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An exchangeable-tip scanning probe instrument for the analysis of combinatorial libraries of electrocatalysts
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View: Figures


Image of FIG. 1.
FIG. 1.

Block diagram of the SDEMS/SECM with connections: (1) mass spectrometer controller to PC Ethernet communications cable, (2) data acquisition (DAQ) card connector, (3) motor controller to PC communications serial cable, (4) mass spectrometer analog outputs to DAQ card, (5) potentiostat analog outputs to DAQ card, (6) motor controller digital output to DAQ digital input, (7) radio frequency generator control and power cable, (8) ionization source and ion optics cable, (9) radio frequency cables, (10)–(13) SECM tip electrode and substrate electrode connections, counter, and reference electrode, (14) motor power cables, (15) SDEMS tip to high vacuum chamber connection hose, (16) angle valve connecting SDEMS hose to high vacuum chamber, and (17) vacuum hose and angle valve connecting diaphragm pump.

Image of FIG. 2.
FIG. 2.

(a) Potential program applied to substrate electrode for SDEMS tip height calibration and (b) corresponding Faradaic current for a Pt substrate. (c) H2 mass spectrometric currents at m/z = 2 collected at tip-substrate separations (ztip) ranging from 0.0 mm (tip in contact with the substrate) to 0.4 mm. The arrow in the color code points in the direction of increasing ztip. The inset shows a plot of the square root of the time delays to the onset and peak mass spectrometric current vs. tip-substrate separation. The solution was 0.1 M H2SO4.

Image of FIG. 3.
FIG. 3.

(a) Photograph of an array of Pt strips sputter deposited on a Ti-coated Si wafer. (b) SDEMS image of H2 evolution, monitored at m/z = 2, over the array at a potential of 0.02 V vs. RHE in 0.1 M H2SO4 solution, (ztip ∼ 75 μm). Scan rate: 80 μm/s. A secondary electron multiplier was used for amplification. (c) SECM image collected with a 25 μm diameter tip poised at −0.55 V vs. RHE in 0.05 M H2SO4 with 0.1 M Na2SO4, (ztip ∼ 14–22 μm). The height varies due to residual sample tilt. Substrate was at open circuit. Scan rate: 80 μm/s. Images were constructed from 18 line scans in the x direction.

Image of FIG. 4.
FIG. 4.

(a) Voltammogram of a 25 μm diameter Pt SECM tip far from the substrate in 0.005 M H2SO4 with 0.1 M Na2SO4 at 20 mV/s. (b) Experimental (points) and calculated (lines) SECM approach curves with a polycrystalline Pt substrate held at several potentials and open circuit, illustrating positive, and intermediate feedback. Complete negative feedback was obtained with a glass substrate. Data were normalized to it∞, the transport-limited current at a large tip-substrate separation. Apparent electron transfer rate constants obtained through fitting the data are shown in the legend. The diffusion coefficient used in the fitting was that of H2, 4.7 × 10−5 cm2/s.

Image of FIG. 5.
FIG. 5.

(a) Cyclic voltammogram of a polycrystalline Pt disk in 0.1 M H2SO4 with 1.0 M methanol at 1 mV/s, (b) formic acid detection current at a PtPb-modified 50 μm Pt SECM tip held at +0.35 V, and ztip ∼8 μm, substrate generation-tip collection mode, (c) CO2 mass spectrometric current, and (d) methyl formate mass spectrometric current recorded with the SDEMS tip approximately 25 μm above the surface. A secondary electron multiplier was used to amplify the methyl formate signal. Note that the SECM tip and SDEMS tip data were collected in two separate measurements at the same substrate.

Image of FIG. 6.
FIG. 6.

(a) Photograph of an array of PtRu rectangles sputter deposited on a 3-in. Si wafer. (b) WDS analysis of the Ru content in each of the rectangles. (c) Photograph of the fluorescence over the array in 5.0 M methanol, 0.1 M KO3SCF3, and 0.3 mM quinine at the end of a potential ramp to 0.61 V. Only the blue channel of the photograph is displayed. (d) Relative quinine fluorescence intensity over the array during a potential sweep at 5 mV/s. Intensities were extracted as rows of pixels from the blue channel of digital photographs collected at the indicated potentials. (e) CO2 mass spectrometric current during SDEMS line scans across the substrate, holding its potential at increasingly more positive values in 1.0 M methanol, 0.1 M H2SO4. ztip ∼ 70 μm. Secondary electron multiplier used for amplification. Scan rate: 120 μm/s.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: An exchangeable-tip scanning probe instrument for the analysis of combinatorial libraries of electrocatalysts