Trapping Behaviors of Photogenerated Electrons on the (110), (101

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Trapping Behaviors of Photogenerated Electrons on the (110), (101) and (221) Facets of SnO2: Experimental and DFT Investigations Yucheng He, Wenxiu Que, Xiaobin Liu, and Chao Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11220 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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ACS Applied Materials & Interfaces

Trapping Behaviors of Photogenerated Electrons on the (110), (101) and (221) Facets of SnO2: Experimental and DFT Investigations Yucheng He a, Wenxiu Que a,∗ , Xiaobin Liu a, Chao Wu b,* a

Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic & Information

Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China b

Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China Abstract

Spatial separation of photogenerated charges between different crystal facets has been observed in some semiconductor photocatalysts, however, the charge separation mechanism is still ambiguous. As a characteristic parameter of crystal facet, surface energy may be a crucial factor to dictate the flow of photogenerated charges. In this work, the relationship between surface energy and the flow mode of photogenerated charges is investigated by using model photocatalysts, including lance-shaped SnO2 particles and dodecahedral SnO2 particles. The former are enclosed by two kinds of crystal facets with a big gap in surface energy, while the latter are composed of two types of crystal facets with nearly equal surface energy. However, the experimental results exhibit that the photogenerated electrons flow to the all exposed crystal facets randomly in both two kinds of SnO2 nanocrystals, which is opposite to what has been observed in extensively investigated semiconductor photocatalysts including TiO2, SrTiO3, BiVO4, BiOCl, Cu2O. Our results disqualify surface energy as an appropriate descriptor of preferential charge flow. Furthermore, the experimental results are confirmed by trapping energies and work functions calculated with the firstprinciples methods, which are proved to be more relevant parameters for describing the charge flow direction. Additionally, the trapping sites on each crystal facet are determined by charge analysis. Keywords: Photogenerated charge; Trapping energy; Surface energy; DFT calculations; Preferential flow

∗ Corresponding author: Tel.: +86-29-83395679; Fax: +86-29-83395679 Email address: [email protected], [email protected]

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1. Introduction Promoting charge separation and migration to the surface of photocatalysts have been attracting considerable attention because of its key role in photocatalytic reactions.1-3 To this end, constructing heterojunction structure based on semiconductor band structure theory is a powerful methodology, which has been employed in experiment. Generally, heterojunction structure is formed by combining more than two semiconductor materials together.4-9 In recent years, thanks to the development of facet engineering, semiconductors with novel surface heterojunction have been proposed, which are single crystalline with different crystal facets.10-18 Due to different surface atom arrangement and coordination, electrons and holes excited by light are driven to different crystal facets. Therefore, recombination of carriers is inhibited and the performance of photocatalysts is substantially improved. However, whether different surface atom arrangement and coordination are the critical factors to guide the photogenerated electrons and holes to migrate to different crystal orientations? Whether charge separation is a general rule for any two crystal facets with different thermodynamic properties? Up to now, the origin for the preferential flow of photogenerated carriers to specific crystal facet is still confusing scientists. Li has verified the fact that the preferential flow of photogenerated electrons and holes can be realized between different crystal facets in both low symmetry (monoclinic phase)19 and high symmetry (cubic phase) nanocrystals.20 Kudo et al. have observed that structure anisotropy induced by alkali earth metal cations can promote charge separation in BaLa4Ti4O15.21 In addition, wavelength and intensity of exciting lights have been proved to be unrelated to this phenomenon.22 It has been reported that in anatase TiO2 nanocrystals the preferential flows of photogenerated electrons and holes were to the exposed (101) facet [surface energy 0.44 J/m2] and the (001) facet [surface energy 0.90 J/m2],23 respectively,24 which was further confirmed by Tachikawa et al. by using single-molecule imaging and kinetic analysis.25 At the same time, Ohno provided the evidence that the spatial separation of photogenerated electrons and holes occurred between the (110) [surface energy 0.47 J/m2] and (011) [surface energy 0.95 J/m2] 26 facets in rutile TiO2.27 Besides TiO2, Li observed that the accumulations of electrons and holes were on the (010) [surface energy 0.502 J/m2] facet and the (110) [surface energy 0.552 J/m2] 28 facet in BiVO4, respectively. 19 Recently, the exposed (001) [surface energy 1.36 J/m2]

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and (110) [surface energy

1.54 J/m2] 30 facets of SrTiO3 nanocrystals exhibited the preferences of electrons and holes correspondingly.20


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The above results indicate that a small gap in surface energy between two kinds of facets drives the preferential flow of photogenerated carriers, but counter examples have been reported as well. For anatase TiO2 nanocrystals, the preferential flow was not observed for the (010) [surface energy 0.53 J/m2] 31 and (001) [surface energy 0.90 J/m2] facets.32 Moreover, Zhang and his co-workers reported that a big gap in surface energy between the exposed (001) [surface energy 0.026 J/m2] and (110) facets [surface energy 1.426 J/m2] 33 of BiOCl could initiate the preferential flow.34 The relationship between the difference of surface energy and the preferential flow still need more careful analysis. In the present work, two kinds of SnO2 nanocrystals were chosen as the models to investigate the effect of surface energy difference between two crystal facets on the flow mode of photogenerated carriers. One model is composed of the (110) [surface energy 1.401 J/m2] and (221) [surface energy 2.280 J/m2] crystal facets, which have a difference of 0.879 J/m2 in surface energy. The second model consists of the (101) [surface energy 1.554 J/m2] and (110) [surface energy 1.401 J/m2] facets, whose surface energies are nearly equal.35 In experiments, photo-reduction of noble metals (Au, Pt, and Ag) and photo-oxidation of Pb2+ were used to track the migration of electrons and holes, respectively. In addition, in order to better understand the experimental observations, the first-principles density functional theory (DFT) calculations were used to investigate the factors concerning the flow mode of photogenerated electrons and holes. 2.

