Register      Login
Australian Journal of Chemistry Australian Journal of Chemistry Society
An international journal for chemical science
REVIEW

Novel design strategies of three-dimensional MXene structures and their applications in metal-ion hybrid capacitors

Lingfang Li https://orcid.org/0000-0002-4085-5535 A , Bin Zeng https://orcid.org/0000-0002-1014-6316 A * , Chuang Xiang A and Wen Liu A
+ Author Affiliations
- Author Affiliations

A College of Mechanical Engineering, Hunan University of Arts and Science, Changde 415000, China.




Lingfang Li received her PhD from Hunan University in 2016, and then worked as an Associate Professor at the Hunan University of Arts and Science (HUAS) in Changde, Hunan. She then became a full Professor at HUAS in 2020. Her research interests include the controllable fabrication and assembly of electrode materials, and the fundamentals, principles, and applications of supercapacitors and lithium-ion batteries.



Bin Zeng received his PhD from Hunan University in 2013, and then worked as an Associate Professor at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China. He then became a full Professor at HUAS in 2019. His research interests include catalytic materials and electrospinning technology.



Xiang Chuang received his PhD from Central South University in 2020, and then worked as a Lecturer at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China.



Liu Wen received his PhD from Hunan University in 2022, and then worked as a Lecturer at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China.

* Correspondence to: 21467855@qq.com

Handling Editor: Xinhua Wan

Australian Journal of Chemistry 76(11) 746-759 https://doi.org/10.1071/CH23090
Submitted: 19 May 2023  Accepted: 23 August 2023  Published online: 20 September 2023

© 2023 The Author(s) (or their employer(s)). Published by CSIRO Publishing.

Abstract

MXene is a novel two-dimensional material that exhibits excellent competitive performance in energy storage and conversion applications due to its high electrical conductivity, good dispersibility, and abundant surface functional groups. However, the van der Waals interactions between MXene nanosheets tend to lead to stacking, which limits the number of active sites and ion dynamics. Constructing MXene materials into three-dimensional (3D) porous structures is an effective strategy to improve energy storage performance by increasing specific surface area and porosity, and decreasing ion transport distance. This review provides an overview of four novel design strategies for preparing three-dimensional MXene materials, including template-based, 3D printing, electrospinning, and gas-assisted methods, over the last 5 years (2019–2023), and explores the potential applications of 3D MXene structures in the new-type energy storage systems of metal-ion hybrid capacitors. Finally, the authors provide prospects for the future development of 3D MXene structures.

Keywords: 3D structures, design strategies, energy storage applications, hybrid capacitors, MXene materials, perspective, porous materials, template method.

Biographies

CH23090_B1.gif

Lingfang Li received her PhD from Hunan University in 2016, and then worked as an Associate Professor at the Hunan University of Arts and Science (HUAS) in Changde, Hunan. She then became a full Professor at HUAS in 2020. Her research interests include the controllable fabrication and assembly of electrode materials, and the fundamentals, principles, and applications of supercapacitors and lithium-ion batteries.

CH23090_B2.gif

Bin Zeng received his PhD from Hunan University in 2013, and then worked as an Associate Professor at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China. He then became a full Professor at HUAS in 2019. His research interests include catalytic materials and electrospinning technology.

CH23090_B3.gif

Xiang Chuang received his PhD from Central South University in 2020, and then worked as a Lecturer at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China.

CH23090_B4.gif

Liu Wen received his PhD from Hunan University in 2022, and then worked as a Lecturer at the Hunan University of Arts and Science (HUAS) in Changde, Hunan, China.

References

Ding H, Li Y, Li M, et al. Chemical scissor-mediated structural editing of layered transition metal carbides. Science 2023; 379(6637): 1130-1135.
| Crossref | Google Scholar | PubMed |

Mojtabavi M, Tsai WY, VahidMohammadi A, et al. Ionically active MXene nanopore actuators. Small 2022; 18(11): 2105857.
| Crossref | Google Scholar | PubMed |

VahidMohammadi A, Rosen J, Gogotsi Y. The world of two-dimensional carbides and nitrides (MXenes). Science 2021; 372(6547): eabf1581.
| Crossref | Google Scholar | PubMed |

