Black Phosphorus: Preparation, Properties, Structure & Uses

White Phosphorus - Preparation, Properties, Structure & Uses | NEET JEE GATE CSIR

Black Phosphorus – Preparation, Properties, Structure, Uses and Previous Year Questions

Highly Important for NEET, JEE Main/Advanced, GATE & CSIR-NET

1. Preparation of Black Phosphorus

Method	Description	Exam Importance
High Pressure Method (Bridgman Method)	White phosphorus is heated at 200–250°C under very high pressure (~12,000–15,000 atm) for several hours.	JEE Advanced & CSIR-NET favourite Most asked method in exams
From Red Phosphorus	Red P is heated in a sealed tube at 530–600°C under high pressure or with mercury as catalyst.	GATE & CSIR-NET level
Bismuth Flux Method	White P + Bismuth melt → cooled slowly → black P crystals separate.	Advanced level (rarely asked)

JEE Trick: Only high pressure + moderate heat on white P gives black P. Normal heating gives red P.

2. Structure of Black Phosphorus

Orthorhombic Black Phosphorus – Puckered layered structure (similar to graphite)

Orthorhombic crystal system
Layered structure – each layer consists of puckered (corrugated) sheets
Each P atom is covalently bonded to 3 other P atoms (sp³ hybridized)
3 bond pairs + 1 lone pair per P atom
Layers held by weak van der Waals forces → can be exfoliated to phosphorene (2D material)
Similar to graphite in layered arrangement

JEE Advanced & CSIR-NET Classic:
Black P has puckered layer structure with each P atom forming 3 covalent bonds.

3. Physical Properties of Black Phosphorus

Property	Black Phosphorus	Comparison
Appearance	Shiny, black, metallic lustre	Looks like graphite
Density	Highest → 2.69 g/cm³	White: 1.82, Red: ~2.1–2.34
Melting Point	~610°C (under pressure)	Highest among allotropes
Electrical Conductivity	Best conductor (semiconductor, band gap ~0.3 eV)	White & Red are insulators
Solubility	Insoluble in water & CS₂	Like red P
Ignition Temperature	Very high (>400°C)	Does not catch fire easily
Reactivity	Least reactive allotrope	Most stable
Stability	Thermodynamically most stable	Stability: Black > Red > White

NEET/JEE Most Repeated:
• Most stable allotrope
• Highest density & melting point
• Best electrical conductor
• Least reactive
• Metallic lustre

4. Chemical Properties

Highly resistant to air and water at room temperature
Burns only when heated strongly (>400°C) → P₄O₁₀
Does not glow in dark (no chemiluminescence)
Reacts very slowly with halogens, alkalis, etc.
With hot conc. NaOH → very slow reaction (unlike white/red P)

5. Uses of Black Phosphorus

Use	Reason
Emerging 2D material – Phosphorene	Single-layer black P → excellent semiconductor
High-performance transistors & sensors	Tunable band gap, high carrier mobility
Lithium-ion & sodium-ion batteries	High theoretical capacity
Photodetectors & solar cells	Broadband photoresponse
Potential replacement for graphene in some applications	Has natural band gap (unlike graphene)

CSIR-NET & GATE Modern Topic:
“Phosphorene” (monolayer black P) is the phosphorus analogue of graphene.

Quick Comparison Table (High Yield for Exams)

Property	White P	Red P	Black P
Structure	P₄ tetrahedron	Polymeric chain	Puckered layers
Stability	Least stable	Moderate	Most stable
Reactivity	Most reactive	Less reactive	Least reactive
Density	1.82	2.1–2.34	2.69 g/cm³
Conductivity	Insulator	Insulator	Semiconductor
Ignition Temp	30°C	260°C	>400°C
CS₂ Solubility	Soluble	Insoluble	Insoluble
Glow in dark	Yes	No	No

High-Yield One-Liners for Last-Minute Revision

Black P is the most stable and least reactive allotrope
Prepared by heating white P under very high pressure
Has puckered layered structure like graphite
Each P atom bonded to 3 others
Highest density (2.69 g/cm³) and melting point
Best conductor (semiconductor) among allotropes
Exfoliated form = Phosphorene (2D material)
Does not glow in dark, insoluble in CS₂
Used in next-gen electronics and batteries

Must Read Black Phosphorus MCQs asked in NEET, IIT-JEE, GATE, IIT-JAM & CSIR-NET

Black Phosphorus: Preparation, Properties, Structure & Uses