Fig. 1. Design schematic of the in situ fiber-optic detection for photoelectrochemistry. (a) Schematic diagram of the lab-on-fiber photoelectrochemical platform composed of an Au nanofilm-coated silica optical fiber on which electrochemical reactions are controlled by inputting an electrical signal. (b) 2D simulated electric field at the Au nanofilm-surrounding interface at wavelength of non-matched and phase-matching condition. Norm(T) is the normalized electric field intensity. (c) Corresponding 1D electric field at the interface. (d) Schematic diagram of photoelectrochemical reaction paths associated with photoexcitation of CuO on the optical fiber.

Fig. 2. (a) Resonant wavelength shift of SPR as a function of refractive index. (b) CV curves of the optical fiber probe in 0.5 mol/L KCl solution with 5 mmol/L [Fe(CN)6]3−/4− redox probe at different scan rates. (c) Oxidation and reduction current peaks versus the square root of the scan rate. (d) Real-time electrochemical current, synchronous resonant wavelength variations Δλ, and differential wavelength variations d(Δλ)/dt of SPR under periodic potential sweeping between −0.4 and 0 V during cyclic voltammetry.

Fig. 3. (a) CV behaviors of the bare optical fiber probe over the range of −0.4 to 0 V in 0.1 mol/L NaCl solution containing 1 mmol/L CuCl2 at a scanning rate of 50 mV/s and (b) corresponding anodic and cathodic peak currents with respect to the times of cycle. (c) Fiber-optic transmission spectra measured after each cycle, which are presented as stacked images. (d) Transmission spectra measured at cycle 0 and cycle 10, showing typical spectral changes before and after electrochemical growth of Cu-based nanomaterials. Inset: photographs obtained from one section of device samples with and without Cu/Cu2O nanomaterials. (e) Complex refractive index changes, including the change in the real part (Δn) and the imaginary part (Δk, k is the extinction coefficient), calculated from the measured spectra.

Fig. 4. Chronoamperogram with stepped potentials of (a) 0.4 to −0.2  V and (b) 0.4 to −0.4  V in 0.1 mol/L NaCl solution containing 1 mmol/L CuCl2 and (c) and (d) corresponding real-time spectra plotted against time as 2D maps. The spectra are scaled in logarithmic coordinates of the transmission light power after normalization. (e) Transmission spectra extracted from (c) and (d) that are measured after applying the potentials of 0.4 V, −0.2  V, and −0.4  V for 600 s. (f) Measured real-time spectra during the first 300 s after the potential step from 0.4 to −0.4  V. (g) Complex refractive index changes calculated from the measured spectra. (h) XPS and (i) high-resolution Cu 2p XPS of the electrochemically grown Cu-based nanomaterials. (j) Spectral shift during chronoamperometry test at different ambient temperatures.

Fig. 5. (a) CV behaviors of the optical fiber probe deposited with Cu/Cu2O nanomaterials over the range of −0.5 to 0.1 V in 1 mol/L NaOH solution at a scanning rate of 50 mV/s and (b) corresponding anodic and cathodic peak currents with respect to the times of cycle. (c) Fiber-optic transmission spectra measured after each cycle, which are presented as stacked images. (d) Transmission spectra measured at cycle 0 and cycle 10, showing typical spectral change during composition transitions of copper oxides. Inset: photographs obtained from one section of device samples deposited with Cu/Cu2O and CuO nanomaterials. (e) Complex refractive index changes calculated from the measured spectra.

Fig. 6. (a) SEM image of the bare optical fiber probe. (b) SEM image of the optical fiber probe with CuO and (c) corresponding zoomed-in region. (d) Raman spectra of the optical fiber probe with and without CuO. (e) Top: photocurrent–time curves in 0.1 mol/L NaCl solution at a constant potential of 0 V under chopped lighting with a period of 2 s. Bottom: chopped photocurrent over three on/off cycles with a period of 600 s. The currents for the light-on and light-off processes are plotted in different colors. (f) High-resolution XPSs of (left) Cu 2p and (right) Cu LMM of the Cu photocathode with different reaction times under continuous illumination. (g) Fiber-optic transmission spectra measured during the photocurrent tests. The spectra are scaled in logarithmic coordinates after normalization, presented as stacked images, and plotted in different colors for light-on and light-off processes. (h) Corresponding wavelength shift of SPR during three consecutive on/off light cycles. The photocurrent experiments are performed in triplicate. The average results (dots) are calculated together with the error bars (shadings). (i) Average photocurrent and the loss of photocurrent during light illumination for each cycle. (j) Average wavelength of SPR and wavelength shift during light illumination for each cycle.