Revisiting Pt/TiO2 photocatalysts for thermally assisted photocatalytic reduction of CO2

Fei Yu a, Changhua Wang *a, He Ma a, Miao Song b, Dongsheng Li b, Yingying Li a, Songmei Li a, Xintong Zhang *a and Yichun Liu a
aKey Laboratory of UV-Emitting Materials and Technology of Chinese Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China. E-mail: wangch100@nenu.edu.cn; xtzhang@nenu.edu.cn
bPhysical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99352, USA

Received 15th November 2019 , Accepted 7th February 2020

First published on 11th February 2020


Artificial photosynthesis by a semiconductor–oxide-based photocatalysis is presently challenging due to low CO2 conversion rates and poor product selectivity. To promote CO2 reduction, Pt/TiO2 has been deemed as a classic photocatalyst. In this study, we restudy Pt/TiO2 for the thermally assisted photocatalytic reduction of CO2 and reveal a different story between photocatalysis and photothermal catalysis. For example, when using disordered Pt/TiO2−x, the CO2 conversion via photocatalysis at 298 K is not impressive. However, when the system temperature is increased to 393 K, the CO2 conversion rate is significantly enhanced by a factor of 155 as compared to that obtainable from pristine TiO2; further, surprisingly high selectivity of CH4 (87.5%) could be achieved. Thermally coupled photocatalysis yields the enhanced evolution of H2 side products over Pt (4.06 nm)/TiO2 and promoted H2 splitting over Pt (2.33 nm)/TiO2, which is seldom observed in conventional Pt/TiO2 photocatalysis. The synergy of improved charge separation at the Pt/TiO2−x interface induced by surface disordering and accelerated H2 consumption near smaller Pt nanoparticles by thermal assistance are believed to be critically important for the simultaneous enhancement of CO2 conversion rates and CH4 product selectivity. This study inspires revisiting not only Pt/TiO2 but also reactivating other semiconductor–oxide-based photocatalysts for use in thermally assisted photocatalysis.


1. Introduction

The production of hydrocarbons from the photoreduction of CO2 using semiconductor–oxide-based photocatalysts is a promising approach for addressing the issues of both fossil fuel depletion and global warming.1,2 In the recent years, tremendous efforts have been devoted toward searching for candidates that can overcome the issue of sluggish CO2 conversion rates, and the family of semiconductor–oxide-based photocatalysts has been rapidly growing to this end.3,4 Significant advances have been made in materials engineering for achieving enhanced photocatalysis, such as heterojunction engineering,5 defect engineering,6 and other strategies.7–10 In long-term materials research and engineering, the thermal effect is deemed to be negligible and rarely considered for boosting photocatalysis. However, very recent studies have highlighted the fact that thermal assistance can remarkably enhance the CO2 conversion rate over several semiconductor oxides.11–13 It is highly anticipated to restudy photocatalysts as photothermal catalysts and uncover the veil of secrecy behind the thermally coupled photocatalytic (PC) reduction of CO2.

Earlier works have demonstrated that the loading of Pt nanoparticles on TiO2−x supports (i.e., Pt/TiO2−x) offers significant potential for enabling high CH4 selectivity in standard PC CO2 reduction processes.14 Here, the oxygen vacancies in TiO2−x have been shown to promote the wide dispersion and size reduction of Pt nanoparticles deposited via the photochemical route. Moreover, electrons trapped in such oxygen vacancies are more likely to be released to the surface of the catalyst by thermal activation and hence the process of CO2 reduction on the catalyst surface can be accelerated.15–17 However, our recent study involving the thermally coupled photoreduction of CO2 with H2O using TiO2−x revealed that oxygen vacancies enhanced the rate of CO evolution from CO2 conversion, involving the transfer of two electrons, which was fairly independent of the evolution rates of the other products involving a higher number of transferred electrons.18 Other studies have reported similar results (Table S1). As such, increasing the CH4 selectivity of thermally coupled PC processes using TiO2 is not expected to be achieved solely by the introduction of oxygen vacancies. The Pt nanoparticles not only facilitate the separation of electrons and holes and promote the transfer of multiple electrons, but also act as atomic hydrogen reservoirs that supply sufficient and readily available protons for CH4 formation.19–21 On considering the consistently reported enhanced PC performance for CO2 reduction, Pt/TiO2−x serves as an ideal model to be restudied as a photothermal catalyst, which would stimulate broader interest in other semiconductor–oxides-based photocatalysts for use in photothermal catalysis.

On restudying Pt/TiO2−x photocatalysts for thermally assisted PC reduction of CO2 with H2O, two competitions between photocatalysis and thermally assisted photocatalysis should be taken into consideration. Firstly, as stated above, Pt/TiO2 can promote charge separation in photocatalysis. However, the imposed thermal energy in thermally assisted photocatalysis can result in serious electron–hole recombinations due to the enhanced random thermal motion of the charge carriers. Secondly, from a thermodynamics standpoint, a higher reaction temperature tends to move the step reaction of H2O splitting forward due to the nature of endothermic reactions (ΔH > 0). Combining the role of Pt cocatalyst to promote H2 evolution as reported elsewhere,22–24 there will be a positive impact of heat on PC H2O splitting over Pt/TiO2−x. Despite this, the subsequent CO2 hydrogenation for obtaining the CH4 product is an exothermic reaction (ΔH < 0); therefore, a higher reaction temperature will tend to move the step reaction of CO2 hydrogenation backward. On the basis of this data, heat will have a negative impact on PC CO2 reduction. With regard to the above two competitions between photocatalysis and thermally assisted photocatalysis, it is imperative that the Pt/TiO2−x interface be more precisely designed and engineered than that when applied in standard photocatalysis.

