity in the TiO₂ structural lattice modifies the positions of the conduction and valence bands of the band gap (E_g). Therefore, the energy levels introduced by doping with cobalt (impurity) have the function of donors or acceptors, which prevents the recombination of charge carriers and improves the efficiency of photogenerated charge separation. The incorporation of cobalt as a dopant is effective for semiconductor oxides, because it increases the density of surface oxygen vacancies and these function as surface electron donors [20,21].

In this sense, cobalt has two valence states (Co³⁺ and Co²⁺) and these states can directly affect the migration of charge carriers (e⁻/h⁺), which can serve as an electron trap and thus reduce the recombination of charge carriers and increase photocatalytic efficiency [21,22,23]. Therefore, in this work, nanomaterials based on TiO₂ modified with different mol% of cobalt were synthesized by the sol–gel method. Nanomaterials were tested by photocatalytic reduction of CO₂ to obtain chemical compounds with high added value, such as CH₄, C₂H₆, CO, and H₂ at room temperature under a UV light source. The objective is studying the effect of adding different mol% of cobalt to TiO₂ on its structural, optical, and morphological properties and to correlate them with the photocatalytic reduction of CO₂. Likewise, the possible photocatalytic mechanism by which the CO₂ reduction reaction is carried out in the Ti1Co nanomaterial is proposed.

2. Results

2.1. X-Ray Diffraction (XRD)

X-ray diffraction (XRD) patterns of the photocatalysts calcined at 400 °C cobalt-free Ti0Co (TiO₂) and doped with different TiXCo contents, where X = 0, 1, 3, 5, and 10% mol% cobalt, can be observed in Figure S1 (Supplementary materials). In the diffractograms (see Figure S1) obtained for the TiXCo nanomaterials, seven peaks located at 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, 62.69°, and 75.03° can be observed as a function of the angle 2θ, which correspond to the planes (101), (004), (200), (105), (211), (204), and (215) that are characteristic of TiO₂ in anatase phase whose diffraction card is JCPDS 21-1272 Quality Star (*). Likewise, a peak located at 30.8° can be observed as a function of the angle 2θ, which corresponds to the (121) plane that is characteristic of TiO₂ in the brookite phase whose diffraction card is JCPDS 29-1360 Quality Star (*). In our photocatalysts, a mixture of TiO₂ phases was obtained, this mixture of crystalline phases of TiO₂ depends on the synthesis method, dopant content, precursor type, and the temperature at which the different nanomaterials are thermally treated, as reported by several authors [17,24,25,26]. In nanomaterials, the incorporation of different mol% of cobalt to TiO₂ does not significantly influence the crystallization process of the crystalline phases of TiO₂ (anatase and brookite); the production of these phases is due to the preparation conditions, aging time, and calcination temperature of our photocatalysts synthesized by the sol–gel method compared to what has been reported [24,27]. Also, in the different X-ray diffraction patterns of the TiXCo photocatalysts, no other secondary phases correlated to the production of rutile phase, metallic cobalt, or cobalt oxides (CoO, Co(OH)₂ and Co₃O₄) were observed. In the diffraction patterns of our photocatalysts, a shift in the anatase peak (101) was not clearly observed because there is a mixture of phases (anatase and brookite), attributed to the overlap of the planes (101) of anatase and (120) and (111) of brookite, contrary to what was previously reported in other works where only the anatase phase was obtained [16,17,28]. On the other hand, a notable change in the decrease and widening of the diffraction peak was observed in the (101) plane of anatase and (121) of brookite due to the incorporation of different contents in mol% of cobalt (TiXCo); this effect is correlated to the incorporation of Co²⁺ into the structural lattice and to the structural irregularity of TiO₂ as a consequence of cobalt doping [28,29,30,31]. The existence of a small proportion of cobalt oxides can also be considered, since they are not detectable due to the detection limit and the normal measurement of XRD.

In order to confirm the insertion of cobalt to the crystalline structure of TiO₂ and the non-segregation of cobalt oxides, and verify the respective lattice parameters, crystal size, and relative composition of each of the phases identified by XRD, it was necessary to perform the Rietveld refinement to the TiXCo photocatalysts.

2.2. Rietveld Refinement

Characterization of diffraction patterns by Rietveld refinement is used to understand the effect of cobalt doping on TiO₂ in TiXCo nanomaterials. To obtain the refined diffraction patterns, their symmetry and respective space group were used: tetragonal and I41/amd (anatase) and orthorhombic Pcab (brookite). In Figure 1, the diffraction patterns obtained from the Rietveld method of Ti0Co (Figure 1a) and Ti1Co (Figure 1b) are shown, and for Ti3Co, Ti5Co, and Ti10CO they are shown in Figure S2. The information obtained from the refined patterns confirms that there is only a mixture of TiO₂ phases (anatase and brookite) and that there is no segregation or detection of cobalt oxides (CoO, Co(OH)₂, Co₃O₄, or metallic cobalt) due to the synthesis method and thermal treatment in the photocatalysts (TiXCo) [28,30,31,32,33].

Table 1. Obtained parameters from Rietveld refinement, cell parameters, crystal size, and relative composition (wt(%)) of TiO₂ crystalline phases in the different TiXCo photocatalysts.

	Anatase					Brookite
	Cell Parameters (nm)		Crystal Size (nm)	Relative Phase Conc. wt(%)	Volume Unit Cell (Å3)	Cell Parameters (nm)			Crystal Size (nm)	Relative Phase Conc. wt(%)	Volume Unit Cell (Å3)
Sample	a	c				a	b	c
Ti0Co	0.37891 (3) *	0.94908 (1)	11.9 (6) *	78.2 (3) *	136.26	0.92020 (5) *	0.54365 (3)	0.51828 (2)	9.0 (2) *	21.7 (3) *	259.28
Ti1Co	0.37906 (3)	0.94810 (1)	10.4 (6)	81.1 (4)	136.22	0.91892 (8)	0.54338 (5)	0.52032 (3)	8.6 (2)	18.8 (3)	259.80
Ti3Co	0.37927 (4)	0.94752 (1)	9.5 (6)	80.8 (4)	136.29	0.91793 (9)	0.54419 (6)	0.52033 (4)	8.0 (2)	19.9 (4)	259.92
Ti5Co	0.37928 (4)	0.94853 (1)	10.6 (7)	82.4 (5)	136.44	0.91775 (1)	0.54460 (8)	0.52056 (6)	7.0 (3)	17.5 (5)	260.18
Ti10Co	0.37969 (5)	0.94799 (1)	9.3 (7)	79.3 (6)	136.66	0.91471 (1)	0.54463 (8)	0.52365 (7)	6.9 (2)	20.7 (6)	260.87

* The number in parentheses corresponds to the standard deviation cell parameters, crystal size, and relative phase concentration.

shows the results of cell parameters, crystal size, cell volume, and relative composition of each of the phases in the TiXCo photocatalysts obtained from the Rietveld refinement.

