Research Article | Open Access
M. V. Nogueira, G. M. M. M. Lustosa, Y. Kobayakawa, W. Kogler, M. Ruiz, E. S. Monteiro Filho, M. A. Zaghete, L. A. Perazolli, "Nb-Doped TiO2 Photocatalysts Used to Reduction of CO2 to Methanol", Advances in Materials Science and Engineering, vol. 2018, Article ID 7326240, 8 pages, 2018. https://doi.org/10.1155/2018/7326240
Nb-Doped TiO2 Photocatalysts Used to Reduction of CO2 to Methanol
In pursuit of higher photoactivity, Nb-doped TiO2 powders were evaluated in the reduction of CO2. The replacement of Ti by Nb in the crystalline structure of TiO2 promoted methanol formation. Nb-doped TiO2 powders were successfully synthesized in Nb concentrations of 0.0, 0.5, 1.0, and 2.5% (w/w = weight/weight) using the Pechini method. The materials were calcined at 500°C for two hours to promote the formation of the anatase crystalline phase. After characterization, the powders were modified through an Nb0 magnetron sputtering deposition using a metallic target in vacuum conditions of 2 × 10−3 torr, with a deposition time of 10 minutes, and calcination again at 500°C for two hours. The resulting powders showed a surface area up to 30 m2/g. The Pechini method promoted the substitution of Ti4+ for Nb4+ as observed using XRD and XPS techniques at the crystalline structure and at the surface of the powder. Furthermore, the presence of Nb0 was also observed at the powder’s surface. The presence of Nb in the crystalline structure increased the photoactivity of powders when compared to nonmodified TiO2 powders, while the Nb0 deposition at the powder’s surface decreased the photoactivity for all the investigated compositions.
The development of photocatalysis began in 1972 , aiming to create efficient water treatment systems and to produce H2 to use as fuel. Various researchers have conducted investigations in many different fields, such as semiconductor photoelectrochemistry , photocatalysis [3, 4], and photoreduction of CO2 in aqueous environments, to produce methanol . The TiO2 produced by Evonik, known as TiO2–P25 (consisting of 75% anatase phase and 25% rutile phase), is the mostly used for this purpose due to its excellent photoactivity and is also used as a reference to compare results with others news materials. TiO2–P25 has high surface area (∼50 m2/g), and its complex crystalline microstructure is directly related to the synthesis method .
The photocatalysis process consists in the exposition of semiconductor powder to ultraviolet light radiation which will create then the electron-hole pair from the electronic excitation. The hole (h+) is formed in the valence band, and the electrons (e−) are formed in the conduction band. These species can react with adsorbed water or hydroxyl group and produce hydroxyl () radical and superoxide anion () radical, starting an oxidation and reduction process and then degrading the component of interest [7–9].
The addition of transitional metals to TiO2 seeks to increase the recombination time of electron/hole pair during electronic excitation [10, 11]. It is believed that these transitional metals create acceptor/donator electron centers, influencing the electronic recombination. Perazolli et al.  synthesized a ternary mixture of Ti, Sn, and Ag oxides that obtained better Rhodamine B discoloration results than P25 due to the heterojunction TiO2/SnO2 and formation of metallic Ag reduced from Ag2O increasing the recombination time. The utilization of heterostructures has been studied in degradation of organic compounds. Through the solvothermal synthesis, Li et al.  synthesized pure anatase, pure rutile, and mixed anatase-rutile cake-like particles and observed that anatase-rutile particles reached higher photocatalytic activity compared to powders of pure phases (anatase and rutile) and also higher than P25. In the work of Zhou et al. , heterostructures of CdS/TiO2 were synthesized in different concentrations (wt.) of CdS and then used in degradation of methyl orange dye. It was observed that the photocatalyst of concentration 2% (wt.) reached total degradation of dye in 75 minutes, a higher activity when compared to pure TiO2 that reached only 90% of degradation at the same time, showing that the heterojunction promotes better charge separation and increases the recombination time of electron-hole pairs (e−/h+). Through the hydrothermal synthesis, Tao et al.  deposited the heterostructures of TiO2/ZnO/Ag on FTO substrate where Ag0 acts like antennae capturing the electrons photogenerated and then increased the recombination time for improving the degradation of methyl orange dye. The photocatalyst TiO2/ZnO/Ag reached 96% of degradation of dye in 120 minutes, better than the results of pure TiO2 (46%) and of TiO2/ZnO heterostructure (62%) at same conditions.
