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1 CLEAN TECHNOLOGY, Vol. 19, No. 3, September 2013, pp. 191~200 총 설 태양광을활용한물분해수소생산용광촉매재료 김정현 * 서울시립대학교화학공학과 서울특별시동대문구서울시립대로 163 (2013 년 6 월 3 일접수 ; 2013 년 6 월 28 일수정본접수 ; 2013 년 6 월 28 일채택 ) Photocatalysts for Hydrogen Production from Solar Water Splitting Jung Hyeun Kim* Department of Chemical Engineering, University of Seoul 163 Seoulsyripdaero, Dongdaemun-gu, Seoul , Korea (Received for review June 3, 2013; Revision received June 28, 2013; Accepted June 28, 2013) 요 약 미래의무한 청정에너지원으로고려되고있는태양에너지를활용하여수소를생산할수있는광촉매재료에대한연구가활발히진행되고있다. 본총설에서는태양광을이용한물분해수소생산용광촉매재료들에대하여알아보고, 현재까지보고된다양한광촉매재료의특성들을검토하고자한다. 또한, 다양한광촉매재료를활용하여수소생산효율을높이기위해서시행되었던촉매재료개질방법들을통하여향후지속적으로진행될연구방향을모색해보고자한다. 각각의광촉매재료들이활성을가질수있는빛의영역을알아보고, 광촉매작용에필수적인광원, 광밀도, 파장영역등의중요성에대해서도토론한다. 주제어 : 광촉매, 태양광물분해, 반도체, 띠간격, 수소생산 Abstract : Researches on developing photocatalyst materials for hydrogen production from solar water splitting attract great attentions due to the unlimited and clean characteristics of the solar energy. In this review, photocatalysts used for hydrogen production from the solar water splitting are discussed in terms of material characteristics. In addition, various modification techniques applied to the photocatalysts for improving hydrogen production efficiency are summarized. Finally, light characteristics such as intensity, illumination density and wavelength cutoff are also discussed for the importance of hydrogen production rate. Keywords : Photocatalyst, Solar water splitting, Semiconductor, Band gap, Hydrogen production 1. 서론 화석연료의꾸준한사용증가로고갈되어가는에너지자원을 대체하기위한새로운에너지원의개발이중요하게인식되고 있다. 특히무한한에너지원으로평가되고있는태양에너지 를상업적인공정을통하여일상생활에활용하고자하는연구 가최근들에매우활발하게진행되고있다 [1]. 태양에너지는 화석연료의사용으로부터발생되는환경오염을방지할수있 는큰장점이있지만, 화석연료에비하여현저하게떨어지는 에너지전환효율과낮은경제성으로인하여상업적적용이매 우미미한상태에있다고할수있다. 태양에너지의활용은크 * To whom correspondence should be addressed. jhkimad@uos.ac.kr doi: /ksct 게전기에너지로변환하는태양전지재료와화학에너지로변환하는광촉매재료의개발을통하여가능하다. 본연구는태양에너지를화학에너지로변환할수있는광촉매재료에대한총설이며현재까지보고된다양한연구결과들을통하여광촉매재료에따른광반응메커니즘과수소생산효율등의광촉매활용추진방향등에대하여논하고자한다. 광촉매반응은크게두가지로구분이될수있는데, 산소의존재하에서비가역적으로이루어지는유기물분해반응과물을분해하여수소와산소로전환하여화학에너지를생산하는물분해반응으로볼수있다. 광촉매를활용한물분해반응의연구는 1972년 Fujishima와 Honda[2] 에의하여이산화티타늄 (TiO 2) 기반물질이처음보고된이후, 다양한물질들이개발되어수소생산연구에사용되고있다. TiO 2 는 3.2 ev의띠간격 (band gap) 에너지를가지고있어, 전자를원자가띠 (valence band) 191

