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    Hong, Frank T. (2022-11) [Dissertation]
    Advisor: Sarathy, Mani
    Committee members: Castaño, Pedro; Hadjichristidis, Nikos; Kim, Seong H.; Samad, Mohammed Abdul
    Parasitic friction and material wear exist in all moving parts, causing about 20% in global energy loss annually. Machinery startup accounts for a major portion of this loss. This issue involves a boundary lubrication problem, where rubbing surfaces are inadequately covered by lubricating oils. Lubricating oil fluids rely on tribochemical reactions to establish metalorganic tribofilms that protect the contacting surfaces. The improved oil lubrication mechanism can ensure smooth operation, improving efficiency, and extending the mechanical component lifetime. In this thesis, we study tribochemical reactions resulting from various fuel and oil blends. The interactions among blended additives are given particular attention. Lubrication phenomena are simulated using a ball-on-disk linear reciprocation configuration in a standardized tribological test rig, Optimol SRV5. The tribofilm growth patterns are investigated by measuring friction and electrical contact resistance (ECR), followed by a detailed surface analysis. The proposed lubrication mechanisms are verified with experimental and numerical simulation results. Fuel lubrication studies are conducted by investigating a) lubricity loss upon the addition of multiple oxygenated compounds, b) accelerated material wear rates observed in dieselethanol fuel blends, and c) enhanced lubrication performances with carbon-based nanofluid fuels. Lubricity loss is found to correlate with: ● Extended induction periods for ECR rises, ● Reduced average electrical contact resistance values, and ● Inhibitions of protective frictional species formations (e.g., iron oxides and graphite). The developed tribochemical reaction model advances the design of friction and extremepressure modifiers using tribo-active nanomaterials. For instance, adding carbon-based nanomaterials to fuels enhances lubrication performance by serving as tribo-active materials to accelerate tribofilm formation and by replenishing damaged surfaces. In engine oil systems, we demonstrated that the lubrication performance could be enhanced by formulating TiO2 nanoparticles modified by gallic acid esters, and polyether-based co(ter)polymers. Based on the tribochemical reaction mechanisms found in this study, we propose more designs of functionalized nanomaterials for advanced lubricant applications in future work.
  • Molecular engineering of intrinsically microporous polybenzimidazole for energy-efficient gas separation

    Abdulhamid, Mahmoud A.; Hardian, Rifan; Bhatt, Prashant; Datta, Shuvo Jit; Ramirez, Adrian; Gascon, Jorge; Eddaoudi, Mohamed; Szekely, Gyorgy (Applied Materials Today, Elsevier BV, 2021-12-04) [Article]
    Polybenzimidazole (PBI) is a high-performance polymer that exhibits high thermal and chemical stability. However, it suffers from low porosity and low fractional free volume, which hinder its application as separation material. Herein, we demonstrate the molecular engineering of gas separation materials by manipulating a PBI backbone possessing kinked moieties. PBI was selected as it contains NH groups which increase the affinity towards CO$_2$, increase sorption capacity, and favors CO$_2$ over other gasses. We have designed and synthesized an intrinsically microporous polybenzimidazole (iPBI) featuring a spirobisindane structure. Introducing a kinked moiety in conjunction with crosslinking enhanced the polymer properties, markedly increasing the gas separation performance. In particular, the BET surface area of PBI increased 30-fold by replacing a flat benzene ring with a kinked structure. iPBI displayed a good CO$_2$ uptake of 1.4 mmol g$^{−1}$ at 1 bar and 3.6 mmol g$^{−1}$ at 10 bar. Gas sorption uptake and breakthrough experiments were conducted using mixtures of CO$_2$/CH$_4$ (50%/50%) and CO$_2$/N$_2$ (50%/50%), which revealed the high selectivity of CO$_2$ over both CH$_4$ and N$_2$. The obtained CO$_2$/N$_2$ selectivity is attractive for power plant flue gas application requiring CO$_2$ capturing materials. Energy and process simulations of biogas CO$_2$ removal demonstrated that up to 70% of the capture energy could be saved when iPBI was used rather than the current amine technology (methyl diethanolamine [MDEA]). Similarly, the combination of iPBI and MDEA in a hybrid system exhibited the highest CO$_2$ capture yield (99%), resulting in nearly 50% energy saving. The concept of enhancing the porosity of PBI using kinked moieties provides new scope for designing highly porous polybenzimidazoles for various separation processes.
  • Room-temperature multiple ligands-tailored SnO2 quantum dots endow in situ dual-interface binding for upscaling efficient perovskite photovoltaics with high VOC.