Han M, Zhang D, Singh A, et al. Versatility of infrared properties of MXenes. Mater Today 2023; 64: 31-39.
| Crossref | Google Scholar |

Lounasvuori M, Sun Y, Mathis TS, et al. Vibrational signature of hydrated protons confined in MXene interlayers. Nat Commun 2023; 14(1): 1322.
| Crossref | Google Scholar | PubMed |

Naguib M, Barsoum MW, Gogotsi Y. Ten years of progress in the synthesis and development of MXene. Adv Mater 2021; 33: 2103393.
| Crossref | Google Scholar | PubMed |

Anasori B, Lukatskaya MR, Gogotsi Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat Rev Mater 2017; 2(2): 16098.
| Crossref | Google Scholar |

Lim KRG, Shekhirev M, Wyatt BC, et al. Fundamentals of MXene synthesis. Nat Synth 2022; 1(8): 601-614.
| Crossref | Google Scholar |

Ding L, Jiang R, Tang Z, et al. MXene: nanoengineering and application as electrode materials for supercapacitors. J Inorg Mater 2023; 38: 619-633 [In chinese].
| Crossref | Google Scholar |

10  Li G, Wyatt BC, Song F, et al. 2D titanium carbide (MXene) based films: expanding the frontier of functional film materials. Adv Funct Mater 2021; 31(46): 2105043.
| Crossref | Google Scholar |

11  Ghassemi H, Harlow W, Mashtalir O, et al. In situ environmental transmission electron microscopy study of oxidation of two-dimensional Ti3C2 and formation of carbon-supported TiO2. J Mater Chem A 2014; 2: 14339-14341.
| Crossref | Google Scholar |

12  Hu M, Li Z, Zhang H, et al. Self-assembled Ti3C2Tx MXene film with high gravimetric capacitance. Chem Commun 2015; 51: 13531-13533.
| Crossref | Google Scholar | PubMed |

13  Wang Y, Dou H, Wang J, et al. Three-dimensional porous MXene/layered double hydroxide composite for high performance supercapacitors. J Power Sources 2016; 327: 221-228.
| Crossref | Google Scholar |

14  Ren CE, Zhao M-Q, Makaryan T, et al. Porous two-dimensional transition metal carbide (MXene) flakes for high-performance Li-ion storage. ChemElectroChem 2016; 3: 689-693.
| Crossref | Google Scholar |

15  Zhao MQ, Xie X, Ren CE, et al. Hollow MXene spheres and 3D macroporous MXene frameworks for Na-ion storage. Adv Mater 2017; 29: 1702410.
| Crossref | Google Scholar | PubMed |

16  Yue Y, Liu N, Ma Y, et al. Highly self-healable 3D microsupercapacitor with MXene-graphene composite aerogel. ACS Nano 2018; 12: 4224-4232.
| Crossref | Google Scholar | PubMed |

17  Xia Y, Mathis TS, Zhao MQ, et al. Thickness-independent capacitance of vertically aligned liquid-crystalline MXenes. Nature 2018; 557: 409-412.
| Crossref | Google Scholar | PubMed |

18  Shang T, Lin Z, Qi C, et al. 3D macroscopic architectures from selfassembled MXene hydrogels. Adv Funct Mater 2019; 29: 1903960.
| Crossref | Google Scholar |

19  Sambyal P, Iqbal A, Hong J, et al. Ultralight and mechanically robust Ti3C2Tx hybrid aerogel reinforced by carbon nanotubes for electromagnetic interference shielding. ACS Appl Mater Interfaces 2019; 11: 38046-38054.
| Crossref | Google Scholar | PubMed |

20  Lin Z, Liu J, Peng W, et al. Highly stable 3D Ti3C2Tx MXene-based foam architectures toward high-performance terahertz radiation shielding. ACS Nano 2020; 14: 2109-2117.
| Crossref | Google Scholar | PubMed |

21  Bu F, Zagho MM, Ibrahim Y, et al. Porous MXenes: synthesis, structures, and applications. Nano Today 2020; 30: 100803.
| Crossref | Google Scholar |

22  Cheng Y, Li L, Liu Z, et al. 3D porous MXene aerogel through gas foaming for multifunctional pressure sensor. Research 2022; 2022: 9843268.
| Crossref | Google Scholar |