2 192 청정기술, 제 19 권제 3 호, 2013 년 9 월 에서전도띠 (conduction band) 로여기시키기위해서는자외선에해당하는빛이필요하다. 하지만태양광자체에포함된자외선은약 4% 수준으로태양에너지를활용하는효율성측면에서바람직하지않다고할수있다. 따라서 TiO 2 를개선하기위하여다양한물질의도핑 (doping) 으로띠간격을줄이고, 원자가띠와전도띠의위치를바꾸는시도가수행되어오고있다 [3]. 특히, 이러한개선시도를크게나누어보면두가지로구분할수있다. 첫째는금속이온도핑, 둘째는비금속물질도핑이라고할수있다. 금속물질로는대부분전이금속 ( 루테늄 (ruthenium, Ru), 은 (silver, Ag), 백금 (platinum, Pt), 구리 (copper, Cu), 몰리브데늄 (molybdenum, Mo), 니오비움 (niobium, Nb), 바나디움 (vanadium, V), 철 (iron, Fe), 코발트 (cobalt, Co), 니켈 (nickel, Ni), 크롬 (chrome, Cr), 망간 (manganese, Mn)) 들이사용되고있으며 [4,5], 비금속물질로는탄소 (carbon, C), 황 (sulfur, S)[6], 질소 (nitrogen, N)[7], 인 (phosphate, P)[8], 붕소 (boron, B), 요드 (iodine, I), 불소 (fluorine, F) 등이첨가되어띠간격에너지를낮추는역할을하고있다. 또한, 복합재료를형성하여띠간격을낮추려는시도로, 황화카드뮴 (CdS) 입자활용 [9], 염료감응화 [10], 업컨버젼 (upconversion) 용형광물질 [11], 나노구조체도입 [12,13], 띠간격이좁은일산화구리 (Cu 2O) 와같은 p형반도체도입 [14,15], 그래핀이나 C 60 와같은전자를받아들이는물질과복합화 [16-19] 등의연구가보고되었다. 또한, 텅스텐산화물 (WO 3) 에다양한금속성물질들을복합화하여띠간격을조절하는연구도광범위하게수행되었다 [20]. 이외에도구리산화물 (Cu 2O), 카본나이트라이드 (C 3N 4), 금속황화물 (ZnS, CdS), 및철산화물 (Fe 2O 3), 다양한금속산화물 (ZnO, Ta 2O 5, TaON) 등의반도체물질들이있다. Figure 1 은물분해수소생산연구에활발히사용되고있는다양한광촉매재료들의띠간격, 원자가띠, 전도띠의에너지를물분해반응의수소및산소생산전위와연관하여나타내고있다. 이러한다양한반도체물질을활용한물분해수소생산연구결과들을광촉매재료별로구별하여수소생산연구의효율성개선에대하여고찰하고자한다. 2. 광촉매재료 2.1. 이산화티타늄 (TiO 2 ) 이물질은물분해와공기정화를위한효율적인광촉매재료로고려되고있으며재료의표면을스스로정화하는것으로알려져있다. 또한, 강한산화활성 (oxidation activity) 과수친화력 (hydrophilicity) 으로인하여살세균제 (antibacterial agent) 로의사용도활발히고려되고있다. 하지만이물질은상대적으로큰띠간격에너지로인하여자외선이상의가시광선 ( 태양광의약 45%) 을활용함에어려움이있으며, 가시광선을효율적으로이용하고자하는측면에서다양한개선시도가행하여지고있다. 이러한기본적인소재의가시광선흡수향상을위한개선연구는서론에서소개한것과같이금속성물질들과비금속성물질들의도핑을통하여다양하게시도되고있다. Figure 2는 TiO 2 가빛을흡수 (hν 1) 하여전자가여기되어전자-정공쌍을형성하는것을개략적으로나타내고있으며, 전자는전도띠에정공은원자가띠에존재하는것을보여주고있다. 이렇게형성된전자와정공이수소생산반응에효율적으로사용되기위해서는서로재결합하여열을발생할수있는가능성을줄여야한다. 그림에보여주고있는것처럼전자와정공은반응의조건에따라 OH 라디칼, O 2-, H 2O 2 등의 Figure 1. Comparison of the band positions of various selected semiconductor materials. The visible spectrum corresponds to energies from 1.56 ev (800 nm) to 3.12 ev (400 nm).

3 태양광을활용한물분해수소생산용광촉매재료 193 Figure 2. Electron excitation mechanism of TiO 2 photocatalysis: h ν 1: pure TiO 2; hν 2: metal-doped TiO 2 and hν 3: nonmetaldoped TiO 2. It is adaped from the reference[3]. The CB and VB represent conduction band and valence band, respectively. 중간물질을거쳐수소와산소기체를생성하게된다. TiO 2 물질이가지고있는큰띠간격을줄이고가시광선의흡수를높이기위해금속물질이나비금속물질의도핑이수행되는데, Figure 2에서보여주는것처럼금속물질을도핑할경우전도 띠에너지 (hν 2) 를낮추게되며, 비금속물질을도핑할경우원 자가띠에너지 (hν 3) 를높이게된다. 