Engineering Coexposed {001} and {101} Facets in Oxygen-Deficient

Dec 31, 2015 - University of Wisconsin-Milwaukee, Mechanical Engineering Department, Milwaukee, Wisconsin 53211, United States. ‡ Texas A&M Universi...
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Engineering Coexposed {001} and {101} Facets in Oxygen-Deficient TiO2 Nanocrystals for Enhanced CO2 Photoreduction under Visible Light Lianjun Liu,† Yuqiu Jiang,† Huilei Zhao,‡ Jiatang Chen,‡ Jianli Cheng,§ Kesong Yang,*,§ and Ying Li*,†,‡ †

University of Wisconsin-Milwaukee, Mechanical Engineering Department, Milwaukee, Wisconsin 53211, United States Texas A&M University, Department of Mechanical Engineering, College Station, Texas 77843, United States § University of California, San Diego, Department of NanoEngineering, La Jolla, California 92093, United States ‡

S Supporting Information *

ABSTRACT: This work for the first time reports engineered oxygen-deficient, blue TiO2 nanocrystals with coexposed {101}{001} facets (TiO2−x{001}-{101}) to enhance CO2 photoreduction under visible light. The TiO2−x{001}-{101} material demonstrated a relatively high quantum yield (0.31% under UV− vis light and 0.134% under visible light) for CO2 reduction to CO by water vapor and more than 4 times higher visible light activity in comparison with TiO2 with a single {001} plane or {101} plane and TiO2(P25). Possible reasons are the exposure of more active sites (e.g., undercoordinated Ti atoms and oxygen vacancies), the facilitated electron transfer between {001} and {101} planes, and the formation of a new energy state (Ti3+) within the TiO2 band gap to extend the visible light response. An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study was applied to understand the roles of coexposed {001}{101} facets and Ti3+ sites in activating surface intermediates. The in situ DRIFTS analysis suggested that the coexposed {001}{101} facets increased the capacity of reversible CO2 adsorption and that the combination of {001}-{101} and Ti3+ enhanced the activation and conversion kinetics of adsorbed species. The visible light responsive TiO2−x{001}-{101} material is not oxidized after long-term exposure to an air environment. This work is a significant contribution to the design of efficient and stable solar fuel catalysts. KEYWORDS: TiO2, exposed plane, point defect, CO2 photoreduction, visible light {101} for water splitting,26−28 organics degradation,29,30 and dye-sensitized solar cells.31,32 In terms of CO2 photoreduction, so far a few studies have been done to develop TiO2-based crystals with specific facets, including Pt-deposited TiO2{001} nanosheets,8 N-doped TiO2{001}/graphene,17 TiO2 with 95% {001},26 TiO2{010} nanorods,31 fluorinated TiO2 (optimum ratio 72%{001}/28%{101}),33 and TiO2 with a tailored {001}/ {101} ratio (optimum ratio 58% {001}/42% {101}).34 Especially, TiO2 with coexposed {001} and {101} facets has a higher CO2 conversion efficiency than those with {001} and {101} alone.34 This is primarily because (1) the {001} facet may provide additional active sites (5-fold-coordinated Ti atom, Ti5c) and (2) a “surface heterojunction” is formed between {001} and {101}, where photogenerated electrons favorably transfer from {001} to {101}, while holes migrate from {101} to {001}, resulting in the enhanced separation of e−−h+.23,24,35 However, most literature in CO2 photoreduction using the aforementioned TiO2{001}-based nanocrystals reports only

1. INTRODUCTION The process of solar-activated carbon dioxide (CO 2 ) conversion with water (H2O), so-called artificial photosynthesis, is an attractive approach to reduce CO2 emissions and produce energy-rich fuels and useful chemicals.1−5 To achieve this, titanium dioxide (TiO2) has been widely used as a photocatalyst because of its chemical stability, low cost, and nontoxicity.1,4,6 However, applications of TiO2 suffer from a low quantum yield (QY) and limited harvesting of visible light, which result from the rapid recombination of electron−hole (e−−h+) pairs and the wide band gap of TiO2 (e.g., 3.2 eV for anatase), respectively. To tackle the above obstacles, a number of strategies have been applied to modify TiO2, including depositing noble metals (e.g., Pt),7−9 incorporating transitionmetal ions (e.g., Cu2+) or nonmetal elements (e.g., N) into the TiO2 lattice,10−14 adding electron carriers (e.g., graphene),15−18 or coupling small band gap quantum dots (e.g., PdS) with TiO2.19−21 Alternatively, controllable fabrication of TiO2 with specific morphologies and crystal facets has emerged as a promising way to improve its QY.6,22−25 Anatase TiO2 with a {001} or {010} facet has demonstrated efficiency higher than that with © 2015 American Chemical Society

