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Detecting the Photoactivity of Anatase TiO(001)-(1×4) Surface by Formaldehyde 2

Bin Luo, Haoqi Tang, Zhengwang Cheng, Yuanyuan Ji, Xuefeng Cui, Yongliang Shi, and Bing Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04530 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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The Journal of Physical Chemistry

Detecting the Photoactivity of Anatase TiO2(001)-(1× ×4) Surface by Formaldehyde

Bin Luo, Haoqi Tang, Zhengwang Cheng, Yuanyuan Ji, Xuefeng Cui, Yongliang Shi, Bing Wang*

Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, Key Laboratory of Strong-Coupled Quantum Matter Physics (CAS), University of Science and Technology of China, Hefei, Anhui 230026, China

Corresponding Author *E-mail [email protected]; Tel +86 551 63602177 (B. W.).

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ABSTRACT: We report our investigation using temperature programmed desorption to detect the activity and the photoactivity of the reduced anatase TiO2(001)-(1×4) surface by using CH2O as a probe. We obtain the adsorption energy of 0.55 eV for CH2O on the surface, and find that the defect sites in the reduced surface are the only active sites for thermally-driven reaction to produce C2H4. We also identify the pathways for photo-decomposition of CH2O on the anatase TiO2(001)-(1×4) surface, which undergoes through breaking C-H bond of CH2O to form the intermediates of HCOO-. After the ultraviolet light irradiation, the dissociation of HCOO- produces CO and H2O at elevated temperatures. Accompanying with desorption of H2O, we observe higher production of C2H4 than that in the not-irradiated sample. We interpret our experimental results by attributing to the temporary change of the (1×4) ridge structure due to water desorption, that is, the initially oxygen-rich five-fold coordinated Ti atoms change to the four-fold coordinated Ti atoms. The four-fold coordinated Ti sites thus play as the highly active sites for coupling reaction of CH2O to produce C2H4. Our findings here provide insightful understanding for the thermal and photocatalytic reactions of CH2O on the anatase TiO2(001)-(1×4) surface.

1. INTRODUCTION ACS Paragon Plus Environment

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Among the various facets in the different polymorphs of TiO2, the activity of anatase TiO2(001) surface and its reconstructed (1×4) surface is somewhat a mysterious issue. Because of its high surface energy, the (001) surface favors a (1×4) reconstruction.1-10 In various models of the (1×4) reconstruction,3, 7-8, 10-12 the so-called ad-molecule model (ADM)12 has been conventionally used to describe the atomic geometry of the reconstructed (1×4) surface.4, 9, 13-17 Based on the ADM, the (1×4) surface has long been theoretically predicted to be highly active.14-15, 18-19 However, many experimental works showed that the activity of the surface is not active as expected.6,

20-24

Nearly equal

photochemical rate constants were observed in anatase (001) and rutile (110) surfaces.24 The anatase (001) facet even showed a lower photoactivity than the anatase (101) facet,20, 23 using the nanoparticles with coexposed {001} and {101} facets.25 In our previous works,7, 26 we have shown that the anatase TiO2(001)-(1×4) surface are quite inactive for the thermally-driven reaction of H2O and CH3OH using epitaxially grown films with high quality. On the basis of our observations, we proposed an ad-oxygen model (AOM), that is, the Ti atoms at the reconstructed ridges are five-fold coordinated (Ti5c),7 instead of the four-fold coordinated Ti atoms (Ti4c) by modifying the ADM.12, 27. Our experimental results showed that the adsorption energies of H2O and CH3OH are about 0.5-0.6 eV, in good agreement with the calculated values according to the AOM model.7, 26 Whereas, the theoretical calculations indicated that the AOM is energetically less favored than the ADM at the typical experimental conditions in the sample preparations.9,

28

It remains an open question about the atomic geometry and the intrinsic

(photo)activity of the anatase TiO2(001)-(1×4) surface. To address such a situation, more insightful experimental works are obviously needed. In addition to H2O and CH3OH, CH2O can be another probing molecule to detect the activity and photoactivity of the anatase TiO2(001)-(1×4) surface. While the properties of CH2O on rutile TiO2(110) ACS Paragon Plus Environment


