The photoactivity of porous Tio2 anodized at a high voltage

Volume 100, number 3 CHEMICAL PHYSICS LETTERS 9 September 1983

THE PHOTOACTIVITY OF POROUS Ti02 ANODIZED AT A HIGH VOLTAGE

Doug MILLER, Suzanne MAMICHE-AFARA, M J. DIGNAM and hi. MOSKOVITS Department of Chemishy, Lash Miller Chemical Laboratories. University of Toronto, Toronto, Canada J1.S IA1

Rcccited 29 March 19S3: in final form 27 June 19S3

The photoactivity of Ti02 produced b> hi&\oltage anodizrltion as described by Macbenoir et al. was comparred to single-crystal TiOz. The rtnodic o.xide film was found to be highly porous. The photocurrent and quantum yield of the anodic sample is markedly larger on the lonr_wavelcn~th side of the action spectrum as compared to singlecr)stdl rutile. We ascribe this behavior either to the formation of ;lndtasc “pockets” in the film or to the incomplete o.xidation of Ti during the anodization process.

l_ introduction

The photoelectrocheniicril and photocatrilytic prop- erties of TiO, have been studied extensively [ 1-3-l] *. The photoactive TiOz is nomraliy placed in a photo- electrochemic~ cell with either an aqueous or non- aqueous electrolyte. where it fomrs the anode.

Photocatalytic reactions have two general classifica- tions. as suggested by hlatsumoto [ 1] : reactions in which light energy is stored (AC > 0), and reactions where the rate is photoassisted by the catalyst (2.G < 0). The reactions where AC > 0 are important lie in the ares of solar energy conversion. Examples of these are: the photolysis of water [3--91. photorcac- tions of NO [lo] and CO [I 11, photosynthesis of amino acids [ 121, photocleavage of hydrogen sulfide [ 131. and the photoo.tidation of carbon with water [ 14,151. The other chrss of reactions includes pro- cesses such as the photodecomposition of carboxylic acids [ 161. the photoo_xidation of cyanide [ 17,l S] ) sulfite [ 1 S] , hydroquinones [ 191. and hydrocarbons [20]. the photo Koibe reaction [21--241. the photoreactions of dichromate [25], photoreactions of water with alcohols [2,3], and in the photooxidation of polyolefins and polyamides. In the second class of reactions, the TiOz acts as an energy absorber. The

* A comprehensive series of papers on the study of TiOp can be found in ref. [ 35 ] _

stored ener,T is then imparted to the system, thereby enabling sunlight to be used as an activation energy substitute.

The wide forbidden band of TiOZ (~3 eV) implies that ultraviolet photons are required for photoreactions on TiO,. Unfortunately, though, sunlight is low in UV photon content: hence, many of these reactions will be quite inefficient in photoprocesses depending upon sunlight as the energy source.

In this paper, we report an improvement in the quantum efficiency of a TiO-, PEC, at wavelengths longer than 390 nm. The action spectra and current -voltage plots for the anodized TiO2 and rutile single- crysnal TiOZ were obtained.

2_ Experimental

Photosensitive TiOl tihns were prepared using the high-voltage anodization technique reported by Marchenoir et al. [29,30] _ The voltage employed was well above the sparkling potential of Ti (-100 V), BS- sisting the production of a highly porous TiO?/Ti sample.

The Ti foil was purchased from the A-D MacKay Co., thickness O.OOl”, purity 999%. Ti foil was cut into 10 cm X 1 cm strips, and was successively polished with time carborundum paper, polished with an alumina-ethanol slurry, and ultrasonically degreased in CH,CI, for 30 min.

236 0 009-2614/83/0000-0000/S 03.00 0 1983 North-Holland


The electrochem@l system consisted of a variable- voltage (O-150 V, 8 A) power supply connected to the electrochemical cell consisting of a cylindrical Pb foil cathode surrounding the Ti foil anode draped over

a copper rod. The electrolyte was 5.0 N HzS04 to which OS g of lithium nitrate was added to increase the conductance of the electrolyte_ It was reported

[31] that the addition of LiNO, does not affect the composition of the anodic oxide.

The voltage applied was raised slowly, so that the current was never greater than 8 A, and the maximum voltage attained (150 V), was applied for 5 min. The current density through the film was 020 A/cm’.

The anodized sample was rinsed with triply distilled water. Prior to the study of the photoelectric proper- ties, the film was soaked in 0.1 N NaOH for several hours.

The TiO2/Ti sample was cut into 2 cm X 0.5 cm strips which were supported on a glass slide by epoxy

cement. The sample was masked using silicone rubber cement, leaving an illumination area of =O.lO cm2.

The singlecrystal TiO, was polished with 1 pm dia- mond paste. The polished crystal was annealed in oxygen at a temperature of 900°C. pure hydrogen was used to dope the crystal, the temperature being 500°C. Finally the TiO2 was etched in sulphuric acid/ammo- nium sulphate (one to one by volume)_ Ohmic contact to the back of the crystal was established with a gallium-indium alloy.

