Unraveling the Mechanisms of Beneficial Cu-Doping of NiO-Based Photocathodes

Abstract

Dye-sensitized photoelectrochemical (DSPEC) water splitting is an attractive approach to convert and store solar energy into chemical bonds. However, the solar conversion efficiency of a DSPEC cell is typically low due to a poor performance of the photocathode. Here, we demonstrate that Cu-doping improves the performance of a functionalized NiO-based photocathode significantly. Femtosecond transient absorption experiments show longer-lived photoinduced charge separation for the Cu:NiO-based photocathode relative to the undoped analogue. We present a photophysical model that distinguishes between surface and bulk charge recombination, with the first process (∼10 ps) occurring more than 1 order of magnitude faster than the latter. The longer-lived photoinduced charge separation in the Cu:NiO-based photocathode likely originates from less dominant surface recombination and an increased probability for holes to escape into the bulk and to be transported to the electrical contact of the photocathode. Cu-doping of NiO shows promise to suppress detrimental surface charge recombination and to realize more efficient photocathodes.

Dual Role of Surface Hydroxyl Groups in the Photodynamics and Performance of NiO-Based Photocathodes

“…The P1 excited state (P1*) is known to give a strong and broad positive absorbance above 550–560 nm and a weak absorbance around 410 nm. , Due to hole injection from P1* into NiO, the intensity of P1* decreases, whereas the characteristic spectrum of P1 •– arises (positive bands around 420 and 610 nm) . Photoinduced hole injection is known to cause a red shift in the TA spectrum due to overlapping signals of P1 GSB, P1*, P1 •– , and Ni 3+/4+ ,− (Supporting Information Figure S3). It is notable that the spectra for NiO/P1 in air and especially in PBS at early times (<1 ps) are red-shifted compared to those of NiO/P1 in MeCN, which can be explained by more photoinduced hole injection within the instrumental response time (IRT, 100–150 fs) in air/PBS.…”

“…A possible reason for this fast charge recombination after hole injection is hole accumulation. Transition metal hydroxyl oxides are well-known supercapacitor materials, and they have the ability to store charges, either from the additional bias − or from photogenerated charges. , In addition, NiO is a p-type material but suffers from a poor hole mobility, which means that after ultrafast hole injection, most of the holes likely remain at, or close to the OH – -terminated surface. This is in agreement with literature studies reporting holes to be mainly pinned at the NiO surface, increasing the chance to recombine with P1 •– .…”

Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes

“…However, whether an applied potential has the same effect on DSPEC cells with their configuration based on lightinduced charge separation at the dye-semiconductor interface rather than inside the semiconductor is unknown, although some dye-sensitized photocathodes show surprisingly low photocurrents at more negative bias potentials. [17][18] The composition of the electrolyte may also play an important role here.…”

Long-Lasting Panchromatic Electrochromic Device and Energy-Dense Supercapacitor Based on Zn-Doped NiO Microstars and a Redox-Active Gel

“…This concern can be addressed by using doped or multimetal TMOs. Cu-doping of NiO was found to reduce ultrafast charge recombination, which improved the photocatalytic conversion properties by lowering the band gap . In La-doped (Ni 3 (NO 3 ) 2 (OH) 4 nanoflowers, La cations with large diameters functioned as pillars, increased the c-axis spacing, and led to an effective enhancement of ionic diffusivity and electronic conductivity …”

“…Cu-doping of NiO was found to reduce ultrafast charge recombination, which improved the photocatalytic conversion properties by lowering the band gap. 10 In La-doped (Ni 3 (NO 3 ) 2 (OH) 4 nanoflowers, La cations with large diameters functioned as pillars, increased the c-axis spacing, and led to an effective enhancement of ionic diffusivity and electronic conductivity. 11 In this study, we report the synthesis, characterization, and application of Zn-doped NiO (Zn:NiO) microstars to high performance supercapacitors and high-contrast ECDs with panchromatic visible light absorption capability.…”