Enhanced photocatalytic water oxidation on ZnO photoanodes in a borate buffer electrolyte

Abstract

Solar fuels can be produced from photocatalytic, photoelectrochemical or photovoltaic-electrochemical splitting of H 2 O and conversion of CO 2 . In these processes, the water oxidation reaction is involved. The water oxidation reaction is complicated with transfer of 4 electrons and 4 protons. Calculation results showed that the free energy change of the rate-controlling step of water oxidation is as high as B0.6 eV at the overpotential of 0.7 V on the TiO 2 surface, 1 and transient absorption spectroscopy revealed that the water oxidation is as slow as on the scale of sub-seconds. 2 On the other hand, the valence bands of semiconductors like TiO 2 are still sufficiently positive to meet the energy requirement. As an analogue of TiO 2 , ZnO is similar in band gap and positions of CB and VB. 3 ZnO is has been studied as a photoanode for water oxidation as well, 4-6 but gives insufficient efficiency. There are some other reasons for low efficiency of water oxidation on photoanodes. Most photo-generated holes are trapped by defect sites or surface species except for intrinsic recombination. 2,7,8 The trapping reduces the energy of holes and thus the oxidizing ability. Furthermore the trapped sites and surface species may act as recombination centers which results in a much shorter lifetime of the holes. Most holes are recombined before driving water oxidation reaction, so the quantum efficiency (QE) is very low. Applying high potential on the photoanode inhibits recombination by extracting electrons, but it requires external bias and thus consumes external energy. 9In order to increase the QE, one way is to remove the recombination centers by improving the crystallinity of the semiconductor and by surface modification. 10,11 Another way is to change the path of water oxidation to lower the kinetic barrier and reduce the overpotential required for water oxidation.Cocatalysts are deposited on the semiconductor photoanode to reduce the overpotential of water oxidation reaction, such as IrO 2 , 12,13 Co 3 O 4 , 13-15 FeOOH, 16 and a Co-based catalyst deposited with phosphate buffer (known as Co-Pi). 6,[17][18][19] In the studies using Co-Pi as a cocatalyst, usually phosphate buffer solution is also employed as an electrolyte, following the original research on electrocatalytic water oxidation using Co-based catalysts deposited with various buffers including phosphate, methylphosphate and borate. 20,21 A few studies showed that the activity of the assisted photoanodes in a buffered phosphate electrolyte is higher than that in an unbuffered sulfate electrolyte with the same pH. 22In this work, we found that even without adding a source of valence-variable metals, such as Fe, Co or Ni, water oxidation on ZnO photoanodes was enhanced by in situ photoelectrochemical reaction in B BS or treatment at 75 1C in concentrated B BS . ZnO photoanodes used in this study were fabricated on FTO with a seed layer by galvanostatic cathodic deposition from a solution of 5 mM Zn(NO 3 ) 2 and 50 mM KNO 3 at 70 1C. A base forms from the re...

Photoelectrochemical Water Oxidation Efficiency of a Core/Shell Array Photoanode Enhanced by a Dual Suppression Strategy

“…[16] A recent study also showed that the activity of photoanodes in ab uffered phosphate electrolyte, as ap roton-accepting electrolyte (PAE), was higher than that in an unbuffered sulfate electrolyte. [18] These results highlight the important role of the PAE during PEC water oxidation. [18] These results highlight the important role of the PAE during PEC water oxidation.…”

“…[17] Another study reported that the water oxidation onset potentialh ad an egative shift of about 0.2 Vb yi nsitu treatment or immersion of ZnO photoanode in borate buffer. [18] These results highlight the important role of the PAE during PEC water oxidation.…”

“…The TiO 2 -P photoanode exhibits as hift of 430 mV towardst he cathodic direction, which suggests that the phosphate may play ac riticalr ole in decreasing the overpotential of water oxidation, similart ot hat of borate reported. [18] Moreover, phosphate, as the PAE, facilitates the PCET process in the water oxidation reactions. [3a, 28] Recent studies reported that doping of phosphate into semiconductors (e.g.,B iVO 4 ,T iO 2 ,a nd Fe 2 O 3 )i mprovedt heir PEC photocurrent response.…”

Solar Water Oxidation by Multicomponent TaON Photoanodes Functionalized with Nickel Oxide

“…Among the many inorganic photocatalysts applied for water splitting (TiO 2 , WO 3 , ZnO, α ‐Fe 2 O 3, BiVO 4 , etc. ), tantalum‐based oxynitrides among the few n ‐type semiconductors with close to an ideal electronic configuration, as illustrated in Scheme .…”

Enhanced photocurrent by the co-sensitization of ZnO with dye and CuInSe nanocrystals

“…7,8 ZnO is well known to have energy-band structure and physical properties similar to TiO 2 . [9][10][11] In addition, ZnO has the advantages of low-temperature preparation, controllable structural design, and high carrier transport, especially its good electron mobility (for ZnO, it is 115-155 cm 2 V −1 s −1 ; for TiO 2 , it is 10 −5 cm 2 V −1 s −1 ). [12][13][14][15][16][17][18] ZnO has the capability of growing with many different types of morphologies including wires, rods, tubes, particles and power shapes at the nanoscale, [14][15][16][17][18][19] which can be prepared through different approaches, for example, chemical vapor deposition, 20 sputtering, 21 thermal spraying, 22 and the commonly used wet-chemical routes, 23 while the preparation of porous ZnO nanoparticles by a chemical template-free method has been scarcely reported.…”