Letter Cite This: J. Phys. Chem. Lett. 2017, 8, 5430-5437
pubs.acs.org/JPCL
In Situ Monitoring of Chemical Reactions at a Solid−Water Interface by Femtosecond Acoustics Chih-Chiang Shen,†,‡ Meng-Yu Weng,† Jinn-Kong Sheu,§ Yi-Ting Yao,† and Chi-Kuang Sun*,†,‡,∥,⊥ †
Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan ‡ Molecular Imaging Center, National Taiwan University, Taipei 10617, Taiwan § Institute of Electro-Optical Science and Engineering and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan ∥ Institute of Physics and Research Center for Applied Science, Academia Sinica, Taipei 115, Taiwan ⊥ Graduate Institute of Biomedical Electronics and Bioinformatics and Center for Optoelectronics Medicine, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *
ABSTRACT: Chemical reactions at a solid−liquid interface are of fundamental importance. Interfacial chemical reactions occur not only at the very interface but also in the subsurface area, while existing monitoring techniques either provide limited spatial resolution or are applicable only for the outmost atomic layer. Here, with the aid of the time-domain analysis with femtosecond acoustics, we demonstrate a subatomic-level-resolution technique to longitudinally monitor chemical reactions at solid−water interfaces, capable of in situ monitoring even the subsurface area under atmospheric conditions. Our work was proven by monitoring the already-known anode oxidation process occurring during photoelectrochemical water splitting. Furthermore, whenever the oxide layer thickness equals an integer number of the effective atomic layer thickness, the measured acoustic echo will show higher signal-tonoise ratios with reduced speckle noise, indicating the quantum-like behavior of this coherent-phonon-based technique.
S
microscopy (STM) based on the concept of quantum tunneling encounters the limitation of conductive samples and the intrusion of the metal tips. STM and atomic force microscopy are not able to provide information below the outmost atomic layer of the solid surface. The limitation for transmission and scanning electron microscopy is the requirement of a microcell to seal the liquid in vacuum. Furthermore, beyond a time period the reaction will stop by depletion of liquid.6 Here, we show that femtosecond acoustics imaging, a technique based on the time-domain analysis with femtosecond acoustic pulses, is able to noninvasively monitor in situ the chemical reactions occurring at a solid−liquid-water interface and the subsurface area under atmospheric conditions while providing subatomic layer sensitivity. By converting a femtosecond optical pulse into a femtosecond acoustic pulse, one can take advantage of the ultraslow sound velocity, which is usually on the order of 3000 m/s and the ultrahigh temporal resolution which is usually on the order of 100 fs. It can be noted that within 100 fs, the femtosecond acoustic pulse travels only a distance of 3 Å, assuming a 3000 m/s velocity. Femtosecond time-resolved
olid−liquid-water interface plays a critical role in various natural phenomena, such as catalytic processes,1 wetting of surfaces,2 metal corrosion,3 and protein folding.4 Examples for chemical reactions at a solid water interface include photoelectrochemical (PEC) water splitting, one of the well-known methods for hydrogen generation, which has an electrochemical energy conversion event occurring at the interface between semiconductor and electrolyte.5 PEC water splitting, taking advantage of free solar energy to split water, is regarded as one potential solution to our energy crisis. Improving the conversion efficiency and increasing the electrode lifetime are crucial for its future. Like all other interfacial chemical reactions, having a nondestructive subsurface imaging technique with an atomic resolution and the capability of in situ monitoring of a solid−liquid interface can help to reveal the dynamic change of electrodes in the early stage. With the assistance of such a technology, it is then possible to improve the performance of the next-generation energy conversion and electrochemical energy storage devices by understanding the early stage corrosion or oxidation processes. In situ imaging a chemical reaction at a solid−liquid interface has been reported by a few researchers.6−10 These previous efforts either provide limited spatial resolution or face experimental restrictions. For example, scanning tunneling © XXXX American Chemical Society
Received: September 7, 2017 Accepted: October 23, 2017 Published: October 23, 2017 5430
DOI: 10.1021/acs.jpclett.7b02384 J. Phys. Chem. Lett. 2017, 8, 5430−5437
Letter
The Journal of Physical Chemistry Letters
Figure 1. Femtosecond acoustic pulses and femtosecond acoustics imaging of PEC water splitting. (a) Typical optically measured waveform for the acoustic echo pulse reflected from a GaN−air interface. The waveform can be well-described by a Gaussian function (denoted by the red dashed line). A full-width-at-half-maximum (FWHM) pulsewidth of 580 fs can be found by fitting. The acoustic pulse was photogenerated in a InGaN single quantum well through femtosecond laser excitation. (b) Autocorrelation of the femtosecond laser pulse with an autocorrelation pulsewidth of 230 fs. (c) Deconvoluted acoustic pulse with a FWHM pulsewidth of 580 fs. The signal-to-noise ratio of the acoustic pulse obtained after deconvolution was much improved by filter artifact. (d) Schematic illustration of a PEC water-splitting cell by using n-GaN as photo anode and platinum as counter electrode. Femtosecond acoustic pulses were used to image the anode oxidation process during water splitting by using time-resolved echo imaging with a femtosecond resolution.