Research of efficient processes in the conversion of CO2 into methanol, ethanol, CH4, H2, and other compounds has been made since the late 1980s . The use of Pd as a dopant in TiO2 was evaluated on CO2 hydrogenation under UV irradiation generating the amount of 355.62 µmol CH4/g-cat by the increase of recombination time of generated electrons and adsorption and activation of CO2 molecules on surface of catalyst . Cheng et al.  used Cu2+–TiO2 nanorod thin film photocatalysts reducing CO2 into methanol and ethanol obtaining, respectively, 36.18 µmol/g-cat·h and 79.13 µmol/g-cat·h in which Cu2+ acted as active sites of electron traps and could suppress the electron-hole recombination allied to high surface area of TiO2 nanorods. Pan et al.  produced Pt nanoparticle-dispersed gallium oxide (Pt/Ga2O3) catalysts calcined in different atmospheres at 600°C in comparison with Pt/P25; the oxygen vacancies promoted better CO2 adsorption leading more efficient separation of the photo-induced electron-hole pairs and reaching 2.1 and 1.9 µmol CH3OH using Pt/Ga2O3 and Pt/P25, respectively.
The use of a substituent that has a higher electron valence number than Ti4+, as Nb5+ an example, can lead to a level of electron donation that behaves like an extrinsic n-type semiconductor, which can act in the photoreduction reaction. One way to introduce the elements of the TiO2 ceramic matrix in order to improve the photocatalytic properties by substitution of Ti4+ cation for Nb5+ is the chemical synthesis through Pechini’s method. This method is useful to obtain ceramic powders since it is possible to control parameters as composition and crystalline phase through the immobilization in a complex organic matrix and controlled calcination, decreasing the segregation of metals and then providing homogenous distribution of the components [20, 21]. The sputtering method is a powerful tool of deposition of metals on surfaces and can be used in superficial modification of catalysts improving optical and electrical properties [22, 23].
Brazil has the biggest reserve of niobium of the world, possessing 98% of world niobium ore reserves . This work chose niobium due to the abundance and low cost of niobium (Nb2O5, Nb0) allied with the advantages brought with the structural modification by doping (n-type extrinsic semiconductor/excess of electrons and defects generation) and superficial modification (oxygen vacancies and formation of antennae) both improving charge separation and increasing the electron-hole pair recombination time. The goal of this research was to synthesize and characterize TiO2-based photocatalytic powders modified with Nb in two ways: into the crystalline structure Nb4+ (by chemical synthesis Pechini) and upon the surface Nb0 (through sputtering deposition), as well as aiming at improving the photocatalyst properties by combining these two methods (the novelty). The concentration of dopants from 0.0 to 2.5% (w/w) was adopted to guarantee the formation of a solid solution and avoid the appearance of other phases . It also intended to verify the photocatalytic activity of these powders in the reduction of CO2 to methanol.