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surfaces have been widely studied and well addressed,29-35 less is known about the photocatalytic reaction of CH2O on the anatase (001)-(1×4) surface. The knowledge about the reactivity of CH2O on the anatase (001)-(1×4) surface is of course necessary for the fundamental understanding of TiO2 photocatalysis. In this work, we investigate the photocatalytic reactions of CH2O adsorbed on reduced anatase TiO2(001)-(1×4) surface with epitaxially grown anatase TiO2(001) films on SrTiO3(001), characterized by temperature programmed desroption (TPD). We find that the desorption energy of CH2O is as small as 0.55 eV, and the thermally-driven reaction of CH2O occurs at the reduced defect sites. Our results can be well interpreted by adopting the AOM, in which the Ti5c sites have lowered activity. The photocatalytic decomposition of CH2O to produce CO and C2H4 is observed, and CH3OH is also observed as a by-product after ultraviolet (UV) light irradiation. The mechanism of the photocatalytic reaction of CH2O is discussed.

2. EXPERIMENTAL SECTION All experiments were conducted in an ultrahigh vacuum (UHV) chamber with a base pressure below 5 × 10-11 mbar, equipped with a quadrupole mass spectrometer (QMS, MAX-120, Extrel) for TPD measurements and a low-energy electron diffraction and Auger electron spectrometer (LEED/AES, SPECTALEED, Omicron) for surface structure characterization. The anatase TiO2(001) film sample was prepared in another UHV multifunctional system, composed with a pulsed laser deposition (PLD) chamber (base pressure of 1 × 10−10 mbar), an STM chamber (3 × 10−11 mbar, Omicron) and an x-ray/ultraviolet photoemission spectroscopy (XPS/UPS) analysis chamber (8 × 10−11 mbar, VG Scienta). The procedure for sample preparing was reported previously elsewhere.7 The


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anatase thin films were deposited by KrF excimer laser (Coherent, 248 nm, 4 Hz, pulse duration of 20 ns, ∼10 mJ/pulse) on a 0.7 wt% Nb-doped SrTiO3(001) substrate, with a size of 10 × 4 × 0.5 mm3, which was kept at 920 K in O2 atmosphere with pressure of 1.5 × 10-5 mbar during the sample preparing and cooled down to room temperature slowly in O2 atmosphere after depositing. The as-grown sample was then transferred to TPD chamber in N2 atmosphere and mounted on a Ta foil for resistive heating, using high temperature ceramic adhesive (Ceramabond 552, Aremco). Before TPD measurements, the sample was cleaned and reduced by Ar+ ion sputtering at 1 keV for 2 min and annealing at 900 K for 20 min. The cleanness and reconstruction were examined by AES and LEED pattern, respectively. The CH2O vapor was obtained by thermal decomposition of paraformaldehyde (96%, J&K Scientific). In the experiment, the paraformaldehyde powder in a quartz tube, which was connected with the TPD chamber through a leak valve, was pumped to 4×10−2 mbar, and then the powder was degassed at 330 K overnight. Followed, the paraformaldehyde powder was heated to 450 K for 2 min, and cooled back to room temperature. In the heating process, the tube was not pumped, and after cooling to room temperature the tube was pumped to recover the pressure (4×10−2 mbar). After the purifying treatments for several cycles, the paraformaldehyde powder was then decomposed at 450 K, by keeping the pressure of CH2O vapor at 4.0 mbar in the tube for dosing. The CH2O vapor was dosed through the leak valve. In the UHV chamber, the valve connected a tube (4 mm in diameter) to conduct the vapor to the sample surface with the tube outlet away from the surface by about 10 mm during dosing. The UV light for photoreaction experiments was generated by a Nd:YAG laser (wavelength of 266 nm at 20 mW, 10 Hz, Quanta-Ray, Spectra-Physics). The incident direction was perpendicular to the surface, and the light spot was about 8 mm in diameter. During UV light irradiation, the sample was ACS Paragon Plus Environment


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kept at ~110 K. In the TPD measurements, a constant rate of 1 K/s was used. The details of our TPD were described elsewhere.26