The results obtained from the Rietveld refinement show that the incorporation of different cobalt contents (mol%) into TiO₂ modifies the lattice parameters, crystal size, relative composition, and cell volume of both TiO₂ phases (anatase and brookite) as the mol% of cobalt increases. The increase in the lattice parameters in anatase (a and c) and in brookite shows a decrease in parameter (a) and (b and c) an increase; it is attributed to the substitution of the dopant in the TiO₂ structure (see Table 1) by the following: the ionic radius of Ti⁴⁺ in an octahedral coordination is 0.605 Å and the ionic radii of Co²⁺ and Co³⁺ in an octahedral coordination are 0.65 Å and 0.545 Å, respectively [28,31,34,35]. The increase in the lattice parameters can also be associated with a higher ratio of Co²⁺/Co³⁺ species within the TiO₂ structure due to the difference in their ionic radii compared to Ti⁴⁺ [23,35]. Cobalt substitution, regardless of the species incorporated into the structural lattice, promotes oxygen vacancies to compensate for the charge difference in TiO₂ [23,31,32]. Another effect that must be considered is the decrease in crystal size due to the increase in the mol% of cobalt, which is in the range of 9.3–10.6 nm compared to the Ti0Co reference; this decrease is correlated to the insertion of cobalt into the TiO₂ structure (see Table 1). The relative compositions of the anatase and brookite phases range from 80% to 20%, respectively. This is correlated with the addition of cobalt to the TiO₂ structure, the synthesis method, reaction conditions, and aging time. Another factor that must be considered is the increase in the volume of anatase and brookite as a function of the cobalt content (mol%); this is attributed to the incorporation of cobalt into the TiO₂ structural lattice, so this effect coincides with Vegard’s law [23,31].

2.3. X-Ray Photoelectron Spectroscopy (XPS)

The measurements carried out using XPS were to determine the chemical states (oxidation states) of the elements that make up the TiXCo photocatalysts. Survey spectra of the TiXCo series (X = 0, 1, 3, 5, and 10% mol cobalt) are shown in Figure S3a. In the TiXCo survey spectra, peaks are presented that are correlated to the elements C, Ti, and O, and only the peak based on the cobalt (Co) content in the nanomaterials is presented, and there is no other element detected in the survey spectra that functions as an impurity in our photocatalysts. The calibration performed to obtain the high-resolution spectra was with respect to C 1s (see Figure S3b), with a binding energy of 284.5 eV.

High-resolution spectra of Ti 2p, Co 2p, and O 1s are shown in Figure 2a–c. The high-resolution XPS spectrum of Ti 2p (Figure 2a) of TiXCo photocatalysts shows two peaks corresponding to the Ti 2p_3/2 and Ti 2p_1/2 doublets. Subsequently, the deconvolution of the high-resolution spectrum in Ti 2p_3/2 of our reference (Ti0Co) is performed and three peaks are obtained that are located at 458.7, 458.5, and 456.9 eV (see

Table 2. Binding energy, oxidation state, and relative composition of Ti2p, Co 2p, and O 1s in the TiXCo nanomaterials, where x = 0, 1, 3, 5, and 10% mol of cobalt.

	Ti 2p3/2				Co 2p3/2		O 1s
Binding Energy (eV)
Sample	Ti4+		Ti3+	TixOy	Co3+	Co2+	OLatt	OSurf	Ovac
	Anatase	Brookite
Ti0Co	458.7	458.5	456.9	-	-	-	530.1	531.7	-
	(77.3) *	(3.7) *	(19.0) *	-	-	-	(89.2) *	(10.8) *	-
Ti1Co	458.5	458.3	456.8	-	782.9	781.9	530.0	531.4	531.0
	(74.9)	(5.9)	(19.2)	-	(29)	(71)	(88.7)	(10.2)	(1.1)
Ti3Co	458.5	458.3	456.9	459.9	782.8	781.7	529.8	530.8	531.8
	(48.2)	(9.3)	(19.8)	(22.7)	(30.5)	(69.5)	(58.1)	(21.3)	(20.6)
Ti5Co	458.3	458.2	456.6	459.6	782.7	780.8	529.4	530.2	531.1
	(65.2)	(17.8)	(5.1)	(11.9)	(36.7)	(63.3)	(62.7)	(15.8)	(21.5)
Ti10Co	458.1	458.0	456.6	459.5	782.4	780.7	529.2	529.7	530.6
	(59.5)	(22.9)	(2.4)	(15.2)	(31.5)	(68.5)	(49.8)	(19.7)	(30.5)

* Values between ( ) are the relative abundances of each species found with their respective binding energy.

). The first two binding energies (BEs) are assigned to the anatase and brookite crystalline phase of TiO₂ (Ti⁴⁺) and the third is assigned to Ti₂O₃ (Ti³⁺). The third contribution present in the reference nanomaterial is due to the preparation of the sample under ultra-high vacuum conditions and cleaning of the sample by Ar⁺, as reported in the literature [36,37]. The separation line in our analysis (DE) between Ti 2p_3/2 and Ti 2p_1/2 is 5.7 eV; this result is comparable with that reported by several authors in the literature [31,38,39,40]. Binding energies (BEs) obtained in the high-resolution spin–orbit coupling spectrum of Ti 2p as a function of the cobalt content (mol%) of the nanomaterials (TiXCo) decompose into four components with their respective relative abundances of each of them and exhibit a slight shift at low energy compared to our reference Ti0Co (see Table 2). The first two energies are associated with anatase and brookite TiO₂ and are in the range of 458.5–458.1 eV and 458.3–458.0 eV, respectively. The third in the range of 456.8–456.6 eV is associated with Ti₂O₃ and, finally, the energies located at 459.9–459.5 eV are attributed to the Ti_xO_y species. The slight shift in binding energies in the different nanomaterials with different mol% cobalt is attributed to the insertion of cobalt into the lattice in both TiO₂ crystal structures (anatase and brookite), as previously reported by various authors [29,40,41]. On the other hand, the incorporation of cobalt presents two phenomena: the first is reflected below 3% mol, where an increase in the proportion of Ti³⁺ and the generation of the Ti_xO_y species is observed, and the second is observed above 5% mol of cobalt in which the proportions of Ti³⁺ and Ti_xO_y decrease and the proportion of brookite increases with the content of the mol% of cobalt; this can be attributed to the generation of oxygen vacancies by the insertion of cobalt into the TiO₂ structure and in this way can generate surface and structural defects in nanomaterials. These results corroborate what was observed in XRD-Rietveld refinement.

