Langmuir 1999, 15, 8569-8573
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Tapping Mode Atomic Force Microscopy Studies of the Photoreduction of Ag+ on Individual Submicrometer TiO2 Particles William E. Farneth* and R. Scott McLean DuPont Co. Experimental Station, Wilmington, Delaware 19880
John D. Bolt DuPont Co., Iler Research Center, New Johnsonville, Tennessee 37134
Eleni Dokou and Mark A. Barteau Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received July 6, 1999. In Final Form: October 7, 1999 In this paper we describe an experimental procedure that can be used to study the photoreduction of metal ions on the surfaces of individual TiO2 particles. The submicrometer-sized particles are isolated on porous membranes, characterized by atomic force microscopy (AFM), removed from the microscope, exposed to AgNO3 solution and light, and reimaged. Photoreduction of Ag+ produces 5-50 nm particles of Ag0 distributed over the surfaces of the TiO2 particle. This procedure allows the spatial distribution of the chemical reactivity of particle surfaces to be mapped at the nanometer scale resolution of the atomic force microscope. Using different excitation wavelengths (366, 400 and ∼600 nm), we demonstrate by AFM that the photoreduction of Ag on single anatase or rutile particles is consistent with the differences in the band gaps of the bulk samples. We also show that the Ag0 particles are not uniformly distributed over the TiO2 crystallite surfaces implying that a closer study of the unreacted particle surfaces may reveal insights into the nanometer-scale structures that are responsible for variations in photoactivity of TiO2 samples of different microstructure or composition.
Introduction Chemical reactions at the surfaces of solids leave topographic signatures. These morphological changes can often be followed with atomic force microscopy (AFM). For example, anisotropic growth of organic crystals,1 dissolution of minerals,2 and vapor-phase reduction of metal oxides3 are recent examples in which AFM has been used to study differences in chemical reaction rates among different faces or along different axes of crystals. AFM methods have also been used to study reaction-induced changes to the surfaces of heterogeneous catalysts. We have studied the crystal face specificity of the photocatalytic reduction of Ag(I) to Ag on TiO2 polycrystalline films this way.4 Others have used the technique to examine coking of selective oxidation catalysts.5 Most studies of surface chemical reactivity with AFM have been conducted either as in situ reactions on model materials, crystals, or films with relatively large flat terraces or as ex situ chemistry in which AFM is used to compare different regions of sample surface before and after treatment.6 In this paper we describe another method. We show how the AFM-based protocol can be applied directly to study surface structure/activity relationships on individual (1) Mao, G.; Lobo, L.; Scaringe, R.; Ward, M. D. Chem. Mater. 1997, 9, 773. (2) Britt, D. W.; Hlady, V. Langmuir 1997, 13, 1873. (3) Smith, R. L.; Rohrer, G. S. J. Catal. 1998, 180, 270. (4) Morris-Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E.; Lowekamp, J. B.; Rohrer, G. S. J. Phys. Chem. B 1998, 102, 3216. (5) Gaigneaux, E. M.; Ruiz, P.; Wolf, E. E.; Delmon B. J. Catal. 1997, 172, 247. (6) Bickmore, B. R.; Hochella, M. F.; Bosbach, D.; Charlet, L. Clays Clay Miner., in press.
submicrometer-sized particles such as those used as catalysts, catalyst supports, pigments or fillers in standard industrial use. Individual TiO2 particles are isolated on porous membranes, characterized by AFM, removed from the microscope, surface modified in an aqueous photoreaction, and reimaged, a procedure that allows the spatial distribution of the chemical reactivity of particle surfaces to be mapped at the nanometer scale resolution of the AFM. Scanning electron microscopy (SEM) has recently been used in a similar way to monitor local photoreduction reactions on porous TiO2 films.7 However, exposure to the electron beam degraded the photoreactivity of the film. Experimental Section Samples suitable for single particle AFM imaging were obtained by preparing a dilute (10 ppm) dispersion of a TiO2 powder in purified, deionized water (Millipore Milli-Q system) and filtering through Nuclepore 0.1 µm polycarbonate membranes. After overnight drying at 60 °C in a vacuum oven, the membrane was removed from the filter holder and mounted onto a microscope slide using doublesided tape. Several puncture marks were made in the membrane surface to facilitate reproducible positioning of the AFM tip at a precise location. Tapping mode AFM (TMAFM) was used to obtain height and phase imaging data simultaneously on a Nanoscope IIIa AFM, Dimension 3100, from Digital Instruments, Santa Barbara CA. Microfabricated cantilevers or silicon probes (Nanoprobes, (7) Matsushita, S. I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Langmuir 1998, 14, 6441.