The photosensitive characteristics of the TiO, fti were studied using a PAR-170 potentiostat. The refer- ence electrode was a saturated calomel electrode (SCE). The photoresponse curves were obtained using a 150 1%’ xenon lamp white light source. The bias applied to the system ranged from -1 _O V SCE to +1-O V SCE. The intensity at various wavelength settings of monochromator was measured, allowing quantum efii- ciency to be calculated_

3. Results

The quantum efficiency, 9, was determined using the following equation:

Q= 1_2408jffi,

where j, the current density is defined as the current/ illuminated area bA/cm2), and 12408/h is a term re-

Fig. 1. Photocurrent versus bii for the anodized TiOs samples;Ti(l-a)l81,Ti(l-e)181 and Ti(2)182 refer to three different samples; and for singlecrystal TiOa.

suiting from the conversion of photon fhrx (photons/ cm?) to power flux (mW/cmQ The intensity of the light source is determined in mW/cm?.

Fig. 1 represents current-voltage curves for samples Ti(l-a)lSl, Ti(l-c)lSl, Ti(2)182 and singlecrystal TiO,. Fig. 2 is the action spectra for the four TiO2 samples. The sample Ti(2)182 prepared in August 1982 exhibited a small dark current, which increased slightly in the region of oxygen evolution. The samples Ti(l-a)181 and Ti(l-c)lSl initially exhibited a signifi- cant dark current in the oxygen evolution region. However, the dark current settled to a low level after

rig. 2. Action spectra (quantum yield versus wavelength) for three different samples of anodized TiOa, compared to singlecrystal TiOz. The bias was held at +I_0 V SCE for all samples.

237


l--ig. 3. (a) Scanning electron micrograph of anodized TiO2 formed at 150 V (sample (1-a)181). The scale is 30 gill be tween tics. (b) lligll-magnifk-alion scanning micrograph o f the same sample us ill (a): scale; 1 ~m between tics.

tliirty minutes of immersion In the electrolyte.

Evidence for the porosity of the fJnx is provided in the scanning electron micrographs (fig. 3). The hi&

voltage teclmique creates a bigbly porous o.Gdc film as

shown in fig. 3x The mean pore diameter is of the or-

der of 1000 A (fig. 3). An X-ray cbemicsl analysis wws also performed. Tbc

only element detected other than Ti and 0 was S. prc- sumably absorbed from the sulfuric acid anodizing so-

lution_ No lithium wx detected in the analysis.

4. Discussion

The large initial dark current observed in the study

of tbeTi(l-a)lSl and Ti(l-c)lSl samples may be due

to substantial amounts of oxygen absorbed in the po-

rous TiOz _ III the UV region. singlecrystal TiOZ 113s ;1 much

grcatcr qumrunl cfiiciency t11m does anodircd TiO?

(fig. 2). A good quantum efficiency in tlic UV region

is not surprising as. tbc band-gap of TiO? corresponds

to a cutoff \mvelcngtll of400 nm. Tllc action spectra

for tbc anodized TiO, xc reproducible in tile region

360100 nm. Even tbougb there are apparent tliffer-

ences in the pllotosensitivity between samples it is sig-

nikmt tlmt in tllc near-visible region, tbc quantlull

cfficicncies become conlparable. In the blue part of‘

tile spcctrunl. tlw polous TiOz possesses a qumtunl

cfficitncy superior to that of the rutile single tryst-xl. This may be due to 3 number of leasons of wbicb WC

present two 3s most lihely. Tlw first involves tile for-

nmtion of matase, 3x1 allotrope of TiO-, along the sides of rhe pore during bigb-voltage anodiz&

hlarcbennir et al. [29jO] suggest that the higil volt-

age applied to tlw Ti foil during anodiGlg, 3 voltage

wbicb exceeds the sparking potential of the oxide, creates 3 hot likunent or “‘spark” wbicli is sufficiently

hot to convert tlw rutile to anatase. The anatase-rutile transforni3tion occurs at temperatures between 700 and 1200°C. depending upon the method employed. 1 lence. during slow annealing conin~only used in sin-

gle+xystsl preparation there is sufficient time for Al

the 3nat;Lse to convert to rutile which has 9 lower pllo-

toactivity than anatase. This may 1101 be the case during tlie “‘sparking” process and as a result pockets of

the matase Ti02 will rem-tin, presumably lining the

pores. These pockets may contribute to the estension

of the photoactivity of the PEC into the blue region

of the visible spectrum. Alternatively the improvement

of the quantum efficiency in the 300 xun region could

be due to incomplete osidation of the titaniuni during

medic o.sidation. Gautron et al. [34] llavesbown that

non-stoicbiometric TiO, (i.e. TiOl_,) shows improved

qumtum efficiency and improved near-UV--\-isible rc-

spnnse for ~1 range ofs rangins from 2 X IO-” to 47

X 1 0v4 _ with optimum efficiency at around 7 X 1 O-‘_

Altl~ougl~ this mxkmism is 3 possible contributor we

prefer the first since the quantum yield measured in

the 400 nin region of the spectrum ~3s quite rcpro-

ducible for 3 number ofsamples prepared under sonx- ~1 bat vaving anodizing conditions in wliicb circuni-

stances one would have espccted the degree of non-

sloicbionietly. if present at 311. to vaq.