High-resolution Co 2p spectra for all nanomaterials (TiXCo) can be observed in Figure 2b. The spectrum can be decomposed into two correlated contributions to Co 2p_3/2 and Co 2p_1/2 spin–orbital coupling. Deconvolution of Co 2p_3/2 spin–orbital coupling reveals three peaks in each of the samples analyzed (see Figure 2b). The attribution of the first of the peaks found in the range of 782.9–782.4 eV (see Table 2) corresponds to Co³⁺ in the Co 2p_3/2 spectrum. On the other hand, it is observed that there is another contribution in the range of 781.9–780.7 eV, which is related to Co²⁺ in the same Co 2p_3/2 spectrum and the last peak found at higher binding energy in Co 2p_3/2 is associated with a satellite shake-up, which is attributed to the characteristic high spin signal of Co²⁺ [29,31,42,43]. The afore mentioned results of our analysis show us that we have a mixture of valences (Co²⁺ and Co³⁺); the separation between the spin–orbit coupling of Co 2p_3/2 and Co 2p_1/2 is 15.5 eV and is comparable with that reported in the literature by several authors [31,35,42]. No other contribution to the binding energy was detected from Co⁰ (metallic cobalt), Co(OH)₂, CoO, Co₂O₃, and Co₃O₄ [40,44], and this fact is confirmed since no other surface segregated phase was observed in the different nanomaterials analyzed by XRD-Rietveld refinement analysis.

In Figure 2c the high-resolution O 1s spectra of our TiXCo nanomaterials can be observed. In Figure 2c, it can be observed that in the high-resolution O 1s spectrum of reference it is deconvoluted into two peaks that are located at 530.1 eV, associated with the structural oxygen (O_L) belonging to TiO₂, and 531.7 eV, attributed to the hydroxyl groups (O_surf) on the Ti0Co surface. In Figure 2c it can be observed that by adding different contents (mol%) of cobalt two effects can be seen and are attributed as follows: The first is in the oxygen associated with the structure (O_L) is found in the range of 530.0–529.2 eV and the surface hydroxyls (O_surf) 531.4–529.7 eV when compared with Ti0Co. The second effect that can be observed is the formation of a species corresponding to the peak found in the range of 531.0–530.6 eV, which is associated with the formation of oxygen vacancies and defects (O_vac) in the TiO₂ structure. The high-resolution spectrum of O 1s shows the effect of the decrease in the binding energy, which is attributed to the insertion of cobalt in the structural lattice, formation of oxygen vacancies in TiO₂, and the formation of the Ti-O-Co bond, as can be seen in Table 2; our results agree with what has been reported in the literature [35,40,45].

2.4. Raman Spectroscopy

Raman spectroscopy was used as an additional tool to observe the crystalline phases, disorder, and defects that are generated by incorporating a dopant into the TiO₂ (Ti0Co) structural lattice. Measurements were made at room temperature in a range between 100 and 800 cm⁻¹. The six characteristic vibration modes of the anatase phase according to the literature are located at 144, 197, 399, 514, 519, and 640 cm⁻¹ [23,31,46] and the characteristic bands of the brookite phase are reported at 128, 135, 153, 172, 195, 214, 247, 288, 322, 412, 454, 461, 502, 545, 585, and 636 cm⁻¹ [47,48]. Figure 3 shows the Raman spectra of TiXCo nanomaterials. For our reference Ti0Co (TiO₂), the vibration modes located at 145, 398, 517, and 639 cm⁻¹ are observed, corresponding to TiO₂ in the anatase phase (see Figure 3, symbol A). Also in Ti0Co, three additional vibration modes are observed and are located at 248, 287, and 324 cm⁻¹ (see Figure 3, symbol B), which correspond to the brookite phase of TiO₂; these results confirm that there are only two phases and that there is no segregation of cobalt oxides or any other phase of TiO₂, so these results coincide with what was observed in DRX-Rietveld refinement. For nanomaterials doped with different contents (mol% of cobalt TiXCo, see Figure 3), it is observed that the intensity of the most intense peak around 145 cm⁻¹ decreases as a function of the cobalt content and exhibits a blue shift when compared to Ti0Co (Figure 3 inserted). This effect is attributed to the formation of oxygen vacancies, distortion of the structural lattice, phonon confinement effect, and the broadening of vibration modes because of the incorporation of cobalt atoms into the TiO₂ structure [23,31,38]. Moreover, it is worth mentioning that despite the decrease in the intensity of the maxima as Co concentration increases, their relative intensities remain almost unchanged, as it is unveiled in the normalized Raman spectra depicted in Figure S4 (Supplementary Information). This is consistent with the results of the X-ray diffraction pattern analysis; namely, there are similar concentrations and crystallite sizes of the anatase and brookite phases for all samples.

2.5. Diffuse Reflectance (DRS)

UV–Vis spectroscopy allows the study of optical transitions and defects caused by the incorporation of a dopant into the crystalline structure. The diffuse reflectance spectra of the synthesized TiXCo nanomaterials, where X = 1, 3, 5, and 10% mol of cobalt, are shown in Figure 4a. The absorption spectrum of the Ti0Co nanomaterial has an absorption edge around 380 nm, which is related to the charge transfer from the valence band (O 2p) to the conduction band (Ti 3d) and commonly has an absorption edge in the UV region and no absorption in the visible region, as has been reported in the literature [31,35,40].

In the different TiXCo nanomaterials with different mol% of cobalt, it can be observed (see Figure 4a) that the absorption edges are modified with a slight shift towards the visible region. This shift in cobalt-modified nanomaterials (TiXCo) is associated with the incorporation of cobalt into the TiO₂ structural lattice and has been commonly explained because of exchange interactions between electrons located in the sp-d band and electrons located in the d band of the dopant cation (cobalt). These interactions between the s-d and p-d bands cause a decrease in the conduction band edge and an increase in the valence band edge, causing a decrease in the band gap, as reported in the literature [29,31,49].

Two effects of the incorporation of cobalt to TiO₂ can also be observed in Figure 4a: The first is the increase in the absorption in the range of 400–700 nm as a function of the cobalt content in the visible region compared to Ti0Co, this is correlated to the transition of the ligand field of Co²⁺/Co³⁺ in an octahedral coordination and will experience a strong crystalline force due to the surrounding oxygens so that the states of the d band will be divided into fundamental and excited states, generating an electronic d-d transition that is localized in the visible region, since the Ti⁴⁺ coordination has an octahedral coordination in TiO₂ [29,40,42]. The second can be observed below 400 nm; it is observed that below 3% mol of cobalt there is an increase in absorption in the UV region and above 5% mol of cobalt there is a decrease in UV absorption; this can be correlated to the relative concentration of the Co²⁺/Co³⁺ species according to the XPS analysis and to the different species with their relative concentrations of TiO₂.