Received: September 19, 2015 Revised: December 25, 2015 Published: December 31, 2015 1097

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reduction.40 Briefly, the as-prepared TiO2 was mixed with NaBH4 powder with a weight ratio of 5:1, ground in a mortar, and then calcined in a tube furnace under an Ar atmosphere at 300 °C for 30 min, during which NaBH4 was decomposed to produce active hydrogen to reduce Ti4+ to Ti3+ (as evidenced by XPS and EPR described later in this paper). After calcination, the powder was washed with distilled H2O until pH 7 to remove Na and B impurities. The three TiO2−x samples with different facets are denoted as TiO2−x{001}, TiO2−x{001}-{101}, and TiO2−x{101}, respectively. 2.2. Materials Characterization. The crystal structures of the catalyst samples were identified by X-ray diffraction (XRD, Scintag XDS 2000) using Cu Kα irradiation at 45 kV and a diffracted beam monochromator at 40 mA. Scanning electron microscopy (SEM) (Hitachi S4800) was used to obtain the surface morphology. The lattice structure of TiO2 was visualized by phase-contrast high-resolution transmission electron microscopy (HRTEM) carried out with 300 keV electrons in a Hitachi H9000NAR instrument with 0.18 nm point and 0.11 nm lattice resolution. UV−vis diffuse reflectance spectra were obtained by a UV−vis spectrometer (Ocean Optics) using BaSO4 as the background. The chemical states of Ti and O elements were identified by X-ray photoelectron spectroscopy (XPS), a PHI 5000 Versaprobe system using monochromatic Al KR radiation (1486.6 eV). All binding energies were referenced to the C 1s peak at 284.6 eV. Electron paramagnetic resonance (EPR) experiments were carried out using a BrukerA300 spectrometer operating in the X-band (9.4 GHz) and equipped with an Oxford CF935 helium flow cryostat with an ITC-5025 temperature controller. The g factors were calibrated against a coal standard, g = 2.00285(±0.00005), and the EPR spectra were recorded at 110 K. 2.3. Photocatalytic Activity Measurement. The photocatalytic reduction of CO2 with H2O vapor was conducted in a homemade photoreactor operating in a continuous flow mode. For each test, 40 mg of catalyst was used and evenly dispersed onto a glass-fiber filter that was placed at the center of the photoreactor. CO2 (99.999%, Praxair) was continuously passed through a water bubbler to bring a CO2 + H2O gas mixture (with 2.3 vol % of H2O) into the photoreactor. A high gas flow rate was used initially to purge out air inside the reactor, and then a flow rate of 4.0 mL/min was maintained during photoreaction. A 100 W mercury vapor lamp was used as the UV−vis light source, and the light intensity was about 10 mW/ cm2 in the UV range ( TiO2−x{001}-{101} (0.31) > TiO 2−x {001} (0.28). The higher Ti 3+ /Ti 4+ ratio on TiO2−x{101} indicates that VO sites (or Ti3+ species) are more easily formed on the {101} exposed facet. This is consistent with our theoretical calculations that the formation energy of VO (surface or subsurface) in the {101} facet (4.46, 4.39 eV) is lower than that in the {001} facet (5.98 eV, 5.18 eV), which is also in agreement with previous results.58 Low-temperature EPR was conducted to further investigate the relative concentration and distribution of defects in the reduced TiO2, since EPR is highly sensitive to characterize both surface and bulk Ti3+ that has unpaired electrons. The location of Ti3+ can be distinguished by the difference of the EPR

Figure 4. XPS spectra for TiO2{001}, TiO2{001}-{101}, TiO2{101}, and their corresponding TiO2−x nanocrystals.

subsurface of TiO2−x, which may change the surface chemical bonding environment of TiO2. It should be noted that the 1101