Charge carrier recombinations in reactions photocatalyzed by TiO2 have been demonstrated to be reduced by the introduction of disorder at the surface of the photocatalyst.25–27 In addition to promoting charge carrier separation, surface disorders have been demonstrated to extend the range of visible-light absorption of the photocatalyst relative to that with standard TiO2.28,29 However, while this approach offers substantial potential for use in thermally assisted photocatalysis processes based on Pt/TiO2−x photocatalysts, this approach has not been applied for the photoreduction of CO2.

The present work addresses the abovementioned issues regarding thermally assisted photocatalysis processes using Pt/TiO2−x photocatalysts by inducing disorder in the surface layer of TiO2−x using high-power ultrasonic treatment. In addition to promoting charge carrier separation and extending the range of visible-light absorption of the photocatalyst, the surface layer in the disordered TiO2−x (D-TiO2−x) is found to also play a key role in narrowing the size distribution of Pt nanoparticles; these are very important factors for enhancing the CO and CH4 evolution rates in the thermally coupled PC process. First, the electronic structure and CO2 reduction mechanism of a pristine TiO2 surface were investigated by density functional theory (DFT) calculations. Then, the performances of the thermally assisted PC process using the proposed Pt/D-TiO2−x photocatalyst for the reduction of CO2 with H2O as well as the formation of CH4 were experimentally evaluated. The CH4 evolution rate was observed to increase by a factor of approximately 71 as the reaction temperature increased from 298 to 393 K; further, high CH4 selectivity of 87.5% could be obtained. Moreover, the CH4 formation rate obtained using the Pt/D-TiO2−x photocatalyst at 393 K was greater than that obtained using Pt/TiO2−x and TiO2−x photocatalysts at 298 K by factors of approximately 100 and 155, respectively. The reasons for the superior thermally assisted PC performance of the proposed Pt/D-TiO2−x photocatalyst were discussed in detail.

2. Experimental

2.1. Fabrication of TiO2−x nanoparticles

We prepared mixed-phase TiO2−x nanoparticles by a hydrothermal method. This resulted in the production of a light yellow powder. The target phases were anatase (TiO2 (A)) and TiO2 (B). These target phases were adopted due to the fact that the TiO2 (A)/TiO2 (B) interfaces were better at suppressing charge recombinations than anatase/rutile interfaces.30 In addition to the formation of mixed phases, oxygen vacancies can be naturally introduced in this step. The formation mechanisms of TiO2−x and D-TiO2−x photocatalysts are shown in Scheme S1.

2.2. Ultrasonication treatment

We dispersed 0.5 g TiO2−x nanoparticles into 100 mL deionized water and placed the solution in an XH-300 UL ultrasonic excitation apparatus (Beijing Xianghu Science and Technology Development Co., Ltd, China). This apparatus provides ultrapowerful ultrasonication with effects extending well beyond the dispersion of nanoparticles associated with standard ultrasonication.31,32 Here, ultrapowerful ultrasonication generates minute bubbles around the ultrasonic probe that can produce instantaneous temperatures (as high as 5000 °C) and extremely high pressures (as high as 500 atmospheres), which are conditions sufficient to break chemical bonds at the surfaces of nanoparticles.33,34 The ultrasonic process was conducted for 8 h in the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 pulse mode at a power of 1500 W and frequency of 25 kHz. The ultrasonic process was controlled by means of a titanium alloy ultrasonic probe inserted into the solution. After this ultrasonic treatment, the powder was collected and dried in air at 80 °C. The color of the resulting D-TiO2−x powder was bluish gray. This color could be maintained for several months under ambient conditions.

2.3. Photodeposition of Pt on TiO2−x and D-TiO2−x

We dispersed 50 mg TiO2−x or D-TiO2−x with 50 μL H2PtCl6·6H2O (1 wt%) in a solution containing methanol (10 mL) and deionized water (90 mL). Dissolved oxygen was removed by bubbling N2 gas into the mixture for 15 min. Then, the mixture was irradiated under full-spectrum radiation for 1 h using a 300 W Xe lamp under constant stirring. Finally, the obtained products were collected by centrifugation and washed with water. The formation processes of Pt/TiO2−x and Pt/D-TiO2−x photocatalysts are shown in Scheme S1.

3. Results and discussion

3.1. DFT results

We first examined the electronic structures of the TiO2 (A) and TiO2 (B) supercells without oxygen vacancies, as shown in the insets of Fig. 1a and b, respectively. These insets also show the determined densities of states (DOS) for the TiO2 (A) and TiO2 (B) models, respectively. The obtained results indicate that the valence and conduction states are derived mainly from the O 2p and Ti 3d orbitals, respectively. In addition, the band structure exhibits no midgap states in the absence of oxygen vacancies. However, the DOS results shown in Fig. 1c based on the modelled TiO2 (A)/TiO2 (B) interface with oxygen vacancies (inset of Fig. 1c) reveal the midgap electronic states below the conduction band (CB). These results indicate that these can be mainly attributed to the Ti 3d orbitals. The individual roles contributed by TiO2 (A), TiO2 (B), and the TiO2 (A)/TiO2 (B) interface for generating the midgap electronic states are shown in Fig. 1d, which demonstrates that all the components have a significant impact on the charge carriers. Song and co-workers reported that the surface junction can promote photogenerated carrier separation and offer more strongly reductive electrons for the photoreduction reaction or more strongly oxidative holes for the photooxidation reaction.35 Accordingly, the TiO2 (A)/TiO2 (B) structure with oxygen vacancies is shown to introduce midgap electronic states that can extend the wavelength range of light absorption and enhance the lifetimes of the charge carriers. Wang and co-workers also demonstrated that the oxygen vacancies had two important roles: enhancing the charge carriers and easily adsorbing reactants.36
image file: c9nr09743k-f1.tif
Fig. 1 DOS data for (a) the (001) plane of TiO2 (A), (b) the (100) plane of TiO2 (B), and (c), (d) the TiO2 (A)/TiO2 (B) interface.