따라서, 두가지경우모두 에서띠간격을줄여주며 TiO 2 물질자체에비하여도핑을시 도할경우가시광선의흡수를향상시킬수있는것을보여주 고있다. 또한, 금속성물질을도핑할경우전자의이송을용이 하게하여전자가이동시물질내부에갇혀있거나재결합하 는현상을줄일수있는것으로알려져있으며, 결과적으로광 흡수활성을향상시키는것을보고하고있다. 다양한도핑물질 을도입하여 TiO 2 광촉매를제조하는방법들의예를 Table 1 에보여주고있다. Table 1의예들에서볼수있듯이, 도핑을 수행하기위해서는고온에서의반응과소결과정이요구되는 것을알수있다. 금속도핑된 TiO 2 를제조하는공정은주재료인 TiO 2 입자를 만들고여기에도핑하고자하는금속물질을수용액상에첨가 하여주재료인 TiO 2 와혼합하여이를고온공정에서소결하는전형적인과정을거치게된다. 공정상으로살펴보면, 금속물질을도핑하기위한방법으로이온주입법 (ion-implantation)[32], 스퍼터링 (sputtering)[24], 수열법 (hydrothermal)[25], 졸겔 (solgel)[26] 등이사용되고있다. 각각의공정으로도핑된 TiO 2 를제조하고이의광활성을증진시키기위하여공정조건에따른고온의소결을진행한다. 또한, 비금속도핑공정으로제조하는 TiO 2 는가시광활성을향상시키기위해다양한도핑물질을함유한타이타늄원료물질 (precursor) 의수분해 (hydrolysis)[29, 33,34], 기상박막증착법 [35], 산화분위기에서소결법 (TiN[36], TiS 2[36], TiC[37]), 대기압하에서플라즈마법을통한나노입자제조 [38] 등의방법을통하여제조되고있다. 이렇게다양한도핑방법으로개질된 TiO 2 광촉매의가시광반응효율은제조방법에크게의존하며, 장시간사용과반복적인사용에의한광활성손실을어떻게극복할것인가가중요한문제점으로알려져있다. 또한, 도핑이되지않은 TiO 2 와비교하여금속이도핑된 TiO 2 가가시광하에서활성이없거나자외선하에서낮은활성을보이는경우도나타나는데, 이는금속이온이전하의재결합을촉진하는역할을하는것으로알려져있기때문이다. 따라서, 상업적활용을목적으로한광범위한광촉매시스템확대를위해서는가시광하에서화학적, 물리적안정성과향상된표면특성을소유하는새로운소재의개발이나기존에개발된다양한소재의최적화가매우중요할것으로판단된다. 추가적으로, TiO 2 를기반으로하는가시광반응촉매의활성을향상시키기위한새로운도핑물질이나도핑물질을도입하기위한새로운방법을개발하는것과광반응의새로운응용분야를확보하는것이중요하게인식되고있다 텅스텐 (W) 계열도핑을통한 TiO 2 기반광촉매재료의개발은상대적으로큰띠간격을줄여줌으로써가시광선의활용효율을향상시키는 Table 1. Examples of the various doping materials used for manufacturing doped-tio 2 photocatalysts Doping materials Preparation method Ref. Metal Nonmetal Pt Au Fe Ag N Photoreduction (125 W Hg lamp for 1 h): TiO 2 particls are suspended in a mixture of hexachloroplatinic acid in methanol. After photoreduction, Pt-TiO 2 particles are filtered, washed with distilled water, and dried at 100 for 24 h. Titanium butoxide dissolved in absolute ethanol is added to solution including HAuCl 4 4H 2O, acetic acid and ethanol. The resulting suspension is aged for 48 h, and then dried, grinded and calcined at 650. Reactive magnetron sputtering (99.99% titanium target, 99.9% iron pieces in the reaction chamber) with a gas mixture (argon and oxygen) during discharging Silver nitrate/reduction agent (sodium citrate tribasic dihydrate) at 80, then TIP and HNO 3 are added and maintained at 50 for 24 h. The prepared sol is dried at 105 for 24 h and calcined at 300. TiO2 powder in the ammonia atmosphere at 600 for 3 h TiN oxidation at for 2 h in air [21,22] [23] [24,25] C Tetrabutyl orthotitanate is hydrolyzed in the presence of ethanol, water, and nitric acid, and then the titanium hydroxide is dried at 110 and calcined in air at [29,30] S Titanium disulfide (TiS 2) oxidized and sintered at [31] [26] [27] [28]