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Figure 5. EPR spectra for (a) TiO2{001} and TiO2−x{001} and for (b) TiO2−x{101}, TiO2−x{001}-{101}, and TiO2−x(P25).

parameters. Generally, the values of the g factors for surface Ti3+ are significantly lower than those found in the bulk. In addition, the signal shape for surface Ti3+ is usually broad, but inner (bulk) Ti3+ has a narrow axially symmetric signal.59 As shown in Figure 5a, TiO2{001} presented a very weak signal at a g value of 1.977, which was ascribed to the existence of trace Ti3+−oxygen vacancy association in the TiO2.57 TiO2−x{001} displayed a strong response at g = 1.918, corresponding to the surface Ti3+ species.41,60 Again, this result confirmed the XPS observations that more Ti3+ species were generated by NaBH4 treatment. In Figure 5b, TiO2−x{101} and TiO2−x(P25) (rich in {101} facet as well) exhibited broad and strong EPR signals at an average g value of 1.943 that was related to VO stabilized bulk Ti3+, indicating that some Ti3+ sites could be diffused into the bulk in the {101} facets because the formation energy of VO in the bulk is smaller than that in the surface.40,46,61 In contrast to TiO2−x{101} and TiO2−x{001}, TiO2−x{001}-{101} showed a dominant sharp signal at g1 = 1.963 for bulk Ti3+ and two very weak signals at g2 = 1.945 (subsurface Ti3+) and g3 = 1.918 (surface).41,61 Combining the results of UV−vis, XPS, and EPR analyses, we can infer that the NaBH4 reduction tends to cause the formation of surface and subsurface Ti3+/VO in the TiO 2−x {101}, surface/subsurface/bulk Ti 3+ /V O in the TiO 2−x {001}-{101}, and only surface Ti3+ /V O in the TiO2−x{001}. 3.3. First-Principles Electronic Structure Calculations. We have conducted DFT calculations to further understand the electronic structure of reduced {101} and {001} facets and the formation of Ti3+/VO on different facets. It is already known that a VO can introduce two additional electrons into the bulk anatase TiO2, which preferably occupy the Ti 3d orbital on the nearest-neighboring Ti ions of the VO and lead to the formation of two Ti3+.62 Herein, to confirm whether the {101} and {001} facets have a Ti3+-like electron configuration similar to that of the bulk anatase TiO2, we also calculated the partial DOS of the nearest-neighboring Ti ions of the VO in each facet model. The calculated total and partial density of states (DOSs) of (101) and (001) facets of anatase TiO2 with a VO are shown in Figure 6. The calculated total DOS plot of the pristine {101} facet model indicates that the TiO2{101} has a band gap of about 3.2 eV (Figure S4 in the Supporting Information), which agrees well with the experimental value of 3.16 eV from UV−vis spectra measurements in Figure 3b. In addition, there are two discrete occupied up-spin and down-spin gap states in the band gap, which are pinned above the valence band maximum at about 0.6 and 2.0 eV, respectively. As a result, the electron transition energies from these two occupied gap states to the

Figure 6. Total (I) and partial (II and III) density of states (DOS) of (a) (101) and (b) (001) slab model of anatase TiO2 with VO. The partial DOS plots for (101) belong to two nearest-neighbor Ti3+ ions of the VO, while those for (001) belong to the nearest-neighbor Ti3+ and Ti3.5+ ions of the VO, respectively.

conduction band minimum are about 2.6 and 1.2 eV, respectively. In agreement with literature reports,41,63,64 the formation of the occupied gap states (due to the presence of VO/Ti3+) could lead to visible-light absorption as observed in the experiment. It is interesting to note that the two discrete gap states of the two Ti3+ ions around the VO in the (101) TiO2 slab model are significantly different from the case in the bulk TiO2 model.62 In the later, the spin-up and spin-down gap states are located at the same energy level within the band gap, about 1.2 eV below the CBM. This is because the two Ti ions around the VO in the bulk TiO2 are symmetrical and share the same local structures. In the slab model, the two Ti ions around the VO (see Figure S2 in the Supporting Information) are no longer symmetrical due to the existence of the dangling bond on the surface Ti ion, which leads to the splitting of the Ti 3d gap states. Further partial DOS analysis shows that these two 1102