2.1. Synthesis of Photocatalytic Powders
In this work, we prepared two polymer precursor solutions through the Pechini method:(a)The titanium polymeric solution was obtained by slow addition of titanium isopropoxide (Aldrich) in ethylene glycol (Synth) at 80°C with constant stirring, then the heating was raised to 110°C, and the citric acid (Carlo Erba) was added. The molar ratio for the preparation of this stock titanium solution was 1 : 4 : 16 (metal : citric acid : ethylene glycol).(b)The niobium polymeric solution was obtained by dissolution of niobium oxide (Nb2O5) (Carlo Erba) in hydrofluoric acid aqueous solution under stirring and heating (80°C). The pH of the solution was increased to 8 by addition of ammonium hydroxide (Synth), causing precipitation as hydroxide of niobium [Nb(OH)5]. The precipitate was washed with distilled water to eliminate fluoride ions (F−) until the test with calcium carbonate (CaCO3) to give negative fluoride ions. The niobium hydroxide was dissolved in aqueous solution of citric acid under stirring and heating (±80°C), followed by addition of ethylene glycol. The molar ratio for the preparation of stock niobium solution was 1 : 4 : 16 (metal : citric acid : ethylene glycol) . The niobium and titanium solutions were standardized gravimetrically at 900°C for 2 hours.
After preparation of polymeric solutions, new solutions were obtained from the mixture of the previously prepared solutions according to addition of niobium: 0, 0.5, 1, and 2.5% (w/w) and then submitted to calcination in a muffle furnace at 500°C for 1 hour and milling by 500 rpm for 1 hour.
For superficial decoration by deposition of Nb0 through sputtering, circular pellets of 1.0 g of each powder were prepared using a hydraulic press (BOVENAU-P15500) applying a load of 2500 kg for 5 minutes and then introduced into the deposition chamber of sputtering (Sorensen DCS 600–1.7) for deposition according to the following conditions: metallic spherical target of niobium, time of deposition of 10 minutes, vacuum of 2 × 10−3 torr with argon gas plasma, potential of 370 V, and a current of 0.055 A . The pellets were macerated into powders again and submitted to heat treatment at 500°C for 1 hour promoting a more effective adhesion of the metallic Nb particles on TiO2 surface obtaining the powders modified superficially by sputtering.
2.2. Sample Characterization
The powders were analyzed utilizing TGA/DTA (Netszch Thermische Analyse-STA 409) to evaluate the temperature in which all organic matter can be removed. Furthermore, phase transits from anatase to rutile could be detected using this method.
In order to investigate the crystallinity of the material and identify the present phases, X-ray Diffraction (XRD) and the Rietveld method (XRD-Rigaku Rint-2000) with radiation source of copper emission line (Cu Kα, λ = 0.154 nm) and voltage acceleration of 42 kV, 120 mA current, and scan rate of 2θ min−1 were used.
The Raman spectroscopy was adopted to evaluate the different phases present in the powders utilizing HORIBA-HR 800, with laser 632.8 nm. Infrared spectroscopy measurements were made in Shimadzu FTIR 8300 using the transmission technique (resolution: 4 cm−1, spectral range: 4000 to 400 cm−1, and 50 scans). The XPS technique (UNI-SPECS UHV) was used to analyze surface changes in the chemical composition due to the substitution to Nb.
The spectrophotometer Varian Cary model 500 was used for quantification of the bandgap values. The SBET technique was used to verify the specific surface area (Micrometrics ASAP 2010). The particle size of the powder and its morphology were evaluated in a field emission scanning electron microscopy (FE-SEM JEOL 7500F model).
2.3. Photocatalytic Activity
The photocatalytic activity of reduction was carried in a reactor (Heraeus) made out of quartz tube housing for the cooling of the mercury UV lamp of 150 W. In this reactor, a magnetic stirrer, a CO2 bubbling tube, an inlet and outlet for water cooling, and a sample collector tube were coupled as illustrated in Figure 1.