4 194 청정기술, 제 19 권제 3 호, 2013 년 9 월 것에주요점이있다. 이처럼띠간격이작은반도체성질의새 로운물질을발견하고광촉매의효율을높이고자하는노력 이계속되고있다. Figure 1 에서보여준띠간격에너지준위 를참고해보면, 효율적인수소생산을위한재료의조건으로 는전도띠의에너지준위가프로톤이전자를받아서수소가 형성되는것보다음준위가되어야하며, 또한이에비례하여 물로부터산소가생산되는준위는더양준위로분포해야한 다. 하지만이런조건을모두만족할경우띠간격에너지가 너무크게되어태양광에의한전자 - 정공의발생이어렵게 된다. 따라서한종류의금속산화물반도체광촉매를바탕으 로이러한모든성질을만족시키는것보다는두종류의재료 를결합하여각각으로부터수소와산소를생산하는역할을 담당하게하는촉매, 즉 Z-scheme 촉매의개념이등장하게되 었다 [39]. 이러한개념은수소를생산하는촉매와산소를생산 하는촉매를따로구성하는이며좁은띠간격에너지를가지 는다양한광촉매를혼합할수있다는큰장점이있다. 이러 한개념은수소와산소를생산하는데관여하는포톤 (photon) 의 개수로크게구별할수있는데, 단일광촉매의경우는하나의 포톤을흡수하여하나의전자-정공쌍을만들지만 Z-scheme 의경우는두개의포톤을흡수하여두개의광반응에관여하는것이며이를 Figure 3에나타내고있다. 이러한개념을바탕으로텅스텐산화물 (WO 3) 은물을산화 시켜산소를생산하는데매우유리한전위구조를가지고있 으며, 다양한화학반응용액조건에서광및화학적안정성을 보여주고있다. 또한, 이미지적한것처럼낮은띠간격 (2.7 Figure 3. Illustration of the one photon (a), and the two photon (Zscheme) water splitting (b), on a single and a dual semiconductor photocatalyst[4]. ev vs. 3.2 ev for TiO 2) 으로인하여태양광의잠재적활용가능성이크다고할수있다. 따라서텅스텐산화물의우수한산소발생능력을유지하면서수소생산성을높일수있도록텅스텐산화물의전자수조를조절하기위한다양한연구가수행되어오고있다. TiO 2 의경우와같이금속이나비금속물질을도핑하여수소생산효율을높이고자하는연구들이수행되었으나큰효율의향상이이루어지지않았으며텅스텐산화물을활용하는수소생산의연구는이성분이나삼성분물질을함유하여광촉매의효율을향상시키고자하는다양한연구가보고되고있다. Figure 4는다양한종류의이성분계텅스텐산화물반도체광촉매의띠간격과전위수준을보여주고있다. 기본물질인 WO 3 와비교하여첨가물의종류에따라원자가띠의에너지준위와띠간격이다양하게변화되는것을잘보여주고있다. 예를들어, Kudo와공동연구자들 [40-42] 이보고하였던비스뮤스텅스텐산화물 (Bi 2WO 6) 의밴드에너지를보면, WO 3 와비교하여띠간격에서는큰변화가없지만전도띠의에너지준위가음준위방향으로개선됨에따라수소생산이용이할수있음을나타낸다. 이러한이성분계텅스텐산화물을제조하는방법은고온공정이필수적으로동반되어에너지관점에서큰단점으로작용하기도한다. 따라서, 전기증착법, 화학적용액성장법 (chemical bath deposition, CBD), 졸겔법등의에너지와시간의관점에서재료합성의효율성을향상시키고자하는다양한시도가이루어지고있다. 또한삼성분을혼합하여밴드에너지준위를개선하기위한시도들도다양하기이루어졌으나이성분시스템들과유사한영역에서의밴드에너지준위들을확보하는것으로판단된다. 또한, WO 3 를활용한수소생산효율을높이기위하여 Z- scheme 광촉매를구성하는다양한연구가보고되었다. WO 3 를활용한수소생산용혼합광촉매로보고된물질로는스트론튬타이타늄산화물 : 로듐 (SrTiO 3 : Rh)[43-46], 탄탈륨옥시나이트라이드 (TaON)[47,48], 구리인듐셀레늄 (CuIn 3Se 5)[49] 등의다양한물질들이있다. 이상에서살펴본바와같이텅스텐산화물의전자구조를바꾸기위한노력, 수소생산을용이하게하기위한전도띠를음전위로이동하기위한노력, 띠간격을줄이기위한노력등의다양한시도등이텅스텐계열광촉매재료의주요연구방향이었다. 혼합형광촉매를위한 Z-scheme Figure 4. Band positions of various tungsten based binary oxide semiconductors[4].