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that the Kohn−Sham DFT electronic structure calculations can accurately predict the position of the band gap center (BGC) that is almost independent of the choices of the exchangecorrelation functional and the U values, while the VBM is dependent on these choices.66,67 Accordingly, the band edges can be better determined by locating BGC than by locating VBM relative to the vacuum level, particularly for different types of semiconductors.67 Nevertheless, in this work, the calculated band gap of the (101) slab model is about 3.2 eV, which is very close to the experimental value of 3.16 eV, and thus the above two approaches will yield nearly the same results for the band edge positions. Our results show that the CBM of the (101) anatase slab model is about 0.38 eV above the H+/H2 redox level, which is in excellent agreement with the experimental value.68 For the (001) model, the calculated CBM position is about 0.72 eV above the H+/H2 redox level, and by using its experimental bulk band gap of 3.33 eV, the VBM position is then determined as −2.61 eV relative to the NHE. The calculated band edges of the two facets are shown in Figure 7. Our calculations show that the band alignment

gap states come from two nearest-neighboring Ti ions of the VO, and each Ti ion has a magnetic moment of about 1.0 μB. This implies that the two electrons introduced by the neutral VO are captured by the two neighboring Ti ions, forming two Ti3+ ions with an electronic configuration of 3d14s0, which is consistent with our experimental XPS and EPR measurements. Similar electronic structure character also occurs in the {001} facet model with a VO, in which two discrete occupied up-spin and down-spin gap states emerge. This indicates that, as in the case of the {101} facet, the electron transition from these occupied gap states to the conduction band can also lead to visible-light absorption. However, there are still two major differences in the {001} facet model from the {101} facet model. On the one hand, we found that the band gap of the pristine {001} facet model is significantly less than that of the {101} facet model: i.e., about 2.3 eV vs 3.2 eV. This could be explained by our layer-resolved partial DOS analysis for the {101} and {001} facet model (see Figures S5 and S6 in the Supporting Information). The results show that for the {101} slab there is no band bending but that for the {001} slab there is a significant band bending on the top layer because of a significant upward shift of the O 2p orbital to high energy from the inner O to surface O atoms. This leads to a significant reduction of the calculated band gap in the TiO2{001} surface. It is worth mentioning that the calculated band gap of the {001} facets is also smaller than the experimental band gap from the UV−vis spectra, probably because the actual TiO2{001} surface is covered by about 8% {101} facets (calculated from the SEM and HRTEM results). Since no band bending exists on the {101} facets, the presence of 8% {101} facets may prevent the surface band bending effects on the real TiO2{001} surface. Actually, the deep layers in the {001} slab still have a band gap of about 3.1 eV, very close to the experimental value of 3.33 eV. However, it is still not clear what the real reasons are for the difference between the experimental band gap and the theoretically calculated value. Further exploration is ongoing. On the other hand, we found that the two additional electrons are shared by the three neighboring Ti ions in the {001} facet instead of the two neighboring Ti ions in the {101} facet model. To clearly show the distribution of the two additional electrons on the three Ti ions, we plotted the spin density around the VO in Figure S7 in the Supporting Information. Our calculations indicate that one Ti4+ ion is reduced to a Ti3+ with a magnetic moment of 1.0 μB, while the other two Ti4+ ions share one remaining electron and each of them has a magnetic moment of about 0.5 μB; thus, one can assume that the two equivalent Ti ions exist as Ti3.5+ (3d1/24s0). To determine whether the band alignment between the {001} and {101} facets can support the formation of the surface heterojunction, we also calculated the band edges of the {001} and {101} facets of the anatase TiO2. For convenience, the energy scale was plotted with respect to the normal hydrogen electrode (NHE). The absolute NHE potential of 4.44 eV with respect to the vacuum level was used,65 which is widely used in the study of the electrochemistry of semiconductors. For the {101} facet, we first determined its valence band maximum (VBM) position relative to the vacuum level from DFT calculations by calculating the average electrostatic potential along the slab direction. Next, we calibrated its conduction band minimum (CBM) position using the experimental band gap. This is because the standard DFT calculations even within the GGA+U approach often cannot reproduce the accurate band gap value. However, it is noted

