功能化奈米半導體硫化亞銅/球型氫氧化鎳粒子使用於光催化水分解產氫的應用

Abstract

利用太陽光進行光催化水分解產氫的研究近年來越來越受重視，水分解產氫配合儲氫材料的發展被視為解決能源問題的一大方法。此方法主要是利用具有合適能隙(bandgap)半導體粒子當作光觸媒材料吸收太陽光，半導體粒子價帶(valence band)上的電子吸收能量之後躍遷到傳導帶(conduction band)上，原本價帶上產生一電洞(h+)達到電子電洞對分離，吸收能量後躍遷的電子和水中的氫離子反應，產生的電洞亦被傳遞到表面和水反應產生氧氣，一般情況下我們會加入一犧牲試劑將電洞消耗掉，以避免電子電洞對快速的再結合。 Cu2S硫化亞銅為一P型半導體材料，且早已被運用於光催化降解水汙染方面上的研究，另外它具有較小的bandgap(1.5eV)，和水的還原電位(1.23ev)極為接近，但卻沒有使用來做水分解方面的應用。Ni(OH)2常被使用於鋰電池和鎳氫電池的cathode材料上，具有好的穩定度特性，近年來亦有報導指出氫氧化鎳在適當的條件之下可以當作催化材料，大幅度的增加光催化產氫的效率，其催化機制和目前已知的使用貴金屬沉積在半導體表面捕捉和傳遞電子的機制相異，主要為： Ni2+ /Ni (Ni2+ + 2e- → Ni, Eo = -0.23 V) (1) 2H+ + 2e- → H2, Eo = -0.00 V (2) 鎳二價離子會先和吸收光能重半導體粒子的價帶躍遷到傳導帶的電子反應，鎳被還原出來，如反應式(1)，被還原出來的鎳開始扮演活性中心並傳導來自半導體粒子吸收光能產生的電子以還原水溶液中的氫離子產稱氫氣，如反應式(2)。氫氧化鎳的高催化效率以及和貴金屬(如：鉑)相比較為低廉的價格可望在將來被應用於共催化材料的應用方面。One of the major needs for the 21st century is the development of alternative energy sources to fossil fuels that do not contribute to greenhouse gas emissions. Hydrogen has considerable potential as an alternative fuel because it is carbon-free, facilitates use of more efficient power generation systems. Using a single precious metal electrode and an ECPB to generate hydrogen and oxygen from water would allow much more economically-viable large-scale generation of hydrogen than is currently possible. The primary steps of natural photo-synthesis involve the absorption of sunlight and its conversion into spatially separated electron-hole pairs. The photocatalyst uses a photon to excite an electron from the valence band to the conduction band: resulting in an excited state, and the energy difference between the valence band and conduction band is known as the “band gap”. This must correspond to the wavelength of light for it to be effectively absorbed by the photocatalyst. After photoexcitation , the excited electrons and holes separate and migrate to the surface of photocatalyst .The two protons, which are needed to generate hydrogen gas. The hole in the valence band can be filled with an electron produced by the oxygen generation, in the photocatalytic water-splitting reaction, they act as reducing agent and oxidizing agent to produce H2 and O2 respectively. Efficient charge separation are fundamentally important for photocatalytic hydrogen generation through water splitting. So we can via avoiding charge recombination by adding sacrificial agent to improve hydrogen generate efficiency. Cu2S has been widely used in solar cell devices and identified as p-type semiconducting materials because of the copper vacancies within the lattice. Cu2S is both an indirect and direct band gap materials, with Egbulk≈ 1.2eV and 1.8eV, respectively. In order to absorb light, it is necessary to narrow the band gap. The conduction band is fairly close to the reference potential for H2 formation, there are few reports on the photocatalytic H2 production. Nickel hydroxide (Ni(OH)2) has received increasing attention since it is a very important cathode material in a number of alkaline rechargeable batteries . Because of the stability in strong alkaline electrolyte and excellent reversibility when charged to NiOOH, it’s also reported the enhanced photocatalytic H2-production efficiency. The enhanced mechanism is because thepotential of Ni2+/Ni (Ni2+ + 2e- → Ni, Eo = -0.23 V) is slightly lower than conduction band (CB) (-0.25 V) of Cu2S, meanwhile higher than the reduction potential of H+/H2 (2H+ + 2e- →H2, Eo = -0.00 V), which favors the electron transfer from CB of Cu2S to Ni(OH)2 and the reduction of partial Ni2+ to Ni. The function of Ni is to help the charge separation and to act as cocatalyst for water reduction, thus enhancing the photocatalytic H2-production activity. Its higher efficiency and lower cost than noble metal can be applications of co-catalyst with another semiconductor material.