The role of Pt loading on TiO2−x in CO2 reduction was also evaluated using DFT calculations.37 Fig. S1 shows the total energy difference between the products and reactants of each elementary step of the overall reduction reaction when employing the TiO2−x and Pt/TiO2−x photocatalysts. The obtained results indicate that the CO2 adsorption and further reduction to CO over Pt/TiO2−x is superior than that obtained from TiO2−x, which is the rate-limiting step in the entire reaction process. Hence, Pt/TiO2−x can considerably enhance the efficiency of CO2 reduction.

3.2. Structures of TiO2−x and D-TiO2−x

The XRD patterns shown in Fig. 2a reveal the formations of mixed phases of TiO2 (A) (PDF no.: 21-1272) and TiO2 (B) (PDF no.: 46-1237) in both TiO2−x and D-TiO2−x. The Raman spectra of the TiO2−x and D-TiO2−x samples shown in Fig. 2b reveal vibration modes at 147, 201, 400, 517, and 636 cm−1, which are characteristic of both TiO2 (A) and TiO2 (B) phases.38 In addition, the peaks at 220 and 246 cm−1 can be attributed to TiO2 (A) and TiO2 (B), respectively, which further confirm the mixed-phase composition of the samples. The ESR spectra of the TiO2−x and D-TiO2−x samples shown in Fig. 2c reveal a single signal peak at g = 2.002, demonstrating the presence of oxygen vacancies in the samples.39 The signal intensity of ESR for TiO2−x yields a marginal enhancement when compared with that for D-TiO2−x, which indicates the concentration of oxygen vacancies enhanced by the ultrasonication treatment.28 The TEM image (Fig. S2a) shows that the TiO2−x sample comprises uniformly sized nanoparticles (average diameter: 5 nm). The HRTEM images shown in Fig. 2d for TiO2−x and D-TiO2−x clearly reveal interplanar spacing values of d = 0.62 nm and d = 0.35 nm, corresponding to the (001) facets of TiO2 (B) and (101) facets of TiO2 (A), respectively, both before and after ultrasonic treatment. Further, it should be noted that the applied ultrasonic treatment induces no obvious changes in the TiO2 (A) and TiO2 (B) crystalline phases (Fig. 2a and b). Moreover, the ultrasonic treatment is observed to have no effect on the nanoparticle morphology (Fig. S2a and d). A comparison of the data shown in Fig. 2e and f (Fig. S3 and S4) reveal the presence of a disordered surface layer on the crystalline core of TiO2−x. Moreover, the straight lattice lines of the TiO2 (A) and TiO2 (B) crystallites are bent at the edge of the nanoparticle, as shown in Fig. 2f and S4, demonstrating a deviation from the standard crystalline structure of the phases on the outer layer. For our as-synthesized TiO2−x, it should be noted that the preexistence of oxygen vacancies also contributes toward the disordered surface, as confirmed by the vague and disordered appearance of the edge (shown in Fig. 2e). On careful comparison between the degree of disorder in TiO2 before and after ultrasonic treatment (Fig. 2evs.Fig. 2f), this degree is indeed enhanced. Therefore, the above results confirm that the as-adopted ultrasonic treatment successfully induces disorder on the TiO2−x nanoparticle surfaces.
image file: c9nr09743k-f2.tif
Fig. 2 Characterizations of TiO2−x and D-TiO2−x photocatalysts: (a) XRD patterns; (b) Raman spectra; (c) ESR spectra; (d)–(f) HRTEM images.

Subtle structural changes in the TiO2−x photocatalyst after ultrasonication treatment can be evaluated from a comparison of the XPS results for the Ti 2p doublet and O 1s states of the photocatalyst obtained before and after treatment, as shown in Fig. 3a and b, respectively. From the fitted peaks shown in Fig. 3a, it is evident that the binding energies corresponding to the 2 p3/2 and 2 p1/2 states of the Ti 2p doublet are 458.75 and 464.38 eV, respectively, which are indicative of a valence state of Ti4+.40 In addition, the binding energies of the Ti 2p states are observed to uniformly shift to higher values after ultrasonication. From Fig. 3b, the O 1s spectrum of TiO2−x is deconvoluted into a single main peak located at 530.04 eV and a shoulder peak at 531.68 eV that correspond to O–Ti and O–H surface hydroxyls, respectively.41 As observed for the O 1s states, the binding energies shift to higher values after ultrasonication. It is believed that this shift to a higher value of binding energy of the Ti 2p and O 1s states could be attributed to the increased concentration of surface O vacancies, which is consistent with the results of the ESR spectra.42 In addition, the ratios of the areas under the deconvoluted O 1s peaks representative of O–Ti and OH (i.e., AO–Ti/AOH) for TiO2−x and D-TiO2−x were calculated to be 0.33 and 0.40, respectively, indicating that the disordering process tends to increase the concentration of OH groups on the surface of TiO2−x, as reported elsewhere.29,43 The combined results of the HRTEM images and XPS data clearly demonstrate that the ultrasonic treatment of TiO2−x produces measurable disorder on the top surface layer.