Results and discussion

Lance-shaped SnO2 nanocrystals enclosed by 8 (221) and 4 (110) facets, and dodecahedral SnO2 nanocrystals enclosed by 8 (101) and 4 (110) facets were prepared by the hydrothermal method as reported previously. 36-37 Fig. 1 shows that the as-synthesized lanced-shaped SnO2 particles has a crystal size of 500 nm, with pyramidal tips bound by the (221) facets and middle section by the (110) facets. Two tips of relatively small size dodecahedral SnO2 particles (～20 nm) are bound by the (101) facets and middle parts by the (110) facets. The in-situ photo-reduction of noble metals Au, Pt, and Ag were carried out to probe the paths of electrons transfer. Water or methanol was used as hole scavenger and they could be simultaneously oxidized. HAuCl4, H2PtCl6, and AgNO3 were used as precursors correspondingly. Surface functional groups of all as-prepared SnO2 samples were removed before performing photo-reduction and photo-oxidation, which were verified by FTIR spectra of lanced-shaped SnO2 dodecahedral SnO2 particles. As shown in Fig. S1, except the broad


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bands of framework vibrations of SnO2 located between 400-800 cm-1,38-39 no other surface functional groups were detected. The role of the amount of precursors was investigated firstly. The photo-reduction of Pt in TiO2 nanoparticles verified that when the amount of precursors was too large, the preferential flow of photogenerated carriers could not be clearly observed.24 Figs. 2a-2c show TEM images of the lance-shaped SnO2 particles with different Au-loading amounts. It can be clearly observed that as the amount of the precursor increases from small to large, Au particles (～5 nm) were photo-reduced on the (221) and (110) facets at the same time. Furthermore, the time of photo-reduction was also investigated. When the time of photo-reduction was prolonged to 4 hours, more Au particles were loaded on the (221) and (110) facets randomly and the Au particle size remained nearly unchanged (comparing Fig. 2a with Fig. 2d), which is similar to the observation of Ag particles in situ photo-deposited on the surface of BaLa4Ti4O15.40 When we substituted water with methanol as hole scavenger, Au particles with bigger sizes (～20 nm) were reduced on the (221) and (110) crystal facets as seen in Fig. 2e and Fig. 2f. Different from the dispersive distribution of Au particles reduced by using water as hole scavenger, only a few Au particles distributed on the surface of lance-shaped SnO2 particles when methanol solution was used. The experiments indicate that the growth of Au particles is easier than the nucleation in methanol, while the opposite is true in water. Compared with water, methanol is known to accelerate the growth rate of Au nanoparticles.19, 41 Moreover, AuCl4- in the precursor solution was fixed at only 1% when either methanol or water was used as hole scavenger. Therefore, as shown in Figs. 2c, 2e and 2f, when larger Au particles form, their numbers decrease. These results confirm that for SnO2 materials, the preferential flow of photogenerated electrons cannot be realized on two crystal facets with a big difference in surface energy. Besides Au, we also carried out Pt and Ag as probes at the same experiment conditions, but we could not find any Ag and Pt particles in the TEM images as shown in Fig. S2. Still, the metal ions were probably reduced as the lance-shaped SnO2 particles changed color. For example, the lance-shaped SnO2 particles became grey after being irradiated in a suspension consisting of lance-shaped SnO2 particles, H2O and H2PtCl6 for 1 hour under a 250 W Xe lamp. The evident color change indicates the reduction of Pt, but the underlying reason needs a systematic further study. Actually, the similar phenomena have also been observed in anatase TiO2. Large quantities of photoreduced Pt particles were detected on the (101) crystal facet, while it was difficult to detect for the photoreduced Ag particles.24, 32