The values of the band gap energies (E_g) of the TiXCo nanomaterials are used in Figure 4b using the Kubelka–Munk (K-M) function at F(R). Obtaining F(R) uses Equation (1):

α/s = [(1 − R)²/2R] = F(R)

(1)

where R is the reflectance and F(R) are proportional to the ratio α/s. To obtain the Tauc’s plot, Equation (2) is used to obtain Equation (3).

(αhν)^1/n = A(hν − E_g)

(2)

[F(R) × hν]^1/n = A(hν − E_g)

(3)

where h is the Planck constant (J.s), hν is the photon energy, and n is the coefficient associated with an electronic transition for an interband transition (n = 2 for indirect transitions). To find the values of E_g, they are calculated by extrapolating the absorption edge using a line that intercepts the X axis. The values found for the different TiXCo nanomaterials are shown in

Table 3. Textural properties (BET area, average pore size, and pore volume) and optical properties of TiXCo nanomaterials.

Sample	BET Surface Area (m2g−1)	Average Pore Diameter (nm)	Pore Volume (cm3/g)	Band Gap (eV)
Ti0Co	114.8	15.2	0.323	3.16
Ti1Co	116.5	4.6	0.149	2.98
Ti3Co	110.5	4.5	0.110	2.85
Ti5Co	99.5	4.7	0.096	2.78
Ti10Co	95.6	7.2	0.313	2.22

. The decrease in the band gap as a function of the cobalt content modifies the values and they are in the range of 2.98–2.20 eV (see Figure 4b and Table 3). This decrease in the forbidden band width causes a slight blue shift and improves the absorption in the visible region compared to the pure TiO₂ material with a value of E_g = 3.16 eV (Ti0Co). This is due to the pseudo-octahedral coordination that Co²⁺ possesses when it is replaced by Ti⁴⁺ in the TiO₂ structural lattice, inducing intermediate states in the forbidden band and the exchange interactions between the bands (sp-d) of the structural network (sp) and the dopant (d). The E_g values obtained from TiXCo nanomaterials in our work are like the values reported in the literature [32,35,38,41,50].

2.6. Energy-Dispersive X-Ray-Scanning Electron Microscopy (SEM-EDS)

In Figure 5, the SEM images of the Ti0CO (Figure 5a) and Ti1Co (Figure 5c) materials can be observed. The morphology of the particles in the Ti0Co nanomaterial shows that they are agglomerated particles with semi-spherical and non-regular shapes. The incorporation of 1% mol of cobalt to TiO₂ (Ti1Co) is observed in the same way: the particles are of non-regular semi-spherical shape and there is no significant change in the morphology due to the incorporation of 1% mol of cobalt to the TiO₂ structure; this can be correlated to the fact that there is a mixture of phases and that a heterojunction (anatase and brookite) is formed between them, not allowing a notable morphological modification, contrary to what has been reported in the literature, where the addition of cobalt to TiO₂ is morphologically modified and suffers an increase in the size of the nanoparticles [35,41,44,49].

The nanomaterials Ti0Co (Figure 5b) and Ti1Co (Figure 5d) are analyzed using EDS. The analysis provides information on the composition and presence of each of the atoms that make up the nanomaterials studied. The elemental analysis of the TiO₂ nanomaterial can be observed in Figure 5b and only the presence of Ti and O is obtained with a composition of 80% and 20% by weight and 57.2 and 42.8% atomic; no other element corresponding to impurities that could come from the precursors used in the synthesis of our nanomaterial were presented.

Elemental analysis by EDS of the nanomaterial containing 1% mol of cobalt (Ti1Co) confirms that it only contains the presence of Ti, O, and Co, with the corresponding compositions of 17.1%, 81.8%, and 1.0% by weight and 38.3%, 61.1%, and 0.6%, and there is no evidence of external impurities to the nanomaterial. The value obtained from the EDS analysis with respect to the mol% of cobalt (Ti1Co) is below that added (1% mol cobalt); this can be correlated to the fact that the cobalt is dispersed in the structural lattice and does not form segregated cobalt oxide on the TiO₂ surface. This fact is corroborated by Rietveld refinement and Raman, by not presenting reflection intensities as a function of 2q and vibration modes corresponding to cobalt oxide.

2.7. TEM-HRTEM

Figure 6 shows the TEM micrographs of the nanomaterials Ti0Co (a) and Ti1Co (c). From the TEM analysis of Ti0Co and Ti1Co (see Figure 6a,b, it can be observed that both samples have semi-spherical particles with uniform sizes; this can be attributed to the synthesis method and the low calcination temperature (400 °C). To obtain an estimate of the crystal size distribution, approximately 250 particles were counted in both TEM micrographs of Ti0Co and Ti1Co (see Figure 6c,d). The results obtained from the TEM analysis show that the average crystal size distribution lies approximately between 11.0 nm (Ti0Co) and 10.1 nm (Ti1Co).

Therefore, the addition of 1% mol of cobalt to TiO₂ induces a reduction in crystal size when compared to Ti0Co. This fact is consistent with the results obtained by XRD-Rietveld refinement, where it is observed that the crystal size decreases as a function of the cobalt content. The effect of decreasing crystal size by incorporating an impurity as a dopant into the TiO₂ crystal structure has been previously reported in the literature [24]. However, it has been reported in the literature that when cobalt is incorporated into TiO₂, two effects are observed: the increase in crystal size with respect to the reference material [41,44,49] and the decrease in crystal size, as in our case. This effect shows that the incorporation of cobalt to TiO₂ could be correlated with the following variables: precursors used, synthesis method, calcination temperature, and the different oxidation states of each of the species that make up the nanomaterials [30,33].

HRTEM micrographs (Figure 7) of the nanomaterials Ti0CXo (Figure 7a) and Ti1Co (Figure 7f) are shown. The analysis for each of the images is performed separately; subsequently, the different areas marked for Ti0Co, as shown in Figure 7b–e, and for Ti1Co, which are labeled as Figure 7g–j, were analyzed. For each of the selected areas, FFT-HRTEM is performed to obtain information on the different crystallographic planes belonging to each of the crystalline phases that constitute the nanomaterial. The first four areas analyzed in Figure 7b–e show six crystalline planes in the Ti0Co nanomaterial: the first three planes (012), (121), and (201) correspond to the interplanar distances 0.2476, 0.2900, and 0.2409 nm of TiO₂ in the brookite phase (labeled as B in Figure 7), whose diffraction card is JCPS PDF-029-1360 Quality: Star (*) and the other three planes (200), (105), and (004) have distances of 0.1892, 0.1699, and 0.2378 nm that correspond to the TiO₂ in the anatase phase (labeled as A in Figure 7), whose diffraction card is JCPDS PDF 021-1272 Quality: Star (*). Figure 7f shows the micrograph of the Ti1Co nanomaterial with the different analyzed areas labeled as Figure 7g–j and on the right side of the image the corresponding FFT-HRTEM of each analyzed area can be observed. In the four analyzed areas, four crystalline planes were obtained, as can be seen in Figure 7g–j, and they correspond as follows: the interplanar distances with values located approximately at 0.2480, 0.2988, and 0.2400 nm correspond to the planes (012), (121), and (201), belonging to TiO₂ in brookite phase (B) associated with the aforementioned diffraction card. The (101) plane corresponds to the approximate interplanar distance of 0.3529 nm of TiO₂ in anatase phase (A), which can be associated with the diffraction card. The values found in the FFT-HRTEM analysis of Ti1Co are slightly modified when compared with the values found in the JCPDS of brookite and anatase. No planes associated with crystallographic phases of CoO, Co₃O₄, or Co⁰ (metallic cobalt) were found; this is further evidence that cobalt is present in the structural lattice of TiO₂, which is confirmed by XRD-Rietveld refinement and Raman, by the absence of cobalt oxides on the surface of TiO₂.

