Novel strategies to design and construct efficient semiconductor-based photocatalyst for enhancing photocatalytic hydrogen evolution and nitrogen fixation under sunlight irradiation

L'énergie solaire est la source d'énergie la plus abondante au monde et elle peut être convertie en énergie chimique via des processus photocatalytiques. Au cours des dernières décennies, la photocatalyse sous la lumière du soleil est apparue comme une alternative innovante aux combustibles fossiles afin de résoudre et prévenir des problèmes graves liés à la crise environnementale et énergétique. Actuellement, les matériaux à base de semi-conducteurs (tels que TiO₂, C₃N₄, In₂O₃, WO₃) sont intensivement étudiés pour diverses applications photocatalytiques, y compris la réaction d’évolution d'hydrogène (HER) et la réduction de l'azote en ammoniac (NRR). Par conséquent, diverses approches telles que l'ingénierie structurelle, les hétérojonctions nanocomposites ont été étudiées afin de surmonter les problèmes de ces matériaux et ainsi augmenter l'activité catalytique. Dans le cadre de cette thèse, nous avons développé des nouvelles stratégies pour la synthèse des quatre types de photocatalyseurs efficaces pour la production d'hydrogène et la fixation de l'azote sous la lumière du soleil. Nos matériaux présentent une structure unique, qui favorise l'absorption de la lumière visible, la séparation des charges électrons-trous et l’augmentation du nombre de sites actifs.Pour l'application de la génération d'hydrogène photocatalytique, nous avons d'abord synthétisé les sphères de type éponge CdI₂nS₄ monophasées via une méthode solvothermique suivie d'un traitement au gaz contenant H₂S. La formation du complexe Cd/In avec une distribution uniforme de Cd²⁺ et In³⁺ a joué un rôle crucial dans la formation du spinelle monophasé CdIn₂S₄. L'énergie de la bande interdite s'est avérée être significativement réduite, ce qui permet une absorption étendue de la lumière visible jusqu'à 700 nm, ceci est principalement attribué à la dispersion d'espèce sulfure sur la bande de valence du CdIn₂S₄ monophasé. Avec le dépôt de Ni métallique comme cocatalyseur de réduction, le photocatalyseur hybride Ni-CdIn₂S₄ a montré une efficacité améliorée pour la production d'hydrogène sous la lumière solaire, ce qui représente une augmentation de l’activité d’environ, respectivement, 5,5 et 3,6 fois que celle des échantillons Pt-CdIn₂S₄ et Pd-CdIn₂S₄. Le deuxième système photocatalytique développé implique la préparation de nitrure de carbone graphitique dopé au S (Ni-SCN). Ce dernier est chimiquement ancré au nickel par une technique connue sous le nom de processus de photo-dépôt assisté par sulfuration. L'origine de la structure distinctive du Ni-SCN est dû à l'existence de liaisons chimiques NiS-C-N dans le système, ce qui favorisait la séparation des charges photogénérées et améliorait la capacité d’absorption lumineuse du photocatalyseur. Par conséquent, l’échantillon NiSCN synthétisé présente une excellente activité photocatalytique pour la production d'hydrogène sous la lumière du soleil. En effet, ce système présente une activité beaucoup plus élevée que celle des systèmes g-C₃N₄ dopés au S, Ni supporté g-C₃N₄ et Pt supporté g-C₃N₄ dopés au S. Pour une application photo (électro) catalytique de fixation de l'azote, nos travaux sont les premiers à rapporter la synthèse de nanoparticules d'Au chargées de nanoparticules W₁₈O₄₉ dopées au Fe (notées WOF-Au) par une synthèse solvothermique suivie d'un dépôt in situ des nanoparticules d'Au. L'incorporation de dopants Fe peut non seulement guérir les états de défaut de masse dans les réseaux non stœchiométriques W₁₈O₄₉, mais également favoriser la séparation et la migration interfaciale des électrons du photocatalyseur vers les molécules N₂ chimisorbées; tandis que les nanoparticules Au décorées sur la surface dopée au Fe W₁₈O₄₉ ont fourni les électrons à haute énergie pour la réduction de N₂ via l'effet de résonance plasmonique de surface localisé (LSPR). Le système WOF-Au plasmonique résultant montre un rendement amélioré pour la production de NH₃, beaucoup plus élevé que celui du W₁₈O₄₉ pur ainsi qu'une très grande stabilité. L'amélioration des performances photoélectrocatalytiques est principalement due à l'effet synergique des dopants Fe et des nanoparticules Au dans l'hôte W₁₈O₄₉. Enfin, les cacahuètes creuses de In₂O₃ dopées au Ru (dénotées Ru-In₂O₃ HPN) ont été fabriquées par la nouvelle stratégie d'auto-matrice suivie de la calcination des précurseurs synthétisés. Les nanoparticules uniformes In₂O₃ sont étroitement agglomérées ensemble pour former une structure de cacahuète creuse, ce qui facilite la séparation et le transport des l'électrons-trous photoexcités, améliorant l’absorption de la lumière par multi-réflexion. De plus, l'introduction des dopants Ru induit de nombreuses lacunes en oxygène à la surface et réduit l'énergie de la bande interdite du système photocatalytique. Ces lacunes d'oxygène agissent comme des centres de piégeage, facilitant la séparation