Diamond Annual Review 2019/20
36 37 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Structures and Surfaces Group Beamline I09 Engineering a new class of photocatalysts for clean fuel production Related publication: Andrews J. L., Cho J.,Wangoh L., Suwandaratne N., Sheng A., Chauhan S., Nieto K., Mohr A., Kadassery K. J., Popeil M. R., Thakur P. K., Sfeir M., Lacy D. C., LeeT. L., Zhang P.,Watson D. F., Piper L. F. J. & Banarjee S. Hole extraction by design in photocatalytic architectures interfacing CdSe quantumdots with topochemically-stabilized tin vanadiumoxide. J. Am. Chem. Soc. 140, 17163-17174 (2018). DOI: 10.1021/jacs.8b09924 Publication keywords: Photocatalyst; metal oxide; X-ray Photoelectron Spectroscopy; Electronic Structure T he generation and storage of clean energy is one of the grand challenges of the 21st century. Water splitting, using sunlight to split water into hydrogen and oxygen, is a promising strategy for clean hydrogen generation. However, it requires the concerted action of absorption of photons, separation of excitons and charge diffusion to catalytic sites and catalysis of redox processes. A newgeneration of photocatalystswill be needed that employ hybrid systems, where different components perform light-harvesting, charge separation and catalysis in synergy. Chalcogenidematerials contain one ormore chalcogen elements (e.g. sulfur, seleniumor tellurium). Quantumdots (QDs) are semiconductor particles just a few nanometres in size, which have different optical and electronic properties to larger particles of the same material. Chalcogenide QDs have high absorption coefficients that can be easily optimised in the visible spectrum for efficient light harvesting. However, they suffer fromphoto-corrosion due to the build-up of photo-generated holes. Researchers developing a new class of hybrid photocatalyst for hydrogen generation used Hard X-ray Photoelectron Spectroscopy (HAXPES) measurements at the Surface and Interface Structural Analysis beamline (I09). The high energy HAXPES enabled them to detect new states induced by intercalating (‘doping’) post-transitionmetal ions in V 2 O 5 semiconductors nanorods interfaced to the QDs. The added ions facilitated rapid, efficient hole transfer from the quantumdot to suppress photo-corrosion and enhance hydrogen generation. In 1972, Fujishima and Honda first demonstrated photolysis with UV light using a wide band gap semiconductor, TiO 2 , to generate photoexcited holes to oxidise the water 1 . The corresponding electrons were transferred to a platinum electrode for the hydrogen evolution. For TiO 2 , the filled O 2p shell is situated at a very negative potential compared to the oxidation potential of water therefore requiring an undesirable 2 V over-potential. Since then there has been considerable research efforts towards developing cheaper platinum- free systems using visible light. From a materials perspective these activities have focused largely on reducing the band gap of the semiconductor i.e. band gap engineering, by modifying the electronic structure by altering the composition. One promising route to band engineer the semiconductor photocatalyst is to combine d 0 transition metals (e.g. V 5+ ) with lone pair-active post-transition N-2 metal ions e.g. Bi 3+ e.g. BiVO 4 . Post-transition metal oxide incorporating N-2 ions can exhibit lone pair distortions where the lone pair s 2 electrons are mediated by hybridising with oxygen 2p orbitals 2 . The oxygen-mediated lone pair orbital raises the ionisation potential closer to the elusive H 2 O/O 2 , while reducing the hole effective band-edge mass. A synthetic lone pair active metal oxide was first demonstrated in 2014, where Pb 2+ was intercalated into the tunnel framework of metastable ζ-phase polymorph of V 2 O 5 3 . When the lone pair active ion is inserted into tunnels of the ζ-V 2 O 5 structure the oxygen mediated lone pair states push out from the void, as illustrated in Fig. 1a, b.The intercalation of the lone pair active ions reduces the band gap and ionisation potential by 0.8 eV compared to the parent ζ-V 2 O 5. The rich playground afforded by the topochemical synthesis of M x V 2 O 5 enables the rational design of p-type metal oxide semiconductors that can be interfaced with efficient visible-light QDs 4 . The ζ-V 2 O 5 