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In order to follow the migration path of holes, we selected photo-oxidation of Pb2+, with IO3- as electron scavenger. Due to the precipitation-dissolution equilibrium of Pb(IO3)2 in the solution as follows: Pb(IO ) ⇔ Pb + 2IO We took ratio of IO3- to Pb2+ into account in photo-oxidation of Pb2+ in the first step. Although nothing was found on the surface of lance-shaped SnO2 particles in the TEM images as shown in Figs. 3a-3c, Pb4+ was detected in the energy spectrum as shown in Fig. 3d at the ratio of 1:2 for Pb2+ and IO3- in the precursor. These results indicated that the photogenerated holes in the bulk can migrate to the surface, while the surface structure of lance-shaped SnO2 particles is unfavorable for the adsorption of oxidation products. Hence, the ratio of Pb2+ to IO3- was fixed at 1:2 in the following investigation of other factors. It has been reported that the pH value of the precursor should be adjusted when photo-oxidation of Pb2+ was performed.42 Thus, we carried out the experiments using precursor solution with the pH value of 1.0, 2.5 and 4.5, respectively. Results indicated that when the pH value of precursor solution was 4.5, aggregations were formed on both the (221) and (110) facets of lance-shaped SnO2 particles and the composition was identified as PbO2 (Fig. 4a-4d.) Then, the photo-oxidation of Pb2+ was performed by using lance-shaped SnO2 particles previously loaded with Au particles. As shown in Fig. 5a-5b, the aggregation can be observed on some samples, while nothing on other samples. When we examined the samples that endured photo-irradiation for a longer time (1 h), no aggregation was found yet as shown in Fig. 5c. However, the binding energy of Pb 4f in the XPS (Fig. 5d) reveals that the deposited lead matters should be assigned to PbO2, which confirms that PbO2 is unable to bond on the surface of lance-shaped SnO2 particles again. Following the photo-reduction of Au and photo-oxidation of Pb2+ sequentially, we further investigated the simultaneous reduction of Au and oxidation of Pb2+. The morphology of particles on the surface of lanceshaped SnO2 particles varies with the amount of precursor as seen in Fig. 6a-6d. Specifically, a larger number of precursors promote the size increase of the reduction and oxidation products. However, on the surface of SnO2 particles, Au as shown in Fig. 6f but Pb can be identified. Meanwhile, we also found that the oxidation products PbO2 fell off from lance-shaped SnO2 particles (Fig. 6e and 6g). The photo-reduction products confirm again the fact that when the reduction of AuCl4- and oxidation of Pb2+ occur simultaneously, the photogenerated electrons will flow to either the (221) facet, or the (110) facet, or both the facets, randomly.


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The above experimental results show that for SnO2, no matter how the common factors (the amount and kind of precursors, the sort of hole acceptors, the photo-deposition time and sequence) are varied, the spatial separation of the photogenerated electrons and holes cannot be achieved between crystal facets, in spite of the big difference in surface energy. In the following section, we investigated the flow mode of the photogenerated electrons between two crystal facets with a nearly equal surface energy by means of in-situ photo-reduction of Pt on the surface of dodecahedral SnO2 particles enclosed by the (110) and (101) crystal facets. Fig. 7 shows that the Pt particles (about several nanometers) were photo-reduced on the (110) and (101) facets of dodecahedral SnO2 particles simultaneously, whether a small or a large number of precursors was used. The content of the Pt particles on the surface of dodecahedral SnO2 particles increased with increasing precursor concentration, which is similar to the photo-reduction of Ag on the surface of BiVO4.43 Even at a low concentration, the preferential photodeposition of Pt particles on either the (110) or (101) facets was not detected. These results suggest that a little difference in surface energy also does not mediate the preferential flow of photogenerated electrons in the semiconductor material like SnO2. In addition, we also used dodecahedral SnO2 particles to carry out the photo-reduction of Au particles and the corresponding result is shown in Fig. S3. As a summary of experiments, the flow of charges along two facets in SnO2 behaves different from other semiconductors mentioned above when surface energy difference is used as a caliber. In order to better understand the above phenomena observed in SnO2 in atomic scale, we employed the DFT calculations to investigate the factors that govern the flow of the photogenerated electrons along the (110), (101) and (221) crystal facets. It is often reported that a photogenerated electron tends to be trapped by surface atom to form a polaron, which consists of a localized electron and an accompanying local lattice distortion. Hence, it is critical to evaluate the trapping energy of the photogenerated electrons on different crystal facets. The trapping energy is defined as the energy difference between a fully relaxed polaronic solution and a fully relaxed free-carrier solution.44 To simulate a photogenerated electron excited in the SnO2 bulk, an extra negative charge was added to the slab model with a compensating uniform background. The slab models of the (110), (101) and (221) facets were created by cleaving the optimized SnO2 bulk structure (a=4.888 Å, c/a=0.672, and u=0.307). The stable oxide surface is formed by the dynamic balance of surface oxygen adsorption and