image file: c9nr09743k-f3.tif
Fig. 3 Characterizations of the TiO2−x and D-TiO2−x photocatalysts: XPS spectra of the (a) Ti 2p doublet states and (b) O 1s states; (c) UV–vis spectra, where the insets show photographs of the respective samples employed during testing; (d) PL emission spectra obtained under 325 nm excitation; (e) Mott–Schottky plots obtained at a frequency of 1 kHz in the absence of incident light; (f) Nyquist plots obtained under illumination with 1 M NaOH electrolyte at DC potential of 1.23 V relative to a reversible hydrogen electrode (RHE) and an AC voltage amplitude of 10 mV with frequencies ranging from 100 kHz to 0.01 Hz, where the inset shows the equivalent circuit model employed during analysis.

The UV–vis diffuse absorbance spectra of the TiO2−x and D-TiO2−x powders are shown in Fig. 3c. Here, it is evident that the range of optical absorbance values for TiO2−x can be extended from about 380 nm for standard TiO2 to about 600 nm, which is induced by the oxygen vacancies introduced in the synthesis process.39,44 In addition, a comparison of the TiO2−x and D-TiO2−x spectra in the UV region indicates that the D-TiO2−x spectrum leading to the maximum absorbance increases at a smaller rate than that of the TiO2−x spectrum, as well as the fact that ultrasonication introduces a blue-shift in the UV portion of the absorbance spectrum. The smaller slope correlates with decreased crystallinity, which can be associated with the disordered layer of D-TiO2−x.42 This blue-shift can be explained by the Moss–Burstein effect.45,46 Here, the Fermi level is shifted upward when the electron carrier concentration exceeds the DOS at the CB edge, which usually corresponds to degenerate doping in the semiconductors. However, from the DOS results shown in Fig. 1, it is evident that the lattice disorder generated by the TiO2 (A)/TiO2 (B) interface in conjunction with oxygen vacancies introduces midgap states and forms a continuum of energy levels extending to the CB edge, thereby yielding an upward shift in the Fermi level.

The separation of photogenerated electrons and holes can be readily evaluated according to the PL emission spectra of the materials because the probability of recombination between the photogenerated electron–hole pairs is proportional to the PL emission intensity.47 According to the PL emission spectra of the TiO2−x and D-TiO2−x materials (Fig. 3d), it is evident that the peak PL intensity of D-TiO2−x is much less than that of TiO2−x in the wavelength range of 350–700 nm. This clearly indicates that the disordered layer in the D-TiO2−x photocatalyst reduces the probability of charge carrier recombinations in TiO2−x. This issue was further explored by analyses of the Mott–Schottky and EIS (Nyquist plots) results for the TiO2−x and D-TiO2−x photocatalysts, as shown in Fig. 3e and f, respectively. The data in Fig. 3e reveal that the slope of the Mott–Schottky plot for the D-TiO2−x electrode was substantially less than that of TiO2−x, indicating the higher charge carrier densities for D-TiO2−x than those for TiO2−x.48,49 This clearly indicates that the disordering process effectively improves the charge carriers of TiO2−x. Similarly, the Nyquist plots (Fig. 3f) indicate that the D-TiO2−x electrode yields a smaller charge transfer resistance (Rct) than that of TiO2−x based on the equivalent circuit model shown in the inset of the figure, which further verifies that the disordering process effectively improves the charge transport characteristics of TiO2−x.50 The PL emission spectra coupled with the results of the Mott–Schottky and EIS analyses confirm that the disordered layer in TiO2−x promotes charge carrier separation.

3.3. Pt-Loaded TiO2−x and D-TiO2−x supports

In addition to the TEM images (Fig. S5), we obtained the HRTEM images (Fig. 4a–d) of Pt/TiO2−x and Pt/D-TiO2−x nanoparticles. These figures clearly reveal that Pt nanoparticles are loaded on both TiO2−x and D-TiO2−x without any agglomeration. The particle size distributions of the loaded Pt particles are shown in the insets of Fig. 4a and c, which indicate that the Pt particles loaded on TiO2−x had a diameter of 2.5–6 nm (average diameter: 4.06 nm), while those loaded on D-TiO2−x had a diameter of 1.5–3.1 nm (average diameter: 2.33 nm). As such, we can conclude that the Pt loaded on D-TiO2−x had a narrower size distribution and smaller average size than that loaded on TiO2−x. In other words, the degree of disorder after high-power ultrasonic treatment can be obviously enhanced, which substantially facilitates the decrease in Pt size on the TiO2 surface. In addition, the higher-magnification HRTEM images (Fig. 4b and d) reveal that the interplanar spacing of the loaded Pt particles along the (111) plane is 0.23 nm, as well as the fact that Pt loading does not affect the d-spacing value of the TiO2−x and D-TiO2−x supports (when compared with the data shown in Fig. 2). The HAADF-STEM images shown in Fig. 4e and f also demonstrate that the Pt particles on the D-TiO2−x surface are smaller and more uniform than those on TiO2−x. Elemental mapping was further conducted for D-TiO2−x based on the HAADF-STEM image (top, Fig. 4g) to determine the distributions of the elemental components. Evidently, elemental Pt was uniformly distributed on the D-TiO2−x surface. We can conclude that the narrower particle size distribution and smaller average particle size obtained by D-TiO2−x are the result of the disordered surface layer because this is the only significant difference between the two materials. Moreover, the loading of Pt nanoparticles by photodeposition suggests that the disordered surface layer of D-TiO2−x yields a more photoactive surface than that of TiO2−x. This is supported by the improved charge carrier separation obtained by the disordered surface layer of D-TiO2−x, as discussed above.
image file: c9nr09743k-f4.tif
Fig. 4 Imaging results for Pt-loaded TiO2−x and D-TiO2−x supports: (a), (c) HRTEM images, where the insets show the histograms of the particle size-frequency distributions of Pt; (b), (d) higher-magnification HRTEM images showing the interplanar spacing values of the various components and phases; (e), (f) HAADF-STEM images; (g) HAADF-STEM image of Pt/D-TiO2−x and its corresponding elemental mappings for Pt, Ti, and O.