nanowires decorated with CdSe QDs exhibit a staggered band gap or type II band offset, Fig. 1c. The photogenerated electrons from the CdSe migrate to the unoccupied conduction band and can be used to reduce protons to hydrogen if a sacrificial agent is used to address the buildup on holes on the quantumdot. By screening M x V 2 O 5 candidates with lone pair active ions, it was possible to band engineer the interfacial alignment to promote hydrogen evolution at the preferred QD site and hole transfer to the nanowire at energies close to the H 2 O/O 2 redox level, Fig. 1d. 4,5 The HAXPES measurements at I09 were critical for both screening the M x V 2 O 5 candidates and measuring the band offset at the nanowire/QD hybrid photocatalysts. As the photon energy is increased the relative orbital cross- sectionsoftheejectedphotoelectronsvariesto increasethesensitivityofmetal s orbitals compared to oxygen p orbitals. As a result, the measured valence band spectra from HAXPES can more easily identify the lone pair states than traditional photoemission spectroscopy. Meanwhile, the increased kinetic energy of the escaping photoelectrons afforded by HAXPES increased the effective probing depth of the technique and facilitates the detection of signals from deeper within the solid compared to transitional photoemission studies. As a result, the band offset of the buried nanowire/QD interface could be measured. From our studies we identified β-Sn 0.23 V 2 O 5, where the hole barrier height with the quantumdot could be reduced to zero, thereby facilitating fast (< ps) hole transfer 5 . In addition, the reduction of the vanadium resulted in the partial filling of the conduction band states suppressing electron transfer and promoting the hydrogen evolution at the preferred QD site. References 1. Fujishima A. et al. Electrochemical photolysis of water at a semiconductor electrode. Nature 238 , 37-38 (1972). DOI: 10.1038/238037a0 2. Walsh A. et al. Stereochemistry of post-transition metal oxides: revision of the classical lone pair model. Chem. Soc. Rev. 40 , 4455-4463 (2011). DOI: 10.1039/C1CS15098G 3. Wangoh L. et al. Electron lone pair distortion facilitated metal-insulator transition in β-Pb 0.33 V 2 O 5 nanowires. Appl. Phys. Lett. 104 , 182108 (2014). DOI: 10.1063/1.4875747 4. Razek, S. A. et al ., Designing catalysts for water splitting based on electronic structure considerations Electron. Struct . 2 023001 (2020). DOI: 10.1088/2516-1075/ab7d86 5. Andrews J. L. et al . Hole extraction by design in photocatalytic architectures interfacing CdSe quantum dots with topochemically stabilized tin vanadium oxide. J. Am. Chem. Soc. 140 , 17163-17174 (2018). DOI: 10.1021/jacs.8b09924 Funding contribution: This research was supported from the National Science Foundation under DMREF grants 1626967, 1627583 and 1627197. We thank Diamond Light Source for access to beamline I09 (SI12764 and SI16005) that contributed to the results presented here. Corresponding authors: Dr Louis Piper, Binghamton University State University of New York, lpiper@ binghamton.edu Ms Sara Abdel Razek, Binghamton University State University of New York, firstname.lastname@example.org ΔE h E Vac (eV) S 3p S 3p, Sn 5s-O 2p H + H 2 e - e - e - h + h + h + h + e - e - e - H + H 2 β-Sn x V 2 O 5 ζ-V 2 O 5 CdSe CdSe No Corrosion CBM CBM VBM (a) (b) H 2 O O 2 (c) (d) Lone Pair VBM O 2 Ni(3-MPA) Ni(3-MPA) S.A Corrosion O 2 /H 2 O H 2 /H + -4.5 -5.5 -6.5 -7.5 -8.5 -9.5 HAXPES HAXPES Figure 1: The calculated spatial distribution of topmost filled orbitals of (a) ζ-V 2 O 5 and (b) β-Sn 0.23 V 2 O 5 , highlighting the lone pair states formed by the intercalation of Sn 2+ ions (black box). (c-d) The measured band offsets of the engineered hybrid photocatalysts formed by decorating the metal oxide nanowires with CdSe quantum dots. The HAXPES data is shown alongside the band diagrams on a common energy scale (w.r.t. vacuum level), the shaded regions highlight the difference in valence band edge alignment for decorated and undecorated nanowrires. The intercalated Sn 2+ is responsible for both suppressing photo-corrosion and inducing hydrogen generation at the preferred quantum dot. A sacrificial agent (SA) was required for ζ-V2O5/CdSe (reproduced from Razek et al., 2020).
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