10.1021/la9908844 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/13/1999
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Langmuir, Vol. 15, No. 25, 1999
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Figure 1. Tapping mode AFM image of a single rutile TiO2 particle on a Nuclepore membrane filter (scan area 500 nm × 500 nm): (a) height contrast mode (z-range 300 nm); (b) phase contrast mode (z-range 60°).
Figure 2. Three-dimensional surface plot of the same rutile TiO2 particle, rotated to show the particle morphology more clearly.
Digital Instruments) with 125 µm long cantilevers were used at their fundamental resonance frequencies which typically varied from 270 to 350 kHz depending on the cantilever. Cantilevers had a nominal tip radius of 5-10 nm. The AFM was operated in ambient conditions with a double isolation system. Extender electronics were used to allow the simultaneous collection of height and phase information. Lateral scan frequencies were about 1.2 Hz. In tapping mode, the level of force applied to the surface can dramatically change the data, especially the phase data. These forces are roughly adjusted by the ratio of the engaged or set point amplitude to the free air amplitude.8 This set point amplitude, which is used in feedback control, was adjusted to 60-75% of the free air amplitude for the images shown here. The images presented here are not filtered. (8) Maganov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. Lett. 1997, 375, L385.
Figure 3. Tapping mode AFM phase image of an anatase TiO2 particle on a Nuclepore membrane filter (scan area 850 nm × 850 nm, z-range 45°).
It is possible to return readily to the same particle after removing and replacing the sample in the AFM by repositioning the sample stage and the AFM scanner to reproduce the distinctive pattern of particles and pores in each image of a set of images taken at progressively finer scales. Optical images were taken at roughly 650 and 125 µm using the microscope camera. Images at 12 µm and below are taken by TMAFM. These membrane preparations yield mixtures of individual particles and multiparticle agglomerates. We have generally chosen to work with individual particles because they tend to be easier to image, and we have focused on particles that are oriented on the membrane with relatively large flat expanses of crystal faces exposed. Beyond that, particles have been chosen for study without bias. We have used both rutile and anatase powders in this work.9
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Figure 4. TMAFM phase images of the same particle through a series of chemical exposures: (a) immediately after mounting the membrane onto the slide; (b) 6 days of storage at ambient conditions; (c) 60 s of irradiation in a 10-4 M AgNO3 solution; (d) an additional 60 s of irradiation; (e) 12 h in a vacuum oven at 60 °C. Scan area 500 nm × 500 nm, z-range 60°. Table 1. Ag Deposition Characteristicsa
Results and Discussion Figure 1 shows a tapping mode AFM image (height contrast mode left, phase contrast mode right) of a single rutile TiO2 particle isolated on a Nuclepore membrane filter. The dark area at the left of the particle is a pore in the membrane. The particle itself can be seen to have two lobes which are connected by a twin boundary oriented roughly vertically in the image. The main face of the lobe on the lower right is inclined away from the imaging plane. The main face of the lobe pointing diagonally to the upper left is nearly parallel to the membrane surface. The edges of the particle are distorted as the tip climbs or descends the steep faces of the particle. Figure 2 is a three-dimensional topographic reconstruction from the same data set rotated to show the shape of the particle more clearly. Figure 3 is a tapping mode AFM phase image of an anatase particle, consisting of several primary particles. Primary particles in the anatase sample are smaller than those in the rutile sample. The anatase primaries are typically aggregated in multiples to give clusters with pigmentary dimensions (0.25-0.30 µm). They therefore generally have more complicated morphologies than the rutile particles. TiO2 particles are very efficient absorbers of ultraviolet radiation ( ) 106 cm-1 at 325 nm).10 In most of their important applications, as pigments, sunscreens, or photocatalysts, for example, the fate of this absorbed energy must be controlled. For the former two applications, (9) The anatase sample was obtained from Aldrich Chemical Co. The rutile sample is from the DuPont Co. Both have BET surface areas around 5 m2/g. Both are free of significant impurities (no metal ions >0.20 wt % as oxide relative to TiO2). The rutile sample has been described in more detail previously. Lusvardi, V. S.; Barteau, M. A.; Farneth, W. E. J. Catal. 1995, 153, 41. (10) Eagles, D. M. Phys. Chem. Solids 1965, 25, 1243.
% TiO2 surface area covered, average Ag particle height (nm) rutile anatase
source a
source a, 400 nm band pass
source b
14%, 6 2%, 1.0
2%, 1.3