des électrons trous photoexcités. Par conséquent, le taux de production d'ammoniac des Ru-In₂O₃ HPNs est 5,6 fois plus élevé que celui des In₂O₃ HPNs purs et largement supérieur au matériau en vrac d'In₂O₃, lorsqu’il est soumis à l’irradiation solaire.Solar energy is the most abundant energy source in the world, and it can be converted into chemical energy via photocatalytic processes. Over the last decades, sunlight-driven photocatalysis has emerged as an innovative alternative to fossil fuels for solving the severe problems related to environmental diseases and the energy crisis. Currently, semiconductorbased materials (such as TiO₂, C₃N₄, In₂O₃, WO₃, BiVO₄) have been intensively studied for diverse photocatalytic applications, including the hydrogen evolution reaction (HER) and the nitrogen reduction reaction (NRR) to produce ammonia. However, the drawbacks of weak visible light absorption, low electron-hole separation with high recombination rate, and lack of surface active-sites have limited the photocatalytic performance of these semiconductorbased photocatalysts. Therefore, various approaches such as structural engineering, nanocomposite heterojunctions have been applied to overcome the limitations of these materials and boosting the catalytic activity. In this thesis, we employed novel strategies to develop four efficient photocatalytic systems for hydrogen production and nitrogen fixation. Our materials possessed a unique structure, which is advantageous to promote the visiblelight absorption, facilitate the separation of charge carrier, and increase the number of surface-active sites. For the photocatalytic hydrogen evolution application, we firstly synthesized the singlephase CdIn₂S₄ sponge-like spheres via solvothermal method followed by H₂S gas treatment. The formation of CdIn-complex with uniform distribution of Cd²⁺ and In³⁺ played a crucial role in achieving the spinel structured-single phase CdIn₂S₄. The bandgap energy was found to be significantly reduced, resulting in the extended visible light absorption up to 700 nm, which was primarily attributed to the sulfide species-mediated modification of the valence band in CdIn₂S₄ single-phase. With the deposition of Ni metal as a reduction cocatalyst, the hybrid Ni-CdIn₂S₄ photocatalyst showed enhanced solar light-driven photocatalytic hydrogen evolution efficiency, which is around 5.5 and 3.6 folds higher than that of Pt-CdIn₂S₄ and Pd-CdIn₂S₄ samples, respectively. The second developed photocatalytic system involved the preparation of chemically bonded nickel anchored S-doped graphitic-carbon nitride (Ni-SCN) through a technique known as sulfidation assisted photo-deposition process. The origin of the distinctive structure of Ni-SCN was due to the existence of Ni-S-C-N chemical bonds in the system, which fundamentally favored the separation of photogenerated electron-hole and improved the light-harvesting capabilities of the photocatalyst. Consequently, the synthesized Ni-SCN exhibited an excellent sunlight-driven photocatalytic activity toward hydrogen evolution, which was several times higher than Sdoped g-C₃N₄, Ni supported g-C₃N₄ and Pt loaded S-doped C₃N₄ systems. For photo(electro)catalytic nitrogen fixation application, our work is the first to report the synthesis of Au nanoparticles loaded Fe doped W₁₈O₄₉ (denoted as WOF-Au) nanorods through a solvothermal synthesis following by in situ deposition of Au nanoparticles. The incorporation of Fe dopants can not only heal the bulk-defect-states in nonstoichiometric W₁₈O₄₉ lattices but also promote the separation and interfacial migration of electrons from photocatalyst to chemisorbed N₂ molecules; while Au nanoparticles decorated on the Fe doped W₁₈O₄₉ surface provided the high energetic electrons for N₂ reduction via the localized surface plasmon resonance effect (LSPR). The obtained plasmonic WOF-Au system shows an enhanced NH₃ yield, which is much higher than that of the bare W₁₈O₄₉, as well as very high stability. The enhancement in photoelectrocatalytic performance is mainly contributed by the synergetic effect of Fe dopants and plasmonic Au nanoparticles on the W₁₈O₄₉ host. Lastly, Ru doped In₂O₃ hollow peanuts (demoted as Ru-In₂O₃ HPNs) were fabricated by the novel self-template strategy followed by the calcination of the as-synthesis precursors. The uniform In₂O₃ nanoparticles were closely packed together to form a hollow peanut structure, which facilitated the separation and transportation of photoinduced electron-hole and favored the light-harvesting ability by the internal multi-reflection process. Furthermore, the introduction of Ru dopants induced numerous surface oxygen vacancies and narrow down the bandgap energy of the photocatalytic system. These oxygen vacancies act as trapping centers, facilitating the separation of photoexcited electrons and holes. Consequently, the ammonia production rate of Ru-In₂O₃ HPNs was 5.6 times and much higher as compared to pure In₂O₃ HPNs and bulk material of In₂O₃ under solar light irradiation