Figure 7. Calculated relative band edges of the {101} and {001} facets of anatase TiO2.

between the {101} and {001} facets can support the formation the surface heterojunction, which is consistent with previous calculations.34 Thermodynamically, the photoinduced electrons tend to transfer from the {001} to the {101} facet while the holes tend to transfer from the {101} to {001} facet, which facilitates the separation of the photoinduced electron−hole pairs and the charge transfer. As a result, the coexposed {001} and {101} facets may show enhanced photocatalytic activity in comparison to each facet alone. 3.4. Photocatalytic Activity for CO2 Reduction. Photocatalytic conversion of CO2 with H2O vapor was conducted under UV−vis light and visible light (the irradiation spectra are shown in Figure S1 in the Supporting Information) while the reactor was simultaneously heated by two infrared (IR) lamps to the designated reaction temperature of 150 °C. Since the photocatalytic activity of TiO2−x at near-ambient temperature (without IR heating) is generally 1 order of magnitude lower than that at 150 °C, the reaction in this work was only conducted at 150 °C (an optimum temperature found in our previous work that balances CO2 adsorption and product desorption).69,70 CO was found to be the major product. The produced CH4 was in a much smaller concentration than CO, and thus, the CH4 production is not reported in this paper. To exclude the thermal-induced catalytic conversion of CO2 and the possibility of surface organic contaminants that may be decomposed into CO, background tests at 150 °C were conducted by introducing He + H2O vapor to the reactor in the presence of TiO2 nanocrystals. The amount of CO produced 1103

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TiO2{001} have a greater reducing power to reduce adsorbed CO2 to CO.26,31 TiO2{001}-{101} shows 15% and 90% higher CO production than TiO2{001} and TiO2{101}, respectively. As suggested by electronic structure calculations in this work and in the previous literature,34 the contacted {001} and {101} facets could form a surface heterojunction due to their appropriate band alignment and mismatched Fermi level (see Figure 7). The photogenerated electrons and holes migrate to {101} (reduction sites) and {001} (oxidation sites) facets, respectively, during the photocataytic process.34,35 Therefore, the enhanced charge separation and transfer between the {001} and {101} interface could explain why TiO2{001}-{101} is more active. Another important finding in Figure 8a is that TiO2−x species always have a CO production higher than that of their corresponding TiO2 crystals. Especially, TiO2−x{001}{101} showed the highest activity, more than 50% higher than TiO2{001}-{101} and nearly 2 times as high as that of TiO2−x(P25). The superiority of TiO2−x{001}-{101} could be ascribed to the formation of more surface Ti3+ active sites, the synergy of the {001}-{101} surface heterojunction, and the enhanced activation and conversion dynamics of adsorbed CO2 (see the detailed evidence by in situ DRIFTS described later). Under visible light irradiation, as shown in Figure 8b, the activity of CO production for TiO2 samples was much weaker than that under UV−vis irradiation and was on the order of TiO2{001}-{101} > TiO2{001} > TiO2{101} ≈ TiO2(P25). Again, coexposed {001}-{101} facets outperformed single {001} or {101} facets in visible light activity. Notably, all of the TiO2−x crystals showed more than 2 times higher activity in comparison to their TiO2 counterparts. The enhancement was mainly attributed to the creation of Ti3+/VO sites that not only extended visible light response (see Figure 3c,d) but also promoted charge carrier transfer. According to the theoretical calculations,40,42,43 the formation of Ti3+ may lead to the formation of a new state below the CBM, but no changes occur in the VB. The visible light excited electrons can be favorably migrated to the intraband Ti3+ state, where the accumulated electrons transfer to the surface to promote the activation and conversion of CO2. In Figure 8b, one can also easily see that TiO2−x{001}-{101} showed much higher visible light activity than TiO2−x{001}, TiO2−x{101}, and TiO2−x(P25). This important finding confirms our original hypothesis that the combination of coexposed {001}-{101} facets and Ti3+/VO sites could improve the visible light activity of TiO2 for CO2 conversion. As evidenced by XPS and EPR results, TiO2−x{101} has the highest density of Ti3+, but excessive Ti3+ sites may serve as a recombination center for photogenerated charges.40,46 Moreover, the {101} facets have a low surface energy and lack of Ti5c active sites, thus leading to the lower activity of TiO2−x{101}. On the other hand, although TiO2−x{001} has a high-surfaceenergy {001} facet, the low concentration of Ti3+ may lead to its lower activity in comparison to TiO2−x{001}-{101}. Hence, it is desirable to generate more Ti3+ species on or near the catalyst surface, which requires an optimum combination of the exposed {101} and {001} facets, which will be our future work. The quantum yield (QY) of CO2 photoconversion has been calculated by the equation

