Laurea in Scienza dei Materiali Materiali Inorganici Funzionali Hydrogen production by photocatalytic water splitting Prof. Dr. Antonella Glisenti -- Dip. Scienze Chimiche -- Università degli Studi di di Padova
PHOTOELECTROLYSIS Photoelectrolysis uses sunlight to directly decompose water into hydrogen and oxygen, and uses semiconductor materials similar to those used in photovoltaics. 1) Electron hole pair photo generation. (2) The holes decompose water at the anode s front surface (3) Hydrogen ions pass through the electrolyte and react with the electrons at the cathode to form H 2 (4) O 2 and H 2 are separated (semi-permeable membrane)
Material Requirements 1) The band gap should fall in the range sufficient to achieve the energetics for electrolysis and yet allow maximum absorption fo the solar spectrum (1.6-2.0 ev for single photoelectrode cells and 1.6-2.0; 0.8-1.2 ev for top/bottom cells in stacked tandem configurations). 2) Have a high quantum yield (> 80%) to reach the efficiency necessary for a viable device; 3) Straddle the redox potentials of the H 2 and O 2 half reactions with its conduction and valence band edges, respectively.
Investigated Materials Photoanode: thin-film WO 3, Fe 2 O 3 and TiO 2, n-gaas, n-gan, CdS, and ZnS Photocathode: CIGS/Pt, p-inp/pt, and p-sic/pt Nano-particles of ZnO, Nb 2 O 5 and TiO 2 (the material of choice) N3 dye = cis-rul 2 (NCS) 2 with L standing for 2,20-bipyridyl-4,40- dicarboxylic acid. Black dye: (tri)cyanto)-2,20200-terpyridyl-4,40400- tricarboxylate) Use of low cost materials and the potential for high efficiencies Low light absorption Unsatisfactory stability in time Limiting factors: imperfections in the crystalline structure, bulk and surface properties of the photoelectrodes, the material s resistance to corrosion from the aqueous electrolytes Energetics of the electrochemical reaction must be harmonized with the solar radiation spectrum,
The mechanism of photocatalytic splitting hν TiO 2 e - TiO2 + h+ TiO2 The photo-generated electrons and holes can recombine in bulk or on surface releasing energy (heat or photons). Electrons and holes that migrate to the surface without recombination can, respectively, reduce and oxidize the reactants adsorbed by the semiconductor. Surface adsorption Photocatalytic reactions (nano-materials)
The mechanism of photocatalytic splitting For hydrogen production: 1. the CB level should be more negative than hydrogen production level (E H2/H2O ) while 2. the VB should be more positive than water oxidation level (E O2/H2O ) for efficient oxygen production from water by photocatalysis. Mechanism of TiO 2 photocatalytic water-splitting for hydrogen production.
The main reasons of low efficiency Recombination of photo-generated electron/hole pairs: CB electrons can recombine with VB holes very quickly and release energy in the form of unproductive heat or photons; Fast recombination reaction: Decomposition of water is an energy increasing process, recombination easily proceeds; Inability to utilize visible light: The band gap of TiO 2 is about 3.2 ev and only UV light can be utilized for H 2 production. UV light = about 4% of the solar radiation energy; visible light = about 50%, the inability to utilize visible light limits the efficiency of solar photocatalytic hydrogen production. chemical additives photocatalyst modification techniques nanotechnologies
Chemical additives: addition of electron donors Electron donors (sacrificial reagents or hole scavengers) react irreversibly with the VB holes enhancing the photocatalytic electron/hole separation (> quantum efficiency). Organic compounds (hydrocarbons) can be oxidized by VB holes. The remaining strong reducing CB electrons can reduce protons to hydrogen molecules. EDTA > methanol > ethanol > lactic acid The decomposition of these hydrocarbons could also contribute to a higher hydrogen yield Photocatalytic decomposition of pollutants and photocatalytic production of clean hydrogen fuel can take place simultaneously when the pollutants are acted as electron donors.
Chemical additives: addition of electron donors Other inorganic ions: S 2- /SO 3 2-, Ce 4+ /Ce 3+, IO 3- /I - CdS: S 2- can react with 2 holes to form S. The aqueous SO 3 2- added can dissolve S into S 2 O 3 2- in order to prevent any detrimental deposition of S onto CdS. IO 3- /I - : CdS + 2h + Cd 2+ + S Photocatalytic hydrogen production under the mediation of I - /IO 3-.