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desorption. Jiang and his co-worker have investigated the stability of various SnO2 crystal facets with all possible terminations,45 based on which, the stable surface structure of the (110), (101) and (221) facets were obtained and depicted in Fig.8. In order to remove the spurious electrostatic interaction in the polaronic solution, the size of vacuum region must be lengthened long enough. As shown in Fig. 9, the calculated polaronic solutions of the (110), (101) and (221) facets were converged within 0.001 Ha when the size of vacuum region reached 32 Å. The calculated trapping energies of the (110), (221) and (101) crystal facets as presented in Table 1 reveal the varying abilities of trapping electrons, but the difference in trapping energies is not large enough to be observed in our experiment. The experiments and calculations implicate the existence of some threshold to differentiate experimentally the trapping ability of various facets. Actually, according to the experimental results, our calculations only give a lower limit of the threshold, that is, when the difference in trapping energy of any two crystal facets of SnO2 is less than the difference of our calculated values, the preferential flow of photogenerated electrons cannot be observed due to the limitation of experiment. Yet, for SnO2, we could not get the upper limit of the threshold, which is powerful enough to initiate the preferential flow of photogenerated carriers, as we do not know which two facets will generate such a big enough trapping energy difference. Moreover, due to the limitation in synthetic method at present stage, such SnO2 crystals have not been identified and synthesized experimentally neither. Actually, the same question concerning the detection threshold also exists in the TiO2 system. In the experiment of photo-deposition of Ag, Lin et al. concluded that photogenerated electrons flew randomly to the (010) and (001) crystal facets.32 Although the author did not provide the trapping energies of the two facets, different atomic arrangements of the (010) and (001) crystal facets must yield different trapping energies. Obviously, the difference in trapping energies of the (010) and (001) facets is below the threshold, and accordingly fails to initiate the preferential flow of photogenerated electrons. Further, Dai et al. reported that the trapping energy of the (101) crystal facet of TiO2 was 0.98 eV more than that of the (001) crystal facet by means of the DFT calculations.46 Such a difference in trapping ability is still not big enough to keep all photogenerated electrons on the (101) crystal facet. In experiment, when the H2PtCl6 concentration was high (about 1%), a few photo-reduced Pt particles also appeared on the (001) crystal facets.24 Moreover, the more precise single-molecule imaging and kinetic analysis revealed that the difference of 0.98 eV in trapping


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energies was indeed not big enough to prevent the (001) facet from trapping a small number of electrons.25 Therefore, we speculate that for TiO2, 0.98 eV is only the lower limit of threshold to differentiate the electron trapping ability of crystal facets, which is perhaps different from SnO2. Additionally, the calculated trapping energies confirm that the (110) facet is the most stable polaronic structure, which agrees with the results reported in other semiconductor photocatalysts, that is, photogenerated electrons tend to be trapped on the crystal facet with lower surface energy. However, our results, together with previous reports, merely support the lowest surface energy criterion as a sufficient condition to form the most stable polaronic structure, not the necessary one. Furthermore, we analyzed the charge localization on the (110), (101) and (221) facets quantitatively by means of Mulliken charge population.47 As shown in Table 2, about 85, 95 and 87% of total charges successfully localize on the outermost layer of the (110), (101), and (221) facets, respectively, which corroborates the experimental observation of the reduction of noble metals on the (110), (101) and (221) facets. Specifically, about 50% or more charges were predicted to reside on the coordinatively unsaturated Sn site, which if with lower coordinations, tends to hold more negative charge. A similar result was also obtained on the crystal facets of TiO2, a popular model photocatalyst.48-51 In addition, we also observed that the threefold-coordinated O atom transferred its charge to neighboring Sn atoms on the (110) facet. The Mulliken charge population suggests that under-coordinated atoms on the surface of photocatalysts govern the localization of photogenerated carries, which will depress the combination of the electron-hole pair and improve the performance of photocatalysts.52 In view of the existing difficulty of evaluating trapping energy of crystal facets, Zawadzki explored the correlation between the work functions of crystal facets and their carrier-trapping stability.53 Due to variations in atomic arrangement and termination, two facets show different work functions. When work functions of two facets are higher than that of bulk, electrons are transferred to the crystal facet with the higher work function.54 A difference of 0.05 eV in CPD work functions of the (111) and (001) facets was measured in the 26-facet Cu2O crystal by Kelvin probe force microscopy, which successfully explained the preferential flow of electrons to the (001) facet.55 The conclusion was also verified by both selective photo-deposition of Pd on the (001) facet and the DFT calculated work function, i. e. 7.25 eV for (001), 4.83 eV for (111).56 As to the most popular model photocatalyst TiO2, the measured work functions were 6.83 eV and 5.10 eV for the (110)


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and (011) facets of rutile nanoparticles, respectively, which supported the Ohno’ experimental observation.5758

For anatase TiO2 nanoparticles enclosed by the (101) and (001) facets, Liu group's experiments were also

rationalized in terms of work function.59 As shown in our previous experiments, noble metal particles were reduced not only on the (110) and (221) facets of lance-shaped SnO2 particles simultaneously, but also on the (101) and (110) facets of dodecahedral SnO2 particles simultaneously. Hence, in terms of work functions, the (110) and (221) facets are approximately equal, and so do the (110) and (101) facets. Fig. 10 shows that the (110), (101) and (221) facets have work functions of 6.91 eV, 6.90 eV, and 6.83 eV, respectively. Together with the work function of the bulk,60 it can be concluded that photogenerated electrons are able to migrate from the bulk to surface, but the small variance in work functions of two exposed facets fails to mediate the preferential flow of photogenerated electrons. 3.

Conclusions

We have successfully investigated the effect of the difference in surface energy on the flow mode of photogenerated electrons in SnO2 nanocatalysts. The photo-reduction of noble metals shows that the difference in surface energy, whether big or small, fails to generate the preferential flow of photogenerated electrons. Therefore, besides crystal symmetry and wavelength and intensity of irradiation, the difference in surface energy should not be considered as a proper handle for designing the preferential flow. Using the firstprinciples calculations, we found that the underlying reason is because the difference in trapping energies of the two facets is small in both lance-shaped SnO2 particles (0.059 Ha) and dodecahedral SnO2 particles (0.022 Ha), which are not powerful enough to initiate the preferential flow of charges. The close values of the calculated work functions of the (110) [6.91 eV], (101) [6.90 eV] and (221) [6.83 eV] facets also support the experimental observed random flow of charges. Additional Mulliken charge population analysis indicates that most part of photogenerated electron resides on Sn atoms with unsaturated coordination number, showing no evident facet-dependency. Our work will be instructive for designing highly-efficient photocatalysts based on semiconductor facet-engineering.