23  Chen N, Duan Z, Cai W, et al. Supercritical etching method for the large-scale manufacturing of MXenes. Nano Energy 2023; 107: 108147.
| Crossref | Google Scholar |

24  Wang D, Zhang D, Li P, et al. Electrospinning of flexible poly(vinylalcohol)/MXene nanofiber‐based humidity senso self‐powered by monolayer molybdenum diselenide piezoelectric nanogenerator. Nano Micro Lett 2021; 13: 1-13.
| Crossref | Google Scholar |

25  Dong Y, Shi H, Wu ZS. Recent advances and promise of MXene‐based nanostructures for high‐performance metal ion batteries. Adv Funct Mater 2020; 30(47): 2000706.
| Crossref | Google Scholar |

26  Thomas SA, Patra A, Al-Shehri BM, et al. MXene based hybrid materials for supercapacitors: recent developments and future perspectives. J Energy Storage 2022; 55: 105765.
| Crossref | Google Scholar |

27  Nahirniak S, Ray A, Saruhan B. Challenges and Future Prospects of the MXene-Based Materials for Energy Storage Applications. Batteries 2023; 9(2): 126.
| Crossref | Google Scholar |

28  Jiang J, Li F, Zou J, et al. Three-dimensional MXenes heterostructures and their applications. Sci China Mater 2022; 65: 2895-2910.
| Crossref | Google Scholar |

29  Zhang J, Jiang D, Liao L, et al. Ti3C2Tx MXene based hybrid electrodes for wearable supercapacitors with varied deformation capabilities. Chem Eng J 2022; 429: 132232.
| Crossref | Google Scholar |

30  Chen B, Zhang L, Li H, et al. Skin-inspired flexible and high-performance MXene@ polydimethylsiloxane piezoresistive pressure sensor for human motion detection. J Colloid Interf Sci 2022; 617: 478-488.
| Crossref | Google Scholar | PubMed |

31  Chang X, Zhu Q, Zhao Q, et al. 3D porous Co3O4/MXene foam fabricated via a sulfur template strategy for enhanced Li/K-Ion storage. ACS Appl Mater Interfaces 2023; 15(6): 7999-8009.
| Crossref | Google Scholar | PubMed |

32  Zhou J, Pei Z, Sui Z, et al. Hierarchical porous and three-dimensional MXene/SiO2 hybrid aerogel through a sol-gel approach for lithium–sulfur batteries. Molecules 2022; 27(20): 7073.
| Crossref | Google Scholar | PubMed |

33  Shi S, Qian B, Wu X, et al. Self‐assembly of MXene‐surfactants at liquid-liquid interfaces: from structured liquids to 3D aerogels. Angew Chem Int Ed 2019; 58(50): 18171-18176.
| Crossref | Google Scholar | PubMed |

34  Zhao S, Li L, Zhang HB, et al. Janus MXene nanosheets for macroscopic assemblies. Mater Chem Front 2020; 4(3): 910-917.
| Crossref | Google Scholar |

35  Fan S, Sun T, Jiang M, et al. Enhanced thermoelectric performance of MXene/GeTe through a facile freeze-drying method. J Alloys Compd 2023; 948: 169807.
| Crossref | Google Scholar |

36  Zhang Y, He Y, Zhou Y, et al. Fabrication of RGO/CNTs/MXene 3D skeleton structure for enhancing thermal and tribological properties of epoxy composites. Tribol Inter 2023; 179: 108172.
| Crossref | Google Scholar |

37  Chen Q, Wei Y, Zhang X, et al. Vertically aligned mxene nanosheet arrays for high-rate lithium metal anodes. Adv Energy Mater 2022; 12(18): 2200072.
| Crossref | Google Scholar |

38  Sun L, Song G, Sun Y, et al. MXene/N-doped carbon foam with three-dimensional hollow neuron-like architecture for freestanding, highly compressible all solid-state supercapacitors. ACS Appl Mater Iterfaces 2020; 12(40): 44777-44788.
| Crossref | Google Scholar | PubMed |

39  Ran F, Wang T, Chen S, et al. Constructing expanded ion transport channels in flexible MXene film for pseudocapacitive energy storage. Appl Surf Sci 2020; 511: 145627.
| Crossref | Google Scholar |

