Zhang Hua and Phase Engineering of 2D Nanomaterials — The Frontier of Noble-Metal Nanosheets at CityU Chemistry

City University of Hong Kong (CityUHK) Comprehensive Information Database · 04 Research Module · Deep-Dive Series on Materials Science For an overview and other landmark achievements, see materials-and-engineering-research.md; Zhang Hua's academic honours and academy memberships also appear in named-centres-and-honours.md.

The one-line verdict: Zhang Hua (张华), Yeung Kin Man Chair Professor of Chemistry at CityUHK, pioneered the research paradigm of "Phase Engineering of Nanomaterials (PEN)", centred on the controllable synthesis of unconventional crystal phases in noble-metal 2D nanosheets. His CityUHK Scholars profile, as of 2026, lists more than 686 papers, over 146,974 Scopus citations and an H-index of 191^※; he was elected a Foreign Fellow of the European Academy of Sciences (Academia Europaea) in 2020^※, and received the 2024 BOCHK Science and Technology Innovation Prize (New Materials and New Energy category)^※.

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# Who is Zhang Hua, and why did he choose CityU?

Zhang Hua received his BSc and MSc from Nanjing University in 1992 and 1995 respectively, and earned his PhD in 1998 at Peking University under the supervision of Liu Zhongfan^※. His postdoctoral work took him to KU Leuven in Belgium and Northwestern University in the United States, followed by positions at NanoInk Inc. and Singapore's Institute of Bioengineering and Nanotechnology. He joined Nanyang Technological University (NTU) in Singapore in 2006 and was promoted to full professor in 2013^※. In 2019, Zhang moved to CityUHK full-time as Yeung Kin Man Chair Professor of Chemistry (Nanomaterials)^※, while also holding a chair professorship in the Department of Materials Science and Engineering and serving as Director of the Hong Kong Institute for Clean Energy (HKICE).

The two research threads he had developed at NTU — ultrathin 2D nanomaterials and unconventional crystal phases in noble metals — moved with him wholesale to CityU, where they were further systematised into the formal conceptual framework of "Phase Engineering of Nanomaterials". The interdisciplinary environment of CityU's Chemistry Department, where chemical synthesis, materials characterisation and clean-energy applications are pursued in an integrated manner, was a major factor in his choice. That same cross-disciplinary soil simultaneously nurtured other flagship materials programmes at CityU, notably perovskite solar cells and high-entropy alloys. Zhang's academy memberships and chair professorships are detailed in faculty-and-leaders.md and academicians-and-awards.md.

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# What is "Phase Engineering of Nanomaterials", and what problem does it solve?

Conventional nanomaterial design tunes parameters such as composition, morphology, size, exposed facets and dimensionality, but has largely overlooked crystal phase itself as an independent variable. Zhang's "Phase Engineering of Nanomaterials (PEN)" establishes crystal phase as the sixth structural parameter determining material properties: nanomaterials of identical composition but different phases can display dramatically different — even entirely new — catalytic activity, optical responses, electronic structures and superconducting behaviour.

In May 2020, Zhang and co-authors published the review "Phase engineering of nanomaterials" in Nature Reviews Chemistry (Vol. 4, pp. 243–256, DOI:10.1038/s41570-020-0173-4)^※. The review states plainly: 「纳米材料中那些块体状态下无法获得的非常规晶相，可能赋予其引人入胜的性质与创新应用」 (unconventional crystal phases that cannot be obtained in bulk materials may endow nanomaterials with fascinating properties and novel applications) — it has since accumulated more than 723 Scopus citations^※. By unifying phase-transition research scattered across different material systems under a single framework, this review became the founding document of PEN as a disciplinary direction.

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# How does the CityU team synthesise noble-metal nanosheets with unconventional crystal phases?

In nature, noble metals (gold, platinum, palladium, etc.) exist almost exclusively in the face-centred cubic (fcc) phase — the thermodynamic ground state of bulk material. Zhang's team broke through this constraint at the nanoscale, achieving the first controllable syntheses of unconventional crystal phases in a range of noble metals. Two landmark results stand out.

