Agrawal, Megha

Chaudhary, Yatendra S

Dass, Sahab

Saroj

Satsangi, Vibha R

Shrivastav, Anubhuti

Shrivastav, Rohit

IACS Institutional Repository

Indian J. Phys. 78A (2), 229-231 (2004) Nanomaterials-2003< U PNanostructured Mn-Doped and undoped CuO thin films-PEC studiesSahab Dass**, Yatendra S Chaudhary*, Megha AgjrawaP, Saro|~, Anubhiiii Shrivastav^, Rohit Shhvastav‘ and Vibhja R Saisangi-'Department o f Chemistry, ^DepartmcMit ol ^ y s ic s  ik  ('ompuicr SticJicc,Faculty o f Science, Dayalbagh Educational Insliiuic, Day^bagh, Agia-282 005. lUiai Fiadcsh. IndiaE-mail sahabdassCa*ya|tc>oAbstract : The photoelectrochcmical properties of copper oxide thiii film (p4ypc) on the SnO, F coated glass (conducting glass) picparcd by spray pyrolysis has been investigated as a (unction dopant and pH o( electrolyic ?<-iay difliactogiain pattern o( thin films confirm the cuprit oxide f('uO) phase. Mn-doped samples exhibited decreased photoelectrochcmical lespDnse as compared to undoped CuO The onset o f photocurieni m uiulopcd CuO IS at ~I20 mV. while in Mn doped ( ’uO films it is -375 mV The observed icvcisc effect o f Mn-doping tin the photoclcetrochermeal response of the films, ean he attiibuted to the possible neutralization of naturally oceunmg majority charge earners (holes) present m undoped coppci oMiIc with the extra eleelrons furnished by the dopant Mn atoms, particulaily. fiom then d orbitals Bcttei photoresponse has been obseived at pH 13Keyword.s Fhotoelectiochcmieal pioperiies, cupric oxide, conducting glass, spray pyiolysisPA(^S Nos. 8 1 1 5  Rs, 82 45.M p,1. IntroductionJ’h(jl(^electrochemicaI (PEC) cells, with active semiconductor electrolyte junction, are considered to be efficient solar energy harvestor, and intensive research is going on to use such systems ui pholosplitting of water for the production of hydrogen ~ a much cleaner substitute of fossil fuels. An ideal semiconductor lor this kind of work should have a band gap in the range of 1.7-2.1 eV (a few tenth of an electron volt larger than theoretical minimum value of 1.23 eV needed for splitting of water), should be abundantly and economically available and should be stable m electrolyte. Fujishima and Honda, in 1972 f 11 reported, for the first time, the possibility of using TiO^ in PEC splitting of water, but, on account of large band gap of Ti02 (3.1 cV), the conversion efficiency of solar energy was limited. After that a bust of other semiconductors e.g. SrTiO^ [2], BaTiO^ [3] and SnO, [41, Fe203 [5], Si [6], GaAs [71, InP [8] and CdS [91 e tc . have been investigated. However, desired results could not be achieved either due to large or small band gap or their instability in the electrolyte.The present investigation deals with the study of copper oxide (CuO) thin film, prepared by spray-pyrolysis, as semiconductor electrode in PEC cell. Oxides of copper, Cu^O having a direct band gap of about 2.1 cV [ 10,11] and is therefore.Corresponding AuthorStrongly absorbing at wavelengths below 6(X) nm, CuO with band gap 1.21-1.51 eV [12, 13) is absorbing throughout the visible region. Besides this, the other reasons for selection of this material are its : (a) natural abundance, (b) low cost production processing, (c) non toxic nature and (d) stability and reasonably good electrical properties. The effect of dopant and its concentration and the pH of electrolyte on the photoelectrochcmical conversion properties have been studied. The optical property, such as, band gap, phase identification, surface morphology have been studied using UV-Visible spectrophotometer, XRD and SEM, respectively.2. Experimental techniqueCopper oxide thin films were deposited on conducting glass (SnO,: E coaled) substrates O’CC), by spraying 0.1 M precursor solution, prepared by dissolving Cu(N03),. 