    Ren, Zhiwei; Liu, Kuan; Hu, Hanlin; Guo, Xuyun; Gao, Yajun; Fong, Patrick W K; Liang, Qiong; Tang, Hua; Huang, Jiaming; Zhang, Hengkai; Qin, Minchao; Cui, Li; Chandran, Hrisheekesh Thachoth; Shen, Dong; Lo, Ming-Fai; Ng, Annie; Surya, Charles; Shao, Minhua; Lee, Chun-Sing; Lu, Xinhui; Laquai, Frédéric; Zhu, Ye; Li, Gang (Light, science & applications, Springer Science and Business Media LLC, 2021-12-03) [Article]
    The benchmark tin oxide (SnO2) electron transporting layers (ETLs) have enabled remarkable progress in planar perovskite solar cell (PSCs). However, the energy loss is still a challenge due to the lack of "hidden interface" control. We report a novel ligand-tailored ultrafine SnO2 quantum dots (QDs) via a facile rapid room temperature synthesis. Importantly, the ligand-tailored SnO2 QDs ETL with multi-functional terminal groups in situ refines the buried interfaces with both the perovskite and transparent electrode via enhanced interface binding and perovskite passivation. These novel ETLs induce synergistic effects of physical and chemical interfacial modulation and preferred perovskite crystallization-directing, delivering reduced interface defects, suppressed non-radiative recombination and elongated charge carrier lifetime. Power conversion efficiency (PCE) of 23.02% (0.04 cm2) and 21.6% (0.98 cm2, VOC loss: 0.336 V) have been achieved for the blade-coated PSCs (1.54 eV Eg) with our new ETLs, representing a record for SnO2 based blade-coated PSCs. Moreover, a substantially enhanced PCE (VOC) from 20.4% (1.15 V) to 22.8% (1.24 V, 90 mV higher VOC, 0.04 cm2 device) in the blade-coated 1.61 eV PSCs system, via replacing the benchmark commercial colloidal SnO2 with our new ETLs.
  • BC6P Monolayer: Isostructural and Isoelectronic Analogues of Graphene with Desirable Properties for K-Ion Batteries

    Tang, Meng; Wang, Cong; Schwingenschlögl, Udo; Yang, Guochun (Chemistry of Materials, American Chemical Society (ACS), 2021-12-03) [Article]
    K-ion batteries are interesting alternatives to Li-ion batteries because of the earth-abundance of K and the similar chemistry between K and Li. However, a lack of high-performance anode materials is a major obstacle to the development of K-ion batteries. We show that the BC6P monolayer, which is isostructural and isoelectronic to graphene due to charge compensation between the constituent elements, can fill this gap. The capacity is found to be 1410 mAh/g (BC6PK6), i.e., about four times that of graphite. The diffusion barrier is as low as 0.13 eV and the average open-circuit voltage is as low as 0.35 V, ensuring high rate performance and high safety, respectively. Metallic states induced by K adsorption provide electrical conductivity during the battery cycle.
  • Growth of Two-Dimensional Materials at the Wafer Scale

    Xu, Xiangming; Guo, Tianchao; Kim, Hyunho; Hota, Mrinal Kanti; Alsaadi, Rajeh S.; Lanza, Mario; Zhang, Xixiang; Alshareef, Husam N. (Advanced Materials, Wiley, 2021-12-03) [Article]
    Wafer-scale growth has become a critical bottleneck for scaling up applications of van der Waal (vdW) layered two-dimensional (2D) materials in high-end electronics and optoelectronics. Most vdW 2D materials were initially obtained through top-down synthesis methods, such as exfoliation, which can only prepare small flakes on a micrometer scale. Bottom-up growth can enable 2D flake growth over a large area. However, seamless merging of these flakes to form large-area continuous films with well-controlled layer thickness and lattice orientation is still a significant challenge. In this review, we briefly introduce several vdW layered 2D materials covering their lattice structures, representative physical properties, and potential roles in large-scale applications. Then, several methods used to grow vdW layered 2D materials at the wafer-scale are reviewed in depth. In particular, we summarize three strategies that enable 2D film growth with a single-crystalline structure over the whole wafer: growth of an isolated domain, growth of unidirectional domains, and conversion of oriented precursors. After that, we review the progress in using wafer-scale 2D materials in integrated devices and advanced epitaxy. Finally, future directions in the growth and scaling of vdW layered 2D materials are discussed.

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