To accentuate the role of pn heterojunctions in composites, the photocatalytic properties of two composites NTC11 (NiO and TiO₂ in 1:1 molar ratio) and NTC36 (0.35:0.65) were investigated in detail for H₂ evolution reaction and compared with the component pure oxides, NiO, TiO₂ and NiTiO₃ prepared by sol-gel route. Among all samples, NTC11 was distinctly most active (greater than pure TiO₂ by ∼22 times, NiO has negligible activity) and yielded reproducible H₂ yields for 60 h under repetititive cycles in sunlight confirming its photostability. Maximum photocatalytic Hydrogen yield @ 0.6 L/h/m² with apparent quantum efficiency (AQE) of 5.4% and solar fuel efficiency (SFE) of 0.8% under sunlight and 1.4 L/h/m² with AQE of 7.8% under UV-visible irradiation along with evolution of bubbles was observed over Pt(1 wt%)/NTC11. Synthesis conditions, calcination temperature and phase compositions were very critical and played an essential role in determining the overall hydrogen yield. Presence of NiTiO₃ was found to be derogatory for the photocatalytic activity of NTC36.The key factors responsible for enhanced rate of sunlight assisted hydrogen generation over NTC11 were 1. Formation of pn nanojunctions in NTC11 evident by enhanced life times of charge carriers monitored by time resolved photoluminescence, uniform distribution of NiO and TiO₂ nanoparticles with Ni/Ti in ratio of 0.99 and 0.95 at bulk and surface, respectively. 2. Favourable morphological characteristics: monodisperse, nanosized faceted particles, higher surface area, better porosity and pore volume. 3. Coformation of minimal NiTiO₃ phase. 4. First principles calculations by density functional theory (DFT) over pure NiO revealed it’s electronic and band structure that helped in understanding its behavior as PL quencher and its contribution in visible light absorption of composites, 5. Valence band offset (ΔE_v) and conduction band offset (ΔE_c) at NiO and TiO₂ heterojunction was calculated to be 0.53 eV and 0.93 eV, respectively. 6. Type-II band alignment was derived at the interface and mechanism was proposed, 7. The effective forbidden gap was deduced to be 3.2(E_g,TiO2) − ΔE_v = 3.6(E_g NiO) − ΔE_c = 2.67 eV. Thus, a composite of pn oxides offers lower band gap energy of 2.67 eV as compared to both pure oxides TiO₂ and NiO with effectiveness in charge separation across the pn junction leading to efficient improved photocatalyst.

中文翻译：

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具有II型能带取向的NiO：TiO ₂复合材料中的pn异质结，辅助阳光驱动的光催化H ₂生成

为了强调pn异质结在复合材料中的作用，详细研究了两种复合材料NTC11（NiO和TiO ₂摩尔比为1：1）和NTC36（0.35：0.65）对H ₂析出反应的光催化性能，并与该组分进行了比较。溶胶-凝胶法制备的纯氧化物NiO，TiO ₂和NiTiO ₃。在所有样品中，NTC11的活性最明显（比纯TiO _2高22倍，NiO的活性可忽略不计），并且在阳光下重复循环60 h产生可重现的H ₂产量，从而证实了其光稳定性。最大光催化制氢量@ 0.6 L / h / m ²在Pt（）上观察到表观量子效率（AQE）为5.4％，太阳燃料效率（SFE）为0.8％，在紫外线可见辐射下为1.4 L / h / m ²，在紫外线可见辐射下的AQE为7.8％ 1wt％）/ NTC11。合成条件，煅烧温度和相组成非常关键，并且在确定总氢产率中起着至关重要的作用。发现NiTiO _3的存在对NTC36的光催化活性是减损的。导致NTC11上阳光辅助氢生成速率提高的关键因素是1.通过随时间监测的电荷载流子寿命的延长，可以明显看出NTC11中pn纳米结的形成。分辨光致发光，NiO和TiO ₂均匀分布Ni / Ti在体积和表面的比例分别为0.99和0.95的纳米颗粒。2.良好的形态特征：单分散，纳米大小的刻面颗粒，更大的表面积，更好的孔隙率和孔体积。3.最小NiTiO ₃相的形成。4.第一原理计算超过纯粹的NiO密度泛函理论（DFT）透露它是有助于理解其作为PL淬灭行为及其复合材料的可见光吸收的贡献电子和带结构，五价带偏移（ΔE _v）和导带偏移（ΔE _ç在的NiO和TiO）₂计算出的异质结分别为0.53 eV和0.93 eV。6. II型能带排列在中导出接口和提出的机制，7.有效禁带推断为3.2（E_{克，二氧化钛}） - ΔE _v  = 3.6（E_克 的NiO） - ΔE _{C ^}  = 2.67电子伏特。因此，与纯氧化物TiO ₂和NiO相比，pn氧化物的复合物提供了较低的2.67eV的带隙能量，并且在跨pn结的电荷分离方面有效，从而导致有效改进的光催化剂。