3. RESULTS ×4) surface. 3.1 Thermally-driven reaction of CH2O on reduced anatase TiO2(001)-(1× Figure 1a shows the TPD spectra of CH2O at various coverage on a reduced anatase TiO2(001)-(1×4) surface, using the TPD signal of mass-to-charge ratio, m/z = 29 (CHO+). In these spectra, we define the red one corresponding to 1 monolayer (ML), in which the shoulder peak from the second adsorption layer just disappear, in comparison with the spectrum labeled by 1.2 ML. The labeled coverages are obtained by integrating the area of each spectrum, with respect to the area of the defined spectrum of 1 ML. We here suggest that a unit cell of the anatase TiO2(001)-(1×4) surface contains one CH2O molecule, that is, 1 ML = 1.73 × 1014 cm−2, by assuming that CH2O should adsorb at the ridge Ti5c sites, similar to the CH3OH adsorption on this surface.26 Our assumption here also considered the fact that the adsorption of CH2O behaves quite similar to that of CH3OH on the rutile TiO2(110) surfaces.30-32, 36-37 Figure1b shows the spectra of m/z = 28, 29, 30, and 31 at the CH2O coverage of 1 ML. The signals of 28, 29, and 30 are from the fragments of CO+, CHO+, and their parent ion of CH2O+, respectively. These signals should reflect the desorption of molecular CH2O at relatively low temperature, with the desorption peaks centered at about 200 K. The weak signal of m/z = 31 can be assigned to the ions such as CHDO+ and/or HC18O+, which are originated from naturally abundant isotopologues of CH2O,34 with about 1% from our measurement (inset in Figure 1b). The almost overlapped spectra of m/z = 29 (in gray) are from several measurements at the same CH2O exposure, which shows the quite repeatable coverage of 1 ML in our experiment. This information is useful for ACS Paragon Plus Environment

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the measurements of the reaction of CH2O under UV light irradiation, as we will discuss below. From the desorption peak at 200 K, the desorption energy of CH2O from the anatase TiO2(001)-(1×4) surface is estimated to be 0.55 eV, by considering the first-order desorption model.38 Our calculations give an adsorption energy of 0.44 eV for the physisorption of molecular CH2O at the Ti5c site based on the AOM model (Supporting Information Figure S1). The calculated adsorption energy of CH2O is a little bit smaller than the value of 0.6 eV for adsorption of CH3OH based on the AOM model.26 Our calculations can well support our assumption that CH2O should also similarly adsorb at the ridge Ti5c sites of the (1×4) reconstructed surface based on the AOM model. Note that in the previous work the calculations gave an adsorption energy of 0.48 eV for the molecular CH2O with the carbonyl group aslant standing on the top of Ti4c site based on the ADM model.18 In comparison, a very recent work reported that the adsorption energy of CH2O on anatase TiO2(101) surface lies in the range from 0.6 to 0.8 eV.39 Here, just from the adsorption energy of CH2O, it is obviously insufficient to determine the exact geometry of the (1×4) ridges. We therefore need to further consider the reactivity of the surface. In addition to the molecular desorption of CH2O, it is observed that there are fragments of m/z = 26 and 27 with the desorption peaks centered at 560 K. These fragments can be most likely attributed to the product of C2H4, by considering the almost identical intensities between signals 26 and 27, in accordance with the TPD signals from pure C2H4, as shown in the inset of Figure 1c. In our measurements by dosing pure C2H4 on the anatase TiO2(001)-(1×4) surface, we find that the adsorbed C2H4 molecules have already been totally desorbed at a temperature below 200 K (Supporting Information Figure S2a). It means that the observed C2H4 production should be from the thermally-driven reaction of CH2O at a higher temperature of about 560 K. It is observed that the ACS Paragon Plus Environment


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production of C2H4 shows dependence on the coverage of CH2O, and become saturated at the coverages higher than 1 ML, judged by the signals of 26 and 27, as shown in Figure 1d (only the signals of m/z = 26 are shown). The saturated production of C2H4 means that C2H4 is mainly produced at the active sites with a well-defined number. We attribute the active sites to the defect sites, that is, reduced Ti4c pairs, which has been characterized in our previous works,7, 26 since in the as-grown sample (no reduced defects) we did not observe the production of C2H4. The reaction of CH3OH has been observed at the reduced defect sites in the similar reduced surface at around 525-600 K.26 Other signals, such as m/z = 2, 4, 15, 18, 32, 44, 45, 46, 59 and 60, were also monitored in our TPD measurements, but no obvious signal was detectable in the TPD spectra. Our results show that C2H4 is the only product in the thermally-driven reaction of CH2O at the defect sites on the reduced anatase TiO2(001)-(1×4) surface. The production of C2H4 should undergo a carbon-carbon coupling reaction, similar to the results reported previously on rutile TiO2(110) surfaces.29, 33-35