2.5H,0 ( 99.99-f%, Aldrich) and the calculated amount of dopant Mn(N03)2.2H,0 (Aldrich) in double-distilled water. The films of copper oxide were deposited over existing coating of SnO, : F. The solution (pH -3.5) was sprayed onto heated (temperature 350 ± 5”C) substrates of dimensions 4 cm x 2.5 cm. The detailed conditions for the spray deposition of copper oxide films have been summarized in Table I . Before spraying, nearly one third length of the substrate was covered by an aluminium foil. The details of experimental set-up is described elsewhere 114].02004 lACS230 Sahab Dass et al'ruble 1 Conditions for the spiay deposition of copper oxide filmsSpray parameiers('opper nitrate solution cone Manganese nitrateSolventSubstiate temperature Carrici gasAngular distance Rate of spiay 'fotal spray timeValues 0 IMcalculated amounts (loi ()5al% . Iat7f. and 2 a t 7 )Distilled wateiC'omprcssed an ( -3  kg/cm^)-25  cm4 4 1 - 0 1  ml/mm 150 secI’ig iirc  1, XRD patterns o f tilm s deposited  at conducting g lass : (a) undoped CuO and (b) 2%Mn-CuO.The surface characterization of films was studied using a Scanning Electron Microscope (Figure 2) and the tentative sizeol‘ grains/particles formed was determined. The particle/gr un size was also calculated from XRD patterns using Schcicr\ equation . The average size of particles was found to be of order o f -'50 nm.Using (3a/In eutectic, silver paint and copper wire, ohmic electrical contact was generated for Pf-ir measurements on the uncoated area (covered by aluminium foil during film dep(3sition) This area was later covered by non-transparent and non­conducting epoxy-resin (Hysol, Singapore). The CuO films, prepared as above were found of p type (as determined by observing Seabeck effect).3. Structural characterizationThe films were characterized using X-Ray Powder DifTractometer (X'PERT Model, Philips, with graphite monochromator) using Cu-Ka wavelength, figure 1 shows the X-ray diftraction patterns of undoped and 2 al%Mn doped CuO films. Which confirms the polycrystallinc nature. The ( 111), (2(X)), (020), (113), ( 220) and (004) peaks of the CuO phase appeared. XRD pattern revealed the shifting of peaks towards higher 20 in Mn-doped films. This implies that in the crystal structure in-plane expansion is taking place. In the obtained X-ray diffractograms, some peaks corresponding to the underlying SnO^ coating were also seen.Figure 2. Seanning lileciion M iciogtaph (oi CuO llim film4. Band gap measurementsOptical reflection and transmission spectra were recorded fot films of cupric oxide deposited at different temperatures The thickness of the films was measured by Telcstep method and u was ~ 1.45 (im. Spectroscopic data were investigated to .sec the evidence of allowed forbidden transitions between indirect valleys. For a single phonon absorption, the ab.sorptum coetticicnl a  for an indirect transitions is given by the equation1151a  = BhvHere, /? = 2 tor an allowed indirect transitions and n = 3 toi a forbidden indirect transitions. 1’he -ve  sign corresponds u» phonon emission and the + vc sign to phonon absorption, B ib a constant, is the indirect energy gap and is the energy ot the phonon assisting in the indirect transition of the electron. When a plot was made between (a /n ’)*^ “ and //v, in the prescni study a linear graph resulted with the intercept of which Uv axis gives band gap of material E^. As shown in the Figure 3 the linear relationship is obtained by plotting v e r s u s  hvThe linear sections of the plot indicate an indirect band gap olN a n o s tr u c tu r e d  M n -d o p e d  a n d  u n d o p e d  C u O  th in  fd m s - P E C  s tu d ie s 231_1 4 eV for thin films. These values fit well with the earlier reported band gap values of CuO 112,13).5. photoelectrochemical measurementsThe photoelectriKhemical measurements involved monitoring of current-voltage (l-V) characteristic, under darkness and illumination, using a three electrode system in a glass cell fitted with pyrex glass window to facilitate the transmittance of light to the photoelectrode surface. The CuO thin film electrode (w o rk in g  electrode, surface