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Enhanced photocatalytic activity of diamond thin films using embedded Ag nanoparticles. Shuo Li, Jason A Bandy, and Robert J Hamers ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13821 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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ACS Applied Materials & Interfaces

Enhanced photocatalytic activity of diamond thin films using embedded Ag nanoparticles Shuo Li †, Jason A Bandy†, and Robert J. Hamers†* Department of Chemistry, †

University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA

*Corresponding Author: [email protected] Keywords: Diamond, silver nanoparticles, plasmonic nanoparticles, ammonia synthesis, solvated electrons

ABSTRACT Silver nanoparticles embedded into diamond thin films enhance the optical absorption and the photocatalytic activity toward the solvated electron-initiated reduction of N2 to NH3 in water. Here we demonstrate the formation of diamond films with embedded Ag nanoparticles 225 nm. Our results suggest that internal photoemission, in which electrons are excited from Ag into diamond’s conduction band, is an important process that extends the wavelength region beyond diamond’s bandgap. Other factors, including Ag-induced optical scattering and formation of graphitic impurities are also discussed.

1. INTRODUCTION While photocatalytic chemical transformations at surfaces are typically limited by the intrinsic electrochemical stability of water, recent studies have demonstrated that diamond surfaces illuminated with ultraviolet light act as solid-state electron sources and are able to initiate chemical reactions that cannot be readily achieved using conventional photocatalysis, including photochemical synthesis of NH3 from N2 in water1-4 and the selective one-electron reduction of CO2 to CO.5 A unique aspect of this approach is that instead of transferring electrons to reactants adsorbed onto the diamond surface, diamond directly emits electrons into the aqueous medium, creating species such as solvated electrons and neutral hydrogen atoms that initiate unusual homogeneous chemistry in solution.1-2 The photocatalytic reduction of N2 to form NH3 is notable because of the very high barrier associated with formation of the critical intermediate N2H,6 and because of the poor adsorption of N2 onto conventional photocatalytic surfaces.7-9 Diamond surfaces are uniquely suited to emit electrons into water because diamond has a very high-lying conduction band (even higher than the vacuum level) and, when the surface is terminated with H-

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atoms, the resulting surface dipole facilitates electron emission by eliminating any barrier to electron emission. This combination of properties is referred to as negative electron affinity (NEA).10-13 While photoemission from the vast majority of materials can only occur from the outer ~ 1 nm of the surface, in NEA materials such as diamond absorption deeper in the bulk can also produce electron emission, as electrons excited into the bulk conduction band can diffuse to the surface and be ejected into vacuum or, as studied here, an adjacent aqueous phase. While bulk absorption is an attractive way of initiating electron emission, it requires wavelengths of 285 nm. In order to ensure that the valence and conduction bands are bend downward at the surface, experiments reported here included a small (-0.5 V) bias applied to the sample.33 The photo-initiated production of ammonia was measured by illuminating the sample for specific lengths of time and characterizing the amount of NH3 present at the end of the experiment using the indophenol blue method.34 Reagents were purchased from Sigma-Aldrich in the highest available purity. A 2 ml aliquot of solution was removed from the reaction vessel. To this solution we added 0.100 ml of a 1 M NaOH solution containing 5% salicylic acid and 5% sodium citrate (by weight), followed by addition of 20 μl of 0.05 M NaClO and 20 μl of an aqueous solution of


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1% (by weight) Na [Fe(NO)(CN)5] (sodium nitroferricyanide). After 1 h, the absorption spectrum was measured using a Shimadzu 2401PC ultraviolet–visible spectrophotometer. The formation of indophenol blue was determined using the absorbance at a wavelength of 700 nm. Absolute calibration of the method was achieved using ammonium chloride solutions of known concentration as standards. In order to ensure that the observed NH3 did not arise from contaminants, a number of different control experiments were performed in this same manner, but leaving out one or more of the critical catalytic factors (e.g., using Ar instead of N2, or leaving the sample in the dark instead of illuminating it) as described below. Photocatalysis experiments were also performed using diamond surfaces with Ag nanoparticles formed in the same manner, but without encapsulating them in another layer of diamond. Under photocatalysis conditions, we found that the Ag dissolves. Therefore, encapsulation of the Ag nanoparticles in a second diamond layer is necessary. RESULTS Figures 2a, 2b, and 2c show SEM data obtained over a single region of the D-Ag-D sample. Figure 2a shows scanning an SEM image (using a SE2 detector to detect the low-energy secondary electrons) of the D-Ag-D sample using 5 kV excitation, while Figure 2b shows the same region using 25 kV excitation and Figure 2c shows an EDS spatial map for Ag (bright color=high emission, white=low emission).


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Figure 2. Physical characterization of D-Ag-D sandwich structure. Panels (a), (b), and (c) show top-view SEM and EDS images viewed perpendicular to the topmost diamond surface, with all three panels showing the identical region and with a 2 μm scale bar. (a) SEM image acquired using 5 kV incident electron energy, showing primarily surface topography. (b) SEM image of the same region, at 25 kV incident energy, now showing bright regions from buried Ag structures. (c) EDS map showing the spatial distribution of Ag; dark coloration corresponds to high Ag X-ray emission. (d) cross-sectional view of D-Ag-D sandwich structure (e) Raman spectra of D-Ag-D and D-Si samples, showing characteristic 1332 cm-1 diamond peak; the D-Ag-D spectrum was shifted vertically for clarity. In Figure 2a, the D-Ag-D surface shows crystallites typically 100-500 nm across, slightly smaller than the grain size of the initial diamond layer. At the 5kV electron energy used in here, the image reflects the surface topography. Figure 2b shows the identical sample region measured using an incident electron beam of 25 kV energy. Under these conditions the D-Ag-D-Si sample shows distinct regions of higher electron yield (bright in images) from the buried Ag layer, and some smaller structures with diameters of < 1 μm are visible. Figure 2c shows EDS data obtained simultaneously with Figure 2b (2.98 keV emission energy), showing the spatial distribution of Ag. In this image, more intense color corresponds to high Ag X-ray emission. The regions where high Ag emission is observed in Fig. 2c correspond to the bright regions where high electron emission occurs in Figure 2b. When the incident beam energy was reduced to 5 kV for EDS mapping, no Ag X-ray emission was detected from the D-Ag-D structure, while emission was observed in control studies using Ag nanoparticles on top of the diamond sample. A complete set of images at intermediate voltages is included in Supporting Information. Figure 2d shows a cross-sectional SEM image of D-Ag-D. Here, the two diamond layers can be identified, as well as the Ag present at the interlayer. The presence of Ag has some effect on the diamond growth morphology, leading to crystallites with less well-defined surface facets, but still exhibiting a columnar growth


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morphology typically of diamond growth. The SEM and EDS data in Figure 2 show that the procedure followed here produces Ag nanoparticles buried between two diamond layers. To investigate the surface composition of D-Ag-D structures, XPS survey spectra and high-resolution spectra of the C(1s), O(1), and Ag regions were measured. As shown in the Supporting Information (Fig. S5), the survey and high-resolution spectra show only carbon with no evidence for Ag or O at the surface. Indeed, the spectra in the SI show among the lowest levels of O contaminants among reported studies of H-terminated surfaces.35-37 Additional characterization data, including EDS maps for other elements, EDS images obtained at different electron beam energies, SEM and EDS images for control samples of diamond on silicon (D-Si) control samples, and both SEM images and EDS maps of cross-sectional views of D-Ag-D sandwich structures and D-Si showing nanoparticles with diameters

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