In Situ Imaging of Photocatalytic Activity at Titanium Dioxide

Subscriber access provided by Iowa State University | Library

Letter

In-Situ Imaging of Photocatalytic Activity at Titanium Dioxide Nanotubes using Scanning Ion Conductance Microscopy Rong Jin, Xiangdong Ye, Jia Fan, Dechen Jiang, and Hong-yuan Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05311 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 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

Analytical Chemistry

In-Situ Imaging of Photocatalytic Activity at Titanium Dioxide Nanotubes using Scanning Ion Conductance Microscopy

Rong Jin1, Xiangdong Ye2, Jia Fan1, Dechen Jiang1*, Hong-Yuan Chen1

1. The State Key Lab of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210093, China. 2. School of Mechanical and Electrical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China

Email: [email protected]

1

ACS Paragon Plus Environment

Analytical Chemistry 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

Page 2 of 13

Abstract In this letter, in-situ imaging of the photocatalytic activity of titanium dioxide (TiO2) nanotubes for the degradation of an organic pollutant (i.e., Rhodamine B (RhB)) is realized with nanometer resolution using scanning ion conductance microscopy (SICM).

Upon illumination, the

separated electrons and holes at the nanotubes induce oxidation of RhB to produce the more positively charged Rhodamine 123 (Rh 123), which leads to increased ionic current through the capillary orifice and an elevated apparent altitude in the SICM image.

Active sites with higher

activity on the nanotubes exhibit significant high spatial-resolution character.

The successful

imaging of the photocatalytic activity of TiO2 nanotubes should provide an in-situ approach for local investigation of the photocatalytic process at the catalyst.

2


Page 3 of 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


Photocatalysis is a process that converts luminous energy into chemical energy via chemical reactions (e.g., hydrogen production and organic pollutant degradation).1-4 improvements in photocatalytic performance have been recently achieved.5

Tremendous

However, a deep

understanding of the mechanisms in these processes is lacking, which is partially due to the challenge in the local characterization of catalytic activity.

Therefore, highly spatial

measurement of the photocatalytic activity of a catalyst is required to elucidate the relationship between the microstructure and the local photocatalytic activity.6 Single-molecule fluorescence microscopes6-8 have been applied to characterize the local activity on the photocatalyst with nanometer spatial resolution.9

This approach utilizes the

radicals generated from the catalytic reaction to convert nonfluorescent resazurin into fluorescent resorufin to characterize the local photocatalytic activity.10,11

To achieve in-situ characterization

of the photocatalytic activity, electrochemical microscopy (i.e., mainly scanning electrochemical microscopy (SECM)) is applied.

This technique uses a micro/nanoelectrode to scan through the

surface and electrochemically oxidize or reduce the catalytic product directly. additional reporter is needed in this process.

Therefore, no

The challenge with achieving high-spatial

resolution characterization is the collection of the weak faradaic current from the nanoelectrode. Scanning ion conductive microscopy (SICM) is another electrochemical imaging approach for characterizing the surface topography of micro- and nanometer scaled structures.12,13

This

approach relies on the hindrance of the ionic current (non-faradaic) flowing in and out of the capillary as the feedback signal to regulate the tip position over the sample surface.

In

comparison to the pico/femtoampere faradaic current from the micro/nanoelectrodes in SECM, the ionic current in SICM is at the nanoampere level, which can be easily recorded.

Therefore, a

spatial resolution as low as a few nanometers has been reported, demonstrating the advantage of this approach in nanoscale characterization.14

In particular, interaction of diffuse double layers at

the nanocapillary and surface produces a permselective region, in which the flux of cations to/from the capillary varies as a function of the position with respect to a charged substrate.13,14 Therefore, the fluctuation of the ionic current facilitates nanometer-scaled imaging of surface charge and topography of materials and cells.12-14

Because most photocatalytic processes

involve electrontransfer from the substrate and generation of a charged intermediate, SICM should provide in-situ characterization of the photocatalytic activity at the catalyst with nanometer spatial resolution. In this letter, the photocatalytic degradation of an organic pollutant (i.e., Rhodamine B (RhB)) by anatase titanium dioxide (TiO2) nanotubes is imaged using SICM to map the photocatalytic activity of the TiO2 surface. shown in eq. 1-9.15-17

The complete degradation process for RhB over TiO2 nanotubes is

Under ultraviolet illumination, TiO2 nanotubes absorb photons and excite

an electron from the valence band to the conduction band, leaving positively charged holes with strong oxidation ability to produce hydroxyl radicals from water. 3


These radicals attack RhB+


Page 4 of 13

species at the surface to generate Rh 123+ (eq. 4), which is quickly hydrolyzed to Rh 1232+ (eq. 5). Since the subsequent decomposition of Rh 1232+ (eq. 6) is reported to be slow (hours),18 its relative fast generation and slow decomposition result in accumulation at the surface.