2.8. Textural Analysis (Specific Areas Determined by BET Method)

Figure 8 shows the N₂ adsorption–desorption isotherms of the different TiXCo nanomaterials. The obtained parameters, such as the estimated BET area, the average pore size, and the pore volume of each of the nanomaterials, are shown in Table 3. The isotherms of TiXCo nanomaterials are associated with a type IV one that corresponds to mesoporous materials according to the IUPAC classification [51]; the same effect of having type IV isotherms with mesopores has been observed in TiO₂ when modified by doping with metals [24,52]. The hysteresis loop for TiXCo nanomaterials is of type H₂, which is characteristic of materials with a pore distribution with wide bodies and narrow necks according to the IUPAC classification [51]. In Figure 8, it is shown that for nanomaterials containing above 5% mol of cobalt, the hysteresis cycle is more pronounced because they have an increase in the narrow distribution of the pore necks, because of the increase in the amount mol% and by the synthesis method. The calculated BET surface area and pore size of the nanomaterials are summarized in Table 3. The incorporation of 1 mol% (Ti1Co) increases the surface area compared to Ti0Co; this same effect has been observed and reported in the literature [24]. However, in nanomaterials above 3% mol of cobalt, a decrease in the specific surface area is observed when compared to Ti0Co (see Table 3), which is attributed to the blocking effect on the TiO₂ pores by the increase in cobalt.

2.9. Photoluminescence (PL)

For the electronic properties, migration, and recombination of charge carriers and nature of defects in materials, it is necessary to perform and study photoluminescence (PL) spectra. Photoluminescence spectra of TiXCo nanomaterials are shown in Figure 9; all measurements were performed with an excitation wavelength of 370 nm at room temperature. Several authors report on the analysis of PL with different excitation lengths [30,31,32,35]. Therefore, in our work, the wavelengths attributed to each of the observed species will suffer a slight shift compared to what was reported and are attributed accordingly to each of them. The assignment of the PL signals obtained from our nanomaterials are assigned (see Figure 9) as follows: the first peak can be observed in all TiXCo nanomaterials located approximately at 418–420 nm, attributed to the excitons located at the TiO₂ band edge; the two signals located in the range of 438–440 nm ARE assigned to self-trapped excitons in TiO₆; the values between 448 and 490 nm are associated with charge transfer from Ti³⁺ to TiO₂ and defects attributed to oxygen vacancies and finally; and the values located between 517 and 520 nm are attributed to F+ centers. Regarding the intensity of the PL spectra (see Figure 9), it decreases according to the following order from highest to lowest: Ti0CO, Ti3Co, Ti1Co, Ti10Co, and Ti5Co. The results obtained according to the reduction in the intensity of the PL spectra indicate that the incorporation of cobalt into TiO₂ inhibits the recombination process of the charge carriers and thus improves the photocatalytic activity of the nanomaterials [30,32].

2.10. Electrochemical Tests

Electrochemical techniques are powerful tools for obtaining information regarding the impact of illumination on the performance of the photocatalysts and their band-edge positions. Figure 10a shows the Nyquist diagrams recorded at open-circuit potential under illumination. Doping with cobalt at 1% mol of TiO₂ (Ti1Co) causes a reduction in the impedance values compared to bare Ti0Co. However, the impedance value continuously increases at higher cobalt contents, hindering the charge transfer reactions at the film/electrolyte interface. It is worth keeping in mind that the generation of value-added products from CO₂ reduction depends on the mechanism of the reduction reaction and not on the rate of the process. On the other hand, photocurrent generation was evaluated using linear sweep voltammetry under chopped illumination, as seen in Figure 10b. TiO₂ doping with cobalt increases the photocurrent generation at low contents, evincing favorable photon harvesting for T1Co. However, at higher cobalt contents, registered photocurrents drop, following a similar trend to what is registered by EIS.

The Mott–Schottky measurements in Figure 10c show that all photocatalysts exhibit a positive slope characteristic of n-type semiconductors [53]. The flat band potential of the TiO₂, derived by extrapolating the linear region towards the potential axis, displaces towards more negative potential values as the cobalt amount increases. This variation indicates that cobalt doping induces energetic states close to the TiO₂ conduction band. The band-edge position of the photocatalysts was schematized in Figure 10d by approaching the flat-band potential to the conduction band of the solid and estimating the valence band position by adding the band gap of the semiconductors [54]. Cobalt doping drives the conduction band toward more reductant potentials, favoring the reduction reaction over the photocatalysts. Meanwhile, the oxidant power of photogenerated holes in the TiO₂ drastically decreases when increasing the cobalt doping, hindering the oxidation reaction of the solid, particularly at the highest cobalt doping amount. The combination of these two effects seems to be well-balanced at lower cobalt doping amounts, which resulted in advantageously obtaining value-added products from the photocatalytic CO₂ reduction.

3. Photocatalytic CO₂ Reduction

Photocatalytic evaluation of TiXCo nanocatalysts is by photoreduction of CO₂ using a UV light source at room temperature for 6 h of reaction and injecting into a chromatograph each hour. Experiments were carried out in which it was verified that the CO₂ reduction reaction does not involve organic remains on the surface of the nanomaterial. For this purpose, experiments were carried out separately: in darkness (the absence of light), without a photocatalyst, in the absence of CO₂, and without a light. The results obtained do not show the generation of any carbon-based by-product after the reaction by organic residues found on the surface of the photocatalyst, as has been previously reported in the literature [24,46,55]. Figure 11a–d show the results of the by-products after photoreduction of CO₂ at 6 h with a UV light irradiation source. The by-products after the photoreduction of CO₂ are methane, ethane, carbon monoxide, and hydrogen. In Figure 11a–d, you can see the photoreduction of CO₂ using Ti0Co (TiO₂) was obtained as a product of the reaction CH₄, H₂, and CO; this can be attributed to the different redox potentials that each species has according to Equations (4)–(6), methane (150.2 mmolg⁻¹_cat), hydrogen (140.1 mmolg⁻¹_cat), and CO (3.9 mmolg⁻¹_cat), according to the literature [8,24,55,56].

$${CO}_2+{8H}^{+}+{8e}^{-} \to {CH}_4+{2H}_{2}O$$

(4)

$${2H}^{+}+{2e}^{-} \to H_2$$

(5)

$${CO}_2+{2H}^{+}+{2e}^{-} \to CO+H_{2}O$$

(6)

The low production of methane as the main product of CO₂ photoreduction is due to the rapid recombination of charge carriers in the reaction because there are no oxygen vacancies or structural defects that function as electron traps, and it also has an intense peak intensity in the photoluminescence spectrum (Ti0Co) compared to other nanomaterials. The intense peak has been reported in the literature to be associated with a high rate in the charge carrier recombination process; therefore, hydrocarbon production is very low using only TiO₂ [28,57].

