Hendewerk, M.

Leygraf, Ch.

Somorjai, G.A.

eScholarship - University of California

Lawrence Berkeley National LaboratoryLBL PublicationsTitleTHE PHOTOCATALYTIC PRODUCTION OF HYDROGEN FROM WATER BY A p- AND n-TYPE POLYCRYSTALLINE IRON OXIDE ASSEMBLYPermalinkhttps://escholarship.org/uc/item/704016jsAuthorsLeygraf, Ch.Hendewerk, M.Somorjai, G.A.Publication Date1982-06-01eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaLBL-14560".d-Preprint Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Materials & Molecular Research Division RECEIVED LAWRENCE BERKELEY LABORATORY !,UG ~ 1982 Submitted for publication to Nature . LIBRARY AND DOCUMENTS SECTION THE PHOTOCATALYTIC PRODUCTION OF HYDROGEN FROM WATER BY A p- AND n-TYPE POLYCRYSTALLINE IRON OXIDE ASSEMBLY Ch. Leygraf, M. Hendewerk and G.A. Somorjai June 1982 TWO-WEEK LoAN COpy This is a Library Circulating Copy which may be borrowed for two weeks. For. a personal retentJon copy} call Tech. Info. Division} Ext. 6782. Prepared for the U.S. Department of Energy under Contract DE-AC03-76SF00098 DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California. nor any of their employees. makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus. product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. LBL-14560 THE PHOTOCATALYTIC PRODUCTION OF HYDROGEN FROM WATER BY A p- AND n-TYPE POLYCRYSTALLINE IRON OXIDE ASSEMBLY r Ch. Leygraf, M. Hendewerk and G.A. Somorjai Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry University of, California Berkeley, California 94720 This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemi~al Sciences Division of the U.S. Department of Energy under Contract Number DE-AC03-76SF00098. -2-Abstract The photodissociation of water to produce hydrogen has been accomplished using light in the solar range of the electromagnetic spectrum and in the absence of any external potential at 300K. The catalyst for the reaction is a polycrys-talline pin diode assembly made out of ~- and Si-doped iron oxide. In 0.1 ¥ Na2S04 solution (pH = 6) the device produces hydrogen catalytically with rates of 1-2 H2 molecules per site per minute and its power conversion effici~ncy is about 0.05%. Iron oxide containing minerals could have played important roles in the photochemical evolution of the planet in the pre-chlorophyl era. -3-We report the successful photochemical conversion of water to hydrogen using light in the solar region of the electromagnetic spectrum and at 300K. The cat-alysts for this reaction are polycrystalline iron oxide discs that are doped with silicon and magnesium. Iron oxide, Fe203, is a semiconductor with a band gap of 2.2 eV. It becomes n-type when doped with silicon and p-type when doped with magnesium. By connecting n-type and p-type iron oxide polycrystalline pressed discs with a conducting wire, a diode assembly is produced as shown in Figure 1. When this assembly is immersed in a 0.1 M Na2S04 aqueous solution and illuminated with visible light both hydrogen evolution and photocurrent across the samples are detectable ~he hydrogen production rate is in the range of 1 to 2 hydrogen molecules per site per minute and the photocurrent is of the order of 10 IJA/cm2 • The reaction is thus catalytic and can be carried out for 6-8 hours without any sign of poisoning. When the photoinduced hydrogen produc-tion and the corresponding photocurrent eventually decline they can be regener-ated'by passing oxygen gas over the catalyst iron oxide surfaces. The poisoning effect is not observed as long as oxygen is continuously passing over.the sur-faces. The apparatus that is used to photodissociate water is shown in Figure 2. It consists of a glass cell that houses the iron oxide assembly immersed in the Na2S04 aqueous solution. Photogenerated hydrogen is transported from the cell through a closed circulation loop to a gas chromatograph. Argon is used as a \f carrier gas and is circulated through the loop by means of a mechanical pump. \ Air leaks in the cell and in the loop hinder accurate detection of photoinduced -., ...... oxygen production. The sample.is illuminated by light from a 500 W lamp which passes through a water filter "and a visible pass filter ( E ~ 