5 태양광을활용한물분해수소생산용광촉매재료 195 재료는텅스텐화합물을산소생산반응에활용함을기본으로 하여이성분이나삼성분계의소재를결합함으로서수소생산 효율을높이고자하는시도들이계속되어왔다. 계속되는나노 수준의다성분계혼합에따른표면공학의진보는 Z-scheme 공정을통한광촉매수소생산연구의효율을지속적으로높여줄것으로기대된다 구리 (Cu) 계열 구리화합물인황동 (chalcopyrite) 계열 (CuInSe 2, CuInS 2, Cu- InGaSe 2) 은좁은띠간격과화합물에따라비교적정교한에 너지준위를가진물질로태양광의특정파장영역을흡수하기 에매우적합한물질이다. 이들은화합물의조성을비교적쉽게 변화시켜밴드구조를 1.1~2.5 ev 범위에서조절이가능하며 우수한전자이송특성을가지고있다. 또한전도띠의전위가 수소를생산하기에용이한음전위에위치하고있으며, 개미산 으로부터수소를생산하기에도충분한광촉매임을보이고있 다. 하지만, 원자가띠의위치에있어서물분해로부터산소를 생산하기에는적합하지않은것으로나타나며, 따라서황동광 계열촉매로부터물분해수소생산은혼합구조광촉매의사용 이필요한실정이다. 황동광계열물질은박막 [50], 나노결정 [51], 나노선 [52], 나노튜브 [53] 등의다양한형태로보고되었 으며나노구조형반도체들이광전환효율의샹항측면에서태 양전지나태양연료로의응용이확대되고있는실정이다. 구리산화물 (Cu 2O) 또한좁은띠간격 (2.0~2.2 ev) 을소유하 고있으며전도띠의전위가 V 수준으로수소생산반응 에적합한것을나타낸다. 이물질은매우광범위하게존재하 는풍부한재료로서경제적인관점에서큰장점이있으며, 환경 적인유해성도적은것으로알려져있다. 하지만, 구리산화물 은빛으로부터생성된전자가효율적으로짧은시간내에활용 되지못할경우구리로환원되는현상이나타날수있다. 따라 서, 이러한전자의광환원작용을줄여줄수있도록양전위전 도띠를가지는 TiO 2 나 ZnO 와같은물질 ( 전자를효율적으로이 송시키는역할을담당 ) 과결합하여전자의광안정성을높여줄 수있다. 최근에 Gratzel 그룹에서는산화구리박막에다층박 Figure 5. Cu 2O photocathode with multiple layers designed by Gratzel[54]. 막을만들고 Pt 나노입자를공용촉매로활용하여수소를생산하는연구를보고하였다 [54]. Figure 5는다층박막이결합된구리산화물접합구조를보여주고있으며, 구리산화물이전해질용액으로부터분리되어스스로광환원작용을줄여주는구조를나타낸다. 이러한구조적특징으로인하여뛰어난광전류안정성을나타내며약 40% 의광전환효율을보였다고보고하였다. 또한 Chen et al.[55] 은구리산화물의광안정성과광촉매효율을높이기위하여나노선에대한연구를제시하였으며, 구리산화물나노선이격자형으로구성된나노입자형구리산화물에비하여매우우수한광안정성과높은광전류를보였다고보고하였다 흑연탄소질화물 (carbon nitride) 흑연탄소질화물 (g-c 3N 4) 은그래파이트와유사한고분자형반도체소재로서우수한화학적, 광화학적안정성을가지고있는것으로알려져있다. 띠간격은약 2.7 ev이며이는가시광선파장영역인 400~450 nm의파장을흡수할수있음을나타낸다. 위에서설명한광촉매재료들과같이흑연산소질화물의경우도띠간격을조절하기위한다양한시도들이있었으며, 여러가지원소를이용한도핑, 소재의열처리등의방법들이보고되었다. 예를들어, 본재료는광반응에의하여생성되는전류의강도는다소약하지만, 그래파이트와유사한 g-c 3N 4 의구조를활용하여다른소재를기능화하여광흡수영역을확장하기에화학적으로매우큰가능성을가지고있다. Domen과동료들은마그네슘프탈로시아닌 (magnesium phythalocyanine, MgPc) 염료를 C 3N 4 의본체에 π-π 적층작용을통하여도입하여더효율적인가시광선의흡수를이루었다고보고하였다 [56]. 또한 π-π 작용으로 C 3N 4 와그래핀복합체가합성되기도하였으며 [57,58], 이때그래핀은전자흡수체역할을함으로서 C 3N 4 영역내에서전하를효율적으로분리시키고광전류를향상시키는작용을하는것으로보고되었다. 최근에보고된연구로는다공성 C 3N 4 (mpg-c 3N 4) 가비표면적을높이고다분산효과에의하여흑연산소질화물의광효율과촉매활성을크게향상키기는가장촉망되는가능성을제시하기도하였다 [59]. 물분해수소생산을위한광촉매로의적용에서 mpg-c 3N 4 는 triethanolamine (TEOA)/ 물의혼합용액에서매우활성이높은것으로나타났으며, 촉매의효율은공극과비표면적에따라크게의존하는것으로알려졌다. 그래핀복합체를사용할경우는전자흡수체역할을하여 mpg-c 3N 4 보다 3배이상의활성을보였다. 또한저가광촉매를개발하기위한노력으로 mpg- C 3N 4/Co 3O 4[60] 를이용한물분해를통한산소반응촉매와같이흑연산소질화물을이용하며귀금속을사용하지않는촉매를개발하기위한시도들이보고되고있다. 또한, 흑연산소질화물에수소생산이나산소생산을일으키는전자촉매를접합하여무선융합광촉매의개발도새로운관심을이끌고있으며, 태양광의효율적인활용과관련한다양한요구사항들을만족시키기위한노력이지속될것으로판단된다.