Figure 1. (a) TPD spectra of m/z = 29 (CHO+) at various coverage of CH2O. (b) TPD spectra of m/z =


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28, 29, 30 and 31 at the CH2O coverage of 1 ML. 4 additional spectra (in gray, for m/z = 29) are also plotted, which are obtained from different measurements at the same CH2O exposure. These overlapped spectra for m/z = 29 show the highly controllable coverage of CH2O at 1 ML (variation within 4%). Inset in (b): the enlarged signal of 31 by a factor of 92, in comparison with the signal of 29. (c) TPD spectra of m/z = 26 and 27, measured at the CH2O coverage of 1 ML. Inset in (c): the normalized relative intensity (blue) of the peaks at 560 K for m/z = 26 and 27 from the product, in comparison with the relative intensity of the signals from the desorption peaks of pure C2H4 (red). (d) Comparison of the TPD spectra of m/z = 26 at various CH2O coverage.

3.2 Photocatalytic reaction of CH2O on reduced anatase TiO2(001)-(1× ×4) surface. Figure 2a-d shows the TPD spectra of the fragments from the CH2O adsorbed samples after UV light irradiation for several minutes. The initial coverage of CH2O is 1 ML in each measurement. It is seen that the desorption peaks of m/z = 28, 29, and 30 at 200 K decrease with the increase of the irradiation time (Figure 2a-c). Meanwhile, the desorption peaks at 560 K for m/z = 28 (Figure 2a) and for m/z = 26 and 27 (only signal 26 is given in Figure 2d) increase with the increase of the irradiation time within 5 min, but the peaks tend to decrease after irradiation for a longer time of 10 min. The enhanced fragments of m/z = 26, 27, and 28 can be attributed to the increased production of C2H4 under UV light irradiation. Unlike the saturated peak for m/z = 26 shown in Figure 1d, in which the production of C2H4 is limited only at the defect sites by thermally-driven reaction, the photo-induced production of C2H4 here should be no longer only at the original defect sites, but can be attributed to the reaction of CH2O at the lattice sites. In Figure 2a-c, the decrease of the peaks after UV light irradiation also reflects the photo-induced reaction of CH2O to other species, although the ACS Paragon Plus Environment


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photo-induced desorption of molecular CH2O may also cause the decrease of the peaks during the UV light irradiation, similar to the case for CH2O on the rutile TiO2(110) surface.34 It is noticed that after UV light irradiation the signals for m/z = 28 in the higher temperature region are obviously increased (Figure 2a and its inset), in comparison, the signal for m/z = 29 is very weak in the higher temperature region (Figure 2b). After UV light irradiation, the enhanced signal for m/z = 28 may contain the fragments of C2H4+ and CO+. To separate their contributions, the simultaneously acquired signals for m/z = 28 and 26 after irradiation for 1 min are plotted in Figure 2e. According to the ratio of 1.4 between the signals of 28 and 26 from pure C2H4 (inset in Figure 1c), we have the signal of 26 multiplied by a factor of 1.4 (dashed red line in Figure 2e), and subtract the multiplied signal from the signal of 28. The result is shown by the purple line, which reflects the CO product, while the shadow area is thus the contribution from the C2H4 product. The CO product in the photocatalytic decomposition of CH2O on the rutile TiO2(110) surface has been observed, which accompanies the fragments of m/z = 29 and 30. The production of CO has been assigned to the thermal decomposition of the intermediates of HCOO-/HCOOH.29, 33-34 Considering the quite similar products in our experiment, we also suggest a similar process on the anatase TiO2(001)-(1×4) surface. To confirm this, we measured the TPD signals by dosing pure formic acid (HCOOH) on the anatase TiO2(001)-(1×4) surface (Supporting Information Figure S3), and compared with the product from the photocatalytic decomposition of CH2O at the high temperature region (see Figure 2b-c), as shown in Figure 2f. It is seen that the relative intensities of the fragments from the irradiated CH2O adsorbed sample are quite comparable with the ones from the sample by directly exposing pure HCOOH. In Figure 2f, we use the derived amount of CO+, where the contribution from the C2H4+ has been subtracted (Figure 2e). In addition, we also compared the other signals ACS Paragon Plus Environment