XPS analysis was conducted to investigate the valence states of the loaded Pt nanoparticles and changes in the chemical environments of Ti and O for TiO2−x and D-TiO2−x before and after Pt loading. As shown in Fig. 5a, the high-resolution Pt spectrum obtained for the 4f7/2 and 4f5/2 states can be deconvoluted into two peaks with binding energies of 71.13 and 74.45 eV, which are representative of Pt0 and Pt2+, respectively.51 However, the high-resolution Pt spectrum shown in Fig. 5b for Pt/D-TiO2−x reveals single Gaussian peaks for the 4f7/2 and 4f5/2 states that are representative of Pt0. In addition, the high-resolution O 1s and Ti 2p spectra obtained for TiO2−x and Pt/TiO2−x (Fig. S6) indicate that these peaks shift to higher binding energies after Pt loading. This may be the result of interactions between Pt2+ and O atoms. In contrast, the O 1s and Ti 2p peaks obtained for D-TiO2−x exhibit no shifts after Pt loading. Consequently, the loaded Pt atoms on D-TiO2−x were completely reduced to metallic Pt, whereas a small percentage of the loaded Pt atoms on the TiO2−x surface remained unchanged. These results further support the fact that D-TiO2−x provides superior photoactivity relative to that by TiO2−x.


image file: c9nr09743k-f5.tif
Fig. 5 XPS spectra of the Pt 4f doublet states of (a) Pt/TiO2−x and (b) Pt/D-TiO2−x. (c) RDB-PAS results for Pt/TiO2−x and Pt/D-TiO2−x. Here, numbers such as 〈59〉 denote the total density of ETs in units of μmol g−1; the inset shows a schematic illustrating the RDB-PAS process, where electrons are excited from the valence band (VB) to ETs lying below the CB. (d) ESR spectra of Pt/TiO2−x and Pt/D-TiO2−x.

The reversed double-beam photoacoustic spectroscopy (RDB-PAS) technology developed by Ohtani et al.52,53 can be used to successfully measure the energy-resolved distribution of electron traps (ERDT) and provide valuable information regarding the density of electron traps (ETs) in materials on the basis of the energy distribution of electron transitions from the top of the valence band (VBT) to the existing ETs. As shown in Fig. 5c and d, the RDB-PAS characterization reveals that the ET density decreases from 59 μmol g−1 for D-TiO2−x to 4 μmol g−1 for Pt/D-TiO2−x. This definitely demonstrates a decrease in surface defects after Pt loading. However, on further characterization by ESR, a subtle decrease in oxygen vacancies related to the ESR signal is evident, as shown in Fig. S7. This subtle decrease in the ESR signal seems to be inconsistent with the significantly decreased ET density. After repeated ESR characterizations, the presence of this subtly decreased ESR signal can be confirmed. Accordingly, we tentatively deduce that the disorder defect in D-TiO2−x is only partly contributed by the oxygen vacancies. The diminishing of other defects in addition to the oxygen vacancy by Pt filling cannot be detected by ESR. In fact, this hypothesis can be also be supported by effecting a comparison between the characterization data of D-TiO2−x and TiO2−x. As shown in the ESR characterization data (Fig. 2c), there is a subtle increase in the oxygen-vacancy-related ESR signal in D-TiO2−x than that in TiO2−x in spite of the introduction of a disordered layer. However, the binding energies of Ti 2p and O 1s are notably shifted in the high-resolution XPS spectra of D-TiO2−x as compared to those in TiO2−x, which is frequently observed in other disordering-engineered semiconductor oxides (Fig. 3a and b). Again, the sharp contrast between the subtly increased ESR signal and significant XPS shift suggests that the disorder in D-TiO2−x is partly contributed by the presence of oxygen vacancies.