# Category one: the first synthesis of hcp-phase gold square sheets

Using graphene oxide (GO) flakes as templates, Zhang's group (then at NTU) reported the first in-situ synthesis of gold square sheets with an unconventional hexagonal close-packed (hcp, i.e. 2H) phase. These sheets measured roughly 200–500 nm on a side and about 2.4 nm in thickness — approximately 16 atomic layers of Au. First-principles and molecular dynamics calculations attributed the stabilisation of the hcp structure to a synergy between the unconventional structure itself and strong surface effects. Later work showed that a complete hcp-to-fcc phase transition could be triggered at room temperature via surface ligand exchange^※, published in Nature Communications (2015) — demonstrating that the transformation is driven by surface chemistry.

# Category two: high-yield synthesis of 4H-hexagonal gold nanoribbons

In 2015, Zhang's team published "Stabilization of 4H hexagonal phase in gold nanoribbons" in Nature Communications (6:7684, DOI:10.1038/ncomms8684)^※. The work achieved high-yield synthesis of gold nanoribbons in the 4H hexagonal polytype with an "ABCB" stacking sequence: yield around 60%, via wet-chemical reduction at 58°C for 16 hours^※, with ribbon thicknesses of 2.0–6.0 nm. The low symmetry of 4H-Au gives rise to anisotropic optical properties — monochromated electron energy loss spectroscopy (EELS) detected two sets of surface plasmon resonance peaks (0.27–0.82 eV and 1.72–1.94 eV), markedly different from conventional fcc gold. More importantly, using 4H-Au nanoribbons as templates, the team achieved the first stabilisation of silver (Ag), palladium (Pd) and platinum (Pt) in the 4H hexagonal phase through direct epitaxial growth, opening a new route to phase-controlled synthesis of multi-element noble-metal nanomaterials.

The table below summarises the core parameters of the two landmark results:

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# How does phase engineering boost catalytic performance? The case of Pd-alloy oxygen reduction

Unconventional crystal phases are not merely a structural curiosity — they directly determine catalytic activity. In 2021, Zhang's CityU team published "Seeded Synthesis of Unconventional 2H-Phase Pd Alloy Nanomaterials for Highly Efficient Oxygen Reduction" in the Journal of the American Chemical Society (JACS, DOI:10.1021/jacs.1c08973)^※, with Yiyao Ge as first author.

The study used a seed-mediated synthesis to produce bimetallic PdCu and trimetallic PdCuPt alloy nanocatalysts in the unconventional hexagonal close-packed (2H) phase. The key benchmark: in the electrochemical oxygen reduction reaction (ORR) under alkaline conditions, the 2H-phase PdCuPt ternary alloy achieved a mass activity of 1.92 A mg⁻¹ (Pd+Pt) at 0.9 V — roughly 8.7 times that of a commercial Pd/C catalyst and 19.2 times that of commercial Pt/C. The work showed that phase engineering can operate independently of composition optimisation as a core dimension for tuning the catalytic selectivity and activity of noble-metal nanomaterials.

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# Can phase engineering outperform commercial platinum catalysts in hydrogen-evolution electrodes? The 2023 Nature paper's answer

The JACS result above validated phase engineering for the oxygen reduction reaction (ORR). For the other half of water splitting, the hydrogen evolution reaction (HER), Zhang's team delivered a heavier-weight answer — landing in Nature itself rather than a sister journal.

In September 2023, Zhang, together with Anthony R. J. Kucernak of Imperial College London, CityU's Chun-Sing Lee (李振声) and others, published "Phase-dependent growth of Pt on MoS₂ for highly efficient H₂ evolution" in Nature (Vol. 621, No. 7978, pp. 300–305, DOI:10.1038/s41586-023-06339-3)^※, with Zhenyu Shi, Xiao Zhang and Xiaoqian Lin among the first authors. The study was the first to systematically reveal how the crystal phase of 2D transition metal dichalcogenides (TMDs) dictates the growth morphology of a second material — platinum — deposited on top: 2H-phase MoS₂ templates promote epitaxial cluster growth of Pt nanoparticles, whereas 1T′-phase MoS₂ supports highly dispersed platinum in single-atom form (single-atom Pt), achieving loadings up to 10 wt%^※.