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corresponding to HCOO-/HCOOH, that is, m/z = 44 (CO2+), 45 (HCOO+), and 46 (HCOOH+) in the photocatalytic decomposition of CH2O. There is almost no obvious signal in the spectra of 45 and 46, but fairly obvious signals can be seen in the spectrum of 44 in comparison with the spectra of 45 and 46 (Supporting Information Figure S4). Overall, their relative intensities are comparable with those in the experiment by dosing pure HCOOH (Supporting Information Figure S3b). Our results suggest that the production of CO on the anatase TiO2(001)-(1×4) surface should follow the mechanism similar to that on the rutile TiO2(110) surface, that is, through the intermediates of HCOO-/HCOOH.29, 33-34 In our TPD experiment, we find that the products from CH2O after UV light irradiation mainly present at temperature higher than 400 K, which behave quite different from the desorption peaks centered at 200 K for molecular CH2O (Figure 1a). As shown above, the product of CO is mainly from the thermal decomposition of the intermediate product of HCOO-/HCOOH. As to the product of C2H4, our experimental results show that the dosed pure C2H4 almost totally desorbs from the anatase TiO2(001)-(1×4) surface at temperature lower than 200 K (Supporting Information Figure S2a), and more, no C2H4 can be produced by exposing pure HCOOH on this surface (Supporting Information Figure S3a). These results suggests that C2H4 is neither directly produced during the UV light irradiation, nor through the intermediate of HCOO-/HCOOH, which will be discussed below.



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Figure 2. TPD spectra recorded after UV light irradiation, (a) m/z = 28, (b) m/z = 29, (c) m/z = 30, (d) m/z = 26, from the initial CH2O coverage of 1 ML. Insets in each panel: the enlarged signals by a factor of 10 in the temperature range from 400 to 700 K. (e) Derived signal of CO+ for the peaks at 560 K, for instance, the signals after UV light irradiation for 1 min are shown. The CO+ signal (purple line) is obtained by subtracting the signal of 26 with a multiplying factor of 1.4 (red dashed line) from the signal of 28. Inset: enlarged signals in the temperature range from 400 to 700 K. (f) Comparison of the normalized integrated intensities for the signals from 400 to 700 K between the CH2O-adsorbed sample after UV light irradiation and the sample with adsorption of pure HCOOH. Inset: TPD signals from the adsorption of pure HCOOH on the anatase TiO2(001)-(1×4) surface.

Besides the products of C2H4 and CO, we also observed a by-product of CH3OH, as shown in Figure 3. In the spectrum before UV light irradiation (black curve in Figure 3a), the peak at 200 K for m/z = 31 originates from naturally abundant isotopologues of CH2O.34 No obvious peak can be seen in the spectrum of m/z = 32 before irradiation (black curve in Figure 3b). After UV light irradiation, additional peaks appear in both sets of the spectra for m/z = 31 and 32 at 280 K. Their intensities tend ACS Paragon Plus Environment

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to increase with the increase of the irradiation time within 5 min, but decrease after irradiation for 10 min. Such behaviors is quite similar to the spectra for the product of C2H4 (Figure 2d). In the spectra of m/z = 31, the decrease of the peaks for the CH2O isotopologues at 200 K is consistent with the desorption of CH2O during UV light irradiation, as seen in Figure 2a-c. We suggest that the appeared peaks in the spectra of m/z = 31 and 32 are from the product of CH3OH during UV light irradiation (Figure 3c). It is noted that in the TPD spectra, the by-product of CH3OH should also have some other fragments, like m/z = 29 (CHO+), 30 (CH2O+), but these signals are not resolvable due to the quite strong background from CH2O around 280 K (Figure 2a-c).

Figure 3. TPD spectra after UV light irradiation, (a) m/z = 31, (b) m/z = 32. (c) Comparison of the spectra of m/z = 31 and m/z = 32 before (black) and after (red) UV light irradiation for 5 min. The spectra are shifted vertically for clarity.