2.7 eV). The apparatus also permits photoelectrochemical measurements of p- and n-type iton oxides separately. This is accomplished by a potentiostat and a standard three -4-electrode configu;ration consisting of the iron oxide sample, a reference elec-trodeand a platinum counter electrode (see Figure 2). Iron oxide discs with n-type behavic:>r were produced by mixing fine powders of a-Fe203 and Si02 so that ° < Si/Si + Fe ( 20 atomic %. Similarly, p-type iron oxide discs were produced by mixing powders of a-Fe203 and ~O so that ° < Yg/ ~ + Fe ( 20 atomic %. By pressing the powders into pellets, heating them in air at 1340-1390 C and allowing them to cool rapidly, the resulting mixed iron oxide discs had resistivities in the range 103 - 104 n·cm. Scanning electron microscopy studies show all mixed iron oxide surfaces to be heterogeneous. The n~type surfaces consist of a silicon doped iron oxide matrix with a grain size of 10-20 ~m and smaller precipitates highly enriched in silicon oxides. The p-type surfaces consist of a magnesium doped iron oxide matrix and precipitates enriched in magnesium oxide. Scanning Auger spectroscopy . measurements reveal the same. heterogeneity and show, furthermore, that the chemi-cal composition of the outermost 5 nm th~ck surface region is not significantly changed upon exposure of the iron oxide discs to the NazS04 aqueous solution. When immersing the pIn-type iron oxide assembly in the Na2S04 aqueous solution a dark current of a few ~A/cm2 is observed which decreases with time. Upon il+umination of both iron oxide samples a photocurrent is measured which ranges between S~10~A/cm2 during the first hours of illumination·and which tends to increase slightly with time. Simultaneous detection of photoinduced hydrogen evolution by gas chromatography reveals a hydrogen production rate of between 5 and 10 x 10 16 molecules/hour from the p-type iron oxide with ar'ea '" 0.6 cm 2 •. After about 6-8 hours of operation in 0.1 MNa2S04 the rate of photoinduced hydrogen evolution declines. Simultaneously the photocurrent of the p-type magnesium doped iron oxide also declines. By flowing oxygen gas through.the reaction cell for a few minutes to an hour the device can be reactivated and -5-both the hydrogen evolution rate and the photo current between the p-and n-type tron oxide discs return to the original values. Periodic flowing of oxygen gas through the reaction cell results in a stable photocatalytic hydrogen pro-duction rate of 1-2 hydrogen molecules per site per minute giving around 2000 monolayers of hydrogen molecules per 24 hours of exposure to the light. X-ray photoelectron spectroscopy studies 'showed that the oxygen treatment resulted in a higher oxygen content of the iron oxide lattice (Figure 3) indicating a reoxidation of the iron oxide surface. No significant change in the Fe 2P peak position was observed after flowing oxygen gas through the cell. The deterioration of the p-type samples is most probably caused by a gra-dual reduction of the iron oxide surfaces by a small fraction of the photopro-duced hydrogen. The oxidation treatment restores the iron oxide at the surface to its higher oxidation state and also regenerates the photoch~mical activity. The deposition of platinum on SrTi03 single crystal surfaces greatly aided the production of hydrogen from water upon irradation with band gap (3.2 eV) or larger energy photons.(l) In an attempt toincrease·the hydrogen production rate and possibly to reduce the gradual reduction of the iron oxide surface a grid of platinum was deposited over the p-type sample having a thickness larger than 20 nm. However, no improvement was observed. The photocurrent between the p-'and n-type samples was reduced and the hydrogen evolution rate declined in proportion. On the other hand the photocurrent of the n-type Si-doped iron oxide does not deteriorate or show any other sign of poisoning. Thus, the sur"" facel at which oxygen is evolving remains stable under present reaction condi-tions. The power conversion efficiency can be calculated to be about 0.05% using the H2 production rate of'S x l016 H2 molecules per hour and incoming photon flux with 17 mW power. This is about an