On the other hand, it can be observed in Figure 11a–d that the CO₂ reduction reaction using nanomaterials with the incorporation of different mol% of cobalt (TiXCo) improves the photocatalytic activity with respect to Ti0CO. It can be observed that in nanomaterials with 1% and 3% mol cobalt, CH₄, C₂H₆, H₂, and CO are generated (see

Table 4. Results after 6 h of the reaction by photoreduction of CO₂ using cobalt-doped TiO₂ nanomaterials (TiXCo), where X = 0, 1, 3, 5, and 10% mol cobalt.

Sample	Methane (mmol g−1cat)	Ethane (mmol g−1cat)	CO (mmol g−1cat)	H2 (mmol g−1cat)
Ti0Co	150.2	-	3.9	140.1
Ti1Co	440.0	14.6	3.1	177.0
Ti3Co	416.5	9.8	3.0	161.5
Ti5Co	354.8	-	2.9	50.5
Ti10Co	345.2	-	3.2	29.8

). In these nanomaterials, according to the results obtained with the different analyses performed, such as DRX-Rietveld, XPS, and UV–Vis, photoluminescence can be correlated to the production of the products by the photoreduction of CO₂. DRX-Rietveld shows that the crystal size of both phases is in the range between 10 and 8 nm; a relative composition between 80 and 20% of anatase and brookite approximately and a modification of the unit cell volume occurs in both phases, causing a distortion of the lattice causing structural defects, so that in this way sites are generated that function as electron traps to improve the photoreduction of CO₂ [27,57]. XPS can be observed (see Table 2) and it was found that Ti 2p species suffer a slight shift at low energy and the relative abundances of each species are affected by the incorporation of cobalt. Additionally, the relative abundance of Co³⁺/Co²⁺ species is in the range of approximately 30–70% and a relative ratio of oxygen vacancies is generated due to the incorporation of cobalt in each of the nanomaterials, all these factors mentioned above cause the recombination rate of the charge carriers to decrease with respect to Ti0Co [28]. Using UV–Vis, it can be observed that materials with 1 and 3% mol have a greater absorption of UV light due to the incorporation of cobalt with respect to Ti0Co. Photoluminescence shows that the incorporation of cobalt modifies the emission spectra, significantly decreasing the peak intensity. This indicates a marked increase in carrier separation efficiency compared to TiO₂ (T0Co). This same effect has been observed using doping of TiO₂ with metals and non-metals [9,24,46,55]. However, when comparing these two nanomaterials that generate the same products, Ti1Co is more active than Ti3Co, due to the lower peak intensity in Ti1Co than in Ti3Co; therefore, it has greater efficiency in the separation and migration of charge carriers (e⁻ and h⁺). Using electrochemical characterization, it can be observed that the conduction and valence band edges are modified by the incorporation of cobalt as a dopant, inducing energy levels below the conduction band, causing them to function as an electron trap and reducing the band gap, as can be seen in Figure 10d. In addition, it can be observed (Figure 10a) that Ti1Co presents a greater decrease in curvature; this indicates that it has less resistance in charge transport, which implies better separation and faster charge transport in the nanomaterial.

For nanomaterials with cobalt content above 5% mol, CO₂ reduction shows that less CH₄, CO, and H₂ are produced, and there is no ethane (see Table 4). Incorporation of cobalt above 5% mol results in modifications to its structural properties, with a decrease in crystal size, crystalline phase composition, and unit cell volume. Using characterization by XPS, it can be observed that the species of Ti⁴⁺, Ti³⁺, and Ti_xO_y, Co³⁺/Co²⁺ are also generated and increase in oxygen vacancies is seen, each of the species mentioned above with its respective relative abundance. Using UV–vis, a decrease in the absorption intensity in the absorption spectra can be observed. Photoluminescence, although at higher content the intensity of the peak, decreases considerably and this results in a more efficient separation of the charge carriers; it implies that these species (observed in XPS) act as recombination centers for the charge carriers and thus reduce the photocatalytic efficiency of the nanomaterial.

Additionally, CO₂ photoreduction experiments were performed to observe the effect of a light source in the visible region (Argon lamp) on TiXCo nanomaterials (same reaction conditions). Results obtained are shown in Table S1, which is included in the Supplementary Information.

The results (see Table S1) show low photoactivity in the photoreduction of CO₂ using the visible light source in the production of methane, ethane, CO, and hydrogen when compared to the results obtained with the UV light source after 6 h of reaction. The low photoactivity of TiXCo nanomaterials using a visible light source is attributed to the fact that the abundance of the Ti³⁺ species decreases as a function of the cobalt content, thus modifying the defects that act as an electron trap during the photocatalytic process and additionally showing a higher resistivity, as shown in Figure 10a. Despite the observations in Figure 4a, where the nanomaterials show increased photon absorption in the visible region, a shift in the band gap (E_g), and decreased photoluminescence as a function of cobalt content (see Figure 9), they are not ideal for CO₂ reduction to obtain methane, ethane, and CO using visible light.

