Unveiling the Origin of Co₃O₄Quantum Dots for Photocatalytic Overall Water Splitting

Abstract

Previous studies reveal that oxygen evolution reaction is the bottleneck in overall water splitting process because of the four electron redox reaction. [8][9][10][11][12] Co 3 O 4 , as a p-type semiconductor with a band gap of ≈2.1 eV, has been demonstrated as a good candidate for photocatalytic water oxidation due to the environmental friendliness, visible light response, and superior oxidation ability. [13][14][15][16][17] Unfortunately, photocatalytic water reduction to hydrogen is difficult to proceed over Co 3 O 4 nanocrystal, because the conduction band edge was more positive than H 2 evolution potential (0 eV versus NHE). Proper design and modification of conduction band edge position are desired for hydrogen evolution over Co 3 O 4 semiconductors. For example, F-doped Co 3 O 4 were prepared via a plasma assisted method and showed photocatalytic hydrogen generation from water/ethanol solution. [18] Recently, quantum semiconductors have emerged as efficient photocatalysts owing to high specific area, short charge transfer distance, and superior light absorption. [19][20][21] Particularly, the band gaps will be enlarged and the conduction band edge will be elevated when the crystal size of prepared photocatalysts was reduced to smaller than its Bohr radius. [22][23][24] Thus, overall water splitting to hydrogen and oxygen is expected over Co 3 O 4 QDs. For example, Zhang et al. adopt a reverse micelle method to prepare Co 3 O 4 QDs and it showed pure water splitting. [22] In addition, photo-induced charge transfer and separation plays an important role in determining the photocatalyst efficiency. However, poor studies of charge transfer were conducted on Co 3 O 4 QDs owing to their complicated electronic structure and ionic bonding character. [25,26] Efficient hole injection from sensitizer p-oligo (phenylenevinylene) molecular to Co 3 O 4 was observed through transient optical absorption spectroscopy study. [27] Owing to the weak coupled, localized d state, long lived charge carrier of d-d excitations was characterized in Co 3 O 4 . [28] In order to propel the wide application of Co 3 O 4 based photocatalysts, solid understanding of charge carrier dynamic in Co 3 O 4 is urgently required.Herein, we adopt a solvothermal method to prepare colloidal Co 3 O 4 quantum dots, and applied transient absorption (TA) spectroscopy to study the charge carrier dynamics of Co 3 O 4 QDs during the water splitting process. The obtained Co 3 O 4 QDs displayed overall water splitting to hydrogen and oxygen without cocatalysts loading. TA spectroscopy results suggested that charge transfer from 2p state of O 2p to 3d state of Co 2+ played

Spin configuration modulation of Co3O4 by Ru doping for boosting the overall water splitting and hydrazine oxidation reactions

“…43 Moreover, the dominant A 1g mode suggested the presence of Co 3+ in the octahedral site of Co 3 O 4 nanocrystals and the F 2g 1 modes revealed Co 2+ in the tetrahedral site. 44 After calculation, the relative intensity ratios of A 1g and F 2g 1 in Ru-Co 3 O 4 |Vo/CF and Co 3 O 4 /CF were 5.36 and 15.97, respectively. This may be because the Co atom of the octahedral CoO 6 unit in Co 3 O 4 was replaced by the Ru atom.…”

“…43 Moreover, the dominant A 1g mode suggested the presence of Co 3+ in the octahedral site of Co 3 O 4 nanocrystals and the F 2g 1 modes revealed Co 2+ in the tetrahedral site. 44 After calculation, the relative intensity ratios of A 1g and F 2g Two shakeup satellites (identified as "Sat.") were observed at around 786.9 and 802.9 eV.…”

Dual Heterogeneous Structures Promote Electrochemical Properties and Photocatalytic Hydrogen Evolution for Inverse Opal ZnO/ZnS/Co3O4 Crystals

“…Co 3 O 4 has been widely investigated as a promising candidate for boosting the photocatalytic activity of host semiconductor because of its low cost, narrow band gap (∼2.1 eV), and superior redox capability. − Yi et al demonstrated that Co 3 O 4 greatly promoted the photoelectrochemical water splitting of Fe 2 O 3 nanorod arrays . Chen et al found that Co 3 O 4 also significantly increased the hydrogen production rate of ZnIn 2 S 4 under visible light .…”

“…17 than H 2 evolution potential. 19 Therefore, the rational design of Co 3 O 4 -modified semiconductors is an important step in promoting photocatalytic hydrogen evolution.…”

“…proved that the Co 3 O 4 -modified Cd 0.6 Zn 0.4 S nanorods effectively accelerated hole migration for photocatalytic water splitting . However, Co 3 O 4 itself cannot drive photocatalytic hydrogen evolution because its conduction band edge is more positive than H 2 evolution potential . Therefore, the rational design of Co 3 O 4 -modified semiconductors is an important step in promoting photocatalytic hydrogen evolution.…”