Chemical additives: addition of carbonate salts Sayama et al. 1992-2000: The addition of Na 2 CO 3 was observed to be effective for enhancement of hydrogen and oxygen production using Pt loaded TiO 2 CO 2-3 + H + HCO - 3 HCO 3- + h + HCO 3 HCO 3 H + + CO 3-2CO 3 - C 2 O 2-6 C 2 O 2-6 + 2h + O 2 + 2CO 2 The evolution of CO 2 and O 2 could promote desorption of O 2 from the photocatalytic surface and thus could minimize the formation of H 2 O through the backward reaction of H 2 and O 2. Desorbed CO 2 was dissolved and converted into HCO 3 -
Rate of production of H 2 and O 2 from ZrO 2 aqueous suspensions containing several additives Chemical additives: addition of carbonate salts Rate of production of H 2 and O 2 over a Pt/TiO 2 suspension with salt additives
Photocatalyst modification techniques 1. Noble metal loading 2. Ion doping Metal ion doping Anion doping 3. Sensitization Dye sensitization Composite semiconductors Photoelectrolysis 4. Metal ion-implantation
Noble metal doping Noble metals (Pt, Au, Pd, Rh, Ni, Cu and Ag) have been reported to be very effective for enhancement of TiO 2 photocatalysis. As the Fermi levels of these noble metals are lower than that of TiO 2, photo-excited electrons can be transferred from CB to metal particles deposited on the surface of TiO 2, while photogenerated VB holes remain on the TiO 2. This reduce the possibility of electron-hole recombination, resulting in efficient separation and stronger photocatalytic reactions. 1. contact with TiO 2 active sites (particularly for Au). 2. optimal loading (for photon absorption, electron-hole recombination) (i) recombination cannot be completely eliminated; (ii) backward reaction of H 2 and O 2 to form H 2 O is thermodynamically favorable. (iii) Pt is very expensive
Metal ion doping doping of metal ions could expand the photo-response of TiO 2 into visible spectrum. As metal ions are incorporated into the TiO 2 lattice, impurity energy levels in the band gap of TiO 2 are formed: M n+ + hυ M (n+1)+ + e - ch M n+ + hυ M (n-1)+ + h + vb But: M n+ + e - ch M(n-1)+ M n+ + h + vb M(n+1)+ Fe, Mo, Ru, Os, Re, V, Cu, Rh ions can increase photocatalytic activity, while Co and Al ions cause detrimental effects. 1. For photocatalytic reactions, carrier transferring is as important as carrier trapping: only if the trapped electron and hole are transferred to the surface, photocatalytic reactions can occur (deep doping = recombination centers) 2. Optimum concentration of doped metal ion
Anion doping Doping of anions (N, F, C, P, S etc.) in TiO 2 crystalline could shift its photo-response into visible spectrum Unlike metal ions (cations), anions less likely form recombination centers and are more effective to enhance the photocatalytic activity. N The nitrogen doped TiO 2 thin film was prepared by 1. By sputtering TiO 2 in an N 2 (40%)/Ar gas mixture, followed by annealing at 550 C in N 2 for about 4 h. Nitrogen 2. By treating TiO 2 in NH 3 (67%)/Ar at 600 C for 3 h. S Ti isopropoxide + thiourea in ethanol Oxidation annealing of TiS 2 C,P Less effective as the introduced states were too deep
Composite semiconductors When a large band gap semiconductor is coupled with a small band gap semiconductor with a more negative CB level, CB electrons can be injected from the small band gap semiconductor to the large band gap semiconductor. Thus, a wide electronhole separation is achieved. The process is similar to dye sensitization. Electron injection in composite semiconductors.
Composite semiconductors (i) semiconductors should be photocorrosion free, (ii) the small band gap semiconductor should be able to be excited by visible light (iii) the CB of the small band gap semiconductor should be more negative than that of the large band gap semiconductor (iv) the CB of the large band gap semiconductor should be more negative than E H2/H2O and (v) electron injection should be fast as well as efficient. CdS (BG 2.4 ev) + SnO 2 (BG 3.5 ev) - Sacrificial agent, EDTA, has to be added to scavenge VB holes on CdS; CdS-ZnS composite semiconductor. Photocorrosion inhibited by Na 2 S/Na 2 SO 3 WO 3 (BG 2.7 ev) and SiC (BG 3.0 ev). As the CB of SiC was more negative, electron transfer to the CB of TiO 2
Metal ion-implantation When TiO 2 is bombarded with high-energy transitional metal ions these ions are injected into the lattice and interact with TiO 2. This process modifies TiO 2 electronic structure and shifts its photo-response to the visible region (up to 600 nm). Presently metal ion implanted TiO 2 is believed to be the most effective photocatalyst for solar energy utilization and is in general referred as the second generation photocatalyst. implanted Cr-ion into TiO 2 V, Mn, Ni, Ar, Mg, Ti and Fe. Except Ar-ions, Mg-ions and Tiions, implantation of all other metal ions resulted in red shift: V>Cr>Mn>Fe>Ni. Only when implantation was followed by calcinations in an O 2 atmosphere (723 823 K), red shift could be realized.
Bibliography 1. J.D. Holladay, J. Hu, D.L. King, Y. Wang; Catalysis Today 139 (2009) 244 260 Review: An overview of hydrogen production technologies 2. Meng Ni, Michael K.H. Leung, Dennis Y.C. Leung, K. Sumathy; Renewable and Sustainable Energy Reviews 11 (2007) 401 425 A review and recent developments in photocatalytic water-splitting using TiO 2 for hydrogen production