Figure 4a and 4b shows the measured coverage of CH2O and the yields of different products as a function of irradiation time, respectively. The monotonic decrease of the CH2O coverage can be caused by two reasons. One is the conversion of CH2O to other species under UV light irradiation, and the other is the photo-stimulated desorption of CH2O during irradiation, as the case for CH2O on the rutile



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TiO2(110) surface.34 We first consider the conversion of CH2O to other species, which can be estimated from the amount of the observed products in our TPD measurements. The yields of C2H4 were obtained by using the areas of signal 26 shown in Figure 2d, in comparison with the area of signal of 26 for 1 ML of C2H4 by exposing pure C2H4 to the same surface (Supporting Information Figure S2a). The C2H4 production from the not-irradiated sample is about 0.016 ML. The CO yield can thus be obtained by using the signals of 28, by separating it from the contribution of C2H4, as shown in Figure 2e. Here, we neglected the difference in ionization cross sections between C2H4 and CO, since their difference is within 10%.40-42 As to the CH3OH yield, it is obtained by comparing the area of CH3OH product (the m/z signals of 31 and 32 in Figure 3) with the signals of 1 ML adsorption of pure CH3OH in the same surface (Supporting Information Figure S2b). The yields of the products are summarized in Figure 4b, in which the gray bars indicate the converted amount of CH2O, estimated from the number of C atoms in the products. After irradiation for 5 min, for example, about 0.18 ML CH2O was photocatalytically converted to 0.1 ML CO, 0.03 ML C2H4, and 0.017 ML CH3OH, where the C2H4 of about 0.016 ML by the thermally-driven reaction at the defect sites was not included. An amount of 0.19 ML CH2O can be due to the photo-stimulated desorption, by considering the decreased total amount of CH2O by 0.37 ML at 5 min. After a longer irradiation time for 10 min, the observed yields of the products all decrease. This can be understood by considering both of the desorption and the conversion of CH2O, leading to a decreasing fraction of CH2O on the surface. As a result, under UV light irradiation, the reverse reaction from the intermediate species to CH2O may become another factor to decrease the observed products. Here, the photo-stimulated desorption of the intermediate species may also lead to the decrease of the observed products. However, the intermediate species of HCOO-/HCOOH present at temperatures ACS Paragon Plus Environment

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higher than the desorption peak of molecular CH2O in the TPD spectra, indicating that the produced intermediate species should have a higher adsorption energy than CH2O. We therefore suggest that the photo-stimulated desorption of the intermediate species should be less significant than that of CH2O. The by-product of CH3OH can be ascribed to the reaction of CH2O with the produced H adatoms (Had) during UV light irradiation or during heating the sample, similar to the reaction between CH2O and Had on rutile TiO2(110) surface.36, 43

Figure 4. (a) CH2O coverage as a function of irradiation time. (b) Yields of CO (red), C2H4 (black), and CH3OH (blue) as a function of irradiation time. The gray bars and the dashed line indicate the amount of converted CH2O during UV irradiation, estimated from the total amount of C in the products.

4. DISCUSSION We now consider the possible reaction processes for CH2O on the surface under UV light irradiation. Based on the results discussed above, we propose that the C-H bond breaking during UV



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light irradiation is the first step for photocatalytic decomposition of CH2O on anatase TiO2(001)- (1×4) surface. The HCO- species would then react with O atoms on the ridge to produce HCOO-, as described by hν

CH2O + Oridge → HCOOridge + Had. During TPD measurement by heating the sample to an elevated temperature, the decomposition of HCOO- leads to the production of CO. As shown in Figure 5a, only after UV light irradiation can a weak peak in the signal of m/z = 18 (H2O+) be observable. In contrast, no product of H2O can be obviously detectable in the CH2O adsorbed sample before UV light irradiation. The desorption temperature of H2O ranges from 200 K to 400 K, which is likely a sign of recombination desorption, similar to the case on rutile TiO2(110) surfaces.33, 44 It should be noticed that the desorption temperature of H2O on anatase TiO2(001)-(1×4) surface is lower than that on rutile TiO2(110) surface. In contrast to the prediction by the ADM model,14 no spontaneous dissociation of water on the anatase TiO2(001)-(1×4) surface was observed.26 This can be explained according to the oxygen-rich surface based on the AOM model,7 where the Ti atoms at the ridge are five-fold coordinated, instead of the four-fold coordinated Ti atoms in the ADM model.12 In our previous work,26 we have shown that the desorption peak of adsorbed H2O centers at about 185 K. In comparison, the produced H2O desorbs at about 240 K in the thermally-driven reaction of CH3OH on the anatase (001) surface. In the thermally-driven reaction of CH3OH, CH3OH dissociates only at defect sites through breaking the O-H bond to produce Had, leading to the formation and desorption of H2O. This process needs to take the lattice O atoms by Had from the surface, which has been confirmed in our previous experiment using isotopically-labelled CH318OH.26 In the case of photocatalytic reaction of CH2O, it is reasonably believed that the produced Had should also take the O ACS Paragon Plus Environment