6 196 청정기술, 제 19 권제 3 호, 2013 년 9 월 Table 2. Examples of H 2-production from metal sulfide photocatalysts under visible light irradiation Materials H 2-production rate Zn 1-xCu xs 450 µmol/h g Cd 1-xZn xs Cd 1-xZn xs (Zn 0.95Cu 0.05) 1-xCd xs Cu-ZnS shell Cu-doped ZnIn 2S 4 Cd xcu yzn 1-x-yS CuS-Zn xcd 1-xS ~2,000 µmol/h g ~830 µmol/h g ~1,690 µmol/h g ~15 µmol/h g ~760 µmol/h g ~1,170 µmol/h g ~1,500 µmol/h g CdS-Zn 1-xCd xs 2,128 µmol/h g CuS/ZnS nanosheet 4147 µmol/h g Light condition 300 W Xe, >420 nm 300 W Hg, >400 nm 350 W Xe, >430 nm 300 W Xe, >420 nm 300 W Xe, >420 nm 300 W Xe, >430 nm 350 W Xe, >430 nm 300 W Xe, >420 nm 350 W Xe, >400 nm 350 W Xe, >420 nm Ref. [63] [64] [65] [66] [67] [68] [69] [70] [71] [67] Figure 6. Schematic illustration for visible light induced IFCT from the valence band of ZnS to the CuS clusters in CuS/ZnS system[72]. µmol/h g) 의수소생산속도를보고하였다. 이들은 CuS가 ZnS 에비하여상대적으로낮은 2.94 ev의띠간격을가지고있어, 가시광선을활용하는효율을높였으며, 또한 ZnS의표면에응집체 (cluster) 형태로존재하는 CuS의전도띠에너지준위로전자가운반되어수소생산에효과적으로활용된것을설명하고있다. Figure 6은이를개략적으로보여주고있는것이며, CuS를표면에도입할경우함량에따라수소생산효율이달라지기때문에적절한함량이존재함을제시하였다 금속황화물 (metal sulfides) 금속황화물광촉매는촉매기능이나띠간격측면을고려하여최근에연구가활발히진행되고있다. 특히, 아연황화물 (ZnS) 은광여기에의하여전자-정공쌍을빨리형성하고여기된전자가높은음전위를가지고있어물분해수소생산반응에서매우높은활성을보이는특징이있는것으로잘알려진광촉매물질중하나이다 [61,62]. 또한, 금속황화물광촉매재료는수소생산효율향상을위하여다양한연구가꾸준히지속되고있다. Table 2는현재까지보고된다양한금속황화물광촉매를활용한물분해수소생산의대표적인예를정리하여보여주고있다. ZnS의띠간격은 3.66 ev로비교적큰편이며가시광에의한광여기를용이하게하기위하여, 구리 (Cu) 와카드뮴 (Cd) 등의금속성물질을동시에도입하는다양한결과들을보여주고있다. 보고된대부분의금속황화물광촉매의제조는사용되는재료의기재 (precursor) 를동시에혼합하여한번의공정으로만드는고체용액 (solid solution) 법을사용하였으며, 고온의열처리를통하여각각의금속성물질의결정특성을개선하여광촉매특성을향상시킨것을나타내었다. 수소생산속도는 10~4,000 µmol/h g 범위에서다양하게보고되었으며, 이는광반응에사용된촉매의양과반응시간을단위시간과단위무게로환산한경우를나타내고있다. 최근에보고된 Zhang et al.[72] 의결과에서는 ZnS와구리황화물 (CuS) 을접합하는나노쉬트 (nanosheet) 형태를두단계공정으로제조하여여기된전자를계면전하이동 (interfacial charge transfer, IFCT) 개념으로해석하였으며매우높은수준 (4, 기타이상에서정리한광촉매재료와더불어적철광 (hematite, α- Fe 2O 3) 는 2.1 ev의띠간격을가지고있어가시광을흡수하기에매우적합한소재로알려져있으며, 금 (Au) 나노입자를도입하여광흡수와전하분리를용이하게하기위한시도가보고되었다 [73]. 또한, CdSe, CdTe, GaP, GaAs, ZnO, InP, N- doped Ta 2O 5 등의다양한반도체특성을가지는광촉매재료들이보고되었으며, 이러한다양한물질들을효율적으로물분해수소생산에활용할수있기위해서는광흡수특성, 광전류변환효율, 광부식등의관점에서지속적인연구가필요할것으로판단된다. 3. 광원에따른전자여기메커니즘광촉매재료의활용에있어서가장중요하게고려되는것은광원이가지고있는에너지와재료의고유한띠간격에너지와의관계이다. 즉, 광촉매재료의띠간격에너지보다큰광원의에너지가재료에조사될경우재료의전도띠에있는전자는여기상태가되어원자가띠로이동하게된다. 이때여기된전자가물분해수소생산에서프로톤 (H + ) 을수소 (H 2) 로전환하는역할을하게된다. 전자의여기에필요한에너지는식 (1) 과같이표현된다. E = hv = hc/λ (1) 여기서 h는플랑크상수 ( m 2 kg/s), c는빛의

7 태양광을활용한물분해수소생산용광촉매재료 197 Table 3. Summary of possible semiconductor photocatalyst materials Light Wavelength (λ, nm) Band Gap (ev) Examples UV C (5.47 ev), BN (7.50 ev), AlN (6.20 ev), GaN (3.40 ev), TiO 2 (3.2 ev), ZnS (3.35 ev) Violet CuS (2.94 ev) Blue SiC (2.86 ev), WO 3 (2.70 ev), C 3N 4 (2.70 ev), ZnSe (2.70 ev) Green CdS (2.45 ev), TaON (2.40 ev) Yellow Orange Cu 2O (2.00 ev) Red InN (1.90 ev) Near IR , GaAs (1.42 ev), CdSe (1.70 ev), InP (1.35 ev), Si (1.11 ev), Ge (0.66 ev) 속도 (299,792,458 m/s) λ는빛의파장 (m) 이다. 식 (1) 을이용하여빛의에너지를계산할경우 m 2 kg/s 2 의단위가나오는데이는 Joule의에너지단위이며띠간격에너지를나타낼때주로사용하는 ev로환산하기위해서는 1 ev를 J의관계를이용할수있다. 