order of magnitude lower than those -6-reported using single crystals and radiation in the ultraviolet range of the electromagnetic spectrum~(2 ,3) The photoche~ical .. conversion efficiency ca,n be improved in several ways. The particle size of the polycrystalline iron oxide grains can readily be increased from the average 10-3 cm size utilized in the present device. While the deposition of a thick (> 20 nm) grid of platinum over thep-type iron oxide surface did not improve its, phot!lcurrent .or hydrogen pro-duction rate, perhaps a thinner coating of Pt or the use of another metal might accelerate the recombination of hydrogen atoms. Improvements of the cell geom-etry, changes of the iron oxide stoichiometry and doping levels and modification of .other experimental conditions are likely to improve the rate of hydrogen production from water over the iron oxide diode assembly using visible light, in the near future. A redesigned, .. leak tight celt will permit us to detect oxygen in addition to hydrogen ,using the gas chromatograph. The successful photoproduction of hydrogen from water using radiation in the solar region over magnesium and silicon containing polycrystalline iron oxide raises the question of the possible, importance of this process during the evolution of our planet in the pre-chlorophyl era. Si, Fe and .~ containing minerals are most abundant in the mantle and cold have been catalysts not only for the photochemical conversion of water to H2 and 02 but for subsequent reac-tio~s of these molecules with C02 and N20 The reactions of hydrogen with C02 and N2 have. negative free energies •. Furthermore, iron and its compounds are excellent catalysts for the formation of organic molecules from gas mixtures of H2,C02 or CO and of ammonia synthesis from H2 and N2. We shall be exploring the feasibility of inorganic photosynthesis over the iron oxide pin diode assem-blies in the near future ) I') -7-Acknowledgment of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under Contract DE-AC03-76SF00098. The assistance of Dr. R. Dodd (Lawrence Berkeley Laborato~y) in performing the X-ray photoelectron spectroscopy measure-ments is greatly appreciated. , References 1. F. Wagner and G.A. Somorjai, Nature 285, 559 (1980). 2. A.J. Nozik, Appl. Phys~ Lett. 30, 567 (1977). 3. K. Ohashi, J. McCann and J.O'M. Bockris, Nature 266, 610 (1977). Figure Captions , Figure 1: pin-type iron oxide assembly. cp ~ 9 mm. Figure 2: Apparatus for simultaneous photocurrent and gas evolution studies. Figure 3: X-ray photoelectron spectra of the 0 Is peak before (lower curve) and after (upper curve) oxidation treatment of a mixed iron oxide with Si/Si + Fe = 10 atomic %. Indicated in the figure are the ! peak positions of 0 Is in the iron oxide matrix and in the silicon enriched precipitates. ,1" n - TY P E ;IRON OXIDE ) . _. , . I. " e . 1 -8-'CONDUCT'I NG ' .. WIR'E , -. i . .. . ' .' p-TYPE 0 IRON OX'IDE . • l ~ XBL 825-5830A • , i .! , , , Fig. 1 . \ L "T1 1-" ()'Q N ) f .l~:-'" ~. FROM Ar ~ --SUPPLY CIRCULATION PUMP GAS CHROMATOGRAPH FROM Ar~ \ ~ CARRIER ~ GAS SAMPLING VALVE t + '~ -POTENTIOSTAT '\ REFERENCE ELECTRODE COLOR FILTER 00 WATER CELL Pt COUNTER a ECTRODE :b>. ~~ o LAMP XBL 822-5133A .. I ID I ---.J W Z Z <t :c u .......... en I-z :::> o u -10-. SILICATE . IRON OXIDE . ¥ Si/Si + Fe =10 at. % • • • 0 Is :~ : ., • q, .0 o 0 • o • • 0 , • • 0 • • 0 • • 0 • • 0 . , 0 ~: o o o , e, ~'¥ .. · , o o • C» o • o o • o o· o ;. AS RECEIVED cf 542 532 522 BINDING ENERGY (eV) x BL825-5836 Fig. 3 .t This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) arid not necessarily those Of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable. ;~!:c,~' TECHNICAL INFORMA4'ION DEPARTMENT LAWRENCE BERKELEY LABORATORY UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA 94720 , ·~c--;~· ;'i-~ 

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THE PHOTOCATALYTIC PRODUCTION OF HYDROGEN FROM WATER BY A p- AND n-TYPE POLYCRYSTALLINE IRON OXIDE ASSEMBLY

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THE PHOTOCATALYTIC PRODUCTION OF HYDROGEN FROM WATER BY A p- AND n-TYPE POLYCRYSTALLINE IRON OXIDE ASSEMBLY

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