REGIONAL LINKAGE IN TOURISM: THE CASE OF VIETNAM

Abstract. This research is conducted for investigating the current situations of regional linkage in tourism development in the areas of Midlands and Northern Mountains in Vietnam. Data were collected from a survey of 755 people, including officials from State management bodies in charge of tourism, officials and staffs at tourism resorts, tourism firms, tourism scientists and tourists. In addition, we conducted focus group and interviewed tourism agency officials and tourism firms in the Midland and Mountainous provinces of Vietnam. The results show that tourism development in Vietnam in general and the Northwest region in particular is extremely fragmented, not yet forming a regional linkage; regional and national tourism development projects are just formalistic. Some causes are the limited regional integration policy, lack of appropriate regional governance mechanisms and inactive participation of the private sector in regional integration. Based on the findings, we propose a tourism sector linkage model; besides, policyimplications are given for fulfilling the linkage policies in Vietnam in particular, and more broadly for emerging countries in general.Keywords: Midlands and Northern Mountains, tourism linkage, Vietnam

Survey on Vietnamese teachers’ perspectives and perceived support during COVID-19

The COVID-19 pandemic has caused unprecedented damage to the educational system worldwide. Besides the measurable economic impacts in the short-term and long-term, there is intangible destruction within educational institutions. In particular, teachers – the most critical intellectual resources of any schools – have to face various types of financial, physical, and mental struggles due to COVID-19. To capture the current context of more than one million Vietnamese teachers during COVID-19, we distributed an e- survey to more than 2,500 randomly selected teachers from two major teacher communities on Facebook from 6th to 11th April 2020. From over 373 responses, we excluded the observations which violated our cross-check questions and retained 294 observations for further analysis. This dataset includes: (i) Demographics of participants; (ii) Teachers' perspectives regarding the operation of teaching activities during the pandemic; (iii) Teachers' received support from their schools, government bodies, other stakeholders such as teacher unions, and parents' associations; and (iv) teachers' evaluation of school readiness toward digital transformation. Further, the dataset was supplemented with an additional question on the teachers' primary source of professional development activities during the pandemic

Economic linkage in key economic zones: The case of Vietnam

This study was conducted to measure the level of economic linkage in key economic zones in central Vietnam generally and to measure the level of economic linkage in the tourism sector. Secondary data were collected from the Statistical Yearbook published by the General Statistics Office and the Local Bureau of Statistics and primary data collected from surveys of 102 hotels and tourism firms in the center of Vietnam. We used the Moran Index (I) to quantify overall economic aggregation across the region and that scoring method was used to measure the degree of linkage in regional tourism. The results show that the level of overall regional economic linkages was very low and this was also relevant when analyzing the tourism sector in some areas of the center of Vietnam. Based on the results, some recommendations are given for strengthening economic linkages in key economic regions of Vietnam

Insights into the Recent Progress and Advanced Materials for Photocatalytic Nitrogen Fixation for Ammonia (NH₃) Production

Ammonia (NH3) is one of the key agricultural fertilizers and to date, industries are using the conventional Haber-Bosh process for the synthesis of NH3 which requires high temperature and energy. To overcome such challenges and to find a sustainable alternative process, researchers are focusing on the photocatalytic nitrogen fixation process. Recently, the effective utilization of sunlight has been proposed via photocatalytic water splitting for producing green energy resource, hydrogen. Inspired by this phenomenon, the production of ammonia via nitrogen, water and sunlight has been attracted many efforts. Photocatalytic N2 fixation presents a green and sustainable ammonia synthesis pathway. Currently, the strategies for development of efficient photocatalyst for nitrogen fixation is primarily concentrated on creating active sites or loading transition metal to facilitate the charge separation and weaken the N⁻N triple bond. In this investigation, we review the literature knowledge about the photocatalysis phenomena and the most recent developments on the semiconductor nanocomposites for nitrogen fixation, following by a detailed discussion of each type of mechanism

Approximation of Capacity for Downlink Multi-User System with Combination of Precoding and NOMA Methods