Key performance figures: at an overpotential of −50 mV, the Pt mass activity reached 85 ± 23 A mg⁻¹, with a mass-normalised exchange current density of 127 A mg⁻¹^※. The hydrogen adsorption free energy on Pt atoms atop Mo sites approached the theoretical optimum — the ideal zone for catalytic activity. The catalyst operated stably at room temperature in both proton exchange membrane (PEM) electrolysers and H-type electrolytic cells. CityU's Chemistry Department news release characterised the work as 「拓展了纳米材料相工程的应用范围，为设计高效催化剂铺平道路，服务清洁能源与可持续发展」 (extending the scope of phase engineering of nanomaterials, paving the way for designing efficient catalysts in service of clean energy and sustainable development)^※.

The significance of this work lies in advancing phase engineering from "synthesising a new crystal phase" to "using phase differences to precisely control how another material grows at the atomic level" — in other words, extending phase engineering from the study of materials in themselves into a general tool for catalyst design. Another CityU materials science achievement named among Hong Kong's "Top Ten Science and Technology News" (the bio-inspired vision sensor led by He Yaowei's team) belongs to the same year's representative breakthroughs from the department; the two are discussed side by side in 科研成就总览.

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# Is phase engineering limited to metals? What breakthroughs have come in transition metal dichalcogenides?

The reach of PEN extends well beyond noble metals; transition metal dichalcogenides (TMDs such as MoS₂ and WS₂) form another major battleground. In 2021, Zhang's team and collaborators published a study in Nature Materials (DOI:10.1038/s41563-021-00971-y) on a universal synthetic method for 1T′-phase TMDs^※, with Zhuangchai Lai, Qiyuan He and others among the first authors.

The work established a general method for synthesising high-purity 1T′ phase materials, successfully producing 1T′ WS₂, WSe₂, MoS₂, MoSe₂ and their alloys. The key finding: 1T′-WS₂ displays thickness-dependent superconductivity — at 90.1 nm thickness, the superconducting transition temperature Tc reaches 8.6 K, while at the monolayer limit it drops to 5.7 K. This behaviour originates in the 1T′ phase's exceptionally high intrinsic carrier concentration and semimetallic character. All the as-synthesised 1T′ materials could be converted back to the 2H phase by thermal annealing, demonstrating that the phase transition is reversible and controllable. This work extended PEN from noble-metal systems into semiconductor and superconducting materials.

In 2024, Zhang's team published a perspectives article in National Science Review (11(9):nwae289, DOI:10.1093/nsr/nwae289)^※, systematically reviewing progress in PEN across noble metals and TMDs, and identifying three key challenges: the formation mechanisms of unconventional phases still rely on empirical approaches, the operational stability of metastable phases, and AI-assisted scale-up synthesis.

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# How did PEN grow from a single review into an entire literature system?

If the 2020 Nature Reviews Chemistry review merely "proposed a concept", then three major reviews appearing in quick succession between 2023 and 2026 mark the point where "Phase Engineering of Nanomaterials" sedimented from Zhang's personal research agenda into a recognised sub-discipline with a complete literature lineage, citable and teachable by peers worldwide.

• In 2023, a 32-member team led by Zhang published "Recent Progress on Phase Engineering of Nanomaterials" in Chemical Reviews (123(23):13489–13692, DOI:10.1021/acs.chemrev.3c00459)^※. Running to over 200 pages, the review systematically catalogued direct synthesis strategies and phase-transition control methods for unconventional phases across material classes (metals, semiconductors, TMDs and more). All 32 authors came from CityU's Chemistry Department, making it the largest single review on PEN to date.
• In 2024, Zhang's team published "Phase Engineering of Nanomaterials: Transition Metal Dichalcogenides" in Chemical Reviews (124(7):4479–4539, DOI:10.1021/acs.chemrev.3c00931)^※, narrowing the focus to TMD systems and covering phase-engineering progress across six application directions: electronics, optoelectronics, catalysis, biomedicine, energy storage and conversion, and ferroelectricity.
• In early 2026, Zhang's team published "Phase Engineering of Nanomaterials (PEN): Evolution, Current Challenges, and Future Opportunities" in Advanced Materials (DOI:10.1002/adma.73260)^※, tracing the evolution of the PEN concept since its proposal in 2020 and looking ahead to the challenges and opportunities of the next phase. A related review appeared in the same year in Acta Physico-Chimica Sinica (物理化学学报), further evidence that the field had reached a mature stage in which international peers continually track and competitively review it.