3.4. Catalytic CO2 reduction using TiO2−x and D-TiO2−x before and after Pt loading

3.4.1. CO yield before Pt loading. The results of PC- and PTC-induced CO2 to CO conversion using TiO2−x and D-TiO2−x photocatalysts are shown in Fig. S8 and listed in Table 1. Here, CO and CH4 were the main reduction products for both the photocatalysts in both the processes, while O2—an oxidation product—remained undetected. This may be owing to the low sensitivity of the TCD used for the detection of O2. From the figures, it should be noted that the formation rates of CO using the D-TiO2−x photocatalyst were greater than those obtained using TiO2−x at both 298 K (i.e., PC reduction) and 393 K (i.e., PTC reduction). In detail, the CO formation rate obtained during PC reduction using D-TiO2−x was greater than that obtained using TiO2−x by a factor of 1.51, while the CO formation rate obtained during PTC reduction using D-TiO2−x was greater than that obtained using TiO2−x by a factor of 1.83. Moreover, the CO formation rate obtained during PTC reduction using D-TiO2−x was notably greater than that obtained during PC reduction using TiO2−x by a factor of 230.3. This indicates that the D-TiO2−x photocatalyst results in particularly increased activity when coupled with thermal energy. However, the results shown in Fig. S8 indicate that the CH4 yield is less than that of CO, and that this difference increases when the D-TiO2−x photocatalyst is used in the reduction process, as well as when it is coupled, in particular, with thermal energy. These results demonstrate that the proposed disordering in the D-TiO2−x photocatalyst is detrimental to the CH4 yield, particularly when coupled with thermal energy.
Table 1 Characteristics of PC- and PTC-induced CO2 conversion processes using different photocatalysts
Catalysts Photocatalysisa Photothermal catalysisa
CO yield (μmol h−1) CH4 yield (μmol h−1) ERRb (μmol h−1) CO yield (μmol h−1) CH4 yield (μmol h−1) H2 yield (μmol h−1) ERRb (μmol h−1) CH4 selectivityc (%)
a Reaction conditions: Solar-light irradiation for 5 h; 20 mg catalyst; 2 mL H2O; 0.1 MPa CO2; temperature: 298 K (room temperature) for the PC process and 393 K for the PTC process. b ERR: electron reaction rate = 2r(H2) + 8r(CH4) + 2r(CO), where r is an empirical rate constant. c CH4 selectivity = {[8r(CH4)]/[ERR]} × 100%.
TiO2−x 0.0027 0.0022 0.0230 0.34 0.0818 1.3344 45.3
D-TiO2−x 0.0041 0.0018 0.0226 0.6218 0.0336 1.5124 17.8
Pt/TiO2−x 0.0026 0.0034 0.0324 0.1714 0.1298 0.84 3.0612 33.9
Pt/D-TiO2−x 0.0037 0.0048 0.0458 0.0256 0.3412 0.17 3.1352 87.5


In order to explore the effect of ultrasonication treatment, we studied the effects of the time of ultrasonic treatments on the photothermal catalytic (PTC) activity and properties of TiO2−x. We treated TiO2−x for 4, 8, and 12 h (termed US-X, where X = 4, 8, and 12, respectively). As shown in Fig. S9, the PTC activity toward CO2 reduction with H2O was tested. The PTC activity results showed a volcanic trend with an increase in the time of ultrasonication treatment. TiO2−x treated for 8 h showed the highest CO yield. To understand the difference between the PTC activity of US-X, Mott–Schottky and EIS (Nyquist plots) analyses were obtained, as shown in Fig. S10. The results shown in Fig. S10a indicate that the slope of the Mott–Schottky plot sequence for the tested samples is in the order of US-8 > US-4 > US-12 > US-0, revealing the highest charge carrier densities for US-8. This clearly indicates that the disordering process effectively improves the density of charge carriers in TiO2−x. Similarly, the Nyquist plots shown in Fig. S10b reveal that the US-8 electrode yields the smallest Rct as compared to those obtained using US-4, US-12, and US-0, which further verifies that the disordering process effectively improves the charge transport characteristics of TiO2−x.

To investigate the relationship between structure and photoactivity, US-X samples were further characterized by XRD, Raman, TEM, XPS, and UV–vis techniques, as shown in Fig. S11–S14. From the XRD, Raman, TEM, and HRTEM data, TiO2−x treated by ultrasonication for different numbers of hours exhibited no obvious changes in the crystal phases and compositions (Fig. S11 and S12). As observed from the XPS spectra of Ti 2p and O 1s, the binding energies of the peaks shift to higher values with prolonging times of ultrasonication treatments (Fig. S13). It is believed that these shifts in the binding energies of Ti 2p and O 1s to higher values could be attributed to the increased concentrations of the oxygen vacancies on the surfaces. The UV–vis diffuse absorbance spectra of US-X are shown in Fig. S14. The decreased slope correlates with weaker crystallinity, which can be associated with the formation of the disordered layer induced by the ultrasonication treatment. Moreover, a blue-shift in the UV region and increased absorption in the visible region can be observed. The blue-shift can be explained by the Moss–Burstein effect. The Fermi level is shifted upward when the electron carrier concentration exceeds the DOS at the CB edge, which usually corresponds to degenerate doping in semiconductors. The gradually increased optical absorption with increasing ultrasonic time indicates the generation of localized band bending (also called band tail) in these samples.