Proposed Reaction Mechanism

Based on the results obtained from the various characterizations described above and the evaluation of the photocatalytic activity, a mechanism by which CO₂ photoreduction with water is carried out on the synthesized nanomaterials can be proposed. The possible pathways that occur during CO₂ photoreduction in our work are through the following reactions [7,10,28,58]:

$${TiO}_2 +hv \to Ti{O}_2{e}^{-} + {Ti{O}_{2}h}^{+}$$

(7)

$${Co}^{3+}+Ti{O}_2{e}^{-} \to {Co}^{2+} +{TiO}_2$$

(8)

$${Co}^{2+}+Ti{O}_2{h}^{+} \to {Co}^{3+} +{TiO}_2$$

(9)

$$2H_{2}O+{4h}^{+} \to O_2+ {4H}^{+}$$

(10)

$${2H}^{+}+{2e}^{-}\to H_2$$

(11)

$$2C{O}_2+2e^{-} \to 2·{CO}_2^{-}$$

(12)

$${·CO}_2^{-} +{·CO}_2^{-}+4H^{+} \to 2CO+2H_{2}O$$

(13)

$$CO+{2H}^{+}+{2e}^{-}\to ·C+H_{2}O$$

(14)

$$·C+{4H}^{+}+{4e}^{-}\to C{H}_4$$

(15)

$$C{O}_2+{8H}^{+}+{8e}^{-}\to C{H}_4 +{2H}_{2}O$$

(16)

$$2C{O}_2+{14H}^{+}+{14e}^{-}\to C_2{H}_6 +4H_{2}O$$

(17)

$$C{H}_4+h^{+} \to ·C{H}_3 +H^{+}$$

(18)

$$·C{H}_3+·C{H}_3 \to C_2{H}_6$$

(19)

For the formation of products such as CO, CH₄, and C₂H₆ during the CO₂ reduction period, the reactions described above (7)–(19) occur and it is necessary to provide the necessary number of electrons for their formation, two, eight, and fourteen electrons individually, according to the literature [10,24,28]. Another route to obtain CO has also been reported in the literature, which involves the oxidation of CO₂ (Equation (12)) and the radical ∙CO is produced and, subsequently, with the oxidation reaction (Equation (13)) CO is produced. In Figure 11a, it can be observed that in Ti1Co there is an increase in the production of methane and a slight decrease in the production of CO; this effect has been observed in the literature and is associated with the fact that there is another reaction route (Equations (14) and (15)) by which methane is produced, which consists of the reduction of CO for the formation of more radicals •C with the general consumption of six electrons for the production of methane [24,59,60]. Another alternative reaction route to promote the production of C₂H₆ are reactions (Equations (18) and (19)) that consist of the oxidation of methane to produce the methyl radical (∙CH₃) and, subsequently, two methyl radicals react and form ethane. Therefore, in all reactions, the electrons (conduction band) and holes (valence band) that are photogenerated when light is absorbed by the nanomaterial (Ti1Co) are important since they play an important role in the oxidation and reduction reactions that promote the production of CO, CH₄, and C₂H₆ [58].

In the literature [27,55] and in our work, it is observed that hydrogen production (H₂) is also an additional product in the CO₂ reduction reaction. Hydrogen production is a reaction present in the photoreduction of CO₂. However, in Figure 11d it is observed that hydrogen production follows the following order: Ti1Co > Ti3Co > Ti0Co > Ti5Co > Ti10Co; in the first two nanomaterials, there is a greater yield of products derived (CH₄ and C₂H₆) from the CO₂ photoreduction than Ti1Co and Ti3Co. The above can be attributed to the fact that the oxygen vacancies in the different nanomaterials, the Co³⁺/Co²⁺ species, and the different Ti⁴⁺, Ti_xO_y, and Ti³⁺ species function as recombination centers and accelerate the recombination process of the charge carriers, despite the fact that the evidence in the photoluminescence spectra shows that they have a relatively low intensity compared to Ti0Co. Figure 12 shows the proposed mechanism by which CO₂ photoreduction is carried out using the Ti1Co nanomaterial, since it was the most active for the production of CH₄ and C₂H₆. Figure 12 shows a mixture of TiO₂ phases (anatase and brookite) modified by the addition of 1% mol cobalt, which presents a heterojunction between both phases. Additionally, by incorporating 1% mol of cobalt, oxygen vacancies are generated and there is a relative abundance of Co³⁺/Co²⁺ species; both species present generate energy levels below the TiO₂ conduction band. After the nanomaterial (Ti1Co) is irradiated with a light source that is equal to or greater than the band gap, many charge carriers (e^-) and holes (h⁺) are generated in the conduction band and valence band, respectively. The species found in the Ti1Co nanomaterial have the function of trapping electrons and thus slow the recombination process of charge carriers and improve the photocatalytic performance in the CO₂ reduction reaction e^-.

4. Materials and Methods

4.1. Materials

The reagents used are titanium isopopoxide (>99.9%), cobalt (III) acetyl acetonate (>99.9%), and hydrochloric acid (HCL > 38%), and were purchased from Sigma-Aldrich Merck KGaA, Darmstadt, Germany and/or its affiliates, (Merck Mexico a subsidiary of Merck KGaA, Darmstadt, Germany, Calle 5, No. 7, Frac. Industrial Alce, Blanco Naucalpan de Juarez, 53370, Mexico; deionized water is used in the synthesis and in all experiments in this work.

4.2. Synthesis of Nanomaterials

The synthesis of the different nanocomposites is by the sol–gel method. The preparation of the reference and the modified TiXCo is carried out as follows: In a flask in an inert atmosphere, 40 mL of isopropanol and 10 mL of isopropanol are added, and 30 mL of titanium isopropoxide are added to a flask. This solution is left stirring for 1 h at room temperature to obtain a homogeneous mixture (step A).

The flask is then transferred to a heating mantle and left under reflux for 24 h with stirring throughout the synthesis of the nanomaterial. At the end of the time period, it is allowed to cool to room temperature to immediately incorporate the amount of alkoxide/isopropanol/water and have a ratio of 1:16:8 (step B). It is then refluxed again for another 24 h. At the end of this time, it is filtered and dried in an oven at 120 °C for 12 h for subsequent grinding, storage, and labeling. For nanocomposites with different mol% of cobalt, the same step A is carried out, and the calculated amount of cobalt precursor is added (1, 3, 5, and 10% mol) and the same time is left to obtain a uniform and reflux mixture. The procedure has the same alkoxide/propanol/water ratio and reflux, to subsequently grind, store, and label. The nomenclature used for the TiO₂- and TiO₂-doped photocatalysts with different mol% of cobalt is TiXCo, where x represents 0, 1, 3, 5, and 10% mol of cobalt. The photocatalysts were subsequently calcined at 400 °C for 4 h.

4.3. Physicochemical Characterization

The structure and crystalline phase of the photocatalysts were identified by diffraction patterns measured at room temperature on a Bruker D8 Advance diffractometer (Bruker AXS SE Östliche Rheinbrückenstr. 49, 76187 Karlsruhe, Germany) with a Cu irradiation source with a Kα (λ = 0.15406 nm) in the range of 10–80° as a function of 2θ. The diffraction structures were refined using the Rietveld method, using the fundamental parameters during the refinements and implementing the TOPAS version 4.2 code, using a step of 0.020 °/s in the range 10–100° as a function of the 2θ angle.

The chemical compositions of the photocatalysts were evaluated by X-ray photoelectron spectroscopy (XPS) using a PHI 5000 Versa Probe II from Physical Electronics (ULVAC-PHI, Inc. 2500 Hagisono, Chigasaki, Kanagawa, 253-8522, Japan). A monochromatic aluminum irradiation source with a Kα (hν = 1486.6 eV) was used during the analysis. The C 1s signal is located at 245.50 eV and is used as a reference for the other binding energies of the other atoms.

