晶體內部與表面摻雜釩離子對二氧化鈦光觸媒物化特性與光催化活性之影響

Abstract

本研究利用溶膠-凝膠法(sol-gel)及表面溶膠-凝膠法(surface sol-gel)製備釩離子摻雜 之二氧化鈦(TiO2)，並探討晶體內部(bulk doping)或表面(surface doping)釩離子對於TiO2 材料及物化特性之影響。研究結果顯示單純TiO2 經300 □C 鍛燒後呈現69.9 wt%的銳鈦 礦(anatase)與31.1 wt%的金紅石，同時銳鈦礦的平均晶粒為6.1 nm，巨體摻雜後並未明 顯改變TiO2 晶粒尺寸，其晶粒大小範圍為6.0 至6.5 nm 間，然而在V/Ti 比例高於1.27□ 10-3 時完全抑制金紅石晶相(Rutile)的形成，此外，由於鍛燒溫度略高於坦曼溫度，所 以釩會遷移至表面形成V2O5 晶相。由低濃度摻雜釩的UV-Vis 圖譜中，發現巨體摻雜 TiO2 於250~320 nm 間有V5+吸收波峰產生，證實摻雜釩於晶體內部會於TiO2 能帶間導 入額外能階，然而當V/Ti 莫爾比高於1.00 %，部分V5+會還原成V4+，而因為V4+會同 時捕捉電子電洞降低表面電荷轉移，所以降解0.01 mM Rhodamine (RhB)之擬一階反應 速率常數隨著晶格內釩離子濃度的增加，而從5.20×10-2 降至1.50×10-2 1/min，利用EPR 偵測觸媒表面OH 自由基，其積分面積從5.40×107 降至5.50×106，證實電子轉移的效率 會隨晶格內釩離子濃度增加而變差。相較下，表面摻雜對TiO2 的微結構及電子結構並 無巨觀的影響，但表面摻雜釩的反應速率常數卻隨著表面V5+濃度增加而從5.20×10-2 提 升至9.80×10-2 1/min，當V/Ti 約為1.00□10-2 時，表面摻雜TiO2 的反應速率數高於內部 摻雜觸媒的六倍，此原因為表面的V5+易使電子累積於TiO2 表面，增進表面電荷轉移速 率(觸媒表面OH 自由基積分面積從2.07□107 提升至4.22□107)，因此表面摻雜比巨體摻雜更可提高TiO2 光催化活性。

The aim of this study was to investigate the effects of bulk and surface lattice dopings on physicochemical properties and photocatalytic activities of V-doped TiO2. The sol-gel-derived TiO2 exhibited 69.9 and 31.1 wt % of anatase and rutile phase, respectively. In addition, the crystallite size of the anatase TiO2 was 6.1 nm. Lattice vanadium ions had no effect on crystal size of TiO2, ranging between 6.0-6.5 nm. However, lattice vanadium ions completely inhibited the formation of rutile as the V/Ti ratio is as high as 1.27□10-3. V2O5 crystals were observed on the surface of TiO2 since vanadium ion diffused to surface when the calcination temperature was higher than its Tammann temperature. The UV-vis spectra show that bulk doping resulted in an additional absorption band centered at 289 nm. This phenomenon indicated that incorporation of V5+ ions into the bulk lattice of TiO2 at low vanadium concentrations (V/Ti ratio < 1.00□10-2) introduced extra energy levels in the conduction band. When V/Ti atomic ratio was higher than 1.00 %, some V5+ were partially reduced to V4+ which acted as charge recombination centers. Pure TiO2 exhibited a rate constant of 5.20×10-2 min-1 for the photocatalytic degradation of Rhodamine B (RhB). Bulk doping decreased the photocatalytic activity to from 5.20×10-2 to 1.50×10-2 min-1 when the V/Ti ratio increased from 4.41×10-5 to 1.22×10-2. In addition, the integrated area of generated •OHon the surface of photocatalysts, which were calculated by EPR, decreased from 5.40×107 to 5.50×106. The results indicated electrons diffuse to surface hardly. In contrast, surface doping had little effects on the micro- and electronic structures of TiO2. Nevertheless, the photoactivity was enhanced from 5.20×10-2 to 9.80×10-2 min-1 upon increasing vanadium concentration. The photoactivity of the surface doped TiO2 was six times higher than that of bulk doped ones at the V/Ti ratio of 1.00□10-2. Such enhancement is due to that surface-V5+ promotes diffusion of electrons to surface that further facility charges transfer to reactants. According to integrated area of surface doped materials, which increased from 2.07□107 to 4.22□107, it indicate the electrons diffuse to surface efficaciously. Therefore, surface doping greatly improve the degradation efficiency, while bulk ones lad to detrimental effects on the photocatalytic activity.