The volume of 700 mL of deionized water was used, and the pH was increased to 12 with NaOH. Previously a blank test (photolysis) was made using no photocatalyst, and posteriorly in each solution, 0.350 g  of each photocatalytic powder was added and the reactions had the duration of 6 hours with removal of aliquots every 30 min that were analyzed through gas chromatography (GC-FID) in the chromatograph Varian CP-3800 with a flame ionization detector in column (Stabilvax-Restec 30 m, 0.25 mm inner diameter) using the technique SPME (solid phase microextraction) that adsorbs volatile organic compounds of aqueous samples on fiber (75 m Carboxen/PDMS, SUPELCO) by heating (headspace method) and flow rate N2 1.0 mL·min−1. To quantify the methanol in the samples, aliquots of 0.5 mL were transferred to 1.5 mL vial and heated for 7 minutes, then the fiber was exposed to the vapor formed for 5 minutes to promote deadsorption, and finally, the fiber was injected into GC.
3. Results and Discussion
3.1. TG/DTA Characterization
Figure 2 showed the results from the photocatalytic powder Nb–TiO2 with 2.5% Nb (w/w). The TG curve of the precalcined powder (“puff”) Nb–TiO2 with 2.5% Nb (w/w) showed a weight loss at two distinct times. The first loss occurred between 30°C and 120°C and represented 4% of the initial weight and was attributed to water loss. The second loss, 40% of initial weight, was observed between 250°C and 480°C and was attributed to the calcination of organic precursors (citrates). There was no significant mass variation over 500°C. The DTA curve showed two significant thermal events, represented at first by an endothermic peak of around 100°C and characteristic of dehydration of the sample, and secondly by an exothermic peak at 413°C, attributed to the decomposition of organic matter. Therefore, in this experiment, the final temperature of 500°C was adopted because it guarantees total elimination of organic matter in the powders.
3.2. XRD and Rietveld Characterization
The XRD (Figure 3) indicates that the photocatalytic powders are mainly made of the anatase crystalline phase in comparison to characteristic peaks reported in the standard (R) JCPDS-73-1764. However, a peak at 2θ = 27.5°, characteristic of the rutile crystalline phase, was observed. This confirms their presence due to the applied calcination temperature of 500°C, which is sufficient to cause partial phase transition to the rutile phase.
Another fact observed in the XRD patterns was the gradual shrinkage of all of the peaks with increase of Nb concentration in TiO2 powder, attributed to the substitution of Nb in the TiO2 crystalline lattice. Thus, considering the peak 2θ = 48° plane (2 0 0), it is found that the surface modification did not change the results obtained by XRD. Another effect observed by increasing the concentration of Nb was the leftward shift of the Δ2θ° peaks. These were justified by the distortion in the crystalline TiO2 lattice generated by vacancies, interstitials, and substitutions  and by the difference of cationic rays in Ti4+ (0.74 Å) substituted by Nb4+ (0.82 Å), which reinforces the results observed in the TG/DTA analysis described previously.
Figure 4 illustrates the refined XRD pattern through the Rietveld method which can be observed beyond the XRD curves. The overlapping calculated setting curves show the positions of the Bragg peaks in the rutile and anatase crystalline phases in the samples and, further below, the residual adjustments.
The results obtained by Rietveld and shown in Table 1 indicate that the percentages of the anatase phase increased from 91.76% to 98.27% in photocatalytic powders Nb–TiO2 0.0% (w/w) and Nb–TiO2 2.5% (w/w), respectively. The percentage of the rutile phase decreased from 8.24% to 1.73%, indicating that the increase in the concentration of Nb in the photocatalytic powders acts as a rutile phase inhibitor.
Considering the change in values of cell parameters (a, b, c) of anatase and rutile crystals that was caused by the replacement of Ti for Nb and comparing them to the Nb–TiO2 photocatalyst 0.0% (w/w), which does not contain Nb, it was observed that there is a decrease in cell volume ranging from 133.74 Å3 to 133.53 Å3 in Nb–TiO2 0.5 Nb% (w/w) and Nb–TiO2 2.5% Nb (w/w), respectively. This implies that Nb substituted in octahedral sites of this phase promotes the compression of the individual cell.