Raman spectroscopy spectra were measured in the range of 100–1000 cm⁻¹ and collected using a Renishaw Invia model with a 532 nm green laser, operating at 0.5% of the laser power, with 10 accumulations of 10 s each.

Absorption spectra and optical properties for all photocatalysts were measured in the range of 190–800 nm using diffuse reflectance spectroscopy. The Kubelka–Munk theory was used to calculate the band gap of nanomaterials using diffuse reflectance spectra measured on a Varian Cary 100 spectrophotometer (Agilent Technologies, Inc. Headquarters 5301 Stevens Creek Blvd, Santa Clara, CA 95051, USA) equipped with an integrating sphere, using Ba₂SO₄ as a reference. The Tauc’s plot was used to calculate the band gap using data from the Kubelka–Munk theory.

Morphological aspects of the photocatalysts were observed by taking images acquired by a field-emission scanning electron microscope (SEM) using JEOL 7600F equipment (JEOL USA, INC. 11 Dearborn Road, Peabody, MA 01960, USA) and, simultaneously, EDS was performed for the evaluation of the composition of the different elements contained in the photocatalysts.

TEM and HRTEM images of the photocatalysts were obtained using a JEOL-JSM7600-F transmission electron microscope ((JEOL USA, INC. 11 Dearborn Road, Peabody, MA 01960, USA). The TEM and HRTEM images were analyzed using DigitalMicrograph 3.6 software. For TEM images, approximately 150 particles were counted to obtain an approximation of the crystal size, and for HRTEM images, different areas of the image were analyzed to obtain the crystalline planes of the different crystalline phases in the material.

The specific surface area of the photocatalysts was determined by the BET method using nitrogen adsorption–desorption isotherms at 77 K. The analyses were performed on a Tristar II Plus Micromeritics instrument (Headquartered in Norcross, GA, USA). Before starting the BET analysis, the samples were degassed at 200 °C for 24 h.

Photoluminescence spectra of the photocatalysts were measured on an Edinburgh Instruments FSP920 Fluorometer with detection in the visible (200–750 nm) through a Hamamatsu R928P photomultiplier tube and a 450 W Xenon lamp (Head office and enquiries Edinburgh Instruments Ltd. 2 Bain Square Kirkton Campus EH54 7DQ UK). Emission spectra were obtained in the range of 400–700 nm using an excitation wavelength of λ_ex = 350 nm.

4.4. Electrochemical Characterization

TiXCo photocatalysts were evaluated through electrochemical techniques to obtain information regarding the charge-transfer resistance under illumination, photocurrent generation, and band-edge position. For this purpose, TiXCo powders were supported by FTO following the procedure previously reported [61]. In brief, 10 µL of a 20 gL⁻¹ suspension of the photocatalyst in an ethanol/Nafion solution was drop-cast onto a delimited area of 0.5 × 0.5 cm² of the substrate and left to dry at room temperature overnight. The (photo)electrochemical characterization was performed in a three-electrode cell with a quartz window to illuminate the film employing Newport Q Housing (Model 60025) equipped with a 100 W Hg arc Xe lamp (Newport Corporation 8 East Forge Parkway Franklin, MA 02038 United States. A graphite bar (99.999%) was used as the counter-electrode, and Ag/AgCl (3 M NaCl) as the reference electrode. All measurements were performed in a 0.1 M K₂CO₃ (pH = 5) electrolyte. The potential reported in this document was not corrected for ohmic losses. The EIS was measured at the open-circuit potential under illumination. For this, the sample was irradiated until a pseudo-stationary open-circuit potential was reached. Then, the spectra were recorded by applying a sinusoidal AC perturbation of ±10 mV between 100 kHz and 100 mHz. Interrupted-light linear swept voltammetry was recorded (v = 10 mVs⁻¹) in the backward direction from 0.7 V to −0.1 V with a light-on and light-off period of 1 s. Mott–Schottky plots were obtained by measuring the space-charge capacitance at 25 mV intervals using an AC perturbation of ±10 mV at 400 Hz.

4.5. Photocatalytic Tests in CO₂ Photoreduction

Experiments on the photocatalytic reduction of the different TiXCo photocatalysts were evaluated using a photoreactor (made in the laboratory) with a capacity of 200 mL and placed in a black box with the respective stirring grid. The reaction mixture contained 0.25% methanol, and the remainder was water; this mixture was bubbled for 30 min with nitrogen to remove air inside the reactor. Subsequently, 50 mg of photocatalyst (nanomaterial) was added to the reactor and subsequently purged with CO₂ for 30 min to saturate the solution in the photoreactor. The container was sealed and subsequently irradiated with a UV light source with a wavelength of 254 nm for 6 h of reaction and the entire experiment was maintained at 25 °C. The products of CO₂ photoreduction were measured at 1 h intervals until completing 6 h of reaction by injecting 1 μL of the mixture into a Shimadzu GC 2014 chromatograph (Shimadzu Inc. Analytical & Measuring Instruments Division, International Operations Department 1-3, Kanda Nishiki-cho, Chiyoda-ku, Tokyo 101-8448, Japan), equipped with a thermal conductivity detector (TCD), flame ionization detector (FID), and metallizer at 380 °C. Subsequently, CO₂ photoreduction tests were performed using a visible light source (Argon lamp (UVP Pen-Ray Light Sources) Analytik Jena US LLC (Formerly UVP LLC) 2066 W. 11th Street Upland, CA 91786, USA) under the same reaction conditions, and the by-products were identified using gas chromatography, as previously described.

5. Conclusions

Nanomaterials doped with different mol% of cobalt (TiXCo) were synthesized by the sol–gel method and were tested in the photoreduction of CO₂ with H₂O under a UV irradiation source for 6 h of reaction. The results obtained show that when using the Ti1Co nanomaterial, the production of CH₄ (440.0 mmol g⁻¹_cat), C₂H₆ (14.6 mmol g⁻¹_cat), and CO (3.1 mmol g⁻¹_cat) is achieved and when compared with Ti1Co, is approximately 3 and 14 times greater than with Ti0Co since it only produces CH₄ (150.2 mmol g⁻¹_cat) and CO (3.9 mmol g⁻¹_cat) and does not present production of C₂H₆. Ti1Co nanomaterial significantly presents a better activity in CO₂ reduction; this is because the incorporation of 1% mol of cobalt introduces energetic states below the conduction band that are produced by oxygen vacancies and the redox properties of Co³⁺/Co²⁺, which function as an electron trap and in this way efficiently improve the separation of charge carriers and present a greater response under UV light. Therefore, the incorporation of transition metals is a good option to improve photoactivity in TiO₂-based nanomaterials.