In

comparison to the surface charges from RhB+, the generated Rh 1232+introduces more positive charges.

When an innercapillary electrode is applied with a positive voltage, more anions (Cl-)

that are adsorbed at the positively charged surface drives the enhanced flow of cations into the diffuse double layer at the capillary tip-surface interact.19-21

Under constant current mode, the

larger apparent altitudes that were observed should be associated with the additional Rh 1232+ produced at the active sites with higher photocatalytic activity, as proposed in Figure 1. Ultimately, the photocatalytic activity at the TiO2 nanotubes for the degradation of an organic pollutant could be mapped using SICM. The scanning electron microscopy (SEM) images (Figure 2A) and X-ray diffraction spectrum (Figure S1, supporting information) indicate that the anatase TiO2 nanotubes are ~ 100 nm in diameter and ~ 2 μm in length.

Using a nanocapillary with an orifice of ~ 130 nm (Figure S2,

supporting information), the SICM image of the nanotube surface (3.2 × 3.2 µm, 100 nm/pixel) shows a relatively flat surface (Figure 2B).

In comparison to the uneven surface observed in the

SEM image, the flat surface obtained using SICM is due to the insufficient spatial resolution of our approach.

In our SICM experiment, the spatial resolution in the x-y direction is controlled by

the diameter of the capillary orifice, and the resolution in the z direction is ~ 50 nm.

Therefore,

the current submicron resolution is not sufficiently high to reveal the fine structure of the nanotubes.

After adsorption of RhB (λex/em 552/610 nm) at the TiO2 nanotubes, fluorescence is

observed from the entire surface (Figure S3A, supporting information), confirming RhB immobilization at the surface.

Some brighter fluorescence regions in the image suggest

increased adsorption of the dye at these special sites (“accumulated regions”).

Rescanning this

RhB adsorbed surface (Figure 2C) results in an elevated apparent altitude in some regions, which is due to uneven adsorption of positively charged RhB.

Scaling up the scanning region to 90 ×

90 µm results in the presence of accumulated RhB at the surface (Figure S4, supporting information), which is similar to the fluorescence observation. Upon illumination of the TiO2 nanotubes for 10 min to degrade RhB into Rh 123, the fluorescence image is recorded, and no obvious change in the fluorescence intensity occurs before or after illumination (Figure S3B, supporting information).

Therefore, the photocatalytic activity

at TiO2 surface is challenging to observe from the fluorescence observation.

For comparison, the

SICM image (Figure 2D) shows a significant increase in the apparent altitude after illumination, which indicates an increase in surface charges from Rh 123 due to this photocatalytic process. To correlate the apparent altitude and the amount of Rh 123 at the surface, hydrogen peroxide, which can provide additional radicals and accelerate the degradation process, is introduced into the solution.

The apparent altitude further increased (Figure S5, supporting information), which 4


Page 5 of 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


is consistent with additional Rh 123 that was produced in the presence of extra radicals.

Because

RhB can be directly degraded by UV light, the control experiment was performed with an RhB-adsorbed titanium slide in the absence of TiO2 nanotubes.

No increase in the altitude

(Figure S6, supporting information) was observed, which suggests that the self-degradation of RhB is weak and does not significantly contribute to the ionic current. To determine the correlation between the apparent altitude in the SICM image and the local photocatalytic activity, the generated Rh 123 must be adsorbed at the surface.

Moreover, Rh 123

should degrade slowly so the charge increases during the photocatalytic process can be recorded by SICM.

Experimentally, TiO2 nanotubes were continuously scanned under illumination, and

each image was completed in 2 min.

In comparison to the SICM image after illumination for 0

min (Figure S7, supporting information), the altitudes in all the pixels in the images increase synchronously along some special sites during this continuous scanning (Figure 3 A-D).

The

local increase in the altitude indicates the accumulation of charged Rh 123 at the active sites during the illumination.

Therefore, the increase in the apparent altitude at each pixel in the

image could provide information on the local catalytic activity at the TiO2 surface. To illustrate the local activity at the surface, the altitude difference between Figures 2D (after the illumination) and 2C (before the illumination) is shown in Figure 4A.