ASSOCIATED CONTENT


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Supporting Information. Detailed experimental procedures, characterization methods, computational details, and additional results of photo-deposition. These materials are available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected]. *[email protected]. ORCID Wenxiu Que: 0000-0002-0136-9710 Chao Wu: 0000-0002-8573-7196 Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China (No.61774122 and 21477096), the Science and Technology Developing Project of Shaanxi Province (2015KW-001), and the 111 Project of China (B14040).


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References 1.

Pacchioni, G.; Freund, H., Electron Transfer at Oxide Surfaces. The MgO Paradigm: From Defects

to Ultrathin Films. Chem. Rev. 2013, 113, 4035-4072. 2.

Akimov, A. V.; Neukirch, A. J.; Prezhdo, O. V., Theoretical Insights into Photoinduced Charge

Transfer and Catalysis at Oxide Interfaces. ChemInform 2013, 44, 4496-4565. 3.

Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C., Titanium Dioxide-Based Nanomaterials for

Photocatalytic Fuel Generations. Chem. Rev. 2014, 114, 9987-10043. 4.

Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z. X.; Tang, J., Visible-Light Driven

Heterojunction Photocatalysts for Water Splitting – a Critical Review. Energy Environ. Sci. 2015, 8, 731-759. 5.

Huang, H.; Han, X.; Li, X.; Wang, S.; Chu, P. K.; Zhang, Y., Fabrication of Multiple

Heterojunctions with Tunable Visible-Light-Active Photocatalytic Reactivity in BiOBr–BiOI Full-Range Composites Based on Microstructure Modulation and Band Structures. ACS Appl. Mater. Interfaces 2015, 7, 482-492. 6.

Li, H.; Yu, H.; Quan, X.; Chen, S.; Zhao, H., Improved Photocatalytic Performance of

Heterojunction by Controlling the Contact Facet: High Electron Transfer Capacity between TiO2 and the {110} Facet of BiVO4 Caused by Suitable Energy Band Alignment. Adv. Funct. Mater. 2015, 25, 3074–3080. 7.

Zhao, H.; Qian, L.; Guan, X.; Wu, D.; Zhao, G., Continuous Bulk Fecuc Aerogel with Ultradispersed

Metal Nanoparticles: An Efficient 3d Heterogeneous Electro-Fenton Cathode over a Wide Range of pH 3-9. Environ. Sci. Technol. 2016, 50, 5225-5233. 8.

Fan, J.; Chen, Z.; Shi, H.; Zhao, G., In Situ Grown, Self-Supported Iron-Cobalt-Nickel Alloy

Amorphous Oxide Nanosheets with Low Overpotential toward Water Oxidation. Chem. Commun. (Camb) 2016, 52, 4290-4293. 9.

Huang,

X.;

Shen,

Q.;

Liu,

J.;

Yang,

N.;

Zhao,

G.,

A

CO2

adsorption-Enhanced

Semiconductor/Metal-Complex Hybrid Photoelectrocatalytic Interface for Efficient Formate Production. Energy Environ. Sci. 2016, 9, 3161-3171. 10.

Fresno, F.; Portela, R.; Suarez, S.; Coronado, J., Photocatalytic Materials: Recent Achievements and

near Future Trends. J. Mater. Chem. 2014, 2, 2863-2884.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.

Page 12 of 29

Yang, X. H.; Li, Z.; Liu, G.; Xing, J.; Sun, C.; Yang, H. G.; Li, C., Ultra-Thin Anatase TiO2

Nanosheets Dominated with {001} Facets: Thickness-Controlled Synthesis, Growth Mechanism and WaterSplitting Properties. Crystengcomm 2010, 13, 1378-1383. 12.

Wu, N.; Wang, J.; Tafen, D. N.; Wang, H.; Zheng, J. G.; Lewis, J. P.; Liu, X.; Leonard, S. S.;

Manivannan, A., Shape-Enhanced Photocatalytic Activity of Single-Crystalline Anatase TiO2 (101) Nanobelts. J. Am. Chem. Soc. 2010, 132, 6679-6685. 13.

Liu, G.; Yu, J. C.; Lu, G. Q.; Cheng, H. M., Crystal Facet Engineering of Semiconductor

Photocatalysts: Motivations, Advances and Unique Properties. Chem. Commun. 2011, 47, 6763-6783. 14.

Pan, J.; Liu, G.; Lu, G. Q.; Cheng, H. M., On the True Photoreactivity Order of {001}, {010}, and

{101} Facets of Anatase TiO2 Crystals. Angew. Chem. Int. Ed. 2011, 50, 2133-2137. 15.