Taken together with the founding 2020 review and the 2024 NSR perspectives piece, the three reviews form a clear literature chain — "concept proposal → systematic consolidation → sub-field refinement → periodic retrospective" — which is uncommon in the fiercely competitive field of nanomaterials, where most "new concepts" cannot sustain the continued output of multiple top-journal reviews.

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# What academic honours and recognition has Zhang Hua received at CityU?

Zhang Hua's principal honours are summarised below:

According to official sources^※, Zhang also serves as co-Editor-in-Chief of SmartMat and sits on the editorial or advisory boards of more than 20 leading journals, including Chemical Reviews and Nature Materials. His Google Scholar H-index had reached 202 with over 170,000 citations^※ (as of the first half of 2026), placing him among the most-cited scholars in nanomaterials globally.

From 20–22 November 2024^※, CityU's Hong Kong Institute for Clean Energy (HKICE, of which Zhang is Director; for more on the institute see institutes-and-labs.md) partnered with Nature to host the "Nature Conference on Phase Engineering of Nanomaterials 2024" in Hong Kong — the first dedicated Nature Conference on PEN, marking the institutional consolidation of the research direction.

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# What does Zhang Hua's research mean for CityU and for Hong Kong?

The practical impact of Zhang's phase-engineering research operates on two fronts. First, catalysis and energy: 2H-phase PdCuPt surpasses commercial Pt/C in ORR by about 19.2 times, and the high intrinsic conductivity of 1T′-phase TMDs makes them candidate electrode materials for the hydrogen evolution reaction (HER). Second, sensing and biomedicine: the large specific surface area of ultrathin 2D nanosheets offers structural advantages in nanozymes and biosensing. According to his CityU Scholars profile, Zhang currently leads 16 active projects under the HKICE platform^※, funded by the Research Grants Council (RGC) and the Innovation and Technology Fund (ITF), and directly supervises more than 16 doctoral students and postdoctoral fellows at CityU.

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# Research trajectory in summary: from 2D nanosheets to a phase-engineering system

Stringing the above results together reveals a clear evolutionary path:

1. 2D noble-metal nanosheets (2011–2014): graphene-oxide-templated synthesis of hcp-phase gold square sheets, overturning the assumption that noble metals exist only in the fcc phase.
2. 4H-phase gold nanoribbons and epitaxial phase stabilisation (2015): Nature Communications first reports 4H-Au nanoribbons and stabilises the 4H phase in Ag, Pd and Pt for the first time via epitaxial growth.
3. The PEN framework is established (2020): the Nature Reviews Chemistry review establishes "crystal phase" as the sixth structural parameter.
4. TMD phase engineering and superconductivity (2021): Nature Materials reports the universal synthesis of 1T′-WS₂, with a maximum Tc of 8.6 K, extending PEN into semiconductors and quantum materials.
5. Catalytic application validated (2021): JACS reports 2H-phase PdCuPt surpassing commercial platinum catalysts by 19.2 times in ORR.
6. A paper in Nature itself (2023): "Phase-dependent growth of Pt on MoS₂" demonstrates that crystal phase can precisely control the loading morphology of platinum atoms on TMD templates, with Pt mass activity reaching 85 A mg⁻¹.
7. The literature system takes shape (2023–2024): two long Chemical Reviews reviews (one covering PEN across the board, one focused on TMDs) appear in succession; PEN matures from a concept into a sub-discipline.
8. Institutional consolidation (2024): the Nature Conference on Phase Engineering of Nanomaterials 2024 is held in Hong Kong, establishing PEN as a formal research direction.
9. Periodic retrospective (2026): Advanced Materials publishes a review of PEN's evolution and future prospects, marking the field's entry into a mature phase of sustained international attention.

Note on sources and figures: all figures in this article are drawn from published papers or official CityU materials. The "H-index 191/192/202" discrepancy reflects the different counting methodologies and update schedules of Web of Science versus Google Scholar; all are valid values at different points in time. When citing data from this article, please refer to the most recent version under the relevant counting methodology. The "1.92 A mg⁻¹ ORR mass activity at 0.9 V" figure is a laboratory measurement under alkaline conditions; performance under real fuel-cell engineering conditions is subject to multiple additional factors.

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