The different increases

in the altitude correspond to the varied degradation of RhB at the surface.

Because the difference

in the adsorption amount affects the degradation rate, a small region with a similar apparent altitude in Figure 2B is randomly selected to ensure an even adsorption of RhB.

Prior to

illumination, the altitudes in this region are very similar, as shown in Figure S8 (supporting information).

Importantly, after the illumination, two sites with higher altitudes are observed, as

shown in Figure 4B, which indicates the presence of active areas with enhanced catalytic activity. The altitude analysis in Figure 4C indicates that the size of these active sites is ~ 200 nm.

To

further support the presence of these sites, these regions are continuously scanned three times, and these sites are always observed (Figure S9, supporting information).

These results clearly

demonstrate in-situ observation of active sites at TiO2 nanotubes that provides a direct evidence about the difference in the catalytic feature at the catalysis.

The exact size of active sites is still

uncertain, which could only be obtained in a higher spatial resolution image using a thinner capillary. In conclusion, SICM is adopted to realize in-situ characterization of photocatalytic activity at TiO2 nanotube with a nanometer resolution.

Rh123 generated from the photocatalytic

degradation of RhB is adsorbed at the surface, and the degradation process associated with catalytic activity is indicated by the fluctuation of surface altitude in SICM image.

Therefore,

active sites with higher catalytic activity are visualized exhibiting the significance of high spatial resolution characterization of catalyst.

The continuous improvement of spatial resolution in

SICM imaging using narrower tips could reveal more delicate information of catalytic feature that 5



benefit the development of high efficient catalyst. Acknowledgments. This work was supported by National Natural Science Foundation of China (nos. 21575060 and 51475353). Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental section; XRD results of nanotubes; more SICM images References 1.

Yu, H.; Shi, R.; Zhao, Y.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; Zhang, T., Smart

Utilization of Carbon Dots in Semiconductor Photocatalysis. Adv. Mater. 2016, 28, 9454-9477. 2.

Jia, H.; He, W.; Wamer, W. G.; Han, X.; Zhang, B.; Zhang, S.; Zheng, Z.; Xiang, Y.; Yin,

J.-J., Generation of Reactive Oxygen Species, Electrons/Holes, and Photocatalytic Degradation of Rhodamine B by Photoexcited CdS and Ag2S Micro-Nano Structures. J. Phys. Chem. C 2014, 118, 21447-21456. 3.

Dhandole, L. K.; Kim, S.-G.; Seo, Y.-S.; Mahadik, M. A.; Chung, H. S.; Lee, S. Y.; Choi, S.

H.; Cho, M.; Ryu, J.; Jang, J. S., Enhanced Photocatalytic Degradation of Organic Pollutants and Inactivation of Listeria monocytogenes by Visible Light Active Rh–Sb Codoped TiO2 Nanorods. ACS Sustain. Chem. Eng.2018, 6, 4302-4315.

4.

Tada, H.; Kiyonaga, T.; Naya, S., Rational design and applications of highly efficient

reaction systems photocatalyzed by noble metal nanoparticle-loaded titanium (IV) dioxide. Chem. Soc. Rev. 2009, 38, 1849-1858.

5.

Liu, J.; Liu, B.; Ni, Z.; Deng, Y.; Zhong, C.; Hu, W., Improved catalytic performance of

Pt/TiO2 nanotubes electrode for ammonia oxidation under UV-light illumination. Electrochim. Acta 2014, 150, 146-150.

6.

Schmidt, J. E.; Oord, R.; Guo, W.; Poplawsky, J. D.; Weckhuysen, B. M., Nanoscale

tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts. Nat. Commun.2017, 8, 1666. 7.

Nagaiah, T. C.; Maljusch, A.; Chen, X.; Bron, M.; Schuhmann, W., Visualization of the local

catalytic activity of electrodeposited Pt-Ag catalysts for oxygen reduction by means of SECM. Chemphyschem 2009, 10, 2711-2718. 8.

Chen, X.; Maljusch, A.; Rincon, R. A.; Battistel, A.; Bandarenka, A. S.; Schuhmann, W.,

Local visualization of catalytic activity at gas evolving electrodes using frequency-dependent scanning electrochemical microscopy. Chem. Commun. 2014, 50, 13250-13253. 9.