3.3. IR and Raman Characterization
The results obtained by IR spectroscopy (Figure 5) indicated that the photocatalytic powders show characteristic bands (3450 and 1640 cm−1) attributed to the presence of water in atmospheric humidity. The O-Ti-O bonds are attributed to strong band at 979 cm−1 and the Ti-O-Ti bonds to weak band at the range of 800–465 cm−1. These bands are characteristics of the TiO2-based photocatalysts. It was also observed that the presence of CO2 at 2349 and 1337 cm−1 and CO at 2070 cm−1 is due to the capacity of the photocatalytic powders to adsorb some compounds that come from the decomposition of organic precursors present in the “puff,” adsorbed in the calcination process. The comparison of the photocatalytic powders and commercial anatase TiO2 powder in IR analysis confirms that all organic matter was eliminated after heat treatment.
Figure 6 illustrates the Raman spectra of the photocatalytic powders. It is possible to notice that the characteristic peaks matched the theoretical frequencies of TiO2 anatase and that these results also matched with the XRD diffractograms. In contrast, there was a reduction in the intensity of the characteristic peaks at 395, 515, and 638 cm−1, respectively, while the concentration of Nb increased in the photocatalytic powders. This indicated the incorporation of Nb into the TiO2 lattice, which confirmed the results obtained by XRD and Rietveld.
3.4. Diffuse Reflectance and SBET Characterization
The results of diffuse reflectance spectroscopy using Tauc extrapolation  showed values from 3.33 eV to 3.38 eV for the photocatalytic powders Nb–TiO2 0.0% (w/w) and Nb–TiO2 2.5% (w/w), respectively, indicating that the addition of Nb to the TiO2 lattice did not increase the bandgap value significantly. The diffuse reflectance results of all photocatalytic powders analysed were similar and close to the value of 3.38 eV, which indicates that the photocatalytic powders modified through sputtering did not able to promote and/or increase the formation of new defects near the conduction band of the photocatalysts when compared with the photocatalysts not modified through sputtering  (Figure 7).
The SBET surface area results for the photocatalytic powders are shown in Table 2. The results indicate a gradual increase from 22.6 to 31.6 m2·g−1 in photocatalytic powders Nb–TiO2 0.0% Nb (w/w) to Nb–TiO2 2.5% Nb (w/w), respectively. A similar behavior was observed in photocatalytic powders modified by deposition of Nb0 through sputtering. A slight increase in the surface area of photocatalytic powders was observed, which is justified by the presence of deposited Nb0, which can act as a deagglomerator of particles due to the electrostatic repulsion effect, which increases the surface area.
SPT: modification through sputtering.
3.5. SEM Characterization
The SEM images obtained of the photocatalytic powder Nb–TiO2 0.0% Nb (w/w) without modification, by deposition of Nb0 through sputtering, are illustrated in Figure 8. All the compositions (with or without surface modification) showed similar microstructures. It was found that the photocatalytic powders are formed by plates of irregular shapes and sizes, ranging from 1 to 7 µm as seen in Figure 8(a). They are composed of clusters of rounded particles with diameters between 11.3 and 16.2 nm, as shown in Figure 8(b).
3.6. XPS Characterization
The XPS technique was used to analyze the chemical surface structure of the photocatalytic powders and verify changes in chemical composition that happened due to the substitution of Ti for Nb. In the XPS spectra on the Ti2p transition, Figure 9(a), the oxidation state of titanium Ti4+ is represented by the characteristic peak of Ti2p (458.5 eV) and the Nb substitution not resulted in modification of the TiO2 oxidation original state. Regarding Nb (Figure 9(b)) represented by characteristic peaks NbO2 3d3/2–Nb4+ in 209.5 eV, NbO2 3d5/2–Nb4+ in 206.7 eV, and Nb2O5 in 207.4 ± 0.4 eV, it was observed that the oxidation state of niobium in the photocatalytic powders is Nb4+. However, this paper was based on the fact that the Nb replaced in the TiO2 crystalline lattice is found in the oxidation state 5+. If that should happen, an electron donor level could be generated in the forbidden region of the bandgap, which would be easily removed, becoming a free electron promoted and used in the reaction of CO2 reduction.