was not detectable either in the dark or under photoirradiation. This confirms that the produced CO was indeed derived from photocatalytic conversion of CO2. Figure 8a compares the overall CO production by various TiO2 and TiO2−x samples under UV−vis irradiation for 5 h.

Figure 8. CO production over TiO2 and TiO2−x nanocrystals with different exposed facets (a) under UV−vis light (200−700 nm) and (b) under visible light (400−700 nm) irradiation.

TiO2{101} and TiO2(P25) showed a comparable but low CO production. Even though the band gap of TiO2{001} (3.33 eV) is larger than that of TiO2{101} (3.16 eV), TiO2{001} exhibited a 65% higher activity than TiO2{101}, probably due to the following two reasons. First, TiO2{101} has a lower surface energy (0.44 J/m2) than TiO2{001} (0.9 J/m2) and TiO2{001} has more five-coordinated Ti atoms (Ti5c) exposed on the surface.26,71 Ti5c could act as active sites to enhance CO2 activation and conversion. Second, TiO2{001} has a larger band gap, while the measured XPS valence band position of TiO2{001} (2.10 eV) was lower than that of TiO2{101} (2.18 eV) (see Figure S8 in the Supporting Information). Hence, the CBM of TiO2{001} is more negative than that of TiO2{101} by 0.25 eV (calculated by (2.10 − 3.33 eV) − (2.18 − 3.16 eV) = 0.25 eV), indicating the CB electrons in the

ϕCO (%) = 1104

2(mol of CO yield) × 100% mol of photon adsorbed by catalyst DOI: 10.1021/acscatal.5b02098 ACS Catal. 2016, 6, 1097−1108

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Figure 9. In situ DRIFTS spectra for CO2 + H2O interaction with TiO2 nanocrystals in the dark and under UV−vis irradiation: (a) TiO2{101}; (b) TiO2{001}-{101}; (c) TiO2{001}; (d) TiO2−x{101}.

reports for defective TiO2 anatase−rutile (0.0289%),47 Fe,S,Ndoped Sr3Ti2O7 (0.011%),74 and graphene-modified TiO2 (0.0178%).75 Most importantly, our engineered material did not apply any cocatalyst or doping of foreign elements. Doing so in our future work is anticipated to result in an even more promising catalyst for visible-light-driven CO2 photoreduction. 3.5. In Situ DRIFTS for CO2 Adsorption/Activation/ Conversion. In situ DRIFTS has been conducted to identify the surface intermediates and explore the roles of exposed

when CO is the major product. CO2 conversion to CO requires two electrons (see the details in Supporting Information). The QY based on CO production over TiO2−x{001}-{101} was calculated to be 0.31% under UV−vis light, which is 3, 179, and 2 times higher than those in literature reports for fluorinated Ti(001)-(101) (QY = 0.0755%),33 noble metal Pt-deposited TiO2 (QY = 0.00172%),72 and In/TiO2 (QY = 0.1%),73 respectively. Under visible light irradiation, its QY can reach 0.134%, almost 4, 11, and 7 higher than those in literature 1105