Zheng, J. Y.; Haider, Z.; Van, T. K.; Pawar, A. U.; Kang, M. J.; Chang, W. K.; Kang, Y. S., Tuning

of the Crystal Engineering and Photoelectrochemical Properties of Crystalline Tungsten Oxide for Optoelectronic Device Applications. Crystengcomm 2015, 17, 6070-6093. 16.

Huang, K.; Yuan, L.; Feng, S., Crystal Facet Tailoring Arts in Perovskite Oxides. Inorg. Chem.

Frontiers 2015, 2, 965-981. 17.

Huang, Z. F.; Pan, L.; Zou, J. J.; Zhang, X.; Wang, L., Nanostructured Bismuth Vanadate-Based

Materials for Solar-Energy-Driven Water Oxidation: A Review on Recent Progress. Nanoscale 2014, 6, 14044-14063. 18.

Zhang, Y.; Qin, N.; Li, J.; Han, S.; Li, P.; Zhao, G., Facet Exposure-Dependent Photoelectrocatalytic

Oxidation Kinetics of Bisphenol a on Nanocrystalline {001} TiO2/Carbon Aerogel Electrode. Appl. Catal., B 2017, 216, 30-40. 19.

Li, R.; Zhang, F.; Wang, D.; Yang, J.; Li, M.; Zhu, J.; Zhou, X.; Han, H.; Li, C., Spatial Separation

of Photogenerated Electrons and Holes among {010} and {110} Crystal Facets of BiVO4. Nat. Commun. 2013, 4, 66-78. 20.

Mu, L.; Zhao, Y.; Li, A.; Wang, S.; Wang, Z.; Yang, J.; Wang, Y.; Liu, T.; Chen, R.; Zhu, J.,

Enhancing Charge Separation on High Symmetry SrTiO3 Exposed with Anisotropic Facets for Photocatalytic Water Splitting. Energy Environ. Sci. 2016, 9, 2463-2469.


12

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


21.

Miseki, Y.; Kato, H.; Kudo, A., Water Splitting into H2 and O2 over Niobate and Titanate

Photocatalysts with (111) Plane-Type Layered Perovskite Structure. Energy Environ. Sci. 2009, 2, 306-314. 22.

Hotsenpiller, P. A. M.; Bolt, J. D.; Farneth, W. E.,, Orientation Dependence of Photochemical

Reactions on TiO2 Surfaces. J. Phys. Chem. B 1998, 102, 3216-3226. 23.

Lazzeri, M.; Vittadini, A.; Selloni, A., Erratum: Structure and Energetics of Stoichiometric TiO2

Anatase Surface. Phys. Rev. B 2002, 65, 119901. 24.

Liu, C.; Han, X.; Xie, S.; Kuang, Q.; Wang, X.; Jin, M.; Xie, Z.; Zheng, L., Enhancing the

Photocatalytic Activity of Anatase TiO2 by Improving the Specific Facet-Induced Spontaneous Separation of Photogenerated Electrons and Holes. Asian J. Chem. 2013, 8, 282-289. 25.

Tachikawa, T.; Yamashita, S.; Majima, T., Evidence for Crystal-Face-Dependent TiO2

Photocatalysis from Single-Molecule Imaging and Kinetic Analysis. J. Am. Chem. Soc. 2011, 133, 71977204. 26.

Barnard, A. S.; Zapol, P., Effects of Particle Morphology and Surface Hydrogenation on the Phase

Stability of TiO2. Phys. Rev. B 2004, 70, 235403. 27.

Ohno, T.; Sarukawa, K.; Matsumura, M., Crystal Faces of Rutile and Anatase TiO2 Particles and

Their Roles in Photocatalytic Reactions. New J. Chem. 2002, 26, 1167-1170. 28.

Zhao, Z.; Li, Z.; Zou, Z., Structure and Energetics of Low-Index Stoichiometric Monoclinic

Clinobisvanite BiVO4 Surfaces. RSC Adv. 2011, 1, 874-883. 29.

Eglitis, R. I.; Vanderbilt, D., First-Principles Calculations of Atomic and Electronic Structure Of

SrTiO3 (001) and (011) Surfaces. Phys. Rev. B 2008, 77, 195408. 30.

Heifets, E.; Goddard, W. A.; Kotomin, E. A.; Eglitis, R. I.; Borstel, G., Ab initio Calculations of the

SrTiO3 (110) Polar Surface. Phys. Rev. B 2004, 69, 035408. 31.

Lazzeri, M.; Vittadini, A.; Selloni, A., Structure and Energetics of Stoichiometric TiO2 Anatase

Surfaces. Phys. Rev. B 2001, 63, 155409. 32.

Ye, L.; Liu, J.; Tian, L.; Peng, T.; Zan, L., The Replacement of {101} by {010} Facets Inhibits the

Photocatalytic Activity of Anatase TiO2. Appl. Catal., B 2013, 134, 60-65. 33.

Zhang, H.; Liu, L.; Zhou, Z., First-Principles Studies on Facet-Dependent Photocatalytic Properties

of Bismuth Oxyhalides (BiOXs). RSC Adv. 2012, 2, 9224-9229.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

34.