shows a femtosecond-time-resolved acoustic echo pulse reflected from the GaN−air interface. Based on a Gaussian function fitting, our result illustrated a full-width-at-halfmaximum (FWHM) of 0.58 ps for the measured echo pulsewidth, which was the convolution of the acoustic pulsewidth with the autocorrelation width of the optical pulse. With the optical excitation pulse measured (Figure 1b) to be with a 230 fs autocorrelation pulsewidth, the pulseshape of the acoustic echo can then be obtained (Figure 1c), following the principle of deconvolution.12 To study the performance of femtosecond time-domain acoustics applied to in situ monitoring a chemical reaction at a solid−liquid interface, the well-known anode-oxidation process occurring during the GaN-based PEC water splitting was taken as our model (Figure 1d). An n-GaN thin film was used as the unprotected photoanode, and a platinum wire was used as the counter electrode. A 254 nm ultraviolet (UV) pencil lamp was used to excite photocarriers in the n-GaN. A positive bias was applied between the n-GaN film and the platinum to drive the reduction reaction. Because of energy band bending at the nGaN surface,13,14 the electron−hole pairs were spatially separated and photoexcited holes were accumulated on the surface. Holes were then consumed in the oxidation reaction (2GaN + 6OH− + 6h+ → Ga2O3 + 3H2O + N2).15 Related to the stability of water splitting, photocorrosion thus occurred when the photogenerated holes oxidized the unprotected photoanode (n-GaN), resulting in the product of Ga2O3 and N2. In addition, photoexcited electrons were transported to the counter electrode (platinum) through the external circuit and got involved in the reduction reaction for hydrogen generation (6H2O + 6e− → 6OH− + 3H2).15 A detailed sample cell design is shown in Figure 2, including a 3 nm thick InGaN SQW and a 42 nm thick n-GaN anode cap
sound propagation imaging can thus provide a spatial resolution down to 1.5 Å supposing an acoustic round trip path and 100 fs temporal resolution. Through the generation of femtosecond acoustic pulses using a piezoelectric nanolayer and by femtosecond time-resolved acoustic echo imaging, also known as nanoultrasonics scan, our presented study indicates that femtosecond acoustics can indeed noninvasively monitor the dynamic changes of physical structures longitudinally at a solid−water interface and the subsurface area with a subatomiclevel resolution, thus providing the capability of revealing key physical information on reactions occurring at solid−liquid interfaces, ideal for next-generation energy conversion and electrochemical energy storage device developments as well as for the study of early stage corrosion or oxidation processes. Furthermore, whenever the to-be-monitored layer thickness equals an integer number of the effective atomic layer thickness, the measured acoustic echo will show a higher signal-to-noise ratio, not only indicating the discrete quantum behavior of this coherent-phonon-based technique but also providing justification of the applicability of the impedance-matching-layer model even down to a single atomic layer regime. Here we also discuss the physical meaning of noninteger atomic layer thickness for ultrasonic imaging, especially when in this study we push the ultrasonic imaging to its ultimate resolution limitation. The femtosecond acoustics technique was based on the timedomain analysis of a traveling femtosecond acoustic pulse, launched from a piezoelectric semiconductor single-quantumwell (SQW) due to the excitation by an above-bandgap ultrashort optical pulse. This acoustic pulse then traveled toward a solid−liquid interface, got reflected by the multiple interfaces, and was finally femtosecond-time-resolved by an optical probe pulse when it reached the SQW again.11 For details, please see Femtosecond Acoustics System. Figure 1a 5431