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Research Article

ACS Catalysis planes and defective sites (Ti3+/VO) for CO2 activation and conversion. The in situ DRIFTS analysis was carried out in two sequential steps in a continuous-flow mode. First, CO2 adsorption on the sample surface was studied by introducing a CO2/H2O mixture to the IR cell at 150 °C for 60 min in the dark when the intensities of adsorption peaks reached saturation (step 1). Next, the UV−vis light was turned on for 60 min to investigate the photocatalytic conversion of reaction intermediates (step 2). The in situ DRIFTS spectra recorded in each of the two steps are shown in Figure 9. It should be noted that, prior to the CO2 adsorption, we collected the IR spectrum of the photocatalyst first as the background. When introducing the CO2 onto the photocatalyst, we obtained the IR spectra of the adsorbed species by subtracting the background. During the adsorption process in the dark and desorption/conversion process under the photoirradiation, the background did not change due to the good stability of TiO2. Hence, the quick decrease or disappearance of the peaks presented below in Figure 9 was induced by photoirradiation and was not caused by the change of the background. We first compared CO2 interactions with TiO2{101}, TiO2{001}, and TiO2{001}-{101} to investigate the crystal facet effect. As shown in Figure 9a, exposure of TiO2{101} to CO2/H2O resulted in the formation of carboxylate (CO2−, 1671 and 1242 cm−1), bicarbonate (HCO3−, 1422 and 1223 cm−1), monodentate carbonate (m-CO32−, 1510−1560 cm−1), and bidentate carbonate (b-CO32−, 1626 and 1361 cm−1).76−81 Introducing CO2/H2O onto TiO2{001}-{101} formed CO2− at 1662 cm−1, HCO3− at 1406 cm−1, m-CO32− at 1551 cm−1, and b-CO32− at 1625 and 1340 cm−1 (Figure 9b), the positions of which shifted to a low wavenumber range in comparison to TiO2{101}. On the other hand, the CO2/H2O interaction with TiO2{001} only produced very weak CO2− at 1690 cm−1, mCO32− at 1548, 1513, and 1466 cm−1, and b-CO32− at 1640 cm−1 (Figure 9c). As suggested in the literature,77 the formation of CO2−, HCO3−, m-CO32−, and b-CO32− requires the participation of surface anionic Ti sites, cationic Ti−OH−, O2−, and Ti−O2− sites, respectively. As illustrated in the scheme (right panels of Figure 9), the {101} facet exposes both O3c−Ti6c−O3c (i.e., six-coordinated Ti bonded with threecoordinated O atoms) and O2c−Ti5c−O2c sites (i.e., fivecoordinated Ti bonded with two-coordinated O atoms),24,54 while the {001} facet has 100% O2c−Ti5c−O2c sites.26,82 In this case, on TiO2{101} CO2 could interact with both O3c−Ti6c and O2c−Ti5c sites, whereas on TiO2{001} and TiO2{001}-{101} CO2 predominately coordinates with O2c−Ti5c sites. The different coordination structures of Ti and O may influence the vibration frequency of the C−O bond, thus causing the shift in its peak position. In addition, comparison of the spectra in Figure 9a−c reveals that the peak intensities of m-CO32− and bCO32− are on the order of TiO2{101} > TiO2{001}-{101} > TiO2{001} and TiO2{001} has negligible CO2− and HCO3− species, possibly because the TiO2{101} surface contains more adsorption sites and has a stronger interaction energy with CO2 than does TiO2{001}.83 Another important finding in Figure 9 is that, after saturated adsorption in the dark, irradiating TiO2{101} by UV−vis light for 10 min induces a decrease in b-CO32− at 1626 cm−1, mCO32− at 1560 cm−1, and CO2− at 1671 cm−1/1242 cm−1 (Figure 9a) but, in contrast, photoillumination of TiO2{001} and TiO2{001}-{101} within 1 min decreases almost all of the peaks (Figure 9a,b), some of which even completely disappeared (for example, m-CO32− at 1551 cm−1). The