5. Conclusion

Tbc results 01-a preliminxy study indicate tlmt

I@-volt;lgc anodizarion of Ti produces 3 ‘Mew” type

of~l~l~ot~elfctrodr wliicli li3s 31i improved qumtuiii d- ficirncy in tlic ncttr-visible region of the spectrum olcr

sin$x-Tstltl rurlle. Tbc appxcnt increase in pbo~oef-

ficicncy in rbe blue region may be due to either the in-

corporiition of amtasc in the prcdoniinantly rutilc

medic oxide libn or IO oxidation during rbe lli$-volt-

3s~‘ rinodizalion.

Acknowltulgenient

We rbanh NSERC and the donors of tlw Petralcunl

liesrarcb Fund for Iinmicia] assistance.

References

I-S] S. Sllto and J.Xl_ \Vhlte, Clwn. f’hys. Ldters 72 (19SO) -

]6] kto Jnd J_hl. I\‘hitc, J. I’hgb. Chctm. 65 (1981) 592.

239


[ 7 ] F.T. Wagner and G-A. Somorjai, J. Am. Chem. Sot. 102 (1980) 5494.

[S] I;. Kogo, H. Yoneyama and H. Tanura, 3. Phys. Chem. 84 (1980) 1705.

[9] M.V. Rao, R. Rajeshwar, V.R.P. Vemeker and J. Dubow, J. Phys. Chem. 84 (1980) 1987.

[ 101 H. Yoneyama, H. Shiota and H. Tamura, Bull. Chem. Sot. Japan 54 (1981) 1308.

[ 111 T_ Inoue. A. Fujishima. S. Konishi and K. Honda, Nature 277 (1979) 637.

[ 12 j Ii. Rciche and AJ. Bard, J. Am. Chem. Sot. 101 (1979) 3127.

[ 131 81. Grfitzei, Chem. Erg. News 59 (July 17.1981) 40. [ 141 T. Kawai and T. Sakata. Nature 282 (1979) 283. [ 151 S. Sato and J.M. White, J. Phys. Chem. 85 (1981) 3136. [ 161 B. Kraeutler and AJ. Bard, J. Am. Chem. Sot. 100

(1978) 5985. 1171 S.N. Frank and AJ. Bard, J. Am. Chcm. Sot. 99 (1977)

303. [lS] S.N. Frank and A.J. Bard, J. Phys. Chem. 81 (1977)

1484. [19] F.F. Fan and AJ. Bard. J. Am_ Chem. Sot. 102 (1980)

2592. [ 70 1 WV_\V. Dann, K.O. Willbourn, F.F. Fan and AJ. Bard, J.

Phys. Chem. 84 (1980) 3207. ]2l] B. Kracutlcr and AJ. Bard. J. Am. Chem. Sot. 100

(1978) 2239. [22] B. Kraeutirr, CD. Jaqer and A-l. Bard, J. Am. Chcm.

Sot. 100 (1978) 4903.

[23 ] B. Kraeutler and AJ. Bard, Nouv. 1. Chim. 3 (1979) 31. 1241 I. Izumi, F.F. Fan and AJ. Bard, J. Phys. Chem. 85

(1981) 218. [2S] H. Yoneyama, Y. Yamashita and H. Tamura, Nature 282

(1979) 817.

[26] L. Tang, D. Sallet and J. Lemaire, Macromolecules 15 (1982) 1437.

[ 27 ] R. Arnaud and J _ Lemaire, in: Developments in polymer degadatron, Vol. 2, ed. V_ Grassie (Applied Science Publishers, London, 1981) p_ 159.

[28] R. Amaud and J. Lemaire, in: Developments in polymer photochemistry, Vol. 2, ed. N. Allen (Applied Science Publishers_ London. 1981) p_ 135.

[29] J-C. hiarchenoir, J. Gautron and J--P. Loup, hletaux Corrosion Ind. 619 (1977) 83.

1301 J-C. hlarchenoir. J.-P. Loup and J. hlasson, Thin Solid Films 66 (1980) 357.

1311 WTL. B&n, Formation of Porous Films onTitanium Alloys by Anodization Treatments, Air Force Material Lab, Wright-Patterson, OH (1979), hlicrofim.

[32] PJ. Boddy, J. Electrochcm. Sot. 115 (1968) 199. [ 33 ] R J _H. Clark, The chemistry of titanium and vanadium

(Elsevier, Amsterdam, 1968) p_ 271. 134 ] J. Gautron, P. Lemasson and J.-F_ M~ucco. Discussions

Faraday Sot. 70 (1980) 81. [35] J. Electrochem. Sot. 115, No. 2 (1968);

hletaus Corrosion lnd. 619 (1977); Discussions Faraday Sot. 70 (1980).

The photoactivity of porous Tio2 anodized at a high voltage

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