Burdyny, T.; Riordon, J.; Dinh, C. T.; Sargent, E. H.; Sinton, D., Self-assembled

nanoparticle-stabilized photocatalytic reactors. Nanoscale 2016, 8, 2107-2115. 6


Page 6 of 13

Page 7 of 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


10. Maretzky, T.; Yang, G.; Ouerfelli, O.; Overall, C. M.; Worpenberg, S.; Hassiepen, U.; Eder, J.; Blobel, C. P., Characterization of the catalytic activity of the membrane-anchored metalloproteinase ADAM15 in cell-based assays. Biochem. J. 2009, 420, 105-113. 11. Tian, C.-Y.; Xu, J.-J.; Chen, H.-Y., Enhanced Electrochemiluminescence of TiO2 Nanoparticles Modified Electrode by Nafion Film and Its Application in Selective Detection of Dopamine. Electroanalysis 2013, 25, 1294-1300. 12. Hansma, P. K.; Drake, B.; Marti, O.; Gould, S. A.; Prater, C. B., The scanning ion-conductance microscope. Science 1989, 243, 641-643. 13. McKelvey, K.; Kinnear, S. L.; Perry, D.; Momotenko, D.; Unwin, P. R., Surface charge mapping with a nanopipette. J. Am. Chem. Soc.2014, 136, 13735-13744. 14. Kang, M.; Perry, D.; Bentley, C. L.; West, G.; Page, A.; Unwin, P. R., Simultaneous Topography and Reaction Flux Mapping at and around Electrocatalytic Nanoparticles. ACS Nano 2017, 11, 9525-9535. 15. Zhang, J.; Yan, S.; Fu, L.; Wang, F.; Yuan, M.; Luo, G.; Xu, Q.; Wang, X.; Li, C., Photocatalytic Degradation of Rhodamine B on Anatase, Rutile, and Brookite TiO2. Chinese J. Catal. 2011, 32, 983-991. 16. Park, H.; Choi, W., Photocatalytic reactivities of Nafion-coated TiO2 for the degradation of charged organic compounds under UV or visible light. J. Phys. Chem. B 2005, 109, 11667-11674. 17. Liu, G.; Li, X.; Zhao, J.; Hidaka, H.; Serpone, N., Photooxidation Pathway of Sulforhodamine-B. Dependence on the Adsorption Mode on TiO2 Exposed to Visible Light Radiation. Environ. Sci. Tech. 2000, 34, 3982-3990. 18. Yu, K.; Yang, S.; He, H.; Sun, C.; Gu, C.; Ju, Y., Visible light-driven photocatalytic degradation of rhodamine B over NaBiO3: pathways and mechanism. J. Phys. Chem. A 2009, 113, 10024-10032. 19. Perry, D.; Al Botros, R.; Momotenko, D.; Kinnear, S. L.; Unwin, P. R., Simultaneous Nanoscale Surface Charge and Topographical Mapping. ACS Nano 2015, 9, 7266-7276. 20. McKelvey, K.; Perry, D.; Byers, J. C.; Colburn, A. W.; Unwin, P. R., Bias modulated scanning ion conductance microscopy. Anal. Chem.2014,86, 3639-3646. 21. Perry, D.; PauloseNadappuram, B.; Momotenko, D.; Voyias, P. D.; Page, A.; Tripathi, G.; Frenguelli, B. G.; Unwin, P. R., Surface Charge Visualization at Viable Living Cells. J. Am. Chem. Soc.2016, 138, 3152-3160.

7



Page 8 of 13

Figures and Captions. Figure 1.The schematic imaging process of catalytic activity at TiO2 surface. Figure 2. (A) SEM images of TiO2 nanotubes (top left: top view; top right: side view); (B-D) SICM images of (B) bare TiO2 nanotubes, (C) RhB adsorbed TiO2 nanotubes and (D) RhB adsorbed TiO2 nanotubes after the illumination for 10 min. and each pixel size is 100 nm.

The scanning region is 3.2 × 3.2 µm,

The labeled region in image C is the selected region with

similarly adsorbed amount of RhB. Figure 3. SICM images of RhB adsorbed TiO2 nanotubes after the illumination for (A) 2 min, (B) 4 min, (C) 6 min and (D) 8 min. Figure 4. (A) The image to show the difference in the apparent altitude at TiO2 surface before and after the illumination; (B) the amplified image of the selected region in Figure 4A; (C) the altitude across an active spot in Figure 4B.

8


Page 9 of 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


Figure 1.

9



Figure 2.

10


Page 10 of 13

Page 11 of 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


Figure 3.

11



Figure 4.

12


Page 12 of 13

Page 13 of 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


TOC

13


In Situ Imaging of Photocatalytic Activity at Titanium Dioxide

Recommend Documents