40  Yang M, Yuan Y, Li Y, et al. Anisotropic electromagnetic absorption of aligned Ti3C2Tx MXene/gelatin nanocomposite aerogels. ACS Appl Mater Iterfaces 2020; 12(29): 33128-33138.
| Crossref | Google Scholar | PubMed |

41  Li K, Wang X, Li S, et al. An ultrafast conducting polymer@ MXene positive electrode with high volumetric capacitance for advanced asymmetric supercapacitors. Small 2020; 16(4): 1906851.
| Crossref | Google Scholar | PubMed |

42  Song Y, Sun Z, Fan Z, et al. Rational design of porous nitrogen-doped Ti3C2 MXene as a multifunctional electrocatalyst for Li-S chemistry. Nano Energy 2020; 70: 104555.
| Crossref | Google Scholar |

43  Jia X, Shen B, Zhang L, et al. Waterproof MXene-decorated wood-pulp fabrics for high-efficiency electromagnetic interference shielding and Joule heating. Compos B Eng 2020; 198: 108250.
| Crossref | Google Scholar |

44  Qiao H, Qin W, Chen J, et al. AuCu decorated MXene/RGO aerogels towards wearable thermal management and pressure sensing applications. Mater Design 2023; 228: 111814.
| Crossref | Google Scholar |

45  Feng L, Qin W, Wang Y, et al. Ti3C2Tx MXene/Graphene/AuNPs 3D porous composites for high sensitivity and fast response glucose biosensing. Microchem J 2023; 184: 108142.
| Crossref | Google Scholar |

46  Lee J, Kim J. Enhancing the thermal conductivity of PEG composites with freeze-drying and surface treatment of MXene and CNT. Mater Today Chem 2023; 27: 101305.
| Crossref | Google Scholar |

47  Li X, Li H, Fan X, et al. 3D‐printed stretchable micro‐supercapacitor with remarkable areal performance. Adv Energy Mater 2020; 10(14): 1903794.
| Crossref | Google Scholar |

48  Yang W, Yang J, Byun JJ, et al. 3D printing of freestanding MXene architectures for current‐collector‐free supercapacitors. Adv Mater 2019; 31(37): 1902725.
| Crossref | Google Scholar | PubMed |

49  Wei C, Tian M, Wang M, et al. Universal in situ crafted MOx-MXene heterostructures as heavy and multifunctional hosts for 3D-printed Li-S Batteries. ACS Nano 2020; 14(11): 16073-16084.
| Crossref | Google Scholar | PubMed |

50  Wang Z, Huang Z, Wang H, et al. 3D-printed sodiophilic V2CTx/rGO-CNT MXene microgrid aerogel for stable Na metal anode with high areal capacity. ACS Nano 2022; 16(6): 9105-9116.
| Crossref | Google Scholar | PubMed |

51  Fan Z, Jin J, Li C, et al. 3D-printed Zn-ion hybrid capacitor enabled by universal divalent cation-gelated additive-free Ti3C2 MXene ink. ACS Nano 2021; 15(2): 3098-3107.
| Crossref | Google Scholar | PubMed |

52  Yang C, Wu X, Xia H, et al. 3D printed template-assisted assembly of additive-free Ti3C2Tx MXene microlattices with customized structures toward high areal capacitance. ACS Nano 2022; 16(2): 2699-2710.
| Crossref | Google Scholar | PubMed |

53  Zhou G, Li MC, Liu C, et al. 3D printed Ti3C2Tx MXene/cellulose nanofiber architectures for solid‐state supercapacitors: Ink rheology, 3D printability, and electrochemical performance. Adv Funct Mater 2022; 32(14): 2109593.
| Crossref | Google Scholar |

54  Redondo E, Pumera M. MXene-functionalised 3D-printed electrodes for electrochemical capacitors. Electrochem Commun 2021; 124: 106920.
| Crossref | Google Scholar |

55  Ghosh K, Pumera M. MXene and MoS3−x Coated 3D-Printed Hybrid Electrode for Solid-State Asymmetric Supercapacitor. Small Methods 2021; 5(8): 2100451.
| Crossref | Google Scholar | PubMed |