area of 1.5 cm^) was used in con)uction with platinum and saturated calomel electrcxles ( SCE), w hich were used as counter and reference electrodes, icspectively. PEC measurements were performed using a poientiostat (Model ECDA-001, Con-Serv Entcrpri.ses) and a 2S0 W-Xe Lamp (Bentham) was used as light source. The intensity of light was adjusted and fixed at 0.16 W cm"’. The electrolyte used was NaOH.Fipurc 4(1). ( ’urvcb of undoped and M n-doped ( ’uO/SnO, elccirodes at wirious doping concentrations (a) undoped, (b) 0  3 al%Mn, (c ) I at%Mn .md (d) 2 al^^>MnIt IS evident from Figure 4( 1) that the photoelectrochemical response in the doped sample is less than the undoped sample. The onset of photocurrent in undoped CuO is a t-120 mV, while m Mn-doped CuO films at -375 mV. The observed reverse effect of Mn doping on the photoelectrochemical response of the films, can be attributed to the possible neutralization of naturally^Rure 4(2). Curves of undoped CuO/SnO^ electrodes at various pH of ‘Electrolyte (NaoH) ; (a) pH 13 and (b) pH I i.occurring majority charge carriers (holes) present in undoped copper oxide with the extra electrons furnished by the dopant Mn atoms, particularly, from their d orbitals. The decrease m majority charge carriers is probably adversely affecting the formation of depletion layer at the film-electrolyte junction, which might be responsible for rapid recombination of photogenerated electron-hole pairs. Belter photoresponse of CuO films was observed at pH 13 as is evident from Figure 4(2).6. Conclu.sionCuOfhin films arc found to be good candidate for PFC studies. Howttver, exhaustive work is still needed to observe the long^ity {i.e. life span) of films in contact with electrolyte, particularly, under illumination. Besides, the effect in changes in el^trolyte composition on photosensitivity of film and the role 0f catalysts/scnsitizcrs also need to be investigated. Work on these lines is m progress.AcknowledgmentsWe thanks to UGC, Govt, of India for financial assistance [ref. no. 12-109/2001 (SR-Dj. T'he authors also thank Dr. Shyama Prasad, National Institute of Oceanography, Goa and Dr, Umesh Tiwari, Solid State Physics Laboratory, Delhi for the SFM and XRD analysis of films respectively.R eferencesm  A Fujishima and K K Honda N n t ia v  2.38 37 (1972)(2) P S alvador, (. G u tc iic z , G Cam pcl and P H agcnm eller J  E le c t ro c h e m  S ov  131 550 (1984)[3] K H Yoon and K S Chung J  A p p i  P h \s  72 5743 (1992)(4) D C  Slilwcll and S M Park ./ E U r t r o d w m  S<>< 129 1501 (1982)[5] S U M  Khan and J Akikusa J  P liy \  C h en i B103 7184 (1999)16) r  l.cvy-C lcm cnl. A Lagooubi, M Ncwmann-Spullari, M Robot and R Tcnnc J  E U r t m c l ic m  S(H'. 13 L69 (1991)[7] Fu-Rcn, F Fan and A J Bard J  A m  C hvm  Soc  102 3677 (1980)[8] N Chandra, B L Wheeler and A J Raid J  P I m  C h e m  89 5037 ( 1985)f9)  S M Herman, G Franz. S M Mark, B Slomn, S U Robert and D N Furlong J  Sov E a ra d a v  Trans  91 665 (1995), A Hcnglein C h em  Rev. 89 1861 (1989)[ lOJ T D Golden, M G Shumsky, Y Zhou. R A Nander Werf, R A Van Leeuwen and J A Switzer C h e m  M a t  8 2499 (1996)[11]  M T S  Nan, L Guerrero, O L Arena.s and P K Nair A p p I S u rf Set 150 143 (1990)(12]  F Marabelli, G B Parraviciny and F S Drioli Phys R ev  B52 1433 (1 9 9 5 )(1 3] J Cihijscn, L H Tjeng, J V Elp. H Eskes, J Westerink, G A .Sawat/ky and M T Czyzyk P h y s . R e v  B 38 11322 (1988)[14]  Benny Joseph, K G Gopchandran, P V Thomas. Peter Koshy and V K Vaidyan M a t e r  C h e m  P hys  5 71 (1999)f I 5]  J I Pankove O p t ic a l  P rocesse .s  in  S e m ic o n d u c to rs  (Englewood  Cliffs Prentice Hall) (1071)[16] F P Koffyberg and F A Benko J  A p p i P hys. 53 1173 (1982)

Nanostructured Mn-Doped and Undoped CuO Thin Films-PEC Studies

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Nanostructured Mn-Doped and Undoped CuO Thin Films-PEC Studies

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