3.4.2. CH4 selectivity before and after Pt loading. Fig. 6a and b show the results of PC- and PTC-induced CO2 to CO conversion using TiO2−x and D-TiO2−x photocatalysts before and after Pt loading, respectively. The results for the supports prior to Pt loading are duplicated here from Fig. 6 to facilitate a comparison. In addition, the results obtained for the PC and PTC processes using all the photocatalysts under consideration are listed in Table 1 for convenience. The results shown in Fig. 6a indicate that the CO formation rates obtained during PC reduction using either support remained largely unaffected by Pt loading. However, the formation rates of CH4 clearly increase after Pt loading, particularly for the D-TiO2−x photocatalyst. Here, the CH4 formation rate using Pt/D-TiO2−x was greater than that obtained using D-TiO2−x by a factor of 2.67, while the CH4 formation rate using Pt/TiO2−x was greater than that obtained using TiO2−x by a factor of 1.55. Moreover, the CH4 formation rate using Pt/D-TiO2−x was greater that obtained using Pt/TiO2−x by a factor of 1.41. These results demonstrate that the combination of Pt deposition and the disordered surface layer facilitates the multielectron reduction of CO2, thereby enhancing the CH4 yield. Nonetheless, the CO and CH4 formation rates obtained in the PC process using any of the proposed photocatalysts remain fairly low.
image file: c9nr09743k-f6.tif
Fig. 6 (a) PC- and (b) PTC-induced CO2 to CO conversion using different photocatalysts with and without Pt loading. The results obtained without Pt loading are reproduced from Fig. 6 for convenience. H2, CO, and CH4 evolution during PTC-induced CO2 conversion with respect to reaction time using (c) Pt/TiO2−x and (d) Pt/D-TiO2−x. (e) Remaining H2 obtained with respect to H2 decomposition time using Pt/TiO2−x and Pt/D-TiO2−x catalysts under thermally assisted conditions in the absence of light, where the inset shows H2-splitting process facilitated by the Pt nanoparticles.

The results shown in Fig. 6b reveal that the products obtained during PTC reduction included CO and CH4, as well as H2 under conditions of Pt loading, for both the supports. From the results shown in the figure, it is evident that for the Pt/D-TiO2−x-facilitated process, the CH4 formation rate increases to 0.3412 μmol h−1 and the CH4 selectivity increases to 87.5% from the values of 0.0336 μmol h−1 and 17.8%, respectively, obtained using D-TiO2−x. The relatively high performance obtained when using the PTC process employing the Pt/D-TiO2−x photocatalyst is particularly well illustrated by the fact that the obtained CH4 formation rate is greater than the corresponding formation rates obtained by the PC process using the Pt/D-TiO2−x and TiO2−x photocatalysts by factors of 71.08 and 155.09, respectively. In fact, the selectivity of CH4 should be fairly evaluated by the consideration of the H2 product. To report high selectivity, many works have neglected the yield of H2. Herein, we calculated the selectivity of CH4 by including the CO, H2, and CH4 products. The calculation can be expressed as follows: selectivity of CH4 = r(CH4)/{r(CH4) + r(CO) + r(H2)}.20 Meanwhile, we compared the selectivity of CH4 over Pt/D-TiO2−x with other Pt-loaded photocatalysts, as listed in Table S2. This comparison reveals that both yield and CH4 selectivity obtained in this work remain at higher levels among the currently reported photocatalysts.

The factors leading to the increased CH4 selectivity of the Pt/D-TiO2−x photocatalyst were further evaluated by monitoring the increasing yields of CO, CH4, and H2 obtained in the PTC process with an increase in the reaction time using Pt/TiO2−x and Pt/D-TiO2−x. These results are shown in Fig. 6c and d, respectively. Here, it is evident that the CO, CH4, and H2 yields obtained using the Pt/TiO2−x photocatalyst (Fig. 6c) monotonically increased uniformly with increasing reaction time. However, the CH4 and H2 yields monotonically increased nonuniformly and the CO yield decreased slightly with increasing reaction time when using the Pt/D-TiO2−x photocatalyst (Fig. 6d). The different formation rates of the products lead to different product selectivities. According to the experimental results of PTC (Fig. 6b), a decrease in the Pt size from 4.06 to 2.33 nm results in the enhancement of CH4 selectivity from 33.92 to 87.46%. This means that decreasing the Pt size is a key point in improving the CH4 selectivity. Furthermore, in comparison with the experimental results of photocatalysis (as shown in Fig. 6a), decreasing the Pt size marginally improves the CO and CH4 yields and the selectivity is hardly increased. This means that in addition to decreasing the Pt size, thermal assistance plays an important role in improving the CH4 selectivity. Upon closer observation of the formation rate of the products shown in Fig. 6b, it is evident that the H2 formation rate is sharply decreased from Pt (4.06 nm)/TiO2 to Pt (2.33 nm)/TiO2. It is well known that Pt can serve as a good catalyst for H2 activation and hydrogen spillover.21 Here, to understand the synergistic effect of Pt size and thermal assistance, we determine the thermal activation of H2 in mixed N2/H2 gas in the dark at 393 K. As shown in Fig. 6e, H2 can be thermally decomposed over Pt/TiO2−x in the dark. There is a higher decomposition rate of H2 over Pt (2.33 nm)/TiO2 than that over Pt (4.06 nm)/TiO2. This demonstrates that the integration of thermal effects and Pt size could easily promote H2 splitting into H, which is beneficial for the hydrogenation of CO2 and therefore can improve CH4 selectivity.