이를바탕으로하여다양한광원에따른파장, 띠간격에너지, 이에해당되는반도체광촉매재료의예들을 Table 3에정리하여보여주고있다. 광원으로부터충분한에너지를받아서여기된전자가원자가띠로부터분리되어전도띠로이동한후, 전자와정공이다시재결합하는현상이없이효율적으로사용될경우수소생산효율은높아지게된다. 이렇게되기위해서는분리된전자와정공이재결합을하지않는것이매우중요한데, 이는분리된전자가효율적으로이송되어야한다. 띠간격이매우좁은물질의경우는전자의여기를위한에너지는적게필요하지만반면에분리된전자와정공이재결합을이룰가능성또한매우높아서효율적인전자이송과정을반드시고려해서광촉매를설계해야한다. 4. 광촉매활용물분해수소생산연구의제안사항비록다양하게매력적인반도체형광촉매소재군이개발되었지만, 물분해반응으로부터충분한생산속도로수소와산소의자발적인반응을일으키는광촉매의제조는여전히필요한실정이다. 더군다나좁은띠간격을소유한황동, 산화구리, 산화철등은가시광을흡수하는우수한특성을가진반면, 원자가띠의전위가물분해반응을통하여산소를발생하기에는부적절한것으로알려져있다. 또한, 이들은통상적으로장기간사용시에광부식을일으키는것으로도알려져있다. 물분해수소생산에적합한밴드구조를소유하였으며상대적으로안정한 TiO 2 와카본나이트라이드물질등의반도체물질은태양광중에서매우적은양인자외선영역을주로흡수하여광전환효율이상대적으로매우낮은것으로알려져있다. 따라서광안정성과좁은띠간격을소유한반도체형소재의지속적인개발이꾸준히요구되고있으며, 광촉매를이용한물분해수소생산으로의응용을위해서는탠덤구조, 외부전극의응용, 소재의복합화, 산화환원매개물질의응용등다양한공학적인접근이필요할것으로판단된다. 또한, 광촉매연구에서매우중요한변수는광원이라고할수있다. 광원을구성하는중요한인자로는빛의세기 (W), 단위면적당광조사에너지 (W/m 2 ), 파장 (λ) 을선택하는차단 (cutoff) 범위 (nm) 등에따라광촉매를통한수소생산반응은매우다른결과를가져올수있다. 예를들어, Table 2에서보여주고있는금속황화물광촉매를통한수소생산속도는사용된빛의조건이서로다르기때문에직접적인비교가어려운실정이다. 같은빛의세기를가지고있다고하더라도단위면적당광조사에너지밀도를알수없으며, 같은빛의세기를가지고있지만서로다른차단파장범위를가지고있다면수소생산결과도당연히달라질수있기때문이다. 일반적으로태양광을활용한연구의경우상호신뢰비교를위해서는기준조건이필요하다고판단된다. 예를들면, 태양광을활용하는태양전지연구에있어서태양광을모사할수있는장치 (solar simulator) 를이용하여지구표면에도달하는태양광파장을가장유사하게구현할수있는차단파장범위를선택하여태양광밀도 (1,000 W/m 2 ) 를표준조건으로설정하여태양전지효율을보고하고있다. 하지만, 미래무한에어지원인태양광을활용하고자하는일환으로다양하게진행되고있는태양광활용광촉매수소생산연구는, 가장중요한실험변수인광원의표준화가없음으로인하여서로다른연구결과들을상호비교하기가매우어려운실정이다. 따라서가능하다면표준광원과광밀도를사용할수있도록하여다양한연구결과들을비교판단하는것이용이하게하고각기보고된결과의상호신빙성을높여연구결과를모든연구자들이쉽게파악할수있도록하는것이중요할것으로판단된다. 또한, 광촉매를활용한수소생산속도표현방법에있어서도특정한기준조건을사용하여앞으로보고되는다양한연구결과를서로상대비교할수있게함으로써광촉매수소생산연구개선방향을공통적으로찾아갈수있도록하는것이중요할것으로판단된다. 5. 결론물분해수소생산용광촉매재료로연구되어오고있는 TiO 2 계열, 텅스텐계열, 구리계열, 흑연질소산화물계열, 금속황화물등의다양한반도체물질들에대하여알아보았다. 광촉매

8 198 청정기술, 제 19 권제 3 호, 2013 년 9 월 재료와태양광을이용하여물분해수소생산에효율적으로활용되기위해서는띠간격에너지가전자를여기시키기에충분한범위를가져야한다. 띠간격이클경우가시광의흡수를통하여전자의발생이어렵게되며따라서금속물이나비금속물을도입하는방법등을통하여띠간격을낮추는연구가광범위하게수행되었다. 대부분의촉매들이자외선에의한활성이크게나타났지만, 태양광의큰영역을차지하고있는가시광선 (λ: 400~700 nm) 을효과적으로활용하지못한다면효율을높이는면에서한계가있다고할수있다. 따라서이러한관점에서가시광을활용하기위한다양한연구들이수행되었으며, 수소생산효율의향상을위하여다양한공용촉매들을도입하는방법이나, 태양광을흡수하여여기된전자와정공을효율적으로분리하여재결합을방지함으로써프로톤을수소로전환하는성능을높이는방법들이연구되었다. 다양한연구자들이광촉매연구에서활용하는광원에통일성이없는것으로타나났으며, 상호비교를통해보다신뢰성있는연구결과를공유하기위해서는사용되는광원의통일이필요할것으로판단된다. 감사이논문은 2012년도서울시립대학교연구년교수연구비에의하여수행되었으며이에감사드립니다. 참고문헌 1. Winter, C. -J., Hydrogen Energy-abundant, Efficient, Clean: A Debate over the Energy-system-of-change, Int. J. Hydrogen Energy, 34, S1-S52 (2009). 2. Fujishima, A., and Honda, K., Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 238, (1972). 3. Zaleska, A., Doped-TiO 2: A Review, Recent Patents Eng., 2, (2008). 4. Fuerte, M. D. H. A., Maira, A. J., Martinez-Arias, A., Fernandez- Garcia, M., Conesa, J. C., and Soria, J., Visible Light-activated Nanosized Doped-TiO 2 Photocatalysts, Chem. Commun., 24, (2001). 5. Anpo, M., Use of Visible Light. Second-generation Titanium Dioxide Photocatalysts Prepared by the Application of an Advanced Metal Ion-implantation Method, Pure Appl. Chem., 72, (2000). 