Page 14 of 29

Ling, Z.; Wang, W.; Sun, S.; Dong, J.; Gao, E., Selective Transport of Electron and Hole among

{0 0 1} and {1 1 0} Facets of BiOCl for Pure Water Splitting. Appl. Catal., B 2015, 162, 470-474. 35.

Slater, B.; Catlow, R. A. C.; Gay, D. H.; Williams, D. E.; Dusastre, V., Study of Surface Segregation

of Antimony on SnO2 Surfaces by Computer Simulation Techniques. J. Phys. Chem. B 1999, 103,1064410650 36.

Han, X.; Jin, M.; Xie, S.; Kuang, Q.; Jiang, Z.; Jiang, Y.; Xie, Z.; Zheng, L., Synthesis of Tin

Dioxide Octahedral Nanoparticles with Exposed High-Energy {221} Facets and Enhanced Gas-Sensing Properties. Angew. Chem. Int. Ed. 2009, 48, 9180-9183. 37.

Wang, X.; Han, X.; Xie, S.; Kuang, Q.; Jiang, Y.; Zhang, S.; Mu, X.; Chen, G.; Xie, Z.; Zheng, L.,

Controlled Synthesis and Enhanced Catalytic and Gas-Sensing Properties of Tin Dioxide Nanoparticles with Exposed High-Energy Facets. Chem. Eur. J. 2012, 18, 2283-2289. 38.

Amalric-Popescu, D.; Bozon-Verduraz, F., Infrared Studies on SnO2 and Pd/SnO2. Catal. Today

2001, 70, 139-154. 39.

Ma, C.; Sun, X., Preparation and Characterization of SnO2 Nanoparticles with a Surfactant-Mediated

Method. Nanotechnology 2002, 13, 565-569. 40.

Iizuka, K.; Wato, T.; Miseki, Y.; Saito, K.; Kudo, A., Photocatalytic Reduction of Carbon Dioxide

over Ag Cocatalyst-Loaded ALa4Ti4O15 (A = Ca, Sr, and Ba) Using Water as a Reducing Reagent. J. Am. Chem. Soc. 2011, 133, 20863-20868. 41.

Park, S.; Kim, H.; Lee, J.; Kim, Y. C.; Lee, J. C.; Chung, Y., Photoreaction of Gold Ions from

Potassium Gold Cyanide Wastewater Using Solution-Combusted ZnO Nanopowders. J. Eur. Ceram. Soc.2010, 30, 177-180. 42.

Matsumoto, Y.; Masayuki Noguchi, A.; Matsunaga, T., Deposition of Metallic Oxides on TiO2

Electrode Using the Photoelectrochemical Epitaxial Growth Technique (PEEG), and Electrochemical Behavior of the PbO2/TiO2 Electrode. J. Phys. Chem. B 1999, 103, 7190-7194. 43.

Booshehri, A. Y.; Goh, C. K.; Hong, J.; Jiang, R.; Xu, R., Effect of Depositing Silver Nanoparticles

on BiVO4 in Enhancing Visible Light Photocatalytic Inactivation of Bacteria in Water. J. Mater.Chem. A 2014, 2, 6209-6217.


14

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44.

Selcuk, S.; Selloni, A., Facet-Dependent Trapping and Dynamics of Excess Electrons at Anatase

TiO2 Surfaces and Aqueous Interfaces. Nat. Mater. 2016, 15, 1107-1112. 45.

Xu, C.; Jiang, Y.; Yi, D.; Sun, S.; Yu, Z., Environment-Dependent Surface Structures and Stabilities

of SnO2 from the First Principles. J. Appl. Phys. 2012, 111, 063504. 46.

Ma, X.; Dai, Y.; Guo, M.; Huang, B., Relative Photooxidation and Photoreduction Activities of the

{100}, {101}, and {001} Surfaces of Anatase TiO2. Langmuir 2013, 29, 13647-13654. 47.

Sanchez-Portal, D.; Artacho, E.; Soler, J. M., Projection of Plane-Wave Calculations into Atomic

Orbitals. Solid State Commun. 1995, 95, 685-690. 48.

Berger, T.; Sterrer, M.; Diwald, O.; Knözinger, E.; Panayotov, D.; Thompson, T. L.; Jr, Y. J., Light-

Induced Charge Separation in Anatase TiO2 Particles. J. Phys. Chem. B 2005, 109, 6061-6068. 49.

Morgan, B. J.; Watson, G. W., Polaronic Trapping of Electrons and Holes by Native Defects in

Anatase TiO2. Phys. Rev. B 2009, 80, 308-310. 50.

Kowalski, P. M.; Camellone, M. F.; Nair, N. N.; Meyer, B.; Marx, D., Charge Localization

Dynamics Induced by Oxygen Vacancies on the TiO₂ (110) Surface. Phys. Rev. Lett. 2010, 105, 2028-2031. 51.

Deskins, N. A.; Rousseau, R.; Dupuis, M., Localized Electronic States from Surface Hydroxyls and

Polarons in TiO2 (110). J. Phys. Chem. C 2009, 113, 14583-14586. 52.

Liu, T.; Tranca, I.; Yang, J.; Zhou, X.; Li, C., Theoretical Insight into the Roles of Cocatalysts in the

Ni–NiO/β-Ga2O3 Photocatalyst for Overall Water Splitting. J. Mater. Chem. A 2015, 3, 10309-10319. 53.