The smaller Pt nanoparticles loaded on D-TiO2−x indeed influence the catalytic conversion of CO2 and CH4 selectivity. First, the smaller Pt loaded on D-TiO2−x can enhance the CH4 yield.14 At the same time, the H2 yield is also enhanced, which unfortunately decreases the CH4 selectivity.20,54,55 In this work, the effect of Pt size is restudied in the PTC reduction of CO2. In detail, thermally coupled photocatalysis reveals the enhanced evolution of H2 side products over Pt (4.06 nm)/TiO2 (Fig. 6b). However, promoted H2 splitting over Pt (2.33 nm)/TiO2 is observed (Fig. 6e), which is seldom reported in conventional Pt/TiO2 photocatalysis. The synergy of this improved charge separation at the Pt/TiO2 interface induced by surface disorder and accelerated H2 consumption near smaller Pt nanoparticles by thermal assistance are believed to be critically important for the simultaneous enhancement of CO2 conversion rates and CH4 product selectivity.

The stability of Pt/D-TiO2−x toward CO2 reduction can be evaluated in five cycles, and the obtained results are shown in Fig. S15. Evidently, the CH4 yield exhibited no significant decrease, suggesting the worthwhile stability of Pt/D-TiO2−x. In order to verify the stability of Pt nanoparticles, we perform the XPS analysis on Pt/D-TiO2−x before and after PTC. From the Pt 4f spectra, there is no obvious shift in the binding energy (Fig. S16). This indicates that the Pt nanoparticles are subject to no oxidation after the catalytic test and hence exhibit good stability.

The PC and PTC test results conducted with TiO2−x and D-TiO2−x photocatalysts both with and without Pt nanoparticle loading support the following conclusions.

(1) The PTC process outperforms the PC process with respect to CO2 to CO conversion rate for all the photocatalysts under consideration.

(2) The use of the D-TiO2−x photocatalyst significantly increases the formation rate of CO rather than CH4 in both PC and PTC processes.

(3) The integration of oxygen vacancies, surface layer disordering, and Pt nanoparticle deposition selectively promotes CH4 formation, particularly in the PTC process.

3.5. Proposed mechanisms facilitating enhanced CH4 selectivities

Based on the above results and discussion, we propose the Pt/TiO2−x and Pt/D-TiO2−x models shown in Scheme 1a. Firstly, the disordered surface layer of D-TiO2−x facilitates the photodeposition of Pt nanoparticles with a narrower size distribution and smaller average size than those obtained on the TiO2−x surface, and the Pt nanoparticles are more widely dispersed over the surface of D-TiO2−x because the disordered layer induces a higher density of active sites that promotes wider dispersion of Pt nanoparticles. Meanwhile, Scheme 1b shows that the coupling of thermal energy with the PC process increases the rate of electron and hole transfers to the Pt nanoparticles at the surface of the supports, resulting in the increased formation rates of the products. Moreover, the thermal energy in the PTC process assists in the splitting of H2 molecules by the Pt nanoparticles, which subsequently facilitates the reaction between monatomic H with CO. Scheme 1b shows how multiple factors synergistically work in the proposed PTC system to promote rapid CO2 reduction, low H2 formation, and high CH4 selectivity. These can be specified as follows.
image file: c9nr09743k-s1.tif
Scheme 1 (a) Models of Pt/TiO2−x and Pt/D-TiO2−x nanocatalysts with Pt nanoparticles of varying sizes and degrees of dispersion over the nanosupport surfaces. (b) Schematics illustrating the fundamental mechanisms of PC and PTC reactions using Pt/TiO2−x and Pt/D-TiO2−x nanocatalysts.

(1) Increased charge carrier transfer to the surface is facilitated by the enhanced generation of electrons and holes via thermal activation, while their lifetimes are enhanced by the disordered layer in conjunction with the Pt nanoparticle loading via reduced charge carrier recombination.

(2) The disordered layer of D-TiO2−x promotes the wider dispersion of Pt nanoparticles, while ensuring that the Pt nanoparticles are composed of fully reduced metallic Pt atoms in the Pt0 state.

(3) Thermal energy promotes the rapid decomposition of molecular H2 to monatomic H by the catalytic activity of the fully metallic Pt nanoparticles.

4. Conclusion

In summary, we have developed an efficient PTC system that promotes rapid CO2 reduction, low H2 formation, and high CH4 selectivity by coupling thermal energy with an advanced surface-engineered Pt/TiO2−x photocatalyst with oxygen vacancies. Here, disordering was induced on the surface layer of TiO2−x by means of a high-powered ultrasonic treatment and highly dispersed Pt nanoparticles were loaded on the surface. Comprehensive analyses revealed that the surface disordering decreased the average size of the Pt nanoparticles from 4.06 to 2.33 nm, resulting in improved charge carrier separation and increased carrier lifetimes. The adoption of the as-fabricated Pt/D-TiO2−x photocatalyst in the thermally assisted process conducted at 393 K provided a CH4 formation rate that was greater than that obtained by the standard PC process conducted at room temperature using the TiO2−x photocatalyst by a factor of 155. Meanwhile, the proposed PTC system provided CH4 selectivity of 87.5%. The superior performance of the proposed system could be attributed to multiple factors such as thermal assistance, oxygen vacancies, surface disorder, and Pt nanoparticle loading, which function synergistically. This work presents a proven strategy for the design of highly efficient photothermal catalysts that can enhance the yield and selectivity of CH4 in the CO2 reduction process.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (91833303, 51072032, 51102001 and 51872044), Jilin Province Science and Technology Development Project (20180101175JC), and the 111 project (No. B13013). TEM characterization and analysis were supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # 67037. Electron traps density measured by RDB-PAS technology was supported by Professor Bunsho Ohtani from Graduate School of Environmental Science, Hokkaido University.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr09743k

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