56  Huang X, Wang R, Jiao T, et al. Facile preparation of hierarchical AgNP-loaded MXene/Fe3O4/polymer nanocomposites by electrospinning with enhanced catalytic performance for wastewater treatment. ACS Omega 2019; 4(1): 1897-1906.
| Crossref | Google Scholar | PubMed |

57  Zhang Y, Wu D, Liao S, et al. A multi-mode cellulose acetate/MXene Janus film with structure enhanced self-reflection, selective emission and absorption for cooling and heating. Compos Part A Appl Sci Manuf 2023; 172: 107622.
| Crossref | Google Scholar |

58  Li Y, Zhang X. Electrically conductive, optically responsive, and highly orientated Ti3C2Tx MXene aerogel fibers. Adv Funct Mater 2022; 32(4): 2107767.
| Crossref | Google Scholar |

59  Jiang C, Wu C, Li X, et al. All-electrospun flexible triboelectric nanogenerator based on metallic MXene nanosheets. Nano Energy 2019; 59: 268-276.
| Crossref | Google Scholar |

60  Li Y, Meng F, Mei Y, et al. Electrospun generation of Ti3C2Tx MXene@ graphene oxide hybrid aerogel microspheres for tunable high-performance microwave absorption. Chem Eng J 2020; 391: 123512.
| Crossref | Google Scholar |

61  Xu X, Wang S, Wu H, et al. A multimodal antimicrobial platform based on MXene for treatment of wound infection. Colloids Surf B Biointerfaces 2021; 207: 111979.
| Crossref | Google Scholar | PubMed |

62  Fu X, Li L, Chen S, et al. Knitted Ti3C2Tx MXene based fiber strain sensor for human-computer interaction. J Colloid Interface Sci 2021; 604: 643-649.
| Crossref | Google Scholar | PubMed |

63  Cheng Y, Xie Y, Liu Z, et al. Maximizing electron channels enabled by mxene aerogel for high-performance self-healable flexible electronic skin. ACS Nano 2023; 17(2): 1393-1402.
| Crossref | Google Scholar | PubMed |

64  Feng H, Tian Q, Huang J, et al. Supercritical CO2-assisted solid-phase etching preparation of mxenes for high-efficiency alkaline hydrogen evolution. Green Chem 2023; 25: 3966-3973.
| Crossref | Google Scholar |

65  Chang X, Zhu Q, Zhao Q, et al. 3D Porous Co3O4/MXene Foam Fabricated via a Sulfur Template Strategy for Enhanced Li/K-Ion Storage. ACS Appl Mater Interface 2023; 15(6): 7999-8009.
| Crossref | Google Scholar | PubMed |

66  Chen J, Ding Y, Yan D, et al. Synthesis of MXene and its application for zinc‐ion storage. SusMat 2022; 2(3): 293-318.
| Crossref | Google Scholar |

67  Feng M, Wang W, Hu Z, et al. Engineering chemical-bonded Ti3C2 MXene@carbon composite films with 3D transportation channels for promoting lithium-ion storage in hybrid capacitors. Sci China Mater 2023; 66(3): 944-954.
| Crossref | Google Scholar | PubMed |

68  Dong S, Lv N, Wu Y, et al. Lithium‐ion and sodium‐ion hybrid capacitors: from insertion‐type materials design to devices construction. Adv Funct Mater 2021; 31(21): 2100455.
| Crossref | Google Scholar |

69  Nasrin K, Sudharshan V, Subramani K, et al. Insights into 2D/2D MXene heterostructures for improved synergy in structure toward next‐generation supercapacitors: A review. Adv Funct Mater 2022; 32(18): 2110267.
| Crossref | Google Scholar |

70  Luo J, Fang C, Jin C, et al. Tunable pseudocapacitance storage of MXene by cation pillaring for high performance sodium-ion capacitors. J Mater Chem A 2018; 6(17): 7794-7806.
| Crossref | Google Scholar |

71  Kajiyama S, Szabova L, Iinuma H, et al. Enhanced li‐ion accessibility in mxene titanium carbide by steric chloride termination. Adv Energy Mater 2017; 7(9): 1601873.
| Crossref | Google Scholar |