DOI: 10.1021/acs.jpclett.7b02384 J. Phys. Chem. Lett. 2017, 8, 5430−5437
Letter
The Journal of Physical Chemistry Letters
Figure 2. Femtosecond acoustics imaging of anode oxidation during PEC water splitting. (a) The ultrasound amplitude-scan (A-mode) by femtosecond acoustics was performed with a 2 V negative bias applied on the n-GaN cap layer. During the A-mode scan, blue femtosecond optical pulses were used to generate and detect the femtosecond acoustic pulses. The sample structure includes a 3 nm thick InGaN SQW and a 42.5 nm thick GaN cap layer. Both layers were grown on a c-plane sapphire substrate with a GaN buffer layer. On top of the n-GaN cap layer, we deposited a microfluidic channel, which allows a better control of the water thickness and stability during the experiments. (b) PEC water splitting with a 2 V positive bias applied on the n-GaN cap layer through illumination of a UV pencil lamp. (c) Measured ultrasound A-scans at different oxidation stages (from 0 to 45 min). Black dashed line indicates the 10.7 ps in pulse echo time, which is the measured roundtrip time to sample surface before oxidation.
layer. Both layers were grown on a c-plane sapphire substrate with a GaN buffer layer. On top of the n-GaN cap layer, we deposited a microfluidic channel, which allows a better control of the water thickness with much improved imaging stability during the experiments. In general, PEC reaction is slow. In order to accelerate the oxidation of the n-GaN anode surface, we did not provide a protective layer. The thickness and the growth rate of the oxide layer can be controlled by changing the applied bias.16 During PEC water splitting, 2 V bias was applied through an ohmic contact17 deposited on the n-GaN cap layer, as the working anode, for a moderate reaction rate. With a positive bias, UV-lamp-excited holes drifted to the n-GaN− water interface to get involved in the oxidation process, as shown in Figure 2b. The light source used to drive femtosecond acoustics imaging was with a photon energy of 3.0 eV, which was closed to the GaN bandgap energy (3.4 eV).18 To avoid any possibility for the femtosecond light pulses to get involved in the oxidation process, we stopped the PEC water splitting by turning off the UV pencil lamp and by varying the external bias from 2 to −2 V, as shown in Figure 2a, during femtosecond acoustics imaging acquisition. PEC reaction was thus stopped when acoustics scanning (equivalent to ultrasound A-scans) was conducted. The experimental protocol was as follows: the duration time of femtosecond acoustics imaging was 90 s, and the imaging was taken at 3 min intervals between PEC water-splitting events. After injection of water into the PEC cell, negative bias was first applied and then the femtosecond A-scan was
performed, which was the 0 min (black) trace shown in Figure 2c. Subsequently, PEC water splitting was run for 3 min by illuminating 254 nm UV light and with a bias voltage of positive 2 V. Afterward, the femtosecond A-scan was performed again for another 90 s, which is the 3 min trace (red) shown in Figure 2c, with UV light off and with a negative bias. The total accumulated time for PEC water splitting was 45 min. During the PEC water-splitting process, 16 fs A-scans were taken, as shown in Figure 2c. At 0 min, we can observe a dip signal at 10.7 ps, which was the acoustic echo pulse reflected from the nGaN−water interface. The thickness of the n-GaN cap layer, 42.5 nm, can be obtained by LGaN = (t1 × vGaN)/2, where t1 is the pulse transit time and vGaN ∼ 7950 m/s is the sound velocity in GaN.19 The pulse transit time did not change significantly until the PEC reaction reached 18 min. Before 18 min, the echo pulses were found to get wider with time, and details of the echo width analysis in the early stage (