disappearing peaks will reappear after turning off UV−vis light and readsorption for a few minutes (IR spectra not shown here). The much faster disappearance of the adsorbed CO2 species on the TiO2{001} and TiO2{001}-{101} (0.5−1 min) in comparison to those on the TiO2{101} (10 min) demonstrates that the presence of the {001} facet can facilitate the activation and conversion of the adsorbed CO2. Theoretical calculations demonstrated that TiO2{101} facet clusters have stronger acid/base characters of surface cations and anions than {001} clusters, which causes a stronger interaction of CO2 with {101} surface atoms.83 If the bonding is too strong, CO2 becomes irreversibly chemisorbed on the {101} surface and is difficult to convert, whereas, if the bonding is too weak, CO2 adsorption capacity is limited on the {001} surface. The coexposed {001}-{101} facets may simultaneously warrant a reversible adsorption process and a good CO2 adsorption capacity, which could explain its higher activity in comparison to TiO2{101} and TiO2{001} in Figure 8. We also performed a CO2 interaction with TiO2−x{001} and compared it with TiO2{101} to explore the surface defect effect. As shown in Figure 9d, on the surface of TiO2−x{001} the adsorbed species were CO2− (1684 and 1274 cm−1), HCO3− (1400 and 1201 cm−1), b-CO32− (1642 and 1372 cm−1), m-CO32− (1568, 1502, and 1310 cm−1), and a new chelating bridged carbonate (c-CO32−, 1723 cm−1) (spectrum 1). Comparison of the IR features of TiO2{101} and TiO2−x{001} clearly reveals that CO2 interaction with TiO2−x{001} induces not only the appearance of some new species such as c-CO32− but also the shift of peak positions. This result suggests that the presence of Ti3+/VO could provide additional adsorption sites to absorb CO2 and affect the chemical bonding of adsorbed CO2. To further illustrate the role of Ti3+/VO sites on the activation and conversion of CO2, the dynamics of photoinduced evolution of CO2−, c-CO32−, b-CO32−, m-CO32−, and HCO3− within the initial 10 min was characterized by calculating the normalized peak area ratio (i.e., peak area in spectrum 2 divided by peak area in spectrum 1 in Figure 9a or Figure 9d). It was found that within 10 min of photoillumination CO2−, c-CO32−, b-CO32−, and m-CO32− on TiO 2−x {101} decreased by 55−70% while those on TiO2{101} only decreased by 30−50%. The results indicate that it is easier and faster to activate and convert the adsorbed CO2 species on the defective TiO2 surface, because Ti3+/VO could serve as active sites, harvest visible light, and enhance charge separation. Again, the in situ DRIFTS results are well correlated with the photocatalytic activity measurements.

4. CONCLUSIONS We have demonstrated that engineering Ti3+/VO sites and coexposed {101}-{001} facets on TiO2 nanocrystals could favorably promote CO2 activation and conversion to CO under visible light. We found that NaBH4 reduction induced the formation of both surface and subsurface Ti3+ on the TiO 2−x {101} and surface/subsurface/bulk Ti 3+ on the TiO2−x{001}-{101} but only surface Ti3+ species on the TiO2−x{001}. The relative concentration of Ti3+ was on the order of TiO2−x{101} > TiO2−x{001}-{101} > TiO2−x{001}. Regarding the photocatalytic reduction of CO2 with H2O vapor, TiO2−x{001}-{101} demonstrated a much higher visible light activity than TiO2−x{101} and TiO2−x{001} alone and nontreated TiO2{001}-{101}. Although no cocatalyst and foreign dopants were applied, TiO2−x{001}-{101} demonstra1106

DOI: 10.1021/acscatal.5b02098 ACS Catal. 2016, 6, 1097−1108

Research Article

ACS Catalysis

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ted a quantum yield (0.31% under UV−vis light and 0.134% under visible light) higher than those in many literature reports. The presence of coexposed {001}-{101} facets and point defects could provide more active sites (e.g., unsaturated Ti5c atoms, Ti3+/VO sites), enhance charge separation and transfer by the {001}-{101} surface junction, extend visible light response by intra band gap Ti3+/VO energy state, and remarkably promote the activation and conversion of CO2 molecules. Future work will include the investigation of the optimum fraction of coexisted {001} and {101} facets to further enhance the CO2 photoreduction activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b02098. Irradiation spectra of UV−vis and visible light sources, slab models of anatase TiO2 {001} and {101} facets, calculated density of states (DOSs) and spin densities, XPS valence band spectra, and the procedure of exposed facets and QY calculations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*K.Y.: e-mail, [email protected]; tel, (858) 534-2514; fax, +1 858-534-9553. *Y.L.: e-mail, [email protected]; tel, +1 979-862-4465; fax, +1 979-845-3081. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.L. acknowledges support by the National Science Foundation (NSF) Early Faculty CAREER Award (CBET-1254709 & 1538404). K.Y. acknowledges support by start-up funds from the University of California, San Diego. The authors thank Dr. Chuanzhi Sun at Shandong Normal University for the EPR measurements and Dr. Fei Gao at Nanjing University for the XPS measurements.

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REFERENCES

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