72  Li C, Zhang D, Cao J, et al. Ti3C2 MXene-encapsulated NiFe-LDH hybrid anode for high-performance lithium-ion batteries and capacitors. ACS Appl Energ Mater 2021; 4: 7821-7828.
| Crossref | Google Scholar |

73  Yu L, Xiong Z, Zhang W, et al. SnO2/SnS2 heterostructure@ MXene framework as high performance anodes for hybrid lithium-ion capacitors. Electrochim Acta 2022; 409: 139981.
| Crossref | Google Scholar |

74  Fan Z, Wei C, Yu L, et al. 3D printing of porous nitrogen-doped Ti3C2 MXene scaffolds for high-performance sodium-ion hybrid capacitors. ACS Nano 2020; 14(1): 867-876.
| Crossref | Google Scholar | PubMed |

75  Mashtalir O, Lukatskaya MR, Kolesnikov AI, et al. The effect of hydrazine intercalation on the structure and capacitance of 2D titanium carbide (MXene). Nanoscale 2016; 8: 9128-9133.
| Crossref | Google Scholar | PubMed |

76  Luo J, Zhang W, Yuan H, et al. Pillared structure design of MXene with ultra-large interlayer spacing for high performance lithium-ion capacitors. ACS Nano 2017; 11: 2459-2469.
| Crossref | Google Scholar | PubMed |

77  Ahmed B, Anjum DH, Gogotsi Y, et al. Atomic layer deposition of SnO2 on MXene for Li-Ion battery anodes. Nano Energy 2017; 34: 249-256.
| Crossref | Google Scholar |

78  Zhao S, Liu Z, Xie G, et al. Achieving high performance 3D K+-pre-intercalated Ti3C2Tx MXene for Potassium-ion hybrid capacitors via regulating electrolyte solvation structure. Angew Chem Int Ed 2021; 60: 26246-26253.
| Crossref | Google Scholar | PubMed |

79  Melchior SA, Raju K, Ike IS, et al. High-voltage symmetric supercapacitor based on 2D titanium carbide (MXene, Ti2CTx)/carbon nanosphere composites in a neutral aqueous electrolyte. J Electrochem Soc 2018; 165: A501-A511.
| Crossref | Google Scholar |

80  Li L, Wang F, Zhu J, et al. The Facile Synthesis of layered Ti2C MXene/carbon nanotube composite paper with enhanced electrochemical properties. Dalton Trans 2017; 46: 14880-14887.
| Crossref | Google Scholar | PubMed |

81  Song F, Hu J, Li G, et al. Room-temperature assembled MXene-based aerogels for high mass-loading sodium-ion storage. Nanomicro Lett 2022; 14(1): 37.
| Crossref | Google Scholar | PubMed |

82  Li G, Lian S, Song F, et al. Surface chemistry and mesopore dual regulation by sulfur‐promised high volumetric capacity of Ti3C2Tx films for sodium-ion storage. Small 2021; 17(49): 2103626.
| Crossref | Google Scholar | PubMed |

83  Shi H, Yue M, Zhang CJ, et al. 3D flexible, conductive, and recyclable Ti3C2Tx MXene-melamine foam for high-areal-capacity and long-lifetime alkali-metal anode. ACS Nano 2020; 14(7): 8678-8688.
| Crossref | Google Scholar | PubMed |

84  Lian P, Dong Y, Wu ZS, et al. Alkalized Ti3C2 MXene nanoribbons with expanded interlayer spacing for high-capacity sodium and potassium ion batteries. Nano Energy 2017; 40: 1-8.
| Crossref | Google Scholar |

85  Wang Q, Wang S, Guo X, et al. MXene‐reduced graphene oxide aerogel for aqueous zinc‐ion hybrid supercapacitor with ultralong cycle life. Adv Electron Mater 2019; 5(12): 1900537.
| Crossref | Google Scholar |

86  Maughan PA, Tapia-Ruiz N, Bimbo N. In-situ pillared MXene as a viable zinc-ion hybrid capacitor. Electrochim Acta 2020; 341: 136061.
| Crossref | Google Scholar |

87  Li X, Li M, Yang Q, et al. Vertically aligned Sn4+ preintercalated Ti2CTX MXene sphere with enhanced Zn ion transportation and superior cycle lifespan. Adv Energy Mater 2020; 10(35): 2001394.
| Crossref | Google Scholar |