Enhancing performance of downlink MU systems is an attractive and important research for future wireless systems. The non-orthogonal multiple access (NOMA) method was proposed to improve the performance of MU systems. In order to further improve the outage probability (OP) and ergodic capacity (EC) of downlink NOMA MU systems, we propose the combination of precoding and NOMA methods, and then the OP and EC of MU systems with our novel method are derived in scenarios of perfect and imperfect successive interference cancellation (SIC) scheme. Moreover, the closed-form expression of OP and EC for both scenarios is theoretically derived and compared with Monte Carlo simulations. The results show that, the analysis method is accurate, and the proposed combining precoding and NOMA can further enhance the performance of MU systems in comparing with the original orthogonal multiple access method

Chemically Bonded Ni Cocatalyst onto the S Doped g‑C₃N₄ Nanosheets and Their Synergistic Enhancement in H₂ Production under Sunlight Irradiation

Nickel deposited S-doped carbon nitride (Ni–S:g-C₃N₄/Ni-SCN) nanosheets have been synthesized using calcination followed by a sulfidation process. X-ray photoelectron spectra revealed that the doped S atoms are successfully introduced into the 301 lattices of the host g-C₃N₄. XPS spectra indicated that the deposited Ni species are chemically bonded onto the host SCN nanosheets through sulfur bonds. The sunlight-driven photocatalytic hydrogen production efficiency of the synthesized Ni-SCN nanosheets is found to be 3628 μmol g^–1 h^–1, which is around 1.5 folds higher than that of Pt-SCN that synthesized in the present study. The observed efficiency is attributed to the chemical bonding of Ni through S that largely favored the photocatalytic process in terms of charge-separation as well as self-catalytic reactions. The apparent quantum efficiency of the photocatalyst at 420 nm is estimated to be 17.2%, which is relatively one of the higher values reported in the literature. The photocatalytic recyclability results showed consistent hydrogen evolution efficiency over 4 cycles (8 h) that revealed the excellent stability of the photocatalyst. This work has demonstrated that the chemical bonding of cocatalyst onto the host photocatalyst is relatively an effective strategy as compared to the conventional deposition of cocatalyst by means of electrostatic or van der Waals forces

Top economics universities and research institutions in Vietnam: evidence from the SSHPA dataset

Economic research is vital for creating more suitable policies to facilitate economic growth. Employing a combination of descriptive and Bayesian analyses, this paper investigates the research landscape of the economics discipline in Vietnam, in particular, the leading affiliations in the field and how these institutions compare to each other in terms of productivity, the number of lead authors, new authors and publications' journal impact factor. We also examine the differences in the authors' productivity based on their age and gender. The dataset extracted from the SSHPA database includes 1,444 articles. The findings show that among top producers of economic research in Vietnam, seven are universities, leaving only one representative of research institutes. These top producers account for 52% of research output among 178 institutes recorded in the database. We also find a correlation between a researcher's affiliation, sex, and scientific productivity in Vietnam's economic discipline. Overall, publications by male researchers outnumber those by female ones in most of the top affiliations. The findings also indicate that 40–44 is the age group with the highest scientific productivity. Researchers' collaboration, which is observed through co-authorship, is on the rise in all of the top eight economic research affiliations. However, the quality of current Vietnam's scientific works in the discipline is questionable. Therefore, it is suggested that in order to sustain scientific productivity, economic researchers might need to balance the quantity and quality of their contributions.Scientific collaboration; scientific publishing; economics discipline; Vietnam; Bayesian analysis.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Adsorptive Removal of Copper by Using Surfactant Modified Laterite Soil

Removal of copper ion (Cu2+) by using surfactant modified laterite (SML) was investigated in the present study. Characterizations of laterite were examined by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma mass spectrometry (ICP-MS), and total carbon analysis. The optimum conditions for removal of Cu2+ by adsorption using SML were systematically studied and found as pH 6, contact time 90 min, adsorbent dosage 5 mg/mL, and ionic strength 10 mM NaCl. The equilibrium concentration of copper ions was measured by flame atomic absorption spectrometry (F-AAS). Surface modification of laterite by anionic surfactant sodium dodecyl sulfate (SDS) induced a significant increase of the removal efficiency of Cu2+. The surface modifications of laterite by preadsorption of SDS and sequential adsorption of Cu2+ were also evaluated by XRD and FT-IR. The adsorption of Cu2+ onto SML increases with increasing NaCl concentration from 1 to 10 mM, but at high salt concentration this trend is reversed because desorption of SDS from laterite surface was enhanced by increasing salt concentration. Experimental results of Cu2+/SML adsorption isotherms at different ionic strengths can be represented well by a two-step adsorption model. Based on adsorption isotherms, surface charge effects, and surface modification, we suggest that the adsorption mechanism of Cu2+ onto SML was induced by electrostatic attraction between Cu2+ and the negatively charged SML surface and nonelectrostatic interactions between Cu2+ and organic substances in the laterite