The substitution of Ti4+ for Nb5+ is a subject for further studies which will evaluate the influence of temperatures higher than 500°C in the presence of an oxidizing atmosphere and the Nb oxidation state behavior replaced by TiO2 lattice. Furthermore, the phase transition anatase/rutile seeking favorable conditions for the formation of Nb5+ and avoiding the formation of the rutile crystalline phase as much as possible will be observed.
3.7. CO2 Reduction Assays
The results of the CO2 reduction and the formation of methanol, as shown in Table 3, indicate that the addition of structural Nb favored the formation of methanol. It was found that the photocatalytic powder Nb–TiO2 0.0% Nb (w/w) did not promote the formation of methanol, and the surface modification through sputtering showed lower results in comparison with the powders not modified by this technique. The concentrations of methanol obtained can be explained by XPS wherein Nb replaced was Nb4+ instead of Nb5+. If the replacement by Nb5+ occurs in the lattice of TiO2, this replacement can lead to a level of electron donation that could be used in reduction reaction; however, the substitution by Nb4+ can act in the reduction reaction in the same way, but with lower activity expected, once there is no remaining electron. The concentration of methanol formed is directly proportional to the Nb concentration in the powders, and the formation of methanol is due to the presence of Nb in the lattice of TiO2. Concerning the formation of metallic antennae, the reaction of reduction occurs in some preferential sites, and those sites can be covered by the sputtered antennae decreasing photocatalytic activity and the methanol formation when compared with same powders not modified through sputtering. The yield of methanol reached by catalyst Nb–TiO2 2.5% (w/w) (1.00 µmol/g-cat·h) is superior in comparison with all the catalysts but P25 (1.83 µmol/g-cat·h), demonstrating the doping with Nb4+, has potential effect in formation of defects which increases the electron-hole pair recombination time improving the catalytic properties of TiO2.
As discussed above, the formation of methanol is directly proportional to Nb concentration which can open new promising researches to investigate higher concentrations (above 2.5%) of Nb that can reach yields of methanol equal or higher than P25.
We obtained nanostructured TiO2 photocatalytic powders structurally modified with Nb using the Pechini method and superficially modified with Nb0 through sputtering. The photocatalytic powders are mainly formed by the anatase phase of TiO2. Utilizing the Rietveld technique, the Δ2θ° displacement peaks, observed by the XRD measurement, confirmed the replacement of Ti for Nb in the TiO2 lattice. Using SEM, the experiment investigated the morphology of the photocatalytic powders, which were arranged in plates formed by nanosized particles. The SBET technique showed that the surface area of the photocatalytic powders tends to increase with the concentration of Nb. Followed by the reduction of agglomerate formation. The diffuse reflectance technique quantified the average value of bandgap at 3.35 eV. The XPS indicated that the oxidation state of Ti and Nb in the photocatalytic powders is 4+. The substitution to Nb0 through sputtering does not increase photoactivity compared to photocatalytic powders with the same concentration of Nb not modified through sputtering. This is justified by the metallic particles of Nb that assist in the transportation of photogenerated electrons. The formation of methanol was proved by GC-FID in which concentrations up to 1.00 µmol/g-cat·h were obtained by the replacement of Ti4+ by Nb4+ in the lattice of TiO2. Future investigations about higher concentrations of Nb can lead to reach yields of methanol equal or higher than P25.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors thank the LMA-IQ for providing the FEG-SEM facilities and also thank the Brazilian research funding agencies CAPES, CNPq, and FAPESP (2008/544136-R) and FAPESP-CDMF (2013/07296-2) for providing the financial support of this research project.
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