6. Ohno, T., Mitsui, T., and Matsumura, M., Photocatalytic Activity of S-doped TiO 2 Photocatalyst under Visible Light, Chem. Lett., 32, (2003). 7. Liu, Y., Chen, X., Li, J., and Burda, C., Photocatalytic Degradation of Azo Dyes by Nitrogen-doped TiO 2 Nanocatalysts, Chemosphere, 61, (2005). 8. Yu, J. C., Zhang, L., Zheng, Z., and Zhao, J., Synthesis and Characterization of Phosphated Mesoporous Titanium Dioxide with High Photocatalytic Activity, Chem. Mater., 15, (2003). 9. Hirai, T., Suzuki, K., and Komasawa, I., Preparation and Photocatalytic Properties of Composite CdS Nanoparticles- Titanium Dioxide Particles, J. Colloid Inteface Sci., 244, (2001). 10. Chatterjee, D., and Mahata, A., Demineralization of Organic Pollutants on the Dye Modified TiO 2 Semiconductor Particulate System using Visible Light, Appl. Catal. B Environ., 33, (2001). 11. Zhou, W., Zheng, Y., and Wu, G., Novel Luminescent RE/ TiO 2 (RE = Eu, Gd) Catalysts Prepared by In-situ Sol-gel Approach Construction of Multi-functional Precursors and Their Photo or Photocatalytic Oxidation Properties, Appl. Surf. Sci., 252, (2006). 12. Ai, G., Sun, W. T., Zhang, Y.-L., and Peng, L.-M., Nanoparticle and Nanorod TiO 2 Composite Photoelectrodes with Improved Performance, Chem. Commun., 47, (2011). 13. In, S.-I., Nielsen, M. G., Vesborg, P. C. K., Hou, Y., Abrams, B. L., Henriksen, T. R., Hansen, O., and Chorkendorff, I., Photocatalytic Methane Decomposition over Vertically Aligned Transparent TiO 2 Nanotube Arrays, Chem. Commun., 47, (2011). 14. Zhang, S., Zhang, S., Peng, F., Zhang, H., Liu, H., and Zhao, H., Electrodeposition of Polyhedral Cu 2O on TiO 2 Nanotube Arrays for Enhancing Visible Light Photocatalytic Performance, Electrochem. Commun., 13, (2011). 15. Xiang, W., Liu, X., Liu, H., Tong, D., Yang, J., and Peng, J., Coaxial Heterogeneous Structure of TiO 2 Nanotube Arrays with CdS as a Superthin Coating Synthesized via Modified Electrochemical Atomic Layer Deposition, J. Am. Chem. Soc., 132, (2010). 16. Chen, C., Cai, W., Long, M., Zhou, B., Wuu, Y., Wuu, D., and Feng, Y., Synthesis of Visible-light Responsive Graphene Oxide/TiO 2 Composites with p/n Heterojunction, ACS Nano, 4, (2010). 17. Yu, J., Ma, T., Liu, G., and Cheng, B., Enhanced Photocatalytic Activity of Bimodal Mesoporous Titania Powders by C 60 Modification, Dalton Trans., 40, (2011). 18. Fan, W., Lai, Q., Zhang, Q., and Wang, Y., Nanocomposites of TiO 2 and Reduced Graphene Oxide as Efficient Photocatalysts for Hydrogen Evolution, J. Phys. Chem. C, 115, (2011). 19. Lightcap, I. V., Kosel, T. H., and Kamat, P. V., Anchoring Semiconductor and Metal Nanoparticles on a Two-dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide, Nano Lett., 10, (2010). 20. Janaky, C., Rajeshwar, K., de Tacconi, N. R., Chanmanee, W., and Huda, M. N., Tundsten-based Oxide Semiconductors for Solar Hydrogen Generation, Catal. Today, 199, (2013). 21. Li, X. Z., and Li, F. B., The Enhancement of Photodegradation Efficiency Using Pt-TiO 2 Catalyst, Chemosphere, 48,

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