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atom from the lattice site of the surface. As a result, the ridge site may be changed from the AOM structure to the ADM structure temporally, as schematically shown in Figure 5b and described by ∆

2Had + OAOM-ridge → H2O (g) + ADM-ridge, where (g) indicates the desorption of gaseous H2O. Considering the pure HCOOH adsorbed sample does not produce C2H4, we suggest that the production of C2H4 can be ascribed to the coupling reactions between CH2O adsorbed near the temporally produced ADM sites when the sample is heated to about 560 K, possibly through a species of -OH2C-CH2O-. The ADM sites (Ti4c sites) thus play as the active sites for the production of C2H4, in which the process is somewhat similar to the production of C2H4 at the reduced defect sites in the not-irradiated sample (Figure 1c). Finally, the ridge can be recovered to the AOM structure after releasing C2H4 and leaving the O atoms at the surface. Our observations here provide another piece of evidence that the anatase TiO2(001)-(1×4) surface should originally be in the AOM structure, at least a certain oxygen-rich structure. The reactions can be described by ∆

2CH2O + ADM-ridge → -OH2C-CH2O- → C2H4 (g) + AOM-ridge. In the process, the production of C2H4 is thus a consequence of the activation of the surface sites due to the temporal change from the AOM structure to the ADM structure. This process is different from the one on rutile TiO2(110) surface.34 On the rutile TiO2(110) surface, it was observed that photocatalytic decomposition of CH2O at a Ti5c site undergoes through breaking the C-O bond and transferring the methylene to a bridge-bonded oxygen atom (BBO), and the O atom remaining on the Ti5c site is responsible for the formation of HCOO-. As a result, the formation of C2H4 and CH3 undergoes through reactions of methylene on the BBO rows on the rutile TiO2(110) surface.34



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Figure 5. (a) TPD spectra of m/z = 18 before (black) and after (red) UV light irradiation for 5 min, from the initial CH2O coverage of 1 ML. (b) Schematic drawing of the ridge change from the AOM structure to the ADM structure after the desorption of H2O, where Had atoms are produced from photocatalytic decomposition of CH2O on the anatase TiO2(001)-(1×4) surface with an initial AOM structure. Noted that the desorption of HCOO- species should be dissociative.

5. CONCLUSIONS In conclusion, our experimental results strongly suggest that the AOM structure can describe the adsorption and the reaction of CH2O better than the ADM structure for the anatase TiO2(001)-(1×4) surface, probed by CH2O molecules using TPD. We experimentally obtain the adsorption energy of 0.55 eV for CH2O on the anatase TiO2(001)-(1×4) surface, in agreement with our calculated value ACS Paragon Plus Environment

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based on the AOM structure. In the not-irradiated sample, the product of C2H4 can only be formed at the reduced defect sites. While, under UV light irradiation, CH2O can be photocatalytically dissociated by breaking the C-H bond to produce Had and HCOO- species on the surface. At elevated temperatures, the intermediates of HCOO- species can be thermally dissociated to CO, and accompanying with the process, the formation and desorption of H2O by Had through picking up the O atoms from the surface may lead to the temporal change of the AOM sites to the ADM sites. The ADM ridge sites could then play as the active sites for dissociative adsorption of CH2O, leading to the formation of -OH2C-CH2Ospecies near the ADM ridge sites, which should be responsible for the production of C2H4 at the temperatures around 560 K. Our findings show that the anatase TiO2(001)-(1×4) surface is photoactive for decomposition of CH2O, even the surface is thermally inactive for the reaction of CH2O at the perfect lattice sites, which can be well explained by adopting the AOM model.

ASSOCATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS publications websites at DOI Figure S1(a), TPD spectra obtained by dosing ethylene (C2H4) on the anatase TiO2(001)-(1×4) surface at 80 K, at C2H4 coverages of 1 and 1.9 ML. The TPD sprectra show that the adsorbed C2H4 almost totally desorb at a temperature below 160 K; Figure S1(b), TPD spectra obtained by dosing CH3OH on the anatase TiO2(001)-(1×4) surface at 110 K at methanol coverages of 1, 1.1 and 1.3 ML. Figure S2, TPD spectra of different m/z signals obtained by dosing HCOOH to anatase TiO2(001)-(1×4) surface, showing that the dissociative desorption of HCOOH mainly produces CO, but does not produce ethylene, since no any obvious peak can be observed in the signal of 26. ACS Paragon Plus Environment


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the MSTC (Grants 2016YFA0200603, 2013CB834605), the “Strategic Priority Research Program” of CAS (grant XDB01020100), and the NSFC (Grants 91421313, 21421063, 21573207).


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Yang, X. M. Effect of the Hydrogen Bond in Photoinduced Water Dissociation: A Double-Edged Sword. J. Phys. Chem. Lett. 2016, 7, 603-608.

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Figure 1. (a) TPD spectra of m/z = 29 (CHO+) at various coverage of CH2O. (b) TPD spectra of m/z = 28, 29, 30 and 31 at the CH2O coverage of 1 ML. 4 additional spectra (in gray, for m/z = 29) are also plotted, which are obtained from different measurements at the same CH2O exposure. These overlapped spectra for m/z = 29 show the highly controllable coverage of CH2O at 1 ML (variation within 4%). Inset in (b): the enlarged signal of 31 by a factor of 92, in comparison with the signal of 29. (c) TPD spectra of m/z = 26 and 27, measured at the CH2O coverage of 1 ML. Inset in (c): the normalized relative intensity (blue) of the peaks at 560 K for m/z = 26 and 27 from the product, in comparison with the relative intensity of the signals from the desorption peaks of pure C2H4 (red). (d) Comparison of the TPD spectra of m/z = 26 at various CH2O coverage. 131x99mm (300 x 300 DPI)



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Figure 2. TPD spectra recorded after UV light irradiation, (a) m/z = 28, (b) m/z = 29, (c) m/z = 30, (d) m/z = 26, from the initial CH2O coverage of 1 ML. Insets in each panel: the enlarged signals by a factor of 10 in the temperature range from 400 to 700 K. (e) Derived signal of CO+ for the peaks at 560 K, for instance, the signals after UV light irradiation for 1 min are shown. The CO+ signal (purple line) is obtained by subtracting the signal of 26 with a multiplying factor of 1.4 (red dashed line) from the signal of 28. Inset: enlarged signals in the temperature range from 400 to 700 K. (f) Comparison of the normalized integrated intensities for the signals from 400 to 700 K between the CH2O-adsorbed sample after UV light irradiation and the sample with adsorption of pure HCOOH. Inset: TPD signals from the adsorption of pure HCOOH on the anatase TiO2(001)-(1×4) surface. 150x74mm (300 x 300 DPI)


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Figure 3. TPD spectra after UV light irradiation, (a) m/z = 31, (b) m/z = 32. (c) Comparison of the spectra of m/z = 31 and m/z = 32 before (black) and after (red) UV light irradiation for 5 min. The spectra are shifted vertically for clarity. 109x48mm (300 x 300 DPI)



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Figure 4. (a) CH2O coverage as a function of irradiation time. (b) Yields of CO (red), C2H4 (black), and CH3OH (blue) as a function of irradiation time. The gray bars and the dashed line indicate the amount of converted CH2O during UV irradiation, estimated from the total amount of C in the products. 69x92mm (300 x 300 DPI)


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Figure 5. (a) TPD spectra of m/z = 18 before (black) and after (red) UV light irradiation for 5 min, from the initial CH2O coverage of 1 ML. (b) Schematic drawing of the ridge change from the AOM structure to the ADM structure after the desorption of H2O, where Had atoms are produced from photocatalytic decomposition of CH2O on the anatase TiO2(001)-(1×4) surface with an initial AOM structure. Noted that the desorption of HCOO- species should be dissociative. 61x111mm (300 x 300 DPI)



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Detecting the Photoactivity of Anatase TiO2(001 ... - ACS Publications

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