Zawadzki, P.; Laursen, A. B.; Jacobsen, K. W.; Dahl, S.; Rossmeisl, J., Oxidative Trends of TiO2—

Hole Trapping at Anatase and Rutile Surfaces. Energy Environ. Sci. 2012, 5, 9866-9869. 54.

Bai, S.; Jiang, J.; Zhang, Q.; Xiong, Y., Steering Charge Kinetics in Photocatalysis: Intersection of

Materials Syntheses, Characterization Techniques and Theoretical Simulations. Chem. Inform. 2015, 44, 2893-2939. 55.

Li, R.; Tao, X.; Chen, R.; Fan, F.; Li, C., Synergetic Effect of Dual Co-Catalysts on the Activity of

p-Type Cu2O Crystals with Anisotropic Facets. Chem. Eur. J. 2015, 21, 14337-14341. 56.

Wang, L.; Ge, J.; Wang, A.; Deng, M.; Wang, X.; Bai, S.; Li, R.; Jiang, J.; Zhang, Q.; Luo, Y.,

Designing p-Type Semiconductor-Metal Hybrid Structures for Improved Photocatalysis. Angew. Chem. Int. Ed. 2014, 53, 5107-5111.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

57.

Page 16 of 29

Vogtenhuber, D.; Podloucky, R.; Neckel, A.; Steinemann, S. G.; Freeman, A. J., Electronic Structure

and Relaxed Geometry of the TiO2 Rutile (110) Surface. Phys. Rev. B 1994, 49, 2099-2103. 58.

Di Valentin, C.; Tilocca, A.; Selloni, A.; Beck, T. J.; Klust, A.; Batzill, M.; Losovyj, Y.; Diebold, U.,

Adsorption of Water on Reconstructed Rutile TiO2 (011)-(2×1): Ti=O Double Bonds and Surface Reactivity. J. Am. Chem. Soc. 2005, 127, 9895-9903. 59.

Zhao, Z.; Li, Z.; Zou, Z., Surface Properties and Electronic Structure of Low-Index Stoichiometric

Anatase TiO2 Surfaces. J. Phys-Condens. Mat. 2010, 22, 175008-175025. 60.

He, J. H.; Wu, T. H.; Hsin, C. L.; Li, K. M.; Chen, L. J.; Chueh, Y. L.; Chou, L. J.; Wang, Z. L.,

Beaklike SnO2 Nanorods with Strong Photoluminescent and Field-Emission Properties. Small 2006, 2, 116120.


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Figure 1. (a), (b) SEM images of lance-shaped SnO2; (c), (d) TEM images of dodecahedral SnO2.


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Figure 2. TEM images of lanced-shaped SnO2 loading (a) 0.25%, (b) 0.5%, and (c) 1.0% Au for 1h; (d) loading 0.25% Au for 4 h; (e) and (f) loading 1 % Au and methanol as holes scavenger.


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Figure 3. TEM images of photo-oxidation of Pb2+ on lance-shaped SnO2 for c (Pb2+) : c (IO3- ) equals (a) 1:1, (b) 1:4, (c) 1:2; (d) XPS spectrum of Pb 4f.


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Figure 4. TEM images of photo-oxidation of Pb2+ on lance-shaped SnO2 at pH of (a) 1, (b) 2.5, (c) 4.5; (d) XPS spectrum of Pb 4f.


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Figure 5. TEM images of photo-oxidation of Pb2+ on lance-shaped SnO2 loading Au (a), (b) for 0.5h; (c) for 1h; (d) XPS spectrum of Pb 4f.


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Figure 6. TEM images of lanced-shaped SnO2 (a) loading 0.5% Au and PbO2; (b), (c), (d) loading 1% Au and PbO2; (e) PbO2; (f), (g) EDX spectrum of Au, PbO2.


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Figure 7. TEM images of dodecahedral SnO2 particles loading (a) 3% Pt, (b) 6% Pt, (c) 9% Pt, (d) 12% Pt.


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Figure 8. Illustration of surfaces (a) (110), (b) (101), and (c) (221).


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Figure 9. Change of polaronic solution as a function of vacuum width: (a) (110), (b) (101), (c) (221).


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Figure 10. Potential diagrams of (a) (110), (b) (101), and (c) (221). Electronic Fermi levels are set to zero.


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Table 1. Trapping Energy (Ha) for Electrons on the (110), (101) and (221) facets of SnO2 (110)

(101)

(221)

GGA

-0.079

-0.101

-0.138

LDA

-0.091

-0.113

-0.150


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Table 2. Mulliken charge population for the (110), (101) and (221) facets of SnO2 Site

(110)

(101)

(221)

O3c

-0.08

0.05

0.02 0.06 ( )

O2c

0.24

0.18

0.08 ( ) 0.08 ( )

Sn6c

0.20

—

Sn5c

0.49

0.72

Sn4c

—

—


— 0.13 ( ) 0.15 ( ) 0.35

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Table of Contents Graphic


29

Trapping Behaviors of Photogenerated Electrons on the (110), (101

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